Depositional-architecture-of-growth-fault-related-wave-domin

Depositional architecture of growth-fault related wave-dominated shelf edge deltas of the Oligocene Frio Formation in Corpus Christi Bay,Texas

Mariana I.Olariu *,Ursula Hammes,William A.Ambrose

Bureau of Economic Geology,The University of Texas at Austin,Austin,TX 78713,USA

a r t i c l e i n f o

Article history:

Received 24June 2013Received in revised form 16September 2013

Accepted 18September 2013

Available online 26September 2013Keywords:

Frio Formation Growth fault Gulf of Mexico

Wave-dominated deltaic shorelines Shelf edge deltas Oligocene

a b s t r a c t

Growth faults within the Frio Formation de ?ne six subbasins on the South Texas shelf and add to the complexity of the sediment dispersal along the shelf margin.These growth faults in ?uenced sediment pathways,controlled sediment partition and provided increased accommodation for deltaic depocenters.Rollover and thickening of sediments occur on the downthrown side of growth faults with offsets of at least 150m and up to 750m.Growth strata are indicated by intervals that thicken landward (by several tens of meters)from anticline crests.Inpidual 4th-order (100e 500ky)regressive cycles expand about ?ve to seven times across growth faults;expansion ratios as great as 10are observed.The shelf edge is associated with the largest displacement,expansion ratios and thickness of prograding deltaic sequences.Sedimentary structures and trace e fossil associations identi ?ed in cores from Nueces,Encinal and Red Fish Bay subbasins indicate deposition on a wave-dominated deltaic shoreline.The cores exhibit highly bioturbated muddy sandstones alternating with decimeter-thick clean sandstones with hummocky cross-strati ?cation.Alternation of fair-weather wave deposits with sandy storm beds in two successive fault zones suggests that all subbasins developed under similar conditions during growth-fault devel-opment.High sediment supply conditions favored accumulation of thicker sediments on the down-thrown sides of the faults.Wave dominance of deltas on the outer shelves in the growth compartments suggest that building of the upper slope rather lowers slope margin,and decreases the likelihood of the presence of sandy deep water fans.

Correlation of about 700well logs integrated with seismic data provided the areal distribution and relative timing of sediment ?lling of the growth-faulted compartments to unravel the evolution of higher-order (fourth and ?fth)sequences affected by these growth faults.Core data were used to recognize depositional environments,to calibrate lithology to well logs and to characterize seismic amplitude anomalies.Within this shoreface succession the clean sandy storm beds will make attractive hydrocarbon reservoirs especially on the crest of rollover anticlines in downthrown compartments.These sandstone bodies pinch out seaward into open marine shelf mudstones and are sealed by overlying shelf mudstones deposited during transgressive episodes.

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1.Introduction

Major deltas along the Gulf Coast delivered sediment to deep water during the Tertiary (Winker,1982;Galloway,1989).These deltas were associated with growth-fault systems that distorted the topset foreset geometry of the prograding shelf margin,hence making it dif ?cult to recognize the paleo-shelf edge (Winker,1982;Edwards,1995).Changes in eustatic sea level,regional and local

subsidence rates and sediment supply affect the distribution of sediments and sedimentary facies.Changes in facies and deposi-tional patterns re ?ect regional changes in shoreline position that is also in ?uenced by accommodation created by local syndepositional fault movement.Processes of deposition control both time and type of growth fault (Winker and Edwards,1983).Large shelf edge deltas (e.g.,Niger,Nile,and Mahakam deltas)are affected by gravity driven deformation related to shale detachment and differential loading due to deltaic sedimentation (Sestini,1989;McClay et al.,2000;Back et al.,2006;Magbagbeola and Willis,2007).Exten-sional deformation usually remains localized below the delta front and the longer the progradation the greater the associated

*Corresponding author.Tel.:t151********;fax:t151********.E-mail address:mariana.olariu@94f8eb46dd88d0d233d46ae0 (M.I.

Olariu).Contents lists available at ScienceDirect

Marine and Petroleum Geology

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Marine and Petroleum Geology 48(2013)423e 440

extension(Rouby et al.,2011).In the Frio section faulting is believed to be related to deeply-buried ridges of low-density and high-pressured shales(Bruce,1972).Vicksburg and Frio sediments were deposited over an existing Jackson continental slope which is primarily composed of shale(Boyd and Dyer,1964).The burden of these sediments caused the Frio e Vicksburg Flexure where large down-to-the-coast faults rapidly expand both the Frio and Vicks-burg Formation(Fig.1).

