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Marine and Petroleum Geology 28 (2011) 1517e1529

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Marine and Petroleum Geology

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Architecture of a forced regressive systems tract in the Turonian Ferron “NotomDelta”, southern Utah, U.S.A.

Weiguo Li a,*, Janok Bhattacharya b, Yijie Zhu b

aBP America Inc, 501 Westlake Part Blvd., Houston, TX 77079, USAbDepartment of Earth and Atmospheric Sciences, University of Houston, 4800 Calhoun Rd., Houston, TX 77204-5007, USA

a r t i c l e i n f o

Article history:Received 24 October 2010Received in revised form19 April 2011Accepted 15 May 2011Available online 23 May 2011

Keywords:Forced regressionFerron “Notom delta”Facies architectureDepositional processesCretaceous interior seaway

* Corresponding author.E-mail addresses: Weiguo.Li@bp.com, wgliuh@gm

0264-8172/$ e see front matter Published by Elseviedoi:10.1016/j.marpetgeo.2011.05.004

a b s t r a c t

The large-scale stratigraphic architecture of forced regressive deposits has been documented in manyprevious studies. Bed-scale facies architectural analyses of these deposits, however, are still very limited.The Cretaceous Ferron “Notom Delta” in southern Utah, U.S.A. contains a 20 km dip-oriented exposure ofa stepped, forced regressive systems tract. The main focus of this paper is to reconstruct the paleoge-ography and depositional history of the systems tract based on detailed stratigraphic and facies archi-tectural analysis using 23 geological sections, photomosaics, and walking out of beds.

Internally, the systems tract consists of 6 parasequences, 11f to 11a from the oldest to the youngest.During the progradation of parasequences 11f to 11b the paleoshorelines were wave-dominated, asindicated by the abundance of HCS and/or SCS beds, wave-ripple cross-laminated beds, and the occur-rence of diverse and robust ichnological suites attributable to the Skolithos and Cruziana Ichnofacies.Progradation of the wave-dominated shorelines resulted in more homogeneous and laterally continuoussand bodies. From 11b to 11a, however, there is a distinct change in paleoshoreline regime from wave-dominated to tide-influenced as indicated by the common occurrence of tidal facies in 11a, including:(1) lenticular, wavy, and flaser bedding and bidirectional dipping cross strata; (2) reactivation surfaces,double-mud drapes, and ripple cross lamination with opposing dips at the toe of large dune-scale crosssets; (3) inclined heterolithic strata (IHS) and sigmoidal bedding with tidal rhythmites; and (4) cyclicvertical variation in facies and bed thickness and the common occurrence of sand-mud couplets. Thesetide-influenced facies show overall lower bioturbation intensity (BI 0-3). Progradation of the tide-influenced shoreline results in more heterolithic delta-front facies. Tidal and/or tidal-fluvial channelsfurther dissect delta-front sandstones, forming more isolated sand bodies.

Data from this study, as well as previous work, show that width and thickness of the forced regressiveparasequences are small, typically less than 5 km and 20 m respectively. In subsurface studies, identi-fying and correlating such small-scale parasequences using sparse data involve significant uncertainties.A combination of the diagnostic features indicating forced regression and different data sets is essentialto better constrain the geometry and architecture these small-scale bodies.

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1. Introduction

Forced regression is defined as shoreline retreat under relativesea-level fall (Posamentier et al., 1992; Ainsworth and Pattison,1994; Posamentier and Morris, 2000). In recent years, there hasbeen considerable increase in studies of forced regressive depositsdue to their importance as hydrocarbon reservoirs (Posamentieret al., 1992; Plint and Nummedal, 2000; Posamentier and Morris,2000; other papers in Hunt and Gawthorpe, 2000; Lee et al.,

ail.com (W. Li).

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2007). Large-scale stratigraphic architecture of forced regressivedeposits has been addressed in many previous studies (e.g.,Posamentier et al., 1992; Posamentier and Allen,1999; Posamentier,and Morris, 2000). Very few of these studies, however, attempt todocument bed-scale facies architecture to identify changes insedimentology and reservoir architecture from one stage of relativesea-level fall to another. Lee et al. (2007), using integrated outcropand GPR data, recognized a change from river-dominated tostrongly tide-influenced deltaic deposition in the forced regressivedeposits in the Cretaceous Wall Creek Sandstone, Wyoming.Mellere and Steel (2010) identified down-dip changes in offlappingdeltas that change from tide-influenced to fluvial- and wave-influenced in the Cretaceous outcrops in Wyoming. Despite this,

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the documentation of changes in depositional processes duringforced regression based on detailed facies architecture analysis isstill limited.

