Anatomy of Esdolomada laterally migrating tidal bar Tremp-Graus Basin Spain Sedimentology_2012...

Anatomy of a laterally migrating tidal bar in front ofa delta system: Esdolomada Member, Roda Formation,Tremp-Graus Basin, Spain

MARIANA I. OLARIU*, CORNEL OLARIU*,§, RONALD J. STEEL*,ROBERT W. DALRYMPLE� and ALLARD W. MARTINIUS�*Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin,TX 78712, USA (E-mail: [email protected])�Geological Sciences and Geological Engineering, Queen’s University, Room 342, Bruce Wing, Kingston,ON K7L 3N6, Canada�Statoil Research Center, Arkitekt Ebbellsvei 10, N-7005 Trondheim, Norway§National Institute for Marine Geology and Geoecology, 23-25 Dimitrie Onciul St., Bucharest 024053,Romania

Associate Editor – Charlie Bristow

ABSTRACT

The first sandstone unit of the Esdolomada Member of the Roda Formation

(hereafter referred to as ‘Esdolomada 1’) was formed by a laterally-migrating,

shelf tidal bar. This interpretation is based on detailed mapping of the bedding

surfaces on the digital terrain model of the outcrop built from light detection and

ranging data and outcrop photomosaics combined with vertical measured

sections. The Esdolomada 1 sandbody migrated laterally (i.e. transverse to the

tidal currents) towards the south-west along slightly inclined (1.6� to 4.6�)master bedding surfaces. The locally dominant tidal current flowed to the north-

west. This current direction is indicated by the presence of stacked sets of high-

angle (average 21�) cross-stratification formed by dunes that migrated in this

direction, apparently in an approximately coast-parallel direction. The tidal bar

contains sets and cosets of medium-grained cross-stratified sandstone that stack

to reach a thickness of about 5Æ5 m. Individual cross-bed sets average about

50 cm thick (with a range of 10 to 70 cm) and have lengths of ca 130 to 250 m in a

direction perpendicular to the palaeocurrent. Set thickness decreases in the

direction of migration, towards the south-west, and the degree of bioturbation

increases, so that the cross-bedded sandstones gradually change into highly

bioturbated finer-grained and thinner-bedded sandstones lacking any cross-

stratification. The rate of thinning of individual dune sets as they are traced

down any obliquely-accreting master surface is some 40 cm per 100 m (0Æ004)

for the older, thicker sandstones, whereas the younger, thinner beds thin at a rate

of 15 cm over 100 m (0Æ0015). The tidal bar has a sharp base and top and is

encased in finer-grained bioturbated, marine sandstones. The Esdolomada bar

crest was oriented north-west to south-east, parallel to the tidal palaeocurrents

and to the nearby palaeoshoreline, but built by lateral accretion towards the

south-west. Lateral outbuilding generated a flat-topped bar with a measured

width of about 1700 m, and a preserved height of 5Æ5 m. The bar, disconnected

from a genetically related south-westward prograding delta some 2 km to the

north-east, developed during the transgressive phase of a sedimentary cycle.

The tidal bar was most probably initiated as a delta-attached bar at the toesets of

the delta front and during transgression evolved into a detached tidal bar.

Keywords Lateral accretion, LIDAR, roda sandstone, tidal bar, transgression.

Sedimentology (2012) 59, 356–378 doi: 10.1111/j.1365-3091.2011.01253.x

356 � 2011 The Authors. Journal compilation � 2011 International Association of Sedimentologists

INTRODUCTION

This article documents the facies and architectureof a laterally-accreting ancient offshore tidal barin the Lower Eocene deposits of the RodaFormation in the Tremp-Graus Basin. Despitefacies similarities, the architecture described dif-fers from the classic forward-accreting ancienttidal bars that have been described by Mutti et al.(1988) in the Eocene deposits of the Ager Basin.

Dunes that migrate under strong tidal currentscan stack together in at least two ways, generatingtidal bars or tidal compound-dunes, sedimentarybodies with different internal architectures. Bothtidal bars and compound dunes have receivedrecent attention in modern tide-dominated envi-ronments (Dalrymple, 1984; Dalrymple & Rhodes,1995; Snedden & Dalrymple, 1999; Berne et al.,2002; Dalrymple & Choi, 2007) due to the impor-tance of these morphological elements in modernenvironments and their significant reservoir po-tential in ancient successions. Tidal bars, in con-trast to equally large compound dunes, accretelaterally with respect to the net sediment transportdirection (Houbolt, 1968) as shown by the smallersuperimposeddunes.However, it canbedifficult todistinguish laterally accreting tidal bars fromdownstream accreting compound dunes (Dalrym-ple & Rhodes, 1995) of equivalent height andsimilar facies in ancient outcrops with only two-dimensional (2D) exposure. As a result, dunecomplexes can be misinterpreted as tidal bars.This study demonstrates how this distinctioncan be made in the field.

Relatively straight-crested, elongate tidal bars(sometimes also called ‘tidal ridges’) are commonon many continental shelves today, but have notbeen widely used as analogues for ancient deposits(Snedden & Dalrymple, 1999). Isolated sandstonebodies encased in marine shales have traditionallybeen interpreted as shelf sand ridges that formedlong distances seaward of their contemporaneousshoreline (Snedden & Dalrymple, 1999), but diffi-culty in explaining how the sand was transportedlong distances offshore has led to reinterpretationof many of these ancient sandstone bodies aslowstand shoreface or delta deposits. Therefore,there are not many interpreted ancient shelf ridges,perhaps also due to the difficulty of fully recon-structing the geometry of a three-dimensional bar,as they are typically much larger than the size of anormal outcrop (Wood, 2003). Tidal bars that havebeen described from modern environments arelarge, elongate and asymmetrical features whichcan reach several kilometres in width, tens of

kilometres (25 to 100 km) in length and tens ofmetres (25 to 50 m) in height (Dalrymple & Rhodes,1995; Berne et al., 1998, 2002; Dyer & Huntley,1999; Reynaud et al., 1999; Wood, 2003; Dalrym-ple & Choi, 2007). Such bars usually generate asand body characterized internally by stacked setsof cross-strata, separated by lateral-accretion mas-ter bedding that would tend to be interpreted aschannel and/or channel-bank deposits in ancientsuccessions (Dalrymple & Choi, 2007). However,the present models of modern shelf ridges arebased on high-resolution 2D and three-dimen-sional (3D) seismic imaging, their detailed internalarchitecture is not always resolved and their faciesare rarely described. Undisturbed deep vibracoreshelp with the description of internal facies archi-tecture, but vibracoring in tidal ridges is difficultdue to ambient wave and current conditions(Snedden & Dalrymple, 1999).

