Eolian Dune, Interdune, Sand Sheet, and Siliciclastic Sabkha ...

The American Association of Petroleum Geologists Bulletin 

V. 67. No. 2 (February, 1983). P.280-312, 29Figs., 5 Tables 

Eolian Dune, Interdune, Sand Sheet, and Siliciclastic 

Sabkha Sediments of an Offshore Prograding 

Sand Sea, Dhahran Area, Saudi Arabia^ 

STEVEN G. FRYBERGER,^ ABDULKADER M. AL-SARI/ and THOMAS J. CLISHAM^ 

ABSTRACT 

An offshore prograding sand sea exists along portions of 

the Arabian Gulf coastline near Dhahran, Saudi Arabia. 

In this region, sediments of eolian dune, interdune, sand 

sheet, and siliciclastic sabkha intercalate with marine 

deposits. This depositional setting is characterized by 

strong offshore winds which supply abundant sand to the 

coastline, and cause at present time the outbuilding of the 

dune system. This quartz-detrital dominant setting contrasts 

markedly with the carbonate dominant setting 

resulting from onshore winds in the Trucial Coast area to 

the south. The broad intercalation of eolian and marine 

deposits which results creates ideal potential for subregional 

stratigraphic petroleum traps, due to pinch-out of 

porous and permeable dune sands into impermeable 

marine mudstones. Within the eolian system itself are 

potential reservoir rocks (dunes), sources (organic-rich 

sabkha and Interdune deposits), and seals (zones of early 

cementation in all deposits). Early cementation is very 

common in all fades of the eolian sand sea. The early 

cementation occurs owing to (1) soil formation, (2) deposition 

of pore-filling gypsiferous cements from saturated 

solutions near water table, and (3) addition of sand-size 

windblown evaporitic material to sands downwind of sabkhas. 

INTRODUCTION 

The purpose of our studies in the Jafurah sand sea near 

Dhahran was to identify and describe sedimentary facies 

of the complex offshore prograding eolian sand sea near 

Dhahran. We felt this would be of great interest to geologists 

working with petroleum-bearing rocks of similar origins. 

As our work progressed, it became apparent that the 

©Copyright 1983. The American Association of Petroleum Geologists. All 

rights reserved. 

'Manuscript received, February 1,1982; accepted, August 26,1982. 

^Research Institute, University of Petroleum and Minerals, Dhahran, Saudi 

Arabia. Present address: McAdams, Roux and Associates, Suite 530,73017th 

St., Denver, Colorado 80202. 

^Research Institute, University of Petroleum and Minerals, Dhahran, Saudi 

Arabia. 

We owe considerable gratitude to Arabian American Oil Co. (ARAMCO) for 

generous access to technical data, for permission to conduct experiments on 

the ARAMCO reservation, and for access to facilities. We gratefully acknowledge 

the financial assistance provided by Amoco Production Co., Cities Service 

Co., and McAdams, Roux and Associates in the preparation of this report. 

We acknowledge the assistance of S.A.R. Rizvi of the Research Institute, University 

of Petroleum and Minerals, Dhahran, in identification of floral and faunal 

specimens. We also gratefully acknowledge the assistance of Dan Bean 

and Davis Oil Co. for use of core samples from the Minnelusa Fonnation. 

280 

eolian system deposits near Dhahran represented a relatively 

undescribed "sand dominant" model for arid coastlines 

in contrast to the classical carbonate models of the 

nearby Trucial Coast. 

Methods of Study 

Our studies extended throughout Saudi Arabia during 

1978 to 1980, but were focused in the Jafurah saad sea 

between Dhahran and Abqaiq (Figs. 1, 2), where 14 test 

sites were chosen (Fig. 2). At each site, a sand trap was 

installed which could catch and store windblown sand for 

up to a month. Additionally, ten wooden stakes were 

placed in a grid around each trap, to measure erosion and 

deposition of sand. Wind stations were Installed at stations 

4, 9, 10, and 15 (Fig. 2) to record wind speed and 

direction during the course of a year from about October 

1979, to October 1980. Dune advance near the test sites 

was studied monthly for a year, as were the traps and 

stakes. The test sites were selected to represent typical 

eolian terrains: dune, interdune, sand sheet, and sabkha. 

As a result of this continuous study, distinctive sedimentary 

features could be observed to form at the various test 

sites, providing assistance in interpreting complex 

sequences of sedimentary structures in trenches dug near 

the same study sites. Sediment samples were collected for 

sieving and petrographic study, along with specimens of 

local biota. 

Previous Work on Modern 

Sediments in Study Area 

The principal body of literature on modern sediments of 

the Arabian Gulf Coast describes the carbonatedominated 

coastal sabkhas, lagoons, and reefs of the Trucial 

Coast (Kendall and Skipwith, 1964; llling et al, 1965; 

Butler, 1969; Evans et al, 1969; Taylor and llling, 1969; 

Purser, 1973; Patterson and Kinsman, 1982). These studies 

collectively describe a part of the Arabian Gulf Coast 

characterized by carbonate sedimentation, which exists 

despite the presence of vast areas of quartzose dunes a 

short distance inland. Many of the important aspects of 

sedimentation along the Trucial Coast depend on the wind 

blowing mainly from sea to land. In particular, the 

onshore wind assures that the nearshore and intertidal 

zones with their abundant biota are not buried by quartz 

sand from the nearby desert. As pointed out by Handford 

(1981), these classic studies resulted in a depositional 

model for sabkhas and desert coasts which has been 

extended too far in interpreting ancient rocks.

Steven G. Fryberger, Abdulkader M. Al-Sari, and Thomas J. Clisham 281 

BEDROCK 

SABKHA 

GULF 

RAS 

TANNURAH 

DUNE, SAND 

SHEET INTER- 

DUNE 

lAHRAN 

DOME 

FIG. 1—Map of northern Jafurah sand sea on eastern shore of Arabian Gulf. Dotted areas are 

sandy terrains including dunes, interdunes, and sand sheets of Jafurah sand sea. Dark areas 

are sabkhas. Note distribution of sabkhas along coast, and around margins or downwind of 

gentle highs in region. Map after Steineke et al (1958).

282 Dhahran Area, Saudi Arabia 

FIG. 2—Map of study area and Arabian Gulf coastline near Dhahran showing sand trap sites 1 to 15 (no trap site 8) and cross section 

AA'. Dhahran "sand rose" (Frytierger, 1979) in upper left shows principal directions from which sand drifts. Arrow on sand rose 

indicates resultant drift direction. D shows location of Dhahran.


Another radically different model is equally viable, however, 

for deposition along desert margin shorelines and 

associated dune-sabkha systems. Shinn (1973) suggested a 

model for offshore prograding dune systems: desert 

shorelines in which strong winds blow from the land 

toward the sea, and active quartz sand dunes are present 

on land (Fig. 3). Shinn described cores from a sabkha, created 

in part by the prograding dunes, which revealed a 

transition upward from nearshore marine, calcareous, 

fossiUferous muds to more siliceous cross-bedded beach 

accretion-slope sands. Offshore progradation of dunes 

has also been described by Sarnthein and Walger (1974) 

along the coast of Mauritania. 

FIG. 3—Air view (toward soutlieast) showing dunes prograding 

into sea on west coast of Qatar Peninsula. 

Sedimentation in the Arabian Gulf has been described 

by Sugden (1963) and Diester-Haas (1973). Sarnthein 

(1972) suggested that relict dunes may be preserved on the 

floor of the Gulf, apparently deposited during lowered sea 

levels caused by latest Pleistocene glaciation. An excellent 

summary of data on probable sea level changes in the Gulf 

since the last glaciation is offered by Al-Asfour (1978) 

(Fig. 4). 

The literature dealing with modern eolian and related 

deposits of Saudi Arabia is rather sparse. Regional settings 

and distribution of dunes in Arabia have been described 

by Holm (1953, 1960) and more recently by Breed et al 

(1979). Lacustrine and related deposits of glacial times in 

the Rub al Khali have been described by McClure (1976). 

Holm (1960) recognized the fundamental distinction 

between the two types of sabkhas on the Arabian coast. 

One type (discussed as "siliciclastic" in this report) is 

called by Holm "arenaceous," meaning "filled with 

sand." Holm observed these sabkhas to form by movement 

of dunes and windblown sand into the shoreline 

zone. "Argillaceous" sabkhas, however, form from the 

manufacture of calcareous mud by algae and other organisms 

in areas where windblown sand does not bury these 

organisms. Additional descriptions of eohan deposits of 

Saudi Arabia and environs are available in Bagnold (1951) 

and Glennie (1970); in accounts of early travelers such as 

Thomas (1932), Philby (1933), and Thesiger (1949); and in 

the geologic and geographic map series at 1:500,000 scale 

of the U.S. Geological Survey. Other more recent contributions 

include those of Chapman (1971), who discussed 

the fluvial geomorphology of the Eastern Province, and 

Patterson and Kinsman (1981) who described the dependence 

of sabkha growth in the Trucial Coast area on gradual 

rise of regional ground-water table. The best 

description of the essentially siliciclastic sabkhas in the 

study area is by Johnson et al (1978). Johnson et al make a 

distinction between "coastal sabkhas," with brine derived 

partly from seawater, and "inland sabkhas," where sabkhas 

represent areas of equilibrium between eolian deflation 

and the ground-water table. Some features of 

interdune and sabkha deposits in the Jafurah sand sea are 

discussed in Ahlbrandt and Fryberger (1981). An excellent 

summary of meteorologic and oceanographic data for the 

gulf and Eastern Province is available in WilUams (1979). 