High-resolution seismic analysis of Quaternary shelf edgedeltas, particularly in the Gulf of Lions and Gulf of Mexico, providesinsights into the stratigraphic organization of forced regressivedeposits (Hart and Long, 1996; Tesson et al., 2000; papers inAnderson and Fillon, 2004; Serge and Gorini, 2005; Gwenael et al.,2006). Only a few of these studies, however, are calibrated in termsof lithology and depositional facies due to lack of core data. Gámezet al. (2009) analyzed and linked the onshore-offshore stratigraphyin the Llobregat deltaic system, Spain based on an integrated coreand regional seismic data. Most of these studies emphasize thecontrol of external factors (eustasy and climate) on regional-scalestratigraphy. The internal facies architecture and variability of theforced regressive deposits, which are critical for reservoir charac-terization, remains poorly understood in these seismic-basedanalyses. During forced regression, high-frequency relative sea-level fluctuations, changes in sediment supply and accommoda-tion, and variations in basinal wave and tide energy can result indeposits with very different architecture (Ainsworth and Pattison,1994; Plint and Nummedal, 2000; Posamentier and Morris, 2000).

The Cretaceous Ferron “Notom Delta” in the Henry Mountainsregion southern Utah, U.S.A. contains several examples of forcedregressive deposits that are well exposed along a 30 km outcropbelt oriented approximately along depositional dip (Fig. 1). Thisprovides an ideal opportunity to investigate the relationshipbetween larger scale parasequence stacking patterns and internalfacies variability. The focus of this paper is, thus, to document thestratigraphic and facies architecture of these forced regressivedeposits and to reconstruct depositional history.

2. Regional geology and the study area

The “Notom Delta”, located in the Henry Mountains region insouthern Utah, U.S.A. (Fig. 1), is one of the Ferron fluvial-deltaicwedges deposited in the foreland basin of the Cretaceous InteriorSeaway. The fluvial-deltaic wedge is exposed three dimensionallyalong a 60 km by 35 km outcrop belt and has a gradational contactwith the underlying Tununk Shale and a sharp contact with theoverlying Blue Gate Shale (Fig. 2).

Compared with the widely studied Ferron Last Chance Delta incentral-eastern Utah (papers in Chidsey et al., 2004), the Ferron“Notom Delta” has received very little attention. The earliest workon the “NotomDelta” dates back to the 1940’se1950’s and is mainlylithostratigraphic and biostratigraphic analysis (Hunt and Miller,1946; Hunt et al., 1953; Peterson and Ryder, 1975). Peterson andRyder (1975, 1980; Fig. 2) showed that during the deposition ofthe Upper Cretaceous successions in the Henries there were at leastthree transgressiveeregressive cycles and the emplacement of the“Notom Delta”, from late Middle to Late Turonian, was during thefirst regression after the Greenhorn relative sea-level maximum.Fielding (2010) examined the detailed facies architecture of thewedge in the west of the Henry Mountains and suggested pro-gradation of asymmetric flood-dominated, wave-influenced deltasin this area. Several recent studies established the sequencestratigraphic framework of the fluvial-deltaic wedge along theoblique dip-oriented western outcrop belt (Li, 2009; Li et al., 2010;Zhu, 2010) and showed that it consists of 6 depositional sequences,18 parasequence sets, and 43 parasequences (Fig. 3). The sixsequences are separated by five sequence boundaries (SB1eSB5).Numerous isochronous bentonites are present within the Blue GateShale, the Ferron, and the Tununk Shale members of the MancosShale Formation. The cross section in Fig. 3 is hung on a lowerbentonite within the Tununk Shale. Immediately above SB3 there

are 6 overall basinward- and down-stepping parasequences,11fe11a, which converge toward the lower bentonite, forminga forced regressive systems tract that can be mapped for 20 kmdown-dip (FeF0 in Figs. 1 and 3). The study area lies in the basin-ward part of the forced regressive systems tract, where it showsconsiderable complexity in both stratigraphy and facies architec-ture. The studied cliff face is about 10 kmwide and is marked by thenumbered sections 1e23 in Fig. 1 and by the dashed polygon inFig. 3.

3. Data and methods

To document the stratigraphy and facies architecture, a total of23 vertical sections were measured and the locations of thesections are shown in Figs. 1, 3, 5, and 11a. Correlation betweenmeasured sections was achieved by walking out beds along thetectonically-tilted outcrops. Where measuring sections or walkingout beds was impossible along sheer cliffs, photomosaics weretaken to facilitate correlation and mapping. Facies keys andrepresentative measured sections with both sedimentological andichnological data are presented in Figs. 4 and 5. Different faciestypes are further shown in detail in Figs. 6e8 and are described indetail in the following sections. Convergence or divergence ofparasequences relative to the lower bentonite, as well as lapoutrelationships documented by detailed correlations, was used todetermine the overall shoreline trajectory (Zhu, 2010).