Tidal bars located in offshore settings (on conti-nental shelves) tend to form during lowstand(earliest relative sea-level rise) and transgressiveconditions (transgressive systems tracts) given thepresence of widened shelf areas and sandy coastaldeposits reworked on the shelf as relative sea-levelrises (Dalrymple, 1992; Snedden & Dalrymple,1999). Near river mouths, they usually are shore-normal features (East China and Yellow Sea) but,away from river mouths and in constricted areas(for example, the English Channel and southernbight of the North Sea), they are coast parallel(Stride et al., 1982; Dyer & Huntley, 1999).

Understanding the depositional processes andgeometry of tidal bars is important for predictingtheir extent and internal architecture. Accuratemapping of their architecture requires interpreta-tion of 3D geometries and determination ofsurface orientations preserved in outcrop. In thisstudy digital and conventional outcrop observa-tions are used to generate accurate dimensionaldata that allow reconstruction of the true dip andstrike orientation of both the individual dune setsand the larger master surfaces that make up theEsdolomada 1 tidal bar. Several studies of Esdo-lomada 1 have previously been reported as part ofthe Roda Delta system (Martinius & Molenaar,1991; Torricelli et al., 2006; Tinterri, 2007) butthe detailed architecture of the tidal bar has notpreviously been documented.

GEOLOGICAL SETTING

The Esdolomada Member of the Roda Formation(Nijman & Nio, 1975; Lopez-Blanco et al., 2003;

Esdolomada tidal bar 357

� 2011 The Authors. Journal compilation � 2011 International Association of Sedimentologists, Sedimentology, 59, 356–378

Tinterri, 2007) crops out along the Isabena Valleyin the north-western part of the Tremp-GrausBasin in the south-central Spanish Pyrenees(Fig. 1). The south Pyrenean foreland basin is aWNW to ESE oriented elongate feature openingand deepening to the west towards the AtlanticOcean (Tinterri, 2007). The Esdolomada unitbelongs to the Figols Group (Mutti et al., 1988,1996) and is part of the Early Eocene shallow-marine succession of the Tremp-Graus Basin, apiggy-back basin carried on the Montsec thrustsheet and bounded to the north and south by theBoixols and Montsec thrusts, respectively (Lopez-Blanco et al., 2003). The Figols Group can bedivided into four unconformity-bounded deposi-tional sequences that record an overall shallow-ing-upward trend related to the southwardmovement of the Boixols and Montsec thrustsand filling of the basin. The Figols Group overliestransgressive shallow-marine carbonates of theAlveolina Limestone and marks the transition topredominantly terrigenous sedimentation in thebasin (Fig. 2).

The Esdolomada Member occurs just above theLower Eocene Roda Sandstone Member (Fig. 2)which has been variously interpreted to representan ebb-tidal delta (Nio et al., 1984; Nio & Yang,1991) or a forced regressive wave-influenced andtide-influenced Gilbert-type delta (Crumeyrolleet al., 1992; Lopez-Blanco et al., 2003). The Rodasandstone has also been interpreted to representtidal-influenced mouth bars deposited by

catastrophic flood events (Mutti et al., 1996;Crumeyrolle, 2003) or a river-flood-dominateddelta system (Tinterri, 2007). The most recentstratigraphic interpretations (Lopez-Blanco et al.,2003; Tinterri, 2007) consider that the Rodasandstone is made up of six (R1 to R6, U to Z)high-frequency depositional sequences character-

Fig. 1. Geological map showing the location of the study area (star) in the northern part of the Tremp-Graus Basin inthe south-central Spanish Pyrenees (simplified from Lopez-Blanco et al., 2003). The south Pyrenean foreland is aWNW to ESE trending elongate feature, opening and deepening to the west towards the Atlantic Ocean. The Tremp-Graus Basin is a piggy-back basin carried on the Montsec thrust sheet and is bounded to the north and south by theBoixols and Montsec thrusts, respectively.

Fig. 2. Lithostratigraphy and context of the RodaFormation (from Lopez-Blanco et al., 2003 ; Tinterri,2007). The Esdolomada 1 sandstone unit occurs at thebase of the Esdolomada Member. The Figols Group(FG), of which the Esdolomada is a part, is divided intofour unconformity-bounded depositional sequencesthat record an overall shallowing-upward trend(Tinterri, 2007).

358 M. I. Olariu et al.


ized by a basal regressive deltaic sandstone wedgecapped by transgressive siltstone and mudstone.Seaward (towards the south-west) progradation ofthe deltas produced steeply dipping clinoformalsand lobes, a few tens of metres high. The basalpart of the Roda sandstone (R1 to R5, U to X) hasbeen interpreted to represent the large-scalehighstand and falling stage deposits of FG1 withthe R6 (Y) unit as the lowstand deposits of FG2(Tinterri, 2007). The Esdolomada 1 (equivalent tothe R7 unit of Tinterri or the Roda Z of Lopez-Blanco) is backstepping relative to R6 (Y) and sois considered to be the transgressive systemstract of FG2 (Fig. 3). Therefore, the large-scaleregressive–transgressive turnaround occurs be-fore the first unit of Esdolomada sandstone –Esdolomada 1.

The Eocene Roda Sandstone (120 to 200 mthick) accumulated during ca 400 kyr, in theTremp-Graus Basin, synchronously with growingfolds along the Montsec thrust front (Lopez-Blanco et al., 2003; Tinterri, 2007). At the timeof Esdolomada 1 deposition, the basin was morestable with no fold development (Lopez-Blancoet al., 2003). Esdolomada 1 occurs above the

Plateau limestone (Nio & Yang, 1991; Crumeyrolleet al., 1992; Lopez-Blanco et al., 2003), a bioclasticfossiliferous grainstone (Mutti et al., 1988, 1996)at the top of the R6 unit which can be traced overtens of kilometres. The limestone developedduring a transgressive period with reduced ter-rigenous supply in this area.

