Sedimentologic and tectonic origin of an incised-valley-fill sequence ...

Sedimentologic and tectonic 

origin of an incised-valley-fill 

sequence along an extensional 

marginal-lacustrine system 

in the Basin and Range 

province, United States: 

Implications for predictive 

models of the location of 

incised valleys 

Jack E. Deibert and Phyllis A. Camilleri 

ABSTRACT 

Incised valleys in extensional lacustrine systems should be common 

and significant petroleum targets, yet documentation and analysis 

of these systems are limited and, hence, so are predictive models 

for their location. Geologic mapping of the Miocene–Pliocene 

Humboldt Formation in Knoll basin, northeastern Nevada, has revealed 

a significant incised-valley system formed along the lacustrine 

margins of an extensional basin. The valley formed during a 

relative lake-level fall and incised into lacustrine shoreface and 

offshore sandstone and subsequently was filled with fluvial and 

eolian sediment as lake level rose. The valley’s location was tectonically 

influenced; it is situated in the hinge zone of a syncline 

near the tip of the range-bounding fault system. Folding of the 

syncline was broadly synchronous with incision and filling, and it 

appears to have localized the valley along the topographically low 

hinge zone. Furthermore, the large relative lake-level change that 

produced the valley is only recorded in strata in the syncline area, 

suggesting that the location and cause of incision was greatly influenced 

by tectonics. Thus, the location of similar incised valleys 

AUTHORS 

Jack E. Deibert Geosciences, Austin Peay 

State University, P.O. Box 4418, Clarksville, 

Tennessee 37044; deibertj@apsu.edu 

Jack Deibert is an associate professor of geology 

at Austin Peay State University with 

research interests in clastic sedimentology, sequence 

stratigraphy, basin analysis, and sedimentation 

and tectonics. He received his B.S. 

degree from Sonoma State University, his 

M.S. degree from the University of Nevada– 

Las Vegas, and his Ph.D. from the University 

of Wyoming. 

Phyllis A. Camilleri Geosciences, Austin 

Peay State University, P.O. Box 4418, Clarksville, 

Tennessee 37044; camillerip@apsu.edu 

Phyllis Camilleri is a professor of geology at 

Austin Peay State University with research interests 

in the structure, tectonics, and metamorphism 

of continental rifts and convergent 

orogens. She received her B.S. degree from 

San Diego State University, her M.S. degree 

from Oregon State University, and her Ph.D. 

from the University of Wyoming. 

ACKNOWLEDGEMENTS 

This study was partially supported by Austin 

Peay State University Tower Research Grants. 

We thank reviewers Stephen Cumella, Richard 

Moiola, and Colin North for helpful suggestions 

and comments. We also thank Alicia Stanfill 

for editorial assistance on an earlier version 

of this manuscript. 

Copyright #2006. The American Association of Petroleum Geologists. All rights reserved. 

Manuscript received February 28, 2005; provisional acceptance May 17, 2005; revised manuscript 

received August 25, 2005; final acceptance September 14, 2005. 

DOI:10.1306/09140505028 

AAPG Bulletin, v. 90, no. 2 (February 2006), pp. 209–235 209

in other extensional basins may be predictable if comparable 

tectonic features and processes are recognized. 

Our study suggests that the best locations to develop 

and preserve incised valleys are near the tips of 

normal faults during periods of overall high tectonic 

subsidence. Specific areas along basin-bounding faults 

where tectonically influenced incised valleys are more 

likely to form include fault-propagation folds, synthetic 

relay ramps near transfer faults, and areas that 

have large changes in fault slip. Although lake volume 

changes caused predominantly by climate change can 

be an important factor in producing incised valleys, 

tectonically influenced incised valleys are likely to be 

larger, better preserved, and more petroleum prone 

compared to climate-controlled incised valleys. 

INTRODUCTION 

Incised valleys form by fluvial erosion during a drop of 

relative base level and fill with sediment as relative 

base level rises (Dalrymple et al., 1994). Although 

incised-valley-fill sequences comprise a volumetrically 

small part of the stratigraphic record, they are extremely 

important as petroleum exploration targets 

(Van Wagoner et al., 1990; Dalrymple et al., 1994). The 

nature and origin of valley-fill sequences formed along 

marginal-marine settings (coastal plain to shelf areas) 

are well documented (e.g., Dalrymple et al., 1994). In 

contrast, valley-fill strata in marginal-lacustrine systems 

(alluvial to shallow lacustrine areas) have received little 

attention and consequently are poorly understood. In 

fact, ancient examples in outcrops have yet to be documented 

in detail. This article focuses on the origin and 

sedimentologic architecture of an incised-valley sequence 

formed along the margin of a lacustrine system 

in an extensional basin and its implications for petroleum 

exploration. 

Previous work on incised-valley-fill sequences 

formed in marginal-lacustrine systems is limited. Studies 

that mention the occurrence of such sequences focus 

on large-scale stratigraphic architecture in extensional 

basins (mostly half grabens) using seismic and well data 

(e.g., Scholz and Rosendahl, 1990; Xue and Galloway, 

1993; Changsong et al., 2001) and computer or conceptual 

modeling (e.g., Olsen, 1990; Gawthorpe and 

Leeder, 2000; Contreras and Scholz, 2001). In addition, 

incised-valley-fill sequences have been inferred in 

some sequence-stratigraphic models (e.g., Cohen, 

1990; Bohacs et al., 2000). These studies provide only 

broad, cursory information about incised valleys and 

their fills and do not specifically address incised-valley 

systems with regard to petroleum exploration. 

Our geologic mapping in Knoll basin, Nevada, an 

extensional basin in the Basin and Range province, 

United States (Figure 1), has revealed a spectacular 

three-dimensional exposure of a Miocene–Pliocene 

marginal-lacustrine incised-valley system. This study 

documents the stratigraphic architecture of this system 

and assesses the origin of incision and filling. Because 

incised valleys and their fills heretofore have 

largely been inferred from subsurface data (e.g., Scholz 

and Rosendahl, 1990; Xue and Galloway, 1993; Changsong 

et al., 2001), this article provides the first detailed 

analog that may be used for petroleum exploration 

and provides a better sedimentologic and 

tectonic understanding of incised valleys and their fills 

in extensional-lacustrine settings. Moreover, we present 

models to help predict the positions of incised 

valleys that could be viable exploration targets in extensional 

basins. 

GEOLOGIC SETTING 

Knoll basin is an informal term that we apply to an 

unnamed Tertiary basin bounded by Knoll Mountain 

(originally called ‘‘HD’’ range by Riva, 1970) on the 

east, the Granite Mountains to the north, and the 

Snake Mountains to the west (Figure 1). This and 

other Cenozoic basins in the region are products of a 

protracted extension (rifting) beginning as early as the 

Paleocene or Eocene and continuing to the Holocene 

(e.g., Snoke and Lush, 1984; Thorman et al., 1990; 

Mueller and Snoke, 1993a, b; Wright and Snoke, 1993; 

McGrew and Snee, 1994; Camilleri, 1996; Camilleri 

and Chamberlain 1997; Mueller et al., 1999). Rocks in 

the region include Proterozoic to Triassic strata locally 

intruded by Mesozoic granite (Coats, 1987). In Knoll 

basin, these rocks are unconformably overlain by the 

Cenozoic Humboldt Formation, which is the focus of 

this study. Regionally, the Humboldt Formation is 

composed of conglomerate, sandstone, shale, minor 

limestone, and volcanic tuff deposited in lacustrine, 

alluvial-fan, and fluvial environments in extensional 

basins (e.g., Schrader, 1912; Sharp, 1939; Van Houten, 

1956; Riva, 1962; Mueller and Snoke, 1993b). In Knoll 

basin, the Humboldt Formation is at least middle 

Miocene to Pliocene in age based on the occurrence of 

the 10.5–12.5-Ma (Perkins et al., 1998) Cougar Point 

210 Sedimentologic and Tectonic Origin of an Incised-Valley-Fill Sequence

Tuff (P. A. Camilleri and J. E. Deibert, unpublished 

data) and the presence of Pliocene vertebrate fauna 

(Stirton, 1940). The Humboldt Formation in Knoll 

basin is presently well exposed at the surface because 

of significant erosion by modern streams of the Snake 

River drainage system. 