Depositional Model 

The purpose of this study is in part to illustrate the sedimentary 

features of the eolian deposits of an offshore prograding 

sand sea. It seems advisable, however, to briefly 

review other basic ideas which follow directly or indirectly 

from the facts which will be presented. 

One important idea is that this study deals with an aqueous 

mechanism of accumulation of eolian deposits, which 

is one of three basic mechanisms for the formation of 

eolian sand seas (Fryberger and Ahlbrandt, 1979). The 

aqueous mechanism refers simply to the role of water in 

causing the accumulation of wind-driven sand. The original 

concept (of Fryberger and Ahlbrandt, 1979) was that 

standing bodies of water can block wind-driven sand, 

causing an accumulation in the shoreline zone. Processes 

associated with high water tables and high ground-water 

salinity in the Arabian shoreUne zone-of-accumulation 

have important effects on sedimentary features of the 

deposits and their geometry. However, the aqueous mechanism 

of formation of sand seas is only one of three mechanisms, 

the other two being topographic and climatic. The 

topographic mechanism produces accumulation of winddriven 

sand by the action of positive or negative topography 

upon wind regime. Thus, sand will tend to be 

deposited upwind or downwind of escarpments, and will 

tend to fill low-lying areas where wind velocity is reduced. 

The chmatic mechanism of sand-sea formation causes 

regional sand deposition due to gradual loss of wind 

energy along the direction of sand drift. 

In summary, this study illustrates in detail the sedimentary 

aspects of only one subtype of eolian sand accumulation. 

Within this aqueous class of accumulation, however, 

are the basic types of eolian terrain common to all deserts 

(dune, interdune, sand sheet, and sabkha deposits), albeit 

with a distinctive suite of sedimentary features and sedimentary 

unit geometries. 

Another significant aspect of this model is the relationship 

of the prograding eolian sediments to the basinward 

marine sediments of the Arabian Gulf, which consist principally 

of biogenic lime muds and sands (Fig. 5). Given a 

series of sea level changes such as those previously 

described for the Arabian Gulf (Fig. 4), appropriate subsidence, 

and the extremely shallow slope of the Arabian


PRESENT 

SEAIFVFI 

AFTER AL-ASFOUR, 

1 1 1 1 I 

1978 

1 1 

+20M 

50ft= 

50 ft- 

20M 

100 

-40 

150 - 

200 60 

250 -_ 

-80 

300 

-lOO 

350 

1 1 

18 16 14 12 10 8 6 4 2 

YEARS B.R X I03 

FIG. 4—Probable eustatic sea level changes in study area since 

the last glacial period (Wisconsin) (after Al-Asfour, 1978). 

Shelf (in an ancient analog), it is probable that eohan 

deposits would develop as a series of tongues pinching out 

basinward. Thus, an excellent reservoir facies (eolian system) 

would be intercalated with potentially excellent 

source rocks (marine carbonate muds). Furthermore, 

upon reversal of regional dip, an excellent subregional 

stratigraphic trap would form (Fig. 5). 

There are two additional concepts which the authors 

hope to illustrate. First, within the prograding eolian system 

itself can be observed potential reservoir rocks, source 

rocks, and seals. The reservoir facies are the dunes and 

other well-sorted eolian deposits, the source facies are 

organic-rich interdune and sabkha deposits, and the seals 

are early cemented eolian deposits which result in various 

ways from the abundance of evaporites in the system. 

Secondly, given the overall conditions of sedimentation 

along the west coast of the Arabian Gulf, the nature of the 

shoreline, whether siliciclastic or carbonate dominant, is 

dependent on wind direction. Slight changes in the orientation 

of the shoreline (given a relatively steady regional 

wind direction oblique to the shorehne) result in radically 

different shoreline deposits. The further implication for 

explorationists is that both the potentially prolific stratigraphic 

hydrocarbon provinces of eolian and carbonatedominant 

prograding systems may exist in close proximity 

in the subsurface as they do presently in the Arabian Gulf. 

Thus, the recognition of one such system in the subsurface 

may imply the possible existence of the other nearby 

Description of Study Area 

Climate.—The climate of the study area (encompassed 

essentially by Fig. 2) is typically desertic (Fig. 6). Mean 

O 

temperatures range from 60°F (16°C) in February to 98°F 

(37°C) in August, with recorded extremes in the study area 

of about 28°F (-2°C) and 125°F (52°C) (Williams, 1979). 

Rainfall is scant, averaging 3.11 in. (7.9 cm) annually at 

Dhahran from 1950 to 1976. Maximum rainfall, sometimes 

in the form of thunderstorms, occurs during winter 

(Fig. 6). Evaporation rates are high, averaging over 30 in./ 

month (76 cm/month) during the summer. Diurnal ranges 

of temperature and humidity are extreme (about 30%) 

during much of the year. Humidities are greater near the 

coast, which commonly has early morning fog in fall and 

spring. 

Wind direction in the study area is primarily from the 

north or northwest. A circular histogram depicting potential 

sand movement amount and direction (Fig. 2) is representative 

of most of the area (Fryberger, 1979). Some wind 

reversals occur, chiefly during winter, causing reversed slip 

faces on the crests of dunes, but apparently having little 

regional effect on sediment transport. 

Wind energy release is highly seasonal in the study area 

(Fig. 6). Maximum drift potentials, an expression of the 

potential sand-moving power of wind (Fryberger, 1979), 

typically occur in June (Fig. 6). This maximum coincides 

roughly with the maximum development of the Indian 

monsoon, and essentially represents air flow from high 

pressure over northern Arabia and Egypt into a low pressure 

cell over the Indian subcontinent. This monsoonal 

aspect of the wind- and sand-moving season (locally 

termed the "shamal") differs from the typical spring 

(April) sand-moving seasons of North Africa, which have 

their origins in wind energy released by strong frontal passages 

(Breed et al, 1979). The result is that sandstorms in 

the study area are less violent, but more sustained than 

those in North Africa, lasting anywhere from one to several 

days, with winds in excess of 20 knots (37 km/hour). 

Nevertheless, the study area lies in one of the windiest desert 

regions of the world (Fryberger, 1979), with annual 

drift potentials in excess of 400 v.u. (30 mVmeter-width/ 

year). As described below, this high energy wind regime is 

in part responsible for the abiUty of the eohan system to 

prograde the coastline in the study area. 

Past climates in the study area have not always been as 

arid or hot. Considerable evidence for a cooler, more 

humid phase, perhaps in the middle-late Pleistocene, has 

been presented by McClure (1976) and Chapman (1971). 

Chapmsm describes extensive duricrusts in the study area, 

and fluvial weathering features of many types which he 

attributes to a cooler, more humid clunate during the Pleistocene 

(100,000 to 60,000 y.b.p.) with more arid conditions 

typical of the present day beginning about 11,000 

years ago (Chapman, 1971). This change roughly coincides 

with the rise in sea level in the area beginning about 

18,000 years ago (Al-Asfour, 1978). We have observed 

some calcareous and gypsiferous Aridosols in older eoUan 

deposits of the study area, which based on associated 

human artifacts could not be more than a few thousand 

years old. Thus, some soil-forming processes are also 

apparently proceeding at present, albeit slowly. 

The occurrence of pluvial periods, and changes in sea 

level in the area affect the conclusions of this study in two 

principal ways. First, pluvial periods and associated soil or


p Sedimentary Basin Center 

Marine 

Deposits 

Tectonic Basin Center 

Subsidence 

^w 

^Stratigraphically Entrapped Petroleum 

(Subregional or Local) Younger 

Rock Units 

FIG. 5—Depositional model of the seaward prograding eolian system. 

A. Offshore directed winds cause eolian outbuilding which at times exceeds sea level rise caused by basin subsidence. Alternation of 

low and high sea levels, perhaps caused by glaciation (or evaporation in restricted basins) would assist process of preserving eolian 

deposits below average base level of sedimentation. Transitional contact would occur where eolian deposits overlie marine deposits 

(as described by Shinn, 1973). Unconformable contact would probably develop where marine deposits overlie eolian deposits in this 

model. 

B. Reversal of dip, perhaps due to formation of tectonic basin, as shown would lead to formation of regional and local stratigraphic 

traps for petroleum at pinch-out of eolian system dunes into marine limestones. Overlying rocks could be continental or marine, 

depending on sedimentary history of original basin. Ideal cap rocks (younger rock units) from standpoint of petroleum accumulation 

would be evaporitic. 

duricrust-forming episodes enhance the possibilities for 

early cementation of the eolian deposits of the study area. 

Second, fluctuations in sea level on this scale enhance the 

potential for intercalation of eohan and marine deposits 

(an example of which is described by Evans et al., 1969). 

Physiography (Onshore) 

The study area lies on the interior platform of the Arabian 

Peninsula (Powers et al, 1966). The interior platform 

is an area in which structural dip is extremely slight, averaging 

about 0°20* toward the east. The flat-lying sediments 

are disturbed by local uplifts, possibly related to 

basement block faulting, which have formed the broad 

gentle anticlines associated with the oil accumulations at 

Ghawar and Abqaiq (Powers et al, 1966). The Dammam 

dome, on which Dhahran rests (Fig. 2) is apparently a salt 

intrusion structure. 

