Eolian Dune, Interdune, Sand Sheet, and Siliciclastic Sabkha ...
Eolian Dune, Interdune, Sand Sheet, and Siliciclastic Sabkha ...
Eolian Dune, Interdune, Sand Sheet, and Siliciclastic Sabkha ...
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The American Association of Petroleum Geologists Bulletin<br />
V. 67. No. 2 (February, 1983). P.280-312, 29Figs., 5 Tables<br />
<strong>Eolian</strong> <strong>Dune</strong>, <strong>Interdune</strong>, <strong>S<strong>and</strong></strong> <strong>Sheet</strong>, <strong>and</strong> <strong>Siliciclastic</strong><br />
<strong>Sabkha</strong> Sediments of an Offshore Prograding<br />
<strong>S<strong>and</strong></strong> Sea, Dhahran Area, Saudi Arabia^<br />
STEVEN G. FRYBERGER,^ ABDULKADER M. AL-SARI/ <strong>and</strong> THOMAS J. CLISHAM^<br />
ABSTRACT<br />
An offshore prograding s<strong>and</strong> sea exists along portions of<br />
the Arabian Gulf coastline near Dhahran, Saudi Arabia.<br />
In this region, sediments of eolian dune, interdune, s<strong>and</strong><br />
sheet, <strong>and</strong> siliciclastic sabkha intercalate with marine<br />
deposits. This depositional setting is characterized by<br />
strong offshore winds which supply abundant s<strong>and</strong> to the<br />
coastline, <strong>and</strong> cause at present time the outbuilding of the<br />
dune system. This quartz-detrital dominant setting contrasts<br />
markedly with the carbonate dominant setting<br />
resulting from onshore winds in the Trucial Coast area to<br />
the south. The broad intercalation of eolian <strong>and</strong> marine<br />
deposits which results creates ideal potential for subregional<br />
stratigraphic petroleum traps, due to pinch-out of<br />
porous <strong>and</strong> permeable dune s<strong>and</strong>s into impermeable<br />
marine mudstones. Within the eolian system itself are<br />
potential reservoir rocks (dunes), sources (organic-rich<br />
sabkha <strong>and</strong> <strong>Interdune</strong> deposits), <strong>and</strong> seals (zones of early<br />
cementation in all deposits). Early cementation is very<br />
common in all fades of the eolian s<strong>and</strong> sea. The early<br />
cementation occurs owing to (1) soil formation, (2) deposition<br />
of pore-filling gypsiferous cements from saturated<br />
solutions near water table, <strong>and</strong> (3) addition of s<strong>and</strong>-size<br />
windblown evaporitic material to s<strong>and</strong>s downwind of sabkhas.<br />
INTRODUCTION<br />
The purpose of our studies in the Jafurah s<strong>and</strong> sea near<br />
Dhahran was to identify <strong>and</strong> describe sedimentary facies<br />
of the complex offshore prograding eolian s<strong>and</strong> sea near<br />
Dhahran. We felt this would be of great interest to geologists<br />
working with petroleum-bearing rocks of similar origins.<br />
As our work progressed, it became apparent that the<br />
©Copyright 1983. The American Association of Petroleum Geologists. All<br />
rights reserved.<br />
'Manuscript received, February 1,1982; accepted, August 26,1982.<br />
^Research Institute, University of Petroleum <strong>and</strong> Minerals, Dhahran, Saudi<br />
Arabia. Present address: McAdams, Roux <strong>and</strong> Associates, Suite 530,73017th<br />
St., Denver, Colorado 80202.<br />
^Research Institute, University of Petroleum <strong>and</strong> Minerals, Dhahran, Saudi<br />
Arabia.<br />
We owe considerable gratitude to Arabian American Oil Co. (ARAMCO) for<br />
generous access to technical data, for permission to conduct experiments on<br />
the ARAMCO reservation, <strong>and</strong> for access to facilities. We gratefully acknowledge<br />
the financial assistance provided by Amoco Production Co., Cities Service<br />
Co., <strong>and</strong> McAdams, Roux <strong>and</strong> Associates in the preparation of this report.<br />
We acknowledge the assistance of S.A.R. Rizvi of the Research Institute, University<br />
of Petroleum <strong>and</strong> Minerals, Dhahran, in identification of floral <strong>and</strong> faunal<br />
specimens. We also gratefully acknowledge the assistance of Dan Bean<br />
<strong>and</strong> Davis Oil Co. for use of core samples from the Minnelusa Fonnation.<br />
280<br />
eolian system deposits near Dhahran represented a relatively<br />
undescribed "s<strong>and</strong> dominant" model for arid coastlines<br />
in contrast to the classical carbonate models of the<br />
nearby Trucial Coast.<br />
Methods of Study<br />
Our studies extended throughout Saudi Arabia during<br />
1978 to 1980, but were focused in the Jafurah saad sea<br />
between Dhahran <strong>and</strong> Abqaiq (Figs. 1, 2), where 14 test<br />
sites were chosen (Fig. 2). At each site, a s<strong>and</strong> trap was<br />
installed which could catch <strong>and</strong> store windblown s<strong>and</strong> for<br />
up to a month. Additionally, ten wooden stakes were<br />
placed in a grid around each trap, to measure erosion <strong>and</strong><br />
deposition of s<strong>and</strong>. Wind stations were Installed at stations<br />
4, 9, 10, <strong>and</strong> 15 (Fig. 2) to record wind speed <strong>and</strong><br />
direction during the course of a year from about October<br />
1979, to October 1980. <strong>Dune</strong> advance near the test sites<br />
was studied monthly for a year, as were the traps <strong>and</strong><br />
stakes. The test sites were selected to represent typical<br />
eolian terrains: dune, interdune, s<strong>and</strong> sheet, <strong>and</strong> sabkha.<br />
As a result of this continuous study, distinctive sedimentary<br />
features could be observed to form at the various test<br />
sites, providing assistance in interpreting complex<br />
sequences of sedimentary structures in trenches dug near<br />
the same study sites. Sediment samples were collected for<br />
sieving <strong>and</strong> petrographic study, along with specimens of<br />
local biota.<br />
Previous Work on Modern<br />
Sediments in Study Area<br />
The principal body of literature on modern sediments of<br />
the Arabian Gulf Coast describes the carbonatedominated<br />
coastal sabkhas, lagoons, <strong>and</strong> reefs of the Trucial<br />
Coast (Kendall <strong>and</strong> Skipwith, 1964; llling et al, 1965;<br />
Butler, 1969; Evans et al, 1969; Taylor <strong>and</strong> llling, 1969;<br />
Purser, 1973; Patterson <strong>and</strong> Kinsman, 1982). These studies<br />
collectively describe a part of the Arabian Gulf Coast<br />
characterized by carbonate sedimentation, which exists<br />
despite the presence of vast areas of quartzose dunes a<br />
short distance inl<strong>and</strong>. Many of the important aspects of<br />
sedimentation along the Trucial Coast depend on the wind<br />
blowing mainly from sea to l<strong>and</strong>. In particular, the<br />
onshore wind assures that the nearshore <strong>and</strong> intertidal<br />
zones with their abundant biota are not buried by quartz<br />
s<strong>and</strong> from the nearby desert. As pointed out by H<strong>and</strong>ford<br />
(1981), these classic studies resulted in a depositional<br />
model for sabkhas <strong>and</strong> desert coasts which has been<br />
extended too far in interpreting ancient rocks.
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 281<br />
BEDROCK<br />
SABKHA<br />
GULF<br />
RAS<br />
TANNURAH<br />
DUNE, SAND<br />
SHEET INTER-<br />
DUNE<br />
lAHRAN<br />
DOME<br />
FIG. 1—Map of northern Jafurah s<strong>and</strong> sea on eastern shore of Arabian Gulf. Dotted areas are<br />
s<strong>and</strong>y terrains including dunes, interdunes, <strong>and</strong> s<strong>and</strong> sheets of Jafurah s<strong>and</strong> sea. Dark areas<br />
are sabkhas. Note distribution of sabkhas along coast, <strong>and</strong> around margins or downwind of<br />
gentle highs in region. Map after Steineke et al (1958).
282 Dhahran Area, Saudi Arabia<br />
FIG. 2—Map of study area <strong>and</strong> Arabian Gulf coastline near Dhahran showing s<strong>and</strong> trap sites 1 to 15 (no trap site 8) <strong>and</strong> cross section<br />
AA'. Dhahran "s<strong>and</strong> rose" (Frytierger, 1979) in upper left shows principal directions from which s<strong>and</strong> drifts. Arrow on s<strong>and</strong> rose<br />
indicates resultant drift direction. D shows location of Dhahran.