4. Architecture of the forced regressive systems tract

4.1. Stratigraphic architecture

The forced regressive parasequences 11fe11a are within theforced regressive systems tract of sequence 3 (SQ3) and have a dipextent of about 20 km (Figs. 1 and 3). The youngest parasequence11f is detached from the preceding highstand parasequence 12a,forming a detached system (e.g. Posamentier et al.,1992; Ainsworthand Pattison, 1994; Plint and Nummedal, 2000; Fig. 3). The para-sequence has a sharp, erosional base in the proximal region anda gradational base in the distal area. Parasequences 11e through 11bare detached from the youngest parasequence 11f, and are attachedto one another, forming a 15e20 m thick, 4 km wide offlappingwedge (Figs. 3 and 5). Each of these parasequences is sharp based inthe proximal areas, floored by a regressive surface of marineerosion (RSME) and becomes gradational based in the more distalregions (Fig. 5). Parasequence 11a is the youngest parasequence inthe forced regressive systems tract and is detached from itspreceding parasequences. The overall stacking patterns of theparasequences 11fe11a suggest a long-distance forced regression(Fig. 3).

4.2. Facies architecture

Two major facies types are recognized in the forced regressivesystems tract: wave-dominated shoreface and tide-influenced deltafront. These facies have very different sedimentological and ich-nological characteristics and are described in detail in thefollowing.

4.2.1. Wave-dominated shoreface4.2.1.1. Description. The coarsening-upward facies successions11fe11b (Figs. 5 and 6A) are erosively overlain by a decimeter- tometer-thick pebbly coarse-grained interval throughout the studyarea (Fig. 6B). The lower parts of these successions are typicallyheterolithic and consist of non-amalgamated centimeter- todecimeter-thick, very fine-grained sandstones and muddy

Figure 1. Base map showing the location of the study area and measured sections. The high-resolution sequence stratigraphic framework along cliff face A-B is shown in Fig. 3. FeF0

marks the start and the end of the forced regressive systems tract shown in Fig. 3. The forced regressive deposits show considerable complexity frommeasured sections 1e23 and isthe focus of this study. The 23 geological sections are marked every five from the first to the last. See Fig. 3 for positions of the stratigraphic sections along section A-B. Location ofthe widely studied Last Chance Delta is shown in the inset for reference. Regional paleocurrent direction is based on dips of dune-scale cross strata within fluvial deposits.

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sandstones interbedded with mudstones. Hummocky and swaleycross stratififcation (HCS/SCS), planar stratification, andwave-ripplecross-lamination are common (Fig. 6D). These facies grade upwardsinto amalgamated very fine-grained sandstones (Fig. 6A). Many of

these amalgamated sandstones show “laminated-to-burrowed”bedding with sporadically distributed bioturbation. Common sedi-mentary structures are planar stratification and wave-ripple crosslamination, with subordinate hummocky cross stratification (HCS).

Figure 2. Upper Cretaceous stratigraphy in the Henry Mountains region and their relations to the time-equivalent rocks on the Wasatch Plateau in central Utah. The FerronSandstone was deposited under the first major regression after the Greenhorn relative sea-level maximumwithin the Cretaceous seaway. In the study area the Ferron Sandstone isbounded below by the Tununk Shale with a gradational contact and is overlain by the Blue Gate Shale with an erosional sharp contact (after Peterson and Ryder, 1975; Petersonet al., 1980).

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Further upward there is common occurrence of unburrowed orsparsely burrowed, fine- to medium-grained, dune-scalecross-stratified sandstones (Fig. 6A and C). Overlying these facies,there is very local occurrence of fine-grained planar stratifiedsandstones dipping seaward at low angles.

These coarsening-upward successions are variably burrowedbutshow overall high burrowing intensity (Figs. 5 and 6F). The heter-olithic facies in the lower part of these successions are commonlyintensely burrowed, with a common bioturbation index (BI, sensuBann et al., 2004) of 2e4. Ichnogenera in these facies consist ofa diverse and diverse assemblage of grazing and deposit-feedingstructures, including Asterosoma, Chondrites, Palaeophycus, Phyco-siphon, Planolites, Teichichnus, and Thalassinoides with uncommonoccurrences of Ophiomorpha and fugichnia (Fig. 6F). Upward, theamalgamated sandstones are unevenly burrowed, with a commonoccurrence of “laminated-to-burrowed” bedding and BI range of1e5. The dune-scale cross strata are unburrowed or sparsely bur-rowed with Arenicolites, Diplocraterion, Ophiomorpha, and Skolithos.

