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

AUTHORS

Jack E. Deibert � Geosciences, Austin PeayState University, P.O. Box 4418, Clarksville,Tennessee 37044; [email protected]

Jack Deibert is an associate professor of ge-ology at Austin Peay State University withresearch interests in clastic sedimentology, se-quence stratigraphy, basin analysis, and sedi-mentation and tectonics. He received his B.S.degree from Sonoma State University, hisM.S. degree from the University of Nevada–Las Vegas, and his Ph.D. from the Universityof Wyoming.

Phyllis A. Camilleri � Geosciences, AustinPeay State University, P.O. Box 4418, Clarksville,Tennessee 37044; [email protected]

Phyllis Camilleri is a professor of geology atAustin Peay State University with research in-terests in the structure, tectonics, and meta-morphism of continental rifts and convergentorogens. She received her B.S. degree fromSan Diego State University, her M.S. degreefrom Oregon State University, and her Ph.D.from the University of Wyoming.

ACKNOWLEDGEMENTS

This study was partially supported by AustinPeay State University Tower Research Grants.We thank reviewers Stephen Cumella, RichardMoiola, and Colin North for helpful suggestionsand comments. We also thank Alicia Stanfillfor editorial assistance on an earlier versionof this manuscript.

Sedimentologic and tectonicorigin of an incised-valley-fillsequence along an extensionalmarginal-lacustrine systemin the Basin and Rangeprovince, United States:Implications for predictivemodels of the location ofincised valleysJack 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 re-

vealed a significant incised-valley system formed along the lacus-

trine 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 tec-

tonically 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 in-

fluenced by tectonics. Thus, the location of similar incised valleys

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

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

Manuscript received February 28, 2005; provisional acceptance May 17, 2005; revised manuscriptreceived August 25, 2005; final acceptance September 14, 2005.

DOI:10.1306/09140505028

in other extensional basins may be predictable if com-

parable tectonic features and processes are recognized.

Our study suggests that the best locations to de-

velop 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, syn-

thetic 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 volumetrical-

ly small part of the stratigraphic record, they are ex-

tremely important as petroleum exploration targets

(VanWagoner 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 docu-

mented in detail. This article focuses on the origin and

sedimentologic architecture of an incised-valley se-

quence formed along the margin of a lacustrine system

in an extensional basin and its implications for petro-

leum exploration.

Previous work on incised-valley-fill sequences

formed in marginal-lacustrine systems is limited. Stud-

ies thatmention 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 con-

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

Leeder, 2000; Contreras and Scholz, 2001). In addi-

tion, 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 sys-

tem and assesses the origin of incision and filling. Be-

cause incised valleys and their fills heretofore have

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

and Rosendahl, 1990;Xue andGalloway, 1993; Chang-

song et al., 2001), this article provides the first de-

tailed analog that may be used for petroleum ex-

ploration and provides a better sedimentologic and

tectonic understanding of incised valleys and their fills

in extensional-lacustrine settings. Moreover, we pre-

sent models to help predict the positions of incised

valleys that could be viable exploration targets in ex-

tensional 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; VanHouten,

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 STRUCTURALARCHITECTURE OF KNOLL BASIN

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 nor-

mal 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 recon-

naissance study of the Humboldt Formation, he noted

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

mity, 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-the-

west 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 con-

sists of tuffaceous sandstone, conglomerate, and vol-

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

only the upper 80 m (262 ft) of the member is de-

scribed. A distinctive welded-tuff unit of the Cougar

Figure 1. Maps showing location of the Basin and Rangeprovince, study area, incised-valley system, and range-boundingnormal faults in the region. The Basin and Range province isan area of overall east–west extension. Range-bounding nor-mal faults are shown as bold lines with a ball and bar on thehanging wall. EHR = East Humboldt Range; GM = GraniteMountains; 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 High-way 93. Data for faults shown in Knoll Mountain are simplifiedfrom unpublished mapping by P. A. Camilleri; elsewhere, dataare modified from Coats (1987).

Deibert and Camilleri 211

Point Tuff occurs near the top of the Blanchard mem-

ber (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 out-

crops of the Humboldt Formation. The Knoll member

consists dominantly of tuffaceous sandstone with mi-

nor 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 sand-

stone 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 tuff-

aceous sandstone with minor conglomerate. The top

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

(98 ft) of the member is described.