During the past three decades information has been acquired about growth faults in the Frio Formation,mostly in the form of seismic re?ection pro?les(e.g.,Winker and Edwards,1983;Zeng et al.,2007;Zeng and Loucks,2007;Ambrose et al.,2010)and well-log correlations(Bebout et al.,1975;Galloway et al.,1982; Edwards,1986,1995;Brown et al.,2004;Hammes et al.,2007; Bonnafféet al.,2008).Most of the high-resolution seismic data have sparse well and core control(Zeng and Loucks,2007).As

a

Figure1.Location map of the Corpus Christi Bay showing the six growth-faulted Frio subbasins.More than500digital well logs have been used for subsurface correlation. Stratigraphic cross sections(black lines)and3D seismic volumes(white dashed lines)are indicated.

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424

result they are useful for predicting the fault geometry and struc-tural history,but not the lithology or depositional environments (Winker and Edwards,1983).For this reason there have been many publications with poor scienti?c method,e.g.,making sequence stratigraphic conclusions directly from seismic or well data without arguing through a’sedimentary process’step.Little work has been published on cores(Berg and Powell,1976;Tyler and Han,1982; Kerr and Jirik,1990).In order to be able to identify the paleoshelf-edge,microfossils and sedimentary structures should be used(Winker,1982).Gravity-dominated versus traction-dominated sedimentary structures indicate an important process threshold and therefore the position of the shelf break,whereas microfaunal assemblages re?ect the paleo water depth.

This paper presents a study of detailed facies architecture reconstruction based on integration of cores with well logs sup-plemented by seismic data.The identi?ed fourth-order sedimen-tary successions are interpreted to represent wave-dominated shorelines,in all likelihood wave-dominated deltas with along-strike shoreface reaches.

2.Geologic setting

The Frio Formation is a major clastic depositional unit in the Texas Gulf Coast underlain by the Lower Oligocene Vicksburg For-mation and overlain by the transgressive Miocene Anahuac Shale (Galloway et al.,1982).The Frio sedimentary clastic wedge composed of interbedded sandstone and mudstone is up to4.5km thick and was deposited during late Oligocene over an interval of10 million years.The average sedimentation rate was about0.5mm/ year(Hardin and Hardin,1961).The Frio sedimentary wedge thickens basinward from only60m near the outcrop to over2745m in the subsurface over a distance of more than160km(Bebout et al.,1975).The Norias delta system is the major depositional depocenter of the Frio Formation in south Texas within the Rio Grande Embayment and is separated from the northern Gulf Coast Tertiary depocenter by the San Marco Arch(Bruce,1972).As the Norias delta prograded across the Frio shelf,sediment was supplied to the southeast(Duncan,1983).Longshore currents transported sediment from the delta system to the adjacent Greta-Carancahua barrier/strandplain system to the east(Galloway and Morton, 1989).The extensive barrier-bar-system is analogous to modern Padre,Mustang and Matagorda Islands in the South Texas Gulf Coast(Boyd and Dyer,1964;Wilkinson,1975;Tyler and Ambrose, 1985;Galloway,1986;Galloway and Cheng,1985).

In the Corpus Christi Bay area six subbasins controlled by growth-faults developed during Oligocene(Brown et al.,2006; Brown et al.,2004;Hammes et al.,2007;Bonnafféet al.,2008; Edwards,2006).Structurally,the subbasins are located on the basinward downthrown hanging wall side of regional growth faults(Brown et al.,2004).Faults are concave in a basinward direction displaying a NE e SW trend(Fig.1).Because growth faults are active during sedimentation,considerable thickening of the sedimentary units occurs on the downthrown side of the faults though the surface expression of this may have been barely noticeable.During the lower Frio deltas prograded over the pre-existing unstable shelf margin(Galloway et al.,1982)and built a thick sedimentary succession of deltaic and shoreface deposits. Wave processes reworked sediments into narrow,strike elongate barriers and strandplains.Vertically amalgamated,aggradational wave-dominated sandstone bodies formed the sedimentary suc-cession of the lower Frio.The middle and upper Frio successions in the studied area consist of?uvial sediments in the form of channel?ll,splay and?ood-plain deposits(Galloway et al.,1982) also con?rming the likely importance of delta supply to the lower Frio.3.Methodology