The bedrock underlying the eolian deposits of the study 

area consists of Tertiary sandstone, sandy limestones, and 

marls of primarily continental origin (Powers et al, 1966). 

These deposits are in places poorly consoUdated, and thus 

may supply some of the sediment carried by the wind 

(white areas. Fig. 1). Some of the bedrock limestones (e.g., 

the Eocene limestones of the Dammam dome) have been 

sculptured by wind into yardangs, and bear evidence of 

both wind and water erosion. 

The dune, interdune, and sand sheet deposits extend 

continuously in a 40-mi (64 km) wide strip along the coast- 

Une from Jubayl southward (Fig. 1). These deposits are 

thinnest, or absent, over topographic highs. Sabkha 

deposits are most common along the coast and along the 

western margin of the Jafurah. Distribution of sabkhas is 

controlled by topography, with sabkhas occupying low 

areas along the coast or developing around the margins of 

structures such as the Dhahran dome and the Abqaiq antichne 

(Fig. 2).


Z 

ID 

9 

U 

LU 

> 

< 

z 

LU 

B 

o Q- 

90 

80 

70 

60 

50 

40 

30 

20 

10 

-0- 

^^UbLT^p 

\4_HUhi^ 

J ' F ' M A ' M ' J ' J ' A ' S ' O ' N ' D 

MONTH 

FIG. 6—Summary of Dbahran climate including mean monthly temperature, evaporation, and humidity. Drift potential, a measure 

of sand moving effectiveness of the wind, is at a maximum during the June "shamal" season of northwest winds. Graph is based on 

long-term climate records of ARAMCO and Dhahran International Airport. 

The abrupt termination of the Jafurah sand sea against 

the coastline near Jubayl indicates that the present shoreline 

may contribute sediment to the Jafurah system. This 

could occur through deposition of sand in the littoral zone 

by longshore drift (southward in the area) and subsequent 

removal by wind deflation. Additionally, it is inviting to 

speculate that the sand sea extended across the present 

floor of the Arabian Gulf possibly to the Tigress- 

Euphrates delta region during lowered sea levels of the 

Pleistocene. Well-rounded, red, "eolian" sand grains have 

been retrieved from the present gulf floor in the regions 

north of Jubayl by Emery (1956). Another source of sand 

for the Jafurah may have been the extensive (presently 

inactive) Wadi Al Batin system (described by Holm, 1960) 

which crosses the Dhahna northeastward into Kuwait. 

The discussion of possible sources of sand for the Jafurah 

is of interest primarily (for our purposes) in terms of distal 

versus proximal sand supply. The apparent upwind cutoff 

of the Jafurah at Jubayl means that distal sources available 

during the Pleistocene owing to lowered sea levels and 

active wadi systems are not now available with the exception 

of sediment supplied by longshore drift. Thus, resupply 

of sand must presently be from local sources such as 

older eolian deposits or bedrock. Our impression is that 

the Jafurah system is slightly undersaturated—that is, 

there is an excess of available wind energy over sand resup- 

>tz 

Q 

ID 

I 

ply. This may account in part for the widespread occurrence 

of stable sand sheet terrain and thinner stratigraphic 

units we observed. 

Sources of water.—Perhaps surprisingly, water is abundant 

in the recent eoUan sediments of the study area, and 

with wind plays a definitive role in the depositional process. 

There are three principal sources of water in the area: 

fossil water, meteoric water, and seawater. Brackish fossil 

water from subsurface aquifers rises to the surface in artesian 

wells near Hofuf, and is pumped to the surface for 

irrigation and other uses. This source of water accounts, in 

part, for the flow across the area near Abqaiq (Fig. 2) and 

for the relative freshness of the water in the interdunal 

wadi shown in Figure 22b. 

Despite the low annual rainfall, fresh meteoric water is 

an important component of the ground-water system in 

the study area. Sand dunes are extremely effective storage 

devices for water, which enters the dunes during rains and 

cannot run off and evaporate (Bagnold, 1954). This process 

contributes to the high ground-water tables in the study 

area. We observed a dramatic illustration of this process 

during the winter of 1980, when water tables in sabkhas at 

traps 11 and 7 (Fig. 2) rose 2 to 4 in. (5 to 10 cm) after winter 

rains which could not have totaled more than a few 

inches. 

Seawater is an important contribution to the ground-


FIG. 7—Sediments on floor of the Arabian Gulf. Contour lines show percentage of CaCO, in sediments. After Emery (1956). Arrow 

shows study area. 

water table in locations near the coast, and provides a 

source of evaporite minerals. 

These various sources of water for the ground-water 

table seem to be imperfectly mixed. This resuks in interesting 

contrasts in vegetation and sedimentary features 

within relatively small areas, both as a function of amount 

of water and salinity. 

Physiography (Offshore) 

The floor of the Arabian Gulf is an extension of the Arabian 

interior platform. The gently sloping basin floor is 

interrupted by geologic structures (such as salt domes), 

coral reefs, mud mounds, and shallow terraces before 

reaching a maximum depth of 250 ft (75 m) in the center of 

the basin opposite Dhahran. Water salinities are about 

36.5°/ooin the center of the gulf, and exceed 42°/oo in some 

restricted bays and lagoons. Sediments offshore along the 

Arabian Coast are mainly calcareous sands and muds 

(Fig. 7). The highest percentage of calcium carbonate in 

the sediments corresponds to highest salinities (Emery, 

1956) but may also be seen to be associated with the 

onshore wind province of the Trucial Coast (Fig. 7). 

Sources of carbonate material offshore include shells and 

comminuted shell debris including mollusks, corals, and 

algae, inorganically precipitated material, and airborne 

dust, which is highly calcareous (Emery, 1956). Emery 

noted that medium-grained carbonate sands occur on 

bathymetric highs in the gulf, whereas lower areas are 

commonly muddier. The proximity of the nearby prograding 

dune system is indicated by the presence of quartz sand 

in the sediments and by the well-rounded shape and reddish 

color of some grains found by Emery. 

Tidal range is 8 ft (2.5 m) and waves may be up to 12 ft 

(3.5 m) high during storms. Water temperatures at the surface 

range from 60 to 90°F (15 to 32°C) in winter and summer, 

respectively 

Eolian Terrains and Associated 

Wind Microenvironments 

Within the broad prograding sand sea near Dhahran are 

four basic types of wind-laid deposits: dunes, interdunes, 

sand sheets, and sabkhas. These four basic types of eohan 

terrain can occur in varying proportions in any desert and 

so are not unique to the study area. However, the sedimentary 

features and geometries of each type of deposit can 

differ greatly. We hope that sufficient detail will emerge in 

this study to allow recognition of similar systems preserved 

as ancient rocks. Nevertheless, it should be recog-


nized that even many of the sedimentary features of the 

distinctive eolian sabkha deposits (for instance) are not 

unique to the coastal setting. 

Sand dunes.—Sand dunes in the area are of the barchanoid 

type (McKee, 1979), including barchan, barchanoid 

ridge, transverse ridge, dome, parabolic, and various 

types of coppice or "obstacle" dunes (Fig. 8). Dune 

heights average 16 to 40 ft (5 to 12 m). Dune advance measured 

during our one-year study ranged from 89 ft (27 m) 

for a barchan dune 10 ft (3 m) high, to 27 ft (8 m) for a barchanoid 

ridge dune 40 ft (12 m) high. Crests of dunes are 

usually very active and unvegetated; however, some vegetation, 

mainly Cyprus conglumeratus, commonly grows 

on the windward slope of some dunes. This leads to development 

of parabolic dunes, particularly in the northern 

part of the study area near Jubayl. The microenvironment 

of the dune terrain is the most exposed, because of wind 

and sand blast, and the least stable because of the advance 

of the dune. Drift rates which we measured atop a dome 

dune at site 15 (Fig. 9) illustrate the very active nature of 

the dune microenvironment compared to the other eolian 

terrains. Wind flow over dunes is steady on the upwind 

sides and over the crest, eddying and breaking into vortices 

as it flows over the brink. Wind reversals frequently result 

in attached horizontal vortices at the brink of the dune 

which scour out huge troughs, 3 to 13 ft (1 to 4 m) in circumference. 

These troughs fill quickly upon return of 

wind to the usual northwest direction. 

Interdunes.—Geomorphically, interdunes are flat or 

gently sloping areas between dunes, as shown in Figure 8a, 

b. Interdunes may be enclosed by dunes (Fig. 8b) or exist 

as rather extensive areas between widely spaced dunes 

(Fig. 8a). In general, the subenvironment we associate 

with true interdunes and associated deposits occurs in 

enclosed interdunes. Further, the deposits seem more distinctive 

as the associated dunes become larger. The larger 

size allows more time for deposition due to slower movement 

and results in greater variability wind regime. 

The wind microenvironment of interdunes is more directionally 

variable than that of dunes owing to eddying 

around the upwind dune. Wind strengths are less in the 

interdunes due to the sheltering effect of the upwind dune, 

but are frequently very gusty, with sand flying about horizontally 

at eye level and falling from above. Drift rates in 

interdunes generally reflect the sheltering effect (Fig. 9). 