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 283<br />
Another radically different model is equally viable, however,<br />
for deposition along desert margin shorelines <strong>and</strong><br />
associated dune-sabkha systems. Shinn (1973) suggested a<br />
model for offshore prograding dune systems: desert<br />
shorelines in which strong winds blow from the l<strong>and</strong><br />
toward the sea, <strong>and</strong> active quartz s<strong>and</strong> dunes are present<br />
on l<strong>and</strong> (Fig. 3). Shinn described cores from a sabkha, created<br />
in part by the prograding dunes, which revealed a<br />
transition upward from nearshore marine, calcareous,<br />
fossiUferous muds to more siliceous cross-bedded beach<br />
accretion-slope s<strong>and</strong>s. Offshore progradation of dunes<br />
has also been described by Sarnthein <strong>and</strong> Walger (1974)<br />
along the coast of Mauritania.<br />
FIG. 3—Air view (toward soutlieast) showing dunes prograding<br />
into sea on west coast of Qatar Peninsula.<br />
Sedimentation in the Arabian Gulf has been described<br />
by Sugden (1963) <strong>and</strong> Diester-Haas (1973). Sarnthein<br />
(1972) suggested that relict dunes may be preserved on the<br />
floor of the Gulf, apparently deposited during lowered sea<br />
levels caused by latest Pleistocene glaciation. An excellent<br />
summary of data on probable sea level changes in the Gulf<br />
since the last glaciation is offered by Al-Asfour (1978)<br />
(Fig. 4).<br />
The literature dealing with modern eolian <strong>and</strong> related<br />
deposits of Saudi Arabia is rather sparse. Regional settings<br />
<strong>and</strong> distribution of dunes in Arabia have been described<br />
by Holm (1953, 1960) <strong>and</strong> more recently by Breed et al<br />
(1979). Lacustrine <strong>and</strong> related deposits of glacial times in<br />
the Rub al Khali have been described by McClure (1976).<br />
Holm (1960) recognized the fundamental distinction<br />
between the two types of sabkhas on the Arabian coast.<br />
One type (discussed as "siliciclastic" in this report) is<br />
called by Holm "arenaceous," meaning "filled with<br />
s<strong>and</strong>." Holm observed these sabkhas to form by movement<br />
of dunes <strong>and</strong> windblown s<strong>and</strong> into the shoreline<br />
zone. "Argillaceous" sabkhas, however, form from the<br />
manufacture of calcareous mud by algae <strong>and</strong> other organisms<br />
in areas where windblown s<strong>and</strong> does not bury these<br />
organisms. Additional descriptions of eohan deposits of<br />
Saudi Arabia <strong>and</strong> environs are available in Bagnold (1951)<br />
<strong>and</strong> Glennie (1970); in accounts of early travelers such as<br />
Thomas (1932), Philby (1933), <strong>and</strong> Thesiger (1949); <strong>and</strong> in<br />
the geologic <strong>and</strong> geographic map series at 1:500,000 scale<br />
of the U.S. Geological Survey. Other more recent contributions<br />
include those of Chapman (1971), who discussed<br />
the fluvial geomorphology of the Eastern Province, <strong>and</strong><br />
Patterson <strong>and</strong> Kinsman (1981) who described the dependence<br />
of sabkha growth in the Trucial Coast area on gradual<br />
rise of regional ground-water table. The best<br />
description of the essentially siliciclastic sabkhas in the<br />
study area is by Johnson et al (1978). Johnson et al make a<br />
distinction between "coastal sabkhas," with brine derived<br />
partly from seawater, <strong>and</strong> "inl<strong>and</strong> sabkhas," where sabkhas<br />
represent areas of equilibrium between eolian deflation<br />
<strong>and</strong> the ground-water table. Some features of<br />
interdune <strong>and</strong> sabkha deposits in the Jafurah s<strong>and</strong> sea are<br />
discussed in Ahlbr<strong>and</strong>t <strong>and</strong> Fryberger (1981). An excellent<br />
summary of meteorologic <strong>and</strong> oceanographic data for the<br />
gulf <strong>and</strong> Eastern Province is available in WilUams (1979).<br />
Depositional Model<br />
The purpose of this study is in part to illustrate the sedimentary<br />
features of the eolian deposits of an offshore prograding<br />
s<strong>and</strong> sea. It seems advisable, however, to briefly<br />
review other basic ideas which follow directly or indirectly<br />
from the facts which will be presented.<br />
One important idea is that this study deals with an aqueous<br />
mechanism of accumulation of eolian deposits, which<br />
is one of three basic mechanisms for the formation of<br />
eolian s<strong>and</strong> seas (Fryberger <strong>and</strong> Ahlbr<strong>and</strong>t, 1979). The<br />
aqueous mechanism refers simply to the role of water in<br />
causing the accumulation of wind-driven s<strong>and</strong>. The original<br />
concept (of Fryberger <strong>and</strong> Ahlbr<strong>and</strong>t, 1979) was that<br />
st<strong>and</strong>ing bodies of water can block wind-driven s<strong>and</strong>,<br />
causing an accumulation in the shoreline zone. Processes<br />
associated with high water tables <strong>and</strong> high ground-water<br />
salinity in the Arabian shoreUne zone-of-accumulation<br />
have important effects on sedimentary features of the<br />
deposits <strong>and</strong> their geometry. However, the aqueous mechanism<br />
of formation of s<strong>and</strong> seas is only one of three mechanisms,<br />
the other two being topographic <strong>and</strong> climatic. The<br />
topographic mechanism produces accumulation of winddriven<br />
s<strong>and</strong> by the action of positive or negative topography<br />
upon wind regime. Thus, s<strong>and</strong> will tend to be<br />
deposited upwind or downwind of escarpments, <strong>and</strong> will<br />
tend to fill low-lying areas where wind velocity is reduced.<br />
The chmatic mechanism of s<strong>and</strong>-sea formation causes<br />
regional s<strong>and</strong> deposition due to gradual loss of wind<br />
energy along the direction of s<strong>and</strong> drift.<br />
In summary, this study illustrates in detail the sedimentary<br />
aspects of only one subtype of eolian s<strong>and</strong> accumulation.<br />
Within this aqueous class of accumulation, however,<br />
are the basic types of eolian terrain common to all deserts<br />
(dune, interdune, s<strong>and</strong> sheet, <strong>and</strong> sabkha deposits), albeit<br />
with a distinctive suite of sedimentary features <strong>and</strong> sedimentary<br />
unit geometries.<br />
Another significant aspect of this model is the relationship<br />
of the prograding eolian sediments to the basinward<br />
marine sediments of the Arabian Gulf, which consist principally<br />
of biogenic lime muds <strong>and</strong> s<strong>and</strong>s (Fig. 5). Given a<br />
series of sea level changes such as those previously<br />
described for the Arabian Gulf (Fig. 4), appropriate subsidence,<br />
<strong>and</strong> the extremely shallow slope of the Arabian
284 Dhahran Area, Saudi Arabia<br />
PRESENT<br />
SEAIFVFI<br />
AFTER AL-ASFOUR,<br />
1 1 1 1 I<br />
1978<br />
1 1<br />
+20M<br />
50ft=<br />
50 ft-<br />
20M<br />
100<br />
-40<br />
150 -<br />
200 60<br />
250 -_<br />
-80<br />
300<br />
-lOO<br />
350<br />
1 1<br />
18 16 14 12 10 8 6 4 2<br />
YEARS B.R X I03<br />
FIG. 4—Probable eustatic sea level changes in study area since<br />
the last glacial period (Wisconsin) (after Al-Asfour, 1978).<br />
Shelf (in an ancient analog), it is probable that eohan<br />
deposits would develop as a series of tongues pinching out<br />
basinward. Thus, an excellent reservoir facies (eolian system)<br />
would be intercalated with potentially excellent<br />
source rocks (marine carbonate muds). Furthermore,<br />
upon reversal of regional dip, an excellent subregional<br />
stratigraphic trap would form (Fig. 5).<br />
There are two additional concepts which the authors<br />
hope to illustrate. First, within the prograding eolian system<br />
itself can be observed potential reservoir rocks, source<br />
rocks, <strong>and</strong> seals. The reservoir facies are the dunes <strong>and</strong><br />
other well-sorted eolian deposits, the source facies are<br />
organic-rich interdune <strong>and</strong> sabkha deposits, <strong>and</strong> the seals<br />
are early cemented eolian deposits which result in various<br />
ways from the abundance of evaporites in the system.<br />
Secondly, given the overall conditions of sedimentation<br />
along the west coast of the Arabian Gulf, the nature of the<br />
shoreline, whether siliciclastic or carbonate dominant, is<br />
dependent on wind direction. Slight changes in the orientation<br />
of the shoreline (given a relatively steady regional<br />
wind direction oblique to the shorehne) result in radically<br />
different shoreline deposits. The further implication for<br />
explorationists is that both the potentially prolific stratigraphic<br />
hydrocarbon provinces of eolian <strong>and</strong> carbonatedominant<br />
prograding systems may exist in close proximity<br />
in the subsurface as they do presently in the Arabian Gulf.<br />
Thus, the recognition of one such system in the subsurface<br />
may imply the possible existence of the other nearby<br />
Description of Study Area<br />
Climate.—The climate of the study area (encompassed<br />
essentially by Fig. 2) is typically desertic (Fig. 6). Mean<br />
O<br />
temperatures range from 60°F (16°C) in February to 98°F<br />
(37°C) in August, with recorded extremes in the study area<br />
of about 28°F (-2°C) <strong>and</strong> 125°F (52°C) (Williams, 1979).<br />
Rainfall is scant, averaging 3.11 in. (7.9 cm) annually at<br />
Dhahran from 1950 to 1976. Maximum rainfall, sometimes<br />
in the form of thunderstorms, occurs during winter<br />
(Fig. 6). Evaporation rates are high, averaging over 30 in./<br />
month (76 cm/month) during the summer. Diurnal ranges<br />
of temperature <strong>and</strong> humidity are extreme (about 30%)<br />
during much of the year. Humidities are greater near the<br />
coast, which commonly has early morning fog in fall <strong>and</strong><br />
spring.<br />
Wind direction in the study area is primarily from the<br />
north or northwest. A circular histogram depicting potential<br />
s<strong>and</strong> movement amount <strong>and</strong> direction (Fig. 2) is representative<br />
of most of the area (Fryberger, 1979). Some wind<br />
reversals occur, chiefly during winter, causing reversed slip<br />
faces on the crests of dunes, but apparently having little<br />
regional effect on sediment transport.<br />
Wind energy release is highly seasonal in the study area<br />
(Fig. 6). Maximum drift potentials, an expression of the<br />
potential s<strong>and</strong>-moving power of wind (Fryberger, 1979),<br />
typically occur in June (Fig. 6). This maximum coincides<br />
roughly with the maximum development of the Indian<br />
monsoon, <strong>and</strong> essentially represents air flow from high<br />
pressure over northern Arabia <strong>and</strong> Egypt into a low pressure<br />
cell over the Indian subcontinent. This monsoonal<br />
aspect of the wind- <strong>and</strong> s<strong>and</strong>-moving season (locally<br />
termed the "shamal") differs from the typical spring<br />
(April) s<strong>and</strong>-moving seasons of North Africa, which have<br />
their origins in wind energy released by strong frontal passages<br />
(Breed et al, 1979). The result is that s<strong>and</strong>storms in<br />
the study area are less violent, but more sustained than<br />
those in North Africa, lasting anywhere from one to several<br />
days, with winds in excess of 20 knots (37 km/hour).<br />
Nevertheless, the study area lies in one of the windiest desert<br />
regions of the world (Fryberger, 1979), with annual<br />
drift potentials in excess of 400 v.u. (30 mVmeter-width/<br />
year). As described below, this high energy wind regime is<br />
in part responsible for the abiUty of the eohan system to<br />
prograde the coastline in the study area.<br />
Past climates in the study area have not always been as<br />
arid or hot. Considerable evidence for a cooler, more<br />
humid phase, perhaps in the middle-late Pleistocene, has<br />
been presented by McClure (1976) <strong>and</strong> Chapman (1971).<br />
Chapmsm describes extensive duricrusts in the study area,<br />
<strong>and</strong> fluvial weathering features of many types which he<br />
attributes to a cooler, more humid clunate during the Pleistocene<br />
(100,000 to 60,000 y.b.p.) with more arid conditions<br />
typical of the present day beginning about 11,000<br />
years ago (Chapman, 1971). This change roughly coincides<br />
with the rise in sea level in the area beginning about<br />
18,000 years ago (Al-Asfour, 1978). We have observed<br />
some calcareous <strong>and</strong> gypsiferous Aridosols in older eoUan<br />
deposits of the study area, which based on associated<br />
human artifacts could not be more than a few thous<strong>and</strong><br />
years old. Thus, some soil-forming processes are also<br />
apparently proceeding at present, albeit slowly.<br />
The occurrence of pluvial periods, <strong>and</strong> changes in sea<br />
level in the area affect the conclusions of this study in two<br />
principal ways. First, pluvial periods <strong>and</strong> associated soil or
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 285<br />
p Sedimentary Basin Center<br />
Marine<br />
Deposits<br />
Tectonic Basin Center<br />
Subsidence<br />
^w<br />
^Stratigraphically Entrapped Petroleum<br />
(Subregional or Local) Younger<br />
Rock Units<br />
FIG. 5—Depositional model of the seaward prograding eolian system.<br />
A. Offshore directed winds cause eolian outbuilding which at times exceeds sea level rise caused by basin subsidence. Alternation of<br />
low <strong>and</strong> high sea levels, perhaps caused by glaciation (or evaporation in restricted basins) would assist process of preserving eolian<br />
deposits below average base level of sedimentation. Transitional contact would occur where eolian deposits overlie marine deposits<br />
(as described by Shinn, 1973). Unconformable contact would probably develop where marine deposits overlie eolian deposits in this<br />
model.<br />
B. Reversal of dip, perhaps due to formation of tectonic basin, as shown would lead to formation of regional <strong>and</strong> local stratigraphic<br />
traps for petroleum at pinch-out of eolian system dunes into marine limestones. Overlying rocks could be continental or marine,<br />
depending on sedimentary history of original basin. Ideal cap rocks (younger rock units) from st<strong>and</strong>point of petroleum accumulation<br />
would be evaporitic.<br />
duricrust-forming episodes enhance the possibilities for<br />
early cementation of the eolian deposits of the study area.<br />
Second, fluctuations in sea level on this scale enhance the<br />
potential for intercalation of eohan <strong>and</strong> marine deposits<br />
(an example of which is described by Evans et al., 1969).<br />
Physiography (Onshore)<br />
The study area lies on the interior platform of the Arabian<br />
Peninsula (Powers et al, 1966). The interior platform<br />
is an area in which structural dip is extremely slight, averaging<br />
about 0°20* toward the east. The flat-lying sediments<br />
are disturbed by local uplifts, possibly related to<br />
basement block faulting, which have formed the broad<br />
gentle anticlines associated with the oil accumulations at<br />
Ghawar <strong>and</strong> Abqaiq (Powers et al, 1966). The Dammam<br />
dome, on which Dhahran rests (Fig. 2) is apparently a salt<br />
intrusion structure.<br />
The bedrock underlying the eolian deposits of the study<br />
area consists of Tertiary s<strong>and</strong>stone, s<strong>and</strong>y limestones, <strong>and</strong><br />
marls of primarily continental origin (Powers et al, 1966).<br />
These deposits are in places poorly consoUdated, <strong>and</strong> thus<br />
may supply some of the sediment carried by the wind<br />
(white areas. Fig. 1). Some of the bedrock limestones (e.g.,<br />
the Eocene limestones of the Dammam dome) have been<br />
sculptured by wind into yardangs, <strong>and</strong> bear evidence of<br />
both wind <strong>and</strong> water erosion.<br />
The dune, interdune, <strong>and</strong> s<strong>and</strong> sheet deposits extend<br />
continuously in a 40-mi (64 km) wide strip along the coast-<br />
Une from Jubayl southward (Fig. 1). These deposits are<br />
thinnest, or absent, over topographic highs. <strong>Sabkha</strong><br />
deposits are most common along the coast <strong>and</strong> along the<br />
western margin of the Jafurah. Distribution of sabkhas is<br />
controlled by topography, with sabkhas occupying low<br />
areas along the coast or developing around the margins of<br />
structures such as the Dhahran dome <strong>and</strong> the Abqaiq antichne<br />
(Fig. 2).