STRATIGRAPHIC AND STRUCTURAL 

ARCHITECTURE OF KNOLL BASIN 

Figure 1. Maps showing location of the Basin and Range 

province, study area, incised-valley system, and range-bounding 

normal faults in the region. The Basin and Range province is 

an area of overall east–west extension. Range-bounding normal 

faults are shown as bold lines with a ball and bar on the 

hanging wall. EHR = East Humboldt Range; GM = Granite 

Mountains; KM = Knoll Mountain; MRV = Mary’s River Valley; 

SCR = Summer Camp Ridge; SM = Snake Mountains; WH = 

Wood Hills; I 80 = Interstate 80; US 93 = United States Highway 

93. Data for faults shown in Knoll Mountain are simplified 

from unpublished mapping by P. A. Camilleri; elsewhere, data 

are modified from Coats (1987). 

Knoll basin is a north-trending half graben bounded 

on the east by north- to northeast-trending, west-dipping 

normal faults and locally by a southeast-dipping normal 

fault along its northern margin (Figures 1, 2). The 

Humboldt Formation fills the basin and overall dips 

east (Figure 2). 

Our study focuses on the northeastern part of the 

Knoll basin (Figure 2). The Humboldt Formation in 

this area was previously mapped by Schrader (1912) 

and Riva (1962, 1970) as a single unit. In Riva’s reconnaissance 

study of the Humboldt Formation, he noted 

the presence of large cross-beds, an angular unconformity, 

and minor folding. He also inferred that most of 

the contacts between the Humboldt Formation and 

Paleozoic strata were depositional. We have mapped 

the northeastern part of the basin in detail, and what 

follows is a new stratigraphic and structural framework 

that we have established for the Humboldt Formation 

in this area. 

Three basin-bounding faults — the down-to-thewest 

Knoll Mountain, Hice, and Valder faults — are 

present along the eastern margin of the basin (Figure 2). 

The 24-km (14.9-mi)-long Knoll Mountain fault is 

the main range-bounding fault, whereas the Hice and 

Valder faults are restricted to the northern margin of 

the basin and are likely synthetic faults related to the 

Knoll Mountain fault. The Hice fault is 6 km (3.7 mi) 

long, and the 3-km (1.9-mi )-long Valder fault appears 

to be a small splay of the Hice fault (Figures 2, 3); for 

simplicity, we will refer to this basin-bounding fault 

as the Hice-Valder fault. 

We have divided the Humboldt Formation into 

four informal members. From oldest to youngest, they 

are the Blanchard, Knoll, Cave, and Bloody Gulch 

members (Figures 4, 5). The Blanchard member consists 

of tuffaceous sandstone, conglomerate, and volcanic 

tuff. The base of the member is not defined, and 

only the upper 80 m (262 ft) of the member is described. 

A distinctive welded-tuff unit of the Cougar 

Deibert and Camilleri 211

Figure 2. Simplified 

geologic map and cross 

section of the northeastern 

corner of Knoll basin 

(see Figure 1 for location). 

Strike and dip symbols 

represent attitudes 

of bedding. Data derived 

from 1:24,000 to 

1:8000 scale unpublished 

mapping by P. A. Camilleri 

and J. E. Deibert. 

Point Tuff occurs near the top of the Blanchard member 

(Figure 4) . The tuff is the only welded tuff in the 

eastern part of the basin, and it serves as an important 

regional isochronous marker bed that enables accurate 

chronostratigraphic correlations between isolated outcrops 

of the Humboldt Formation. The Knoll member 

consists dominantly of tuffaceous sandstone with minor 

amounts of limestone and ranges in thickness from 

24 to 54 m (79 to 177 ft). The Cave member consists 

mostly of large-scale, cross-stratified, tuffaceous sandstone 

with minor conglomerate and occupies a 50-m 

(164-ft)-deep incised valley cut into the Blanchard and 

Knoll members. The Bloody Gulch member overlies 

both the Knoll and Cave members and consists of tuffaceous 

sandstone with minor conglomerate. The top 

of the member is not defined, and only the lower 30 m 

(98 ft) of the member is described. 

The stratigraphic and structural geometry of the 

Humboldt Formation in the east-central part of the basin 

is relatively simple. There, the members of the 


Humboldt Formation are conformable and, in general, 

dip 15–20j east in the center of the basin and then reverse 

dip direction and dip gently west near the Knoll 

Mountain fault (see cross section AA 0 in Figure 2). 

The stratigraphic and structural geometry of the 

Humboldt Formation in the northeastern corner of the 

basin is complex. There, all members of the Humboldt 

Formation overall define a broad syncline (Hice syncline; 

Figure 3) in the hanging wall of the Hice-Valder 

fault, with bedding adjacent to the fault dipping in the 

same direction as the fault (see cross section BB 0 in 

Figure 3). The hinge of the syncline trends northeast, 

and to the south, it locally steps east across a small, 

gentle anticlinal flexure (Figure 3). Three major erosion 

surfaces consisting of two tectonically tilted angular 

unconformities and one incised valley are present in 

the Humboldt Formation in the syncline (Figure 6). 

From oldest to youngest, angular unconformity 1 forms 

the base of the Knoll member; angular unconformity 2 

is within the Knoll member; and the youngest erosion 

surface (unconformity 3) is an elongate, northeast-trending, 

concave-upward surface that truncates 

the Knoll member and the upper Blanchard member 

(Figure 6). We interpret the latter surface to be an 

incised valley formed by fluvial erosion. The incised 

valley is located approximately along the syncline’s 

hinge (Figure 3). A similar but smaller erosive surface 

is exposed 2 km (1.2 mi) southeast of the syncline 

and may represent a small tributary of the larger valley 

(Figure 3). 

SYNTECTONIC RELATIONSHIPS BETWEEN 

FOLDING AND VALLEY INCISION AND FILLING 

The position of the incised valley along the hinge of 

the Hice syncline suggests a possible syntectonic relationship 

between these two features. To evaluate 

this relationship, we assess the origin of the Hice syncline 

and its age relative to valley incision and filling. 

The timing of folding relative to the age of the members 

of the Humboldt Formation can be determined 

in part by structurally assessing the two tilted angular 

unconformities in the syncline. Both unconformities 

are exceptionally well exposed along a 2-km (1.2-mi)- 

long, northeast-trending ridge (Cave ridge; Figure 7A) 

that straddles the syncline’s hinge (Figure 6A–E). 

Cave ridge contains the most complete stratigraphic 

section in the syncline; therefore, it is the data from 

this ridge that we have analyzed to determine the 

relative age of the syncline. 

Relative Age of Folding 

Structural data indicate that the Hice syncline began 

to form before, and folding continued after, incision 

and filling of the valley. Assessment of angular unconformity 

1 indicates that the syncline began to form 

after the development of this unconformity. Unconformity 

1 is only present in the southeastern limb and 

hinge area of the Hice syncline, where it forms the 

contact between the Blanchard and Knoll members 

(Figure 6A–C; 7A, B). This unconformity and the 

Knoll member are not present on the northwestern 

limb of the syncline because they were erosionally 

removed during valley incision (see cross sections in 

Figure 3). To determine if the syncline began to form 

before the tilting of unconformity 1, bedding attitudes 

of strata beneath the unconformity were rotated to 

their pretilt orientation using standard stereographical 

methods (see Figure 7 for details). The results indicate 

that prior to the tilting of the unconformity, the Blanchard 

member had an overall east to northeasterly 

dip punctuated by a few small flexures with northwestto 

north-northeast–trending hinges (Figure 7C). From 

these data, it is clear that pretilt attitudes do not define 

the syncline, and therefore, the syncline had not 

begun to form prior to the development of angular 

unconformity 1. 

