Architecture and trace-fossil characteristics of a 10-20,000 yr fluvial to marine sequence

JOURNAL OF SEDIMENTARY RESEARCH, VOL. 69, NO. 2, MARCH, 1999, P. 365–383Copyright q 1999, SEPM (Society for Sedimentary Geology) 1073-130X/99/069-365/$03.00

ARCHITECTURE AND TRACE-FOSSIL CHARACTERISTICS OF A 10,000–20,000 YEAR,FLUVIAL-TO-MARINE SEQUENCE, SE EBRO BASIN, SPAIN

ERLING I. HEINTZ SIGGERUD1 AND RONALD J. STEEL2

1 PGS Reservoir A5, PO Box 354/383, N-1324 Lysaker, Norway2 Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, U.S.A.

FIG. 1.—Simplified geological map ofCatalunya in NE Spain, showing the location ofthe Sant Llorenc del Munt study area.

ABSTRACT: The Eocene Sant Llorenc del Munt fan-delta complex lieson the southeastern edge of the Ebro Basin in NE Spain. Alluvial fansand fan deltas prograded northwestwards from the Catalan CoastalRange to form a series of clastic wedges that were interrupted by ma-rine transgressions of period varying from 60,000 to a few hundredyears, resulting in a highly cyclic succession. The cyclicity of the suc-cession was controlled by a combination of syndepositional basin-mar-gin tectonics, high sediment supply, and eustatic sea-level changes.

Analysis of one of the component (25–40 m thick) transgressive-to-regressive sequences (10,000–20,000 year duration) shows that withinits transgressive part the alluvial and coastal-plain deposits are exten-sive and thickly developed whereas shoreline lithosomes are narrowand thin. In the overlying regressive part the opposite relationship isdocumented. The transgressive and regressive tracts thus have an over-all landward-expanding and seaward-expanding geometry, respective-ly. Six ichnofabrics have been documented in the succession, rangingfrom nonmarine to offshore-transition in origin. These ichnofabrics areused to help define sequence boundaries, flooding surfaces, and maxi-mum flooding surfaces, as well as to increase the resolution of thestratigraphic analysis.

Within the transgressive tract of the main sequence, nonmarine de-posits are separated from marine deposits by a steep, landward-in-clined complex of wave-ravinement surfaces. Successive ravinementsurfaces have a vertical spacing of about a meter, are offset landwardsfrom each other by less than 100 m, and diverge slightly from eachother as they climb through the section. These same erosion surfacesmerge with each other basinwards, causing the transgressive tract tothin significantly.

Five high-frequency sequences are recognized within the main se-quence. The middle one is nearly symmetrical, the lower two are asym-metric and dominated by their transgressive tract, and the upper twoare asymmetric but dominated by their regressive tract.

INTRODUCTION

The development of a sequence stratigraphic framework for any rocksuccession relies on identification of a hierarchy of stratal units separatedby chronostratigraphic surfaces (Van Wagoner et al. 1990). Such key sur-faces have generally been mapped by registering facies changes (water-depth changes) across them. This has been done most successfully in shore-line and nearshore successions, where changes in relative sea level leavetheir mark most clearly (e.g., Posamentier and Vail 1988; Bhattacharya andWalker 1991). It has been difficult to trace key sequence stratigraphic sur-faces from nearshore into alluvial successions, though there are some ex-ceptional studies in this respect (Shanley and McCabe 1994; Rogers 1994).The present study area was chosen because of its potential to contribute tothe debate around this problem.

Excellent exposures of the Eocene Sant Llorenc del Munt fan-delta suc-cession allow stratal units and key stratal surfaces to be walked out betweenthe alluvial and nearshore facies belts. Although our exposure is mainlytwo-dimensional, we do also have some control locally on the third di-mension. The transgressive and overlying regressive components of a singlestratigraphic sequence are documented, as well as the architecture of fivehigh-frequency sequences contained therein. An important additional aimhas been to evaluate the potential of trace fossils (individual traces andmore complex ichnofabrics) in improving our understanding of the archi-tecture of such sequences.

REGIONAL GEOLOGY AND SEQUENCE STRATIGRAPHIC FRAMEWORK

The Eocene Sant Llorenc del Munt fan-delta complex is in that part ofthe Ebro Basin located in Catalunya, NE Spain (Fig. 1). The Ebro Basinis an Alpine foreland basin bounded by the Pyrenees to the north, theIberian Range to the southwest, and the Catalan Coastal Range to the east-southeast. The Sant Llorenc del Munt fan-delta complex was dispersedfrom the southeast margin of the Ebro Basin close to the Catalan CoastalRange (Fig. 2). The latter range is dominated by a series of older basement

366 E.I.H. SIGGERUD AND R.J. STEEL

FIG. 2.—A schematic paleogeographicalreconstruction of the Sant Llorenc del Munt fan-delta complex, in relation to the syndepositionaltectonic elements in Eocene time.

FIG. 3.—The sequential architecture of part ofthe Sant Llorenc del Munt fan-delta succession(after M. Lopez-Blanco 1993). The Manresa,Vilomara, and Sant Vincent composite sequencesare shown, each consisting of a series offundamental sequences that group into sequencesets. The studied fundamental sequence is thelowermost one in the El Marcet transgressivesequence set, one of the most markedtransgressive levels in the succession.

faults that moved during the late Paleozoic and were reactivated in theTertiary (Anadon et al. 1985). Two main fault systems have been described:(1) a set of nearly vertical en echelon left-lateral strike-slip basement-in-volved faults, the Valles Fault system (Fig. 2), extending for about 200 kmin a NE–SW orientation, and (2) transverse NW-striking basement-involvedfaults with only minor offsets (Fig. 2) (Guimera 1988; Anadon et al. 1985).During the Mesozoic, the transverse NW-striking, basement-involved faultswere active as normal faults; during the Paleogene they were reactivatedas strike-slip faults. The sinistral slip along the bounding faults of theCatalan Coastal Range has been interpreted in terms of regional north–south compression due to the convergent motion of the European (andIberian) and African plates during the Alpine orogeny (Guimera 1988).

This generated local transpressive conditions along the strike-slip system,resulting in emplacement of basement slices and development of folds witha northwest vergence. Uplifted areas, e.g., the ‘‘les Pedritxes’’ area, whichforms part of the source areas for the Sant Llorenc del Munt complex (Fig.2), shed detritus to form the coarse-grained alluvial fans and fan deltas ofthe present study.

The Sant Llorenc del Munt fan-delta complex extends approximately 20km into the basin and forms a 1000-m-thick, coarse-grained alluvial suc-cession that grades basinwards into marine sandstones, siltstones, and lime-stones (Fig. 3). A lithostratigraphy was established for the Paleogene sed-iments along the Catalan Coastal Range by Anadon et al. (1985). However,because of the great repetition and variety of facies within the Sant Llorenc

367FLUVIAL-TO-MARINE SEQUENCE, EBRO BASIN, SPAIN

FIG. 5.—Location of the measured profiles (1–7) along the correlation panel (Fig. 4) within the Sant Llorenc del Munt National Park.

del Munt fan-delta complex and their rapid lateral changes, a conventionallithostratigraphic subdivision was deemed impractical and an allostrati-graphic subdivision (North American Commission on Stratigraphic No-menclature 1983) was suggested by Lopez Blanco (1993) (Fig. 3). ThePriabonian stratigraphic evolution of the Sant Llorenc del Munt fan-deltacomplex was characterized by a general progradation of alluvial fans andfan deltas to the northwest. This progradation was punctuated by a seriesof transgressive episodes of varying frequency, resulting in a highly cyclicsuccession. The sequences that are best expressed in the field are approx-imately third and fourth order (sensu Vail et al. 1977; Haq et al. 1988) andwere used by Lopez Blanco (1993) to establish a series of allostratigraphictransgressive–regressive units, termed by him fundamental sequences(fourth order) and composite sequences (third order). A typical compositesequence is defined by the stacking pattern of its component fundamentalsequences, and consists of a transgressive sequence set (TSS) overlain bya regressive sequence set (RSS) (Fig. 3).

