Shelf Edge Deltas _offshore Trinidad

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Shelf-edge deltas alongstructurally complex margins:A case study from eastern

offshore TrinidadLorena Moscardelli, Lesli J. Wood, andDallas B. Dunlap

A B S T R A C T

A 15,000-km2 (5792-mi2) three-dimensional seismic data sur-vey that covers the shelf-slope transition of the eastern offshore

Trinidad continental margin reveals the geometry and depo-sitional history of the last maximum glacial lowstand shelf-margin succession. Despite the lack of well information at theseshallow depths, the quality and continuity of the seismic dataallow us to pursue a detailed seismic stratigraphic interpre-tation of the last lowstand margin succession. The basin-fillstratal architecture of the studied stratigraphic interval showsa great deal of lateral and vertical variability along the con-tinental margin during the Pleistocene to Holocene. Threegeomorphological elements controlled the character of theaccommodation within the basin and were crucial in trans-porting, delivering, and storing sediment supply from shelf toslope areas: (1) the Columbus sedimentary pathway on theshelf, (2) bypass zones in the shelf-break region, and (3) deep-water depocenters. The location and geometry of these geo-morphological elements within the basin are clearly controlledby underlying structures, transpressional to the north andgravity driven to the south. Migration of the paleo-Orinocodelta system across the shelf was also a key factor in definingthe stratigraphic geometries that are observed within the shelfbreak. Development of shelf-edge versus outer-shelf deltaic

systems on the continental margin was controlled by the na-ture of sediment supply at specific times, as well as by theavailability of accommodation, although, to a lesser extent, torelative sea level fluctuations. The interpretation also showed

A U T H O R S

Lorena Moscardelli Bureau of Econom-ic Geology, Jackson School of Geosciences,University of Texas at Austin, Austin, Texas;[email protected]

Lorena Moscardelli is a research associate andlecturer at the Bureau of Economic Geologyand the co-principal investigator of the Quan-titative Clastics Laboratory Industrial Associatesprogram. Her interests include seismic geo-morphology of deep-water deposits with spe-cial emphasis on mass-transport complexes,regional hydrocarbon prospectivity, and appli-cation of novel seismic visualization techniquesto tackle a variety of geologic problems. Sheholds a B.S. degree in geology from the CentralUniversity of Venezuela, and a Ph.D. in geologyfrom the University of Texas at Austin.

Lesli J. Wood Bureau of Economic Geol-ogy, Jackson School of Geosciences, Universityof Texas at Austin, Austin, Texas;[email protected]

Lesli J. Wood is a senior research scientist andlecturer at the Bureau of Economic Geologyand the principal investigator of the Quantita-tive Clastics Laboratory Industrial Associatesprogram. Her interests include seismic geo-morphology, clastic depositional systems, andMartian sedimentology. She holds B.S. and M.S.degrees in geology from the Arkansas TechUniversity and the University of Arkansas, and aPh.D. in earth resources from Colorado StateUniversity.

Dallas B. Dunlap Bureau of EconomicGeology, Jackson School of Geosciences, Uni-versity of Texas at Austin, Austin, Texas;[email protected]

Dallas B. Dunlap received his B.S. degree ingeology from the University of Texas at Austinin 1996. That year, he joined the Bureau ofEconomic Geologys (BEG) international proj-ects group as a research scientist associatefocused on reservoir characterization studies inAustria, Mexico, and Venezuela. In 2006, hemoved to BEGs Quantitative Clastics Labora-tory studying various marine depositionalsystems.

Copyright 2012. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received March 30, 2011; provisional acceptance November 28, 2011; revised manuscript

received January 9, 2012; final acceptance January 24, 2012.

DOI:10.1306/01241211046

AAPG Bulletin, v. 96, no. 8 (August 2012), pp. 14831522 1483


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that, for time-equivalent units, parts of the shelf-edge regioncould develop as an erosional margin (sediment bypass zones),whereas other parts of the shelf edge could behave as an ac-cretionary margin (sediment accumulation). The sequence-stratigraphic interpretation that was attempted in this workalso demonstrated that the characteristics of systems tracts can

abruptly change along strike in the shelf-edge region for time-equivalent units. These changes can be misleading if a geneticinterpretation is pursued only on the basis of the definition ofsystem tracts in the shelf-edge region without the consider-ation of a complete regional framework.

INTRODUCTION

Numerous scientists have used two-dimensional (2-D) seismicsections along various modern and ancient paleocontinental

margins to discuss the dynamics of sediment transport alongthese settings from shelf edge to slope (Johannessen and Steel,2005). However, 2-D seismic sections can be sparse, and abruptchanges on margin architecture along strike can pose a chal-lenge when trying to select representative seismic sections for anentire margin. Likewise, a multitude of regional sedimentationmodels are based on outcrop analyses that provide a valuableinsight into large-scale stratigraphic relationships (Posamentierand Vail, 1988; Posamentier and Allen, 1999; Johannessen andSteel, 2005). Outcrop studies also provide information that is

crucial to understand the small-scale architecture of basin-filldeposits but escaping the observation that such analysis ismostly 2-D is not possible. True sediment distribution anal-ysis, process analysis, and source-sink analysis of even lim-ited (10 km2 [4 mi2]) areas are best attempted with a three-dimensional (3-D) data set. Such models will allow one toreturn to 2-D studies emboldened with a new perspective andwill enable placement of those observations in a much moremeaningful context. Three-dimensional data, such as 3-D seis-mic volumes, integrated with well-log data provide the type

of dense understanding of the relationships between structureand strata that may lead to an ability to decipher the interplayof tectonic, eustatic, and sediment-supply signatures withinindividual clinoform packages across the basin.

Most depositional models that try to explain sedimenttransport from shelf to deep-water locations assume that theseprocesses occur along a bidimensional plane that is perpendic-ular to the shelf-edge trajectory (Karner and Driscoll, 1997).Despite these practical and commonly necessary assumptions,subsurface and modern environmental studies highlight the

A CKNOWLE DGE M E NTS

The member companies of the QuantitativeClastics Laboratory Industrial Associates pro-gram provided funding for this research. Wethank the Ministry of Energy and Energy Indus-tries for Trinidad and Tobago for the ongoing

partnership in the study of the geology ofTrinidad and Tobago. We also thank Exxon-Mobil, ChevronTexaco, Shell, BP, BHP Billiton,and their partner companies for the generousdonation of seismic data, as well as the Land-mark University Grant software, for the ongoingsupport. We thank the Jackson School of Geo-sciences for providing partial funding to coverpublication costs and the director of the Bureauof Economic Geology for providing similarfunding. We also thank the Computing andMedia departments of the Bureau of Economic

Geology for providing excellent technical sup-port. We thank AAPG editor Stephen E. Laubachas well as Mourad M. Bedir and additionalanonymous reviewers for providing valuableinput that improved the technical content ofthis manuscript . The publication was authorizedby the director of the Bureau of Economic Ge-ology, University of Texas at Austin.The AAPG Editor thanks the following reviewersfor their work on this paper: Mourad M. Bedirand three anonymous reviewers.

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importance of alongshore and current-controlledprocesses for sediment transport into deep-waterenvironments (Martinsen and Helland-Hansen,1995; Boyd et al., 2008; Carvajal et al., 2009;Elliott et al., 2010; Zhu et al., 2010; Dickinson et al.,2011; Georgiopoulou et al., 2011; Hubble et al.,

2011). A study based on outcrop and subsurfacedata covering the Maastrichtian depositional cyclesfrom the Lewis-Fox Hills shelf margin in Wyomingconcludes that waves and tides can contribute sandto canyon heads via alongshoredrift (Carvajal et al.,2009). Dickinson et al. (2011) described the mor-phology of modern canyons on the northern SouthChina Sea margin and concluded that canyon for-mation in this region was related to the action ofthe Kuroshio current and not to shelf processes.

The lack of evidence for canyon linkages to theshelf in the South China Sea margin suggests thatstrong bottom currents interacting with the mid-slope region have triggered the formation and lateralmigration of canyons since the Miocene (Zhu et al.,2010; Dickinson et al., 2011). The study of modernouter-shelf environments, such as theone conductedby Boyd et al. (2008) in Fraser Island (southeasternAustralia), also highlights the importance of along-shore tidal transport systems to deliver sedimentsinto deep-water locations during highstand condi-

tions. Slope stability analysis from the eastern Aus-tralian margin indicate that the slope should bestable; however, bottom-water currents managedto erode the slope triggering pervasive sedimentmass failures across the margin (Hubbleet al., 2011).The Rockall Bank mass flow located in eastern off-shore Ireland was generated by the action of bot-tom water currents that eroded the lower part of theslope and triggered the 24,000 km2 (9266 mi2) masswasting event (Elliott et al., 2010; Georgiopoulou

et al., 2011). This realization of the importance ofalongshore, tidal and bottom water currents em-phasizes the idea that sediment dynamics alongcontinental margins is clearly a 3-D problem.