Assessment of angular unconformity 2 in the Knoll 

member indicates that the Hice syncline clearly started 

to form before the development of this unconformity 

and, hence, before valley incision. Unconformity 2 is 

only present in the central part of the southeastern limb 

of the syncline. Strata above and below the unconformity 

become conformable toward the hinge of the syncline 

and along strike to the southwest; to the northeast, 

this unconformity was erosionally removed during valley 

incision (Figure 7A). The strata immediately above and 

below the unconformity both dip to the northwest with 

little variance in strike; however, an angular dip discordance 

of 7–10j is present (Figure 7D). The stereographic 

determination of pretilt attitudes of strata beneath 

unconformity 2 indicates that strata dipped gently 

to the northwest and defined the southeastern limb 

of the syncline prior to tilting of this unconformity 

(Figure 7D). These data indicate that the syncline began 

to form during the deposition of the Knoll member. 

Moreover, folding must have continued through, or at 

least after, the deposition of the Cave and overlapping 

Bloody Gulch members because they also are folded 

(Figure 3). In summary, the data imply that valley incision 

and filling were broadly coeval with folding. 


Figure 3. Geologic map 

and cross sections of the 

incised-valley area (see 

Figure 2 for location). 

Strike and dip symbols 

represent attitudes of 

bedding. Note that the 

Quaternary alluvium is 

too thin to be shown in 

cross sections. 

Origin of the Hice Syncline 

Large-scale hanging-wall synclines similar to the Hice 

syncline have been recognized in several other extensional 

terrains. These synclines are generally inferred 

to have formed during fault-propagation folding above 

a blind normal fault (e.g., Gawthorpe et al., 1997; Corfield 

and Sharp, 2000; Maurin and Niviere, 2000; Sharp 

et al., 2000; Khalil and McClay, 2002). The hinges of 

fault-propagation synclines are approximately parallel 

to the fault, and the deposition of sediment during 

folding produces a distinct stratal geometry adjacent 

to the fault, such that units thin toward the fault, dip 

in the same direction as the fault, and contain angular 

unconformities reflecting progressive rotation of 

strata (Figure 8) (e.g., Gawthorpe et al., 1997; Maurin 

and Niviere, 2000; Sharp et al., 2000). Once the fault 

breaches the surface, subsequent deposition generates 

a reversal in stratal patterns wherein strata thicken and 

dip toward the fault. 

We infer that the Hice syncline is a product of 

fault-propagation folding along the Hice-Valder fault. 

This interpretation is based on four observations that 

are consistent with fault-propagation folding: (1) the 

parallelism of the hinge of the syncline to the fault; 

(2) the localization of angular unconformity 2 in the 

syncline’s limb adjacent to the fault; (3) the orientation 

and type of stratal rotation at unconformity 2, consistent 

with fault-propagation folding (cf. Figures 6, 8); 

and (4) the lack of a stratal geometry wherein strata 

thicken toward and dip into the fault in the Knoll 

member and overlying strata in the syncline (e.g., Gawthorpe 

et al., 1997), suggesting that the sediment presently 

preserved in the syncline was deposited while 


Figure 3. Continued. 

the Hice-Valder fault was blind. Moreover, the lack 

of coarse-grained detritus in the Humboldt Formation 

near the fault suggests that the fault was not emergent 

during deposition. Although the thinning of stratigraphic 

units toward the fault is characteristic of faultpropagation 

folds, we note that we cannot determine 

whether strata in the Hice syncline thin toward the 

Hice-Valder fault because the Knoll member has been 

largely eroded in proximity of the fault during valley 

incision and modern erosion (see cross sections in 

Figure 3). 

LITHOFACIES AND DEPOSITIONAL ENVIRONMENTS 

The Humboldt Formation can be divided into six 

lithofacies and four general depositional environments 

(Figures 4, 5; Table 1). The vertical and lateral distribution 

of these lithofacies and environments serve 

as the basis for the sedimentary history of the Knoll 

basin. 

Four composite stratigraphic sections were measured 

along the eastern margin of the basin, with each 

section representing stratigraphic observations along a 

1-km (0.6-mi) or longer lateral exposure (Figures 2, 4). 

Strata in the measured sections were divided into six 

distinct lithofacies, from which depositional environments 

were interpreted. A succinct description of the 

lithofacies and their occurrence in the measured sections 

are given in Table 1 and Figure 4. Detailed descriptions 

of the measured sections are given in the Appendix. 

The four general depositional environments are 

(1) gently sloping interchannel plains with sparse small 

lakes; (2) broad, intermittent braided streams; (3) eolian 

dune fields; and (4) perennial lakes. As a whole, the 

depositional features indicate an overall environmental 

setting of a semiarid climate with moderately vegetated 

grassland, small braided stream channels, periodic perennial 

lakes, and small eolian dune fields. The occurrence 

of large perennial lakes in a semiarid climate may 

seem somewhat contradictory; however, they are not 

uncommon in tectonically active areas (Carroll and 

Bohacs, 1999). The lakes in the East African rift system 

are examples of such perennial lakes. 

SEDIMENTARY AND TECTONIC HISTORY 

OF THE EASTERN KNOLL BASIN 

In this section, structural data and the architecture of 

the lithofacies and depositional environments are synthesized 

to derive the sedimentologic and tectonic 

history of Knoll basin and to specifically address the 

origin of the incised-valley system. The synthesis will 

be done by comparing the depotectonic history in the 

northeastern and southeastern parts of the basin during 

the deposition of each of the members of the Humboldt 

Formation. 


Figure 4. Measured statigraphic sections and correlations of the Humboldt Formation. Vertical scale of columns is in meters. The 

horizontal spacing of columns is proportional to the true distance between measured sections. See Figure 2 for location of sections. 

The measured section S1 includes both the column and the Cave member (valley fill) shown directly to the right of the column. 

Divisions on grain-size scale (width of columns) are in whole phi with m = mud, f = fine sand, c = coarse sand, g = gravel. Cave Mb. = 

Cave member; L = 1.5-m (4.9 ft)-thick limestone unit (top of Knoll member in section S4); WTD = welded-tuff datum; arrows next to 

box 3 indicate thin units at the top of the Cave member assigned to lithofacies 3. The welded tuff averages 30 cm (1 ft) in thickness 

and is too thin to show as a separate bed in the columns. The welded tuff is not present in section S4, and this section is correlated 

with S3 using the top of the Knoll member as a datum. Detailed stratigraphic columns are presented in the Appendix. 

Blanchard Member 

The Blanchard member largely consists of laterally adjacent 

and vertically stacked units of tuffaceous sandstone 

and clast-supported conglomerate (lithofacies 1 

and 2; Figure 4), indicating deposition dominated by 

vertically aggrading, gently sloping vegetated plains 

with minor fluvial channels (Figure 9A). Abundant 

pyroclastic debris was delivered to the basin by direct 

volcanic air fall and by stream transportation from 

nearby highlands. The composition and angular texture 

of the gravel clasts indicate that they were derived from 

a local source of Paleozoic sedimentary rocks and Mesozoic 

granitic rocks of the Contact pluton (Figures 2, 9A). 

