Revista de la Asociación Geológica Argentina 62 (4): 521-529 (2007) 521

INTRODUCTION

The lower Maastrichtian basal part of theLópez de Bertodano Formation is wellexposed on Seymour and Snow Hill islands,northeastern Antarctic Peninsula (Fig. 1).The section is dominated by thick, friablemudstone-rich deposits, with variable pro-portions of clay, silt, and sand particles

(Macellari 1988). The friable texture andresulting lack of preservation of diagnosticsedimentary structures imposes a homoge-neous appearance to these deposits. Con-sequently, the distinction of sedimentary fa-cies is difficult and the interpretation of thedepositional environments problematical.Based mainly on paleoecological interpreta-tions, two contrasting depositional settings

were proposed: a) very shallow marine envi-ronment, near a delta or estuary (Macellari1988); and b) deep water, outer shelf set-tings (Crame et al. 2004). The recognition oflarge, lenticular bodies of heterolithicmudstones with complex cut and fill struc-tures, interpreted as tide-influenced estuari-ne channels (Olivero 1998) supported thefirst interpretation, but the discussion was

DEPOSITIONAL SETTINGS OF THE BASAL LÓPEZ DE BERTODANO FORMATION, MAASTRICHTIAN,ANTARCTICA

Eduardo B. OLIVERO1, Juan J. PONCE1, Claudia A. MARSICANO2, and Daniel R. MARTINIONI1

¹ Laboratorio de Geología Andina, CADIC-CONICET, B. Houssay 200, CC 92, 9410 Ushuaia, Tierra del Fuego, Argentina.E-Mail: emolivero@ciudad.com.ar2 Dpto. Cs. Geológicas, Universidad Buenos Aires, Ciudad Universitaria Pab. II, C1428 DHE Buenos Aires.

ABSTRACT:In Snow Hill and Seymour islands the lower Maastrichtian, basal part of the López de Bertodano Formation, rests on a high relief, erosi-ve surface elaborated in the underlying Snow Hill Island Formation. Mudstone-dominated beds with inclined heterolithic stratificationdominate the basal strata of the López de Bertodano Formation. They consist of rhythmical alternations of friable sandy- and clayey-mudstone couplets, with ripple cross lamination, mud drapes, and flaser bedding. They are characterized by a marked lenticular geometry,reflecting the filling of tide-influenced channels of various scales and paleogeographic positions within a tide-dominated embayment orestuary. Major, sand-rich channel fills, up to 50-m thick, bounded by erosive surfaces probably represent inlets, located on a more centralposition in the estuary. Minor channel fills, 1- to 3-m thick, associated with offlapping packages with inclined heterolithic stratification pro-bably represent the lateral accretion of point bars adjacent to migrating tidal channels in the upper estuary. Both types of channel fills bearrelatively abundant marine fauna, are intensively bioturbated, and are interpreted as a network of subtidal channels. In southwestern SnowHill Island, the minor offlapping packages have scarce marine fossils and bear aligned depressions interpreted as poor preserved dinosaurfootprints. They represent the lateral accretion of point bars adjacent to intertidal creeks, probably located on the fringes of a mud-domi-nated estuary or embayment. The basal unconformity was produced by subaerial erosion; hence the inferred estuarine settings are consis-tent with the beginning of a new transgressive sedimentary cycle.

Keywords: Antarctica; Snow Hill-Seymour islands; Maastrichtian; Dinosaur footprints; Tidal settings.

RESUMEN: Ambientes de depositación de la parte basal de la Formación López de Bertodano, Maastrichtiano, Antártida.. En las islas Snow Hill y Sey-mour, la Formación López de Bertodano apoya en relación de discordancia erosiva de gran relieve sobre la Formación Snow Hill Island.La parte basal, Maastrichtiano inferior, de la Formación López de Bertodano está dominada por una alternancia rítmica de fangolita are-nosa y fangolita arcillosa friables, con estratificación heterolítica inclinada, laminación ondulítica, flaser y cortinas de fangos. Los depósi-tos presentan a diferentes escalas una marcada geometría lenticular, que refleja el relleno de canales mareales con distintas posiciones paleo-geográficas dentro de un engolfamiento o estuario dominado por mareas. Los canales mayores, hasta 50 m de espesor, con superficies ero-sivas basales y relleno más arenoso, probablemente representan colectores mayores situados en una posición central del estuario. Los cana-les menores, 1 a 3 m de espesor, asociados con traslape lateral de capas con estratificación heterolítica inclinada, representan estructurasde acreción lateral -migración de barras de punta mareales- adyacentes a canales menores del estuario superior. Ambos tipos de relleno tie-nen abundante fauna marina y densa bioturbación y se interpretan como un sistema de canales submareales. En el SO de la isla Snow Hill,los canales menores definen capas con estratificación heterolítica inclinada y escasos fósiles marinos, que preservan depresiones alineadasinterpretadas como probables icnitas de dinosaurios. Estas capas representan la acreción lateral de barras de punta de canales intermare-ales menores, probablemente situados en los márgenes fangosos del estuario. La discordancia basal tiene origen subaéreo y en consecuen-cia los ambientes estuáricos inferidos son consistentes con el inicio de un nuevo ciclo sedimentario transgresivo.