The scarcity of models rigorously addressingthe issue of sediment movement along strike seemsto favor the concept of along-dip sediment-transportmechanisms functioning as almost exclusive pro-cesses in most continental margins. Many modelsalso rely heavily on relative sea level fluctuations

as the primary driver to explain how and whensediments are delivered into deep-water settings.The general assumption is that, if the shoreline isable to regress entirely onto the shelf edge, sandcould easily be delivered beyond the outer shelfand, therefore, the shelf margin should prograde

basinward (Posamentier and Vail,1988;Posamentierand Allen, 1999; Johannessen and Steel, 2005). Thisconcept might be adequate for some parts of theshelf margin but, even under passive margin con-ditions, local and semiregional structuring can gen-erate shelf proximal sediment sinks (e.g., growthsediment traps) that prevent sediments from beingtransferred into deep-water locations during low-stand conditions. Many numerical models also en-counter difficulties in addressing the issue of sedi-

ment transfer along strike, in part because of thedifficulty of dealing with the three-dimensionalityof the problem (Jordan and Flemings, 1991; Steckleret al., 1993; Ross et al., 1994; Karner and Driscoll,1997; Wolinsky and Pratson, 2007). Therefore, mostnumerical models assume that stratigraphic stack-ing patterns are solely the result of depositional andsediment-transport processes operating within themodeling section(2-D approach;KarnerandDriscoll,1997). Karner and Driscoll (1997, p. 443) recognizedthese limitations by indicating thatwhile diffusion

models provide a physical basis for the generationof clinoforms, the resulting model predictions ap-pear to do little more than track the transgressiveand regressive movements of the shoreline. Theintroduction of 3-D interactions by allowing along-margin sediment diffusion to redistribute the sed-iment brought across the margin by a combinationof diffusion (e.g., 3-D progradation and switching ofdepositional lobes along the margin) and advection(e.g., gravity flows across the margin) is a step in the

right direction to improve the predictive capabil-ities of these models (Karner and Driscoll, 1997).

The conceptualization of models that attemptto explain the mechanisms of sediment transferfrom shelf edge to slope poses a big challenge toresearchers. The complexity of the problem causedin part by the multitude of variables involved isclear. Such complexity requires a delicate bal-ance of simplification and detailed data, the latterof which is commonly lacking in some studies.

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Existing models seem to rely too heavily on theinfluence of relative sea level changes as the maindriver of sediment transport to deep-water loca-tions, underestimating the impact of underlyingstructural configuration, as well as that of along-shore, current-controlled, and tidal transport sys-

tems (Ryan et al., 2009). Structures can controlaccommodation by influencing the morphology ofthe shelf edge so that bypass zones and areas ofsediment retention are commonly related to thepresence or absence of certain geometric arrange-ments that are defined by fault architecture. Totruly understand these systems and pinpoint sinks,sources, and transfer pathways for sediments, wemust look at the 3-D nature of these margins andtheir deposits. The study area, located in eastern

offshore Trinidad, offers a unique look at both thetemporal and spatial changes of a margin under-going regions of extension and transpression withina 100-km (62-mi) span along the shelf break. Suchan examination would allow for comparison andcontrast of these structural settings (extension vs.transpression). Thisarticle concentrates on the studyof the paleo-Orinoco shelf-edge delta that devel-oped during the last maximum glacial lowstand ineastern offshore Trinidad. The main objective ofthis work is to pursue a detailed geomorphological

study of the last glacial lowstand shelf-margin sys-tem (100-km [62-mi] length) along eastern off-shore Trinidad. We seek to address the followingquestions: (1) How does the geomorphology andarchitecture of shelf-margin and shelf-edge deltasvary laterally along the structurally complex mar-gin of eastern offshore Trinidad?; (2) How does thenature of clinoform development and interactionwith different structural domains along this studyarea relate to sediment escape into the downslope

region?; and (3) What aretheseismic-based criteriathat can be used to characterize the different typesof sediment-transport mechanisms operating in theouter-shelf to upper slope region?

STUDY AREA

The area of study is located in the southeasternmargin of the tectonically active Caribbean plate

boundary zone (CPBZ) in eastern offshore Trini-dad (Figure 1). This study uses a 15,000-km2

(5792-mi2) 3-D seismic survey that extends fromthe eastern part of the Columbus Basin (modernshelf) to the deep-water exploratory blocks of east-ern offshore Trinidad (modern slope) to examine

the architecture and character of the last maximumglacial lowstand shelf-margin succession (Figure 1).The Trinidad and northeastern Venezuela area ischaracterized by active tectonism driven by thetranspressive character of the CPBZ, which passesthrough the north edge of the study area, in whichboth compressional and extensional forces operate(Escalona and Mann, 2011). High sediment ac-cumulation rates in the area, associated with sup-ply from the Orinoco delta system and accom-

modation enhanced by tectonics, have dominatedthe character of the stratigraphic succession in thestudy area since the Miocene (Wood, 2000). TheOrinoco River is the second largest river in SouthAmerica; most of the sediment transported by thissystem is derived from the Neogene-uplifted ter-rains of the Eastern Cordillera of Colombia, theMerida Andes of Venezuela, and the CaribbeanMountain Range, whereas a smaller percentagecomes from the Guayana shield to the south (Diazde Gamero, 1996). A series of studies have docu-

mented the existence of a paleo-Orinoco delta thatmigrated from the Maracaibo Basin to its present-day position throughout the Neogene (Diaz deGamero, 1996; Mann et al., 2006; Escalona andMann, 2011). This system shifted its outlet con-sistently eastward in response to barriers emplacedby the uplift of successively more and more east-ward located mountain ranges along the northernSouth American margin (Rod, 1981; Diaz deGamero, 1996; Mann et al., 2006; Escalona and

Mann, 2011). The paleo-Orinoco River shiftedto a position similar to its present-day course dur-ing the late Miocene, whereas the active lobe ofthe paleo-Orinoco delta migrated through the Co-lumbus Basin during the late Pliocene, reaching theshelf edge during the Pliocene-Pleistocene (paleo-Orinoco shelf-edge deltas) (Diaz de Gamero, 1996;

Wood, 2000).Significant framework mapping has been done

by a variety of authors in the slope and deep-water



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areas of the basin, but these efforts concentratedon the study of deep-water, gravity-induced de-posits that occurred below the shelf break (Bramiet al., 2000; Moscardelli et al., 2006; Moscardelliand Wood, 2008; Wood and Mize-Spansky, 2009).Maher (2007) also examined the influence thatshelf-edge structures had in defining the bypasszones of deep-water sands in the southern growth-

fault domain (Figure 2B), as well as the relation-ship between delta architecture, fault character,and slope morphology. This later work tackles someof the basic questions related to lateral variationsof shelf-edge deltas along the southern growth-fault domain of our study area (Figure 2B) (Maher,2007). In this article, we present results of a moredetailed interpretation of shelf-edge architecture

Figure 1. Map showing the area of study located in northeastern South America along the Caribbean plate boundary zone. Contoursrepresent bathymetry in meters. The area of three-dimensional (3-D) seismic data is outlined. The light-grayshadowed area in the maphighlights the deep-water blocks where gravity-induced deposits were studied and documented by Brami et al. (2000), Moscardelli et al.(2006), Moscardelli and Wood (2008), and Wood and Mize-Spansky (2009). The dark-grayshadowed area highlights the 3-D seismicdata that have been incorporated into this work and where fluvial and deltaic sequences were briefly documented by Maher (2007) andAlvarez (2008). Lines X and Y represent the location of two regional seismic lines (line drawings) that are shown in Figure 4.



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Figure 2. (A) Composite northeast-southwestoriented three-dimensional (3-D)seismic line along dip within the northern structural domain, showing the shallowstructural deformation associated with the transpressive forces related to the SouthAmericaCaribbean plate boundary zone. Notice the relative high associated with theDarien Ridge and the paleocanyon incision located to the west. (B) Compositenortheast-southwestoriented 3-D seismic line along dip within the southern struc-tural domain, showing the character of the growth and counterregional fault province.Notice the sea-floor displacement associated with one of the counterregional faultsnear the shelf break. The 3-D seismic lines also show the outer shelf to upper slopetransition, and the data coverage associated with the 3-D seismic megamerge isshown in the inset map. The red and yellow interpreted horizons on the seismic linescorrespond to the P10-SB1 and P4-SB2 sequence boundaries, respectively. The mapdisplayed within the inset map corresponds to the P10-SB1 horizon. TWTT = two-waytraveltime.

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Figure 3. (A) Composite northwest-southeastoriented 3-D seismic line along strike, showing the shelf-edge region in eastern offshore Trlocation). Three main horizons were mapped across the study area to reconstruct the stratigraphic architecture of the basin. From base tofloor. (B) Line drawing from 3-D seismic line above, showing seismic facies (SF) distribution and sequence-stratigraphic interpretation. Thesequence-stratigraphic units significantly vary from south to north; these variations are thought to be associated with underlying structural across the margin. Terms in quotation marks indicate that these units and surfaces do not present the typical seismic characteristics definhowever, these intervals are time equivalent to units to the south that fit the traditional sequence-stratigraphic definition for key surfaces anLST = lowstand systems tract; HST = highstand systems tract; TST = transgressive systems tract; mfs = maximum flooding surface; lw =

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in the southern growth-fault domain (Trinidad-Venezuela border; Figure 2B). We also integrateour interpretation of the time-equivalent units tothe north (Darien Ridge area), where transpres-sional deformation was dominant at the time ofdeposition (Figures 2A, 3). In addition, this work

seeks to characterize the association between theshelf-margin dynamic during the last maximumglacial lowstand in the eastern offshore Trinidadcontinental margin and its impact on definingthe location of downslope sediment fairways anddepocenters.

DATA SETS AND METHODS

The primary data set that was used to conduct thisstudy of the eastern offshore Trinidad continentalmargin during the last maximum glacial lowstandperiod was 3-D seismic reflection data. The avail-able 3-D seismic coverage was large enough tocover most of the shelf, a 100-km (62-mi) extentof the shelf-margin region, and the upper part ofthe slope in the eastern offshore Trinidad area(Figures 1, 2). Figure 1 shows the extension of the3-D seismic data set within the study area and

how the overlapping of the seismic surveys pro-vides a continuous lateral coverage of the shelf-edge region. Six separate 3-D seismic volumes wereavailable and merged into a single continuous vol-ume covering 15,000 km2 (5792 mi2). Imagingdepths vary between individual surveys but rangefrom a minimum of 1 s two-way traveltime (TWTT)in the westernmost part of the study area (shelf) toa maximum of 5 s TWTT of coverage in the deep-water blocks (Figure 2). Bin spacing of the time-

migrated merged 3-D volume is 25 12.5 m (82 41 ft), and the vertical sample rate is 4 ms. Three-dimensional seismic volumes were processed tozero phase, and the average frequency content ofthe full-stack seismic data was 30 to 35 Hz. Ap-proximate time-depth conversion at shallow depthis 1 ms equivalent to 0.75 m. We also had access to325 km (202 mi) of 2-D seismic reflection data(GULFREX) collected by Gulf Oil Company in1974, which have a 4-ms vertical sample rate and

a total data record of 18 s (Figure 4). The 3-Dseismic volumes were used to map detailed archi-tecture of the last glacial lowstand shelf margin(Figures 3, 5), whereas the regional 2-D seismiclines were used to provide an overview of the mainstructural elements in the study area (Figure 4).