The deposits immediately above the Plateaulimestone consist of blue-grey, calcareous mud-stone. Within this muddy succession there is athin and laterally continuous bioturbated bioclas-tic limestone with abundant marine fauna (stormbed) which has been used as a regional marker.Above the storm bed there are several metres ofhighly bioturbated fine-grained sandstone whichare, in turn, overlain by up to 6 m of medium-grained, cross-bedded sandstone that is the Esdo-lomada 1 (Fig. 4). This sandstone, in turn, isoverlain by several metres of muddy bioturbatedfine-grained sandstone and topped by medium-grained sandstone with more than 90% forami-nifera and abundant Thalassinoides burrows.

The Esdolomada 1 sandbody was chosen forthis study because its setting within a fullymarine succession indicated that it was a marine

Fig. 3. The Roda Sandstone Member consists of a large-scale regressive to transgressive clastic wedge made up of aseries of thinner tongues or wedges (from Tinterri, 2007). The FG2 depositional sequence is made up of basal fluvio-deltaic deposits of the R6 unit (lowstand systems tract) overlain by transgressive facies (R7 unit = Esdolomada1 – tidal sandstone and the encasing marine mudstones) and bounded above by a maximum flooding surface,marking the passage into highstand mudstone.



sand body. In addition, the abundance of med-ium-grained, stacked sets of cross-strata suggestedstrong, probably tidal, currents. Reconnaissanceobservations suggested that it contained

large-scale master-bedding surfaces that dippedtransverse to the local palaeocurrent direction(north-west) as indicated by the smaller-scaledunes; thus, it appeared to be an example of anelongate tidal bar. The purpose of this study wasto test this hypothesis and to document thecharacteristics of this sand body.

METHODS

In this study, light detection and ranging(LIDAR), photomosaics and vertical measuredsections have been used to characterize thearchitecture of the Esdolomada 1 sandbody. Theoutcrop is located along the eastern bank ofthe Isabena River, near Roda de Isabena, betweenthe Roda anticline and the Las Forcas syncline(Fig. 5). The axis of the Roda anticline is locatedin the western part of the studied outcrop and hasa crest line oriented 173/7 S on average. The axisof the Las Forcas syncline is located approxi-mately 1 km to the north-east of the Roda anti-cline and has a crestline oriented 169/8 S onaverage (Lopez-Blanco et al., 2003). The mainoutcrop, on the eastern side of the Isabena River,is about 1Æ5 km long and oriented roughly north–south, but has local east–west oriented sections.On the western side of the river the sandbodycrops out for about 100 m; there is a lack ofexposure of about 350 m between the two out-crops (Fig. 5).

LIDAR

Using an Optech Ilris 3D terrestrial laser scanner(Optech Incorporated, Vaughan, ON, Canada), thesurface morphology of the outcrop was capturedand expressed in a global reference system withcentimetre accuracy. The Ilris 3D has a typicalaccuracy of 8 mm at 100 m and an acquisitionrate of about 2500 points per second, with ahorizontal resolution of a few millimetres. Eachscanned point has horizontal coordinates (x andy) and elevation (z) information. The rapid,automatic acquisition of data points creates ahighly detailed point cloud that can be used tocreate digital models of the terrain (Bellian et al.,2005; Buckley et al., 2008). Multiple scans takenfrom various locations (different angles) are inte-grated using Polyworks software. The alignmenttechnique is based on the geometrical shape ofthe scans. Overlapping portions of adjacent high-resolution scans should contain at least threefeatures in common. Once these features are

Fig. 4. Detailed lithological column of Esdolomada 1–measured section 8 (for location see Fig. 7) showing thesandstone body (ca 5.5 m thick) with stacked sets andcosets of cross-strata (palaeocurrents are indicated byblack arrows – north is towards the top) separated bylateral-accretion master bedding surfaces (M1 to M6;dip direction shown by coloured arrows). The mea-sured section displayed here does show a fining-up-ward grain-size trend, but overall there is no preferredvertical grain size or bed thickness trend (see alsoFig. 7). The Esdolomada 1 tidal bar is interpreted tohave been deposited as part of a transgressive systemstract. The tidal bar overlies finer bioturbated sandstone.As the water depth increased during transgression,finer sediments were deposited on top of the aban-doned tidal bar. These finer, bioturbated sandstones arein turn capped by a hardground (maximum floodingsurface).



extracted, a six-parameter transformation (threerotations and three translations) can be estimatedand all the points of a scan can be transformedinto a common coordinate system. The best sets ofnon-redundant data points in the overlappingregions are identified in the aligned 3D scans andmerged into a unified data set. Key stratigraphicfeatures such as bedding planes and faciesboundaries, are highlighted on the 3D terrainmodel of the outcrop and their orientation (strike/dip) was determined (Fig. 6). Master beddingplanes were digitized in Polyworks and a planewas fit to pass through the 3D points using a leastsquare approach. Lateral continuity and morpho-logy (variability of bed thickness) of sedimentarybodies in three dimensions allow the architectureof the tidal bar to be reconstructed. The strike anddip of both dune foresets and master beddingsurfaces were corrected for the regional structuraldip, using the underlying storm bed (N50Æ5E/4�)as a reference horizon (Fig. 7). The thinning rateof sets and co-sets was measured as the thicknessof the beds at different locations divided by thedistance (see method description in Olariu et al.,2010).

Measured sections

Seventeen vertical sections were measured atdifferent locations along the outcrop over adistance of more than 1 km (Fig. 5). Field obser-vations and measurements were concentrated onthe stratigraphic interval between the storm bedbelow the sandstone body and the bioclasticsandstone above (Fig. 7). Sedimentary sectionsrecord grain size, physical and biological sedi-mentary structures and palaeocurrents based oncross-strata foreset orientation. The degree ofbioturbation is described by the bioturbationindex (BI) range 0 to 6 (bioturbation has blurredprimary structures) according to Taylor & Gold-ring (1993).

RESULTS

Facies description and interpretation

The Esdolomada 1 and the encasing strata consistof five lithofacies: (i) cross-stratified sandstone;(ii) bioturbated sandstone; (iii) thin-bedded sand-

Fig. 5. Geological map of the Rodaformation showing the Esdolomada1 sandstone – ca 2 km of outcropexposure (thick black line) along theIsabena River in Roda de Isabena,Spain (after Yang & Nio, 1989). Theoutcrop is located between the axesof the Roda anticline and Las Forcassyncline (Lopez-Blanco et al., 2003).Locations of measured sections areindicated by numbers. Thin blacklines are faults.