The fundamental and composite sequences in the Sant Llorenc del Muntfan-delta complex were believed by Lopez Blanco (1993) to be related tovariations in sediment supply, eustasy, climate, and subsidence. The Vi-lomara composite sequence is calculated, on the basis of integrated mag-netostratigraphy and biostratigraphy, to have had a duration of about 90,000years (see Lopez-Blanco et al., in press, for details). Because there are aboutseven fundamental sequences within the Vilomara composite sequence, thisgives the El Marcet fundamental sequence, the object of this study, a du-ration of some 10,000–20,000 years.

THE STUDY UNIT

The study unit, equivalent to one fundamental sequence of Lopez Blanco(1993), is part of the El Marcet transgressive sequence set (Fig. 3), has an

overall transgressive-to-regressive character (Fig. 4, foldout), and is ex-posed along the valley of the river Santa Creu (Fig. 5). The sequence is25–40 m thick and bounded below by an erosion surface that marks abasinward shift of sedimentary facies with respect to the underlying strata.The sub-units within the sequence (named 1–5 in Figure 4) have a slightlandward-stepping stacking pattern in the lower half of the sequence, anda basinward stepping in the upper half. The middle part of the sequence(middle of sub-unit 4 in Figure 4) shows the greatest degree of open-marineinfluence and is termed the maximum flooding zone. The sequence is alsocapped by an erosion surface, above which the abrupt occurrence of non-marine facies denotes a new basinward shift of deposition.

The five sub-units within the fundamental sequence also contain bothtransgressive and regressive components, and for this reason they are con-sidered to be high-frequency sequences. They are thin and of short duration(several thousand years), and our use of the term ‘‘sequence’’ does notnecessarily imply association with a relative fall of sea level.

The study area is exceptionally well exposed, allowing the sequenceboundaries and other key surfaces to be walked out along facies belts. Thisenabled easy correlation between measured sections.

SEDIMENTARY FACIES

Analysis of the presence, thickness, and character of individual sedi-mentary facies in the study unit showed these facies to be strongly depen-dent on the sequential organization of the succession, in two ways: (1) thepresence and volumetric distribution of the various alluvial and marinefacies within a particular time interval is dependent on whether the suc-cession is regressive or transgressive, and (2) the geometry and internalarchitecture of some facies belts, e.g., the coarse-grained nearshore units,vary depending on whether they are in a regressive or transgressive setting.


FIG. 6.—A) Conglomeratic fluvial channel with cross-stratified channel infill, cutting down into red, fine-grained floodplain/floodbasin deposits. B) Fluvial conglomeratesin channels that have amalgamated to form thick conglomeratic bodies

Fluvial Conglomerates

Description.—Fluvial conglomerates are relatively coarse grained (clastsup to 10–15 cm), poorly sorted, and clast-supported, and they never containbored pebbles or other fossils. They occur in erosionally based, lenticular

bodies that usually cut into red sandstones and siltstones, and they arepresent in most of the investigated profiles through the sequence (Fig. 4).The conglomeratic bodies are, in some cases, contained by relatively nar-row, steep-walled channel forms (Fig. 6A) but are usually more extensive(tens to hundreds of meters). The depth of individual channels rarely ex-


FIG. 6.—C) Small fluvial channel that has suffered soft-sediment deformation. There is some mixing of conglomerate and adjacent red siltstone. D) Debris flow markedhorizon showing bi-modal texture, and mixed angular and rounded clasts.


FIG. 7.—A) Lag conglomerate. Note the borings on some of the uppermost peb-bles in the layer, to the right of the lens cap. B) Vertical repetition of marineconglomerate lags, with finer-grained nonmarine deposits between.

FIG. 8.—Detailed sedimentary log through the transgressive tract of El Marcet 1at locality 7a. Note the multiple wave-ravinement surfaces (R), with their overlyingbored-pebble lags. The upward-coarsening trends in the deposits between the bored-pebble lags indicate coastal-plain or shoreface progradation between ravinement ep-isodes. The ‘‘parasequences’’ in this log form groups that are mainly nonmarine(A), mixed nonmarine and marine (B), and mainly marine (C). The upward transitionfrom fluvial to marine, as well as the increase in Ophiomorpha (bioturbation symbol)upwards, indicates the overall transgressive nature of the tract.

ceeds 1–1.5 m, although amalgamation of channels creates more wide-spread bodies some 5–6 m thick (Fig. 6B). The conglomerates can be large-scale cross-stratified (Fig. 6A), show patchily developed clast imbrication,and usually show no obvious internal trend of upward fining or coarseningwithin the bodies. Where the conglomerates occur in relatively small chan-nels, within a section otherwise dominated by fine-grained floodplain de-posits, there is commonly a significant amount of load-induced soft-sedi-ment deformation (Fig. 6C). In some cases this causes mixing of conglom-eratic and silty deposits. The clasts consist of up to 70% light-coloredlimestone and quartzite, and are in a fine- to medium-grained sandstonematrix. The remaining 30% are metamorphic clasts, and these give thefacies a dark green appearance, in contrast with the lighter-colored shore-face gravels.

Interpretation.—The poorly sorted but clast-supported texture of theconglomerates, their channeled geometry with fairly low width/depth ratios,their close association with red siltstones and mudstones, and the lack ofmarine trace fossils suggest an alluvial origin (see also Rasmussen 1993),probably as low-sinuosity, streamflood channels (Steel 1974; Nemec andSteel 1984) incised into fine-grained floodbasin deposits. The coarsenessof the conglomerates and their tendency to amalgamate landwards into aconglomerate-dominated succession (Lopez-Blanco 1993) further suggestthat the river channels were distributaries of an alluvial-fan system. Thedowncutting bases of the channels probably reflect catastrophic events onalluvial fans in areas where there was little or no vegetation to inhibit rapidrunoff after sudden rain storms.

Floodplain and Coastal Floodbasin Deposits

Description.—Massive or ripple-laminated red siltstones, mudstones,and fine sandstones (Fig. 6A, C) are present in most of the measured sec-tions and are in units that vary in thickness from less than 1 m in distalareas (profile 7A, Fig. 4) and up to 12 m in more proximal areas (profiles3–5, Fig. 4). In profiles 1 and 2 (Fig. 4) and in even more landward areasthe thickness of this facies is again reduced, thinning as a result of thedominance of fluvial conglomerates. In some cases the beds of this faciesform as upward-fining units (5–15 cm thick) in small channels that are


FIG. 9.—A) Marine shoreface or mouth-bar sandstone and conglomerate bedsforming an upward-coarsening unit. B) Marine conglomerate–sandstone upward-fining couplets form the building blocks of larger scale, upward-coarsening units.

capped by reddish sandy mudstones. The deposits of this facies are usuallyhighly bioturbated with Taenidium, Scoyenia, and Planolites traces (ich-nofabric 1). Where these deposits are associated with conglomeratic chan-nels, there is a often a high degree of soft-sediment deformation.