4.2.1.2. Interpretation. Each of the coarsening-upward successions11fe11b is interpreted as a shallowing-upward parasequence andthe pebbly coarse-grained interval erosively overlying these para-sequences are interpreted as a transgressive lag. Deposits in theseparasequences are interpreted as a wave-dominated shorefacebased on: (1) the overall sandy coarsening-upward characteristics(Figs. 5 and 6A); (2) the wide occurrence of wave- and/or storm-induced structures including HCS, SCS, wavy bedding, and wave-ripple cross lamination (Fig. 6D); and (3) the occurrence ofdiverse and robust trace fossil suites attributable to the archetypalCruziana and the Skolithos Ichnofacies (Figs. 5 and 6E). The heter-olithic facies and the amalgamated very fine-grained sandstoneswith “laminated-to-burrowed” bedding are interpreted as lowershoreface (lSF) deposits. Ichnogenera assemblage within thesedeposits suggests a suite attributable to the Cruziana Ichnofacies.The occurrence of a diverse and robust ichnogenera suite in thesefacies indicates low sedimentation rates, sufficient nutrients andoxygen near the water bottom induced by mixing of water by

Figure 4. Facies legend for Figs. 5e8.

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waves, and a relatively stable substrate. All these are typical inwave-dominated environments (MacEachern et al., 2005; Bannet al., 2008; MacEachern and Bann, 2008). The unburrowed orslightly burrowed, fine- to medium-grained, dune-scale cross-stratified sandstones further upward are interpreted as uppershoreface (uSF) deposits. The common occurrence of Arenicolites,Diplocraterion, Ophiomorpha, and Skolithos indicates a suite prob-ably attributable to the Skolithos Ichnofacies. The lack of bio-turbation in these facies is interpreted as the result of high-energyconditions in the surf zone under persistent wave agitation. High-energy conditions typically retard the activities of trace makersand also make the preservation of bioturbation structures lesslikely. The locally developed seaward-dipping low-angle stratifiedsandstones are interpreted as foreshore (beach) deposits. Theoverall sedimentological and ichnological characteristics docu-mented here are very similar to those observed in many otherwave-dominated prograding shoreface to foreshore facies succes-sions (Howard and Reineck, 1981; McCubbin, 1982; Clifton, 1981,2006). The lag overlying these wave-dominated successions indi-cates shoreline erosion during ensuing transgression, which mostprobably removed most of the foreshore strata and the fluvial,coastal, and delta plain facies.

Overall the proximal parts of these wave-dominated para-sequences primarily consists of amalgamated proximal lowershoreface and upper shoreface sandstones (Fig. 6A); the distalparts, in contrast, typically consists of significant proportions ofnon-amalgamated highly burrowed heterolithic lower shorefacefacies.

4.2.2. Tide-influenced delta front4.2.2.1. Description. As in parasequences 11fe11b, 11a also consistsof a coarsening-upward succession (Figs. 5 and 7A). Detailedexamination of facies, however, shows that deposits in it are more

Figure 3. Cross section showing the stratigraphy of the Ferron “Notom Delta” along the outcregressive systems tract FeF0. The fluvial-deltaic complex is bounded above by the Bluesequences, 18 parasequence sets, and 43 parasequences. FeF0 is a forced regressive systems trdetailed facies and stratigraphic architecture of the area outlined by the polygon. Dots atnostratigraphically significant ash layers both below, within, and above the fluvial-deltaicsystems tract relative to the lower bentonite in the Tununk Shale was used to determine thforced regressive systems tract and the measured sections along the dip-oriented outcropet al. (2010), and Zhu (2010).

heterolithic than the preceding parasequences. The lower part ofthe coarsening-upward succession typically consists of heterolithicthinly interbedded mudstones and very fine-grained sandstones(Fig. 7A). Current ripple cross-lamination, lenticular, wavy, andflaser bedding are the most common sedimentary structures inthese heterolithic facies (Fig. 7B). Grading into the upper part of thesuccession, decimeter- to meter-thick fine- to medium-grainedcross strata with reactivation surfaces and double-mud drapes arecommon (Fig. 7C and D). Ripple cross lamina sets with opposingdips are rather distinct in the foreset toes of many of the largerdune-scale cross strata. (Fig. 7E).

rop belt slightly oblique to depositional dip and the stratigraphic position of the forcedGate Shale and below by the Tununk Shale and internally consists of 6 depositionalact in sequence 3 (SQ3). The dashed polygon shows the focus of this study. See Fig. 5 forthe bottom showing positions of measured sections. Notice the occurrence of chro-complex. Convergence or divergence of the parasequences in the forced regressive

e overall shoreline trajectory shown in Fig. 9 (Zhu, 2010). See Fig. 1 for locations of thebelt. SB: sequence boundary; MFS: maximum flooding surface. Data from Li (2009), Li

Figure 5. Stratigraphic and facies architecture of the forced regressive systems tract. Internally, 6 parasequences are identified. From the oldest parasequence 11f to the youngest11a there is an overall basinward and downward parasequence stacking pattern. Each of the parasequences is sharp-based in the proximal part, bounded below by a regressivesurface of marine erosion (RSME), and become more gradational based in the distal part. Notice the lack of fluvial, coastal, and delta plain facies in the forced regressive systemstract and the change in facies from typical wave-dominated shoreface facies in 11f through 11b to tide-influenced delta-front facies in 11a. Each of the coarsening-upwardsuccession is a meso-scale flow unit, with an upward increase in reservoir porosity and permeability (from light to dark color). Notice that stratigraphic architecture of the off-lapping parasequences is the key control on reservoir connectivity and flow units. The geological sections, showing the characteristics of the forced regressive deposits, can bedeemed as cored intervals in subsurface studies. Detailed description of each of them would contribute to identify these small-scales wedges. See Figs. 1 and 3 for position of thediagram. See Fig. 4 for facies keys and Figs. 6 and 7, and 8 for more facies details.