286 Dhahran Area, Saudi Arabia<br />
Z<br />
ID<br />
9<br />
U<br />
LU<br />
><br />
<<br />
z<br />
LU<br />
B<br />
o Q-<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
-0-<br />
^^UbLT^p<br />
\4_HUhi^<br />
J ' F ' M A ' M ' J ' J ' A ' S ' O ' N ' D<br />
MONTH<br />
FIG. 6—Summary of Dbahran climate including mean monthly temperature, evaporation, <strong>and</strong> humidity. Drift potential, a measure<br />
of s<strong>and</strong> moving effectiveness of the wind, is at a maximum during the June "shamal" season of northwest winds. Graph is based on<br />
long-term climate records of ARAMCO <strong>and</strong> Dhahran International Airport.<br />
The abrupt termination of the Jafurah s<strong>and</strong> sea against<br />
the coastline near Jubayl indicates that the present shoreline<br />
may contribute sediment to the Jafurah system. This<br />
could occur through deposition of s<strong>and</strong> in the littoral zone<br />
by longshore drift (southward in the area) <strong>and</strong> subsequent<br />
removal by wind deflation. Additionally, it is inviting to<br />
speculate that the s<strong>and</strong> sea extended across the present<br />
floor of the Arabian Gulf possibly to the Tigress-<br />
Euphrates delta region during lowered sea levels of the<br />
Pleistocene. Well-rounded, red, "eolian" s<strong>and</strong> grains have<br />
been retrieved from the present gulf floor in the regions<br />
north of Jubayl by Emery (1956). Another source of s<strong>and</strong><br />
for the Jafurah may have been the extensive (presently<br />
inactive) Wadi Al Batin system (described by Holm, 1960)<br />
which crosses the Dhahna northeastward into Kuwait.<br />
The discussion of possible sources of s<strong>and</strong> for the Jafurah<br />
is of interest primarily (for our purposes) in terms of distal<br />
versus proximal s<strong>and</strong> supply. The apparent upwind cutoff<br />
of the Jafurah at Jubayl means that distal sources available<br />
during the Pleistocene owing to lowered sea levels <strong>and</strong><br />
active wadi systems are not now available with the exception<br />
of sediment supplied by longshore drift. Thus, resupply<br />
of s<strong>and</strong> must presently be from local sources such as<br />
older eolian deposits or bedrock. Our impression is that<br />
the Jafurah system is slightly undersaturated—that is,<br />
there is an excess of available wind energy over s<strong>and</strong> resup-<br />
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ply. This may account in part for the widespread occurrence<br />
of stable s<strong>and</strong> sheet terrain <strong>and</strong> thinner stratigraphic<br />
units we observed.<br />
Sources of water.—Perhaps surprisingly, water is abundant<br />
in the recent eoUan sediments of the study area, <strong>and</strong><br />
with wind plays a definitive role in the depositional process.<br />
There are three principal sources of water in the area:<br />
fossil water, meteoric water, <strong>and</strong> seawater. Brackish fossil<br />
water from subsurface aquifers rises to the surface in artesian<br />
wells near Hofuf, <strong>and</strong> is pumped to the surface for<br />
irrigation <strong>and</strong> other uses. This source of water accounts, in<br />
part, for the flow across the area near Abqaiq (Fig. 2) <strong>and</strong><br />
for the relative freshness of the water in the interdunal<br />
wadi shown in Figure 22b.<br />
Despite the low annual rainfall, fresh meteoric water is<br />
an important component of the ground-water system in<br />
the study area. <strong>S<strong>and</strong></strong> dunes are extremely effective storage<br />
devices for water, which enters the dunes during rains <strong>and</strong><br />
cannot run off <strong>and</strong> evaporate (Bagnold, 1954). This process<br />
contributes to the high ground-water tables in the study<br />
area. We observed a dramatic illustration of this process<br />
during the winter of 1980, when water tables in sabkhas at<br />
traps 11 <strong>and</strong> 7 (Fig. 2) rose 2 to 4 in. (5 to 10 cm) after winter<br />
rains which could not have totaled more than a few<br />
inches.<br />
Seawater is an important contribution to the ground-
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 287<br />
FIG. 7—Sediments on floor of the Arabian Gulf. Contour lines show percentage of CaCO, in sediments. After Emery (1956). Arrow<br />
shows study area.<br />
water table in locations near the coast, <strong>and</strong> provides a<br />
source of evaporite minerals.<br />
These various sources of water for the ground-water<br />
table seem to be imperfectly mixed. This resuks in interesting<br />
contrasts in vegetation <strong>and</strong> sedimentary features<br />
within relatively small areas, both as a function of amount<br />
of water <strong>and</strong> salinity.<br />
Physiography (Offshore)<br />
The floor of the Arabian Gulf is an extension of the Arabian<br />
interior platform. The gently sloping basin floor is<br />
interrupted by geologic structures (such as salt domes),<br />
coral reefs, mud mounds, <strong>and</strong> shallow terraces before<br />
reaching a maximum depth of 250 ft (75 m) in the center of<br />
the basin opposite Dhahran. Water salinities are about<br />
36.5°/ooin the center of the gulf, <strong>and</strong> exceed 42°/oo in some<br />
restricted bays <strong>and</strong> lagoons. Sediments offshore along the<br />
Arabian Coast are mainly calcareous s<strong>and</strong>s <strong>and</strong> muds<br />
(Fig. 7). The highest percentage of calcium carbonate in<br />
the sediments corresponds to highest salinities (Emery,<br />
1956) but may also be seen to be associated with the<br />
onshore wind province of the Trucial Coast (Fig. 7).<br />
Sources of carbonate material offshore include shells <strong>and</strong><br />
comminuted shell debris including mollusks, corals, <strong>and</strong><br />
algae, inorganically precipitated material, <strong>and</strong> airborne<br />
dust, which is highly calcareous (Emery, 1956). Emery<br />
noted that medium-grained carbonate s<strong>and</strong>s occur on<br />
bathymetric highs in the gulf, whereas lower areas are<br />
commonly muddier. The proximity of the nearby prograding<br />
dune system is indicated by the presence of quartz s<strong>and</strong><br />
in the sediments <strong>and</strong> by the well-rounded shape <strong>and</strong> reddish<br />
color of some grains found by Emery.<br />
Tidal range is 8 ft (2.5 m) <strong>and</strong> waves may be up to 12 ft<br />
(3.5 m) high during storms. Water temperatures at the surface<br />
range from 60 to 90°F (15 to 32°C) in winter <strong>and</strong> summer,<br />
respectively<br />
<strong>Eolian</strong> Terrains <strong>and</strong> Associated<br />
Wind Microenvironments<br />
Within the broad prograding s<strong>and</strong> sea near Dhahran are<br />
four basic types of wind-laid deposits: dunes, interdunes,<br />
s<strong>and</strong> sheets, <strong>and</strong> sabkhas. These four basic types of eohan<br />
terrain can occur in varying proportions in any desert <strong>and</strong><br />
so are not unique to the study area. However, the sedimentary<br />
features <strong>and</strong> geometries of each type of deposit can<br />
differ greatly. We hope that sufficient detail will emerge in<br />
this study to allow recognition of similar systems preserved<br />
as ancient rocks. Nevertheless, it should be recog-
288 Dhahran Area, Saudi Arabia<br />
nized that even many of the sedimentary features of the<br />
distinctive eolian sabkha deposits (for instance) are not<br />
unique to the coastal setting.<br />
<strong>S<strong>and</strong></strong> dunes.—<strong>S<strong>and</strong></strong> dunes in the area are of the barchanoid<br />
type (McKee, 1979), including barchan, barchanoid<br />
ridge, transverse ridge, dome, parabolic, <strong>and</strong> various<br />
types of coppice or "obstacle" dunes (Fig. 8). <strong>Dune</strong><br />
heights average 16 to 40 ft (5 to 12 m). <strong>Dune</strong> advance measured<br />
during our one-year study ranged from 89 ft (27 m)<br />
for a barchan dune 10 ft (3 m) high, to 27 ft (8 m) for a barchanoid<br />
ridge dune 40 ft (12 m) high. Crests of dunes are<br />
usually very active <strong>and</strong> unvegetated; however, some vegetation,<br />
mainly Cyprus conglumeratus, commonly grows<br />
on the windward slope of some dunes. This leads to development<br />
of parabolic dunes, particularly in the northern<br />
part of the study area near Jubayl. The microenvironment<br />
of the dune terrain is the most exposed, because of wind<br />
<strong>and</strong> s<strong>and</strong> blast, <strong>and</strong> the least stable because of the advance<br />
of the dune. Drift rates which we measured atop a dome<br />
dune at site 15 (Fig. 9) illustrate the very active nature of<br />
the dune microenvironment compared to the other eolian<br />
terrains. Wind flow over dunes is steady on the upwind<br />
sides <strong>and</strong> over the crest, eddying <strong>and</strong> breaking into vortices<br />
as it flows over the brink. Wind reversals frequently result<br />
in attached horizontal vortices at the brink of the dune<br />
which scour out huge troughs, 3 to 13 ft (1 to 4 m) in circumference.<br />
These troughs fill quickly upon return of<br />
wind to the usual northwest direction.<br />
<strong>Interdune</strong>s.—Geomorphically, interdunes are flat or<br />
gently sloping areas between dunes, as shown in Figure 8a,<br />
b. <strong>Interdune</strong>s may be enclosed by dunes (Fig. 8b) or exist<br />
as rather extensive areas between widely spaced dunes<br />
(Fig. 8a). In general, the subenvironment we associate<br />
with true interdunes <strong>and</strong> associated deposits occurs in<br />
enclosed interdunes. Further, the deposits seem more distinctive<br />
as the associated dunes become larger. The larger<br />
size allows more time for deposition due to slower movement<br />
<strong>and</strong> results in greater variability wind regime.<br />
The wind microenvironment of interdunes is more directionally<br />
variable than that of dunes owing to eddying<br />
around the upwind dune. Wind strengths are less in the<br />
interdunes due to the sheltering effect of the upwind dune,<br />
but are frequently very gusty, with s<strong>and</strong> flying about horizontally<br />
at eye level <strong>and</strong> falling from above. Drift rates in<br />
interdunes generally reflect the sheltering effect (Fig. 9).<br />
<strong>Interdune</strong> sites are increasingly unstable roughly in proportion<br />
to distance above water table (the local "base<br />
level"). Despite lower drift rates, rapid erosion often<br />
occurs in interdunes due to storage of drifting s<strong>and</strong> by the<br />
dune immediately upwind. Thus the wind crossing the<br />
interdune will commonly be very undersaturated (with<br />
respect to s<strong>and</strong> it could potentially carry) <strong>and</strong> will pick up<br />
s<strong>and</strong>. For example, the interdune at site 13 (Fig. 9) was<br />
eroded over 16 in. (40 cm) during our study, whereas the<br />
crest of the dome dune at trap 15 (Fig. 9) was eroded only 6<br />
in. (15 cm). Wind environments in interdunes near the<br />
water table frequently become weakened by the baffling<br />
effect of plants, <strong>and</strong> the lower more shehered position<br />
favors deposition over scour at many localities.<br />