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stone and mud; (iv) bioturbated muddy sand-stone; and (v) bioclastic sandstone (Table 1).

Cross-stratified sandstoneThe sandstone body itself consists of sets ofmedium-grained cross-stratified sandstone (setthickness averages about 50 cm; range 10 to70 cm) that stack to reach a total thickness of upto 6 m (Fig. 8). Set thicknesses decrease in thedownlap direction along the master beddingsurfaces (i.e. to the south-west; Fig. 9). Thesandstone sets typically contain high-angle (4�to 44�, average 21�) foresets (Fig. 10A) orientedtowards the north-west (248� to 358�, average314�) and occasionally display a sigmoidal shape.The bounding surfaces of the cross-strata areparallel to sub-parallel suggesting that most of thecross-beds are 2D dunes. However, compoundtrough cross-bedding is present locally. Withinthe cross-stratified sets, trace fossils (Thalassino-ides, Skolithos, Macaronichnus and Psilonichus)are present, but rare (Fig. 10B); the BI is 0 to 1.Macaronichnus is mostly attributed to highenergy wave-dominated upper shoreface succes-sions (Clifton & Thompson, 1978), lower shore-face (Curran, 1985), toe of the foreshore (Saunderset al., 1994) or, rarely, intertidal environments(Rahmani & Smith, 1988; Ranger & Pemberton,1988; Saunders & Pemberton, 1990; MacEachern& Pemberton, 1992). However, the presence ofMacaronichnus in tidal sandbars has been docu-mented in various papers (Clifton & Thompson,1978; Pollard et al., 1993; Savrda & Uddin, 2005;Gibert et al., 2006; Carmona et al., 2008). Theoccurrence in cross-stratified sands confirmsthe high energy environment for these burrowssuggested by previous workers.

Mud drapes (less than 1 cm thick) occur withincross-beds (Fig. 10C). The mud drapes are thoughtto have been deposited from suspension during thereversal of the tide when the water was stationary.Such mud drapes are usually destroyed by thesubsequent tide, but are preserved locally. Mudchips are sometimes found in the lower parts of thecross-bedded sets at the base of the sandstone body

(Fig. 10D). The presence of compound cross-bed-ding between two master surfaces indicates thatcompound dunes migrated along the surface of thesandstone body and that the troughs of the com-pound dunes would be responsible for the creationof the master surfaces.

Bioturbated sandstoneThe cross-stratified sandstone gradually passesdownward, along any master bedding plane, intohighly bioturbated, carbonate-cemented finer-grained and thinner-bedded sandstones lackingany cross-stratification (Fig. 7). This lateral(towards the south-west) variation in facies mayrepresent lower energy conditions or abandon-ment of the sandstone body. The presence of sandmeans that, at times, the current was sufficientlystrong to carry it into the area but the sedimen-tation rate was sufficiently low that extensivebioturbation destroyed the evidence of the deci-metre-thick cross-strata or ripples.

Thin-bedded sandstone and mudThis facies is characterized by centimetre todecimetre-thick beds that commonly occurbetween the cross-stratified sets, just above themaster bedding planes. Sandstone beds haveasymmetrical, unidirectional current ripples.The grain size is bimodal with fine to mediumsands and mud. The bioturbation is variable, butrelatively high (BI 3 to 6). Planolites and Thalas-sinoides trace fossils were observed. Locally,symmetrical ripples (wave-generated structures)are preserved. The dunes migrate obliquely acrossthe crest of the sandstone body generating masterbedding planes via erosion of the troughs; thefiner sediment is deposited over the erosion(master) surface as the current decelerates. There-fore, master bedding planes are commonlydirectly overlain by lower-energy deposits.

Bioturbated muddy sandstoneHighly bioturbated fine to very fine-grained sand-stones with abundant admixed mud and forami-nifera (Fig. 10E), echinoids and trace fossils

Fig. 7. Outcrop interpretation based on photomosaics and stratigraphic measured sections. (A) Outcrop photomo-saic showing the positions of the stratigraphic columns along the 1.5 km outcrop. (B) Correlation panel based onstratigraphic columns Note that: (i) master surfaces (coloured lines) are truncated by an erosional surface; and (ii)vertical logs are not spaced according to their true locations and the north-eastern side (stoss side) has a dip of about0.4 degrees. Master bedding surfaces (M1 to M9) identified and quantified on LIDAR and outcrop photomosaics areinterpreted on the measured sections. Outcrop photomosaics of: (C) the left side of the gully; (D) the right side of thegully; and (E) and the southernmost part of the outcrop with superimposed bedding diagram and stratigraphiccolumns.



(Thalassinoides, Skolithos and Psilonichnus)(Fig. 10F) occur below and above the cross-bedded sandstone of the tidal bar. The BI is 5to 6 and no physical sedimentary structuresare preserved. Marine fossils and trace-fossilassemblages indicate open marine conditions(Martinius & Molenaar, 1991; Torricelli et al., 2006).The high degree of bioturbation suggests thatenergy levels and sedimentation rates were low.

Bioclastic cemented sandstone (firmground)Above the upper fine-grained, highly bioturbatedsandstone there is a thin (less than 1 m thick) andlaterally persistent medium-grained calcareoussandstone bed with abundant marine fauna con-sisting of foraminifera (90%), molluscs and largeThalassinoides burrows (firmground burrows ofthe Glossifungites ichnofacies). This fossiliferousand intensely bioturbated bed is interpreted as

Fig. 9. Outcrop photographs showing the continuity of the master bedding planes (for location of photographs seeFig. 5) Master surfaces downlap onto the base of the bar and are truncated at their up-dip termination. (A) The cross-bedding between master surfaces (yellow) is mostly formed by simple 2D dunes, but occasionally compound dunesare present. Dune-scale cross-strata dip towards the north-west. (B) On the western outcrop between the masterbedding planes (yellow) there are thinner-bedded sandstones lacking any cross-stratification. This lateral (towardsthe south-west) variation in facies may represent lower energy conditions or abandonment of the bar.