Interdune sites are increasingly unstable roughly in proportion 

to distance above water table (the local "base 

level"). Despite lower drift rates, rapid erosion often 

occurs in interdunes due to storage of drifting sand by the 

dune immediately upwind. Thus the wind crossing the 

interdune will commonly be very undersaturated (with 

respect to sand it could potentially carry) and will pick up 

sand. For example, the interdune at site 13 (Fig. 9) was 

eroded over 16 in. (40 cm) during our study, whereas the 

crest of the dome dune at trap 15 (Fig. 9) was eroded only 6 

in. (15 cm). Wind environments in interdunes near the 

water table frequently become weakened by the baffling 

effect of plants, and the lower more shehered position 

favors deposition over scour at many localities. 

Sand sheets.—Sand sheets are sandy plains (of either 

erosional or depositional origin) whose topographic 

expression is independent of water table (Fig. 8c, d). Sand 

sheets seem to develop from remnant deposits of migrating 

dunes, or from strips and patches of sand deposited 

during storms. Some sand sheet deposits are crossbedded, 

clearly reflecting origins from migrating dunes. 

The sand sheet deposits of interest here, which comprise 

the bulk of those we studied, are horizontally bedded and 

have developed independently of dunes through slow deposition. 

Surface wind velocities, and thus sand-drift rates, 

are lower over vegetated sand sheets than most other types 

of eolian terrain (Fig. 9). Thus sand sheets are relatively 

stable and provide a habitat for diverse flora and fauna. 

As vegetative cover diminishes, sand sheets are proportionally 

less stable because of increased exposure to wind 

currents around scattered clumps of vegetation and 

increased exposure to drifting sand from upwind. 

Sabkhas.—Eolian sabkhas are evaporitic sandy plains 

of erosional or depositional origin, whose topographic 

expression is dependent on a shallow (3 to 6 foot, 1 to 2 m) 

ground-water table below the sabkha surface (Fig. 8c). A 

rising water table causes deposition of wind-laid sediments 

and evaporites. A falling water table (after establishment 

of a sabkha) causes erosion, often to a cemented layer 

which typically forms in the uppermost several feet of the 

saturated zone. Sabkhas are distributed widely in the study 

area (Figs. 1,2), ranging from the extensive, highly evaporitic 

coastal sabkhas such as Sabkha Ar Riyas, to very 

small, interdunal, slightly evaporitic sabkhas far inland. 

Since evaporite minerals are commonly available in the 

sediments, the principal requirement for sabkha development 

in the study area was presence of a shallow water 

table. It is not known whether evaporite minerals in 

ground water are a requirement for the formation of sabkhas, 

although every sabkha we observed was associated 

with moderately to extremely evaporitic conditions. Relatively 

fresh, shallow water tables seem to result in proliferation 

of plant hfe, destroying the flat, open conditions 

associated with typical sabkhas. 

Sabkhas were the most stable terrain we observed, with 

only an inch or two of erosion or deposition during the 

course of the study. Sand drift rates over sabkhas are lowest 

in the center of the sabkhas (which are elongate downwind) 

and higher along the margins (which are closer to 

nearby dunes and sand sheets). During summer sandstorms, 

however, drift rates can be rather high due to the 

exposed nature of the sabkha (Fig. 9). 

The evaporitic nature of typical sabkhas renders them 

unsuitable for even the most salt-resistant higher plants. 

We seldom observed algal or microbial growth at the surface 

of the sabkhas and then only in sabkhas associated 

with interdune ponds. However, some organic-rich or 

darkened zones, about 3 ft (1 m) below the sabkha surface 

(at the interface of the saturated water table and the capillary 

fringe) were associated with microbial growth. Larvae 

and burrows of an unidentified insect were observed at 

Sabkha Ar Riyas. Thus, some insects are capable of surviving 

in the sabkha sediments. 

Microenvironments of Ground-Water Table 

The damp zone, capillary fringe zone, and saturated 

zone (Table 1) associated with the presence of the shallow

^*--" '. rf*- . • 

-^ ^ w ^ '^-. JIBW 

FIG. 8—Air views of study area showing different types of desert terrain. A. Near Dammam looking southwest. Isolated barchan and dome dunes move across vegetated sand sheets 

Palm trees for scale. Eolian sUiciclastic sabkha is dark area in lower left portion of photograph. B. Dune complex consisting of barchanoid dunes near trap 4. Sabkbalike interdune in 

left center, vegetated interdune in background. Utility poles for scale. C. View to southeast. Dome dunes (arrow 1), vegetated sand sheet (arrow 2), and eoUan siliciclastic sabkha 

(arrow 3). Tire tracks on sabkha for scale. Note irregular distribntion of sand sheet. D. View to northeast (Arabian Gulf in background) near Dammam showing extensive, veeetated 

sand sheet with parabolic dunes Oeft center) and blow outs. 

0) 

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09


Table 1 

Interface Processes at Interface Insects 

Wind/dry sediment Mobile sand, wind-formed features, 

contorted strata (loading) 

Dry/damp sand 

Damp/capillary fringe 

Capillary fringe/water 

table 

Scour surfaces to cohesive damp sand 

Sabkha, salt ridges, halite, soils, 

adhesion-produced strata 

Organic layers, micrite, euhedral 

gypsum crystals (croute zonaire), wet 

interdunes, anhydrite, contorted strata 

(liquefaction) 

F M 

1980 

Early 

Grasses Saltbrush Cementation Soils 

Relative abundance 

FIG. 9—Sand drift rates measured on different eolian terrains during the year of study. Note relatively steep increase in drift rates 

during June sandstorms ("shamals") for exposed sabkha compared to other eolian terrains. 

ground-water table in these porous and permeable sands 

each constitutes a distinctive microenvironment of the 

water table. Each zone (microenvironment) is associated 

with certain conditions or processes which ultimately have 

a great impact on all features of the sediments (Table 1). 

Thus, for example, halite accumulation and resulting saltridge 

structures are associated with the capillary fringe 

zone at its interface with the surface. However, cementation 

involving direct precipitation of gypsum and anhydrite 

from water—as opposed to soil formation which 

involves remobilization of gypsum from airborne dust— 

and the formation of euhedral crystals ("croute zonaire") 

is associated with the upper part of the saturated zone. 

The vertical distance from the top of the saturated zone 

to the top of the damp zone is usually less than 6 ft (2 m). 

Since water-table levels are not steady during the course of 

a year in the study area and certainly change over geologic 

time, the result is the constant shifting of these damp, capillary 

fringe, and saturated zones within the sediments. 

The further result is inevitably the overprinting of some of 

the effects of the various zones on each other within sediments 

of dune, interdune, sand sheet, and sabkha terrains. 

The complicating effects of the water table and associated 

aqueous zones are increased in the study area owing to the 

thin sedimentation units typical of the area. Our impressions 

are summarized in Figure 10, which shows the typi-


HIGHEST LOWEST LOWEST 

INFLUENCE OF 

STRATIGRAPHIC 

WATER ON 

POSITION 

SEDIMENTARY 

FEATURES 

PRESERVATION 

POTENTIAL 

LOWEST HIGHEST HIGHEST 

FIG. 10—Schematic diagram illustrating typical stratigrapliic 

positions of eolian deposits in study area, and transition or overlap 

of sedimentary features in respective deposits (circles). Sand 

sheet and interdune deposits are commonly switched in terms of 

stratigrapliic position (that is, sand sheets commonly overlie sabkhas); 

however, interdunes in general resemble sabkhas more 

than sand sheets, mainly because of the abundance of water in 

deposits. 

cal stratigraphic arrangement in the study area. In general, 

the effect of water on sediments as they are deposited is 

greatest in units such as sabkhas which are stratigraphically 

low at the time of deposition. However, sediments 

deposited in a dry enviromnent, such as a dry dune, can 

become damp because of a rising water table. This may 

allow the growth of vegetation which can completely obliterate 

the original cross-stratification. 

In summary, (a) the ground-water table and its associated 

damp, capilleuy fringe, and saturated zones produce 

distinctive effects on both primary and secondary features 

of sediments in the study area; (b) the combination of 

shifting of water-table depth, the shallowness of the water 

table (allowing evaporative processes to proceed), and 

thinness of sedimentary units in the area results in extensive 

overprinting (secondary effects) on the sediments by 

the processes acting in each different zone; and (c) these 

overprinting effects may be seen in dune, interdune, sand 

sheet, and sabkha sediments. Further illustrations of these 

processes are given in sections below. 

Another concept which is derived from the above discussion 

is that the role of wind is mainly to deliver most of the 

sand to the sites of deposition. The roles of water are as 

follows: (1) water affects primary sedimentary features of 

the sediment by interaction with wind deposition; (2) the 

water causes modifications to the sediments through support 

of plant and animal activities and evaporative processes; 

and (3) water distributes early cements in the 

sediment. 

Ebounples of Offshore Prograding Dunes 

Shinn (1973) described a sabkha created by offshore prograding 

dunes on the Qatar Peninsula. Shinn's data are 

summarized in Figures 11 and 12, which show the stratig 

raphy of the sabkha and physical features of the underlying 

nearshore marine deposits. The first set of steeply 

dipping strata encountered above the burrowed marine 

deposits (Figs. 11,12) are marine accretion slope beds. 