<strong>S<strong>and</strong></strong> sheets.—<strong>S<strong>and</strong></strong> sheets are s<strong>and</strong>y plains (of either<br />
erosional or depositional origin) whose topographic<br />
expression is independent of water table (Fig. 8c, d). <strong>S<strong>and</strong></strong><br />
sheets seem to develop from remnant deposits of migrating<br />
dunes, or from strips <strong>and</strong> patches of s<strong>and</strong> deposited<br />
during storms. Some s<strong>and</strong> sheet deposits are crossbedded,<br />
clearly reflecting origins from migrating dunes.<br />
The s<strong>and</strong> sheet deposits of interest here, which comprise<br />
the bulk of those we studied, are horizontally bedded <strong>and</strong><br />
have developed independently of dunes through slow deposition.<br />
Surface wind velocities, <strong>and</strong> thus s<strong>and</strong>-drift rates,<br />
are lower over vegetated s<strong>and</strong> sheets than most other types<br />
of eolian terrain (Fig. 9). Thus s<strong>and</strong> sheets are relatively<br />
stable <strong>and</strong> provide a habitat for diverse flora <strong>and</strong> fauna.<br />
As vegetative cover diminishes, s<strong>and</strong> sheets are proportionally<br />
less stable because of increased exposure to wind<br />
currents around scattered clumps of vegetation <strong>and</strong><br />
increased exposure to drifting s<strong>and</strong> from upwind.<br />
<strong>Sabkha</strong>s.—<strong>Eolian</strong> sabkhas are evaporitic s<strong>and</strong>y plains<br />
of erosional or depositional origin, whose topographic<br />
expression is dependent on a shallow (3 to 6 foot, 1 to 2 m)<br />
ground-water table below the sabkha surface (Fig. 8c). A<br />
rising water table causes deposition of wind-laid sediments<br />
<strong>and</strong> evaporites. A falling water table (after establishment<br />
of a sabkha) causes erosion, often to a cemented layer<br />
which typically forms in the uppermost several feet of the<br />
saturated zone. <strong>Sabkha</strong>s are distributed widely in the study<br />
area (Figs. 1,2), ranging from the extensive, highly evaporitic<br />
coastal sabkhas such as <strong>Sabkha</strong> Ar Riyas, to very<br />
small, interdunal, slightly evaporitic sabkhas far inl<strong>and</strong>.<br />
Since evaporite minerals are commonly available in the<br />
sediments, the principal requirement for sabkha development<br />
in the study area was presence of a shallow water<br />
table. It is not known whether evaporite minerals in<br />
ground water are a requirement for the formation of sabkhas,<br />
although every sabkha we observed was associated<br />
with moderately to extremely evaporitic conditions. Relatively<br />
fresh, shallow water tables seem to result in proliferation<br />
of plant hfe, destroying the flat, open conditions<br />
associated with typical sabkhas.<br />
<strong>Sabkha</strong>s were the most stable terrain we observed, with<br />
only an inch or two of erosion or deposition during the<br />
course of the study. <strong>S<strong>and</strong></strong> drift rates over sabkhas are lowest<br />
in the center of the sabkhas (which are elongate downwind)<br />
<strong>and</strong> higher along the margins (which are closer to<br />
nearby dunes <strong>and</strong> s<strong>and</strong> sheets). During summer s<strong>and</strong>storms,<br />
however, drift rates can be rather high due to the<br />
exposed nature of the sabkha (Fig. 9).<br />
The evaporitic nature of typical sabkhas renders them<br />
unsuitable for even the most salt-resistant higher plants.<br />
We seldom observed algal or microbial growth at the surface<br />
of the sabkhas <strong>and</strong> then only in sabkhas associated<br />
with interdune ponds. However, some organic-rich or<br />
darkened zones, about 3 ft (1 m) below the sabkha surface<br />
(at the interface of the saturated water table <strong>and</strong> the capillary<br />
fringe) were associated with microbial growth. Larvae<br />
<strong>and</strong> burrows of an unidentified insect were observed at<br />
<strong>Sabkha</strong> Ar Riyas. Thus, some insects are capable of surviving<br />
in the sabkha sediments.<br />
Microenvironments of Ground-Water Table<br />
The damp zone, capillary fringe zone, <strong>and</strong> saturated<br />
zone (Table 1) associated with the presence of the shallow
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FIG. 8—Air views of study area showing different types of desert terrain. A. Near Dammam looking southwest. Isolated barchan <strong>and</strong> dome dunes move across vegetated s<strong>and</strong> sheets<br />
Palm trees for scale. <strong>Eolian</strong> sUiciclastic sabkha is dark area in lower left portion of photograph. B. <strong>Dune</strong> complex consisting of barchanoid dunes near trap 4. Sabkbalike interdune in<br />
left center, vegetated interdune in background. Utility poles for scale. C. View to southeast. Dome dunes (arrow 1), vegetated s<strong>and</strong> sheet (arrow 2), <strong>and</strong> eoUan siliciclastic sabkha<br />
(arrow 3). Tire tracks on sabkha for scale. Note irregular distribntion of s<strong>and</strong> sheet. D. View to northeast (Arabian Gulf in background) near Dammam showing extensive, veeetated<br />
s<strong>and</strong> sheet with parabolic dunes Oeft center) <strong>and</strong> blow outs.<br />
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290 Dhahran Area, Saudi Arabia<br />
Table 1<br />
Interface Processes at Interface Insects<br />
Wind/dry sediment Mobile s<strong>and</strong>, wind-formed features,<br />
contorted strata (loading)<br />
Dry/damp s<strong>and</strong><br />
Damp/capillary fringe<br />
Capillary fringe/water<br />
table<br />
Scour surfaces to cohesive damp s<strong>and</strong><br />
<strong>Sabkha</strong>, salt ridges, halite, soils,<br />
adhesion-produced strata<br />
Organic layers, micrite, euhedral<br />
gypsum crystals (croute zonaire), wet<br />
interdunes, anhydrite, contorted strata<br />
(liquefaction)<br />
F M<br />
1980<br />
Early<br />
Grasses Saltbrush Cementation Soils<br />
Relative abundance<br />
FIG. 9—<strong>S<strong>and</strong></strong> drift rates measured on different eolian terrains during the year of study. Note relatively steep increase in drift rates<br />
during June s<strong>and</strong>storms ("shamals") for exposed sabkha compared to other eolian terrains.<br />
ground-water table in these porous <strong>and</strong> permeable s<strong>and</strong>s<br />
each constitutes a distinctive microenvironment of the<br />
water table. Each zone (microenvironment) is associated<br />
with certain conditions or processes which ultimately have<br />
a great impact on all features of the sediments (Table 1).<br />
Thus, for example, halite accumulation <strong>and</strong> resulting saltridge<br />
structures are associated with the capillary fringe<br />
zone at its interface with the surface. However, cementation<br />
involving direct precipitation of gypsum <strong>and</strong> anhydrite<br />
from water—as opposed to soil formation which<br />
involves remobilization of gypsum from airborne dust—<br />
<strong>and</strong> the formation of euhedral crystals ("croute zonaire")<br />
is associated with the upper part of the saturated zone.<br />
The vertical distance from the top of the saturated zone<br />
to the top of the damp zone is usually less than 6 ft (2 m).<br />
Since water-table levels are not steady during the course of<br />
a year in the study area <strong>and</strong> certainly change over geologic<br />
time, the result is the constant shifting of these damp, capillary<br />
fringe, <strong>and</strong> saturated zones within the sediments.<br />
The further result is inevitably the overprinting of some of<br />
the effects of the various zones on each other within sediments<br />
of dune, interdune, s<strong>and</strong> sheet, <strong>and</strong> sabkha terrains.<br />
The complicating effects of the water table <strong>and</strong> associated<br />
aqueous zones are increased in the study area owing to the<br />
thin sedimentation units typical of the area. Our impressions<br />
are summarized in Figure 10, which shows the typi-
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 291<br />
HIGHEST LOWEST LOWEST<br />
INFLUENCE OF<br />
STRATIGRAPHIC<br />
WATER ON<br />
POSITION<br />
SEDIMENTARY<br />
FEATURES<br />
PRESERVATION<br />
POTENTIAL<br />
LOWEST HIGHEST HIGHEST<br />
FIG. 10—Schematic diagram illustrating typical stratigrapliic<br />
positions of eolian deposits in study area, <strong>and</strong> transition or overlap<br />
of sedimentary features in respective deposits (circles). <strong>S<strong>and</strong></strong><br />
sheet <strong>and</strong> interdune deposits are commonly switched in terms of<br />
stratigrapliic position (that is, s<strong>and</strong> sheets commonly overlie sabkhas);<br />
however, interdunes in general resemble sabkhas more<br />
than s<strong>and</strong> sheets, mainly because of the abundance of water in<br />
deposits.<br />
cal stratigraphic arrangement in the study area. In general,<br />
the effect of water on sediments as they are deposited is<br />
greatest in units such as sabkhas which are stratigraphically<br />
low at the time of deposition. However, sediments<br />
deposited in a dry enviromnent, such as a dry dune, can<br />
become damp because of a rising water table. This may<br />
allow the growth of vegetation which can completely obliterate<br />
the original cross-stratification.<br />
In summary, (a) the ground-water table <strong>and</strong> its associated<br />
damp, capilleuy fringe, <strong>and</strong> saturated zones produce<br />
distinctive effects on both primary <strong>and</strong> secondary features<br />
of sediments in the study area; (b) the combination of<br />
shifting of water-table depth, the shallowness of the water<br />
table (allowing evaporative processes to proceed), <strong>and</strong><br />
thinness of sedimentary units in the area results in extensive<br />
overprinting (secondary effects) on the sediments by<br />
the processes acting in each different zone; <strong>and</strong> (c) these<br />
overprinting effects may be seen in dune, interdune, s<strong>and</strong><br />
sheet, <strong>and</strong> sabkha sediments. Further illustrations of these<br />
processes are given in sections below.<br />
Another concept which is derived from the above discussion<br />
is that the role of wind is mainly to deliver most of the<br />
s<strong>and</strong> to the sites of deposition. The roles of water are as<br />
follows: (1) water affects primary sedimentary features of<br />
the sediment by interaction with wind deposition; (2) the<br />
water causes modifications to the sediments through support<br />
of plant <strong>and</strong> animal activities <strong>and</strong> evaporative processes;<br />
<strong>and</strong> (3) water distributes early cements in the<br />
sediment.<br />
Ebounples of Offshore Prograding <strong>Dune</strong>s<br />
Shinn (1973) described a sabkha created by offshore prograding<br />
dunes on the Qatar Peninsula. Shinn's data are<br />
summarized in Figures 11 <strong>and</strong> 12, which show the stratig<br />
raphy of the sabkha <strong>and</strong> physical features of the underlying<br />
nearshore marine deposits. The first set of steeply<br />
dipping strata encountered above the burrowed marine<br />
deposits (Figs. 11,12) are marine accretion slope beds.<br />