Because of confidentiality issues and data-releaseconstraints for publication, the 2-D seismic linesfrom the vintage GULFREX data are presentedin the form of line drawings, and only the first 8 sof data are included in the interpretation becauseof the poor quality of the seismic image at greaterdepths (Figure 4). Several exploratory wells havebeen drilled in the study area, but no available logsor core penetrating within the interval of interestexist because of technical reasons and because this

shallow section has no commercial interest for theoil industry.Conventional seismic interpretation methods,

including seismic facies descriptions and map-ping (Figures 57), were used to generate the keystratigraphic surfaces that define the gross archi-tecture of the continental-margin stratigraphicsuccession of eastern offshore Trinidad (Figure 8).Seismic interpretation techniques included man-ual and automatic picking of key amplitude ho-rizons, interpolation, and merging of horizons to

generate final versions of key stratigraphic surfaces(Figures 7, 8). Extraction of a variety of seismicattributes was also performed to produce seismicattribute maps and visualizations (Figure 9). Seis-mic data were interpreted using Linux worksta-tions and Landmark subsurface interpretation soft-ware (Seisworks and Geoprobe). Structure mapson key stratigraphic horizons and interval isopachmaps of the stratigraphic units were generated todefine the gross architecture of the shelf-margin

region and to assess its sedimentary-fill history(Figures 8, 10, 11). Sequence-stratigraphic unitswere defined using seismic stratigraphic criteria(reflection terminations and geometries) (Vail et al.,1977; Posamentier and Allen, 1999) and integra-tion of the observations from previous workers inthe study area (Figures 3, 5) (Brami et al., 2000;

Wood, 2000; Sydow et al., 2003; Moscardelli et al.,2006; Maher, 2007; Alvarez, 2008; Moscardelliand Wood, 2008).



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Figure 4. (A) Line drawing of a regional southwest-northeast two-dimensional (2-D) seismic line located within the northern structural elements associated with the transpressive forces that affected the north margin of the basin. Notice the structural high associated with the ehow it influences the geometry of the shallow stratigraphic units (light-gray shadow). (B) Line drawing of a regional southwest-northeast structural domain that shows the character of the growth and counterregional faults located in the southern parts of the basin. The light-grepresents the stratigraphic interval under study (the base of this interval is equivalent to the P10-SB1 sequence boundary). The outlinesoverlap between the composite three-dimensional seismic lines shown in Figure 2 and the line drawings of the regional 2-D seismic ltraveltime.

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Figure 5. Three-dimensional seismic lines taken from four different locations along the eastern offshore Trinidad continental marginand line drawings showing seismic facies (SF) distribution and sequence-stratigraphic interpretation (see index map for location). Noticethe variation along strike within the different seismic transects in terms of stratigraphic architectures and seismic facies distribution foreach of the time-equivalent systems tracts. (A) Transect A along the main axis of the paleocanyon located in the northern structuraldomain (erosional shelf margin). (B) Transect B located to the south of the outlet of the Columbus sedimentary pathway, showing activegrowth faults within the southern structural domain (SSD) (accretionary shelf margin). (C) Transect C showing active growth andcounterregional faults that form growth-fault sediment traps in the SSD (accretionary shelf margin). (D) Transect D taken across thesouthern bypass zone and showing higher density of faults and steeper slopes within the SSD (erosional shelf margin). TWTT = two-waytraveltime; LST = lowstand systems tract; HST = highstand systems tract; mfs = maximum flooding surface; lw = lowstand wedge; bff =basin-floor fan; sf = slope fans; LCC = levee-channel complex; MTC = mass-transport complex; TS = transgressive surface.



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Figure 6. Seismic facies (SF) descriptions and depositional environment interpretation.

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Figure 7. Key stratigraphicsurfaces mapped across thestudy area. (A) Sea-floor surface:

the slope values of the sea floorincrease from proximal to distal(shelf to slope direction) andincrease from south to north asthe South AmericaCaribbeanplate boundary zone is ap-proached. Water depths rangefrom 10 m (33 ft) in the shelfregion to 1932 m (6339 ft) in thedeep-water blocks. Lines X and Yindicate the location of the two-dimensional seismic line draw-

ings shown in Figure 4. (B) TheP4-SB2 sequence boundarypresents a 1400-m (4593-ft)vertical relief that ranges from aminimum of 100 m (328 ft) onthe shelf break area to a max-imum of 1500 m (4921 ft) on theslope area. This surface was in-terpreted by Maher (2007) asthe last glacial maximum low-stand surface in the shelf-breakarea in the southern structural

domain (SSD) (Brami et al., 2000;Maher, 2007) and as the base ofa regional levee-channel com-plex in the deep-water blocks(Brami et al., 2000; Moscardelliet al., 2006). (C) The P10-SB1sequence boundary was mappedas the base of mass-transportcomplex MTC_1 in the deep-water blocks (Moscardelli et al.,2006). This surface is equivalentto the TQ85 surface in the shelf-

break region (SSD) (Maher, 2007)and to the top D horizon in theshelf (Alvarez, 2008). The P10-SB1 surface presents a 1540-m(5052-ft) vertical relief that rangesfrom a minimum of 120 m(394 ft) in the shelf to a maximumof 1660 m (5447 ft) in the slope.TWTT = two-way traveltime.



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Figure 8. (A) Isopach map (in two-way traveltime [TWTT]) between the P10-SB1 surface and the sea floor. (B) Sketch highlighting themain structural and geomorphological elements in the study area. The shelf region is dominated by northwest-southeastorientedgrowth faults and northeast-southwestoriented folds associated with transpressive forces. The Columbus sedimentary pathway is themain sedimentary conduit within the shelf region, and it was the main feeder of sediments for the slope area within the northernstructural domain (NSD). Structural elements within the shelf-edge region include northwest-southeast oriented growth and counter-regional faults and semigraben structures located to the north that are related to the eastern termination of the Darien Ridge. Bypasszones in the shelf-edge region are defined to the north (NSD) by a paleocanyon and by the outlet of the Columbus sedimentary pathwayand to the south (southern structural domain [SSD]) by the southern termination of a growth-fault sediment trap. The depocenters arenortheast-southwestelongated minibasins that are flanked to the north and south by mud-volcano ridges that are associated with deep-rooted transpressive faults.



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Figure 9. Visualization of three-dimensional seismic subvolume within the paleocanyon area in the northern structural domain. Crosssection aa highlights the interval of interest and surfaces that are shown in the visualizations. (A) The image shows a perspective of

sequence boundary P10-SB1 from the southeast (looking updip). Notice the location of the paleocanyon axis and paleoshelf break. Theboundaries of the paleocanyon coincide with a counterregional fault that lies against the Darien Ridge (eastern boundary) and a growthfault located on the shelf break (western boundary). (B) Same view as in A, but the figure also shows a surface that represents aclinoform that was mapped within the paleocanyon area (see seismic section for reference). Notice that the clinoform areal extension islimited by the lateral boundaries of the paleocanyon. (C) A root-mean-square (RMS) amplitude extraction using the clinoform surface asa guide horizon was generated and superimposed on top of the original clinoform surface. Notice the high amplitudes (green, red, andyellow) in the topset part of the clinoform. These features have been interpreted as mouth-bar deposits associated with a paleoshelf-edgedelta lobe. There are also moderate to high amplitudes distributed as an elongated body in the foreset part of the clinoform that havebeen interpreted as gravity-induced flows. (D) Same view as in A, but a semblance extraction was generated using the clinoform surfaceas a guide horizon. The red colors correspond to areas of dissimilarities within the seismic volume that expose the presence of small-scalefaulting within the topset part of the clinoform (faults are constrained to the clinoform package) and collapses of the upper parts of theclinoform foreset near the rollover area.



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Figure 10. (A) Isopach map (in time)of lowstand systems tract 2 (LST2),equivalent to the last maximum glaciallowstand paleo-Orinoco shelf-edge delta(the map is constrained to the southernstructural domain). The map shows theinfluence that growth and counter-regional faults located in the shelf-edge

region have in defining sediment trapswithin the outer-shelf and upper sloperegion, as well as the location of sedi-ment bypass zones. (B) Transect Cshowing a cross-sectional view of thegrowth-fault sediment trap. (C) TransectD located to the south end of thegrowth-fault sediment trap and showingan increase in the steepness of the slopeangles (associated with a more intensefaulting system) that favored the bypassof sediments to downslope locations.

TWTT = two-way traveltime; HST =highstand systems tract; TST = trans-gressive systems tract; mfs = maximumflooding surface; lw = lowstand wedge;sf = slope fans; RMS = root meansquare.