Palabras clave: Antártida; Snow Hill-Seymour; Maastrichtiano; Icnitas de dinosurios; Ambientes mareales.

not completely settled due to the inade-quacy of detailed knowledge of sedimen-tary facies.The main aim of this study is to analyze thesedimentary facies of the basal López deBertodano Formation (Fig. 1) and to dis-cuss their significance for the understan-ding of paleoenvironmental settings. In ad-dition, we report putative dinosaur foot-prints recorded in the López de BertodanoFormation, Snow Hill Island.

METHODS

This paper is based mainly on the prelimi-nary results of field studies conducted insouthwestern Seymour and northeasternSnow Hill islands by three of the presentauthors in 2004 and 2005 (EBO, JJP, andDRM). The study of sedimentary facies isbased on four detailed sedimentary logs ofthe basal López de Bertodano Formation inSnow Hill and Seymour islands (Fig. 2).Details of sedimentary structures and bed-ding geometry are mostly visible on freshexposures at the sea cliff; consequentlymuch of the effort in field research was res-tricted to this area. The unconformity bet-ween the Snow Hill Island and López deBertodano formations, as well as the erosi-ve basal surfaces of large channels weremapped by walking along the recognizedsurfaces and recording their geographicposition with a GPS navigator. The recor-ded points were then plotted on publishedtopographic maps of the area (Brecher and

Tope 1988, US Geological Survey 1995). A series of poorly preserved tetrapod foot-prints were found during the austral sum-mer of 2004 while carrying out sedimento-logical fieldwork in Snow Hill Island. A newexpedition in 2005 resulted in the discovery,in the same horizon, of more material. Thedetailed mapping of the interpreted trackswas done with a measuring tape and a com-pass.

GEOLOGICAL ANDSTRATIGRAPHICALFRAMEWORK

In the James Ross archipelago, the UpperCretaceous Marambio Group is about 3 kmthick and consists of highly fossiliferousmarine, shelf deposits originated in a back-arc basin located to the east of the Antarc-tic Peninsula (cf. Pirrie et al. 1997, Oliveroand Medina 2000, and references therein). In the ice-free areas of northeastern SnowHill and southwestern Seymour islands(Figs. 1, 2), the lower Maastrichtian part ofthe Group is well exposed (Crame et al.2004), and includes the upper part of theSnow Hill Island Formation and the basalpart of the López de Bertodano Formation(Macellari 1988, Pirrie et al. 1997). Thesetwo units are characterized by monotonous,friable sandy siltstones and mudstones, withabundant concretionary horizons and occa-sional intercalation of indurate very fine-grained sandstone beds. Due to this homo-geneous appearance, all these fine-grained

deposits were previously included in theLópez de Bertodano Formation (e.g. Rinaldiet al. 1978, Macellari 1988). The differentia-tion of the Snow Hill Island Formation isbased on the recognition of an importantunconformity that separates it from theoverlying López de Bertodano Formation(Pirrie et al. 1997). In this redefined strati-graphy of the studied area, the uppermostpart of the Snow Hill Island Formationincludes the informal Rotularia Unit 1, andthe basal López de Bertodano Formationthe Rotularia Units 2 to 5 of Macellari(1988). Absolute dating based on 87Sr/86Srchronostratigraphy suggests an early Maas-trichtian age for both stratigraphic units inthe studied area (Crame et al. 2004), thussupporting previous data based on ammo-nite biostratigraphy (cf. Crame et al. 2004,Olivero and Medina 2000, and referencestherein).

SEDIMENTOLOGY OF THELOPEZ DE BERTODANOFORMATION

The basal unconformity

According to Pirrie et al. (1997), in SnowHill and Seymour islands the base of theLópez de Bertodano Formation rests on animportant unconformity, which erosivelycuts more than 60 m of the underlyingSnow Hill Island Formation. The results ofour field mapping support this conclusion,but we found that the unconformity is loca-ted at a lower stratigraphic level, within theupper beds of the Snow Hill Island For-mation as originally mapped by Pirrie et al.(1997; cf. their figure 3 with Fig. 2a). Se-veral erosive surfaces characterized thesedimentary bodies stratigraphically locatedeither above or below the main unconfor-mity (cf. Olivero 1998), and recognition of amajor sequence boundary requires properdistinction of erosive surfaces generated byautocyclic or allocyclic processes (Fig. 2b). In our interpretation, the main sequenceboundary is coincident with an irregular,high-relief erosive surface locally markedby a basal bed with abundant pebbles ofreddish concretions, rounded sandstones,and fossils reworked from the underlyingSnow Hill Island Formation, and isolated

E. B . OLIVERO, J . J . PONCE, C . A . MARSICANO, AND D. R . MARTINIONI522

Figure 1: Location map.