The writers also observed dunes prograding offshore at<br />
Doha Dhalum sabkha near Dhahran. Most of the sabkha<br />
(Fig. 13) is overlain by a thin, 1 to 2-in. (2.5 to 5-cm) layer<br />
of eolicm sabkha deposits, which are then underlain by<br />
eolian avalanche (slip-face) deposits from earlier seawardmigrating<br />
dunes. In a few places, the eolian avalanche<br />
strata are directly overlain by serpulid beachrock <strong>and</strong><br />
shells of gastropods <strong>and</strong> marine moUusks. This situation<br />
estabUshed the basic fact of marine superposition above<br />
dune deposits in the area, <strong>and</strong> suggested the model proposed<br />
in this study. The sabkha deposits over much of the<br />
area seem to be a later phenomenon (postdating the beachrock<br />
deposits) <strong>and</strong> are derived in part from s<strong>and</strong> blown in<br />
from the northwest. The s<strong>and</strong> sheet deposits netu' the shore<br />
are unusually coarse grained, <strong>and</strong> probably have developed<br />
by wind scour of dune deposits exposed when thin<br />
sabkha deposits upwind are stripped off. The coarseness<br />
of the s<strong>and</strong> sheet deposits is attributed in part to the<br />
coarseness of underlying dune s<strong>and</strong>s, which are presumably<br />
mostly base of shp-face deposits (these usually collect<br />
the coarsest grains in a dune). The s<strong>and</strong> sheets are also<br />
derived in part from the belt of dunes along the water's<br />
edge. The deposits of the coastal zone are illustrated in<br />
Figure 14, which shows the dunes prograding directly into<br />
the water. A generalized stratigraphic column (Fig. 15)<br />
summarizes our observations regarding the vertical<br />
sequence of genetic units at the Doha Dhalum sabkha.<br />
EOLIAN DEPOSITS OF PROGRADING SYSTEM<br />
Observable Features of Sediments<br />
Our discussion will emphasize those sedimentary aspects<br />
of the deposits which are likely to be of most use to explorationists,<br />
whose view of prospective rocks is usually limited<br />
to cores. Thus, we will emphasize small-scale<br />
sedimentary structures <strong>and</strong> primary eolian strata, as<br />
opposed to arrangements of sets of strata. In particular,<br />
we will refer commonly to avalanche-, grainfall-, ripple-,<br />
<strong>and</strong> adhesion-produced strata as the fundamental units of<br />
eolian deposition. Origins of these deposits are illustrated<br />
in Figme 16 <strong>and</strong> are discussed in more detail in Fryberger<br />
<strong>and</strong> Schenk (1981). We will attempt to summarize as well<br />
our textural <strong>and</strong> compositional data, with special reference<br />
to indications of early cementation of the deposits,<br />
development of organic-rich zones, <strong>and</strong> general geographical<br />
distribution of the fades.<br />
Sedimentary Features of <strong>Dune</strong>s<br />
Sedimentary features of dunes which we trenched are<br />
summarized in Table 2, <strong>and</strong> terminology applicable to barchanoid<br />
dunes <strong>and</strong> associated deposits is summarized in<br />
Figure 17. In general, we observed a suite of primary sedimentary<br />
features similar to those described for eolian<br />
dunes by other workers such as McKee et al (1971) <strong>and</strong><br />
McKee (1966). We observed the formation of ripple-,
292 Dhahran Area, Saudi Arabia<br />
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<strong>Sabkha</strong><br />
.^^i^4gsj.,^
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 293<br />
Table 2. Barchan <strong>Dune</strong> <strong>and</strong> Dome <strong>Dune</strong><br />
Feature/Size Distribution<br />
BARCHAN DUNE<br />
Origin<br />
Avalanche-produced strata<br />
30 to 34° dip<br />
Ripple-produced strata<br />
0 to 30° dip<br />
Grainfall-produced strata<br />
0 to 30° dip<br />
On slip face:<br />
flame structures<br />
break-aparts<br />
fadeout laminae<br />
wavy laminae<br />
s<strong>and</strong>f low toes<br />
Bundles of fine-grained<br />
laminae <strong>and</strong> coarse-grained<br />
beds, steeply dipping<br />
Isolated coarse-grained<br />
layers along gently dipping<br />
surfaces of truncation<br />
(also called "reactivation<br />
surfaces")<br />
Isolated high-index ripples<br />
or ripple trains<br />
1 to 2 in. (2.5 to 5 cm)<br />
thick lenses of coarse s<strong>and</strong>,<br />
inversely graded<br />
Cemented zones,<br />
large scale 3 ft (1 m) thick,<br />
wide lateral extent<br />
Bioturbation<br />
Ripple-produced strata:<br />
thin, gently curving if<br />
followed laterally,<br />
great lateral extent<br />
TUbular bioturbation:<br />
plant-root traces <strong>and</strong><br />
insect burrows (termites,<br />
ants, beetles)<br />
Contorted (wavy, compacted)<br />
layers, 5 in. (12.5 cm) thickness<br />
Isolated coarse-grained<br />
layers ("reactivation<br />
surfaces")<br />
Very coarse layers up to<br />
1.6 in. (4 cm) thick<br />
Shp face of dune Avalanching<br />
Slip face, apron, windward slope,<br />
leeward slope<br />
Apron, slip face, leeward slope,<br />
breaks in slope near top of dune<br />
In avalanche-produced strata<br />
In avalanche-produced strata<br />
In avalanche-produced strata<br />
In avalanche-produced strata<br />
Base of slip face<br />
Slip-face deposits<br />
Anywhere on dune<br />
Anywhere on dune<br />
Slip face of dune—lobes of<br />
s<strong>and</strong> flows viewed parallel<br />
to wind direction<br />
Thin but laterally continuous<br />
zones. Commonly a secondary<br />
feature<br />
Common on slip faces <strong>and</strong> lower<br />
parts of dunes between s<strong>and</strong>storms<br />
Bulk of dune<br />
DOME DUNE<br />
Vegetated parts of dune—<br />
usually around base of dune<br />
or on windward slope. Most<br />
common near moist or vegetated<br />
areas (termites collect plant<br />
materials)<br />
Anywhere, commonly isolated<br />
Anywhere—not nearly as coimnon<br />
as on s<strong>and</strong> sheets because dune<br />
s<strong>and</strong>s are finer <strong>and</strong> better sorted<br />
Top of dune<br />
Migrating ripples (fine<br />
grained)<br />
Flow separation over crest<br />
or protruding part of dune<br />
Avalanching down slip face:<br />
tensional features near top<br />
of slip face, compressional<br />
features near base of slip face<br />
Represents alternate<br />
ripple- <strong>and</strong> avalanche-produced<br />
strata<br />
Wind direction shifts, causing<br />
episode of scour, leaving<br />
lag or layer of coarse grains<br />
Change in wind conditions<br />
causes preservation—commonly<br />
a wind shift or increase in<br />
wind strength preserves ripples<br />
beneath grainfall deposits<br />
Avalanching of dry s<strong>and</strong><br />
(See text). Soils, windblown<br />
detrital cements, evaporites<br />
Reptiles, insects, plants<br />
(usually grass), in places<br />
large animals<br />
Preservation of lower portions<br />
of migrating ripples<br />
Root <strong>and</strong> rhizome disturbance<br />
of s<strong>and</strong>, termite burrowing<br />
along dead-plant remains,<br />
insect scavenging<br />
Camel footprints<br />
Scour episodes, strong winds<br />
Strong winds, grainfall<br />
deposition<br />
Abundance<br />
in Trenches<br />
Common<br />
(most of dune)<br />
Common<br />
Rare<br />
Common<br />
Common<br />
(bulk of dune)<br />
Common<br />
Rare<br />
Common<br />
Common<br />
Common<br />
Common<br />
Common<br />
Rare<br />
Rare<br />
Rare
294 Dhahran Area, Saudi Arabia<br />
TIDAL FLAT<br />
High-angle<br />
Cross-bedding<br />
Low-angle<br />
Cross-bedding<br />
Fine <strong>S<strong>and</strong></strong><br />
No Bedding<br />
Bioturbation<br />
Shell Fragments<br />
Muddy (Carbonate)<br />
Quartz <strong>S<strong>and</strong></strong><br />
Bioturbation<br />
Hinged Mollusks<br />
FIG. 12—Generalized stratigraphic column (after Shinn, 1973)<br />
showing (a) upward fining of calcareous shell debris, (b) loss of<br />
carbonate mud upward, <strong>and</strong> (c) increase in quartz s<strong>and</strong> content<br />
<strong>and</strong> cross-bedding of deposits upward. Based on core in Umm<br />
Said sabkha through units 2 to 4 of Figure 11.<br />
SOUTH<br />
A<br />
1 mile<br />
MARINE DEPOSITS<br />
DUNES<br />
SAND SHEET<br />
THIN SABKHA DEPOSITS<br />
(0-2 feel I<br />
SAND SHEET<br />
NORTH<br />
A<br />
MARINE BEACHROCK \ JAF 23 <<br />
UNCONFORMITIES<br />
MARINE OR<br />
SABKHA<br />
FIG. 13—Schematic drawing illustrating stratigraphy inferred<br />
for Doha Dhalum sabkha <strong>and</strong> vicinity from numerous excavations<br />
<strong>and</strong> h<strong>and</strong>-dug trenches. Thickness of sabkha deposits is<br />
greatly exaggerated for clarity. <strong>Eolian</strong> deposits become highly<br />
bioturbated <strong>and</strong> cemented by gypsiferous rhizocretions as unit<br />
rises toward Dhahran dome (dotted area right side of drawing).<br />
This facies change may have resulted from entrance of fresh<br />
water into dune s<strong>and</strong>s, allowing growth of vegetation. <strong>Dune</strong><br />
s<strong>and</strong>s (below sabkha) between T3 <strong>and</strong> Tl seem to be free of bioturbation,<br />
perhaps because of lower position <strong>and</strong> resulting high<br />
salinity of interstitial water. Modern s<strong>and</strong> sheet above JAF 23 has<br />
less bioturbation than underlying unit <strong>and</strong> no development of<br />
secondary gypsiferous plant root <strong>and</strong> rhizome molds.<br />
careous, with presumably much of the cement derived<br />
from airborne dust. Some gypsiferous cemented zones<br />
contained euhedral, poikilotopic gypsum crystals, apparently<br />
derived from precipitation from saturated solutions<br />
at the top of the water table. Rhizocretions (replacement<br />
of plant roots <strong>and</strong> rhizomes by gypsum) were common in<br />
cemented zones. The region of white s<strong>and</strong> described above<br />
SHORE'<br />
UPM BEACH TEST SITE<br />
DOHA DHALUM<br />
((APPROX)<br />
SABKHA<br />
FIG. 14—<strong>Dune</strong>, s<strong>and</strong> sheet, <strong>and</strong> sabkha terrain along the northern<br />
coastHne of Doha Dhalum (see also Figure 2). <strong>Dune</strong>s are prograding<br />
obliquely into water; both dome dunes (D) <strong>and</strong> barchan<br />
dunes (B) are present. <strong>S<strong>and</strong></strong> sheets (vegetation symbols) are distributed<br />
irregularly atop sabkha, but are elongate upwind<br />
(toward northwest). Position of marine accretion ridge shown by<br />
dashed line. Heavy dark lines are roads. S indicates nearshore<br />
JAF 26<br />
JAF 29<br />
SALT RIDGES<br />
SAND SHEET<br />
DUNE<br />
SABKHA<br />
6 Feet<br />
MARINE BEACHROCK 2 Meters<br />
AND SANDS<br />
DUNE<br />
Ll = Unconformities<br />
FIG. 15—Composite stratigraphic sequence at Doha Dhalum<br />
sabkha, based on several study locations (indicated on left). In<br />
places, due to incomplete preservation or nondeposition of<br />
marine deposits, sabkha facies lies directly on dunes. Upward<br />
sequence of sabkha to dune to s<strong>and</strong> sheet is typical of much of<br />
study area. U = unconformity.<br />
was experiencing early cementation due to remobilization<br />
of gypsum incorporated into the dunes as windblown,<br />
s<strong>and</strong>-size particles. These s<strong>and</strong>s were usually so firm that<br />
we seldom had trouble driving our vehicles across them.