The stratigraphic and structural geometry of the

Humboldt Formation in the east-central part of the ba-

sin is relatively simple. There, the members of the

Figure 2. Simplifiedgeologic map and crosssection of the northeast-ern corner of Knoll basin(see Figure 1 for loca-tion). Strike and dip sym-bols represent attitudesof bedding. Data de-rived from 1:24,000 to1:8000 scale unpublishedmapping by P. A. Camil-leri and J. E. Deibert.


Humboldt Formation are conformable and, in general,

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

verse dip direction and dip gently west near the Knoll

Mountain fault (see cross section AA0 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 syn-

cline; 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 BB0 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). Threemajor 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 ero-

sion surface (unconformity 3) is an elongate, north-

east-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 val-

ley (Figure 3).

SYNTECTONIC RELATIONSHIPS BETWEENFOLDING AND VALLEY INCISION AND FILLING

The position of the incised valley along the hinge of

the Hice syncline suggests a possible syntectonic re-

lationship between these two features. To evaluate

this relationship, we assess the origin of the Hice syn-

cline and its age relative to valley incision and filling.

The timing of folding relative to the age of the mem-

bers 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 un-

conformity 1 indicates that the syncline began to form

after the development of this unconformity. Uncon-

formity 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 Blan-

chard member had an overall east to northeasterly

dip punctuated by a few small flexures with northwest-

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

these data, it is clear that pretilt attitudes do not de-

fine 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 unconfor-

mity become conformable toward the hinge of the syn-

cline 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 dis-

cordance of 7–10j is present (Figure 7D). The stereo-

graphic determination of pretilt attitudes of strata be-

neath 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 in-

cision and filling were broadly coeval with folding.


Origin of the Hice Syncline

Large-scale hanging-wall synclines similar to the Hice

syncline have been recognized in several other exten-

sional terrains. These synclines are generally inferred

to have formed during fault-propagation folding above

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

field and Sharp, 2000;Maurin andNiviere, 2000; Sharp

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

fault-propagation synclines are approximately paral-

lel 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 angu-

lar 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 orienta-

tion and type of stratal rotation at unconformity 2, con-

sistent 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., Gaw-

thorpe et al., 1997), suggesting that the sediment pres-

ently preserved in the syncline was deposited while

Figure 3. Geologic mapand cross sections of theincised-valley area (seeFigure 2 for location).Strike and dip symbolsrepresent attitudes ofbedding. Note that theQuaternary alluvium istoo thin to be shown incross sections.


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 strati-

graphic units toward the fault is characteristic of fault-

propagation 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 dis-

tribution of these lithofacies and environments serve

as the basis for the sedimentary history of the Knoll

basin.

Four composite stratigraphic sections were mea-

sured 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 environ-

ments were interpreted. A succinct description of the

lithofacies and their occurrence in the measured sec-

tions are given in Table 1 and Figure 4. Detailed descrip-

tions of themeasured sections are given in theAppendix.

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 withmoderately vegetated

grassland, small braided stream channels, periodic pe-

rennial lakes, and small eolian dune fields. The occur-

rence 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 HISTORYOF THE EASTERN KNOLL BASIN

In this section, structural data and the architecture of

the lithofacies and depositional environments are syn-

thesized 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 dur-

ing the deposition of each of the members of the Hum-

boldt Formation.

Figure 3. Continued.


Blanchard Member

The Blanchard member largely consists of laterally ad-

jacent and vertically stacked units of tuffaceous sand-

stone 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 theywere derived from

a local source of Paleozoic sedimentary rocks andMeso-

zoic granitic rocks of theContact 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 Blanchardmember (Figure 4). Paleo-

current indicators are not abundant in the Blanchard

member. However, granite clasts are abundant in the

three northernmost measured sections, and the north-

ern provenance of the clasts indicates an axial drainage

Figure 4. Measured statigraphic sections and correlations of the Humboldt Formation. Vertical scale of columns is in meters. Thehorizontal 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 tobox 3 indicate thin units at the top of the Cave member assigned to lithofacies 3. The welded tuff averages 30 cm (1 ft) in thicknessand 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 correlatedwith S3 using the top of the Knoll member as a datum. Detailed stratigraphic columns are presented in the Appendix.