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Figure 11. (A) Structural map of the P4-SB2 sequence boundary (base LST2 unit) in the northern structural domain (NSD). Notice thatthe Darien Ridge (horst) still influences the morphology of the shelf-break region for this interval. (B) Isopach map (in two-way traveltime[TWTT]) of lowstand systems tract 2, equivalent to last maximum glacial lowstand paleo-Orinoco shelf-edge delta (NSD). Thicker intervalsare concentrated in the paleocanyon area, where accommodation was greater. The isopach map also shows a sinuous channel as-sociated with a levee-channel complex that connects with the paleocanyon area. (C) Root-mean-square amplitude extraction map taken20 ms above the P4-SB2 surface. The map shows what seems to be mouth-bar deposits near the shelf break and a series of gullies thatwere transporting sediments downslope. (D) Line drawing of transect A across the paleocanyon axis.



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REGIONAL STRUCTURAL SETTING

Extensive 2-D and 3-D seismic reflection data ac-quired over the eastern offshore Trinidad regionenable the detailed description of the main struc-tural elements that are present in the study area,

revealing the complexities associated with a va-riety of structural styles that coexist along thismargin (Figures 2, 4) (Wood, 2000; Sydow et al.,2003; Sullivan et al., 2004; Moscardelli et al., 2006;Garciacaro et al., 2011). Two distinctive structuraldomains were defined in this work: the northernstructural domain (NSD), where transpressionaltectonics associated with the CPBZ are the domi-nant structural deformational driver (Figures 4A,8), and the southern structural domain (SSD),

where gravitational tectonics controls the forma-tion and arrangement of growth and counterre-gional faults along the southern part of the shelfmargin (Figures 4B, 8). Proper characterization ofgeometric arrangements associated with individualstructural elements in each of these structural do-mains is crucial to understanding the remarkablearchitectural variations that are observed within theshelf-edge stratigraphic succession along the east-ern continental margin of Trinidad. Each structuraldomain is composed of distinctive fault patterns

that have the capacity to control stratigraphic var-iations along strike and influence the final geom-etry of deposits along the shelf break. Moreover,the underlying structural configuration along themargin also influences the location of sedimentbypass zones through which sediments are fun-neled downslope.

Northern Structural Domain

The Darien Ridge bounds the northern edge ofwhat we consider to be the NSD. This ridge is anortheast-southwest narrow zone (20 km [12.43mi] wide) of uplift that defines the northern bound-ary of the Neogene Columbus Basin (Figures 1,2A, 3, 4A, 8). The Darien Ridge is composed of0.5 to 4.5 km (0.32.8 mi) of thick, folded, andthrusted Cretaceous and lower Tertiary carbonatesand clastic rocks that were deformed during theMiocene (Wood, 2000; Garciacaro et al., 2011).

This ridge exhibits a significant sea-floor relief insome areas (20140 m [66459 ft]), and it repre-sents the eastern offshore continuation of TrinidadsCentral Range (Wood, 2000). The Darien Ridgealso defines the present-day plate boundary zonebetween the South American and the Caribbean

plates. The high-angle thrusts associated with plateboundary transpression cut through Cretaceous andearly Pliocene rocks generating the steep walls (1224) of the Darien Ridge (Figure 4A) (Garciacaroet al., 2011). The presence of the uplifted ridgeconfined sedimentation from the shelf to the slopeto areas immediately southwest of the uplift(Figures 4A, 8). This geography promoted growthalong the southwest-facing counterregional fault(Figure 4A), contributing to the formation of a

Pliocene-Pleistocene incised paleocanyon to thesouthwest of the main structure (Figures 4A, 7C,8, 9) (Moscardelli et al., 2006). The NSD was de-scribed by Wood (2000) and Garciacaro et al.(2011) as an area dominated by northeast-strikingthrust faults and associated folds. Thrusts associ-ated with the Darien Ridge are the oldest faultfamily in the Columbus Basin (middle Miocene),although transpressive forces were still activeduring the Pliocene-Pleistocene. The age of thesenorthern structures relative to those developing in

the more southern areas of the basin allows forlonger and larger growth and the generation ofhigher structural relief, creating sea-floor featuresinthe NSDup to 200 m (656 ft) high. Figure 4A isa line drawing of one of the northeast-southwestregional 2-D seismic lines that were acquired overthe Darien Ridge. This line shows the main struc-tural elements associated with the NSD: (1) northwest-southeastoriented regional growth faults and coun-terregional faults to the west of the Darien Ridge

associated with big rollover structures, which arePliocene-Pleistocene in age (Heppard et al., 1998;

Wood, 2000; Boettcher et al., 2003); (2) northeast-southwestoriented, high-angle thrusts arranged ina flowerlike structure that forms the core of thestructural high associated with the Darien Ridge(Garciacaro et al., 2011); (3) tothe northeastern partof the line drawing (slope region), a series of high-angle normal faults associated with the flanks ofnortheast-southwestoriented regional mud-volcano



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ridges can also be observed (Sullivan et al., 2004;Moscardelli and Wood, 2008); and (4) northeast-southwest high-angle transpressive faults that aregenetically linked to the core of the regional mud-volcano ridges (middle Miocene in age) (Moscardelliand Wood, 2008). Mud-volcano ridges located in

the slope area define structural and bathymetrichighs that flank deep-water depocenters, wheresediments that are bypassed through the shelf breakaccumulate (Figures 4A, 8).

The geometric arrangement of the differentstructural elements that are present in the NSD hasa noticeable influence on the architecture of theshelf-margin stratigraphic succession. The presenceand evolution of the regional basinward-dippinggrowth faults along the eastern offshore Trinidad

continental margin, as well as the geometry of theassociated rollover anticlines, define oversteepenedareas that ultimately influence the location ofthe shelf break. In addition, the positioning of aregional, landward-dipping counterregional faultagainst the western flank of the Darien Ridgefacilitated accentuation of a structural low thatfavored the incision of a paleocanyon (Figures 4A,9). This paleocanyon system started to developduring the Pliocene-Pleistocene and continued tofunnel sediments downslope during the last max-

imum glacial lowstand period (Figures 4A, 9;Moscardelli et al., 2006). Within the NSD, theintensity of the transpressional deformation andassociated structures is more noticeable than inthe SSD. The proximity to the CPBZ favored theformation of a higher structural relief in this region(NSD). The geometric arrangements that devel-oped within the NSD between transpressive struc-tures such as the Darien Ridge and growth faultslocated in the shelf break region promoted the

formation of canyons that did not develop withinthe time equivalent section to the south (SSD)(Figures 4, 9).

Southern Structural Domain

The main structural elements along the southerncontinental margin of eastern offshore Trinidad area series of Pliocene-Pleistocene northwest-southeastoriented regional growth faults and counterregional

faults and associated rollover anticlines (Figure 4B).These structures are equivalent to the growth faultsthat can be observed to the west of the DarienRidge on the NSD (Figure 8). The rollover anti-clines in the SSD present a higher degree of com-partmentalization than do equivalent structures

to the north, and a series of secondary faults dis-secting the crest of the anticlines can be observed(Figures 4, 5D). Similar to the NSD, these growth-fault structures are Pliocene-Pleistocene in age, andthey were an important factor in constrainingthe position of the shelf break through time. Inthis part of the Columbus Basin, subsidence in thehanging wall of growth faults is the dominant ac-commodation process and dense faulting in theshelf-break region significantly obscures margin

trajectories (Sydow et al., 2003). The presence oflandward-dipping counterregional faults in someareas of the shelf break favored sequestration ofsediments in the upper slope region (Figures 4B, 5C,8, 10). At the same time, in areas of the shelf breakwhere only basinward-dipping growth faults werepresent, sediments were able to bypass directlyinto deep-water depocenters (Figures 5D, 8, 10).

In the deep-water parts of the basin, a series ofhigh-angle transpressive faults can be interpretedfrom the 2-D seismic lines; however, the intensity

and structural relief associated with these trans-pressive structures are not as abrupt as those in theNSD (Figure 4). Transpressive faults within thedeep-water parts of the basin are commonly asso-ciated with regional mud-volcano trains and buriedanticlines (Figures 4B, 8) whose crestal trends canbe traced southwestward into the modern shelfwhere they form significant hydrocarbon traptrends (Figure 8). In the deep-water parts of thebasin, these anticlines have thus far proven to

have limited liquid hydrocarbons associated withthem, instead they have sea-floor expression in theform of mud volcanoes, which might be responsi-ble for hydrocarbon escape (Figure 8). These mud-volcano trains and buried anticlines define the lateralboundaries of minibasins that act as key pathwaysand depocenters for sediments delivered from theupper slope region (Figure 8) (Moscardelli and

Wood, 2008). Ridges associated with the mud-volcano trains decrease in height and steepness



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to the south, as transpressive deformation forcesassociated with the CPBZ diminish southward(Figure 4B).

DESCRIPTION OF KEY STRATIGRAPHICSURFACES IN THE EASTERN OFFSHOREAREA OF TRINIDAD

Two regional horizons and the sea floor weremapped and correlated across the shelf and intothe slope and deep-marine basin (Figures 2, 3, 5,7). Numerous workers have mapped the differentparts of these horizons during the last 10 yr (seeTable 1) (Sullivan et al., 2004; Moscardelli et al.,

2006; Maher, 2007; Alvarez, 2008; Moscardelliand Wood, 2008); however, final mapping of thesesurfaces required a complete revision of previ-ous seismic interpretations over a 15,000-km2

(5792-mi2) area. In this article, we standardizethe nomenclature according to Moscardelli and

Wood (2008) so that the merged key regional sur-faces are, from oldest to youngest, P10, P4, andthe sea floor (Figure 7) (Table 1). The P10 hori-zon (Figure 7C) was originally mapped as the

base of a regional mass-transport deposit (MTD;MTC_1) that was identified in the deep-waterblocks of the study area and correlated updip tothe northern parts of the shelf margin (NSD) (Bramiet al., 2000; Moscardelli et al., 2006; Moscardelliand Wood, 2008; see figure 5 of Moscardelli et al.,2006). Maher (2007) extended the mapping ofthe P10 horizon in the southern part of the shelfmargin on the basis of seismic correlations. In theshelfal part of the basin (west), the P10 horizon

correlated to the top D surface as interpreted byAlvarez (2008) but additional interpretation wasnecessary to merge these surfaces (Figure 7C).The P4 horizon represents the base of a regionallevee-channel complex (LCC) that overlies MTC_1(Figures 2, 7B). Poor seismic imaging in some areasrestricted interpretation of the P4 horizon to thewestern (shelf) and to the southern parts of thedeep-water blocks, although most of the area of in-terest within the shelf-break region was well rep-resented in the data and final map (Figure 7B).