The writers also observed dunes prograding offshore at 

Doha Dhalum sabkha near Dhahran. Most of the sabkha 

(Fig. 13) is overlain by a thin, 1 to 2-in. (2.5 to 5-cm) layer 

of eolicm sabkha deposits, which are then underlain by 

eolian avalanche (slip-face) deposits from earlier seawardmigrating 

dunes. In a few places, the eolian avalanche 

strata are directly overlain by serpulid beachrock and 

shells of gastropods and marine moUusks. This situation 

estabUshed the basic fact of marine superposition above 

dune deposits in the area, and suggested the model proposed 

in this study. The sabkha deposits over much of the 

area seem to be a later phenomenon (postdating the beachrock 

deposits) and are derived in part from sand blown in 

from the northwest. The sand sheet deposits netu' the shore 

are unusually coarse grained, and probably have developed 

by wind scour of dune deposits exposed when thin 

sabkha deposits upwind are stripped off. The coarseness 

of the sand sheet deposits is attributed in part to the 

coarseness of underlying dune sands, which are presumably 

mostly base of shp-face deposits (these usually collect 

the coarsest grains in a dune). The sand sheets are also 

derived in part from the belt of dunes along the water's 

edge. The deposits of the coastal zone are illustrated in 

Figure 14, which shows the dunes prograding directly into 

the water. A generalized stratigraphic column (Fig. 15) 

summarizes our observations regarding the vertical 

sequence of genetic units at the Doha Dhalum sabkha. 

EOLIAN DEPOSITS OF PROGRADING SYSTEM 

Observable Features of Sediments 

Our discussion will emphasize those sedimentary aspects 

of the deposits which are likely to be of most use to explorationists, 

whose view of prospective rocks is usually limited 

to cores. Thus, we will emphasize small-scale 

sedimentary structures and primary eolian strata, as 

opposed to arrangements of sets of strata. In particular, 

we will refer commonly to avalanche-, grainfall-, ripple-, 

and adhesion-produced strata as the fundamental units of 

eolian deposition. Origins of these deposits are illustrated 

in Figme 16 and are discussed in more detail in Fryberger 

and Schenk (1981). We will attempt to summarize as well 

our textural and compositional data, with special reference 

to indications of early cementation of the deposits, 

development of organic-rich zones, and general geographical 

distribution of the fades. 

Sedimentary Features of Dunes 

Sedimentary features of dunes which we trenched are 

summarized in Table 2, and terminology applicable to barchanoid 

dunes and associated deposits is summarized in 

Figure 17. In general, we observed a suite of primary sedimentary 

features similar to those described for eolian 

dunes by other workers such as McKee et al (1971) and 

McKee (1966). We observed the formation of ripple-,


NW SE 

30Tt 

Imile 

«C » » ' 

eg 0 

Dune 

Dune 

Sabkha 

.^^i^4gsj.,^


Table 2. Barchan Dune and Dome Dune 

Feature/Size Distribution 

BARCHAN DUNE 

Origin 

Avalanche-produced strata 

30 to 34° dip 

Ripple-produced strata 

0 to 30° dip 

Grainfall-produced strata 

0 to 30° dip 

On slip face: 

flame structures 

break-aparts 

fadeout laminae 

wavy laminae 

sandf low toes 

Bundles of fine-grained 

laminae and coarse-grained 

beds, steeply dipping 

Isolated coarse-grained 

layers along gently dipping 

surfaces of truncation 

(also called "reactivation 

surfaces") 

Isolated high-index ripples 

or ripple trains 

1 to 2 in. (2.5 to 5 cm) 

thick lenses of coarse sand, 

inversely graded 

Cemented zones, 

large scale 3 ft (1 m) thick, 

wide lateral extent 

Bioturbation 

Ripple-produced strata: 

thin, gently curving if 

followed laterally, 

great lateral extent 

TUbular bioturbation: 

plant-root traces and 

insect burrows (termites, 

ants, beetles) 

Contorted (wavy, compacted) 

layers, 5 in. (12.5 cm) thickness 

Isolated coarse-grained 

layers ("reactivation 

surfaces") 

Very coarse layers up to 

1.6 in. (4 cm) thick 

Shp face of dune Avalanching 

Slip face, apron, windward slope, 

leeward slope 

Apron, slip face, leeward slope, 

breaks in slope near top of dune 

In avalanche-produced strata 




Base of slip face 

Slip-face deposits 

Anywhere on dune 

Anywhere on dune 

Slip face of dune—lobes of 

sand flows viewed parallel 

to wind direction 

Thin but laterally continuous 

zones. Commonly a secondary 

feature 

Common on slip faces and lower 

parts of dunes between sandstorms 

Bulk of dune 

DOME DUNE 

Vegetated parts of dune— 

usually around base of dune 

or on windward slope. Most 

common near moist or vegetated 

areas (termites collect plant 

materials) 

Anywhere, commonly isolated 

Anywhere—not nearly as coimnon 

as on sand sheets because dune 

sands are finer and better sorted 

Top of dune 

Migrating ripples (fine 

grained) 

Flow separation over crest 

or protruding part of dune 

Avalanching down slip face: 

tensional features near top 

of slip face, compressional 

features near base of slip face 

Represents alternate 

ripple- and avalanche-produced 

strata 

Wind direction shifts, causing 

episode of scour, leaving 

lag or layer of coarse grains 

Change in wind conditions 

causes preservation—commonly 

a wind shift or increase in 

wind strength preserves ripples 

beneath grainfall deposits 

Avalanching of dry sand 

(See text). Soils, windblown 

detrital cements, evaporites 

Reptiles, insects, plants 

(usually grass), in places 

large animals 

Preservation of lower portions 

of migrating ripples 

Root and rhizome disturbance 

of sand, termite burrowing 

along dead-plant remains, 

insect scavenging 

Camel footprints 

Scour episodes, strong winds 

Strong winds, grainfall 

deposition 

Abundance 

in Trenches 

Common 

(most of dune) 

Common 

Rare 

Common 

Common 

(bulk of dune) 

Common 

Rare 

Common 

Common 

Common 

Common 

Common 

Rare 

Rare 

Rare


TIDAL FLAT 

High-angle 

Cross-bedding 

Low-angle 

Cross-bedding 

Fine Sand 

No Bedding 

Bioturbation 

Shell Fragments 

Muddy (Carbonate) 

Quartz Sand 

Bioturbation 

Hinged Mollusks 

FIG. 12—Generalized stratigraphic column (after Shinn, 1973) 

showing (a) upward fining of calcareous shell debris, (b) loss of 

carbonate mud upward, and (c) increase in quartz sand content 

and cross-bedding of deposits upward. Based on core in Umm 

Said sabkha through units 2 to 4 of Figure 11. 

SOUTH 

A 

1 mile 

MARINE DEPOSITS 

DUNES 

SAND SHEET 

THIN SABKHA DEPOSITS 

(0-2 feel I 

SAND SHEET 

NORTH 

A 

MARINE BEACHROCK \ JAF 23 < 

UNCONFORMITIES 

MARINE OR 

SABKHA 

FIG. 13—Schematic drawing illustrating stratigraphy inferred 

for Doha Dhalum sabkha and vicinity from numerous excavations 

and hand-dug trenches. Thickness of sabkha deposits is 

greatly exaggerated for clarity. Eolian deposits become highly 

bioturbated and cemented by gypsiferous rhizocretions as unit 

rises toward Dhahran dome (dotted area right side of drawing). 

This facies change may have resulted from entrance of fresh 

water into dune sands, allowing growth of vegetation. Dune 

sands (below sabkha) between T3 and Tl seem to be free of bioturbation, 

perhaps because of lower position and resulting high 

salinity of interstitial water. Modern sand sheet above JAF 23 has 

less bioturbation than underlying unit and no development of 

secondary gypsiferous plant root and rhizome molds. 

careous, with presumably much of the cement derived 

from airborne dust. Some gypsiferous cemented zones 

contained euhedral, poikilotopic gypsum crystals, apparently 

derived from precipitation from saturated solutions 

at the top of the water table. Rhizocretions (replacement 

of plant roots and rhizomes by gypsum) were common in 

cemented zones. The region of white sand described above 

SHORE' 

UPM BEACH TEST SITE 

DOHA DHALUM 

((APPROX) 

SABKHA 

FIG. 14—Dune, sand sheet, and sabkha terrain along the northern 

coastHne of Doha Dhalum (see also Figure 2). Dunes are prograding 

obliquely into water; both dome dunes (D) and barchan 

dunes (B) are present. Sand sheets (vegetation symbols) are distributed 

irregularly atop sabkha, but are elongate upwind 

(toward northwest). Position of marine accretion ridge shown by 

dashed line. Heavy dark lines are roads. S indicates nearshore 

JAF 26 

JAF 29 

SALT RIDGES 

SAND SHEET 

DUNE 

SABKHA 

6 Feet 

MARINE BEACHROCK 2 Meters 

AND SANDS 

DUNE 

Ll = Unconformities 

FIG. 15—Composite stratigraphic sequence at Doha Dhalum 

sabkha, based on several study locations (indicated on left). In 

places, due to incomplete preservation or nondeposition of 

marine deposits, sabkha facies lies directly on dunes. Upward 

sequence of sabkha to dune to sand sheet is typical of much of 

study area. U = unconformity. 

was experiencing early cementation due to remobilization 

of gypsum incorporated into the dunes as windblown, 

sand-size particles. These sands were usually so firm that 

we seldom had trouble driving our vehicles across them.