D<br />
lin.<br />
Ripple Foreset<br />
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 295<br />
296 Dhahran Area, Saudi Arabia<br />
Type of<br />
<strong>Interdune</strong><br />
Dry<br />
Dry<br />
Dry<br />
Dry<br />
Damp or<br />
Evaporitic<br />
Damp<br />
Damp or<br />
Evaporitic<br />
Dry<br />
Dry<br />
Dry<br />
Dry<br />
Dry<br />
Dry<br />
Dry<br />
Feature/Size<br />
Diastem (erosional) flat<br />
to gently undulating<br />
Troughlike scour depression<br />
(cut <strong>and</strong> fill)<br />
Contorted bedding below<br />
interdune surface<br />
Faults in dune bedding<br />
(avalanche-produced strata)<br />
below interdune surface<br />
Uneven, ridgy erosion<br />
surface—up to 1 ft (0.3 m)<br />
relief<br />
Contorted strata<br />
(liquefaction)<br />
Table 3. Erosional <strong>Interdune</strong> <strong>and</strong> Depositional <strong>Interdune</strong><br />
Distribution<br />
EROSIONAL INTERDUNE<br />
Top of underlying dune<br />
deposits<br />
Along erosion surface<br />
0 to 1 ft (0 to 0.3 m) below<br />
interdune surface<br />
0 to 1 ft (0 to 0.3 m) below<br />
interdune surface<br />
Along halite encrusted<br />
surface of interdune<br />
0to6in. (0tol5cm)thick<br />
below interface<br />
Origin<br />
Wind scour due to undersaturation<br />
of wind<br />
Wind scour around obstacles<br />
or depressions<br />
Loading by animals walking<br />
on interdune (dry soft s<strong>and</strong>)<br />
Loading of damp or dry s<strong>and</strong><br />
below interdune<br />
Seepage of salt to interface<br />
along certain layers—early<br />
cementation causes resistant<br />
ridge<br />
Quick condition of wet sediments,<br />
<strong>and</strong>/or bioturbation or<br />
loading by dune<br />
Microscale irregular ridges Along halite encrusted Very small salt-ridge features<br />
<strong>and</strong> bumps up to 0.5 in. surfaces<br />
<strong>and</strong>/or microscour of halite<br />
(1.3 cm) relief encrusted sediments<br />
DEPOSITION AL INTERDUNE<br />
"Wind" ripple-produced Anywhere in interdune,<br />
strata, commonly thin but comprises most of<br />
laterally continuous, flat sediment<br />
or gently dipping (flat at<br />
base, may have transition<br />
upward into dune without<br />
break—contact with underlying<br />
dune commonly sharp <strong>and</strong><br />
erosional)<br />
"Granule" ripple foresets,<br />
commonly coarse grained<br />
Anywhere, commonly near<br />
downwind margin of<br />
interdune, preserved entire<br />
or as coarse-grained layers<br />
Migrating wind ripples—<br />
commonly thin layers result<br />
due to undersaturated winds<br />
typical of interdunes<br />
Abundance<br />
Common<br />
Common<br />
Common<br />
Common<br />
Common<br />
Rare<br />
Common<br />
Common<br />
Strong undersaturated winds Common<br />
Root traces, 0.08 to 0.16 in. W<strong>and</strong>ering through sediment, Sedge <strong>and</strong> other plant roots<br />
(2 to 4 mm) diameter commonly not visibly branching<br />
(slightly damp s<strong>and</strong>—not related<br />
to water table—commonly<br />
underlies even the driest<br />
interdunes, allowing some plant<br />
growth. This is also true of "dry"<br />
dune <strong>and</strong> s<strong>and</strong>-sheet terrains)<br />
Contorted strata<br />
Gentle, curving scour<br />
surface<br />
Short, irregular, "floating"<br />
laminations in<br />
bioturbated zones<br />
Soils (Aridosols: gypcrete<br />
or calcrete)<br />
Just below surface of<br />
interdune<br />
Entire surface of original<br />
interdune<br />
Zone of extreme bioturbation<br />
near plant<br />
Characteristic of older<br />
more stable terrain—<br />
exist as laterally<br />
extensive zones in<br />
sediments<br />
Rare<br />
Loading by camels Rare<br />
Scour episode due to wind shift Rare<br />
<strong>and</strong> eddying around dune<br />
Bioturbation destroys all but a Rare<br />
few laminae<br />
Early cementation associated Common<br />
with dustfall, ageing, stabilization.<br />
May be a secondary<br />
feature—observed in all<br />
facies except sabkha
Type of<br />
<strong>Interdune</strong><br />
Dry<br />
Dry<br />
Damp<br />
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 297<br />
Feature/Size<br />
"Massive" aspect to trench;<br />
in places relict bedding<br />
visible<br />
Plant "leaves" Zygophyllum<br />
sp., etc<br />
Large <strong>and</strong> small scale<br />
intense bioturbation, mottled,<br />
commonly dark-colored zones<br />
Table 3. Continued<br />
Distribution<br />
Zones in trench (extend<br />
horizontally)<br />
Near top of trench—<br />
reworked from existing<br />
plant community<br />
Most of interdune area:<br />
can affect sediments<br />
several feet below modern<br />
interdune surface<br />
Damp Anticlinal or synclinal Around base (rhizomes) of<br />
depositional drape or scour of plants<br />
of layers around zones of<br />
bioturbation 1.5 ft (0.5 m)<br />
high <strong>and</strong> 3 to 6 ft (1 to 2 m) wide<br />
Damp<br />
Damp<br />
Damp<br />
Wet<br />
Wet<br />
Wet<br />
Intense insect bioturbation<br />
termite colonies<br />
Salt ridges (not as common<br />
as in fully evaporitic<br />
interdunes); exist as<br />
isolated layers<br />
Organic-rich layers<br />
Adhesion-produced strata<br />
Aqueous ripple, bar, <strong>and</strong><br />
point-bar deposits<br />
Salt ridges at margin of<br />
wet interdune—along what<br />
would be called damp—<br />
a transitional facies<br />
Evaporitic Most features of siliciclastic<br />
sabkha (see below)<br />
but thinner, less continuous<br />
Near plants<br />
In places evaporitic<br />
episodes; not a<br />
dominant feature, few<br />
ridges—commonly a crust<br />
Below surface of interdune,<br />
commonly near top of saturated<br />
zone of water table<br />
Wettest part of interdune<br />
("quicks<strong>and</strong>") locally, sets<br />
of adhesion-produced<br />
strata can be several feet thick<br />
Zones of interdune with<br />
flowing water or windagitated<br />
water<br />
Around edge of watersaturated<br />
s<strong>and</strong><br />
Whole surface of interdune,<br />
or preferentially<br />
near base of upwind dune<br />
cally (e.g., a few inches). Moreover, "interdunal sabkhas"<br />
are sometimes created simply by scour to water table (capillary<br />
fringe) owing to the presence of a dune, <strong>and</strong> disappear<br />
once the dune has moved on. The areally extensive<br />
eolian sabkhas however are related to a more stable <strong>and</strong><br />
continued rise of water table, <strong>and</strong> perhaps more importantly<br />
to the absence of a dune complex. Thus, sedimentary<br />
units are thicker for eolian sabkhas, commonly 3 to 10<br />
ft (1 to 3 m) <strong>and</strong> have less tendency to exhibit rapid vertical<br />
facies changes related to water content of the sediments.<br />
Early cementation.—Early cementation was common in<br />
interdunal <strong>and</strong> associated sediments, presumably owing to<br />
Origin<br />
Extensive bioturbation—as<br />
a secondary feature<br />
(observed in all facies<br />
except sabkha)<br />
Plants<br />
Abundant plant growth in<br />
interdune when water is not<br />
too saline, burrowing<br />
animals<br />
Abundance<br />
Common<br />
Rare<br />
Common<br />
Results from scour or deposition Common<br />
around exposed bases of plants,<br />
along with bioturbation by<br />
plant roots<br />
Termites feeding on buried<br />
plant remains<br />
Rapid evaporation<br />
Decaying plants; microbial<br />
growth<br />
Adhesion of drifting<br />
(saltating) s<strong>and</strong> to<br />
wet surface<br />
Flowing water<br />
Seepage-evaporation<br />
Wind scour to water table,<br />
or rise of water table<br />
Common<br />
Common<br />
Common<br />
Rare regionally,<br />
common<br />
in this setting<br />
Common<br />
Common<br />
Common<br />
low stratigraphic position <strong>and</strong> thus increased proximity to<br />
water <strong>and</strong> cements. Typically halite was the most common<br />
cement, although gypsum was also abundant. Carbonate<br />
cement was uncommon, except in places where shells of<br />
gastropods or other organisms accumulated in sufficient<br />
quantities in wet interdunes. This type of occurrence was<br />
most common near the coast, where shelly debris commonly<br />
included marine organisms (mollusks). In one<br />
trench in an interdune about 3 mi (5 km) from the coast,<br />
we observed fresh shells at the surface, partly decomposed<br />
<strong>and</strong> broken shells 16 in. (41 cm) below the surface, <strong>and</strong><br />
blue micrite (evidently derived from decomposed shells) at
298 Dhahran Area, Saudi Arabia<br />
LEEWARD SLOPE ILSI<br />
BRINK<br />
SLIPFAC<br />
APRON<br />
INTERDUNE<br />
CREST<br />
WIND<br />
WINDWARD SLOPE (WS)<br />
\CREST<br />
CRESTLINE (CLl<br />
ELEVATION CONTOUR<br />
FIG. 17—Terminology useful for describing parts of barchanoid dunes <strong>and</strong> associated eolian deposits (after Fryberger <strong>and</strong> Schenk,<br />
1981). A. Cross section of barchanoid dune (stratification not shown). B. Plan view of barchanoid ridge dunes with interdune <strong>and</strong><br />
s<strong>and</strong> sheet deposits. C. Detail plan view of barchanoid dune.<br />
the bottom (less than 3 ft [1 m] below the surface).<br />
Organic enrichment.—We observed numerous places of<br />
apparent organic enrichment in interdunes, usually in the<br />
sediments of evaporitic interdunes at the top of the saturated<br />
water table. Some of the organic material was<br />
undoubtedly derived from remains of higher plants in<br />
well-vegetated interdunes (Fig. 22d). In other places the<br />
organic matter seemed to have been derived from microbial<br />
growth at the top of an extremely saline ground-water<br />
table in the s<strong>and</strong>s underlying the interdune. In general,<br />
however, organic enrichment did not seem as great as in<br />
sabkha sediments.<br />
Erosional interdunes.—Erosional interdunes are common<br />
in the study area. The record of dry erosional interdunes<br />
is usually a scour surface, sometunes separated<br />
from overlying deposits by a coarse-grained layer. Damp<br />
or wet erosional interdunes are evidenced by a highly irregular<br />
or wavy erosional contact (Fig. 23a, e). The irregular<br />
surface of erosion develops because dampness, or in many<br />
places cementation by halite, causes differential resistance<br />
to wind erosion. This process can produce considerable<br />
relief, up to 12 in. (30 cm). Figure 22d (foreground) shows<br />
rather large resistant ridges resulting from halite cementation<br />
in an interdune which is mainly erosional. Microscale<br />
relief also develops on damp erosional interdunes, lending<br />
a bumpy aspect to the gently undulating or ridgy surface<br />
of erosion (Fig. 23e, arrow 2). Erosional interdunes are<br />
also sometimes associated with effects on underlying sediments<br />
which result from processes taking place near the<br />
surface. Resulting sedimentary features include plant bioturbation<br />
<strong>and</strong> contorted strata due to loading by camels or<br />
other organisms.<br />
Dry depositional interdunes.—Dry depositional interdune<br />
sediments consist mainly of subhorizontal rippleproduced<br />
strata (Fig. 23b). Ripple-produced strata include<br />
those (1) which result from passage of small, relatively<br />
fine-grained <strong>and</strong> well-sorted "saltation" or "wind" ripples,<br />
<strong>and</strong> (2) of larger, coarse-grained, high-relief "granule<br />
ripples" (Fig. 23b). Bioturbation was observed in dry<br />
interdune deposits but was less common than in damp or<br />
wet interdunes. Dry interdunes are common in dime complexes,<br />
or in dune areas which occupy a topographically<br />
high position <strong>and</strong> thus commonly have deeper groundwater<br />
tables.<br />
Damp depositional interdunes.—These interdunes are<br />
associated with limited development of salt ridges <strong>and</strong><br />
evaporitic structures, <strong>and</strong> with an abundance of vegetation<br />
if water salinities are not too high <strong>and</strong> the upwind
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 299<br />
tflll lit»<br />
.f-. -fl-<br />
JftliM^ ''-\*^<br />
FIG. 18—<strong>Dune</strong> deposits.<br />
A. Gently downwind-dipping, ripple-produced strata at crest of a dome dune. Thin coarse-grained layers (arrow 1) <strong>and</strong> bioturbation<br />
(arrow 2) are visible in trench. Scale in cm.<br />
B. Avalanche-produced strata in a barchanoid dune. Fadeout laminae (arrow 1) <strong>and</strong> flame structure (arrow 2) as well as gently undulating<br />
nature of strata are typical of avalanche-produced deposits.<br />
C. Fracturing (tensional) in cohesive damp s<strong>and</strong> of avalanche-produced strata of a barchanoid dune, overiain by undisturbed<br />
deposits.<br />
D. Grainfall-produced strata (barely visible in original photograph, unit looked massive in field) (arrow 1). Grainfall-produced unit is<br />
overiain by avalanche-produced strata with flame structures (arrow 2) near crest of barchanoid dune.<br />
E. Base of barchanoid dune. Ripple-produced strata of interdune (arrow 1) overiain by ripple-produced strata of dune apron (arrow<br />
2), in turn overiain by avalanche-produced strata (arrow 3) terminating in s<strong>and</strong>-flow toes (unnumbered arrow). Deposits of present<br />
dune apron (arrow 4) <strong>and</strong> its rippled surface visible at top of photograph.<br />