More sedimentation and basin subsidence occurred in 

the east-central part of the basin (measured section S3, 

Figure 9A) as indicated by thicker strata above the 

welded tuff in the Blanchard member (Figure 4). Paleocurrent 

indicators are not abundant in the Blanchard 

member. However, granite clasts are abundant in the 

three northernmost measured sections, and the northern 

provenance of the clasts indicates an axial drainage 


system flowing to the south. Furthermore, evidence for 

large, long-lived lakes are absent from the Blanchard 

member, indicating that sedimentation was greater 

than subsidence, and that the fluvial system flowed 

through the basin. We infer that the Knoll Mountain 

fault was probably active at this time and produced 

relatively slow subsidence of the basin. 

The angular unconformity at the top of the Blanchard 

member in the northeastern part of the basin 

indicates tectonic rotation and subaerial erosion (angular 

unconformity 1) before the deposition of the Knoll 

member (Figure 9B). This event produced a general 

easterly dip of the Blanchard member. Not enough data 

are available to link stratal rotation to a specific structure; 

however, it may reflect the rotation of the strata 

toward an emergent Knoll Mountain fault. Alternatively, 

strata may have been rotated above a blind fault 

(e.g., as in Figure 8). 

Knoll Member 

The Knoll member consists of laterally continuous 

beds of very fine- to medium-grained sandstone and 

minor beds of limestone (lithofacies 3) and is interpreted 

to represent deposition in a regional shallowlacustrine 

environment (Figure 9C). The presence of 

abundant gastropods and bivalves in limestone units 

and the lack of evaporites and subaerial exposure 

features suggest a perennial freshwater environment. 

The basal contact of the Knoll member is sharp and 

erosional, and the finest grained beds and the lowest 

energy bedforms occur in the lower parts of the member, 

suggesting a rapid lake formation over the entire 

eastern part of the basin. The rapid transition from the 

throughgoing fluvial system of the Blanchard member 

to a regional long-lived lacustrine system indicates 

a change in relative subsidence rates, with the 

rate of subsidence being greater than the rate of sedimentation. 

Increased slip on the Knoll Mountain fault 

may have caused this relative increase in subsidence 

rate. Subsidence may have been less in the southern 

part of the basin (see measured section S4, Figure 4) 

because of the relatively thin amount of lacustrine 

sediment deposited there. Moreover, the southern part 

of the basin is the only area that contains limestone 

units, suggesting that the clastic input into this area 

was significantly lower relative to other parts of the 

basin. 

In the northeastern part of the basin, the Hice 

syncline began to form during the deposition of the 

lower part of the Knoll member (Figure 9C). Erosional 

truncation of lacustrine strata in the syncline’s 

southeastern limb, and subsequent continuation of lacustrine 

deposition, produced angular unconformity 2. 

There is no clear evidence that the erosional surface 

formed subareally or subaqueously. Furthermore, we 

infer that the Hice-Valder fault had not breached the 

surface during the deposition of the Knoll or overlying 

members. 

The middle and upper parts of the Knoll member 

contain an overall coarsening-upward sequence, suggesting 

that the shoreline gradually regressed. The 

regression in the northeast part of the basin was followed 

by 50 m (164 ft) of vertical fluvial erosion, i.e., 

valley incision (Figure 4). The axis (i.e., trunk stream) 

of the fluvial system was located directly along the 

hinge of the syncline and parallel to the blind Hice- 

Valder fault (Figures 9D, 10). The axis of the fluvial 

system was localized in the fold’s hinge area because 

of the topographic low created during syncline development. 

Valley incision occurred in response to a lowering 

of relative lake level, which resulted in a significant 

basinward shift in depositional facies (Figure 9D). 

The vertical magnitude of the relative lake-level drop 

was more than 50 m (164 ft), which is on the scale of 

the entire thickness of the lacustrine deposits of the 

Knoll member. Surprisingly, this large-magnitude basinward 

shift in facies and the resulting erosional event 

are not recognized in the southeastern parts of the 

basin (see measured sections S3 and S4, Figure 4). Instead, 

a gradual shoreline regression and a conformable 

transition to a vegetated-plain environment are 

observed in these areas. These data suggest that the 

large drop in relative lake level occurred only in the 

northeast corner of the basin. 

Cave Member 

The Cave member fills the aforementioned incised 

valley. The incised valley is at least 50 m (164 ft) deep, 

1 km (0.6 mi) wide, and a minimum of 6 km (3.7 mi) 

long (Figures 3, 9E, 10). Part of a small northwesttrending 

incised valley, 10 m (32 ft) deep, at least 150 m 

(492 ft) wide, and 400 m (1312 ft) long, is present to 

the southeast of the main incised valley (Figure 3). The 

smaller valley could have been a tributary of the main 

incised valley based on its orientation and position in 

the basin. 

In general, the lower 0.5–2 m (1.6–6.6 ft) of the 

Cave member is composed of fluvial conglomerate 



(lithofacies 5). The middle parts of member are dominantly 

composed of large-scale cross-stratified eolian 

sandstone (sets up to 11 m [36 ft] thick; lithofacies 4), 

except for a few areas in the valley where intercalated 

fluvial conglomerate and sandstone beds (lithofacies 5 

and 6) form units up to 100 m (329 ft) wide and 25 m 

(82 ft) thick. Paleocurrents of eolian strata are dominantly 

to the northeast, and paleocurrents of fluvial 

strata are dominantly to the southwest (Figure 10). The 

upper part of the member contains a laterally continuous, 

0.5–2.2-m (1.6–7.2-ft)-thick bed of lacustrine 

sandstone (lithofacies 3) that overlaps the top of the 

valley fill (Figure 5E). 

The distribution of lithofacies and environments 

indicate that after incision, deposition of fluvial conglomerate 

occurred in the lower parts of the valley. 

Paleocurrents indicate that the fluvial system was flowing 

to the southwest, presumably issuing into the lake 

in the east-central part of the basin. As deposition continued, 

eolian dune fields with small interdune ponds 

developed adjacent to the fluvial system, and these 

two systems filled most of the valley with sediment. 

Paleocurrents indicate that the source of the eolian 

sand lie southwest of the incised valley. The relatively 

low lake level in this area of the basin may have subaerially 

exposed a large area of lacustrine sediment belonging 

to the Knoll member and, hence, provided an 

abundant source of unlithified volcaniclastic sand for 

eolian transport. Eolian environments may have been 

common along the margin of the basin; however, their 

potential for preservation was limited because of reworking 

by lake and stream processes. The thick accumulation 

of eolian deposits in the valley fill was the 

result of the uncommonly deep subaerial accommodation 

space created by the incised valley. The filling 

of the valley with sediment deposited in two contrasting 

environments, fluvial and eolian, in a well-defined 

concave-upward erosional surface is clear evidence that 

the strata represent a filled valley instead of a large 

fluvial channel associated with the conformable progradation 

of marginal-lacustrine sediment over openlacustrine 

sediment. 

Filling of the valley was followed by a lake shoreline 

transgression across the northeastern part of the 

basin that deposited a thin layer of sand over the top of 

the incised-valley fill. The thin deposit (Figure 5E) 

suggests that the duration of lake deposition was relatively 

short after valley filling. Lacustrine deposition 

directly before and after valley incision and filling indicates 

that the formation of the incised-valley system 

was directly linked to relative lake-level changes. 

No direct evidence of continued folding in the syncline 

during filling of the valley exists. However, beds 

of the upper Cave member and the overlying strata 

in the Bloody Gulch member are folded, suggesting 

that folding of the syncline may have continued during 

valley incision and deposition. 

The large cycle of relative fall and rise of the lake 

level that produced the incised valley and overlying lacustrine 

deposits is present only in the northeastern 

part of the basin, indicating a dominantly local tectonic 

control of this cycle instead of a lake-volume (climatic) 

control. The local tectonic control may be a result of 

differential subsidence along the Knoll Mountain fault 

with greater slip or subsidence in the central part of 

the basin, which would locally increase the depth of 

the lake adjacent to the fault and force the regression 

of the lake’s northern shoreline. The subsequent transgression 

of the lake over the incised valley may have 

been caused by an increased slip along faults in the 

northeastern part of the basin relative to slip along 

faults to the south. 