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 Blan-

chard member in the northeastern part of the basin

indicates tectonic rotation and subaerial erosion (angu-

lar unconformity 1) before the deposition of the Knoll

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

easterly dip of the Blanchardmember. Not enough data

are available to link stratal rotation to a specific struc-

ture; however, it may reflect the rotation of the strata

toward an emergent Knoll Mountain fault. Alterna-

tively, 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 inter-

preted to represent deposition in a regional shallow-

lacustrine 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 mem-

ber, suggesting a rapid lake formation over the entire

eastern part of the basin. The rapid transition from the

throughgoing fluvial system of the Blanchard mem-

ber to a regional long-lived lacustrine system indi-

cates a change in relative subsidence rates, with the

rate of subsidence being greater than the rate of sedi-

mentation. 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). Ero-

sional truncation of lacustrine strata in the syncline’s

southeastern limb, and subsequent continuation of la-

custrine 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, sug-

gesting that the shoreline gradually regressed. The

regression in the northeast part of the basin was fol-

lowed 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 devel-

opment. Valley incision occurred in response to a low-

ering of relative lake level, which resulted in a signifi-

cant 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 ba-

sinward 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). In-

stead, a gradual shoreline regression and a conform-

able 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 northwest-

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

(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 domi-

nantly 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 domi-

nantly to the northeast, and paleocurrents of fluvial

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

upper part of the member contains a laterally contin-

uous, 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 con-

glomerate occurred in the lower parts of the valley.

Paleocurrents indicate that the fluvial system was flow-

ing to the southwest, presumably issuing into the lake

in the east-central part of the basin. As deposition con-

tinued, 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 sub-

aerially exposed a large area of lacustrine sediment be-

longing 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 re-

working by lake and stream processes. The thick ac-

cumulation of eolian deposits in the valley fill was the

result of the uncommonly deep subaerial accommo-

dation space created by the incised valley. The filling

of the valley with sediment deposited in two contrast-

ing 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 pro-

gradation of marginal-lacustrine sediment over open-

lacustrine sediment.

Filling of the valley was followed by a lake shore-

line 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 rela-

tively short after valley filling. Lacustrine deposition

directly before and after valley incision and filling in-

dicates that the formation of the incised-valley system

was directly linked to relative lake-level changes.

No direct evidence of continued folding in the syn-

cline 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 la-

custrine 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 trans-

gression 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 Knollmember (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 Cavemember. The height of the cliff is 30 m (98 ft). Bedding overall is horizontal, and all inclined strata are cross-stratification. (D) Photographof 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 trans-gression over the valley fill. The height of the outcrop is approximately 3 m (9.8 ft). (F) Photograph of lithofacies 6 in the Blanchardmember.



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 dis-

tinct, laterally traceable units, and therefore, it is not

possible to evaluate relative subsidence and sedimen-

tation 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 OFINCISED-VALLEY SYSTEMS AND THEIRROLE IN PETROLEUM EXPLORATIONAND 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 6. (A) Schematic diagram illustrating tectonostratigraphic relationships in the Humboldt Formation in the northeasterncorner of the Knoll basin. Units are the same as in Figure 3. Dashed lines shown in units Thb, Thk, and Thbg represent the trace ofbedding. (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) presentin the outcrop shown in (B). The strata above unconformity 1 are part of the Knoll member, and the strata below the unconformityare part of the Blanchard member. The angular discordance across unconformity 2 is not visible in this photo because the northeast-trending outcrop face is nearly parallel to the strike of the strata above and below this unconformity. The trace of bedding beneathunconformity 1 is shown by thin lines, and the diagonally ruled pattern below unconformity 1 represents a paleosol developed onthe Blanchard member; hence, bedding is obscured in this area. (D) Photograph of outcrop showing unconformity 2 in the Knollmember (lithofacies 3). Outcrop face trends northwest and is at a high angle to the strike of the strata above and below the angularunconformity; consequently, the angular discordance is overt. (E) Diagram showing the location of angular unconformity 2 andthe trace of bedding present in the outcrop shown in (D). (F) Photograph of the incised-valley wall and the Cave member fillingthe valley.