Depositional settings of the basal López de Bertodano Formation, Maastrichtian, Antarctica 523

pebbles of slates and rhyolites derived fromolder rocks cropping out along the Antarc-tic Peninsula (Fig. 3). This bed rests above aslightly angular unconformity that erosivelycuts more than 100 m of the underlyingSnow Hill Island Formation, being thelowest erosional relief located near thePicnic Passage (Figs. 2a, 4a). The position of the unconformity is appro-ximately coincident with the level detectedby Pirrie et al. (1997) in the upper Snow HillIsland Formation, which marks a majorcompositional break in the palynomorphassemblages. The provenance levels of theupper three palynological samples from thisFormation (figure 8 of Pirrie et al. 1997) arenow reassigned to the basal López de Ber-todano Formation. Furthermore, the loca-tion of the unconformity proposed in thispaper indicates that the outcrop belt of theSnow Hill Island Formation is laterally con-tinuous along Snow Hill and Seymourislands. As this fact eliminates the proposedlateral offset of the outcrop belt betweenthese two islands (Pirrie et al. 1997), there is

no need to invoke a major extensional faultalong the Picnic Passage (Fig. 2a).

Sedimentary facies and architecture

The dominant sedimentary facies of theLópez de Bertodano Formation are muds-tone-dominated heterolithics with inclinedbed sets, lenticular sandstone-dominatedheterolithics, and fossiliferous conglomera-tes. Detailed features of individual sedi-mentary facies and architecture are bestseen in fresh exposures at the sea cliffs. In-land, the combination of low-cuesta topo-graphy and the friable character of thesefine-grained sediments prevent detailedobservations. Accordingly, these individualfacies are grouped for ease of description ina single facies association dominated byheterolithic mudstones.The most abundant facies consists of regu-lar alternations of thinly stratified, millime-ter- to decimeter-scale, dark-gray mudsto-nes and pale-gray sandy siltstones to veryfine-grained sandstones showing variations

in thickness of individual laminae or bedsthat could be arranged in vertically cyclicalpatterns. Generally, a dense mottling produ-ced by bioturbation resulted in the loss ofthe original stratification; in less bioturba-ted areas the sandstone laminae or bedspreserve delicate current ripples with muddrapes. Herringbone and flaser bedding arerare. Paleocurrent measurements indicatethat major flows were directed to the east,with minor flow reversals (Figs. 4a, 5a-d). The heterolithic mudstones record thickstratigraphic packages with variable propor-tions of sand-mud ratios, but they are diffi-cult to map because of the poor quality ofthe outcrops. Nonetheless, based on a largenumber of grain-size analyses Macellari(1988) was able to show an intertonguing ofsilt-rich, clay-rich, and sand-rich units inSeymour Island that he distinguished as theinformal units U2, U3, and U4, respectively. The bedding attitude, particularly in theunits U3 and U4 and their equivalents inSnow Hill Island, are also highly variable,but two repetitive motifs are evident. In one

Figure 2: a) Geological sketchof the NE Snow Hill and SWSeymour islands. b) Sedimen-tary log of the Snow HillIsland and the López de Ber-todano formations, Section 3.

motif, lenticular packages of 1- to 3-mthick, are characterized by low dipping tosubhorizontal strata, which define low-an-gle, offlapping bed sets with inclined hete-rolithic stratification that can be followedlaterally for 50- 70-meters. The other motifrecords large, sometimes deeply incised,concave upward erosion surfaces boundingchannel-shaped heterolithic packages, up to50 m thick (Fig. 4). In the mid and upperparts of unit U4 on Seymour Island theseheterolithic packages present multiple ero-sion surfaces, resulting in complex cut andfill structures (Olivero 1998) characterizedby a dominantly aggradational bedding style(Fig. 4b). Other channel-shaped fills, mainlycomposed of thicker very fine-grainedsandstone beds irregularly interbedded withthinner mudstone lenses, form isolatedbodies encased in mudstone-rich heteroli-thic beds (Fig. 4c). The basal contact ofthese channel-shaped fills is an erosive sur-face associated with a lag deposit of rewor-