Bloody Gulch Member 

The Bloody Gulch member is dominantly composed 

of laterally and vertically stacked units of massive 

fine- to coarse-grained sandstone (lithofacies 1 and 2; 

Figure 4). This architecture above the lacustrine units 

Figure 5. Photographs of lithofacies of the Humboldt Formation. (A) Photograph of lithofacies 2 (upper white beds) and lithofacies 1 

(lower massive unit) in the Blanchard member. Jacob staff is 1.5 m (4.9 ft) in length. (B) Photograph of lithofacies 3 in the Knoll 

member (see Figure 6F for location). Scale bar in (B), (D), and (F) is 15 cm (0.5 ft) long. (C) Photograph of lithofacies 4 in the Cave 

member. The height of the cliff is 30 m (98 ft). Bedding overall is horizontal, and all inclined strata are cross-stratification. (D) Photograph 

of lithofacies 5 (upper conglomerate unit) in the Cave member. (E) Photograph of lithofacies 3 (upper white beds) over lithofacies 4 

(lower cross-stratified unit) in the uppermost part of the Cave member. Lithofacies 3 at this location represents the lake transgression 

over the valley fill. The height of the outcrop is approximately 3 m (9.8 ft). (F) Photograph of lithofacies 6 in the Blanchard 

member. 



Figure 6. (A) Schematic diagram illustrating tectonostratigraphic relationships in the Humboldt Formation in the northeastern 

corner of the Knoll basin. Units are the same as in Figure 3. Dashed lines shown in units Thb, Thk, and Thbg represent the trace of 

bedding. (B) Photograph of outcrop showing angular unconformities 1 and 2. Outcrop is along the southeast side of the Cave ridge 

(see Figure 7 for the location of Cave ridge). (C) Diagram showing the locations of angular unconformities 1 and 2 (bold lines) present 

in the outcrop shown in (B). The strata above unconformity 1 are part of the Knoll member, and the strata below the unconformity 

are part of the Blanchard member. The angular discordance across unconformity 2 is not visible in this photo because the northeasttrending 

outcrop face is nearly parallel to the strike of the strata above and below this unconformity. The trace of bedding beneath 

unconformity 1 is shown by thin lines, and the diagonally ruled pattern below unconformity 1 represents a paleosol developed on 

the Blanchard member; hence, bedding is obscured in this area. (D) Photograph of outcrop showing unconformity 2 in the Knoll 

member (lithofacies 3). Outcrop face trends northwest and is at a high angle to the strike of the strata above and below the angular 

unconformity; consequently, the angular discordance is overt. (E) Diagram showing the location of angular unconformity 2 and 

the trace of bedding present in the outcrop shown in (D). (F) Photograph of the incised-valley wall and the Cave member filling 

the valley. 

of the Knoll and Cave members indicates that gently 

sloping vegetated plain and fluvial systems prograded 

over the lake along the entire eastern part of the basin 

(Figure 9F). Granite clasts are abundant in the Bloody 

Gulch member in the three northernmost measured 

sections, indicating an axial drainage system flowing to 

the south and that the basin returned to an overfilled 

state. The Bloody Gulch member does not contain distinct, 

laterally traceable units, and therefore, it is not 

possible to evaluate relative subsidence and sedimentation 

rates. 

The Bloody Gulch member in the northeastern 

part of the basin is folded in the Hice syncline, and in 

the south-central part of the basin, it is significantly 

tectonically rotated toward the Knoll Mountain fault. 

This suggests that the folding and faulting occurred 

after and possibly during the deposition of the Bloody 

Gulch member. 

IMPLICATIONS FOR THE LOCATION OF 

INCISED-VALLEY SYSTEMS AND THEIR 

ROLE IN PETROLEUM EXPLORATION 

AND PRODUCTION 

The origin of the incised-valley system in Knoll basin 

has broad implications for the understanding of 

marginal-lacustrine systems in an extensional setting 

and their petroleum potential. The incised-valley fill 


Figure 7. (A) Detailed geologic map of Cave ridge showing location of the syncline’s hinge and tilted angular unconformities 1 and 2 

(see the geologic map in Figure 3 for the location; cross section CC 0 in Figure 3 shows the structure of Cave ridge.) Dashed lines 

represent topographic contours. (B) Map showing pairs of bedding attitudes taken immediately above and below unconformity 1. 

Each pair represents attitudes that were measured within 10 m (32 ft) of each other. (C) Map showing pretilt attitudes of strata below 

unconformity 1. Pretilt attitudes were stereographically derived for each pair of attitudes by rotating the attitude above the 

unconformity to horizontal while rotating the corresponding attitude of underlying strata to its pretilt orientation. The pretilt attitude 

of strata overlying the unconformity was assumed to be approximately horizontal. (D) Map showing pairs of present attitudes above 

and below unconformity 2 and pretilt attitudes of strata beneath the unconformity. Pretilt attitudes were determined in the same 

manner as described above. 

in Knoll basin has the characteristics of, and is large 

enough to be, a significant petroleum reservoir. Furthermore, 

it is appreciably larger than most reservoirs 

in rift lake systems that are typically less than 15 m 

(49 ft) thick (Sladen, 1994). The processes that formed 

the valley may have been operative in other larger extensional 

basins, creating bigger incised valleys. Although 

the strata of the Knoll basin are mostly volcaniclastic 

sandstone and are not typical of sediment 

containing petroleum deposits, this depositional system 

can contain the more typical petroleum-related 

rocks, such as quartz sandstone and shale. Therefore, 

the incised-valley system in the Knoll basin can serve as 

an analog for petroleum exploration and production. 

The two most important aspects of this study concern 

the control on the location of the incised valley 

and the lithofacies architecture produced by the formation 

and filling of the valley. The location of the 

valley was not an arbitrary place along the margin 

of the basin where a fluvial system entered the basin; 


Figure 8. (A) Diagram illustrating a fault-propagation syncline developed above a normal fault. (B) Schematic diagram illustrating 

stratal patterns developed in a fault-propagation syncline. Stratal patterns include (1) thinning of strata toward the fault and 

(2) intraformational angular unconformities in the fold limb adjacent to the fault. Angular unconformities may develop as strata 

progressively rotate during fault propagation. Modified from Gawthorpe et al. (1997) and Sharp et al. (2000). 

instead, the location was controlled by specific tectonic 

features. Thus, the location of other incised-valley systems 

could be predicted by recognizing similar tectonic 

features. 

An important aspect in the exploration of lacustrine 

petroleum deposits is the presence of a lithofacies 

architecture where reservoir rocks are directly juxtaposed 

against source and seal rocks (Scholz and Rosendahl, 

1990; Sladen, 1994). In lacustrine basins, typical clastic 

reservoir rocks (littoral and fluvial sandstone) are not 

commonly found directly next to source and seal rocks 

(open-lacustrine shale), making it difficult to establish 

the critical elements of source, reservoir, and seal. 

The large relative lake-level change associated with the 

incised-valley sequence in the Knoll basin produced 

dramatic basinward and landward shifts in facies. This 

allowed coarse-grained fluvial conglomerate and eolian 

sandstone to be vertically and laterally encased in lacustrine 

deposits. In a similar depositional system, the 

lacustrine rocks could be open-lacustrine organic-rich 

shale that could act as both petroleum source and seal, 

and the coarse-grained valley-fill strata would serve as 

a reservoir. 