Structural relationships in three Frio subbasins(Nueces,Encinal and Red Fish Bay)were characterized on well-log cross-sections (Fig.2)and seismic pro?les.The stratigraphic and sedimentological character of the Frio Formation was examined in detail in cores (106.5m).Interpretation of about700correlated well logs(Spon-taneous Potential and Resistivity)provided insights into the rela-tive timing of the?lling of the growth-faulted compartments.To correlate well-logs,the“site-speci?c sequence-stratigraphic sec-tion(S5)chart”approach developed by Brown et al.(2005)was 94f8eb46dd88d0d233d46ae0posite wire-line logs were built for each subbasin and captured the most complete succession within a robust biostrati-graphic and sequence stratigraphic framework(Bonnafféet al., 2008).Each S5chart illustrates inferred depositional setting,ages, depositional sequences,and systems tracts.Wells that penetrated the entire Frio were selected except in the downdip areas,where most of the wells do not penetrate the entire Frio section.The top and base of the section were located with the aid of micropaleon-tology(benthic marker foraminifers)e Heterostegina texana and Marginulina vaginata are near the top of the Frio(used as a datum), and Textularia warreni is near the base(top of Vicksburg Forma-tion).Biostratigraphic zones used in regional correlations indicate that the duration of inpidual sequences de?ned here is0.5e1m.y. (fourth order).Description of three slabbed cores was used to characterize facies and calibrate wire-line log curves(Fig.2).

Estimating lithologies(sandstone and shale)requires log normalization and cutoff log values for each lithology.SP curves were normalized to make the maximum and minimum de?ections equal in all wells by rescaling the curves.Normalization was done according to a type of SP curve(withà20MV for sandstone-shale cutoff)and logs inspected to check that cutoff values adequately separate sandstone from shale in the log.Sandstone values be-tweenà90andà20MV were used to construct sandstone maps to delineate sandstone body geometry and to interpret depositional environments.In growth-faulted settings sandstone percentage maps are more meaningful to interpret depositional environments and to locate axes of sediment input as they remove the differential subsidence and emphasize depositional control on lithofacies dis-tribution(Galloway et al.,1982;Winker and Edwards,1983).The thickness calculation was performed with Petra?software and involves the calculation of sandstone thickness between surfaces of interest,and the creation of a grid for the study area using the least squares method.

Serrated,blocky and bell-shaped(upward?ning)log motifs are interpreted to represent heterolithic coastal plains and sandy ?uvial channels.Funnel-shaped(upward coarsening)log motifs are interpreted as prograding delta-front deposits.Deltaic para-sequences are separated by high frequency?ooding surfaces marked by muddy sediments(right de?ection on SP logs).

Correlation of about700well-logs was conducted using the genetic sequence approach of Galloway(1989)mainly because the shale-prone intervals formed during high-frequency basinward marine transgressions are easily identi?able on SP logs.Deltaic complexes(coarsening-upward units)are separated by thin?ning-upward intervals of coastline transgression and high-frequency (fourth-order)?ooding surfaces marked by muddy sediments (right de?ection on SP logs)of local to subregional extent(Fig.2). Mapping of the fourth-order cycles is accomplished through the identi?cation of inpidual regressive deltaic complexes and thin transgressive units bounded by?ooding surfaces within each well log.These?ooding surfaces can be traced laterally to the sur-rounding wells and their correlation is done in3-D.

Seismic and well-log data were combined within Landmark’s Stratworks?and Seisworks?to map stratigraphic surfaces.

M.I.Olariu et al./Marine and Petroleum Geology48(2013)423e440425

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M.I.Olariu et al./Marine and Petroleum Geology 48(2013)423e 440427

Mapped stratigraphic surfaces and faults were converted to depth using Landmark’s TDQ.

4.Results

4.1.Structural deformation

The stratigraphy of the Frio Formation is complicated by structural deformation associated with normal faulting(Trevi?o and Vendeville,2008).Fault displacement is accommodated by movement of underlying mobile prodelta muds(shale diapirs) (Fig.3).During shoreline regression growth faults are developed primarily by differential compaction with some associated gravity adjustments(McClay et al.,2000).Differential loading by deltaic sedimentation caused fault movement and deposition of thicker sediments in the downthrown compartments(Brown et al., 2004).

The examined seismic volume spans the distal part of Nueces, Encinal and Red Fish Bay subbasins and is characterized by continuous,subparallel re?ections in the upper part(upper and middle Frio)which become inclined and increasingly offset by faults in the lower part(lower Frio).Two major faults and associ-ated rollover anticlines on the downthrown side are the dominant structures with several antithetic and synthetic faults(Fig.3A).All faults in the study area exhibit synsedimentary growth,thickened or additional sedimentary units on their downthrown sides.Major faults have broadly basinward-concave,northeast-southwest strike trends and basinward dips toward the southeast.These faults decrease in dip with depth from nearly vertical to less than30 . Offsets become progressively less up-section,suggesting that rates of fault movement decreased over time.Upper horizons show displacements of about200m,whereas lower ones can be dis-placed with more than700m(Fig.3B).Smaller synthetic and antithetic faults that cut the rollover anticline crests have throws of less than100m and rarely exceed few kilometers in length. Thickness changes between seismic re?ectors are in?uenced by structural deformation with stratal thickening on downthrown sides adjacent to faults and thinning over anticlinal crests.Growth strata are indicated by intervals that thicken landward(by several tens of meters)from anticline crests.Inpidual4th-order(100e 500ky)regressive cycles expand about?ve to seven times across growth faults.Expansion ratios(downthrown thickness pided by upthrown thickness)as great as10are observed(Fig.3B).Structure on top of the Frio Formation shows only weak reversal of dip to-ward the fault zone,but thickening of the Frio section on the downthrown side produced increasing northwest dip on deeper horizons.