ked concretions and shell fragments. Thelargest channel-form sandstones, about 2-km wide and up to 50-m thick, are prefe-rentially located near the Picnic Passage(Figs. 2, 4a). Because of the dense ichnofa-bric that has destroyed most original physi-cal structures, it is difficult to discern thebedding style of these channel-form sands-tones, but appears to be dominantly accre-tional. Just above the basal unconformity of theLópez de Bertodano Formation in SnowHill Island an unusual conglomerate faciesis locally developed. This facies, exposed indiscontinuous patches of reduced extend(2- to 5-m), consists of a hard cementedconcentration (20- to 50-cm thick) of peb-ble- to cobble-sized reworked concretionsand sandstone fragments, derived from theSnow Hill Island Formation, occasionalpebbles of acidic volcanics and schists, andabundant shell fragments (Figs. 3b-d). In Seymour Island marine fossil invertebra-

tes are relatively abundant in several strati-graphic horizons within the heterolithicmudstones. In the informal unit U2, abun-dant specimens of the bivalves Nordenskjol-dia, Eselaevitrigonia, Oistotrigonia, and Pinnaare frequently preserved in life position.Relatively large forms of the ammoniteMaorites tuberculatus, together with M. densi-costatus, Grossouvrites gemmatus, and Diplomo-ceras lambi, indeterminate gastropods, echi-noid spines, and serpulids are also relativelyabundant. A more depauperate fauna ispresent in unit U3, where only serpulidsand echinoid spines are abundant throug-hout its thickness (Fig. 5e). However, gas-tropods, oysters, and ammonites could beabundant in certain horizons located at thebase of the offlapping inclined-bed sets.Unit U4 records a similar pattern, but withincreased abundance of M. tuberculatus,Nordenskjoldia, Pinna, and cirripeds (cf. Ma-cellari 1988, Olivero 1998. Olivero and Me-dina, 2000, and Crame et al. 2004).

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Figure 3: a) Panoramic view of the main unconformity (sb) between the Snow Hill Island and the López de Bertodano formations at Snow HillIsland showing the location of the dinosaur footprints. b) The basal conglomerate of the López de Bertodano Formation in Snow Hill Island. c-d)Detailed photos of the basal conglomerate showing fossil fragments and pebbles; arrows point to reworked sandstone pebbles from the under-lying Snow Hill Island Formation (c), and rounded schist pebble derived from the Antarctic Peninsula (d).

Depositional settings of the basal López de Bertodano Formation, Maastrichtian, Antarctica 525

In northeastern Snow Hill Island, marineinvertebrates are restricted to localized stra-tigraphic horizons and are only abundant inlenticular lag deposits. The basal lags of thetwo large, isolated channel-form sandstonesbear abundant specimens of gastropods, bi-valves, and ammonites; the latter dominatedby Maorites tuberculatus. The discontinuousbasal lag resting on the unconformity at the

base of the López de Bertodano Formationrecords abundant specimens of corals, oys-ters, and the bivalve Thyasira townsendi Whi-te. Limited horizons within the mud-richheterolithic beds, which are dominant tothe southwest of Snow Hill Island, could re-cord abundant serpulids and echinoid spines. The trace fossils distribution has a similarpattern to that of the megafauna. In the

mudrich beds of the López de BertodanoFormation in Seymour and northeasternSnow Hill islands the dominant trace fossilsare Phycosiphon, Nereites missouriensis, Paradic-tyodora (cf. Olivero et al. 2004), and "Terebelli-na", whereas Neonereites biserialis, Patagonich-nus stratiforme, and Ophiomorpha dominatethe sandier beds. Bioturbation is scarce orabsent in the clay-rich heterolithics exposed

Figure 4: a) Alongstrike correlation ofsections 1 to 4 sho-wing the high-reliefsequence boundary,the architecture ofsedimentary facies,and the distributionof channels of dif-ferent scale and fos-sil-rich horizons inthe basal López deBertodanoFormation. Notethe concentration oflargest channels andfossil horizons nearthe Picnic Passage,between sections 1and 3. b)Sedimentary fillingsand internal erosionsurfaces (es) of thelarge channel indi-cated in Section 1,Seymour Island.Height of the cliffis about 50 m. c)Medium-scale,muddy-filled chan-nel with inclinedheterolithic stratifi-cation (ihs), Section4, Snow Hill Island.Bedding is gentlydipping to thenorth. d) Small-scale channel filledwith offlapping pac-kages (inclinedheterolithic stratifi-cation). The basalerosion surface (es)and some offlap-ping beds are indi-cated.

toward the southwest of Snow Hill Island,but sandier beds locally record abundantScolicia, Neonereites biserialis, and Teichichnusisp. Large "Rosselia"-like traces are relativelyabundant at the base of the offlapping,inclined-bed sets (Figs. 5f-h).