Presently, few recognized petroleum accumulations 

exist in incised valleys in extensional lacustrine 

systems. Incised valleys have been documented in the 

Jurassic–Cretaceous Songliao and Erlian extensional 

lacustrine basins of northeast China (Xue and Galloway, 

1993; Changsong et al., 2001), and they produce 

significant quantities of petroleum. For example, the 

largest oil field in China, one of the few giant nonmarine 

oil fields in the world, occurs in the Songliao 

basin, where incised-valley deposits are a principal reservoir; 

and the adjacent open-lacustrine shale serves as 

source and seal rock (Xue and Galloway, 1993). The 

limited number of recognized incised-valley systems in 

the world suggests that they maybe an underrecognized 

and unrealized exploration target in nonmarine extensional 

basins. More incised valleys may be recognized 

if geologists specifically look for them and gather highquality 

seismic data over areas specifically prone to 

incised-valley formation. 

PREDICTIVE MODELS FOR LOCATING 

PETROLEUM-SIGNIFICANT INCISED 

VALLEYS IN MARGINAL-LACUSTRINE 

EXTENSIONAL SYSTEMS 

Incised-valley systems can form anywhere along the 

margin of an extensional-lacustrine basin as streams 

erode and deposit sediment in response to changes in 

relative base level. However, specific locations are 

present in extensional basins, where incised valleys are 

more likely to form and would be preserved, that are 

more advantageous with regard to petroleum exploration. 

Significant factors that make an incised valley a 

potential exploration target include a volumetrically 

large valley and fill; large shifts in depositional facies 

such that coarse valley fill is encased in fine-grained, 

organic-rich lacustrine strata; and long-term preservation 

of the incised valley in the basin. 

We have developed general and specific conceptual 

models for predicting the location of petroleumsignificant 

incised valleys using previous work as well 


Table 1. Lithofacies and Depositional Environments* 

Lithofacies 1 

Fine- to coarse-grained tuffaceous sandstone with a minor amount of clast-supported conglomerate and siltstone. Beds range in 

thickness from 1 to 5 m (3.3 to 16 ft) and are mostly internally texturally homogenous or mottled without evidence of 

stratification. Rare stratification includes horizontal laminations and unidirectional trough cross-stratification in sets 2–20 cm 

(0.78–7.8 in.) thick. Other sedimentary structures include root casts, insect burrows, rodent burrows, and bone fragments of 

large vertebrate animals. The only identifiable fragments are horse teeth. (1) 

Depositional environment: broad, gently sloping, vegetated plain environment with minor fluvial channels. 


Fine- to medium-grained, horizontally stratified, tuffaceous sandstone and minor siltstone and welded tuff. Beds range in 

thickness from 0.5 to 5 cm (0.2 to 2 in.) and are normal to reverse graded. Other sedimentary features include sparse wave 

ripple-laminations, trough stratification in sets 2–20 cm (0.78–7.8 in.) thick, and rodent burrows. (2) 

Depositional environment: distal volcanic air-fall tuffs deposited on broad, gently sloping plains with small shallow lakes. 

Sand reworked locally by wind and water action and burrowing animals. 


Very fine- to medium-grained, horizontally and cross-stratified sandstone with sparse limestone. Beds range in thickness from 

0.5 to 17 cm (0.2 to 6.7 in.) thick and average 2 cm (0.8 in.) thick. Beds are laterally continuous and can be traced up to 

1 km (0.62 mi) in length. Internally, beds are ungraded to normal graded and contain wave ripple cross-stratification 

and multidirectional trough cross-stratification in sets 2–20 cm (0.78–7.8 in.) thick. Limestone units are gastropod and 

bivalve wackestone in beds 1–8 cm (0.4–3.1 in.) thick. (3, 4) 

Depositional environment: lacustrine shoreface to shallow-offshore environments. 


Fine- to medium-grained, large-scale trough cross-stratified, volcaniclastic sandstone. The sandstone is mostly composed of 

tabular to lenticular, trough cross-stratified sets that average 3 m (10 ft) thick and can be as thick as 11 m (36 ft). 

Individual sets of cross-strata can be traced for 200 m (660 ft) in both transverse and longitudinal directions. Typical angles 

of cross-stratification range from 15 to 20j, with a maximum angle of 30j. Paleocurrents are dominantly unidirectional 

but have a wide range of flow directions. Other sedimentary features include sparse mudcracks in siltstone lenses that 

range in thickness from 1 to 20 cm (0.4 to 7.8 in.). (5) 

Depositional environment: eolian dune field with small ephemeral interdune ponds. 


Clast-supported pebble conglomerate. Crudely horizontally stratified into beds 0.2–1 m (0.6–3.3 ft) thick that have lateral 

continuities ranging from 5 to 200 m (16 to 660 ft). Beds display crude low-angle planar and trough cross-stratification in 

sets 20–60 cm (7.8–23.6 in.) high and are dominantly unidirectional. Framework grains are subrounded to subangular 

and are composed of variable amounts of granite and Paleozoic chert and siltstone. The matrix of the conglomerate is fine 

to very coarse, poorly sorted, subangular to subrounded grains of sand composed of variable amounts of volcanic glass 

shards, quartz, and feldspar. (6) 

Depositional environment: intermittent braided stream channels. 


Medium- to coarse-grained, trough cross-stratified, tuffaceous sandstone and minor granule conglomerate. Internally, the 

lithofacies is well stratified into unidirectional tough cross-stratified sets that range in height from 5 to 100 cm (2 to 39 in.). 

In thicker sections, the lithofacies show distinct fining-upward and decreasing bedform-size trends. (1, 7) 

Depositional environment: intermittent braided stream channels. 

*Numbers in parentheses indicate examples of similar lithofacies and depositional environment: 1 = lithofacies F2-MS and F1-XS of Hunt (1990); 2 = lacustrine ash 

layers of Fisher and Schmincke (1984), and distal pyroclastic fall deposits of Cas and Wright (1987); 3 = upper and lower shoreface of Castle (1990); 4 = Sr and St 

lithofacies of Horton and Schmitt (1996); 5 = eolian deposits of Smith and Katzman (1991); 6 = Gm, Gp, and Gt lithofacies of Miall (1978); 7 = St lithofacies of Miall 

(1978). 


Figure 9. Paleogeographic 

maps. (A) Map 

for the deposition of the 

lower Blanchard member. 

(B) Map for the deposition 

of the upper Blanchard 

member. (C) Map 

for the deposition of 

the lower Knoll member. 

(D) Map for the deposition 

of the middle Knoll 

member. (E) Map for the 

deposition of the upper 

Knoll member and Cave 

member. (F) Map for 

the deposition of the 

Bloody Gulch member. 

Triangles represent measured 

section locations; 

HS = Hice syncline; HVF = 

Hice-Valder fault (blind); 

dashed-dot lines = fluvial 

systems. Stratal rotation 

shownin(B)represents 

the general attitude of the 

Blanchard member prior 

to the formation and tilting 

of angular unconformity 

1. 

as our own studies of extensional basins. The models 

are intended to aid in the exploration of incised valleys 

and to call attention to these features that theoretically 

should be common in extensional-lacustrine basins but 

are rarely recognized. 