Fig. 8. Stacked sets (of 2D dunes)and cosets (of compound dunes)which characterize the Esdolomadatidal bar. Individual sets typicallycontain high-angle cross-stratifica-tion and stack to reach a total barthickness of up to 5 to 6 m. Masterbedding planes (yellow) separatestacked sets and cosets of cross-strata.



a maximum flooding surface at the top ofthe Esdolomada 1 transgressive systems tract.A time-equivalent hardground with very similar

characteristics has been described in detail (Mar-tinius & Molenaar, 1991; Molenaar & Martinius,1996; Torricelli et al., 2006) in a more proximal

Fig. 10. Facies, ichnofacies and sedimentary structures. (A) Medium-grained, cross-bedded sandstone with high-angle cross-stratification (yellow) oriented towards the north-west (303/26). The master surface (red) dips towards thesouth-west (239/2.7). (B) Bedding-plane view of medium-grained, cross-bedded sandstone with large Thalassinoidestrace fossil (arrow). (C) Thin-bedded sandstone and mud between sets of cross-bedded sandstone. (D) Mud chipsoccasionally occur in sandstone layers in the basal part of the bar. (E) Highly-bioturbated, fine-grained sandstonewith foraminifera. (F) Highly-bioturbated, fine-grained sandstone with inclined burrow of Psilonichus.



area to the north at the top of deltaic lobes andinterpreted to represent short periods of clasticabandonment due to lateral switching of deltas.This hardground does not mark regional discon-tinuities in a stratigraphic frame.

In summary, the deposits are interpreted as atidal sandstone body in a marine setting. Theinterpretation is supported by facies that indi-cates a marine environment with strong unidi-rectional and bi-directional (tidal) currents. Thestrength of the currents, the sedimentation ratesand the intensity of the bioturbation variedthroughout the deposits during the building ofthe Esdolomada 1 unit. Below, analysis of thearchitecture reveals the nature of this sandbody.

Tidal sandbody architecture

Since it is critical to the analysis and interpreta-tion of the Esdolomada sandstone body, either asa tidal bar or a tidal compound-dune complex,the internal geometry of the beds is described indetail from the attitude of simple dunes to that oflarger master surfaces. The Esdolomada sand-stone body overlies a non-erosional surface thathas a rather abrupt weathered appearance (Figs 7and 9) and some grain-size change (highly bio-turbated muddy – lower fine sandstone to upperfine-medium sandstone). The Esdolomada 1 is asandbody that was laterally accreting along mas-ter bedding surfaces (Fig. 11) and, at the sametime, was covered by dunes that produced thestacked cosets of cross-strata that occur betweenthese bedding surfaces. A single set of cross-stratais considered from the surface on which theforesets are downlapping to the thicker surface atthe top or to the contact with another cross-strataset that is truncating the top (Figs 8 and 10A).Cross-stratal foreset dip directions (about 70measurements) document flow consistently to-wards the north-west (Fig. 11), essentially at rightangles to the south-west dipping master surfacesof the bar (Fig. 12). The landward end of themaster surfaces (towards the north-east) are trun-cated (Figs 6, 7B, 7D and 9B) suggesting that thesandbody had an erosional surface towards thenorth-east. The length of the master surfaces asmeasured along the outcrop ranges from 285 to450 m. However, when the length of any exposedmaster surface is projected along its direction ofmigration the range is 130 to 250 m (Fig. 12).Sandstone set thicknesses decrease towards thesouth-west down any master bedding plane andthe degree of bioturbation increases so that thecross-bedded sandstones (facies 1) change into F

ig.

11.

Rose

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ms

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ing

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sof

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the

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on

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ou

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highly bioturbated finer-grained and thinner-bedded sandstones lacking any cross-stratifi-cation (facies 2). Thin-bedded sandstone andmudstone (facies 3) that represent the toesets ofthe cross-strata underlie facies 1 throughout theentire unit. The cross-bedded sandstone (facies 1)is only exposed in the main outcrop (Fig. 5). Thelocal rate of thinning of sets and cosets betweenmaster beds (mostly facies 1) is some 40 cm per100 m (0Æ004) for the thicker sandstones exposedin the main outcrop, whereas the thinner sand-stone beds exposed on the western outcrop(predominantly facies 2) thin at a rate of 15 cmover 100 m (0Æ0015) (Figs 13 and 14). The sand-body is encased in highly bioturbated muddysands with marine body fossils (facies 4).

Facies interpretation of Esdolomada 1 and trun-cation (erosion) of the master surfaces towards thenorth-east (Fig. 7), as well as the accretion of themaster surfaces at a high angle to the palaeocurrentdirection, suggests that the sandstone body formedby lateral accretion and that it is, by definition, atidal bar (Houbolt, 1968; Stride et al., 1982). Thesandbody shows no clear vertical grain-size trendoverall, although there are some measured sec-tions that display a fining-upward or coarsening-upward grain-size trend (Fig. 7); there is also nopreferred vertical bed thickness trend. The basalcontact is sharp and best exposed in the centre ofthe bar, and less so towards the edges of the bar.This variation might be explained by the relativeabundance of facies 2 and the higher degree ofbioturbation towards the south-west, as the barwas becoming moribund in the latest phases of itsexistence. The top of the bar is sharp; masterbedding surfaces are truncated at their up-dip(north-east) termination (Fig. 15). The erosionsurface at the top of the bar represents theerosional stoss side of the bar formed by thecontemporary flood tidal currents. Lateral out-building generated a preserved flat-topped barwith a measured width of 1700 m and a preservedheight of 5Æ5 m.

Depositional setting of the Esdolomada tidalbar

During deposition of the Roda Formation, theTremp-Graus Basin lay near the edge of a tropicalclimate zone (Martinius & Molenaar, 1991). Basedon the fauna found in the hardground at the topof Esdolomada 1, the palaeoenvironment wasinterpreted as a shallow open inner shelf withnormal-marine conditions (Martinius & Mole-naar, 1991). Water depth in the vicinity of La

Puebla de Roda, several kilometres north of thestudy area, was estimated to be 10 m or less(Martinius & Molenaar, 1991). Therefore, it isinferred that the water in the present study areawas deeper, given its location further offshore.Dune height tends to scale to flow depth at a ratioof ca 1:6 and about one third of the height of adune tends to be preserved as a cross-stratified set(Dalrymple & Rhodes, 1995; Dalrymple & Choi,2007). Maximum measured thickness of com-pound dunes (ca 70 cm) indicates that the waterwould have been about 13 m deep, which isconsistent with the estimate of Martinius &Molenaar (1991).