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Near the top of the parasequence, there are meters-thick het-erolithic fining-upward bodies confined within U-shaped erosionalscours. The erosional scours range from tens of meters to 300 mwide (Fig. 5). Many of the fining upward bodies consist of distinctlyinclined heterolithic strata (IHS) of interbedded very fine- to fine-grained sandstones and mudstones (Fig. 7F). Individual inclinedstrata are commonly centimeters to decimeters thick. Commonsedimentary structures include current ripple cross-laminationwith wavy and flaser bedding in the thicker sandier beds andlenticular bedding in the muddy intervals. Dune-scale cross stratawith bidirectional dips typically occur near the bottom of the scourfills and may also consist of sigmoidal cross bedding.

The coarsening-upward succession in 11a is overall less intenselyburrowedandwithcommonBIof0e3. Locally, however, bioturbationcan be high with BI up to 4. The heterolithic facies in the lower partthe coarsening-upward succession consists of Cylindrichnus, Palae-ophycus, Planolites, Teichichnus, and Thalassinoides (Fig. 8A). Theoverlying dune-scale cross strata are largely unburrowed. Theinclined heterolithic strata (IHS) also have an overall low to mediumbioturbation intensity (BI 0-3), but contain distinct fugichnia andequilibrichnia (Fig. 8B). Decimeter-thick sigmoidal sandstones andinterbeddedmudstones are common inmany of the erosional scoursand these can be widely burrowed with a suite of fugichnia,Cylindrichnus, Diplocraterion, Ophiomorpha, Palaeophycus, Planolites,Skolithos, and Thalassinoides (Fig. 8C). In many cases, Planolites andTeredolites are common at the base of the scour fills (Fig. 8D).

4.2.2.2. Interpretation. The coarsening-upward succession in par-asequence 11a is interpreted as tide-influenced delta-front

deposits. The U-shaped scours are interpreted to represent tidalchannel and/or tidal-fluvial channels, and the inclined heterolithicstrata (IHS) contained within them are interpreted as tidally-influenced point bar deposits. Diagnostic features indicatingtidal process include: (1) the common occurrence of lenticular,wavy, and flaser bedding in the delta-front heterolithics (Fig. 7B);(2) the occurrence of reactivation surfaces and double-mud drapesin the delta-front cross strata formed during dominant andsubordinate flows (Fig. 7C and D); (3) cyclic vertical variation infacies and bed thickness and the common occurrence of sand-mudcouplets, most probably indicating peak flow/stillstand cycles(Fig. 8A); (4) Bidirectional cross strata in the tidal and/or tidal-fluvial channel deposits and ripple cross lamina sets withopposing dips in the foreset toes of large dune-scale cross strata(Fig. 7E); and (5) Inclined heterolithic strata (IHS) and sigmoidalbedding with tidal rhythmites and tide-induced structures(Figs. 7F and 8C).

Bioturbation intensity in parasequence 11a is overall low tomedium (BI 0-3) and many burrowing structures are diminutive.Ichnogenera show low to medium diversity and are dominated bymorphologically simple, facies-crossing structures. Burrowingintensity and ichnogenera diversity in strongly tide-influenceddeltaic systems are typically low because the activities of manytrace makers are suppressed by energetic tidal processes(MacEachern et al., 2005, 2007; Gani et al., 2007). Those observedin the studied parasequence are higher. Similarly, McIlroy (2007)also documented a tide-dominated delta with fairly high bio-turbation indices and diverse ichnological suites in the MiddleJurassic Lajas Formation in the Neuquén Province of Argentina.

Figure 6. Wave-dominated shoreface facies in parasequences 11fe11b. (A) Sharp based, amalgamated coarsening-upward shoreface deposits in the attached parasequences 11d and11e. uSF: upper shoreface, lSF: lower shoreface. (B) Transgressive lag overlying the forced regressive parasequences, indicating shoreface erosion during transgression. Thetransgressive surface of erosion (TSE) is marked by the white dashed line. (C) Dune-scale cross strata in the upper shoreface deposits. Hammer is 18 cm long. (D) Hummocky crossstratification (HCS) in the heterolithic lower shoreface facies. (E) Highly burrowed lower shoreface muddy sandstone. As, Asterosoma; Ch, Chondrites; Op, Ophiomorpha; Pa,Palaeophycus; Pl, Planolites; Ph, Phycosiphon; Te, Teichichnus; Th, Thalassinoides; and fu, fugichnia. The overall trace assemblage suggests an ichnogenera suite attributable to theCruziana Ichnofacies. Coin diameter is about 2.5 cm.