Depotectonic Framework for Predictive Models 

The following generalized depotectonic framework 

of extensional basins serves as the foundation for our 

predictive models for the occurrence of petroleumsignificant 

incised valleys. The framework is developed 

from studies that have addressed the complex 

sedimentary and tectonic development of extensional 

basins (e.g., Leeder and Gawthorpe, 1987; Cohen, 

1990; Olsen, 1990; Scholz and Rosendahl, 1990; 

Prosser, 1993; Gawthorpe and Leeder, 2000; Changsong 

et al., 2001; Contreras and Scholz, 2001). From 

the aforementioned studies, three main principles concerning 

valley-fill sequences in marginal-lacustrine 

systems can be inferred. First, incised valleys form 

during falls of relative lake level as river systems shift 

basinward and erode into older marginal-lacustrine 

deposits. Incised valleys are subsequently filled with 

sediment when relative lake level rises enough to produce 

deposition in the valley. Second, changes in relative 

lake level and stratal architecture are largely 

controlled by the combined effects of tectonic subsidence 

and climate, with climate principally affecting 

the amount of precipitation and consequently controlling 

changes in lake volume. Furthermore, climate 

change can be an important factor in creating incised 

valleys because it can cause rapid and large changes in 


Figure 10. Structure contour map of 

the incised-valley wall and rose diagrams 

depicting paleocurrent data from fluvial 

and eolian strata in the valley fill (Cave 

member). Note that the structure contours 

represent the base of the Cave 

member. Paleocurrent rose diagrams 

shown with 10j class intervals. n = 

number of readings. Arrows indicate 

vector mean directions and uncertainty 

values (95% confidence level). Eolian 

paleocurrent data are from McKelvey 

and Deibert (2000) with measurements 

from trough cross-sets 60 cm (2.0 ft) or 

greater in height. Fluvial paleocurrents 

are from trough cross-sets 20–40 cm 

(0.7–1.3 ft) in height. 

lake levels. Third, large river systems tend to flow 

around footwall blocks and enter the lake and deposit 

sediment along the axis of the basin, parallel to 

the basin-bounding fault (Figure 11). These three principal 

factors influence the formation of incised valleys 

differently in the proximal, distal, and axial-end areas 

of extensional basins. In proximal areas (Figure 11), 

high tectonic subsidence along the basin-bounding 

fault tends to overwhelm the effects of small lakevolume 

reductions, producing a stratigraphic record 

of nearly continuous relative lake-level rise. Consequently, 

incised valleys in proximal areas will be 

sparse and, if present, are generally produced from 

large changes in the rate of tectonic subsidence. 

Conversely, in distal areas (Figure 11), incised valleys 

and subaerial erosional truncation of strata are common 

because small changes in lake volume produce 

a greater amount of relative base-level change because 

of lower tectonic subsidence. Axial-end areas 

(Figure 11) have relatively low to moderate rates of 

tectonic subsidence, and incised valleys here will be 

common and a product of changes in climate and tectonic 

subsidence. 

Figure 11. Diagram depicting tectonic and depositional elements 

in a lacustrine system developed in a simple half graben. 

Subsidence in proximity of the fault ranges from low near the 

tips of the fault to high near the center of the fault. The proximal 

part of the basin refers to the area adjacent to the rangebounding 

normal fault. The axial-end area spans the margin of 

the basin near the tips of the fault. The distal area represents 

the margin of the basin opposite the proximal area. 


General Predictive Model 

The best locations to develop and preserve incised 

valleys suitable for petroleum exploration in an extensional 

basin are the axial-end areas during periods 

of overall high tectonic subsidence in the basin. Incised 

valleys and associated sedimentary deposits in 

the axial-end area should be relatively common, have 

good long-term preservation potential, be volumetrically 

large, contain the best reservoir strata, and have 

large enough shifts in depositional facies to juxtapose 

reservoir, source, and seal rocks. Low to moderate 

subsidence rates in the axial-end area, in conjunction 

with climate changes, will produce significant relative 

lake-level changes and result in the formation of incised 

valleys that have good potential for complete filling and 

long-term preservation. The largest and longest river 

systems in the basin area flow into and deposit sediment 

in the axial-end region. Consequently, these large 

rivers have the ability to incise bigger valleys during 

lake-level lowstands and produce and deposit the most 

texturally and compositionally mature fluvial sediment 

that ultimately fills incised valleys. Both of these factors, 

valley size and sediment maturity, enhance reservoir 

quality and size. In addition, axial-end areas will likely 

be places where wind currents are focused because of 

flow around the topographic high formed by uplifted 

footwall blocks. Such flow in an arid climate results in 

the transport of eolian sand, derived from subaerially 

exposed lacustrine and fluvial sediment, into the axialend 

zone and potentially deposited in incised valleys. 

The eolian sand may be significantly better sorted than 

the fluvial and lacustrine sediment and serves as an 

excellent reservoir. Incised valleys developed in the 

axial-end region should have large depositional-facies 

shifts during relative lake-level changes caused by the 

low topographic gradient of the fluvial-lake shoreline 

system in that area. The low gradient is the result of high 

sedimentation rates from fluvial deposition in concert 

with the low to moderate subsidence rate. The shifts in 

depositional facies can be potentially large enough to 

encase coarse, fluvial valley fill in fine-grained, offshore 

lacustrine strata resulting in the direct juxtaposition of 

reservoir, source, and seal strata. 

Axial-end incised valleys formed dominantly by 

lake-volume (climatic) changes in our general model 

would be located parallel and adjacent to the basinbounding 

fault near the axial ends of the basin because 

axial river systems in active half grabens follow the 

locus of maximum subsidence, which is located near 

the boundary fault (Mack and Seager, 1990). 

Incised valleys developed in distal areas may be 

more numerous relative to those in the axial-end area 

because of a lower rate of subsidence. However, deposits 

in these incised valleys may get removed by 

erosion and, hence, have a lower potential for preservation. 

Furthermore, river systems in both the proximal 

and distal areas tend to have short lengths and 

deposit texturally immature sediment (Prosser, 1993), 

suggesting that incised valleys in these areas may be 

smaller and contain less mature sediment compared to 

axial-end incised valleys. Incised valleys in distal areas 

would have large shifts in depositional facies caused by 

their low gradients, but their lower chance of preservation, 

small size, and poor reservoir rocks rank them 

as being lower potential petroleum targets compared 

to axial-end incised valleys. 

Incised-valley systems developed in the proximal 

area probably have the best chance of being preserved 

in the basin fill but are uncommon because of the 

high rate of tectonic subsidence in this zone. Steep 

topographic gradients in the proximal area would generate 

the smallest shifts in depositional facies during 

relative lake-level changes, making it unlikely that these 

systems will contain the necessary juxtaposition of reservoir, 

source, and seal strata and, therefore, will have 

the lowest potential as petroleum targets. 

During the history of an extensional basin, there 

may be specific periods that favor the development 

of petroleum-significant incised valleys. We suggest 

that the most likely time to form such incised valleys 

would be during periods of high tectonic subsidence. 

During these periods, large long-lived lake systems tend 

to form because subsidence rates are much greater than 

sedimentation rates (Lambiase, 1990; Sladen, 1994). 

The lake systems commonly produce large amounts 

of organic-rich shale necessary for petroleum source 

and seal. Furthermore, incised-valley deposits formed 

during periods of high tectonic subsidence have a 

good chance of being preserved in the basin fill. This 

period of high tectonic subsidence is similar to the 

middle or rift climax phase described in the evolution 

of extensional basins (e.g., Prosser, 1993; Sladen, 

1994). 

Predictive Models for the Location of Tectonically 

Influenced Incised Valleys 

Incised valleys can form, in large part, because of tectonicsubsidence 

variations along the margins of a basin. We 

refer to such valleys as tectonically influenced incised 


Figure 12. (A) Sketch 

of a fault-propagation 

syncline at the tip of a 

normal fault. (B) Diagram 

illustrating the valley incision 

in a fault-propagation 

syncline along the end 

of a range-bounding fault. 

(C) Diagram showing the 

mode of filling and preservation 

of the incised 

valley shown in (B). 

valleys. Tectonically influenced incised valleys and 

their associated fills are likely to be of larger scale, have 

larger facies shifts, and have a better long-term preservation 

potential than incised valleys formed by dominantly 

lake-volume (climatic) changes. 