The cross-stratified sandstone of the Esdolo-mada 1 tidal bar has a low bioturbation index(BI = 0 to 1), contains a diverse assemblage ofrelatively large burrows belonging to the Cruzi-ana ichnofacies and is encased in fine-grainedfossiliferous sandstones and mudstones; all theseand a minor presence of mud drapes within thesand body suggest a depositional setting in ashelf area with respect to any landward-siteddelta. Mud drapes are less common in a shelfsetting because suspended sediment concentra-tions are low, and the presence of rotary tidesthat almost never have a true slack water periodmake the accumulation of mud drapes unlikely(Dalrymple, 1992). Based on the trace fossils(Cruziana ichnofacies) the influence of freshwater input appears to have been negligible.The bioturbated muddy sandstones (severalmetres thick) were deposited during periods oflow tidal energy (the current speeds were com-monly below the threshold of sediment move-ment and, as a result, these sandstones wereintensely bioturbated and contain a substantialamount of mud), whereas the sandy tidal bar wasformed during periods of higher tidal energy.

The Esdolomada 1 tidal bar formed within thetransgressive systems tract according to the regio-nal stratigraphy (Tinterri, 2007). The transgressivetidal bar probably originated from the underlyingtidal-dominated delta, is sharply-based and over-lies finer bioturbated sandstones. As the waterdepth increased during transgression finer sedi-ments were deposited on top of the abandonedtidal bar. These finer, bioturbated sandstones are,in turn, capped by a bioclastic sandstone (maxi-mum flooding surface). The geometry of the over-lying bioclastic sandstone mimics the uppersurface of the sand body, suggesting that the sandbody created a positive topographic relief that wasmantled and only very slowly lost as the overlyingdeposits accumulated.



Fig

.12.

Map

vie

wof

the

ou

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psh

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ing

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DISCUSSION

Based on the sedimentary facies and internalarchitecture, the Esdolomada 1 sandstone bodymight be interpreted as a tidal bar (as suggestedhere), the distal part of a tidally reworked delta ora tidal point bar adjacent to a subtidal channel.Arguments for each one of these possible inter-pretations are given below.

Esdolomada 1: A transgressive, shelf tidal bar

The Esdolomada tidal bar is asymmetrical incross-section with a steep south-western slope(depositional, lee side) and gentle (erosional,stoss side) north-eastern slopes (Figs 7B, 15 and16). Figure 7B shows the gentle stoss side (north-east end) of the bar (dip less than 0.5�) thattruncates the master surfaces versus the steeperlee side (1Æ5 to 3�) towards the south-west. The leeside shows steeply dipping master surfacesthat extend towards the south-west (Fig. 15).The underlying older Roda delta deposits havewest/north-west to south-east oriented tidal pal-aeocurrents (Tinterri, 2007). The tide-influencedmouth-bar has mostly north-west oriented

currents, whereas the distal tidal delta showsdominantly south-east oriented currents (seefig. 17 of Tinterri, 2007).The presence in outcropof mostly north-west oriented palaeocurrents inthe Esdolomada deposits suggests that the dom-inant tidal flow (flood oriented) was towards thesouth-east whereas the preserved north-westernpalaeocurrents represent regionally subordinatecurrent (ebb oriented). Digital and outcrop obser-vations and measurements suggest that two pro-cesses contributed to the growth of the tidal bar:(i) tidal currents along the gentle north-easternflank transported sand obliquely towards thecrest; from the bar crest the sand moved downon the steep south-western slope and gave rise tothe south-west dipping master bedding planes;and (ii) sand was generally transported towardsthe north-west along and over the lee face by thesubordinate tidal currents, as indicated by thenorth-west orientation of the superimposeddunes. In this way the Esdolomada tidal barmigrated laterally towards the south-west, but hasits crest oriented north-west to south-east, paral-lel to the tidal palaeocurrents (Figs 16 and 17).This account of the Esdolomada tidal bar isanalogous to the Houbolt (1968) model and

Fig. 13. Isopach maps of the deposits between two master bedding surfaces. Cross-bedded sandstone sets (i.e. masterforesets) thin and downlap at a rate of about 40 cm per 100 m (rate = 0.004) towards the south-west and towards thesouth-east (offshore). The lateral extent of individual master surfaces (M1 to M7), as well as the mean direction of barmigration (black arrow), are indicated.



observations of shelf bars by Dalrymple (1992)(see also Stride et al., 1982; Snedden & Dalrym-ple, 1999).

The proposed Houbolt (1968) model for thebehaviour of shelf tidal bars suggests that they areelongate with their crest sub-parallel to tidal-current flow, but migrate by erosion of the stoss(gentler) side and accretion on the lee (steeper)side of the bar (see also fig. 29 of Dalrymple &Choi, 2007). Smaller scale dunes migrate obli-quely up the stoss side and progress in theopposite, subordinate-tide direction on the leeside of the bar. Dalrymple (1992) observed thatthe stoss side is mostly erosional and this modelpredicts preferential preservation of dune scalecross-strata dipping in the regionally subordinate

flow direction. The model proposed here suggeststhat the Esdolomada tidal bar originated byreworking of the front of a nearby delta butmigrated away from the delta during transgres-sion (Fig. 17). The model is further developedfrom a previous interpretation associated with theRoda Delta (Lopez-Blanco et al., 2003).

The Tremp-Graus Basin lies in the eastern part ofthe south Pyrenean foreland basin, a WNW to ESEtrending elongate feature opening and deepeningto the west towards the Atlantic Ocean (Tinterri,2007). Based on this physiographic setting, theflood tide would have come in from the west.Therefore, the inferred dominant south-east flow-ing current around the Esdolomada bar is inter-preted to have been the flood current, whereas thepreserved subordinate one was the ebb current.The bar was oriented parallel to the shoreline, butaway (in an offshore direction) from the coevalshoreline (Fig. 17). In a similar way, the Holocenetidal ridges from the North Sea have been observedto migrate away from the closest shoreline (Off,1963; Houbolt, 1968; Berne et al., 1998).