Interpretation.—The relatively fine-grained nature of this facies, thenonmarine trace fossils, and its close association with the fluvial conglom-erates suggest a low-energy alluvial environment. The general location ofthese red beds, between conglomerate-dominated alluvial-fan deposits up-slope and marine shoreline facies downslope suggests a coastal setting, andthe environment was probably a series of coastal floodbasins. The smallsandy channels probably reflect low-sinuosity streams emerging from thelower reaches of alluvial fans and crossing the floodbasins. Some of thefinest-grained and better-laminated intervals may also represent lagoonaldeposits, though there is no specific trace-fossil evidence to support this.

Debris-Flow Marker Horizon

Description.—This facies is a poorly sorted (often bimodal), matrix-supported conglomerate (Fig. 6D) that is present as a single and easilyrecognizable bed over much of the study area (Fig. 4). The conglomeratehas clasts (0.3–4 cm in size) of quartzite and limestone as well as darkmafic rocks, and these tend to ‘‘float’’ in a matrix of red mudstone/siltstone(Fig. 6D). There is usually an inverse clast-size grading at the base of thebed (and in some cases through the whole bed) as well as a marked increasein the matrix percentage in the uppermost level of the bed. Bed boundariesare sharp and planar, with only minor basal erosion. In the proximal area

(profile 1, Fig. 4) this marker bed was observed to be partly eroded byfluvial channels.

Interpretation.—The well-defined nature of the bed, its matrix-supportedtexture, as well as lack of internal lamination or channeling suggests that theunit is a sheet-like sediment gravity flow (Gloppen and Steel 1981). Thefiner-grained and inversely graded basal carpet and upward increase in matrixcontent further suggest a cohesive debris flow in a water-rich environment(Nemec and Steel 1984). The larger particles were supported by the buoyancyand cohesiveness of the water-saturated sediment. The marker debris flow islocated in the middle levels of the transgressive tract of the studied sequence,and is closely associated with fine-grained, red floodbasin deposits. We sug-gest that high rates of sea-level rise at the contemporary shoreline (less than1 km basinward) caused high water-table levels and high rates of sedimentaccumulation within the coastal floodplains and floodbasins. Local pore-waterpressure buildup, together with the relatively steep slopes of this setting, arelikely to have caused sediment instability. Occasional mixing of gravels andsilts, as a result of instability and slope failure, would have caused remobi-lization and generation of debris flows.

Transgressive Lag Conglomerates

Description.—There is a distinctive conglomerate facies in thin (up to10 cm) beds a few clasts thick. A characteristic feature is the abundantmarine-organism borings on the upper surfaces of the upper clasts (Fig.7A, B). These beds usually sharply overlie nonmarine conglomerates (with-out bored clasts) or sandstones (Fig. 8). The clasts are mainly white tolight-colored limestone or quartzitic pebbles and cobbles, generally wellsorted and very well rounded. The thin beds, consisting of clast-supportedframeworks with occasional matrix-rich pockets, are abruptly draped byvery fine-grained conglomerates or well-bioturbated marine sandstones/silt-stones.

Interpretation.—Conglomerate beds of this facies are clearly associatedwith (1) transgression (marine above nonmarine), (2) erosion (to create thesharp, underlying surface), and (3) marine abandonment (to produce themarine borings on the pebble surfaces). This, together with the well-sortedand tight framework nature of the gravel assemblage, indicates that theconglomerate beds are lag deposits, created by marine erosion and sedimentconcentration in a nearshore zone, and subsequently drowned by deeperwater in a setting conducive to rock-boring organisms. This strongly sug-gests shoreface ravinement during transgression, the accumulation of a thintransgressive lag, and finally abandonment of this surface during fartherlandward retreat of the shoreline. The lag deposits are thus a product ofrelative sea-level rise and transgression (see also Kidwell 1991), and con-trast strongly with other marine conglomerates that accumulated duringshoreface regression, as described below.

It is important to note that the wave-ravinement surface underlying eachlag reflects destruction of the beachface and upper-shoreface zones duringtransgression. The conglomerate beds were therefore not deposited as fore-shore gravel, but represent gravel moved by storm waves farther downslopeonto a previously created erosion surface.

Regressive Marine Conglomerates

Description.—Another type of marine conglomerate, seen in the upper(regressive) parts of the study sequence (profiles 3–7 in Figure 4), interfin-gers and associates closely with marine sandstones. This facies consists ofa series of thin (up to 15 cm) beds of clast-supported conglomerate, withrelatively well rounded (but not bored) clasts of mainly limestone andquartzite (Fig. 9A). There are usually very few metamorphic clasts, givingthe conglomerate a very light appearance, in contrast to the associateddarker, fluvial conglomerates. Interlayered fine to medium sandstones areplane-parallel or low-angle laminated, and contain marine trace fossils(plain Ophiomorpha fabric) (Fig. 9A). The conglomeratic beds are some-


times normally graded, and each bed tends to grade upwards into an over-lying, bioturbated sandstone (Fig. 9). In some of the thicker conglomerateunits, large-scale, basinward-directed, low-angle foresets were observed, aswell as low-angle, erosional bounding surfaces. Where the conglomeratebeds form thicker units they generate upward-coarsening trends, shownboth by upward-decreasing sandstone interbeds and by upward-increasingclast sizes (Fig. 9A). In some areas, the conglomerates show a well devel-oped shape sorting of clast populations in beds, with sphere/rod-dominatedlayers alternating with disc/blade-dominated layers.

Interpretation.—The fairly mature textures of these conglomerates andtheir association with Ophiomorpha-bearing sandstones indicate a marinesetting. Where the conglomerates show an upward-coarsening (prograda-tional) trend and are associated up-dip with more poorly sorted fluvialconglomerates, a gravelly deltaic mouth-bar setting is likely (e.g., seeKleinspehn et al. 1984). This interpretation is consistent with the erosionalbases seen below some conglomerate units, as well as with the associatedlow-angle cross-stratification (see also Rasmussen 1993). Where there areno updip-associated fluvial facies the regressive conglomeratic units maysimply represent gravelly shoreface progradation (see Bourgeois and Leit-hold 1984). The shape-sorted clast populations seen in some conglomerateslook similar to those produced by the sorting processes operating on thesteep beachface zones (Bluck 1967).

Marine Fan-Delta-Front and Shelf Sandstones

Description.—This facies is dominated by very fine- to medium-grainedsandstones, in sets (Fig. 9B) that are 10–15 cm thick and show horizontallamination or hummocky cross-stratification, commonly with a capping ofwave-ripple lamination. Bioturbation in the sandstone varies from a highdegree in the lower part of upward-coarsening units (see description ofichnofabric 6 below) to more sparse bioturbation with only solitary verticalOphiomorpha traces in the middle and upper parts of such units. Sandstonesof this type are present in the upper parts of small-scale (20–50 cm thick)upward-fining units (Fig. 9B), but these in turn form a part of thicker,upward-coarsening units.

Interpretation.—Because of the hummocky and horizontal strata, thetrace fossils, and the upslope association with nearshore conglomerates, weinterpret this facies in terms of wave-generated deposits on a delta-front/shoreface to shelf setting, mainly below fair-weather wave base (see anal-ogous successions described by Kleinspehn et al. 1984). The associatedfine conglomerate beds are likely to have been introduced onto the deltafront from mouth-bar systems, possibly later redistributed by longshore orrip currents (e.g., Walker and Bergman 1993).