4.2.Sandstone maps

Subsurface maps of forth-order cycles bounded by?ooding surfaces illustrate several deltaic complexes in each of the three subbasins.Sandstone and sandstone-percent maps have been built to depict the depositional environments.Overall the sand-stone and sandstone-percent maps display a strike elongate ge-ometry(NNE e SSW;Figs.4,5and6).Sandstone thickness increases towards the fault with thinning laterally toward the east and seaward.Progressive basinward shift of depositional environments are thought to be best illustrated on sandstone-percent maps since they best re?ect primary facies and reser-voir trends particularly where abrupt thickness change results from syn-depositional structural movement.However,since both types of maps have a similar appearance the strike-oriented nature of the sandstone body is interpreted to form as a response to the process regime of the delta rather than as a response to the geometry of the growth-fault structure which de?ned the depocenter.Therefore sandstone maps were considered for a better representation of the real thickness of the deposits.

Seven deltaic complexes about100m thick each(53m e162m) have been identi?ed in the Nueces subbasin(Fig.4).The whole deltaic wedge of the3rd-order sequence thins away from the fault from about580m to490m in the west and from295m to260m in the east.The sandstones within each delta are about3e4km wide, 25e35km long and reach a maximum thickness of about60m.The sandstone body geometry is complicated by the presence of a series of closely spaced(fault spacing ranges between2km and up to 3km)normal(crestal)faults.The faults dip in a basinward(south-eastern)direction(Fig.4).Increased sandstone deposition occurs also on the downthrown side of the normal fault orthogonal(trends NW e SE)to the main growth fault.

The3rd-order succession in Encinal subbasin comprises eight deltaic complexes about80m each(60m e105m)(Fig.5).The whole deltaic wedge thins away from the fault from about650m to 500m in the west and from500m to400m in the east over a distance of3km.The sandstone bodies are continuous along strike and are about2e3km wide,20e30km long and reach a thickness of about40e50m.

Seven deltaic complexes about100m each(90m e136m)have been identi?ed in the Red Fish Bay subbasin(Fig.6).They become increasingly sandier with time.The sandstone in the oldest delta reaches a thickness of only30m,whereas the youngest has75m of sandstone.The whole3rd-order deltaic wedge thins away from the fault from about645m to380m in the west and from305m to 280m in the east.Increased sandstone deposition occurs also on the downthrown side of the normal fault orthogonal(trends NW e SE)to the main growth fault.

4.3.Shoreline migration

Strong sediment transport parallel to the coastline by long-shore currents reduces shore normal delta progradation rate and concentrates sandstone in elongate belts along the shelf margin. There is slight evidence of landward or seaward migration of the shoreline,rather deltas steeply aggraded during regression (Fig.7).Strong aggradation stacking is due to a combination of high subsidence rates and high sediment supply enhanced by extensive growth faulting.This implies that accommodation is continually generated and?lled and the maximum regressive point of the shoreline remains roughly in the same position. Shoreline advance and retreat of successive sequences occurs across small distances of some2e4km(Fig.7).There is a slightly variable shoreline behavior for inpidual shoreline excursions, but when combined it results in an overall steeply stacked shoreline pattern.

4.4.Facies description and interpretation

Delta deposits were examined in three cores located in Nueces, Encinal and Red Fish Bay subbasins(Figs.8,9and10)covering a total of106.5m.The sampled cores are located within the pro-grading wedges described by Brown et al.(2004).Sedimentary sections record physical and biologic sedimentary structures.The degree of bioturbation is described by the bioturbation index(BI)of Taylor and Goldring(1993);BI ranges from0(no bioturbation)to6 (bioturbation has blurred primary structures).The sedimentary succession in the three cores consists of upward-coarsening para-sequences that are each several meters thick and composed of as-sociations of the following facies.

M.I.Olariu et al./Marine and Petroleum Geology48(2013)423e440 428

Figure 5.Sandstone maps for shelf edge deltas in Encinal subbasin Narrow elongate sandstone belts are developed along the depositional strike which suggests wave dominance.The sandstone is thicker towards the fault and thins laterally toward east and seaward.The sandstone bodies are about 2e 3km wide,20e 30km long and reach a thickness of about 40e 50m.There is slight evidence of landward or seaward migration of the shoreline;the shoreline remains roughly in the same position.M.I.Olariu et al./Marine and Petroleum Geology 48(2013)423e 440431

maintaining the shoreline roughly in the same position with the overall result of a“steady”shoreline and vertical stacking of successive shoreface sequences.