Tetrapod footprints

In Snow Hill Island, one thin, very fine-grained, well-cemented sandstone bed be-ars decimeter-scale depressions surrounded

by a raised rim of convoluted sediment.These depressions have been found withinone of the offlapping packages with incli-ned heterolithic stratification (Figs. 2, 3a,4a), consisting of thin, indurated sandsto-nes and thick, massive, friable mudstones,

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Figure 5: Main sedi-mentary facies andfossils of the Lópezde BertodanoFormation. a) Generalview of rhythmicalternation of sandy(light-gray, arrowhead)and muddy (dark-gray)mudstone couplets.Field notebook (encir-cled) for scale. b)Detailed view of asandy siltstone bed(arrowhead) with rip-ple cross-laminationand mud drapes. c)Current ripple cross-lamination in fine-grained sandstones. d)Fully bioturbated silts-tone and mudstonecouplets. The burrowsof Paradictyodora isp.(arrow) cross-cut themottled background,full relief, verticalview. e) Mudstone bedwith abundant speci-mens of Rotularia sp.f) Neonereites biserialis(ne), upper beddingplane view. g) Scoliciaisp. (sc), full relief,upper view. h)Phycosiphon isp. (phy) ,full relief, upper view.

with abundant specimens of the serpulidRotularia (Austrorotularia) fallax (Wilckens).The circular to oval depressions are preser-ved in a single horizon within a thin, hard-cemented very fine-grained sandstone bed(3 to 5 cm thick) interbedded with friable,clay-rich mudstones. The rounded depres-sions, at least 3 to 4 cm deep, are not sim-ple holes on the bedding surface, as usually

produced by differential weathering in non-uniformly cemented beds. Instead, closeinspection of the internal structure showsthat the sandy laminae bent downward nearthe center and it conforms a raised rim ofconvoluted sediments toward the margin ofthe depressions (Fig. 6). Because the inter-nal laminae were deformed before cemen-tation of the sandstone bed, deformationby modern cryogenic processes, such asfrost heaving, are ruled-out. Most of thedepressions are irregularly distributed, butin one case up to seven of them were alig-ned in almost a straight line (Fig. 7). Thebedding plane distribution, the associateddeformation pattern, and their relative sizesuggest that these depressions could bebest interpreted as poorly preserved dino-saur tracks and trackways. The putativefootprints are preserved as natural moldsand the raised rim of convoluted sedimentwas probably produced by the compressionof the substrate by the animals (Fig. 6). The assemblage includes many isolatedprints randomly distributed, of highly varia-ble sizes (from 20 to 60 cm), thus sugges-ting a trampling surface. Three trackwayswere identified probably representing bothbipedal (one) and quadrupedal (two) pro-gressions (Fig. 7). Seven steps compose along trackway of a biped, nearly crossingthe entire surface from north to south.Each print is slightly anteroposteriorly elon-gated (approximately 60 cm in length); di-gits are not distinguishable probably relatedto the high water content of the sediment

during impression. The two remainingtrackways apparently correspond to qua-drupeds but in only one of them this con-dition is certain. It includes approximatelyfour manus-pes sets where the pedal prints(approximately 30 cm in length) are largerthan those of the manus (approximately 20cm in length) and partially overprint theposterior half of the former (Figs. 6, 7).

DISCUSSION ANDCONCLUSIONS

As previously mentioned, the interpretationof the depositional settings of the basalpart of the López de Bertodano Formationin Seymour and Snow Hill islands based onpaleoecological analyses is controversial.The depauperate invertebrate macrofaunadominated by the serpulid genus RotulariaDefrance and echinoid spines, typical ofthe basal López de Bertodano Formation,was interpreted to reflect shallow marinedepositional settings, with deltaic or estuari-ne influence (Macellari 1988). On the con-trary, Crame et al. (2004) interpreted that thelowest, mud-dominated part of the Lópezde Bertodano Formation, with sparse ma-crofauna, actually represents a deeper-wa-ter, outer shelf setting. The later authorsconsidered that the whole formation repre-sents a large-scale shallowing-upwardstrend, with most taxa occurring in nearsho-re, shallow-shelf settings by the latest Ma-astrichtian, i.e. in the upper part of the unit.The homogeneous character of these de-posits causes difficulty in the analysis of thesedimentary facies. Nevertheless, the stratalgeometry and heterolithic mudstones, withsedimentary structures typical for tidalites(Figs. 5 a-c), suggest shallow-water, tide-in-fluenced depositional settings. The basalstrata of the López de Bertodano Forma-tion are characterized by a marked lateralvariability originally interpreted as an inter-tonguing of clayey- and sandy-mudstonebodies, with their main axis aligned parallelto the paleoshore, i.e. in a northern direc-tion (Macellari 1988). However, the lateralvariability of these strata appears to reflectthe lenticular filling of large channels, up to1 km in width and 50 m in thickness, withtheir main axis oriented normally to thepaleoshore (Olivero 1998). Moreover, these

Depositional settings of the basal López de Bertodano Formation, Maastrichtian, Antarctica 527

Figure 6: a) General view of the track-bearing surface; the arrow indicates the position of thefootprints in b. b) Detailed view of a natural mold of manus-pes pair surrounded by raised rimsof convoluted sediment produced by compression of the substrate by the animal; scale bar in cm.