Sea-Floor Surface

In general, the slope of the sea floor decreases from

the NSD near the plate boundary zone to the SSDwhere growth faults are the dominant structuralfeatures. Water depths in thestudy area range fromaminimumof10m(33ft)intheshelfregiontothewest to maximum depths of 1932 m (6339 ft) inthe deep-water region to the east (Figure 7A).Slope values associated with the sea floor in theshelf region tend to be low over most of the areaand have increasing dips toward the east near theshelf break. The low-amplitude values associated

with the sea-floor reflector in the shelf region do notallow for good recognition of geomorphologicalelements; however, small channels can be identi-fied in some areas (Alvarez, 2008). In the southernpart of the modern shelf break within the SSD,active basinward-dipping growth faults and asso-ciated counterregional faults break through themodern sea floor, generating vertical displacementsthat can reach up to 48 m (157 ft; Figure 5C, D)(Maher, 2007). A low-sinuosity, east-westtrending

Table 1. Previous Mapping of Key Surface in the Study Area

Horizon* Shelf Shelf Break (North) Shelf Break (South) Slope

Sea floor Sullivan et al. (2004) Sullivan et al. (2004) Sullivan et al. (2004) Sullivan et al. (2004)

P4 (base LCC)* Poor seismic imaging Moscardelli et al. (2006) Maher (2007) Moscardelli et al. (2006),

Wood and Mize-Spansky (2009)

P10 (base MTC)* Alvarez (2008) Moscardelli et al. (2006),

Moscardelli and Wood(2008)

Maher (2007) Brami et al . (2000),

Moscardelli et al. (2006),Moscardelli and Wood (2008)

*The P4 and P10 horizons were remapped as necessary and merged to generate the continuous surfaces that are shown in this work. LCC = levee-channel complex; MTC =

mass-transport complex.



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incised channel located to the east of a majorcounterregional fault within the SSD that appearsto be transferring sediments from the west of thecounterregional fault into deeper slope regions ex-ists (Figure 7B) (Maher, 2007).

Basinward of the shelf-edge region, the slope of

the sea floor progressively increases from west to east(shelf break to upper slope transition; Figure 7A).Localized increases in slope angles also occur aroundindividual mud volcanoes and mud-volcano ridgesin the deep-water blocks(Figure 7A) (Sullivan et al.,2004; Wood and Mize-Spansky, 2009). Sullivanet al. (2004) reported the presence of 161 mudvolcanoes in the deep-water blocks of eastern off-shore Trinidad that have a clear sea-floor expres-sion. Flanks of individual mud volcanoes can reach

maximum slope angles of 12 and total heightsof 76 m (249 ft) (Sullivan et al., 2004). Mud vol-canoes are important components of the sea-floorgeomorphology in the slope area because these fea-tures have the capacity to act as bathymetric bar-riers that can control sediment transport and dis-tribution in the deep-water region (Sullivan et al.,2004; Moscardelli et al., 2006; Wood and Mize-Spansky, 2009). Wood and Mize-Spansky (2009)also showed the most recent channel activity affect-ing the nearsea-floor stratigraphic interval within

the deep-water blocks of the study area (see theirfigure 2).

P4 Horizon

The P4 horizon defines the base of a well-imagedLCC and the top of an MTD (MTC_1) in thedeep-water blocks of the study area. This surfacewas also mapped to the south (SSD) in the shelf-

margin region, and it correlated with the last gla-cial maximum lowstand surface, as interpreted bySydow et al. (2003) and Maher (2007) (Figure 7B).The P4 horizon presents a 1400-m (4593-ft) ver-tical relief that ranges from a minimum of 100 m(328 ft) in the shelf-break area to the west to amaximum of 1500 m (4921 ft) in the deep-waterblocks to the east (Figure 7B). The P4 horizon isrecognizable in the southern part of the shelf-edgeregion (SSD) because it truncates an underlying

progradational package (Figure 5C, D), and to thenorth, the overlying unit onlaps against this sur-face (Figure 5A, B). One of the most notable fea-tures of the P4 horizon in the SSD is the presenceof a northwest-southeastelongated trough imme-diately basinward of the shelf break (Figures 5C,

10). This trough is formed by the coalescence ofa basinward-dipping listric growth fault and acounterregional fault, forming a growth-fault sed-iment trap near the shelf break (Figure 10; Maher,2007). Also, several straight channels originateon the western side of the counterregional fault inthe slope region and incise the P4 horizon (Maher,2007).

In the northern part of the shelf edge (NSD),the architecture of the P4 horizon is dominated

by an older paleocanyon (Figures 7B, 11). Trun-cations of older stratigraphic successions and on-lapping against the P4 horizon in the NSD arecommon and easily visible features (Figure 5A).In the NSD, the P4 horizon also shows verticaldisplacement in a landward-dipping counterre-gional fault that dies out near the Darien Ridge(Figure 11A). However, contrary to what was re-ported by Maher (2007) in the SSD, this coun-terregional fault did not generate a growth-faultsediment trap, which is evident from examining

the isopach map shown in Figure 11B. Severalhigh-sinuosity channels also originate on the west-ern side of this counterregional fault in the slopearea and connect to a well-developed LCC down-dip (Figure 11) (Brami et al., 2000; Moscardelliet al., 2006; Wood and Mize-Spansky, 2009). Ba-sinward, the P4 horizon is represented by a high-amplitude continuous reflector that separates thelow-amplitude chaotic facies associated with anunderlying MTD (MTC_1) from the overlying

high-amplitude continuous seismic reflectors asso-ciated with the LCC (Figure 7B; also see figure 3of Moscardelli et al., 2006).

P10 Horizon

Figure 7C depicts a structural map of the P10 ho-rizon. In the shelf region, the P10 horizon has var-iable reflection-amplitude strength and continuity



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and is systematically offset by a series of northwest-southeastoriented growth faults (Figure 7C)(Alvarez, 2008). In some areas near the shelf breakto the southeast (SSD), the seismic reflector equiv-alent to the P10 horizon becomes slightly discon-tinuous because of seismic data distortion caused

by the presence of shallow gas (Figure 2; Maher,2007). On the shelf, the P10 horizon is on topof a seismic package that generally presents con-cordant reflections, although some truncationscan be seen in the Coralita area to the southeast,suggesting that the P10 horizon is erosive nearthe shelf break (Figure 5D). Onlap relationshipsagainst the P10 horizon have also been docu-mented to the north on the outer shelf near theDarien Ridge (NSD) and to the south in the

Southeast Galeota (SEG) area (SSD; Figure 5).Maher (2007) reported that the P10 horizon isheavily cut by faults and shows post-formationsubsidence near the paleoshelf edge in the south-eastern part of the study area (SSD; Figure 5C, D).The P10 horizon presents a 1540-m (5052-ft) ver-tical relief that ranges from a minimum of 120 m(394 ft) on the shelf to a maximum of 1660 m(5446 ft) in the deep-water blocks (Figure 7C).One of the most important geomorphological ele-ments associated with this surface in the shelf re-

gion is a northeast-southwestoriented depressionthat we refer to as the Columbus sedimentarypathway (Figures 7C, 8). This sedimentary conduitwas structurally confined by northeast-southwesttrending anticlines associated with the DarienRidge to the north and to the Southeast Galeota(SEG) high to the south (Figures 7C, 8) (Alvarez,2008). The Columbus sedimentary pathway reachedthe shelf margin, where the throw on down-to-the-east extensional faults allowed for the forma-

tion of an outlet in the shelf break that connectedto the upper slope region (Figures 3, 7C). Theoutlet of the Columbus sedimentary pathway islocated in the area that defines the boundary be-tween the SSD and the NSD, and it representedone of the main sedimentary bypass zones in theshelf-margin region (Figure 8).

In the northern part of the shelf margin nearthe Darien Ridge (NSD), and in the deep-waterblocks to the east, the P10 horizon is equivalent to

the base of a regional MTD (MTC_1) (Figure 7C;Moscardelli et al., 2006). In these regions, the P10horizon is an extensive and irregular erosionalsurface that, on the outer shelf (near the DarienRidge), correlates to the base of a southeast-northwesttrending paleocanyon that is deeply

incised (Figures 7C, 8, 9). This paleocanyon isapproximately 2 km (1 mi) wide on its updipend, but it widens downslope, reaching more than10 km (>6.2 mi) on its lower end, and it is boundedalong both margins by normal faults forming astructural depression that is greater than 500 m(1640 ft) deep (Figure 9A) (Moscardelli et al.,2006). In the deep-water blocks, the P10 hori-zon is represented by a high-amplitude reflectorthat shows steep erosional edges that can reach

250 m (820 ft) in relief and elongated linear scoursthat are greater than 30 m (>98 ft) deep. Addi-tional erosional morphologies that can be observedin the deep-water blocks and that are associatedwith the P10 horizon include deep erosional es-carpments and basal megascours (Moscardelli et al.,2006). The slope of the P10 horizon varies acrossthe depositional dip, becoming steeper on the outer-shelf and upper slope transition and in the paleo-canyon area to the north.