4.4.1.Facies 1e highly bioturbated,very ?ne-grained muddy

sandstone

The sandstone is lower very ?ne to upper very ?ne grained,

decimeter thick,moderately sorted and displays wavy parallel

lamination (Figs.8A and 9A).The trace fossil persity is high with assemblages re ?ecting Cruziana ichnofacies represented by Tha-lassinoides ,Asterosoma ,Teichichnus ,Paleophycus ,and Planolites .The dominant ichnogenera re ?ect deposit feeding or grazing trophic strategies.The intensity of bioturbation is high (BI 4).Organisms colonize the sandy substrate,but are not capable to

homogenize Figure 8.Lithologic column (3362m -3408m)and representative core samples of lithofacies for shoreface successions in Nueces subbasin (each core sample is 7.6cm wide).A Facies 1e Highly bioturbated very ?ne-grained muddy sandstone.The trace fossil assemblage is represented by Thalassinoides (Th .),Paleophycus (Py .),Asterosoma (As .),and Pla-nolites (Pl .).B.Facies 2e Highly bioturbated ?ne-grained sandstone with Ophiomorpha (Oph .),Teichichnus (Te .),Paleophycus (Py .)and Asterosoma (As .).C.Facies 5e Slightly bio-turbated very ?ne-grained sandstone with soft sediment deformations.D.Facies 7e gray structureless mudstone.

M.I.Olariu et al./Marine and Petroleum Geology 48(2013)423e 440

434

the sand and therefore the physical sedimentary structures are only partially preserved.

The predominance of deposit feeding and grazing behaviors,the high persity of trace fossils and the burrowing intensity indicate that these sediments are fair-weather deposits.The presence of oscillation ripples is interpreted to represent wave reworking in an open marine upper offshore to distal lower shoreface environment.

4.4.2.Facies2e highly bioturbated very?ne-grained sandstone

The sandstone is upper very?ne grained,decimeter thick,and slightly cleaner compared to the sandstone of facies1.The trace fossil persity is high with assemblages re?ecting Cruziana ich-nofacies represented by Thalassinoides,Asterosoma,Teichichnus, Ophiomorpha,Paleophycus,and Planolites(Figs.8B and10A).The intense biogenic reworking(BI5)commonly obscures physical sedimentary structures(bedding)although some remnant wavy parallel laminations are preserved.

The high persity of trace fossils and the burrowing intensity is interpreted to represent deposition under fair-weather conditions. The occasional presence of oscillation ripples suggest wave reworking in open marine distal lower shoreface.

4.4.3.Facies3e thoroughly bioturbated very?ne-grained sandstone

The sandstone is very?ne-grained,centimeter-to-decimeter thick,thoroughly burrowed(BI6)with no discernible sedimentary structures(Fig.9C).The trace fossil persity is low with the most common trace fossil being Paleophycus.Pervasive bioturbation obscures any trace of remnant physical sedimentary structure making this facies to have an overall churned fabric.

Intense bioturbation,low trace fossil persity and position of these sandstones above sandstones of facies2suggest deposition in a proximal lower shoreface to distal middle shoreface environment. The thoroughly burrowed sediments imply deposition under fair weather conditions between storm events when the substrate was colonized by organisms.

4.4.4.Facies4e weakly bioturbated?ne-grained sandstone

The sandstones have sharp bases which are overlain by a basal lag consisting of rip-up clasts and shell debris.Sandstone beds are amalgamated(decimeter thick)and appear to be massive(Figs.9B and10B),but sometimes low-angle parallel to sub-parallel lami-nations are visible.The trace fossil persity is low with the dominant trace fossil being Ophiomorpha.The intensity of bio-turbation is low(BI1e2).

The presence of Ophiomorpha mostly towards the top of the beds represents recolonization of the substrate after storms. Amalgamated beds of parallel and low-angle cross-strati?cation are interpreted as hummocky cross strati?cation that suggests depo-sition in a storm-dominated distal to proximal middle shoreface where sustained wave erosion prevented mud deposition.

4.4.