D 

lin. 

Ripple Foreset 



Type of 

Interdune 

Dry 

Dry 

Dry 

Dry 

Damp or 

Evaporitic 

Damp 

Damp or 

Evaporitic 

Dry 

Dry 

Dry 

Dry 

Dry 

Dry 

Dry 

Feature/Size 

Diastem (erosional) flat 

to gently undulating 

Troughlike scour depression 

(cut and fill) 

Contorted bedding below 

interdune surface 

Faults in dune bedding 

(avalanche-produced strata) 

below interdune surface 

Uneven, ridgy erosion 

surface—up to 1 ft (0.3 m) 

relief 

Contorted strata 

(liquefaction) 

Table 3. Erosional Interdune and Depositional Interdune 

Distribution 

EROSIONAL INTERDUNE 

Top of underlying dune 

deposits 

Along erosion surface 

0 to 1 ft (0 to 0.3 m) below 


0 to 1 ft (0 to 0.3 m) below 


Along halite encrusted 

surface of interdune 

0to6in. (0tol5cm)thick 

below interface 

Origin 

Wind scour due to undersaturation 

of wind 

Wind scour around obstacles 

or depressions 

Loading by animals walking 

on interdune (dry soft sand) 

Loading of damp or dry sand 

below interdune 

Seepage of salt to interface 

along certain layers—early 

cementation causes resistant 

ridge 

Quick condition of wet sediments, 

and/or bioturbation or 

loading by dune 

Microscale irregular ridges Along halite encrusted Very small salt-ridge features 

and bumps up to 0.5 in. surfaces 

and/or microscour of halite 

(1.3 cm) relief encrusted sediments 

DEPOSITION AL INTERDUNE 

"Wind" ripple-produced Anywhere in interdune, 

strata, commonly thin but comprises most of 

laterally continuous, flat sediment 

or gently dipping (flat at 

base, may have transition 

upward into dune without 

break—contact with underlying 

dune commonly sharp and 

erosional) 

"Granule" ripple foresets, 

commonly coarse grained 

Anywhere, commonly near 

downwind margin of 

interdune, preserved entire 

or as coarse-grained layers 

Migrating wind ripples— 

commonly thin layers result 

due to undersaturated winds 

typical of interdunes 

Abundance 

Common 

Common 

Common 

Common 

Common 

Rare 

Common 

Common 

Strong undersaturated winds Common 

Root traces, 0.08 to 0.16 in. Wandering through sediment, Sedge and other plant roots 

(2 to 4 mm) diameter commonly not visibly branching 

(slightly damp sand—not related 

to water table—commonly 

underlies even the driest 

interdunes, allowing some plant 

growth. This is also true of "dry" 

dune and sand-sheet terrains) 

Contorted strata 

Gentle, curving scour 

surface 

Short, irregular, "floating" 

laminations in 

bioturbated zones 

Soils (Aridosols: gypcrete 

or calcrete) 

Just below surface of 

interdune 

Entire surface of original 

interdune 

Zone of extreme bioturbation 

near plant 

Characteristic of older 

more stable terrain— 

exist as laterally 

extensive zones in 

sediments 

Rare 

Loading by camels Rare 

Scour episode due to wind shift Rare 

and eddying around dune 

Bioturbation destroys all but a Rare 

few laminae 

Early cementation associated Common 

with dustfall, ageing, stabilization. 

May be a secondary 

feature—observed in all 

facies except sabkha

Type of 

Interdune 

Dry 

Dry 

Damp 


Feature/Size 

"Massive" aspect to trench; 

in places relict bedding 

visible 

Plant "leaves" Zygophyllum 

sp., etc 

Large and small scale 

intense bioturbation, mottled, 

commonly dark-colored zones 

Table 3. Continued 

Distribution 

Zones in trench (extend 

horizontally) 

Near top of trench— 

reworked from existing 

plant community 

Most of interdune area: 

can affect sediments 

several feet below modern 


Damp Anticlinal or synclinal Around base (rhizomes) of 

depositional drape or scour of plants 

of layers around zones of 

bioturbation 1.5 ft (0.5 m) 

high and 3 to 6 ft (1 to 2 m) wide 

Damp 

Damp 

Damp 

Wet 

Wet 

Wet 

Intense insect bioturbation 

termite colonies 

Salt ridges (not as common 

as in fully evaporitic 

interdunes); exist as 

isolated layers 

Organic-rich layers 

Adhesion-produced strata 

Aqueous ripple, bar, and 

point-bar deposits 

Salt ridges at margin of 

wet interdune—along what 

would be called damp— 

a transitional facies 

Evaporitic Most features of siliciclastic 

sabkha (see below) 

but thinner, less continuous 

Near plants 

In places evaporitic 

episodes; not a 

dominant feature, few 

ridges—commonly a crust 

Below surface of interdune, 

commonly near top of saturated 

zone of water table 

Wettest part of interdune 

("quicksand") locally, sets 

of adhesion-produced 

strata can be several feet thick 

Zones of interdune with 

flowing water or windagitated 

water 

Around edge of watersaturated 

sand 

Whole surface of interdune, 

or preferentially 

near base of upwind dune 

cally (e.g., a few inches). Moreover, "interdunal sabkhas" 

are sometimes created simply by scour to water table (capillary 

fringe) owing to the presence of a dune, and disappear 

once the dune has moved on. The areally extensive 

eolian sabkhas however are related to a more stable and 

continued rise of water table, and perhaps more importantly 

to the absence of a dune complex. Thus, sedimentary 

units are thicker for eolian sabkhas, commonly 3 to 10 

ft (1 to 3 m) and have less tendency to exhibit rapid vertical 

facies changes related to water content of the sediments. 

Early cementation.—Early cementation was common in 

interdunal and associated sediments, presumably owing to 

Origin 

Extensive bioturbation—as 

a secondary feature 

(observed in all facies 

except sabkha) 

Plants 

Abundant plant growth in 

interdune when water is not 

too saline, burrowing 

animals 

Abundance 

Common 

Rare 

Common 

Results from scour or deposition Common 

around exposed bases of plants, 

along with bioturbation by 

plant roots 

Termites feeding on buried 

plant remains 

Rapid evaporation 

Decaying plants; microbial 

growth 

Adhesion of drifting 

(saltating) sand to 

wet surface 

Flowing water 

Seepage-evaporation 

Wind scour to water table, 

or rise of water table 

Common 

Common 

Common 

Rare regionally, 

common 

in this setting 

Common 

Common 

Common 

low stratigraphic position and thus increased proximity to 

water and cements. Typically halite was the most common 

cement, although gypsum was also abundant. Carbonate 

cement was uncommon, except in places where shells of 

gastropods or other organisms accumulated in sufficient 

quantities in wet interdunes. This type of occurrence was 

most common near the coast, where shelly debris commonly 

included marine organisms (mollusks). In one 

trench in an interdune about 3 mi (5 km) from the coast, 

we observed fresh shells at the surface, partly decomposed 

and broken shells 16 in. (41 cm) below the surface, and 

blue micrite (evidently derived from decomposed shells) at


LEEWARD SLOPE ILSI 

BRINK 

SLIPFAC 

APRON 

INTERDUNE 

CREST 

WIND 

WINDWARD SLOPE (WS) 

\CREST 

CRESTLINE (CLl 

ELEVATION CONTOUR 

FIG. 17—Terminology useful for describing parts of barchanoid dunes and associated eolian deposits (after Fryberger and Schenk, 

1981). A. Cross section of barchanoid dune (stratification not shown). B. Plan view of barchanoid ridge dunes with interdune and 

sand sheet deposits. C. Detail plan view of barchanoid dune. 

the bottom (less than 3 ft [1 m] below the surface). 

Organic enrichment.—We observed numerous places of 

apparent organic enrichment in interdunes, usually in the 

sediments of evaporitic interdunes at the top of the saturated 

water table. Some of the organic material was 

undoubtedly derived from remains of higher plants in 

well-vegetated interdunes (Fig. 22d). In other places the 

organic matter seemed to have been derived from microbial 

growth at the top of an extremely saline ground-water 

table in the sands underlying the interdune. In general, 

however, organic enrichment did not seem as great as in 

sabkha sediments. 

Erosional interdunes.—Erosional interdunes are common 

in the study area. The record of dry erosional interdunes 

is usually a scour surface, sometunes separated 

from overlying deposits by a coarse-grained layer. Damp 

or wet erosional interdunes are evidenced by a highly irregular 

or wavy erosional contact (Fig. 23a, e). The irregular 

surface of erosion develops because dampness, or in many 

places cementation by halite, causes differential resistance 

to wind erosion. This process can produce considerable 

relief, up to 12 in. (30 cm). Figure 22d (foreground) shows 

rather large resistant ridges resulting from halite cementation 

in an interdune which is mainly erosional. Microscale 

relief also develops on damp erosional interdunes, lending 

a bumpy aspect to the gently undulating or ridgy surface 

of erosion (Fig. 23e, arrow 2). Erosional interdunes are 

also sometimes associated with effects on underlying sediments 

which result from processes taking place near the 

surface. Resulting sedimentary features include plant bioturbation 

and contorted strata due to loading by camels or 

other organisms. 