F. Typical base-of-dune with s<strong>and</strong>-flow toes (arrow) intertonguing with dune apron.
300 Dhahran Area, Saudi Arabia<br />
lS^)kte<br />
\ <strong>S<strong>and</strong></strong><br />
^ I<strong>Sheet</strong>;' • ••<br />
OWIND<br />
FIG. 19—<strong>Dune</strong> complex (schematic) <strong>and</strong> typical association of<br />
other eolian facies. <strong>Sabkha</strong> is downwind in sheltered zone; s<strong>and</strong><br />
sheet deposits (in part a remnant of the dune complex) are<br />
upwind.<br />
dune is not moving too rapidly (Fig. 22d, background).<br />
Underlying dune deposits sometimes exhibit a zone of<br />
contorted strata extending 4 to 12 in. (10 to 30 cm) below<br />
the contact. These zones have the aspect of a combination<br />
of liquefaction <strong>and</strong> loading (Fig. 23c).<br />
Wet depositional interdunes.—lnitidvMss containing<br />
st<strong>and</strong>ing, permanent water were not studied directly<br />
because of the difficulties of working around the quicks<strong>and</strong>s<br />
associated with the ponds. Interdunal ponds<br />
develop an abundant marshy vegetation around the edges<br />
of the pond, <strong>and</strong> are focal points for much of the animal<br />
life nearby, as evidenced by tracks around the shoreline.<br />
Wet interdunes with water-saturated s<strong>and</strong> at the surface<br />
but without permanent st<strong>and</strong>ing water (e.g., in wadis)<br />
(Fig. 22b) exhibit thick sequences of adhesion-produced<br />
strata ("adhesion ripples"). These strata have intercalated<br />
fluvial ripples <strong>and</strong> point bar sequences if sufficient current<br />
is present at time of deposition (Fig. 23d).<br />
Evaporitic depositional interdunes.—These interdune<br />
deposits exhibit numerous salt-ridge features, but are<br />
much thinner than typical sabkha sequences. Despite careful<br />
observation, we did not notice prohferation of algal<br />
mat material in evaporitic interdunes, perhaps because<br />
algal mats may depend upon periodic wetting by less saline<br />
waters such as those of an intertidal zone.<br />
Sedimentary Features of <strong>S<strong>and</strong></strong> <strong>Sheet</strong>s<br />
<strong>S<strong>and</strong></strong> sheets are an areally extensive eoUan facies which<br />
serve to provide transition between dune complexes <strong>and</strong><br />
sabkhas, or between eolian <strong>and</strong> non-eolian facies (see Fig.<br />
8). <strong>S<strong>and</strong></strong> sheets represent both a residual deposit of<br />
advancing dunes, <strong>and</strong> an independent facies capable of<br />
sustained growth (laterally <strong>and</strong> vertically). Our principal<br />
concern here is with s<strong>and</strong> sheet deposits which have developed<br />
independently of dunes, at least in the sense that they<br />
are not composed principally of steeply dipping<br />
avalanche-produced strata. Typically, s<strong>and</strong> sheets grow<br />
vertically <strong>and</strong> laterally during s<strong>and</strong>storms through ripple<br />
<strong>and</strong> grain-fall deposition, usually interactive with plant<br />
growth, which in the region is most abundant on s<strong>and</strong><br />
sheets. Sedimentary features of s<strong>and</strong> sheets we observed<br />
are summarized in Table 4.<br />
In general, sedimentary features of s<strong>and</strong> sheets in the<br />
study area are similar to those described by Fryberger et al<br />
(1979). Chief distinguishing features of s<strong>and</strong> sheets in the<br />
study area are abundance <strong>and</strong> diversity of vegetation<br />
(including the largest plants of the area such as Tamarix<br />
<strong>and</strong> date palms), <strong>and</strong> associated bioturbation due to<br />
insects, reptiles, <strong>and</strong> other creatures. Younger s<strong>and</strong> sheets<br />
(which seem to have less vegetation) have a suite of sedimentary<br />
features virtually identical to those of interdunes<br />
in the study area, but sedimentary sequences are thicker,<br />
less variable, much more laterally extensive, <strong>and</strong> exhibit<br />
no intercalated features related to damp or evaporitic<br />
s<strong>and</strong>s common in interdunal sediments.<br />
Textural properties.—These range to extremes. Some<br />
s<strong>and</strong> sheet deposits are texturally mature, consisting of<br />
well-sorted mainly quartzose s<strong>and</strong>. Other s<strong>and</strong> sheets have<br />
poor sorting in terms of bulk textural properties, with layers<br />
of both fine <strong>and</strong> extremely coarse grains (Fig. 24d, e).<br />
Nevertheless, the coarse <strong>and</strong> fine grains are commonly<br />
segregated into discrete layers having high initial porosity.<br />
The s<strong>and</strong> sheet facies, therefore, must be considered along<br />
with dunes to have some potential as a reservoir facies in<br />
ancient rock sequences.<br />
Topographic relief.—Although s<strong>and</strong> sheets are almost<br />
flat in many places, in a few, highly vegetated areas rehef<br />
of 15 to 25 ft (4.5 to 7.5 m) is present. In such areas the<br />
s<strong>and</strong> sheet consists of broad swells 300 to 1,500 ft (90 to<br />
450 m) in width, with intervening, steep-sided troughs.<br />
This "ridge <strong>and</strong> swale" topography can extend hundreds<br />
of meters, both crosswind <strong>and</strong> parallel to the wind, with<br />
ridges elongate parallel to wind direction.<br />
Residual facies.—To a certain extent, s<strong>and</strong> sheets (which<br />
contain by far the coarsest s<strong>and</strong>s) may represent a residual<br />
facies which is the product of the passage of many dunes<br />
<strong>and</strong> much windblown s<strong>and</strong> through the study area. <strong>S<strong>and</strong></strong><br />
sheet terrain is clearly very stable in terms of erosion or<br />
deposition, due to armoring by a protective mantle of<br />
coarse s<strong>and</strong> <strong>and</strong> vegetation. Given the rapid movement of<br />
dunes in the area <strong>and</strong> the relative immobility of the s<strong>and</strong><br />
sheets, it seems possible that in many areas the s<strong>and</strong> sheets<br />
are much older than nearby dunes.<br />
Early cementation.—The aspect of permanence of s<strong>and</strong><br />
sheets is enhanced by extensive soil development in some<br />
places, particularly in the northern part of the study area.<br />
At one locahty near Qatif, we observed evident Aridosol<br />
(gypsiferous <strong>and</strong> calcareous) horizons amd resulting loss of<br />
porosity in ancient s<strong>and</strong> sheet deposits. These s<strong>and</strong> sheets<br />
contained broken pottery which is probably not more than<br />
several thous<strong>and</strong> years old. Older s<strong>and</strong> sheets also commonly<br />
contain many rhizoliths (gypsiferous replacements<br />
of plant roots <strong>and</strong> rhizomes; Klappa, 1980). The development<br />
of gypsiferous "soils" may be more common in s<strong>and</strong><br />
sheets because of their relative immobility <strong>and</strong> the absence<br />
of direct evaporite deposition more typical of sabkhas <strong>and</strong><br />
evaporitic interdunes.<br />
Sedimentary Features of <strong>Sabkha</strong>s<br />
Both inl<strong>and</strong> <strong>and</strong> coastal sabkhas exist in the Jafurah<br />
s<strong>and</strong> sea (Fig. 2) (Johnson et al, 1978), <strong>and</strong> both types in
It<br />
iv<br />
F i<br />
1<br />
* o<br />
iir-'i S^<br />
the study area seem to be of the " arenaceous" kind (quartz<br />
dominant) as opposed to the "argillaceous" kind (carbonate<br />
mud dominant) of Holm (1960). Regardless of geographic<br />
position, we found it possible to distinguish two<br />
basic end members of the arenaceous, or siliciclastic, sabkhas<br />
of the region, based principally upon preponderance<br />
of wind-derived quartz s<strong>and</strong> versus ground-water-derived<br />
evaporite minerals. We refer to detrital-dominant sabkhas<br />
as those in which transported (quartzose) s<strong>and</strong>s constitute<br />
most of the sediment. On the other h<strong>and</strong>, evaporitedominant<br />
sabkhas are those in which evaporite minerals<br />
constitute most of the sediment. Although both types of<br />
sabkhas are distributed widely in the Jafurah s<strong>and</strong> sea,<br />
evaporite-dominant sabkhas in the study area are best<br />
developed nearest the coast, perhaps owing to proximity<br />
to seawater-supphed minerals. Sedimentary features of<br />
eolian detrital-dominant <strong>and</strong> evaporite-dominant sabkhas<br />
we observed are summarized in Table 5.<br />
Salt ridges.—Salt ridges are a conspicuous sedimentary<br />
feature in the study area which merit separate discussion<br />
primarily because they bear a striking resemblance to algal<br />
mats described in carbonate (argillaceous) sabkha settings<br />
(Fig. 25). Despite repeated <strong>and</strong> careful observations, we<br />
could find no indication that algal growth processes are<br />
responsible for the formation of the salt-ridge structures.<br />
Rather, salt ridges form from concentration of halite at the<br />
air-s<strong>and</strong> interface because of evaporation of saline capillary<br />
water. The resulting excess material at the surface<br />
causes expansion, forcing portions of the cemented layer<br />
to rise (Fig. 25c). The salt ridges can be classified as pustular,<br />
vermiform, or polygonal, based on aspects in plan<br />
view (Fig. 25a). Tops of the salt ridges are thin <strong>and</strong> brittle,<br />
commonly resulting in breakage <strong>and</strong> collapse of the center<br />
of the salt ridge, <strong>and</strong> subsequent infill by windblown s<strong>and</strong><br />
(Fig. 25b, c). Salt-encrusted s<strong>and</strong>s darken to a reddish<br />
brown where exposed to the sun, resulting in a good contrast<br />
of the salt-ridge structure with lighter s<strong>and</strong> which fills<br />
in breaks in the top of the structure or with areas between<br />
salt ridges (Fig. 25b, see also Fig. 26). Salt-ridge structures<br />
are important because stacked sequences of salt-ridge<br />
structures constitute the bulk of the sediments of detritaldominant<br />
sabkhas we studied (Fig. 26).<br />
Detrital-dominant sabkhas.—These sabkhas consist<br />
essentially of wind-deposited quartz s<strong>and</strong> blown across the<br />
sabkhas in the form of streamers <strong>and</strong> deposited in irregular<br />