in Knoll basin has the characteristics of, and is large

enough to be, a significant petroleum reservoir. Fur-

thermore, 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 ex-

tensional basins, creating bigger incised valleys. Al-

though the strata of the Knoll basin are mostly vol-

caniclastic sandstone and are not typical of sediment

containing petroleum deposits, this depositional sys-

tem 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 con-

cern the control on the location of the incised valley

and the lithofacies architecture produced by the for-

mation 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 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 CC0 in Figure 3 shows the structure of Cave ridge.) Dashed linesrepresent 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 belowunconformity 1. Pretilt attitudes were stereographically derived for each pair of attitudes by rotating the attitude above theunconformity to horizontal while rotating the corresponding attitude of underlying strata to its pretilt orientation. The pretilt attitudeof strata overlying the unconformity was assumed to be approximately horizontal. (D) Map showing pairs of present attitudes aboveand below unconformity 2 and pretilt attitudes of strata beneath the unconformity. Pretilt attitudes were determined in the samemanner as described above.


instead, the location was controlled by specific tectonic

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

tems could be predicted by recognizing similar tectonic

features.

An important aspect in the exploration of lacus-

trine petroleum deposits is the presence of a lithofacies

architecturewhere 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 estab-

lish 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 la-

custrine 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 accumula-

tions 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 Gallo-

way, 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 non-

marine oil fields in the world, occurs in the Songliao

basin, where incised-valley deposits are a principal res-

ervoir; 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

theworld suggests that theymaybe an underrecognized

and unrealized exploration target in nonmarine exten-

sional basins. More incised valleys may be recognized

if geologists specifically look for them and gather high-

quality seismic data over areas specifically prone to

incised-valley formation.

PREDICTIVE MODELS FOR LOCATINGPETROLEUM-SIGNIFICANT INCISEDVALLEYS IN MARGINAL-LACUSTRINEEXTENSIONAL 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 explo-

ration. 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 preserva-

tion of the incised valley in the basin.

We have developed general and specific concep-

tual models for predicting the location of petroleum-

significant incised valleys using previous work as well

Figure 8. (A) Diagram illustrating a fault-propagation syncline developed above a normal fault. (B) Schematic diagram illustratingstratal 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 strataprogressively rotate during fault propagation. Modified from Gawthorpe et al. (1997) and Sharp et al. (2000).


Table 1. Lithofacies and Depositional Environments*

Lithofacies 1Fine- 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.

Lithofacies 2Fine- 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.

Lithofacies 3Very 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.

Lithofacies 4Fine- 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.

Lithofacies 5Clast-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.

Lithofacies 6Medium- 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 ashlayers 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 Stlithofacies 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).


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 petroleum-

significant incised valleys. The framework is devel-

oped 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; Chang-

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

the aforementioned studies, three main principles con-

cerning 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 pro-

duce deposition in the valley. Second, changes in rel-

ative lake level and stratal architecture are largely

controlled by the combined effects of tectonic sub-

sidence and climate, with climate principally affecting

the amount of precipitation and consequently con-

trolling 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 9. Paleogeo-graphic maps. (A) Mapfor the deposition of thelower Blanchard member.(B) Map for the deposi-tion of the upper Blan-chard member. (C) Mapfor the deposition ofthe lower Knoll member.(D) Map for the deposi-tion of the middle Knollmember. (E) Map for thedeposition of the upperKnoll member and Cavemember. (F ) Map forthe deposition of theBloody Gulch member.Triangles represent mea-sured section locations;HS = Hice syncline; HVF =Hice-Valder fault (blind);dashed-dot lines = fluvialsystems. Stratal rotationshown in (B) representsthe general attitude of theBlanchard member priorto the formation and tilt-ing of angular uncon-formity 1.


lake levels. Third, large river systems tend to flow

around footwall blocks and enter the lake and de-

posit sediment along the axis of the basin, parallel to

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

cipal 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 lake-

volume reductions, producing a stratigraphic record

of nearly continuous relative lake-level rise. Conse-

quently, 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 com-

mon because small changes in lake volume produce

a greater amount of relative base-level change be-

cause 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 tec-

tonic subsidence.