5.Facies5e deformed very?ne-grained sandstone

The sandstone is very?ne-grained,centimeter-to-decimeter thick,slightly bioturbated(BI0e1)and displays soft sediment deformation such as convolute bedding(Figs.8C and10C).The contorted sand is intercalated with a muddy siltstone which im-plies sedimentary loading caused by the deposition of sand over muddy layers.Reduced burrowing intensity and the presence of small-scale soft sediment deformation suggests rapid deposition of sand during storm events along a wave-dominated delta front. 4.4.6.Facies6e gray calcareous highly bioturbated siltstone

The highly bioturbated(BI5e6)siltstone is decimeter thick (20e50cm)and cemented with calcite(Fig9D).The trace fossil persity is low with assemblages dominated by Planolites and Palaeophycus(Cruziana ichnofacies).

The burrowing intensity indicates that these sediments are fair-weather deposits.Remnant low-angle and wavy parallel lamina-tion suggests wave reworking in a low-energy,offshore environ-ment.The siltstone in the distal basins has more carbonate cement may be due to the greater burial depth(approximate3200m) (Loucks et al.,1977).

4.4.7.Facies7e gray structureless mudstone

The mudstone is light gray,weakly bioturbated(BI0e1)and structureless.The trace fossil assemblage is represented by grazing and feeding traces(Planolites)of the Cruziana ichnofacies.

Sedimentary structures are not common due to the depth which is generally below storm wave base.Sediments are interpreted to be deposited from suspension in an offshore setting(Figs.8D and 10D).

Overall sedimentary structures and trace fossil associations indicate a transition of depositional environments from open ma-rine upper offshore(mostly mudstone and siltstone with rare thin, intensely bioturbated sandstone)to lower shoreface(alternation of fair-weather suites with storm beds)to middle shoreface(clean amalgamated sandy storm beds)(Fig.11).The typical vertical facies succession(Fig.11)consists of mudstone and siltstone at the base followed by highly bioturbated very?ne-grained muddy sandstone with Cruziana ichnofacies(fair-weather conditions)and topped by weakly bioturbated?ne-grained sandstone with Ophiomorpha (storm-dominated conditions).These upward-coarsening succes-sions are capped by offshore mudstone.Shoreface deposits are recognized in the subsurface by an upward-coarsening wire-line log motif(Fig.2).

5.Discussion

Growth-faulted shelf edge deltas are dif?cult to recognize in the subsurface due to the distorted nature of the topset e foreset ge-ometry of the prograding shelf margin(Winker,1982;Edwards, 1995).However,criteria have been developed to help with the interpretation of the paleo-shelf edge(Winker,1982;Cummings and Arnott,2005).Regional growth faults develop on the seaward side of major depocenters during shoreline regression and are characterized by stratal thickening on the basinward(down-thrown)side where rollover(reverse drag)is exhibited(Winker and Edwards,1983).Depositional architecture on both sides of the fault is in?uenced by eustatic sea-level change,regional sub-sidence,local subsidence caused by growth fault and rate of sedi-ment supply(Van Heijst et al.,2002).Hanging-wall and footwall successions may be very different in thickness and may exhibit a different development of system tracts(Van Heijst et al.,2002). When a rate of sediment supply comparable with rate of subsi-dence is maintained through time,sediments tend to accumulate vertically or aggrade rather than prograding(Winker and Edwards, 1983).As a result the period of maximum growth corresponds to time of deposition of thick deltaic/shelf successions on the down-thrown side of the faults.The stratigraphical successions of shelf edge deltas are thick and localized within major growth fault structures and are characterized by short progradation distances (Elliott,1989).The maximum regression point of the shelf edge Mahakam delta remained in the same geographic position during the Pliocene and Pleistocene(McClay et al.,2000).Signi?cant aggradation was also noticed in Niger Delta(Jermannaud et al., 2010;Rouby et al.,2011).Increased subsidence along growth faults in the Champion delta,on the Brunei shelf slowed basinward progradation of the delta complex(Saller and Blake,2012).

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M.I.Olariu et al./Marine and Petroleum Geology48(2013)423e440 436

Thick,upward-coarsening successions(>100m),association with large-scale growth faults,short progradation distances,abrupt facies change across the fault,evidence of storm-wave deposition (wave-ripple lamination and hummocky cross-strati?cation)and occurrence of soft-sediment deformation structures(convolute lamination)support the idea that the deltas described in this study are in the proximity or at the shelf-edge.These criteria have proven useful in identifying shelf margin trends in other passive margin basins(e.g.,Winker and Edwards,1983;Suter and Berryhill,1985; Pore?bski and Steel,2003;Carvajal and Steel,2009;Dixon et al., 2012).