Figure 7: Map of the probable trackways iden-tified on the track-bearing surface; the circleindicates the manus-pes pair in Figure 6 b.

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lenticular bodies are bounded by large ero-sive surfaces; present ample evidence ofcomplex cut and fill structures; and arecomposed of rhythmical alternations ofsandy- and clayey-mudstone couplets, withripple cross lamination, mud drapes, andflaser bedding (Figs. 2b, 4b, 5a-c). Conse-quently, these lenticular bodies are bestinterpreted as the sedimentary filling oflarge, tide-influenced estuarine channels (cf.Olivero 1998). Several of these major chan-nels are indicated in the accompanying map(Figs. 2a, 4a). In addition to the above men-tioned features, these large channels arealso associated with variable bedding attitu-des, with strike orientations almost perpen-dicular to the regional, NNE bedding strikewithin the basin. These variable beddingattitudes, which commonly delineate thechannel margins, reflect the aggradationalcharacter of the sedimentary filling, withsuccessive beds onlapping the channel mar-gins and dipping toward their axis. Thevariable bedding strike in part also reflectsminor scale, offlapping packages of hetero-lithic mudstones, interpreted as lateral ac-cretion deposits, i.e. inclined heterolithicstratification, migrating in a direction orien-ted normally to the axis of minor channels(Fig. 4c).The facies association of the López de Ber-todano Formation is interpreted to repre-sent sedimentation in tidal-dominatedchannels of various scales and paleogeogra-phic positions within an embayment orestuary. Sedimentary controls by tidal pro-cesses are suggested by the dominance ofvertically stacked packages with inclined he-terolithic stratification; current ripple cross-lamination with abundant mud drapes; evi-dence of paleoflow reversals; and the com-plex, channel-shaped geometry of sedi-mentary bodies containing marine fossils.No clear examples of tidal cyclicity wererecorded, however several thinly laminated,interbedded mudstone/sandstone coupletsare arranged in repeated vertically cyclicalpatterns (Figs. 4, 5), which are suggestive oftidal rhythmites (Nio and Yang 1991, Klein1998).The bedding architecture, lithology, andthickness of the fills in the interpreted tidalchannels are highly variable, but the recor-ded motifs appear to characterize tidal pro-

cesses in different paleogeographic loca-tions. The offlapping packages, 1- to 3-mthick, with inclined heterolithic stratifica-tion probably represent the lateral accretionof point bars adjacent to migrating tidalchannels. The large, heterolithic channelfills, up to 50-m thick, with complex cutand fill structures and an overall aggradatio-nal filling pattern, probably reflect fluctua-ting sea-level changes and accompanyingerosion and deposition by large, subtidalchannels (cf. Clifton 1994, Eberth 1996). InSeymour and NE Snow Hill islands thesetwo motifs alternate at different stratigra-phic levels. As both types of channel fillsbear relatively abundant marine fossils andare intensively bioturbated, they are inter-preted as subtidal channels, probably repre-senting a network of channels of differentsizes.In southwestern Snow Hill Island, the mud-rich beds with inclined heterolithic stratifi-cation are poorly fossiliferous and, exceptfor a few levels, they lack an intense biotur-bated ichnofabric. In addition, the topmostbed of one of the offlapping packages ap-pears disturbed by putative dinosaur foot-prints (Fig. 6). These features strongly sug-gest that the offlapping packages representthe lateral accretion of point bars adjacentto intertidal creeks, probably located on thefringes of a mud-dominated estuary or em-bayment.The footprints represent additional eviden-ce for very shallow water settings, with tem-porary subaerial exposure, in the basal Ló-pez de Bertodano Formation. This eviden-ce and the associated sedimentary featuresstrongly suggest that the interpreted track-bearing site represent an intertidal mud flat,dissected by shallow, small tidal creeks. Inthis scenario, the footprints were made on athin wet sand layer covered by soupy muds,probably located on the margins of the ti-dal creek. The poor preservation of theseimperfect tracks is probably original andwas controlled by the high water content ofthe soupy substrate. As indicated by the rimof folded sediment around the footprints(Fig. 6), the slumping back of the surroun-ding sediments into the footprint (cf.Lockley 1986, Milàn and Bromley 2006)modified the original track impression.These small tidal creeks were probably loca-