Dry depositional interdunes.—Dry depositional interdune 

sediments consist mainly of subhorizontal rippleproduced 

strata (Fig. 23b). Ripple-produced strata include 

those (1) which result from passage of small, relatively 

fine-grained and well-sorted "saltation" or "wind" ripples, 

and (2) of larger, coarse-grained, high-relief "granule 

ripples" (Fig. 23b). Bioturbation was observed in dry 

interdune deposits but was less common than in damp or 

wet interdunes. Dry interdunes are common in dime complexes, 

or in dune areas which occupy a topographically 

high position and thus commonly have deeper groundwater 

tables. 

Damp depositional interdunes.—These interdunes are 

associated with limited development of salt ridges and 

evaporitic structures, and with an abundance of vegetation 

if water salinities are not too high and the upwind


tflll lit» 

.f-. -fl- 

JftliM^ ''-\*^ 

FIG. 18—Dune deposits. 

A. Gently downwind-dipping, ripple-produced strata at crest of a dome dune. Thin coarse-grained layers (arrow 1) and bioturbation 

(arrow 2) are visible in trench. Scale in cm. 

B. Avalanche-produced strata in a barchanoid dune. Fadeout laminae (arrow 1) and flame structure (arrow 2) as well as gently undulating 

nature of strata are typical of avalanche-produced deposits. 

C. Fracturing (tensional) in cohesive damp sand of avalanche-produced strata of a barchanoid dune, overiain by undisturbed 

deposits. 

D. Grainfall-produced strata (barely visible in original photograph, unit looked massive in field) (arrow 1). Grainfall-produced unit is 

overiain by avalanche-produced strata with flame structures (arrow 2) near crest of barchanoid dune. 

E. Base of barchanoid dune. Ripple-produced strata of interdune (arrow 1) overiain by ripple-produced strata of dune apron (arrow 

2), in turn overiain by avalanche-produced strata (arrow 3) terminating in sand-flow toes (unnumbered arrow). Deposits of present 

dune apron (arrow 4) and its rippled surface visible at top of photograph. 

F. Typical base-of-dune with sand-flow toes (arrow) intertonguing with dune apron.


lS^)kte 

\ Sand 

^ ISheet;' • •• 

OWIND 

FIG. 19—Dune complex (schematic) and typical association of 

other eolian facies. Sabkha is downwind in sheltered zone; sand 

sheet deposits (in part a remnant of the dune complex) are 

upwind. 

dune is not moving too rapidly (Fig. 22d, background). 

Underlying dune deposits sometimes exhibit a zone of 

contorted strata extending 4 to 12 in. (10 to 30 cm) below 

the contact. These zones have the aspect of a combination 

of liquefaction and loading (Fig. 23c). 

Wet depositional interdunes.—lnitidvMss containing 

standing, permanent water were not studied directly 

because of the difficulties of working around the quicksands 

associated with the ponds. Interdunal ponds 

develop an abundant marshy vegetation around the edges 

of the pond, and are focal points for much of the animal 

life nearby, as evidenced by tracks around the shoreline. 

Wet interdunes with water-saturated sand at the surface 

but without permanent standing water (e.g., in wadis) 

(Fig. 22b) exhibit thick sequences of adhesion-produced 

strata ("adhesion ripples"). These strata have intercalated 

fluvial ripples and point bar sequences if sufficient current 

is present at time of deposition (Fig. 23d). 

Evaporitic depositional interdunes.—These interdune 

deposits exhibit numerous salt-ridge features, but are 

much thinner than typical sabkha sequences. Despite careful 

observation, we did not notice prohferation of algal 

mat material in evaporitic interdunes, perhaps because 

algal mats may depend upon periodic wetting by less saline 

waters such as those of an intertidal zone. 

Sedimentary Features of Sand Sheets 

Sand sheets are an areally extensive eoUan facies which 

serve to provide transition between dune complexes and 

sabkhas, or between eolian and non-eolian facies (see Fig. 

8). Sand sheets represent both a residual deposit of 

advancing dunes, and an independent facies capable of 

sustained growth (laterally and vertically). Our principal 

concern here is with sand sheet deposits which have developed 

independently of dunes, at least in the sense that they 

are not composed principally of steeply dipping 

avalanche-produced strata. Typically, sand sheets grow 

vertically and laterally during sandstorms through ripple 

and grain-fall deposition, usually interactive with plant 

growth, which in the region is most abundant on sand 

sheets. Sedimentary features of sand sheets we observed 

are summarized in Table 4. 

In general, sedimentary features of sand sheets in the 

study area are similar to those described by Fryberger et al 

(1979). Chief distinguishing features of sand sheets in the 

study area are abundance and diversity of vegetation 

(including the largest plants of the area such as Tamarix 

and date palms), and associated bioturbation due to 

insects, reptiles, and other creatures. Younger sand sheets 

(which seem to have less vegetation) have a suite of sedimentary 

features virtually identical to those of interdunes 

in the study area, but sedimentary sequences are thicker, 

less variable, much more laterally extensive, and exhibit 

no intercalated features related to damp or evaporitic 

sands common in interdunal sediments. 

Textural properties.—These range to extremes. Some 

sand sheet deposits are texturally mature, consisting of 

well-sorted mainly quartzose sand. Other sand sheets have 

poor sorting in terms of bulk textural properties, with layers 

of both fine and extremely coarse grains (Fig. 24d, e). 

Nevertheless, the coarse and fine grains are commonly 

segregated into discrete layers having high initial porosity. 

The sand sheet facies, therefore, must be considered along 

with dunes to have some potential as a reservoir facies in 

ancient rock sequences. 

Topographic relief.—Although sand sheets are almost 

flat in many places, in a few, highly vegetated areas rehef 

of 15 to 25 ft (4.5 to 7.5 m) is present. In such areas the 

sand sheet consists of broad swells 300 to 1,500 ft (90 to 

450 m) in width, with intervening, steep-sided troughs. 

This "ridge and swale" topography can extend hundreds 

of meters, both crosswind and parallel to the wind, with 

ridges elongate parallel to wind direction. 

Residual facies.—To a certain extent, sand sheets (which 

contain by far the coarsest sands) may represent a residual 

facies which is the product of the passage of many dunes 

and much windblown sand through the study area. Sand 

sheet terrain is clearly very stable in terms of erosion or 

deposition, due to armoring by a protective mantle of 

coarse sand and vegetation. Given the rapid movement of 

dunes in the area and the relative immobility of the sand 

sheets, it seems possible that in many areas the sand sheets 

are much older than nearby dunes. 

Early cementation.—The aspect of permanence of sand 

sheets is enhanced by extensive soil development in some 

places, particularly in the northern part of the study area. 

At one locahty near Qatif, we observed evident Aridosol 

(gypsiferous and calcareous) horizons amd resulting loss of 

porosity in ancient sand sheet deposits. These sand sheets 

contained broken pottery which is probably not more than 

several thousand years old. Older sand sheets also commonly 

contain many rhizoliths (gypsiferous replacements 

of plant roots and rhizomes; Klappa, 1980). The development 

of gypsiferous "soils" may be more common in sand 

sheets because of their relative immobility and the absence 

of direct evaporite deposition more typical of sabkhas and 

evaporitic interdunes. 

Sedimentary Features of Sabkhas 

Both inland and coastal sabkhas exist in the Jafurah 

sand sea (Fig. 2) (Johnson et al, 1978), and both types in

It 

iv 

F i 

1 

* o 

iir-'i S^ 

the study area seem to be of the " arenaceous" kind (quartz 

dominant) as opposed to the "argillaceous" kind (carbonate 

mud dominant) of Holm (1960). Regardless of geographic 

position, we found it possible to distinguish two 

basic end members of the arenaceous, or siliciclastic, sabkhas 

of the region, based principally upon preponderance 

of wind-derived quartz sand versus ground-water-derived 

evaporite minerals. We refer to detrital-dominant sabkhas 

as those in which transported (quartzose) sands constitute 

most of the sediment. On the other hand, evaporitedominant 

sabkhas are those in which evaporite minerals 

constitute most of the sediment. Although both types of 

sabkhas are distributed widely in the Jafurah sand sea, 

evaporite-dominant sabkhas in the study area are best 

developed nearest the coast, perhaps owing to proximity 

to seawater-supphed minerals. Sedimentary features of 

eolian detrital-dominant and evaporite-dominant sabkhas 

we observed are summarized in Table 5. 

Salt ridges.—Salt ridges are a conspicuous sedimentary 

feature in the study area which merit separate discussion 

primarily because they bear a striking resemblance to algal 

mats described in carbonate (argillaceous) sabkha settings 

(Fig. 25). Despite repeated and careful observations, we 

could find no indication that algal growth processes are 

responsible for the formation of the salt-ridge structures. 

Rather, salt ridges form from concentration of halite at the 

air-sand interface because of evaporation of saline capillary 

water. The resulting excess material at the surface 

causes expansion, forcing portions of the cemented layer 

to rise (Fig. 25c). The salt ridges can be classified as pustular, 

vermiform, or polygonal, based on aspects in plan 

view (Fig. 25a). Tops of the salt ridges are thin and brittle, 

commonly resulting in breakage and collapse of the center 

of the salt ridge, and subsequent infill by windblown sand 

(Fig. 25b, c). Salt-encrusted sands darken to a reddish 

brown where exposed to the sun, resulting in a good contrast 

of the salt-ridge structure with lighter sand which fills 

in breaks in the top of the structure or with areas between 

salt ridges (Fig. 25b, see also Fig. 26). Salt-ridge structures 

are important because stacked sequences of salt-ridge 

structures constitute the bulk of the sediments of detritaldominant 

sabkhas we studied (Fig. 26). 