layers between salt-ridge structures. Despite presence<br />
of water-saturated s<strong>and</strong> a short distance below (see Fig.<br />
26d), the surface of the sabkha is dry in all but the lowest<br />
places during the heat of the day. At night, the hygroscopic<br />
effect of the salts, in addition to fog <strong>and</strong> dew, can cause<br />
the sabkha to be damp. As most s<strong>and</strong>storms occur during<br />
daytime in the hottest part of the year (June), s<strong>and</strong> can<br />
move easily across the crust of the sabkha, <strong>and</strong> one rarely<br />
sees adhesion-produced strata of the kind associated with<br />
wet, non-evaporitic interdunes.<br />
Growth of the sabkhas is slow, usually only 1 to 2 in. (2.5<br />
to 5 cm) per year. Deposition of s<strong>and</strong> sometimes proceeds<br />
gradually faster than rise of water table, or as a sudden episode<br />
resulting in more than a few inches of new sediment<br />
in one storm. When this occurs, an intercalation of rippleproduced<br />
strata <strong>and</strong> salt-ridge structures may produce a
302 Dhahran Area, Saudi Arabia<br />
Table 4. <strong>S<strong>and</strong></strong> <strong>Sheet</strong><br />
Feature/Size Distribution Origin Abundance<br />
Ripple-produced strata, both<br />
"wind" <strong>and</strong> "granule" ripple<br />
deposits<br />
Constitutes most of sediments Partial preservation of migrating<br />
wind <strong>and</strong> granule ripples<br />
Common<br />
Buried plants, roots, <strong>and</strong><br />
rhizomes, commonly carbonized<br />
or replaced by gypsum<br />
Plant <strong>and</strong> insect bioturbation<br />
traces, usually .25 in. (6 mm)<br />
diameters, tubular features<br />
Coarse-grained layers along<br />
shallow surfaces of erosion<br />
6 ft (2 m) long; 0.5 to 1 in.<br />
(1.2 to 2.5 cm) thick,<br />
commonly convex-downward<br />
Cut <strong>and</strong> fill, 1 to 3 in. (2.5 to<br />
7.5 cm) diameter troughs<br />
Isolated high-index ripples<br />
or ripple trains, flat lying<br />
Partial or complete preservation<br />
of granule ripple<br />
foresets<br />
"Massive" mottled s<strong>and</strong>s<br />
Contorted stratification<br />
(commonly loading)<br />
Gypsiferous or calcareous<br />
Aridosols commonly in<br />
laterally extensive zones up<br />
to 10 ft (3 m) thick<br />
Widespread, especially on<br />
well-vegetated older s<strong>and</strong><br />
sheets<br />
Upper 3 ft (1 m) of vegetated<br />
s<strong>and</strong> sheets<br />
Individual layers or zones<br />
in sediment<br />
Isolated occurrences<br />
Along a single horizon<br />
Commonly along single horizon<br />
Horizonally extensive zones<br />
several feet or more in thickness<br />
Anywhere—usually vegetated<br />
areas<br />
Commonly below s<strong>and</strong> surface—<br />
seen in excavation<br />
b<strong>and</strong>ed appearance in the sediment (Fig. 26b). The development<br />
of thick sequences of siliciclastic sabkha sediments<br />
with salt-ridge structures (such as shown in Fig. 26c,<br />
d) seems to depend on a gradually rising water table, an<br />
absence of dunes, <strong>and</strong> an undersaturated s<strong>and</strong> drift regime<br />
(more available wind than s<strong>and</strong>). Despite undersaturation<br />
of s<strong>and</strong> drift (which we measured at two trap sites on sabkhas),<br />
deposition is able to proceed because of the trapping<br />
of s<strong>and</strong> in sheltered areas between salt ridges.<br />
Detrital-dominant sabkhas in many areas were loosely<br />
packed with considerable excess porosity. Reduction in<br />
thickness (10 to 15%?) might be expected as the sabkha<br />
deposits compacted on burial <strong>and</strong> soluble salts were<br />
leached out. Early cementation by halite was common in<br />
detrital-dominant sabkhas, however, little gypsiferous or<br />
carbonate cementation was observed.<br />
Evaporite dominant sabkhas.—Evaporite-dominant<br />
siliciclastic sabkhas exhibit more extreme forms of saltridge<br />
structures, many of which also incorporate euhedral<br />
platy gypsum crystals (Fig. 26e). Large scale polygonal<br />
cracks <strong>and</strong> teepee structures commonly develop (such as<br />
those described by Ahlbr<strong>and</strong>t <strong>and</strong> Fryberger, 1981). Bedded<br />
salts may occur, along with nodular anhydrite (Fig.<br />
26f). Organic enrichment of sediments occurs in<br />
Burial, Aridosol formation, Common<br />
replacement by ground water<br />
Disturbance of sediment by Common<br />
root growth, burrowing<br />
organisms<br />
Wind scour, in places around<br />
granule ripples<br />
Scour around plant roots <strong>and</strong><br />
rhizomes<br />
Wind shift or change in<br />
intensity<br />
Movement of granule ripple—<br />
strong winds followed by<br />
weaker winds<br />
Intense plant-root<br />
bioturbation<br />
Camel footprints; digging or<br />
rooting of other animals<br />
such as goats, foxes, dogs<br />
Airborn dust, rain,<br />
time, indigenous materials<br />
Common<br />
Rare<br />
Common<br />
Common<br />
Common<br />
Rare<br />
Common<br />
evaporite-dominant sabkhas, presumably as a result of<br />
microbial proliferation in the highly saline brines. Such<br />
organic proliferation, incidentally, is not restricted to<br />
coastal sabkhas. Samples from an evaporite-dominant<br />
portion of Al-Awsajiyah sabkha (near Unayzah in the<br />
middle of the Kingdom) contained 0.75% total organic<br />
carbon <strong>and</strong> 8.6 mg/g of free hydrocarbons. Residual<br />
hydrocarbon fraction after pyrolysis was 3.19 mg/g.<br />
Although these figures are not imposing, they are promising,<br />
especially when we consider the considerable areal<br />
extent of sabkhas such as Awsajiyah (in the interior) <strong>and</strong><br />
similar organic-rich facies in sabkhas (such as Ar-Riyas)<br />
along the gulf coastline (Fig. 26f).<br />
EXAMPLES IN ANCIENT ROCKS<br />
Portions of the Pennsylvanian-Permian Tensleep-<br />
Weber-Minnelusa-Leo eolian s<strong>and</strong>stone systems of Wyoming<br />
may represent deposits of an offshore prograding<br />
eolian s<strong>and</strong> sea. The lower part of the Desmoinesian<br />
Tensleep s<strong>and</strong>stone of the Big Horn basin has been interpreted<br />
as representing an intercalation of marine <strong>and</strong><br />
eolian deposits due to offshore progradation by dunes<br />
(Mankiewicz <strong>and</strong> Steidtmann, 1979). Paleogeographic
Steven G. Fryberger, Abdulkader M. Al-Sari, <strong>and</strong> Thomas J. Clisham 303<br />
Table 5. Depositional <strong>Sabkha</strong><br />
Feature/Size Distribution Origin Abundance<br />
DETRITAL DOMINANT<br />
Salt ridges 0.5 to 2 in.<br />
(1.2 to 5 cm) relief<br />
Salt (halite) crystals<br />
gypsum <strong>and</strong> anhydrite<br />
crystals about 0.25 in.<br />
(6 mm) diameter<br />
Grainfall- <strong>and</strong> rippleproduced<br />
strata—as thin<br />
layers or thick layers<br />
separating salt-ridge<br />
structures<br />
Contorted layers due to<br />
loading<br />
Bioturbation<br />
Reddish brown stain<br />
on s<strong>and</strong> grains<br />
Thin layers of halite<br />
crystals, euhedral,<br />
white<br />
Small lenses of lightcolored<br />
s<strong>and</strong> inside saltridge<br />
structure 2 to 4 in. (5 to<br />
10 cm) long; 0.75 in. (2 cm) thick<br />
Small lenses of lightcolored<br />
s<strong>and</strong> separating<br />
sah-ridge structures<br />
"Salt ridges" composed of<br />
euhedral gypsum crystal<br />
"hash" associated with<br />
halite: 2 to 4 in. (5 to 10 cm)<br />
high, 4 to 8 in. (10 to 20 cm)<br />
across<br />
Organic-rich layers<br />
about 3 ft (1 m) thick<br />
B<strong>and</strong>ed (layered) evaporite<br />
bodies (halite) incorporating<br />
windblown s<strong>and</strong> grains, up to<br />
3 ft (1 m) thick<br />
Desiccation <strong>and</strong> expansion<br />
polygons 3 to 6 ft (1 to 2 m)<br />
across<br />
Gypsum s<strong>and</strong> roses "croute<br />
zonaire" (poikilotopic<br />
crystals) up to 8 in. (20 cm)<br />
in diameter<br />
Bioturbation<br />
Nodular or layered thin<br />
white anhydrite layers<br />
Most of sediment<br />
In sediments or on surface.<br />
Small euhedral forms in layers<br />
at distinct horizons, usually<br />
salt above gypsum with gypsum<br />
or anhydrite near water table<br />
Layers in sediments anywhere<br />
on sabkha<br />
Anywhere in sediments, formed<br />
at surface of sabkha<br />
Along surface <strong>and</strong> in sediments Beetles, termites<br />
Surface sediments <strong>and</strong> trencheswidespread<br />
Thin horizontal (intergranular)<br />
layers in s<strong>and</strong>y sediment of<br />
sabkha<br />
Formed at surface but can be<br />
seen typically in whole trench<br />
with salt-ridge structures<br />
Surface of sabkha <strong>and</strong> trenches<br />
EVAPORITE DOMINANT<br />
Surface of sabkha—often infilled<br />
by s<strong>and</strong>—structures contain<br />
as much evaporite mineral as s<strong>and</strong><br />
Top of water table<br />
Entire surface of sabkha <strong>and</strong><br />
in layers several feet or more<br />
thick<br />
Entire surface of sabkha<br />
In horizontally extensive<br />
zones in water-saturated<br />
sediments<br />
Thin, vertical tubular burrows<br />
Near water table in thin<br />
irregular layers<br />
Formed at surface by salt Common<br />
expansion, followed by burial<br />
by windblown s<strong>and</strong><br />
Direct chemical precipitation Common<br />
from capillary fringe (salt)<br />
or water table (gypsum, anhydrite)<br />
Strong s<strong>and</strong>storms. Most common Common<br />
when wind sedimentation temporarily<br />
exceeds rise of water table<br />
Animals walking across sabkha Rare<br />
Exposure of grains to salt<br />
<strong>and</strong> sun (oxidation of some sort?)<br />
Rare<br />
Common<br />
Evaporation—seems to form below Common<br />
surface—crystals larger than<br />
efflorescence seen at surface<br />
<strong>S<strong>and</strong></strong> blows into hole in roof<br />
of hollow salt-ridge structure<br />
Grainfall infill of hollows<br />
between salt-ridge structures<br />
Extreme evaporation<br />
Microbial growth (?)<br />
Evaporation of brines,<br />
direct precipitation<br />
Shrinkage or expansion of<br />
sediments due to desiccation<br />
or salt buildup, respectively<br />
Extreme evaporation<br />
Insect larvae (?)<br />
Extreme evaporation<br />
Common<br />
Common<br />
Common<br />
(in these<br />
settings)<br />
Common<br />
Common<br />
Common<br />
Common<br />
Rare<br />
Common