ted on the lateral sides of a mud-dominatedestuary. Larger channels, with a highersand-grain content and bounded by exten-sive erosive surfaces, probably representmajor subtidal inlets, located on a morecentral position in the estuary. The relati-vely higher abundance of marine fossils insouthwestern Seymour Island, compared tostratigraphically equivalent beds in SnowHill Island is consistent with this scenario.Thus, the main paleoenvironmental settingsare interpreted as a relatively large, tide-do-minated estuary or embayment. The stu-died outcrops of Snow Hill Island wereprobably located on the mud-dominatedintertidal flats, along the margin of the es-tuary, whereas those of the Seymour Islandrepresent subtidal settings located towardthe central to outer parts of the estuary.The major unconformity at the base of theLópez de Bertodano Formation was pro-bably associated with subaerial erosion, asindicated by the erosional downcutting ofmore than 100 m of sediments of the un-derlying Snow Hill Island Formation. Con-sequently, the inferred estuarine settings aretotally consistent with the beginning of anew transgressive sedimentary cycle repre-sented by the López de Bertodano For-mation.Even though the dinosaur groups represen-ted by the ichnites can not be assessed, theysupport the presence of a diverse dinosaurfauna during the latest Cretaceous in thecontinent. Non-avian dinosaur remains inAntarctica are only known from the LowerJurassic of the Transantarctic Mountains(Hammer and Hickerson, 1994) and theUpper Cretaceous of the James Ross archi-pelago. Particularly, the Upper Cretaceousrecord is rather scarce and fragmentary. Thefirst dinosaur material reported from An-tarctica was a single individual of a nodo-saurid ankylosaur from shallow marinedeposits of the late Campanian Santa MartaFormation, exposed on James Ross Island(Olivero et al. 1986, Gasparini et al. 1987,Olivero et al. 1991, Salgado and Gasparini2006). Also from James Ross Island, butfrom a different locality and slightly olderlevels (Coniacian, Hidden Lake Formation),Molnar et al. (1996) described a tibia attribu-ted to a theropod. Additional Cretaceousdinosaur specimens are known from Vega

Depositional settings of the basal López de Bertodano Formation, Maastrichtian, Antarctica 529

Island and they include skeletal elements ofa hypsilophodontid (early Maastrichtian,Snow Hill Island Formation, Hooker et al.1991) and a single tooth of a hadrosaur(Maastrichtian, López de Bertodano For-mation, Case et al. 2000). Recently, two ad-ditional theropod fragments were recoveredfrom the Maastrichtian of Vega and JamesRoss islands (J.E. Martin, pers. comm. toEBO, 2005). Therefore, the combination ofthe known dinosaurs body-fossil recordwith these new ichnites might suggest amore extensive distribution of dinosaurs inAntarctica than previously assumed. Thiscircumstance could be reflecting opportu-nistic dispersal of members of the group atthe end of the Cretaceous among southernGondwanan areas via Antarctica, as waspreviously proposed (Sampson et al. 1998),probably during periods of more favorableclimatic conditions in the region.

ACKNOWLEDGEMENTS

Logistic support by the Instituto AntárticoArgentino and Fuerza Aérea Argentina aregreatly appreciated. A. Sobral helped withthe 2005 fieldwork. We thank Matthew Ca-rrano, Rodolfo Coria and James E. Martinfor helpful comments on the dinosaur foot-prints in an early version of this contribu-tion. This work was partially funded byPICT 8675 (ANCYPT, Argentina) and PIP5100 (CONICET, Argentina). Constructivesuggestions and comments made by thereviewers L. A. Spalletti and J. E. Martinhelp to improve the study.

WORKS CITED IN THE TEXT

Brecher, H.H. and Tope, R.W. 1988. Topogra-phic map of Seymour Island. In Feldmann,R.M., Woodburne, M.O. (eds.) Geology andPaleontology of Seymour Island, AntarcticPeninsula. Geological Society of America,Memoir 169: 17-19.

Case, J., Martin, J., Chaney, D., Reguero, M, Ma-renssi, S., and Santillana, S. 2000. The firstduck-billed dinosaur (Family Hadrosauridae)from Antarctica. Journal of Vertebrate Pa-leontology 20: 612-614.

Clifton, H.E. 1994. Preservation of transgressi-ve and highstand late Pleistocene valley-fill/estuary deposits, Willapa Bay, Washington. In

Dalrymple, R.W., Boyd, R., and Zaitlin, B.A.(eds.) Incised-Valley Systems: Origin and Se-dimentary Sequences, SEPM Special Publi-cation 51: 321-333.

Crame, J.A., Francis, J.E, Cantrill, and Pirrie D.J.2004. Maastrichtian stratigraphy of Antar-ctica. Cretaceous Research 25: 411-423.

Eberth, D.A. 1996. Origin and significance ofmud-filled incised valleys (Upper Cretaceous)in southern Alberta, Canada. Sedimentology43: 459-477.