Heterolithic Units of Alternating Mudstones, Fine Sandstones, andThin Conglomerates

Description.—This heterolithic facies consists of mudstones, alternatingwith very fine- to fine-grained sandstone beds and very thin conglomeraticbeds. The sandstone beds show hummocky cross-strata passing upwards toplane-parallel and wave-ripple lamination, and then to mudstones. Thesandstones grade upwards from the conglomerate layers. This facies issomewhat restricted in its occurrence. Both the sandstone and conglomer-atic beds tend to thin basinwards, whereas the number and thickness ofmudstone beds increase basinwards. All of the beds are heavily bioturbatedby Ophiomorpha, producing complex mazes and boxworks that typify ich-nofabric 5.

Interpretation.—This heterolithic facies occurs as bottomsets and lowerforesets within prograding mouth-bar or shoreface clinothems of the fan-delta front (see also Figure 16).

TRACE FOSSILS AND ICHNOFABRICS

Various methods have been suggested for identification and descriptionof individual trace fossils, and for characterization of trace-fossil assem-

blages (e.g., Bromley and Ekdale 1984; Bromley 1990; Pemberton et al.1992; Taylor and Goldring 1993). The main building block in the meth-odology suggested by Pemberton et al. (1992) is the ability to recognizedifferent trace fossils and assemblages of traces within the ichnofacies con-cept (Seilacher 1964, 1967; Frey and Seilacher 1980; Ekdale et al. 1984).The concept emphasizes that just as various sediments and physical sedi-mentary structures can be grouped together to define sedimentary facies,so recurring trace-fossil assemblages and associated sediments can begrouped into different ichnofacies. Although the main factor controlling thedistribution of the respective traces within ichnofacies was originallythought to be water depth (e.g., Crimes 1975; Frey and Seilacher 1980),more recent studies have stressed the additional importance of substrate,energy levels, and oxygenation. As pointed out by Bromley and Ekdale(1984) and Bromley (1990), however, the fabric resulting from sediment-manipulating organisms may vary depending upon the degree of activitywithin a trace-fossil assemblage. For such a texture Bromley and Ekdale(1984) proposed the term ichnofabric, defined as ‘‘all aspects of the textureand internal structure of a sediment that results from bioturbation at allscales’’. The advantage of the ichnofabric concept is that it involves studyof the trace-fossil-bearing sediments in their entirety rather than analysisof the distinct trace fossils only (Goldring and Pollard 1993). The study ofichnofabric leads to each trace being interpreted with other traces and withany primary sedimentary structures that remain. The time sequence andvertical tiering of the traces can then be determined. Application of theichnofabric concept involves identification of the different ichnofabricswithin the study unit using a bioturbation index and a diagram in whichthe various details of the ichnofabric are graphically plotted with respectto dimensional data of ichnotaxa, and their order of emplacement againstcoverage. Developed mostly for core description, this method proved tooelaborate and time-consuming to be applied to a large field area, wheregood lateral control exists. A pragmatic modification of the methodologysuggested by Taylor and Goldring (1993) was used here. This is based onrecognition and organization of distinct coexisting traces and sedimentaryfacies into different ichnofabrics. Six different ichnofabrics were distin-guished (Fig. 10), and are described as suggested for outcrop work byPollard et al. (1993).

Ichnofabric 1

Description and Occurrence.—This ichnofabric is characterized byTaenidium, Scoyenia, and Planolites, found in very fine- to fine-grainedsandstones and siltstones, interfingering with red mudstones. The parallel-laminated, thin mudstone beds are observed to be variably disrupted bybioturbation that consists of 0.1–1-cm-thick Planolites traces crosscuttingthe bedding in a vertical to subvertical pattern. No wall structure is ob-served, and the infill is somewhat coarser than the surrounding sediment.The Taenidium traces cut down into the more sand-rich beds, and theymeasure 5–10 m in diameter (Fig. 11A), most often with a fine-grainedmuddy infill. The Scoyenia traces are mostly horizontal crawling–feedingtraces (Fig. 11B) with distinct spreiten structure indicating direction ofmovement. The cross-cutting of the traces made accurate measurements ofthe original length of the individual traces impossible, but the width of thetraces ranges from 0.3 to 0.8 cm. Although branching was observed, it isnot common. This ichnofabric was observed in all the logged sections andpreferentially occurs in the red floodplain and floodbasin deposits. Thick-ness development of this ichnofabric varies in the section from about 60cm in profile 7B to up to more than 5 m in profile 2 (Fig. 4).

Comparison.—The ichnology of nonmarine environments has, until re-cently, been little described and therefore poorly understood. Only theScoyenia ichnofacies has been established in the nonmarine realm (Freyand Pemberton 1984; Pemberton et al. 1992). Pemberton et al. (1992)pointed to the restricted evolution of terrestrial organisms prior to the Cre-taceous, making correlation with present-day settings somewhat ambiguous


FIG. 10.—Summary of the six ichnofabrics recognized in the study sequence. The right side shows the positioning and significance of the individual fabrics within arepresentative profile through the transgressive-to-regressive sequence. The irregularity at some levels reflects higher-frequency sequences.

at times. Observations by Bromley and Asgaard (1979), Pollard (1985,1988), Marples and Archer (1989), and Buatois and Mangano (1993) are,nevertheless, showing an increasing number and variety of traces in Pre-Cretaceous strata.

Discussion.—The red color of the host sediment for this ichnofabric, inaddition to the interfingering and lateral correlation with the fluvial con-glomerates, indicates a nonmarine setting. The Scoyenia traces found in thesmall-scale (5–10 cm) fining-upward, sandy to muddy siltstone units prob-ably occur within overbank-levee deposits on a floodplain environment.Similarly the Taenidium burrows observed to cut down from the top of thesandstone beds probably indicate a channel-bank environment where theburrows were filled with red mud and the trace maker presumably drownedduring flooding. The smaller sand-filled Planolites burrows observed in thefinely laminated mudstone and sandstone beds probably indicate ponded orshallow lacustrine areas between the active channel belts.

Ichnofabric 2

Description and Occurrence.—Ichnofabric 2 is characterized by sub-vertical, gravel-filled shafts. The overall form is somewhat irregular (Fig.12A), commonly 5 cm in width, with lengths varying from 25 cm to 128cm. The irregularity of the burrows implies that the individual shafts hada highly variable shape, and in some cases branching is observed (Fig.12B). Most burrows have an opening diameter up to 10 cm wide but be-come narrower downwards. There is no wall structure between the traceand the host sediment. The infill in the traces is gravel and pebbles from

the bed above, mixed with fine- to medium-grained sand (Fig. 12B). Thetraces are seen to cut only into fine-grained sediments within the underlyingbeds, and not where there is a conglomerate subsurface. This ichnofabricusually occurs directly below the marine lag conglomerates and was foundthroughout the studied succession, within horizons some 50–150 cm thick.The thickness of the fabric is conditioned by the vertical length of thetraces.

Comparison.—The traces characteristic of this ichnofabric resemble thePsilonichnus ichnofacies of Frey and Pemberton (1987), which was de-scribed as developed in a mixture of marine, quasi-marine, and nonmarineconditions, probably set in a backshore environment. They resemble pres-ent-day crustacean burrows, many of which are found to be living on tidalflats or within shallow marine settings such as the Atlantic coast of theU.S.A. (Bromley 1990). Other producers of similar traces or burrows arefreshwater crayfish (Hasiotis and Mitchell 1993), or land crabs, which arewell known from recent terrestrial and backshore realms in warmer regions.Some of these land crabs are known to descend to a depth of more than 3m in order to reach the water table (Bright and Houge 1972). In spite oftheir abundance, little is known about their burrow morphology.