Figure 10. Structure contour map ofthe incised-valley wall and rose diagramsdepicting paleocurrent data from fluvialand eolian strata in the valley fill (Cavemember). Note that the structure con-tours represent the base of the Cavemember. Paleocurrent rose diagramsshown with 10j class intervals. n =number of readings. Arrows indicatevector mean directions and uncertaintyvalues (95% confidence level). Eolianpaleocurrent data are from McKelveyand Deibert (2000) with measurementsfrom trough cross-sets 60 cm (2.0 ft) orgreater in height. Fluvial paleocurrentsare from trough cross-sets 20–40 cm(0.7–1.3 ft) in height.

Figure 11. Diagram depicting tectonic and depositional ele-ments in a lacustrine system developed in a simple half graben.Subsidence in proximity of the fault ranges from low near thetips of the fault to high near the center of the fault. The proximalpart of the basin refers to the area adjacent to the range-bounding normal fault. The axial-end area spans the margin ofthe basin near the tips of the fault. The distal area representsthe 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 ex-

tensional basin are the axial-end areas during periods

of overall high tectonic subsidence in the basin. In-

cised valleys and associated sedimentary deposits in

the axial-end area should be relatively common, have

good long-term preservation potential, be volumetri-

cally 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 sedi-

ment 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 axial-

end 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 lowgradient 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 basin-

bounding 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, de-

posits in these incised valleys may get removed by

erosion and, hence, have a lower potential for preser-

vation. Furthermore, river systems in both the proxi-

mal 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 preser-

vation, 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 gen-

erate the smallest shifts in depositional facies during

relative lake-level changes, making it unlikely that these

systems will contain the necessary juxtaposition of res-

ervoir, 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 evolu-

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

1994).

Predictive Models for the Location of TectonicallyInfluenced Incised Valleys

Incised valleys can form, in large part, because of tectonic-

subsidence variations along the margins of a basin. We

refer to such valleys as tectonically influenced incised


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 preser-

vation potential than incised valleys formed by domi-

nantly lake-volume (climatic) changes.

Data from extensional basins, both real and mod-

eled, indicate that rapid and dramatic changes in tec-

tonic 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, in-

cised valleys.

There are several ways that variations in tectonic

subsidence could result in the formation of an incised-

valley 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 breach-

ing 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 subsi-

dence rate, and is commonly a site of axial drainage

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

tively 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 pres-

ervation 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 fault-

propagation fold. However, the synclinal stratal pat-

tern might not remain if the area is rotated by

continued faulting. Areas potentially containing this

type of incised valley could be recognized in seismic

Figure 12. (A) Sketchof a fault-propagationsyncline at the tip of anormal fault. (B) Diagramillustrating the valley inci-sion in a fault-propagationsyncline along the endof a range-bounding fault.(C) Diagram showing themode of filling and pres-ervation of the incisedvalley shown in (B).


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 thick-

ness in strike sections.

Model 2: Transfer Faulting of Synthetic Relay Ramps

Synthetic relay ramps occur between two overstep-

ping 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

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 hang-

ing wall of the transfer fault (Figure 13B). Further-

more, 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 rec-

ognized 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.

Model 3: Changes in Slip along the Basin-Bounding

Fault System

Temporal and lateral variations in slip along a basin-

bounding fault system can create enough local dif-

ferential tectonic subsidence to produce incised valleys.

Basin-bounding fault systems commonly do not fol-

low 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 dif-

ferential subsidence. For example, in a slow subsiding

depocenter near the end of the fault system, sediment

can fill the depocenter completely and create a through-

going fluvial system into an adjacent, faster subsiding

depocenter (Figure 14A). Differential subsidence be-

tween 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

Figure 13. Diagrams illustrating valley incision, filling, and pres-ervation along a synthetic relay ramp. 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

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 depo-

centers along the strike of the basin.

CONCLUSIONS

The Knoll basin contains a large incised-valley system

that developed along the axial end of a marginal-

lacustrine extensional basin. The valley fill and adja-

cent strata record large depositional-facies shifts, and

such shifts can produce stratigraphic conditions ad-

vantageous 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 topo-

graphic 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 ba-

sins 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 basin-

bounding 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.

Figure 14. Diagrams illustrating valley incision, filling, andpreservation along a basin-bounding fault with temporal andlateral changes in subsidence. See text for explanation.


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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