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Figure 8. Bioturbation type and fabric in the tide-influenced delta front and tidal and/or tidal-fluvial channel deposits in parasequence 11a. (A) Moderately burrowed delta-frontsand-mud couplets (marked as white and black arrows). Common ichnogenera include Cy, Cylindrichnus; Pa, Palaeophycus; Pl, Planolites; Te, Teichichnus; and Th, Thalassinoides. (B)Distinct fugichnia and equilibrichnia (marked as black arrows) in the inclined heterolithic strata shown in Fig. 7E. (C) Moderately burrowed heterolithic tidal-channel-fill facies.Typical ichnogenera include fu, fugichnia; Cy, Cylindrichnus; Di, Diplocraterion; Pa, Palaeophycus; Pl, Planolites; Sk, Skolithos; and Th, Thalassinoides. Notice the sigmoidal geometry ofthe sandstones. BI 1-4, with 3 common.(D) Side-on view of Planolites (Pl) and Teredolites (Te) burrowed facies at base of channel fill.

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5. Discussion and implications for exploration andproduction

5.1. Depositional processes and paleogeography

Progradation of parasequence 11f initiated the forced regres-sion, and under a negative shoreline trajectory (sensu Helland-Hansen and Gjellberg, 1994) the paleoshoreline prograded about7e8 km (Figs. 3 and 9A). From parasequence 11f to 11e, there wasa continuous relative sea-level fall, forming an erosional surfaceassociated with sediment bypass. A decrease in the rate of degra-dation allowed parasequence 11e to prograde. The 11e shoreline,however, only prograded 1e2 km under a negative shorelinetrajectory, suggesting a slowing in the rate of relative sea-leveldrop. Following deposition of parasequence 11e, there is a posi-tive shoreline trajectory under a relative sea-level rise (Fig. 9A). Theensuing continuous relative sea-level fall resulted in the develop-ment of parasequences 11be11d. The related shorelines during the

Figure 7. Tide-influenced delta-front facies in parasequence 11a. (A) Coarsening-upward dis(B) Flaser bedding in the thin-bedded delta-front sandstones. (C) Dune-scale cross strata wsurfaces are marked by black arrows and are associated with dominant and subordinate flowRipple cross lamina sets with opposing dips in the foreset toes of large dune-scale delta-Paleocurrent is generally perpendicular and out of the page. This, coupled with the overall beleft during deposition.

progradation of parasequences 11fe11b were wave-dominated, asindicated by their internal facies architecture (Figs. 5 and 6). Par-asequence 11a represents the culmination of the forced regressionand shows significant tidal influence. The paleoshoreline progradedfor about 5 km (Figs. 3, 5 and 7e9). The development of fluvial,coastal, and delta plain facies was inhibited because of negativeaccommodation behind the 11a through 11f paleoshorelines duringthe forced regression (Fig. 9). These facies were likely not depositedor, if developed, most probably have been removed by subsequenttransgressive erosion as indicated by the decimeter- to meter-thicklag (Fig. 6B).

The main stages of this forced regression (parasequences11fe11b) record more wave-dominated shoreface systems. Sand-stone bodies would be predicted to be more homogenous andcontinuous along strike (Fig. 9B). Parasequence 11a, formed at thetermination of the forced regression, may correlate with a time ofvery early lowstand. Change in the extent of the seaway andincrease in bathymetric restriction and tidal resonance all may

tal delta-front deposits with heterolithic thin interbedded sandstones and mudstones.ith reactivation surfaces in the upper part of the delta-front successions. Reactivations of tidal processes. (D) Delta-front dune-scale cross strata with double-mud drapes. (E)front cross strata. (F) Inclined heterolithic strata (IHS) in the tidal point bar deposits.dding geometry, suggest that the IHS within the point bas was laterally accreting to the

Figure 9. Depositional history and paleogeography of the forced regressive systems tract. (A) Shoreline trajectory determined by the convergence or divergence of parasequenceswith respect to the bentonites in the underlying Tununk Shale, as well as the lapout relationships documented by detailed correlations (Zhu, 2010). The shoreline started toprograde under a negative trajectory in the early stage of parasequence 11f. From parasequences 11f to 11e, there was continuous relative sea-level fall until the end of 11e. Undera negative trajectory, the 11e shoreline prograded for about 2 km and then relative sea level started to rise, leading to a positive shoreline trajectory. The ensuing continuous relativesea-level fall resulted in the deposition of parasequences 11b-11d. Parasequence 11a represents the culmination of the forced regression and, under a slight negative trajectory andtide influence, the paleoshoreline prograded for about 5 km (B) and (C) Paleogeography during the progradation of parasequences 11fe11b and parasequence 11a. Parasequences 11fthrough 11b were wave-dominated and the progradation of the associated paleoshorelines is predicted to result in more homogeneous and laterally continuous sandy facies. The11a paleoshoreline, in contrast, was tide-influenced and the associated delta plain and delta front were dissected by tidal channels and/or tidal-fluvial channels. Progradation of the11a shoreline, thus, is argued to generate heterolithic delta-front successions and more isolated sand bodies. Notice the overall lack of fluvial plain, coastal, delta plain facies duringthe forced regression. Scale in the diagram is a reference for the basinward migration of the paleoshoreline during each stage of the forced regression.