Discussion.—Although relatively poorly preserved, this ichnofabric pro-vides a good example of the use of ichnology in sequence stratigraphicinterpretation. Because the burrows occur directly below the ravinementsurface at the base of the marine lag conglomerates it is probable that theywere formed as a result of the initial brackish-water or marine-water influxonto the floodplain/floodbasin areas, immediately prior to encroachment of


FIG. 11.—A) Taenidium traces found within some of the sandier floodplain de-posits. Scale is 6 cm. B) Horizontal to subvertical Skoyenia traces with distinctspreite. These traces are found mainly in the muddier zones of the floodplain de-posits. Scale is 10 cm.

open-marine conditions and wave ravinement. The burrows thus herald thetransgression proper and represent the first clear evidence of initial marineflooding into the back-barrier and lower coastal-plain areas. As sea levelcontinued to rise, the wave-ravinement surface passed landwards across thearea, allowing accumulation of a transgressive lag that tended to cover andfill the burrows with gravel. The fact that these burrows are observed atseveral levels within the floodplain/floodbasin succession of profiles 2 and3 in Figure 4 (showing an overall upward increase in marine influx) reflectsthe overall ‘‘transgressive’’ or back-stepping nature of the nonmarine suc-cession. This relationship between the increasing occurrence of ichnofabric2 and the independently shown transgressive nature of the succession sup-ports a marine origin of the trace makers, though their precise nature isstill uncertain.

Ichnofabric 3

Description and Occurrence.—Ichnofabric 3 occurs as thin beds oftightly packed lag conglomerate, containing bored pebbles (see also Figures7 and 8). The borings occur exclusively on pebbles of whitish limestone.The diameter of the borings does not exceed 10 mm, and most boringsrange from 3 to 7 mm. Borings with a diameter of up to 10 mm werefound not to penetrate through pebbles with a diameter of 70 mm. Theborings are in many cases filled with red, fine-grained sand. This ichno-fabric is restricted to the transgressive lags of the study sequence.

Comparison.—Several different types of recent organisms are known tobore into hard surfaces. Modern analogue studies show a relationship both

between protection and feeding and between protection and living behavior.The bored clasts of this ichnofabric are placed by Pemberton et al. (1992)in the Trypanites ichnofacies of Frey and Pemberton (1984). This developsin fully lithified substrates such as hardgrounds, reefs, rocky coats, beach-rock, and other types of omission surfaces. The recognition of this ichno-facies, as pointed out by Pemberton et al. (1992), when corresponding witha sedimentological discontinuity, may therefore have major sequence strati-graphic significance.

Discussion.—Boring in clasts is probably a response to stressed livingconditions or to danger of predation. The possibility of boring of pebblesin fresh-water conditions (e.g., Ekdale et al. 1989) should not be overlookedbut is unlikely here because of a close association with overlying marinesiltstones. Boring takes place in areas of little or no sediment supply, i.e.,on an abandoned surface in a hardground environment. Bored pebbles be-came concentrated at the tops of conglomerate beds as lags resultant fromshoreface transgression. The lags developed during ravinement, whereasthe boring occurred during the subsequent deepening across the abandonedsurface. The bored ichnofabrics are notably absent from regressive con-glomeratic shoreline deposits in the succession.

Ichnofabric 4

Description and Occurrence.—Ichnofabric 4 consists of solitary, ver-tical to subvertical Ophiomorpha, without galleries, that vary in length from10 to 50 cm (subvertical), with thickness of the shafts varying between 0.5and 2 cm. The traces are mostly confined within beds, and only rarelypenetrate the underlying bed (Fig. 13A). The shafts are usually lined,though in some cases unlined traces are observed in more gravelly beds.Infill rarely shows spreiten but consists of sand or gravel from overlyingbeds, indicating passive infill of the shafts. Walls consist of muddy silt andsand, with generally well-formed pellets. The thickness of the walls variesaccording to the grain size of the surrounding sediment. ‘‘Turn-around inshafts’’ (Frey and Pryor 1978) are common. This ichnofabric is found inall profiles and varies in thickness from less than a meter in the proximalareas (profile 1, Fig. 4) to 11 m in the more distal areas (profile 7, Fig. 4).

Comparison.—From present-day Atlantic coasts of southeast U.S.A.large callianassid species (e.g., Callianassa major) that construct pelletedwalls similar to Ophiomorpha are known to invade the tidal zones of me-dium-energy, sandy beaches. The burrow system of Callianassa major inparticular has been recorded to reach down to a depth of 5 m in the higherareas of the beach (Frey et al. 1978). Earlier considered to be depositfeeders, they are now considered to have a dwelling if not predatory feedinglifestyle (Bromley 1990). In contrast to the traces observed in ichnofabric4, Callianassa major develops a tightly closed aperture on the top of thetunnel, probably to protect the animal from other predators and invasionof sediment (Bromley 1990). The fecal pellets produced by Callianassamajor are compact and do not disintegrate readily. They are made fromincrements of muddy sand bound with a gelatinous mucus, and in somecases make the walls of the shafts as thick as the inside diameter of theburrow itself.

Discussion.—Ophiomorpha-type traces are well known from storm bedsin an offshore to lower-shoreface setting (e.g., Frey 1990; Frey and Howard1990; MacEachern and Pemberton 1992) and from nearshore to upper-shoreface settings (e.g., Howard 1978; Pemberton et al. 1992; Pollard etal. 1993). The apparent lack of aperture within the individual traces in thisfabric, and absence of well-developed horizontal tunnels and galleries, sug-gest high energy and relatively rapid sedimentation, such as would occurin middle-upper shoreface to foreshore settings.

Ichnofabric 5

Description and Occurrence.—This ichnofabric occurs as part of theheterolithic facies described above, and is characterized by a combination


FIG. 12.—A) Arthropod burrows characteristic of ichnofabric 2, penetrating down into nonmarine deposits. Scale is 10 cm. B) Close-up of the same arthropod burrows.The fill consists of marine gravel and sand. Scale is 10 cm.

of single vertical to subvertical Ophiomorpha traces (similar to ichnofabric4) and a more complex Ophiomorpha maze (Fig. 13B). The facies asso-ciation represents the lower reaches of the shoreface, with fine-grainedsandstone layers (sometimes with thin conglomeratic bases) passing up-wards to mudstones. The sandstone beds have hummocky cross-stratifica-tion and wave-ripple lamination and contain vertical to subvertical singleOphiomorpha with shafts 10–15 cm in depth and 0.5–1.5 cm in diameter.The ‘‘turn-around in shaft’’ (Frey et al. 1978) is common in the studysuccession. Within the mudstones a more complex Ophiomorpha mazeoccurs, consisting of tunnels and shafts connected in a branching pattern(Fig. 13B). The single Ophiomorpha traces within the sandstone beds com-monly penetrate into the top of the beds of the unit below. The Ophio-morpha maze within the mudstones was similarly observed to penetrateinto the underlying sand beds. The degree of bioturbation is fairly constantwithin the mudstone beds but varies from absent to abundant in other beds(Fig. 13A). This fabric was found in profiles 4, 5, 6, and 7 (Fig. 4). Itsthickness varied from 50 cm in the medial areas (profile 4, Fig. 4) toapproximately 3.5 m in a basinward direction (profile 7, Fig. 4).

Comparison.—The Ophiomorpha fabric described by Pollard et al.(1993) was interpreted in terms of deposition and colonization of estuarinepoint bars. However, ichnofabric 4 here (Fig. 4) is developed on a high-energy shoreface.