DESCRIPTION OF SEISMIC FACIES ANDDEPOSITIONAL ENVIRONMENTS

Seismic Facies 1: Steep Clinoform Package

Intervals containing seismic facies 1 (SF1) are con-strained mainly to areas in the SSD. This seis-mic facies is composed of steep clinoform pack-ages in the outer-shelf region near the shelf break

(Figures 5, 6). We adopt the classic definition of aclinoform as a depositional shape characterized indip section by a gently sloping topset, a steeplysloping foreset, and a gently sloping bottomset(Rich, 1951; Pirmez et al., 1998). The height ofSF1 clinoforms is always greater than 10 m (>33 ft),but it rarely exceeds the 200-m (656-ft) mark(Figures 5, 6). The topset parts of the clinoformsare characterized by low-angle, high-amplitude,and relatively continuous reflectors; however, in



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Figure 12. (A) Structural map of the P4-SB2 sequence boundary (base lowstand systems tract 2 [LST2] unit) in the southern structuraldomain. Notice the location of shelf break with respect to growth and counterregional faults. (B) Root-mean-square amplitude extractionmap generated in a 40-ms window below the P4-SB2 surface and targeting the upper part of highstand systems tract 1 (HST1). Notice theclear channelized features in the southern part of the outer shelf and the change in seismic character in the upper slope region, wheredownslope mass movement (slumps) seems to be the dominant process. (C) A blown-up view of the southern part of the amplitudeextraction map showing the character of the distributary channel system and mouth-bar deposits. The geometry of the channelizedfeatures suggests tidal influence in this part of the delta. Moreover, high amplitudes in the upper slope highlight the location of thegrowth-fault sediment trap and how sediments progressively manage to bypass as they approached the southern termination of thecounterregional fault. (D) Transect D across the bypass zone. (E) Transect C across the growth-fault sediment trap region. TWTT = two-way traveltime.



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some areas, the topsets are incised by channel-like features within which seismic reflections pre-sent a low-amplitude and discontinuous character(Figure 6). The clinoform foresets are steep (5)and present medium- to low-amplitude seismicreflections that sometimes mask their geometry

(Figure 5B). The bottomset parts of the clinoformsare difficult to map in this package because aseismic multiple significantly perturbs the imagingof this particular interval (Figure 5C). However,some areas exist where downlapping of the clino-forms against the underlying unit is better exposed(Figure 5B). The rollover points of individual clino-forms within this unit do not seem to surpass theouter-shelf region, and the overall rollover trajec-tory of individual clinoforms suggests a mostly

progradational to slightly aggradational trajectory(Figure 5B, C). This seismic facies is equivalent towhat was described by Sydow et al. (2003) as hor-izontal topset facies and steep sigmoid oblique fa-cies (foreset part of the clinoforms).

Depositional Environment Seismic Facies 1:Outer-Shelf Delta

Seismic facies 1 has been interpreted as an outer-shelf delta associated with the northeasterly mi-

gration of the paleo-Orinoco delta system. Thetopset incisions within the clinoforms reveal thepresence of channels in the shoreface part of the delta(Figures 6, 12). Amplitude-extraction maps, ob-tained in the SSD near the paleoshelf break, showthat channelized features within the SF1 clino-form topsets have a straight geometry (low sin-uosity) and their trajectories tend to be perpen-dicular to the shelf break (Figure 12). The lowsinuosities observed within the channels and their

geometric arrangements with respect to the shelfbreak suggest that the deltaic system was influencedby the action of tides. Moreover, the geomorpho-logical characteristics observed in the SF1 topsetchannels are remarkably similar to the present-daymorphology that is observed within the southern,tidally dominated part of the Orinoco delta system.Note that, despite these similarities, the SF1 cli-noforms were part of an outer-shelf delta that wasin close proximity to the shelf break, whereas the

modern Orinoco delta is an inner-shelf deltaicsystem. This is a relevant point because it indi-cates that tidal processes can also operate in deltaicsystems that are in close proximity to the shelfbreak and therefore might be an important factor,affecting sediment transfer toward deep-water lo-

cations. In fact, seismic reflection discontinuitiesthat can be observed within the foreset part ofthe SF1 clinoforms on the same time-equivalentamplitude-extraction map (Figure 12C) show thatsediment gravity flows (debris flows and turbi-dites) were of common occurrence in the delta-front part of the system. The areal coverage pro-vided by 3-D seismic data allowed us to observethe direct link between tidally influenced channelmorphologies in the topset part of the clinoforms

and the occurrence of sediment gravity flows inthe corresponding foresets (Figure 12). The progra-dational pattern, architecture, and height (


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to be strongly progradational to aggradational and,commonly, the rollover parts of individual clino-forms surpass the location of the previous shelfbreak basinward (Figure 5). It is also common toobserve slides affecting the upper part of the cli-noform foresets when the rollover trajectory sur-

passes the shelf break (Figures 5A, 9).

Depositional Environment Seismic Facies 2:Shelf-Edge Delta

Clinoform heights reported within SF2 always ex-ceed 200 m (656 ft). The scale of these clinoformsand the fact that their foresets always coincidewith the location of the continental slope stronglysuggest that these units were part of a shelf-edge

deltaic system associated with the paleo-Orinocodelta. Figure 5A is a dip line that was taken acrossthe main axis of the paleocanyon within the NSD.The seismic line shows that gravitational slidingaffected the topset-to-foreset transition (rolloverarea) of the SF2 clinoforms and that sedimentswere bypassed through the paleocanyon axis to-ward the deep-water depocenters (this system wasfeeding MTC_1, as described by Moscardelli et al.,2006) (Figure 9). The foresets of the SF2 clino-forms present high-amplitude continuous reflec-

tors, whereas the bottomsets transition to low-amplitude chaotic reflectors. This change in seismiccharacter along dip within single clinoform pack-ages is an indicator of downslope-flow transforma-tions from cohesive to more disaggregated flows(Mohrig et al., 1998). Transect A (Figures 5A, 9)clearly shows a series of slumps and slides af-fecting the rollovers of clinoform packages at thepaleocanyon head. As sediments were transporteddownslope, the system evolved into a regional MTD

(Moscardelli et al., 2006).Figure 9A shows the paleocanyon associated

with the P10 horizon in the NSD and an indi-vidual SF2 clinoform that was mapped withinthe paleocanyon area (Figure 9B). The lateral ex-tension (alongstrike) of the clinoform was con-trolled by the location of the paleocanyon walls(Figure 9A). An RMS amplitude-extraction wasobtained using the clinoform surface as a referencehorizon (Figure 9C); this map shows high-amplitude

reflections in the topset part of the clinoform thatare interpreted as mouth-bar deposits associatedwith a shelf-edge deltaic system. In the foresetpart of the clinoform (Figure 9C), high ampli-tudes that are aligned with the main axis of thepaleocanyon indicate gravitational flows (turbidites

or debris flows) that bypassed sediments to thedeep-water part of the basin. Figure 9D shows asemblance extraction that was obtained using thesame clinoform surface. In this visualization, seis-mic dissimilarities clearly highlight the location ofcollapse scars on the rollover part of the clinoform.The semblance map of the SF2 clinoform alsoshows small-scale faults affecting the topset part ofthe clinoform (Figure 9D). These faults are con-strained within the clinoform package and are not

connected to the deep-rooted structures associatedwith the Darien Ridge. These small-scale faults onthe topset of the clinoforms tend to define localizedsedimentary pathways that link shelfal sedimentswith point sources in the rollover areas of theclinoforms. The attribute-extraction maps do notshow any clear evidence of the presence of chan-nels within the topset part of the clinoforms(Figure 9). The difficulty in imaging distributarychannels suggests that the shoreface part of the del-taic system was most likely exposed to wave action

and storms that prevented the full preservation ofchannel-like geometries on the shoreface; however,it might also be an issue of seismic resolution.

Seismic facies 2 are equivalent to what wasdescribed by Sydow et al. (2003) as purely sigmoidclinoforms in the SSD; however, these authorsinterpreted the clinoforms as not having bypassedsignificant amounts of sediment basinward. De-spite this previous interpretation, seismic attributeextractions and isopach maps presented in this ar-

ticle (Figures 8, 9) clearly show that SF2 clinoformfacies were in fact responsible for the bypass of sed-iment toward deep-water depocenters (Figures 5A,D; 8, 9). Observations made by Sydow et al. (2003)are concentrated in a limited area of the shelf breakbetween transects B and C where bypass zoneswere absent (Figures 5, 8). Basinward thinning ofthe clinoform packages that can be appreciated inboth transects B and C (Figure 5) supports theinterpretation of sediment retention in the outer



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shelfupper slope region for that particular area(Sydow et al., 2003) (Figure 5). However, our ac-cess to a bigger data set allowed us to identify twobypass zones not previously identified by Sydowet al. (2003) along transects A and D, which caneasily be identified on the isopach map shown in

Figure 8. These areas along transects A and D,which coincide with zones of greater accommo-dation defined by underlying structures, are areaswhere SF2 clinoforms were able to bypass sedi-ments into the deep-water blocks (Figures 5, 8).

Seismic Facies 3: Onlapping Wedge

This facies is mostly present in the SSD, and it es-sentially forms a high-amplitude onlapping wedge

against the slope (Figures 5, 6). Internal reflectors,which exhibit high continuity and dip slightly to-ward the basin, are most likely caused by the localgradient at the time of deposition. In more updippositions, seismic facies 3 (SF3) is composed of athin package of high-amplitude continuous reflec-tors that can extend for several kilometers inland onthe shelf (Figure 5B). This facies is restricted to theSSD (Figure 3) and is not volumetrically significantwhen compared with SF1 and SF2. Thickness ofindividual packages containing SF3 facies can range

from a minimum of 75 m (246 ft) to a maximum of150 m (492 ft).