In the Frio subbasins growth faulting is the mechanism for providing space to thicken the section downdip(Ewing,1986; Brown et al.,2004;Hammes et al.,2007;Bonnafféet al.,2008; Brown et al.,2006;Edwards,2006).The structural and deposi-tional systems migrated with the southeast progradation of the delta as sedimentation rates kept pace with fault displacement rates.The sedimentary succession in the downthrown basin is characterized by repeated vertical stacking of shoreface se-quences and pronounced increase in sediment thickness close to the fault.Most preserved deltaic strata display an aggradational architecture(Figs.4,5,and6).This architecture is interpreted to result from high subsidence rates and high sediment supply combined with extensive growth faulting responsible of pro-ducing cumulatively greater accommodation through time.As a result the Lower Frio regional growth faults are inferred to have been developed in an outer shelf to upper slope setting where the sedimentary load was higher.The shelf edge is associated with the largest displacements(400m e700m),expansion ratios (4e12)and thicknesses of prograding deltaic sequences(Fig.3). High subsidence rates consistent with the structural instability of the shelf edge made the earliest deltas in each subbasin thicker.As the regression proceeded subsidence rates decreased as progradation advanced up the very slightly tilted slopes.The shelf edge deltas of the younger sequences are thinner.A likely decrease of the subsidence rate through time is also re?ected in the shoreline position of the earliest deltas which are slightly progradational,whereas the later deltas are slightly retro-gradational(Fig.7).Inpidual4th-order(100e500ky)cycles expand about?ve to seven times across growth faults.The stratigraphic succession just above the shelf edge deltas in each of the three subbasins experiences less expansion(EI e1.3e1.5). Similar aggrading trends for the deltaic system have been observed in Niger delta(Rouby et al.,2011).The highest displacement rates as calculated in the Red Fish Bay subbasin (0.21mm/yr e735m in3.5m.y.)are equivalent to moderate displacement rates(0.004e1mm/yr)for normal faults in similar tectonic settings(Nicol et al.,1997,2005),and to present-day normal faults in the Mississippi river delta(i.e.,0.1e1mm/yr; Gagliano et al.,2003).

The development of continuous strike-oriented sandstone belts suggests a wave-dominated environment.As the shoreline advanced during deltaic progradation more sediment was depos-ited on the downthrown side of the fault and reworked along shore by wave processes.Shoreline sands were deposited by longshore currents coming from the southwest.A greater thickness of sand-stone in the west is caused by proximity to the deltas of the ancestral Rio Grande River.Longshore sediment drift transported sand eastward from these deltas and deposited it in elongated ridges along strike.

By examining three cores in adjacent subbasins it has been noticed that sediments deposited in two successive fault zones exhibit lithologic and faunal similarities.All three cores display shoreface deposits in upward coarsening parasequences(Figs.8,9 and10).The sediments have been interpreted as wave-dominated deltaic shorelines.The studied core interval in the Red Fish Bay subbasin is part of a longer core that cuts through several wave-dominated deltas(for a detailed description of facies see Zhang, 2013).The presence of similar facies suggests that all subbasins developed under similar conditions during successive growth-fault development.Although the facies is similar,correlation across growth faults is not possible because of the progressively younger development of these subbasins(Hammes et al.,2004; Brown et al.,2004;Hammes et al.,2007).The core in the most proximal basin displays coarsening-upward packages with abun-dant wave structures and at the same time subtle evidence of rapid sedimentation and fresh-water input such as mudstone rip-up clasts,massive bedding and soft sediment deformation which suggest mixed river and wave in?uence.The presence of soft sediment deformation structures(convolute lamination)indicates abrupt deposition of sandstone or oversteepening of a wave-dominated delta front in vicinity of the shelf edge.Shelf edge deltas tend to be more wave-dominated perhaps because wave energy is high when shorelines are at the shelf margin due to unrestricted fetch and water depth(Pore?bski and Steel,2003). Wave-dominated shelf edge deltas are common and long lived features on shelves in growth-faulted,extensional basins.The character of the depositional environments(wave-dominated deltas)of the Niger Delta has persisted since the Miocene(Doust and Omatsola,1989).Growth-faulted basins trapped large amounts of sand and shale in wave-dominated deltaic systems in Baram and Champion deltas on the Brunei shelf(Saller and Blake, 2012).

Wave-dominated deltas at the shelf edge are inef?cient sup-pliers of sediment to adjacent deep water regions,because of longshore drift of the sediment(Carvajal and Steel,2009;Dixon et al.,2012).Wave dominance of deltas on the outer shelves in the growth compartments suggest that building of the upper slope rather lowers slope margin,and decreases the likelihood of the presence of deep water fans.None of the six Frio sub-basins developed deeper water than neritic shelf depths implying aggra-dation of successive deltaic shorelines rather than signi?cant pro-gressive progradation that would have caused the shoreline deposits of the higher levels to overlie deep water deposits of older levels.