Esdolomada 1: Tide-dominated delta-frontdeposits (alternative interpretation)

Esdolomada 1 could be interpreted as the distalpart (toesets) of a delta that was tidally reworked.In support of such an interpretation is thesouth-westerly orientation of the master beddingsurfaces, which is also the direction of theprogradational Roda Delta, although this is strati-graphically lower (Crumeyrolle et al., 1992;Lopez-Blanco et al., 2003; Tinterri, 2007). Themodels of Crumeyrolle et al. (1992), Lopez-Blan-co et al. (2003) and Leren et al. (2010) interpretthe Roda Delta as a tidally influenced deltaformed at a sea-level lowstand, where tidalcurrents have reworked the distal part of deltaiclobes. However, in the well-established strati-graphic framework of the Tremp-Graus Basin(Lopez-Blanco et al., 2003; Tinterri, 2007) theEsdolomada 1 is a transgressive sandbody, andnot part of a regressive package (Figs 3 and 17).The Esdolomada 1 is equivalent to R7 of Tinterri(2007, see fig. 18) and the Roda Z unit of Lopez-Blanco et al. (2003, see fig. 10). The delta that wascoeval to Esdolomada 1 was backstepping relativeto the older R6 and Roda Y (lowstand deltasstratigraphically below). The vertical characterand stratigraphic setting of Esdolomada 1, there-fore, are both consistent with formation duringtransgressive conditions (Fig. 3). The older ero-sive surface at the base of the R6 unit is interpreted

Fig. 14. Histograms showing bar foreset thinning ratesin the downlap direction (towards the south-west) ofthe individual sandstone sets. (A) Thicker sandstonesexposed in the main outcrop (mean bed thick-ness = 75 cm) thin towards the south-west at a rate of0.004. (B) Thinner sandstones (about 52 cm in average)on the western outcrop thin towards the south-west at arate of 0.0015.



Fig

.15.

Sim

pli

fied

arc

hit

ectu

reof

the

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olo

mad

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tid

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bar

Th

esa

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ston

ese

tth

ickn

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decre

ase

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ow

nw

ard

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an

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bio

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ase

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an

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an

dth

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pgeom

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the

top

.



Fig

.16.

Arc

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(A)

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ap

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as a sequence boundary. The Roda 6 sandstone(described by Crumeyrolle et al., 1992), lying justbelow the Esdolomada sandstone, is interpretedas a lowstand deltaic wedge formed by tidallyinfluenced mouth bars and delta-front lobes.

There are also additional arguments against aninterpretation of Esdolomada 1 as delta frontdeposits: (i) the sandstones of the coeval back-stepping shoreline (delta) are disconnected (about2 km) from the tidal bar deposits; (ii) the oldertidally reworked delta-front deposits of Roda 6are steeper (10o to 30o; Leren et al., 2010) com-pared with the measured accretion surfaces (1o to3o) of Esdolomada 1; (iii) there is much lessmudstone in the Esdolomada system comparedwith the muddy river-dominated Roda deltaforesets (Tinterri, 2007); (iv) the marine tracefossil associations and abundant echinoids andforaminifera suggest an open-marine shelf envi-

ronment, away from fresh water influence; and(v) the base of the tidal bar is commonly sharp,more typical for a transgressive shelf bar than fora prograding tide-influenced delta. The lowstanddeltaic wedges of Roda 6 (Roda Y) just belowEsdolomada 1 are formed by tidally influencedmouth bars and delta front lobes. Such depositsoccur in the distal part of river dominated tide-influenced deltas; wave structures are presentand the amount of mud associated with the tidaldeposits is higher when compared with theEsdolomada sand body. Diminished accommoda-tion space during relative sea-level fall favoursthe enhancement of tidal processes which tend toproduce prograding deltaic lobes during latelowstand that can be completely reworked intotidal sand bars during transgression (Fig. 17).Therefore, these sand bodies have more chanceto become isolated on the shelf during transgres-

Fig. 17. Formation of the tidal bar in front of a delta based on the Roda Delta evolution [(A) and (B) based on Lopez-Blanco et al., 2003]. (A) Highstand systems tract: fluvial dominated delta has minimal tidal deposits. (B) Lowstandsystems tract: delta has its foresets and toesets strongly reworked by tidal currents at a high angle relative to deltaprogradation. (C) Transgressive systems tract: tidal currents rework the deltaic toesets into a tidal bar that is migratingaway from the delta.



sion as the lowstand topography becomes sub-dued during relative sea-level rise and the tidallyinfluenced/dominated deltas are replaced by anopen embayment.

Esdolomada 1: Tidal point bar of a subtidalchannel (alternative interpretation)

Another possible interpretation is that Esdolo-mada 1 is a tidal point bar adjacent to a subtidalchannel. This interpretation is supported by thepresence of the lateral-accretion master beddingplanes within the Esdolomada sandbody and thecommonly abrupt base of the bar. The lateralaccretion bedding formed by the migration ofboth tidal point bars and elongate tidal bars iserosively based because of the migration of thethalweg of the adjacent channel. However, theabsence of mud deposits and fluid muds thatmight be expected to occur with such tidalchannels (Dalrymple et al., 2003) dictates againstsuch a scenario. Muddy tidal point bars developsuch inclined heterolithic stratification, whereasfree-standing elongate bars at the seaward end ofestuaries or in distributary mouth bar areas ofdeltas are less likely to display this style ofsedimentation because of lower suspended-sediment concentration (Dalrymple & Choi,2007). Another difference between point barsand elongate tidal bars is the degree of sinuosity;point bars will be moderately to highly curved,whereas elongate tidal bars should generatestraight lateral-accretion bedding. In addition,such tidal channels would most probably havebeen oriented normal to the shoreline, not paral-lel to it. One argument against this being achannel is that there is no evidence of exposure(no subaerial indicators), or of a significant low-stand, which would be required to create an axialchannel in the area seaward of the immediatelyolder delta.