Depositional Environment Seismic Facies 3:Slope-Fan Deposits

The wedgelike geometry of SF3 facies and clearonlapping against the slope (Figure 5) suggest thatthis facies was most likely generated by muddyslope-fan deposits; however, the limited areal ex-

tension occupied by SF3 facies indicates that theslope fans were not well developed and that theywere constrained to the upper part of the slope(Figure 3). On the shelf, a small updip part ofthe slope wedge can be observed in some areas(Figure 5B) and, presumably, some channelizedfeatures might be present, although the thinnessof units containing SF3 facies on the shelf doesnot allow for the proper seismic imaging of prob-able geomorphological features in these areas.

Seismic Facies 4: Continuous and ParallelSeismic Reflectors

This facies is characterized by the presence ofhigh-amplitude, continuous, and parallel seismicreflectors that seem to be aggradational and within

which few lateral or vertical variations can be ob-served (Figures 5, 6). Seismic facies 4 (SF4) ismostly present in the SSD, where onlap termina-tions can be observed occurring against the troughof growth faults and against the P10 horizon(Figures 3, 5). Stratigraphic packages containingSF4 facies tend to have uniform thicknesses thataverage approximately 100 m (328 ft) but thatcan reach as much as 600 m (1969 ft) in some areas.The presence of a seismic multiple at the same ap-

proximate depth at which SF4 occurs makes itdifficult to properly image these intervals usingseismic attribute maps (Figure 5). In addition, thecontinuous character of the seismic reflectors withinthis facies does not provide much of a lateral con-trast for identification of stratigraphic boundariesthat could be associated with geomorphologicalfeatures.

Depositional Environment Seismic Facies 4:Prodelta to Upper Slope Deposits

This seismic facies is equivalent to the prodeltato upper slope facies described by Sydow et al.(2003). These authors reported the presence ofSF4 facies downdip and beneath the delta clino-forms and assigned all series of depositional archi-tecture to the SF4 occurrence (e.g., mass-transportcomplexes, confined nested channel complexes,and channel-levee systems). We agree that the SF4facies might contain prodelta deposits most likely

associated with fine-grained deposition occurringat the outer shelf and upper slope, where discretechannelized features and small-scale slumping mightoccur. However, given the observable seismic char-acteristics, big-scale, gravity-induced processes (e.g.,mass-transport complexes and channel-levee sys-tems) do not seem to be a dominant component oftheSF4 facies because chaotic and gull-wing seismicfacies are not present (Figures 5, 6). Alternatively,homogeneities associated with SF4 facies in terms



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of seismic reflectivity and the lack of internal ar-chitecture also suggest that sediments within theseunits could have been deposited as a product ofsuspended-sediment load. Alongshore and current-controlled processes could have also transportedsome of the sediments along the upper slope re-

gion, and strong currents could have reworked thesediments to the point that internal architectureswere not preserved within the unit.

Seismic Facies 5: Chaotic Seismic Facies

Chaotic facies, which start to become volumet-rically significant in the lower slope region, arecharacterized by the presence of low-amplitude,

semitransparent chaotic reflections that, in someregions, can transition to localized, low- and high-amplitude semicontinuous reflections (Figures 3,6). This particular seismic facies was extensivelydocumented in the study area by previous authorsrepresenting the main constituent of stratigraphicpackages that can reach more than 250-m (820-ft)thickness in the deep-water blocks (Brami et al.,2000; Moscardelli et al., 2006; Moscardelli and

Wood, 2008).

Depositional Environment Seismic Facies 5:Mass-Transport Deposits

This seismic facies has been associated with thedevelopment of MTDs (Moscardelli et al., 2006);the chaotic character of the seismic reflectors ismost likely related to the development of debrisflows. The more continuous, high-amplitude, andlocalized seismic responses are associated with semi-

coherent units that have been transported down-slope in the form of slides, slumps, or hydroplanedblocks. Internal variations within this seismic faciesare also indicative of flow transformations thatcan affect the level of disaggregation of the trans-ported sediments (Mohrig et al., 1998). Detaileddescriptions of these volumetrically important de-posits in the deep-water blocks were provided byBrami et al. (2000), Moscardelli et al. (2006), andMoscardelli and Wood (2008).

Seismic Facies 6: Gull-Wing Seismic Facies

Gull-wing seismic facies in the study area arecharacterized by high- and low-amplitude reflec-tors with variable continuity (Figures 3, 6). It iscommon to see within SF6 facies the interaction

between different gull-wing features that com-monly dissect one another vertically and laterally(see figure 3 of Moscardelli et al., 2006). Cleardevelopment of SF6 facies can be observed in thelower slope area, where the features commonlyare superimposed on SF5 facies. A detailed de-scription of gull-wing facies in the deep-waterblocks of eastern offshore Trinidad were providedby Moscardelli et al. (2006) and Wood and Mize-Spansky (2009).

Depositional Environment Seismic Facies 6:Levee-Channel Deposits

Gull-wing seismic facies have been associated withlevee-channel deposits (LCDs) in the deep-waterblocks of eastern offshore Trinidad (Brami et al.,2000; Moscardelli et al., 2006; Wood and Mize-Spansky, 2009). Levee-channel systems acted asconduits for sediments traveling from the sediment-staging area on the shelf edge to the basin floor

approximately 250 km (155 mi) to the east (Woodand Mize-Spansky, 2009). In the deep-water blocks,SF6 facies are commonly overlapping SF5 faciesthat are associated with MTDs (see figure 3 ofMoscardelli et al., 2006). Thickness of the LCDs(SF6) increases along the main axis of the under-lying MTDs (SF5), where the underfilled spacecreated by the previous erosion has been occupiedby the later leveed channels (Moscardelli et al.,2006).

SEQUENCE-STRATIGRAPHICINTERPRETATION: AN ATTEMPT

Sequence-Stratigraphic Surfaces

Sequence Boundary 1 (P10 Horizon)

In the SSD, a series of continuous, parallel, andhigh-amplitude reflectors (seismic facies SF4) that



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have been interpreted as prodelta to upper-slopedeposits overlie the P10 horizon (Figure 3). In theshelfal parts of transects B, C, and D (Figure 5),onlap terminations of SF4 facies can be observedagainst the P10 horizon. In addition, this horizon(P10) is highly erosional in the NSD (Figure 5A),

defining the base of an incised paleocanyon (notetruncation of underlying units) that is overlain byshelf-edge delta clinoforms (seismic facies SF2)(Figures 5A, 9). The P10 horizon maintains itshighly erosional character in the deep-water partsof the basin, where it defines the base of a regionalMTD (MTC_1; seismic facies SF5) (Moscardelliet al., 2006) (Figure 7C). The erosive character ofthe P10 horizon and its relationship with under-lying and overlying strata (onlap terminations and

truncation relationships) both in the NSD andSSD, makes this surface a good candidate for de-fining a sequence-stratigraphic boundary (P10-SB1; Figure 7C).

Maximum Flooding Surface 1

Maximum flooding surface 1 (MFS1) forms a high-amplitude continuous reflector that can be clearlyidentified in some areas within the SSD, but itsoccurrence is sometimes masked by the presenceof the sea-floor multiple (Sydow et al., 2003)

(Figures 3, 5). In the SSD, MFS1 can be identifiedas a downlap surface associated with overlyingSF1 clinoforms (Figure 5B, C). In the outlet of theColumbus sedimentary pathway, the correlativesurface associated with MFS1 lies above strati-graphic packages that have been interpreted asMTDs (seismic facies SF5; Figure 3). In the paleo-canyon area (NSD), the MFS1 surface correlateswith the upper boundary of a shelf-edge del-taic system (SF2 clinoform package); this surface

is then truncated by a younger erosional event(P4 horizon) to the north near the Darien Ridge(Figure 3). Mapping of the MFS1 surface to thenorth was sometimes hindered both by the oc-currence of the sea-floor multiple and by the poorquality of the 3-D seismic image (Figure 3). More-over, the abundance of MTDs affecting the outershelfupper slope region in the NSD made it dif-ficult to identify the MFS1 surface in this part ofthe continental margin (Figure 3). However, the

composite seismic line that was taken along de-positional strike in the shelf-margin region allowedus to establish a reasonable correlation for MFS1from the south (SSD) to the north (NSD; Figure 3).The use of conventional sequence-stratigraphiccriteria within the SSD (e.g., downlap surface)

allows for the interpretation of MFS1; however,the same characteristics are absent in the NSD(Figure 3). Interpretation of MFS1 in the NSD isonly possible by establishing a direct correlationusing direct mapping of the surface on a key seis-mic line across strike (see Figure 3).

Sequence Boundary 2 (P4 Horizon)

In the SSD, the P4 horizon clearly truncates the up-per part of outer-shelf deltaic facies (SF1), whereas

the overlying units show onlapping relationshipsagainst this horizon (Figures 3, 5). Transects B, C,and D show that an onlapping wedge (SF3 facies)lies on top of the P4 horizon in the slope part ofthe basin (Figure 5). At the same time, transectsC and D show onlapping relationships of SF2 fa-cies (shelf-edge deltaic systems) against the shelfalparts of the P4 horizon (Figure 5). The P4 horizonalso truncated units associated with MTDs (SF5)and shelf-edge deltaic facies (SF2) in the outlet ofthe Columbus sedimentary pathway and in the

paleocanyon area, respectively (Figure 3). In theNSD, the geometry of the P4 horizon continuesto be highly influenced by the underlying archi-tecture of the paleocanyon (Figures 7B, 11A). Inthe deep-water blocks, the P4 horizon definesthe base of an extensive levee-channel complex(SF6), and it also defines the top of MTD MTC_1(Moscardelli et al., 2006) (Figure 7B). The degreeof erosion of the P4 horizon on the shelf andupper part of the slope and its regionally exten-

sive character makes this surface a good candi-date for defining a sequence-stratigraphic bound-ary (P4-SB2).