Discussion.—The heterolithic nature of the facies containing the fabricindicates an environment characterized by variable energy and rapid shiftsin sedimentation from mud to sand and gravel. The simple vertical Ophio-morpha within conglomerate-based sandy beds suggests a high-energy set-ting similar to the situation in ichnofabric 4. However, the complexOphiomorpha maze developed in the mudstone beds suggests a lower-energy, more protected environment setting. The apparent lack of tieringwithin this fabric is consistent with a steady progradation, not giving thetrace makers time to develop more complex tiering patterns. This fabric istherefore interpreted in terms of a setting where wave- and storm-reworkedsands accumulated in the distal reaches of a fan-delta front (Fig. 14).

Ichnofabric 6

Description and Occurrence.—Ichnofabric 6 is characterized by a highdegree of bioturbation (index 3–4), by horizontal tunnels with Thalassi-noides, and more sparse Ophiomorpha. The Thalassinoides were observedas horizontal oval burrow fills 3–6 cm in diameter (Fig. 15). The length ofthese horizontal tunnels was difficult to measure because of a high degreeof overprinting of the individual traces, but traces approximately 50 cmlong were observed. Thalassinoides has no distinct wall structure but iseasily observable because of its lighter-colored sandy infill. In some cases


FIG. 13.—A) Marine shoreface sands containing Ophiomorpha (ichnofabric 4),some of which penetrate into the bed below. Scale is 2 cm. B) Ichnofabric 5 shownby beds in the upper part of the photo, where there are sand-filled Ophiomorphaboxworks and mazes in muddy siltstones. The lower beds contain single, nearlyvertical Ophiomorpha. Scale is 2 cm.

this infill contains small amounts of stacked Nummulites foraminifera.Some traces were observed where the wall consisted partly of a thin, darkermud–sand lining. The Ophiomorpha traces only partly resemble those ofichnofabric 4 and 5, in that the burrows here are predominately horizontalto subvertical and have well-developed galleries. Walls made of muddy siltand sand pellets are in general thicker than the walls in ichnofabrics 4 and5. The high degree of overprinting and interfingering makes precise mea-surement of the individual traces difficult. Ichnofabric 6 is present in allprofiles except in the proximal areas of the sequence. The overall thicknessof the fabric varies from less than 1 m in the proximal areas (profile 3,Fig. 4) to 3 m in the more distal areas (profiles 6 and 7, Fig. 4).

Comparison.—Numerous present-day crustaceans produce burrows andtraces similar to the Thalassinoides traces known from the rock record (seeBromley 1990, his figs. 12.9 and 12.10; Pemberton et al. 1992; Pollard etal. 1993, their fig. 8A). Despite this richness in trace producers, and anumber of different species producing similar burrows, the overall shapeof the burrows is similar and basically rather simple. This makes the cor-relation with particular present-day trace makers and life styles rather am-biguous.

Discussion.—The high degree of disruption of the original bedding andlarge degree of overprinting here suggest that, unlike in ichnofabric 4 and5, the trace makers of this fabric had ample time to colonize the sediment.This situation is consistent with transgressive conditions, where little sed-iment is transported onto the shelf. Thalassinoides suggests a transitional

to offshore environment in keeping with the preferential occurrence of thisfabric within the maximum flooding zone of the sequence (Fig. 4).

TRACE FOSSILS IN SEQUENCE STRATIGRAPHY

Trace fossils have often been neglected, except for indications of degreeof bioturbation (little, medium, or much bioturbated) as a crude indicatorof environmental conditions. Although groups of trace fossils, or ichnofa-cies, are sometimes used as though paleobathymetry were the only con-trolling factor, it has been strongly emphasized (e.g., Crimes 1975; Freyand Seilacher 1980; Ekdale et al. 1984; Bromley 1990; Pemberton et al.1992) that trace fossils are in fact the tangible evidence of the response ofthe animals (the original trace makers) to changes in the environment, andthus provide information about the depositional history of the sediments inwhich they were made. Taylor and Gawthorpe (1993) pointed out thatichnofabric analysis is especially useful in a sequence stratigraphic frame-work because it can aid in identification of stratal surfaces, condensed sec-tions, and facies shifts. The stacking distribution of ichnofabrics in timeand space is especially useful.

Ichnofabrics and Sequence Boundaries

Identification of a sequence boundary (Van Wagoner et al. 1990) in-volves recognition of an unconformity (or of a significant regressive surfaceof erosion) where the sedimentary facies lying above this surface show aclear basinward shift with respect to those lying below. Unfortunately, thisfacies shift may not be obvious along the entire dip extent of the surface,because of variation in width of facies zones above and below the surface,particularly because facies belts of the same environment can have greatlydifferent widths when developed in transgressive tracts as opposed to re-gressive tracts. In the present study, trace fossils and their ichnofabrics haveaided identification of sequence boundaries in the following ways:

(1) Identification of a nonmarine ichnofabric lying above a marine ich-nofabric in a conglomeratic succession (Fig. 16) where it is otherwise dif-ficult to distinguish marine from fluvial textures (e.g., upper sequenceboundary as shown in Figure 4 at localities 2, 3, 4).

(2) Identification of changes in vertically stacked ichnofabrics, such aswhere a stacking pattern of progressively less marine (or shallower marine)ichnofabrics is overlain by a series of progressively deeper or more marineichnofabrics (Fig. 10). This represents the turnaround from a regressive toa transgressive stacking of sedimentary units and is sometimes easier topinpoint by ichnofabrics than by sedimentary facies alone (e.g., boundarybetween sub-units 4 and 5 in Figure 4).

Some of the ichnofabric shifts and trends are illustrated schematically inFigure 17. We suggest that identification of ichnofabrics and of changes inthe stacking patterns of ichnofabrics are a more reliable method of iden-tifying sequence boundaries than identification of particular or individualtraces.

Ichnofabrics and Flooding Surfaces

Flooding surfaces are surfaces across which there is sharply defineddeepening of marine facies, an appearance of marine facies in otherwisenonmarine deposits, or other signs of abrupt base-level rise in the stratig-raphy (see Bhattacharya and Walker 1991 for discussion of the variety ofsuch surfaces). Trace fossils and ichnofabric evidence often proved to bemore useful here than in the case of identifying sequence boundaries, notleast because there are many flooding surfaces present for each sequenceboundary encountered. Flooding surfaces are characterized by the incomingof a particular marine ichnofabric, (e.g., Taylor and Gawthorpe 1993, theirfig. 2b) or by the overprinting of a previous fabric by another, characteristicof deeper or more saline conditions. In the present study, flooding surfaceswere identified by:


FIG. 14.—A schematic development history for ichnofabric 5. It occurs within an upward-coarsening shoreface lithosome (clinothem). The individual shoreface sedi-mentation units are storm-generated and show an abrupt upwards fining.

FIG. 15.—Complex nature of Thalassinoides burrows of ichnofabric 6. Scale is7 cm.

(1) The abrupt appearance of intensely bioturbated fabrics (above lessbioturbated levels), in the distal reaches of the systems tract (e.g. Fig. 18).

(2) The appearance of coarse pebble lags with borings within the con-glomeratic nearshore successions (Fig. 8). The flooding surface does notcorrespond with the level of borings, but usually with an erosion surface

immediately below the pebble lag. The boring occurred during abandon-ment of the area, after the landward translation of the high-energy rework-ing zone that created the pebble lag. In such conglomeratic successions itis difficult to identify repeated transgressive events other than by use ofthe presence of bored-pebble levels (Fig. 8).