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contribute to a greater degree of tidal reworking, as has beendocumented in other Cretaceous lowstand systems (Willis et al.,1999; Bhattacharya and Willis, 2001; Willis and Gabel, 2003;Willis, 2005).

Figure 10. Linear plot of dip width and thickness of sandstone bodies in forcedregressive deposits documented in this study and the published literature. Width andthickness of these sandstone bodies are small, typically less than 5 km and 20 mrespectively. Data from Reynolds, 1999; Mellere and Steel, 2000; Tesson et al., 2000;Garrison and Van den Bergh, 2004; Lee et al., 2007; and this study.

5.2. Geometry of forced regressive deposits and correlationuncertainties

Geometric details of forced regressive sand bodies are limited,despite several recent studies on the dimensions of paralic sand-stones (e.g. Reynolds, 1999; Enge and Howell, 2010; Olariu et al.,2010). Data from this study and others (Reynolds, 1999; Mellereand Steel, 2000; Tesson et al., 2000; Garrison and Van den Bergh,2004; Lee et al., 2007) show that many of the forced regressivedeposits have limited dip width and thickness, typically less than5 kmand20m respectively (Fig.10), despite that sharp-based forcedregressive deposits up to 100 kmwide have been documented (e.g.Plint and Norris, 1991; Plint, 1991, 1996). Parasequence 11f, forexample, is only 5e10 m thick and 8 kmwide, the widest among allthe forced regressive parasequences documented in this study(Fig. 3). Parasequences 11be11e, attached to each other, are evensmaller, only 1e2 km wide and 3e7 m thick. Using a rich data set,the current study effectively constrains the geometry and strati-graphic architecture of these small-scale parasequences, resultingin a correct reservoir model (Figs. 3, 5 and 11A). Spacing of thegeological sections in this study is typically 300me500m (Figs. 2, 5and 11A).Well spacing inmany onshore fields commonly is equal toor smaller than the spacing of the geological sections used in thisstudyand, thus, therewill bemore confidence in subsurface analysisin such fields. In many offshore fields, where the subsurface issparsely sampled by wells spaced 160 acres or higher, identifyingand correlating such small-scale units can be difficult. Over-correlation might be common in the subsurface studies in thesefields, resulting in incorrect reservoir models (Fig. 11B).

Correlation uncertainties in such offshore fields, however, canbe mitigated by integrating key features that suggest forced

regression and different data sets. Features indicating forcedregression as identified in this study include:

A. Progressive basinward- and downward-stepping parasequencestacking patterns (Figs. 3 and 5);

B. Proximal to distal sharp- to gradational-based relationships(Figs. 5 and 6A);

Figure 11. Reservoir models for the forced regressive systems tract based on rich (A) vs. sparse (B) data. Model A, constrained by densely-spaced measured sections, photomosaics,and walking out beds in this study (Figs. 5e8), correctly reflects the stratal and facies architecture of the systems tract and effectively captures reservoir geometry, connectivity, andflow units. Model B, constrained only by data from three locations (assuming three wells drilled along the cliff face), underestimates reservoir stratal architecture and overexaggerates connectivity dramatically. The small-scale sand bodies can not be effective constrained with such sparsely-spaced data. Spacing of measured sections in model A is closeto 40e80 acer well spacing in many of the onshore fields (e.g. Prudhoe Bay, Alaska; Carthage, East Texas; and Dunvegan Formation, Alberta). Spacing of the data points in Model B issimilar to the well spacing in many of the offshore fields (e.g. West Delta, offshore Nile Delta; Grand Isle, offshore Louisiana). Subsurface correlations in such fields, thus, can lead tosignificant uncertainties.

Table 1A summary of typical features indicating forced regression identified in this study and the published literature (Posamentier et al., 1992; Plint and Nummedal, 2000;Posamentier and Morris, 2000; other papers in Hunt and Gawthorpe, 2000) and the utility of different data in identifying these feature in subsurface analysis.

Outcrop Core Well logs Seismic

Sharp-based shoreface deposits bounded below by regressive surface of marine erosion (RSME)in the proximal part and may be gradual-based in the distal part.