Systems Tracts

Transgressive Systems Tract 1

Continuous and parallel seismic reflectors that arepart of seismic facies SF4 overlie sequence bound-ary P10-SB1 in the shelfal part of transects B, C,



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and D in the SSD (Figure 5). The high-amplitudeand continuous character of seismic reflectorswithin this package (SF4 seismic facies) and theonlap terminations against sequence boundaryP10-SB1 on the shelf (SSD) suggest that this pro-delta to upper slope unit was deposited during a

transgressive event (transgressive systems tract 1[TST1]). The base of the TST1 unit is defined bythe P10-SB1 sequence boundary and its upperboundary by MFS1 (Figures 3, 5). Note that SF4facies are not present in transect A (NSD) andthat a series of seismic facies transitions occuralongstrike from south to north within the TST1interval (Figures 3, 5). Figure 3 shows that pro-delta and upper slope facies (SF4) are dominantwithin the TST1 unit in the SSD; however, SF4

facies transition to the north into MTDs (SF5)that occupy the outlet of the Columbus sedi-mentary pathway (Figure 3). Additional changesin seismic facies distribution within the TST1unit occur toward the north. Figure 3 shows howMTDs (SF5) located in the outlet of the Co-lumbus sedimentary pathway transition to shelf-edge deltaic facies (SF2) in the paleocanyon area(NSD) (Figures 3, 5). The availability of a con-tinuous seismic data set along the shelf-break re-gion (Figure 1) allowed us to document in detail

and with a great degree of confidence the characterand lateral variability of seismic facies distributionalongstrike within the TST1 unit (Figure 3).

According to the traditional sequence-strati-graphic model, MTDs or shelf-edge deltaic sys-tems are not commonly associated with the occur-rence of transgressive units (TST). Although someauthors have proposed that MTDs occur in thebasal parts of lowstand systems tracts (Beaubouefand Friedman, 2000), revised interpretations now

suggest that not all mass-transport units conform tosuch sequence-stratigraphic predictions (Beaubouefand Abreu, 2010). As pointed out by Beaubouefand Abreu (2010), MTDs may have sometimes oc-curred in response to allocyclic processes (sequence-stratigraphic significance), but these units can alsobe initiated by a variety of autocyclic processes thatinclude, but are not limited to, changes in sedi-mentation rates, increased seismicity, and local var-iations in slope gradients (Moscardelli et al., 2006;

Moscardelli and Wood, 2008; Beaubouef andAbreu, 2010). Moscardelli et al. (2006) proposedthat the aggradational character of shelf-edge del-taic clinoforms associated with SF2 facies withinthe paleocanyon area (NSD) and their relative strat-igraphic position suggested that this unit had been

generated during times of stillstand conditions orduring the early transgression. Recent addition ofnew data to the south (SSD) (Figures 1, 3) allowedus to obtain a direct correlation across the entiremargin that supports the previous interpretation byMoscardelli et al. (2006) (Figure 3). Moreover, thisnew understanding of how seismic facies variationsoccur alongstrike (Figure 3) confirms the existenceof an early transgressive shelf-edge deltaic systemwithin the paleocanyon area that fed MTD MTC_1

downslope. The existence of a transgressive shelf-edge deltaic lobe in the paleocanyon area can beexplained by an increase in sediment supply asso-ciated with the paleo-Orinoco River, as well as byan increase in accommodation that was controlledby the underlying structures (see the previous de-scription of NSD).

Highstand Systems Tract 1

Highstand systems tract 1 (HST1) is composed of

SF1 clinoforms that have been interpreted as anouter-shelf deltaic system in the SSD (Figures 3,5, 12). The lower boundary of the HST1 unit isdefined by MFS1 (downlap surface) and the topboundary by sequence boundary P4-SB2 (Figures 3,5, 7B, 12). Steep clinoform packages (SF1) withinthe HST1 unit are interpreted to have formed asthe result of the southwest-northeast prograda-tion of the paleo-Orinoco delta to the outer-shelfregion in the SSD (Figure 12). As shown in tran-

sects B and C (Figure 5), the foresets of the clino-forms never merged with the continental slope andthe system never developed into a full shelf-edgedeltaic unit in this area. However, if we move alongstrike only a few kilometers to the south, transect Dshows a notable difference in terms of stratigraphicarchitecture and seismic facies distribution withinthe HST1 unit (Figure 5D). In transect D, seismicfacies associated with SF1 clinoforms (outer-shelfdeltas) transition downdip into SF2 clinoforms



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(shelf-edge deltas) as the shelf break is approached(Figures 5D, 12). Moreover, along transect D, thethickness of the HST1 unit increases downslope,and the clinoform foresets are steeper, presentingmore of a chaotic distribution (Figures 5D, 12).The previous observations suggest that, although

the paleo-Orinoco delta reached only the outer-shelf region in the areas between transects B and C(Figures 8, 12), the system was able to bypass theshelf-break boundary a few kilometers to the south,as shown in transect D (Figures 5, 12).

Figure 3 shows that the HST1 unit signifi-cantly thins alongstrike toward the north (NSD)and that the outlet of the Columbus sedimentarypathway is occupied mainly by MTDs (SF5). Thelower preservation of HST1 sediments in the NSD

can be explained by the erosiveness of the gravitysediment flows affecting this part of the marginor by low sediment supply through the Columbussedimentary pathway at this time. Data suggestthat, during HST1 time, most of the sediment sup-ply was concentrated to the south (SSD), where theouter-shelf deltaic system developed (transects Band C; Figures 3, 5). A southern bypass zone wasactive along transect D and sediments were delivereddownslope into a deep-water depocenter (Figures 3,5). Similar to the observations made for the TST1

unit, differences between the seismic characteris-tics observed within the HST1 unit along strike aresignificant (Figure 3).

Lowstand Systems Tract 2

Most of the elements that have been defined ascomponents of the lowstand systems tract (LST)in the classical sequence-stratigraphic model as de-

fined by van Wagoner et al. (1990) and Mitchumet al. (1993) can be identified within the LST2unit. These elements include (1) basin-floor fans(bff) composed of LCDs (SF6) that occupy theoutlet of the Columbus sedimentary pathway andthat extend to deep-water depocenters to theeast, (2) onlapping wedges (SF3) that define slopefans (sf) within the SSD, and (3) a northwest-southeast progradational to aggradational wedge(pw) composed of shelf-edge deltaic facies (SF2)

that developed within the paleocanyon area in theNSD (Figures 3, 5). Finally, a regional lowstandwedge (lw) composed of a southwest-northeastprogradational to aggradational clinoform package(SF2 facies) downlaps against previously describedunits (Figures 3, 5). This last downlapping package

was part of a shelf-edge delta (paleo-Orinoco),and it has been interpreted as a lowstand wedge(lw) that was deposited during the last maximumglacial lowstand (Figures 3, 5). On top of the low-stand wedge (lw), a strong reflector defines the firstmajor marine flooding event for this cycle that isequivalent to a transgressive surface (Figures 3, 5).Seismic facies associations are more laterally con-tinuous alongstrike for the LST2 unit, and an in-terpretation of an LST for this interval is ubiqui-

tous (both in the SSD and the NSD).

DISCUSSION: SEQUENCE STRATIGRAPHY,ALONGSHORE PROCESSES, ANDSTRUCTURAL CONUNDRUMS

Thick TST1 and An UncommonPaleocanyon Infill

Obvious variations exist in the stratigraphic archi-

tecture observed within the TST1 unit that differfrom the facies successions that are traditionallyassociated with transgressive units by conventionalsequence-stratigraphic models. Classic as well assubsequent models describe the transgressive unitas bounded below by the transgressive surface andabove by the downlap surface or MFS (condensedsection) (van Wagoner et al., 1990; Posamentierand Allen, 1999). These models also state thatparasequences within the TST backstep inland in

the form of a retrogradational parasequence set(van Wagoner et al., 1990; Posamentier and Allen,1999). Condensed sections develop during the timeof transgression to early highstand systems and al-legedly consist of thin hemipelagic or pelagic sed-iments that are deposited as the parasequencesstep landward and as the shelf is starved of ter-rigenous sediments (van Wagoner et al., 1990;Posamentier and Allen, 1999). The most notabledifferences between the deposits described in these



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models and the observed TST deposits in theColumbus Basin are associated with the anom-alously thick occurrence of the transgressive unitin the outer-shelf and upper slope regions of theSSD and the occurrence of MTDs and shelf-edgedeltaic systems in the NSD (Figures 3, 5).

The character and distribution of the seismicfacies succession (prodelta to upper slope deposits),the geometry of the seismic terminations (onlapsagainst P10-SB1), the stratigraphic location of thesedeposits immediately overlying the distal sequenceboundary bypass horizon, and the general archi-tecture of the TST1 unit within the SSD lead toits interpretation as having been deposited by atransgressive event (Figure 5). The presence andrelative thickness of this transgressive package in

the outer-shelf region of the SSD can be explainedby (1) an increased accommodation on the down-thrown side of the main growth faults that causeda stratigraphic wedging effect against main faultplanes (Figures 13B) and (2) an increase in sedi-ment supply along depositional strike resultingfrom the action of current-controlled processes(suspended sediment and bottom contour cur-rents) that were able to transport sediments fromthe south (the Amazon delta is the most likelysource of these sediments) (Eisma et al., 1978;

Kuehl et al., 1986; Warne et al., 2002; Meade et al.,1990).

In contrast to the SSD, the time-equivalentinterval of the TST1 unit in the NSD (Figures 3,5A, 13A) is not easily recognizable as a transgres-sive package. Whereas the SSD was affected mainlyby the action of growth faults during this time(Figure 13B), the NSD was affected by transpres-sional deformation tha

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Shelf Edge Deltas _offshore Trinidad