Comparison between modern tidalenvironments and ancient tidal deposits

The preserved Esdolomada 1 tidal bar sharesmany common features with modern tidal barsdeveloped in shelf environments (Stride et al.,1982). Both modern and ancient tidal barsdemonstrate lateral migration and rest on anerosional surface as a result of transgressiveerosion. Most of the modern tidal bars developedon transgressive shelves (the southern North Sea,English Channel and East China Sea, YellowSea) or at the mouth of a sand dominated estuary

(the Severn Estuary or the Cobequid Bay-SalmonRiver Estuary, Bay of Fundy) are composedentirely of sand and rest on a transgressivesurface (Dalrymple et al., 1990). In the case ofmixed-energy systems with both wave and tidal-current processes (Chesapeake and Delawarebays in the eastern United States and MoretonBay in Australia), barrier islands with numerousand/or wide tidal inlets are common (Harriset al., 1992). Tidal inlets can contain elongatetidal bars as well. However, it is not common forthese bars to become broad and flat topped likethose in tide-dominated estuaries (Harris, 1988).The elongate tidal bars in both of these estuarinesettings tend to be sand-dominated because ofthe retention of mud in the inner parts of theestuary.

The Esdolomada 1 bar is a sandstone bodydominated by lateral accretion surfaces, betweenwhich there are stacked cross-strata with foresetdip directions oriented at right angles to themaster bedding surfaces. The bar has its crestoriented almost parallel to the tidal currents. Theestimated dimensions of the Esdolomada bar(width ca 1700 m, height ca 5Æ5 m) are similarto those reported for offshore bars in modernenvironments. Based on modern examples (Berneet al., 1998, 2002; Dyer & Huntley, 1999; Reynaudet al., 1999; Wood, 2003) the length to width ratiofor shelf tidal bars ranges from 5 to 30; this willplace the length of Esdolomada bar somewherebetween 8Æ5 km and 51 km. However, the lengthof the Esdolomada bar might have been evensmaller than the minimum value of modernexamples, as the westward deepening Tremp-Graus basin was about 20 km wide and 95 kmlong in the axial ESE to WNW direction (Yang &Nio, 1989). The reason why the Esdolomada tidalbar has a low length to width ratio might beexplained in terms of the prior deltaic derivationof the tidal bar (Fig. 17). Most of the modernoffshore tidal bars described in the literature, andtherefore used here for comparison, are ‘fullyevolved ridges’ (or Class III) according to theclassification of Snedden & Dalrymple (1999).However, the Esdolomada tidal bar seems to be a‘juvenile ridge’ (or ‘Class I’; Snedden & Dalrym-ple, 1999) because of its proximity to the back-stepping delta (1 to 2 km) and the relativelyshallow (10 to 20 m) water depth.

Reservoir considerations

Esdolomada-type tidal bars may be attractivehydrocarbon reservoirs because of the coarseness



and the high lateral continuity of the clean cross-stratified sandstone beds in the core of the body.However, both the 3D external geometry andinternal architecture should be considered for abetter reservoir quality prediction. Shelf tidal barsare commonly oriented with their long axis at asmall angle to the general flow direction, areasymmetrical to the steeper face on the down-current side relative to the regionally stronger ofthe ebb and flood flows, and migrate obliquelydown-current in the direction of the dominantcurrent. Beds within the Esdolomada bar diplandward (south-west) but are oriented perpen-dicular to the elongation of sandstone. Thereforecoarser grained, better-quality reservoir facies lieson the upcurrent margin (towards the north) ofthe sandstone body. It is reasonable to assumethat vertical permeability would be good withinthe stacked sandstone beds, although the thin-bedded sandstone and mud between cross-bedswill buffer vertical flow in places. The thin-bedded intervals between cross-strata units havea lateral extent of tens to hundreds of metres inplaces; they are formed in the troughs of migrat-ing dunes and occur along the master surfaces.However, migration of larger superimposeddunes was able to remove the finer sediments inthe trough so the cross-strata are commonlyamalgamated and lack thin-bedded intervals.The overlying highly bioturbated muddy sand-stones may not form a reliable seal; they may haverelatively high permeability due to homogeniza-tion by intense bioturbation (Gingras et al., 2004).Some simple computations show that the sand-stone body (1Æ7 km · 5 km, 5Æ5 m pay thickness,15% porosity and 20% water saturation) couldhold upward of 35 million bbl of oil.

CONCLUSIONS

The Esdolomada 1 sandbody of the Roda Forma-tion has been interpreted as a transgressive tidalbar developed on the Tremp-Graus shelf. Themodel suggests that there are two stages of tidalbar formation: (i) the tidal bar sediments weresourced from the adjacent coeval Roda Deltawhen the delta toesets were reworked into tidaldunes by topography enhanced tidal currents;and (ii) the bar developed further by detachingfrom the delta during the succeeding transgres-sion. The Esdolomada tidal bar has grownthrough lateral accretion in keeping with theHoubolt (1968) model. The preserved tidal barhas an exposed width of 1700 m, a preserved

height of approximately 5Æ5 m and is made up ofstacked sets and cosets of medium-grained, cross-stratified sandstone; mud drapes are thin andrare; salinity was normal marine and bioturbationranges from moderate to high in the youngesttransgressive strata. Cross-stratal set thicknessdecreases laterally on the downlapping mastersurfaces and the degree of bioturbation increases,so that the cross-bedded sandstones graduallypass into highly bioturbated finer-grained andthinner-bedded sandstones lacking any cross-stratification. The rate of thinning of the lateral-accretion growth increments in the bar is some40 cm per 100 m (0Æ004) for the older, thickersandstones, whereas the younger, thinner bedsthin at a rate of 15 cm over 100 m (0Æ0015). Thebar commonly has a sharp base and top; its length(crest orientation) is oriented north-west to south-east almost parallel to the tidal palaeocurrentsand to the palaeoshoreline, but it accreted later-ally towards the south-west through time. Thedetailed sedimentary facies and internal architec-ture (dip angles and thinning rates of the accre-tion packages) of the Esdolomada tidal bar can beused as an analogue for hydrocarbon reservoirsbuilt in similar settings.

ACKNOWLEDGEMENTS

The authors would like to thank bars in tidalenvironments (BITEs) sponsor companies (Total,Nexen, Statoil, Devon, Shell, Saudi-Aramco andConocoPhillips) for their continued support onTidal Research, and sponsor-company represen-tatives for lively discussion about tidal dunes andbars. Brian Willis and an anonymous reviewer arethanked for their constructive comments on themanuscript.

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Manuscript received 4 March 2010; revision accepted27 April 2011



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