Good Fair-Good Poor

Absence of fluvial, coastal, and delta plain facies overlying the forced regressive deposits Good Fair PoorTransgressive lag and transgressive surface of marine erosion on top Good Poor-faira PoorDetachment or attachment from the preceding highstand and presence of a separation zone Fair Fair FairAttached offlaping regressive wedge, suggesting long-distance regression Fair Poor-gooda Poor-gooda

a Depends on width/thickness geometric characteristics of the stratigraphic units as well as the coverage and quality of data.

W. Li et al. / Marine and Petroleum Geology 28 (2011) 1517e1529 1527

C. Lack of fluvial, coastal, and delta plain facies due to bothnegative shoreline trajectory during forced regression anderosion during ensuing transgression (Fig. 5);

D. Observation of regressive surface of marine erosion (RSME),transgressive surface of erosion (TSE), and lags (Figs. 5 and 6B);

E. Detachment or attachment of younger parasequences from thepreceding highstand deposits and presence of a bypass zone;

F. An offlaping regressive wedge, suggesting long-distanceregression (Figs. 3 and 5).

The complex architecture of forced regressive deposits, asdocumented here, may not be imaged by conventional explorationseismic, especially where the units are thinner than the seismicresolution. Individual sandstone bodies in this study are typicallyless than 15 m thick. High-frequency seismic data (e.g. >50 Hz) instrata with a velocity of <2000 m/s would be able to detect but notfully resolve such sandstone beds. Sparsely-spaced well logs mayunder-sample the offlapping forced regressive parasequences,making it difficult to correlate laterally. Logs may detect the prox-imal to distal sharp- to gradational-based relationships fairly well(e.g. Plint, 1991). Log signatures (coarsening-upward blocky,coarsening-upward serrated, fining-upward, etc.) can indicategeneral depositional environments. The homogeneous and wave-dominated shoreface facies, for example, may be separated fromthe more heterolithic tide-influenced delta-front deposits on well

logs, especially if higher resolution image logs (e.g. FMI) are avail-able. Core is valuable in identifying key surfaces (RSME, TSE, andflooding surfaces) and depositional facies. In subsurface analysis, anintegration of different types of data is necessary to better predictthe occurrence and understand the architecture of these small-scale forced regressive deposits (Table 1).

6. Summary and conclusions

(1) Parasequences 11a through 11f of sequence 3 (SQ3) in theTuronian Ferron “Notom Delta” form a forced regressivesystems tract that extends about 20 km along depositional dip.Diagnostic features indicating forced regression include anoverall basinward- and downward-stepping patterns from theoldest to the youngest parasequences, proximal to distal sharp-to gradational-based relationships in individual para-sequences, lack of fluvial, coastal, and delta pain facies, anda negative shoreline trajectory with respect to the underlyingbentonite datum.

(2) The forced regressive systems tract shows considerable strati-graphic complexity. The oldest parasequence 11f is detachedfrom the preceding highstand deposits, forming a detachedforced regression. The younger parasequences 11be11e,detached from 11f, are attached to one another, resulting in

W. Li et al. / Marine and Petroleum Geology 28 (2011) 1517e15291528

a 15e20 m thick, 4 km wide offlapping wedge. The youngestparasequence 11a, detached from parasequence 11b, repre-sents the culmination of the forced regression.

(3) Detailed facies architecture analysis shows that internally theforced regressive systems tract consists of two distinct faciessuccessions: wave-dominated shorefaces and tide-influenceddelta front. Parasequences 11fe11b consist of typical wave-dominated shoreface facies; parasequence 11a, however,shows strong tidal influence and consists primarily of hetero-lithic tide-influenced delta-front facies.

(4) This change in facies suggests a change in sedimentationduring the forced regression from wave-dominated shorelinesin parasequences 11fe11b to a tide-influenced deltaic shorelinein parasequence 11a. Accordingly, two very different types ofreservoirs are formed. The progradation of wave-dominatedshorelines is predicted to form more homogeneous and later-ally continuous sand bodies, while the progradation of thestrongly tide-influenced shoreline would generate far moreheterolithic, laterally dissected and isolated sand bodies.

(5) Width and thickness of the sand bodies within in the forcedregressive systems tract documented in this study, as well asothers, are small, typically less than 5 km and 20 m respec-tively. Correlating these small-scale bodies using sparse datacan lead to significant uncertainties. In subsurface analysis, anintegration of key features indicating forced regression anddifferent data sets will better constrain the geometry andarchitecture of these forced regressive deposits.

Acknowledgments

Generous financial support for the field work was provided byAnadarko, BP, Chevron, and Shell USA. We thank Nick Biddle andPetter Dischington for helping in the field. Reviewers Chris Fielding,Brian Willis, and Guy Plint provided valuable comments andsuggestions to make the paper a better contribution. We also thankEditor James MacEachern for helping to clarify some of the inter-pretations and ideas in the manuscript.

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