Data from extensional basins, both real and modeled, 

indicate that rapid and dramatic changes in tectonic 

subsidence along the strike of a boundary-fault 

zone are common (e.g., Gawthorpe et al., 1994; Morley, 

1995; Gupta et al., 1998; Contreras et al., 2000). Such 

variations create significant shifts in depocenters, 

especially during the early and middle phases of basin 

development (e.g., Prosser, 1993; Morley, 1995; Cowie 

et al., 2000; Gawthorpe and Leeder, 2000). The shifts 

are the result of changes in the rate and location 

of fault slip and related subsidence. Many of these 

changes in tectonic subsidence patterns can occur 

during short periods and can create large-magnitude 

relative lake-level changes and, as a consequence, incised 

valleys. 

There are several ways that variations in tectonic 

subsidence could result in the formation of an incisedvalley 

system. We present three conceptual models for 

the formation and location of tectonically influenced 

incised valleys in half graben extensional basins that 

would be suitable for petroleum exploration. 

Model 1: Fault-Propagation Folding and Fault Breaching along 

the Ends of Basin-Bounding Faults 

Fault-propagation folding and subsequent fault breaching 

at the tips of bounding faults will result in local 

differential subsidence that can produce significant, 

tectonically influenced incised valleys. At the ends of 

a bounding fault, fault tips commonly pass laterally 

into fault-propagation folds (e.g., Gawthorpe et al., 

1997; Sharp et al., 2000) (Figure 12A). The zone of 

folding defines a syncline, has a relatively low subsidence 

rate, and is commonly a site of axial drainage 

entering the basin (Figure 12B). In this situation, relatively 

higher subsidence rates basinward along strike, 

coupled with lake-volume changes, can produce a 

situation where streams in the area of folding will 

incise into older strata (Figure 12B). Filling and preservation 

of the incised valley can occur when the 

blind part of the fault breaches the surface, resulting in 

rapid subsidence (e.g., Gawthorpe et al., 1997) and 

consequent relative base-level rise and filling of the 

valley (Figure 12C). These types of incised valleys 

should be located along the synclinal axis of a faultpropagation 

fold. However, the synclinal stratal pattern 

might not remain if the area is rotated by 

continued faulting. Areas potentially containing this 

type of incised valley could be recognized in seismic 


synthetic faults may develop in the relay ramp. This 

would result in the creation of a depocenter and the 

subsidence of part of the incised valley in the hanging 

wall of the transfer fault (Figure 13B). Furthermore, 

downdropping of the hanging wall will result 

in a local relative rise in base level, and the incised 

valley will fill. This type of faulting and subsidence 

history was documented during the Jurassic extension 

of the North Sea (Young et al., 2001), although incised 

valleys were not recognized in this marginal-marine 

example. 

Incised valleys in relay ramp areas should be located 

roughly parallel and adjacent to the more proximal 

of two overstepping normal faults. Areas potentially 

containing this type of incised valley should be recognized 

in seismic data by delineating overstepping 

synthetic faults along the basin-bounding fault zone. 

Incised valleys of both models 1 and 2 are most 

likely to develop when the tips of boundary faults are 

rapidly lengthening and segmented boundary faults are 

being linked by transfer faults. This is likely to occur 

during the period of maximum rate of slip along the 

basin-bounding fault system, i.e., similar to the middle 

or rift-climax phase of extensional basins. 

Figure 13. Diagrams illustrating valley incision, filling, and preservation 

along a synthetic relay ramp. See text for explanation. 

data by changes in stratal patterns in dip sections 

(see Gawthorpe et al., 1997; Sharp et al., 2000, for 

stratal patterns) and by abrupt changes in stratal thickness 

in strike sections. 

Model 2: Transfer Faulting of Synthetic Relay Ramps 

Synthetic relay ramps occur between two overstepping 

normal faults that dip in the same dip direction 

(e.g., Gawthorpe and Hurst, 1993) (Figure 13A) and 

may be zones favorable for the development and 

preservation of incised valleys. These relay ramps are 

areas of low subsidence situated directly adjacent to 

zones of high subsidence and are common entry points 

of large axial rivers (Figure 13A) (e.g., Gawthorpe and 

Hurst, 1993). Differential tectonic subsidence can 

cause a relative base-level fall in the relay ramp area, 

resulting in incision by the axial drainage (Figure 13A). 

As faulting progresses, a transfer fault that links the 

Model 3: Changes in Slip along the Basin-Bounding 

Fault System 

Temporal and lateral variations in slip along a basinbounding 

fault system can create enough local differential 

tectonic subsidence to produce incised valleys. 

Basin-bounding fault systems commonly do not follow 

a simple pattern of increasing subsidence toward 

the center of the fault (Morley, 1999). Instead, slip 

along individual normal-fault segments may vary in 

magnitude or even vary along the length of a single 

fault, producing differential subsidence. 

Incised valleys can form between two areas of differential 

subsidence. For example, in a slow subsiding 

depocenter near the end of the fault system, sediment 

can fill the depocenter completely and create a throughgoing 

fluvial system into an adjacent, faster subsiding 

depocenter (Figure 14A). Differential subsidence between 

the two depocenters along with lake-volume 

changes can generate a relative base-level fall and 

fluvial incision in the slowly subsiding area. Increased 

subsidence in the incising area can create a rise in 

relative base level that will result in sediment filling the 

valley (Figure 14B). This pattern of basin subsidence 

has been documented using seismic data in the East 

African rift system (Morley, 1999), although incised 


where the incised valley formed and, later, was filled as 

subsidence increased. In addition, the incised valley 

formed along the axis of an active fault-propagation 

syncline in the axial-end area of the basin, which was 

likely the entry point of a large river system into the 

basin, with the syncline locally controlling the position 

of the river. 

Incised valleys that formed as a result of local 

differential subsidence should lie parallel to and near 

the boundary fault system. Potential areas containing 

these types of incised valleys may be recognized in 

seismic data by finding evidence for migrating depocenters 

along the strike of the basin. 

CONCLUSIONS 

Figure 14. Diagrams illustrating valley incision, filling, and 

preservation along a basin-bounding fault with temporal and 

lateral changes in subsidence. See text for explanation. 

valleys were not identified. Furthermore, the incised 

valley in the Knoll basin is interpreted to have formed 

in this manner. In the Knoll basin, the area of the Hice 

syncline was initially the slow-subsiding depocenter 

The Knoll basin contains a large incised-valley system 

that developed along the axial end of a marginallacustrine 

extensional basin. The valley fill and adjacent 

strata record large depositional-facies shifts, and 

such shifts can produce stratigraphic conditions advantageous 

for petroleum source, seal, and reservoir 

components. Relative lake-level changes influenced by 

tectonic subsidence along the strike of the basin-bounding 

fault system were responsible for valley incision and 

filling. The localization of valley incision along the 

hinge of the Hice syncline was controlled by a topographic 

low created by active synclinal folding during 

incision. Thus, the location of similar incised-valley 

systems may be predictable if comparable tectonic 

features and processes are recognized. 

Incised-valley systems in extensional lacustrine basins 

should be fairly common, yet documentation and 

discussions of these features are limited. We suggest 

that the best locations to find large incised-valley 

systems are in the axial-end areas during periods of 

overall high tectonic subsidence. Examples of specific 

areas where they are more likely to develop and be 

preserved include (1) fault-propagation folds at the tips 

of basin-bounding faults, (2) synthetic relay ramps 

transected by transfer faults, and (3) areas that have 

evidence for large differential slip along the basinbounding 

fault system. These predictive concepts 

should aid in the exploration of incised-valley systems 

in similar settings. Moreover, it is our intent that these 

ideas and data from the incised-valley system of the 

Knoll basin will stimulate further research into these 

poorly recognized yet potentially petroleum-significant 

features in extensional lacustrine systems. 


APPENDIX: DETAILED STRATIGRAPHIC COLUMNS S1, S2, S3, AND S4 

Simplified versions of these columns are presented in Figure 4. See Figure 2 for locations of stratigraphic columns. 


APPENDIX: Continued 


APPENDIX: Continued 


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