Detrital-dominant sabkhas.—These sabkhas consist 

essentially of wind-deposited quartz sand blown across the 

sabkhas in the form of streamers and deposited in irregular 

layers between salt-ridge structures. Despite presence 

of water-saturated sand a short distance below (see Fig. 

26d), the surface of the sabkha is dry in all but the lowest 

places during the heat of the day. At night, the hygroscopic 

effect of the salts, in addition to fog and dew, can cause 

the sabkha to be damp. As most sandstorms occur during 

daytime in the hottest part of the year (June), sand can 

move easily across the crust of the sabkha, and one rarely 

sees adhesion-produced strata of the kind associated with 

wet, non-evaporitic interdunes. 

Growth of the sabkhas is slow, usually only 1 to 2 in. (2.5 

to 5 cm) per year. Deposition of sand sometimes proceeds 

gradually faster than rise of water table, or as a sudden episode 

resulting in more than a few inches of new sediment 

in one storm. When this occurs, an intercalation of rippleproduced 

strata and salt-ridge structures may produce a


Table 4. Sand Sheet 

Feature/Size Distribution Origin Abundance 

Ripple-produced strata, both 

"wind" and "granule" ripple 

deposits 

Constitutes most of sediments Partial preservation of migrating 

wind and granule ripples 

Common 

Buried plants, roots, and 

rhizomes, commonly carbonized 

or replaced by gypsum 

Plant and insect bioturbation 

traces, usually .25 in. (6 mm) 

diameters, tubular features 

Coarse-grained layers along 

shallow surfaces of erosion 

6 ft (2 m) long; 0.5 to 1 in. 

(1.2 to 2.5 cm) thick, 

commonly convex-downward 

Cut and fill, 1 to 3 in. (2.5 to 

7.5 cm) diameter troughs 

Isolated high-index ripples 

or ripple trains, flat lying 

Partial or complete preservation 

of granule ripple 

foresets 

"Massive" mottled sands 

Contorted stratification 

(commonly loading) 

Gypsiferous or calcareous 

Aridosols commonly in 

laterally extensive zones up 

to 10 ft (3 m) thick 

Widespread, especially on 

well-vegetated older sand 

sheets 

Upper 3 ft (1 m) of vegetated 

sand sheets 

Individual layers or zones 

in sediment 

Isolated occurrences 

Along a single horizon 

Commonly along single horizon 

Horizonally extensive zones 

several feet or more in thickness 

Anywhere—usually vegetated 

areas 

Commonly below sand surface— 

seen in excavation 

banded appearance in the sediment (Fig. 26b). The development 

of thick sequences of siliciclastic sabkha sediments 

with salt-ridge structures (such as shown in Fig. 26c, 

d) seems to depend on a gradually rising water table, an 

absence of dunes, and an undersaturated sand drift regime 

(more available wind than sand). Despite undersaturation 

of sand drift (which we measured at two trap sites on sabkhas), 

deposition is able to proceed because of the trapping 

of sand in sheltered areas between salt ridges. 

Detrital-dominant sabkhas in many areas were loosely 

packed with considerable excess porosity. Reduction in 

thickness (10 to 15%?) might be expected as the sabkha 

deposits compacted on burial and soluble salts were 

leached out. Early cementation by halite was common in 

detrital-dominant sabkhas, however, little gypsiferous or 

carbonate cementation was observed. 

Evaporite dominant sabkhas.—Evaporite-dominant 

siliciclastic sabkhas exhibit more extreme forms of saltridge 

structures, many of which also incorporate euhedral 

platy gypsum crystals (Fig. 26e). Large scale polygonal 

cracks and teepee structures commonly develop (such as 

those described by Ahlbrandt and Fryberger, 1981). Bedded 

salts may occur, along with nodular anhydrite (Fig. 

26f). Organic enrichment of sediments occurs in 

Burial, Aridosol formation, Common 

replacement by ground water 

Disturbance of sediment by Common 

root growth, burrowing 

organisms 

Wind scour, in places around 

granule ripples 

Scour around plant roots and 

rhizomes 

Wind shift or change in 

intensity 

Movement of granule ripple— 

strong winds followed by 

weaker winds 

Intense plant-root 

bioturbation 

Camel footprints; digging or 

rooting of other animals 

such as goats, foxes, dogs 

Airborn dust, rain, 

time, indigenous materials 

Common 

Rare 

Common 

Common 

Common 

Rare 

Common 

evaporite-dominant sabkhas, presumably as a result of 

microbial proliferation in the highly saline brines. Such 

organic proliferation, incidentally, is not restricted to 

coastal sabkhas. Samples from an evaporite-dominant 

portion of Al-Awsajiyah sabkha (near Unayzah in the 

middle of the Kingdom) contained 0.75% total organic 

carbon and 8.6 mg/g of free hydrocarbons. Residual 

hydrocarbon fraction after pyrolysis was 3.19 mg/g. 

Although these figures are not imposing, they are promising, 

especially when we consider the considerable areal 

extent of sabkhas such as Awsajiyah (in the interior) and 

similar organic-rich facies in sabkhas (such as Ar-Riyas) 

along the gulf coastline (Fig. 26f). 

EXAMPLES IN ANCIENT ROCKS 

Portions of the Pennsylvanian-Permian Tensleep- 

Weber-Minnelusa-Leo eolian sandstone systems of Wyoming 

may represent deposits of an offshore prograding 

eolian sand sea. The lower part of the Desmoinesian 

Tensleep sandstone of the Big Horn basin has been interpreted 

as representing an intercalation of marine and 

eolian deposits due to offshore progradation by dunes 

(Mankiewicz and Steidtmann, 1979). Paleogeographic


Table 5. Depositional Sabkha 

Feature/Size Distribution Origin Abundance 

DETRITAL DOMINANT 

Salt ridges 0.5 to 2 in. 

(1.2 to 5 cm) relief 

Salt (halite) crystals 

gypsum and anhydrite 

crystals about 0.25 in. 

(6 mm) diameter 

Grainfall- and rippleproduced 

strata—as thin 

layers or thick layers 

separating salt-ridge 

structures 

Contorted layers due to 

loading 

Bioturbation 

Reddish brown stain 

on sand grains 

Thin layers of halite 

crystals, euhedral, 

white 

Small lenses of lightcolored 

sand inside saltridge 

structure 2 to 4 in. (5 to 

10 cm) long; 0.75 in. (2 cm) thick 

Small lenses of lightcolored 

sand separating 

sah-ridge structures 

"Salt ridges" composed of 

euhedral gypsum crystal 

"hash" associated with 

halite: 2 to 4 in. (5 to 10 cm) 

high, 4 to 8 in. (10 to 20 cm) 

across 

Organic-rich layers 

about 3 ft (1 m) thick 

Banded (layered) evaporite 

bodies (halite) incorporating 

windblown sand grains, up to 

3 ft (1 m) thick 

Desiccation and expansion 

polygons 3 to 6 ft (1 to 2 m) 

across 

Gypsum sand roses "croute 

zonaire" (poikilotopic 

crystals) up to 8 in. (20 cm) 

in diameter 

Bioturbation 

Nodular or layered thin 

white anhydrite layers 

Most of sediment 

In sediments or on surface. 

Small euhedral forms in layers 

at distinct horizons, usually 

salt above gypsum with gypsum 

or anhydrite near water table 

Layers in sediments anywhere 

on sabkha 

Anywhere in sediments, formed 

at surface of sabkha 

Along surface and in sediments Beetles, termites 

Surface sediments and trencheswidespread 

Thin horizontal (intergranular) 

layers in sandy sediment of 

sabkha 

Formed at surface but can be 

seen typically in whole trench 

with salt-ridge structures 

Surface of sabkha and trenches 

EVAPORITE DOMINANT 

Surface of sabkha—often infilled 

by sand—structures contain 

as much evaporite mineral as sand 

Top of water table 

Entire surface of sabkha and 

in layers several feet or more 

thick 

Entire surface of sabkha 

In horizontally extensive 

zones in water-saturated 

sediments 

Thin, vertical tubular burrows 

Near water table in thin 

irregular layers 

Formed at surface by salt Common 

expansion, followed by burial 

by windblown sand 

Direct chemical precipitation Common 

from capillary fringe (salt) 

or water table (gypsum, anhydrite) 

Strong sandstorms. Most common Common 

when wind sedimentation temporarily 

exceeds rise of water table 

Animals walking across sabkha Rare 

Exposure of grains to salt 

and sun (oxidation of some sort?) 

Rare 

Common 

Evaporation—seems to form below Common 

surface—crystals larger than 

efflorescence seen at surface 

Sand blows into hole in roof 

of hollow salt-ridge structure 

Grainfall infill of hollows 

between salt-ridge structures 

Extreme evaporation 

Microbial growth (?) 

Evaporation of brines, 

direct precipitation 

Shrinkage or expansion of 

sediments due to desiccation 

or salt buildup, respectively 


Insect larvae (?) 


Common 

Common 

Common 

(in these 

settings) 

Common 

Common 

Common 

Common 

Rare 

Common

Eolian Dune, Interdune, Sand Sheet, and Siliciclastic Sabkha ...

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