Within a shoreface succession the clean sandy storm beds will make attractive hydrocarbon reservoirs especially on the crest of rollover anticlines in downthrown compartments.These sand-stone bodies pinch out seaward into open marine shelf mud-stones and are sealed by overlying shelf mudstones deposited during transgressive episodes.The vertical permeability would be good within the stacked sandstone beds(Galloway,1986).The mud within the lower shoreface interval might buffer vertical ?ow in places.However,the overall lateral continuity of the clean sandstone beds will make wave-dominated deltas proli?c reservoirs.

Figure9.Lithologic column(3185.6m e3203.4m)and representative core samples of lithofacies for shoreface successions in Encinal subbasin(each core sample is7.6cm wide).A. Facies1e Highly bioturbated very?ne-grained muddy sandstone.The trace fossil persity is high with assemblages re?ecting Cruziana ichnofacies represented by Thalassinoides (Th.),Asterosoma(As.),Rhizocorallium(Ry.),Paleophycus(Py.),and Planolites(Pl.).B.Facies4e moderately bioturbated?ne-grained sandstone with Ophiomorpha(Oph.)C.Facies3e Thoroughly bioturbated very?ne-grained sandstone with no discernible sedimentary structures D.Facies6e gray calcareous highly bioturbated siltstone with Planolites and Paleophycus.

M.I.Olariu et al./Marine and Petroleum Geology48(2013)423e440437

Figure 10.Lithologic column (3410.7m e 3453.4m)and representative core samples of lithofacies for shoreface succession in Red Fish Bay subbasin (each core sample is 7.6cm wide)A.Facies 1e Highly bioturbated very ?ne-grained muddy sandstone.The trace fossil persity is high with assemblages re ?ecting Cruziana ichnofacies represented by Thalassinoides ,Paleophycus ,Asterosoma and Planolites .B.Facies 4e Unbioturbated massive ?ne-grained sandstone.C.Facies 5e Slightly bioturbated very ?ne-grained sandstone with soft sediment deformations.D.Facies 7e gray structureless mudstone.M.I.Olariu et al./Marine and Petroleum Geology 48(2013)423e 440

438

6.Conclusions

The stratigraphic and sedimentological character of the Frio Formation was examined in detail in cores and interpreted from well-logs and seismic data.Multiple,vertically stacked upward-coarsening parasequences separated by thin ?ning-upward in-tervals of coastline transgression and high-frequency (fourth-or-der)?ooding surfaces indicate a transition of depositional environments from offshore to lower and middle shoreface.The sediments were interpreted to have been deposited on a wave-dominated deltaic shoreline in the vicinity of the shelf edge.Changes in facies and depositional patterns re ?ected regional changes in shoreline position that can be also in ?uenced by ac-commodation created by local syndepositional fault movement.Faults were accompanied by development of rollover anticlines and accumulation of growth strata on downthrown sides.Subsi-dence along growth faults slowed basinward progradation of the delta complexes.A balance between the rate of creation of

accommodation and rate of sediment supply generated thick,ag-gradational deltaic succession.There is slight evidence (small dis-tances of some 2e 4km)of landward or seaward migration of the shoreline.The shelf edge is associated with maximum displace-ment,maximum expansion rations and maximum thickness of prograding deltaic sequences.Despite the fact that cores were located in different subbasins separated by growth-faults they display lithologic and faunal similarities suggesting that all sub-basins developed under similar conditions during growth-fault development.The dominance of wave processes on the outer shelves in growth-faulted depocenters suggest that building of the upper slope rather lowers slope margin,and decreases the likeli-hood of the presence of deep water fans.Acknowledgments

The State of Texas Advanced Resource Recovery (STARR)pro-gram supported this research.The authors would like to

thank

Figure 11.Core-to-log calibration Description of the core in Red Fish Bay subbasin is shown against the SP log (normalized)to illustrate typical shoreface facies succession (for details of the lithologic column see Fig.10).The sequence begins with offshore or lower shoreface deposits (alternation of fair-weather suites with storm beds)and passes upward into middle shoreface deposits (clean amalgamated sandy storm beds).Progressive deepening of the shoreface during transgression favors accumulation of lower shoreface and offshore sediments at the top of the sequence.

M.I.Olariu et al./Marine and Petroleum Geology 48(2013)423e 440439

Oscareni Ogiesoba and Rodolfo Hernandez for their help with seismic processing.We thank Western Geco for letting us work with and show the seismic data set.The seismic data were inter-preted using Landmark software under the University Landmark grant.The prompt response of the Core Research Center team at the Bureau of Economic Geology was greatly appreciated.The authors would also want to thank Ron Steel and an anonymous reviewer for their critical reading,comments and corrections.Publication authorized by the director of Bureau of Economic Geology,Uni-versity of Texas at Austin.

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