Gasparini, Z.; Olivero; E. B., Rinaldi, C. A., andScasso, R. A. 1987. Un ankylosaurio (Reptilia,Ornithischia) campaniano en el continenteantártico. 10º Congreso Brasileiro de Paleon-tología, Anais 1: 131-141, Rio de Janeiro.

Hammer, W.R. and Hickerson, W.J. 1994. A cres-ted theropod dinosaur from Antarctica.Science 264: 828-830.

Hooker, J.J., Milner, A.C., and Sequeira, S. 1991.An ornithopod dinosaur from the Late Cre-taceous of West Antarctica. Antarctic Science3: 331-332.

Klein, G.D. 1998. Clastic tidalites- A partial re-trospective view. In Alexander, C.R., Davis,R.A., and Henry, V.J. (eds.) Tidalites: Proces-ses and Products. SEPM Special Publication61: 5-14.

Lockley, M. G. 1986. The paleobiological and pa-leoenvironmental importance of dinosaurfootprints. Palaios 1: 37-47.

Macellari, C.E. 1988. Stratigraphy, sedimentolo-gy and paleoecology of Upper Cretace-ous/Paleocene shelf-deltaic sediments ofSeymour Island (Antarctic Peninsula). InFeldmann, R.M., Woodburne, M.O. (eds.)Geology and Palaeontology of SeymourIsland, Antarctic Peninsula. Geological So-ciety of America, Memoir 169: 25-53.

Milàn, J. and Bromley, R.G. 2006. True tracks,undertracks and eroded tracks, experimentalwork with tetrapod tracks in laboratory andfield. Palaeogeography, Palaeoclimatology,Palaeoecology 231: 253-264.

Molnar, R.E., Lopez-Angriman, A., and Gaspa-rini, Z. 1996. An Antarctic Cretaceous thero-pod. Memoirs of the Queensland Museum39: 669-674.

Nio, S.D. and Yang, C. 1991. Diagnostic attribu-tes of clastic tidal deposits: a review. InSmith, D.G., Reinson, G.E., Zaitlin, B.A., andRahmani, R.A. (eds.) Clastic Tidal Sedimen-tology. Canadian Society Petroleum Geolo-gists, Memoir 16: 3-27.

Olivero, E.B. 1998. Large mud-filled channels inthe Maastrichtian of the López de BertodanoFormation (Seymour Island, Antarctica):Stratigraphical implications. Revista de la A-sociación Geológica Argentina 53: 553-556.

Olivero, E.B; Buatois, L.A., and Scasso, R.B.2004. Paradictyodora antarctica: A new complexvertical spreite trace fossil from the UpperCretaceous-Paleogene of Antarctica andTierra del Fuego. Journal of Paleonto-logy78: 783-789.

Olivero, E. B., Gasparini, Z., Rinaldi, C. A., andScasso, R. A. 1991. First record of dinosaursfrom Antarctica (Upper Cretaceous, JamesRoss Island): Palaeogeographical implica-tions. In Thomson, M.R.A., Crame, J.A.,Thomson, J.W. (eds.) Geological Evolution ofAntarctica: 617-622. Cambridge UniversityPress, Cambridge.

Olivero, E.B. and Medina, F.A. 2000. Patterns ofLate Cretaceous ammonite biogeography insouthern high latitudes: the Family Kossma-ticeratidae in Antarctica. Cretaceous Research21: 269-279.

Olivero, E.B., Scasso, R.A., and Rinaldi, C.A.1986. Revision of the Marambio Group, Ja-mes Ross Island, Antarctica. ContribucionesCientíficas del Instituto Antártico Argentino331: 1-28.

Pirrie, D., Crame, J.A., Lomas, S.A., and Riding,J.B. 1997. Late Cretaceous stratigraphy of theAdmiralty Sound region, James Ross Basin,Antarctica. Cretaceous Research 18: 109-137.

Rinaldi, C.A., Massabie, A., Morelli, J., Rosen-man, H.L., and del Valle, R. A. 1978. Geolo-gía de la isla Vicecomodoro Marambio. Con-tribuciones Científicas del Instituto AntárticoArgentino 217: 1-44.

Salgado, L. and Gasparini, Z. 2006. Reappraisalof an ankylosaurian dinosaur from the UpperCretaceous of James Ross Island (Antar-ctica). Geodiversitas 28: 119-135.

Sampson, S.D., Witmer, L.M., Forster, C.A.,Krause, D.W., Connor, P. O., Dodson, P., andRavoavy, F. 1998. Predatory dinosaur remainfrom Madagascar: implications for the Cre-taceous biogeography of Gondwana. Science280: 1048-1051.

US Geological Survey. 1995. Topographic mapof Seymour Island, Antarctica. Reston, Vir-ginia.

Recibido: 1 de septiembre, 2006Aceptado: 21 de mayo, 2007