(3) The appearance of marine burrows in red, nonmarine sediments, rep-resenting a marine incursion into a coastal-plain setting (Fig. 19). In thiscase the marine burrows immediately underlie transgressive pebble lags,emphasizing that the nonmarine area was partially flooded by marine watereven before the higher-energy ravinement produced the pebble lags andbefore the even later boring activity (Fig. 19).

Ichnofabrics and Maximum Flooding Surfaces

A maximum flooding surface (Van Wagoner et al. 1990) correlates withthe time of maximum landward transgression of a shoreline and can beidentified at the turnaround in stacking pattern of ichnofabrics from thetransgressive to the regressive parts of the system. In the present study, theobserved change in type of ichnofabric was found to occur within a zone(rather than a surface) characterized by a minor increase in trace-fossildiversity and a major increase in degree of bioturbation and resulting com-plexity of ichnofabric compared to the ichnofabrics immediately above andbelow (Fig. 20). The complex Thalassinoides burrows of ichnofabric 6indicate deeper-water conditions than for ichnofabric 5, found immediatelyabove and below. An important point of this interpretation is that the max-


FIG. 16.—Identification of a sequenceboundary on the basis of a nonmarineichnofabric lying above a marine ichnofabric ina conglomeratic succession (e.g., top of profiles5, 6, and 7 in Figure 4). The conglomeratesalone do not always give a clear indication ofthe environmental change.

imum flooding level is not (or cannot be identified as) a particular surface,but occurs as an ichnofabric maximum flooding zone (cf. interval of max-imum flooding of Kidwell 1989). The zone varies from 0.5 m to 1.2 mthick (from proximal to distal reaches) in the sequence illustrated inFigure 4.

SEQUENCE STRATIGRAPHIC ARCHITECTURE

The sequence illustrated in Figure 4 is some 25–40 m thick, and has anoverall transgressive-to-regressive character as deduced by facies and ich-nofabric criteria. The criteria used to recognize the lower and upper bound-aries of this sequence have been discussed above.

Transgressive–Regressive Symmetry

The transgressive aspect of the lower half of the sequence is evidencedby:

● Landward translation of the marine conglomerates through time, seenat the seaward end of the Figure 4 panel (localities 7 to 4). It is also worthnoting that the frequency of individual transgressive events registered with-in the overall transgressive succession (as detailed in Figure 8) is less than500 years (assuming transgressive events to be of about equal durationduring aggradation of the transgressive tract).

● Upward increase and landward encroachment of brackish and marineinfluence seen within the alluvial reaches of the succession at the landwardend of the panel (localities 2 and 3, Fig. 4).

The regressive aspect of the upper half of the sequence is clear from:● Its overall upward-coarsening character and the offlapping, shoreface

clinoform patterns.● The vertical change in ichnofabrics.In the middle reaches of Figure 4 the sequence can be seen to be nearly

symmetrical as regards the relative thickness of its transgressive and re-gressive parts. However, the regressive part thins radically towards thelandward reaches (because of sediment bypass and nondeposition, and/orerosion by the overlying sequence boundary), whereas unpublished datafrom beyond the basinward part of Figure 4 show that the transgressivepart of the sequence thins significantly in that direction. This implies that

the sequence has a landward geometric expansion in its transgressive sys-tems tract and an overall seaward expansion in its regressive systems tract.

The significant thickness of the transgressive deposits is unusual, and wesuggest that this is in keeping with the sequence being one of a series, ina transgressively stacked set of sequences.

El Marcet Ravinement Complex

Early sequence stratigraphic models emphasized mainly the initial trans-gressive surface and younger flooding surfaces as key stratigraphic surfaces(Vail 1987), despite the fact that Swift (1968) had shown clearly the im-portance of the ravinement surface (also referred to as a transgressive sur-face of erosion by Bhattacharya 1993), generated by shoreface erosion ontransgressive coastlines. Sequence stratigraphic models placing moreweight on the ravinement surface came with Nummedal and Swift (1987),Bhattacharya and Walker (1991), Swift and Thorne (1991), Thorne andSwift (1991a), and Bhattacharya (1993). Swift et al. (1991) documentedclearly that this important surface always falls within the transgressive sys-tems tract and separates a back-barrier lithosome below from a shelf lith-osome above. Swift et al. (1991) emphasized also that the landward tra-jectory of the ravinement surface can be steep, i.e., need not be flat as oftenportrayed. Nevertheless, most portrayals of the ravinement surface (e.g.,Thorne and Swift 1991b, their fig. 10) are as a single master surface ontowhich lesser marine erosion surfaces may onlap during landward translationof the shoreface.

The present study documents a case where ravinement surfaces are themost prominent component of the transgressive systems tract; they aresteeply inclined landwards (rising 20 m in less than 500 m), and they areclosely spaced (every meter vertically), forming an inclined ravinementcomplex rather than a single surface. Moreover, they do not merge land-wards as a single master erosion surface (everywhere separating marineabove from nonmarine below) but diverge landwards so that there is anearly continuous interfingering of back-barrier and shelf deposits betweenthe frequent ravinement events. We interpret this unusual ravinement com-plex in terms of high sediment supply (despite high subsidence) and therelatively steep morphology of the land surface being transgressed. This

FIG. 4.—Correlation panel showing the relationships between the marine and nonmarineparts of the El Marcet 1 fundamental sequence. Note the great volume of alluvial strata inthe lower, transgressive part of the sequence. The architecture and details of the fivehigh-frequency sequences are discussed in the text.

EDITOR’S NOTE: Figure 4 is an oversize foldout and is presented on the next PDF pageas a un-numbered JSR page. Click here to view.


FIG. 17.—Shifts and trends in ichnofabric stacking patterns in the El Marcet 1 sequence, as used to improve identification of sequence boundaries and maximum floodinglevels.


FIG. 18.—Schematic presentation of theincrease in bioturbation from ichnofabric 4 to 6and back to 4. The traces within the fabricschange from simple vertical Ophiomorpha tocomplex Thalassinoides burrow systems.

interpretation is consistent with the unusual thickness of the transgressivepart of the El Marcet fundamental sequence.

Facies Partitioning

The time–space distribution of shoreface and other facies in Figure 4,together with the geometric distribution of the two systems tracts notedabove, suggests that the sediment supplied through time was partitioned(Cross 1988) or allocated differently into the two systems tracts (given therestrictions of mainly two-dimensional exposure, though with some three-

dimensional control). Large volumes of sediment supplied during the trans-gressive growth of the sequence were apparently stored in the coastal flood-basins and in the fluvial reaches of the system. The gravelly nearshorezones were narrow (less than 300 m wide) at this time, and the preservedconglomeratic lithosomes have a nearly aggradational and ‘‘stubby’’ char-acter (Fig. 4), though their strike extent is only partly known. The thinningof the transgressive tract seawards of the shoreline area further suggeststhat relatively little of the sediment supplied to the system actually passedthrough into the offshore areas.

During the regressive growth of the sequence, shoreline progradation


FIG. 19.—Schematic development ofichnofabric 2. The occurrence of this ichnofabricsignals the rising water table and initial marineincursions into a back-barrier or embayment areaprior to the passage of the wave-ravinementsurface. Ichnofabric 2 thus allows identificationof initial, low-energy flooding into this area.

FIG. 20.—Recognition of the maximumflooding level or zone based on the turnaroundin stacking pattern of ichnofabrics. Theirenvironmental implication is shown to the right.

caused the sandy and conglomeratic shoreline lithosome here to be manytimes broader than in the transgressive phase. This relates to the lowertrajectory of the shoreface during its regressive growth, compared to thehigh-angle trajectory of the transgressive shoreline. The thickening of theregressive tract beyond the shoreline zone, leading down into slope-turbi-dite aprons (Lopez Blanco 1993), strongly suggests that, in contrast to thepartitioning during transgression, great volumes of sediment supplied atthis time passed through the fan-delta shorelines onto the fan-delta slopesbeyond. This focus of sediment accumulation in the shoreface and beyondsuggests that there was likely to have been erosion and sediment bypassingin the time-equivalent alluvial reaches of the tract. This cannot be proved,however, because the lack of alluvium in the regressive tract (Fig. 4) couldbe due to later erosion as well as to minimal accumulation.

High-Frequency Sequences

The studied fundamental stratigraphic sequence (Fig. 4) has been sub-divided into five high-frequency sequences, each apparently with a timeduration of several thousand years. It is of further interest to examine thefacies succession vertically and from the shoreline to the alluvial reach forany one of these high-frequency sequences.

High-Frequency Sequences 1–3.—High-frequency sequences 1–3,

which constitute most of the transgressive tract of the low-order studysequence, are fairly similar to each other. In their alluvial reaches, eachhigh-frequency sequence is bounded by a marked fluvial erosion surfacewith relief of up to 2–3 m. This surface can be followed down to theshoreline, where it is time equivalent to a regressive wedge of conglom-eratic mouth-bar or shoreface deposits and associated red siltstones/sand-stones (Fig. 4). Above the basal erosion surface in the alluvial reaches eachhigh-frequency sequence shows a crude upward fining from conglomerates(fluvial belt of channels) to red sandstones and siltstones. The top of eachsequence here shows an influx of brackish marine trace fossils (ichnofabric2) that penetrate down into the fine-grained red beds (Fig. 4). In theirshoreline reaches, each high-frequency sequence shows (1) a thin, mouth-bar or shoreface unit that pinches out by erosion updip (the erosion surfaceeventually becoming the base of the upward-fining alluvial units), and (2)a landward-stepping group of ravinement surfaces with the associated trans-gressive tract of interfingering nearshore and nonmarine deposits (whichtraces landwards into the ‘‘body’’ of the alluvial sequence). Confirmationof the time equivalence of these two parts of each high-frequency sequenceis given by the appearance of brackish-water signatures at the top of thealluvial succession, this saline influx relating to maximum flooding at thecontemporary shoreline. This sequence is interpreted to result from (1)


initial stillstand or slight fall of base level, producing updip erosion inalluvial areas and contemporaneous regressive growth of the shoreline, and(2) subsequent base-level rise, producing shelf deposits and the ravinementcomplex, which aggrade and translate landwards, time-equivalent withbackfilling of coastal floodbasin deposits across the formerly channeledareas. Both the basal erosion surface (and time-equivalent conformableshorezone) and the marine maximum flooding level (at the top in alluvialreaches, but lower in the sequence at the shorezone) can be ‘‘walked out’’,providing constraint and internal consistency for the architecture of thedescribed high-frequency sequences.

High-Frequency Sequence 4.—High-frequency sequence 4 has a lowerpart that resembles the three underlying sequences, but has, in addition, awell-developed upwards-coarsening, regressive upper part (Fig. 4). Themiddle level of sequence 4 is the most thoroughly marine interval (ich-nofabric 6) of the studied succession and is thus referred to as the maximumflooding zone in the main, low-order sequence. As a whole, sequence 4shows a nearly symmetrical upward fining to upward coarsening in itsproximal reaches and a thin interval of upward fining followed by a thickinterval of upward coarsening in the shoreline-offshore reaches.

High-Frequency Sequence 5.—This sequence has a very poorly devel-oped transgressive part, merely a lag 1–2 pebbles thick, overlying a rav-inement surface. Above this abruptly lie deeper marine facies, which arefollowed by a larger-scale, upward-coarsening package of shoreface ormouth-bar sediments. As with the upper part of the underlying high-fre-quency sequence, sequence 5 shows a clear development of shoreface cli-noforms. The progradational clinoforms seen in high-frequency sequences4 and 5 probably represent coarse-grained, steep-fronted deltas. Their strati-graphic position, in conventional sequence stratigraphic terms, would be ashighstand systems tracts. Sequence 5 is thus represented mainly by an ero-sion surface in its alluvial reaches, and by coarsening-upward highstanddeltas in its shoreline reaches.

CONCLUSIONS

Three nonmarine and four marine sedimentary facies and six ichnofa-brics have been used to trace key stratigraphic surfaces from marine shore-face deposits through to the contemporary fluvial deposits of an Eocenefan-delta unit in the Sant Llorenc del Munt area of the SE Ebro Basin,Spain. Ichnofabrics have proved useful in providing increased precision foridentification of sequence boundaries, ravinement surfaces, and other ma-rine flooding surfaces through (1) documentation of changes in ichnofabricstacking patterns, (2) identification and mapping of bored-clast horizons asan indicator of ravinement trajectory, and (3) recognition of marine fabricswithin otherwise nonmarine facies as an indicator of early, low-energyflooding into the nonmarine reaches of the fan-delta system.

Mapping of sedimentary facies and key surfaces by these means withina short-duration transgressive–regressive stratigraphic sequence showedthat:

● The transgressive systems tract shows a landward expansion in itsthickness from the marine to the alluvial reaches of the sequence, as wellas an irregular but clear landward stepping of shoreface, coastal floodbasin,and alluvial facies belts, through time. The regressive or highstand systemstract shows the opposite trend, i.e., a seaward expansion in thickness, aswell as in irregular seaward stepping of shoreface and offshore facies belts.Alluvial deposits are thin or absent here because of sediment bypassingand/or erosion by the overlying sequence boundary.

● Five high-frequency sequences, of only a few thousand years duration,can be mapped from marine to nonmarine reaches within the main trans-gressive-regressive sequence. These are 4–10 m thick, are asymmetrical,and are dominated by transgressive lithosomes within the transgressive sys-tems tract of the main sequence. Within the regressive or highstand systemstract of the main, low-order sequence the high-frequency sequences are 0–

12 m thick, are also asymmetrical, but are here dominated by regressive,steep-fronted deltas shaped by interaction of wave and fluvial processes.

● Marine facies in the transgressive systems tract of the low-order se-quence partly interfinger and are partly separated erosionally from broadlytime-equivalent nonmarine facies, by a complex of ravinement surfaces.The diachronous landward translation of the ravinement complex (and itsseaward tail of shelf deposits) is mappable, and deduced to have been time-equivalent with the backfill of fine-grained coastal floodbasin deposits crossalluvial channel belts. Channel incision in the alluvial reaches, on the otherhand, was time equivalent to the maximum progradation of fine-grainedcoastal floodbasin deposits (seen as thin wedges of red siltstones penetrat-ing the shore-zone lithosomes).

ACKNOWLEDGMENTS

The authors wish to thank Mariano Marzo and Miguel Lopez-Blanco for frequentdiscussion on many of the themes presented here, during the course of fruitfulcooperation between groups at the universities of Bergen and Barcelona. We arealso indebted to Susan Kidwell for a clarifying discussion of the ravinement ge-ometries. The Norwegian Research Council is acknowledged for research supportand for a Ph.D. stipend to ES. Comments by Janok Bhattacharya, John Southard,and two anonymous reviewers helped greatly in the improvement of the paper.

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Received 26 June 1995; accepted 24 May 1998.

Architecture and trace-fossil characteristics of a 10-20,000 yr fluvial to marine sequence

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