Depositional Settings of the Basal López De Bertodano Formation, Maastrichtian, Antarctica

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

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: [email protected] 2 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, erosive surface elaborated in the underlying Snow Hill Island Formation. Mudstone-dominated beds with inclined heterolithic stratification dominate 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 or estuary. Major, sand-rich channel fills, up to 50-m thick, bounded by erosive surfaces probably represent inlets, located on a more central position in the estuary. Minor channel fills, 1- to 3-m thick, associated with offlapping packages with inclined heterolithic stratification probably represent the lateral accretion of point bars adjacent to migrating tidal channels in the upper estuary. Both types of channel fills bear relatively abundant marine fauna, are intensively bioturbated, and are interpreted as a network of subtidal channels. In southwestern Snow Hill Island, the minor offlapping packages have scarce marine fossils and bear aligned depressions interpreted as poor preserved dinosaur footprints. They represent the lateral accretion of point bars adjacent to intertidal creeks, probably located on the fringes of a mud-dominated estuary or embayment. The basal unconformity was produced by subaerial erosion; hence the inferred estuarine settings are consistent 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 canales menores, 1 a 3 m de espesor, asociados con traslape lateral de capas con estratificación heterolítica inclinada, representan estructuras de 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 alineadas interpretadas 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.

INTRODUCTION (Macellari 1988). The friable texture and were proposed: a) very shallow marine envi- resulting lack of preservation of diagnostic ronment, near a delta or estuary (Macellari The lower Maastrichtian basal part of the sedimentary structures imposes a homoge- 1988); and b) deep water, outer shelf set- López de Bertodano Formation is well neous appearance to these deposits. Con- tings (Crame et al. 2004). The recognition of exposed on Seymour and Snow Hill islands, sequently, the distinction of sedimentary fa- large, lenticular bodies of heterolithic northeastern Antarctic Peninsula (Fig. 1). cies is difficult and the interpretation of the mudstones with complex cut and fill struc- The section is dominated by thick, friable depositional environments problematical. tures, interpreted as tide-influenced estuari- mudstone-rich deposits, with variable pro- Based mainly on paleoecological interpreta- ne channels (Olivero 1998) supported the portions of clay, silt, and sand particles tions, two contrasting depositional settings first interpretation, but the discussion was 522 E. B. OLIVERO, J. J. PONCE, C. A. MARSICANO, AND D. R. MARTINIONI

not completely settled due to the inade- Tope 1988, US Geological Survey 1995). deposits were previously included in the quacy of detailed knowledge of sedimen- A series of poorly preserved tetrapod foot- López de Bertodano Formation (e.g. Rinaldi tary facies. prints were found during the austral sum- et al. 1978, Macellari 1988). The differentia- The main aim of this study is to analyze the mer of 2004 while carrying out sedimento- tion of the Snow Hill Island Formation is sedimentary facies of the basal López de logical fieldwork in Snow Hill Island. A new based on the recognition of an important Bertodano Formation (Fig. 1) and to dis- expedition in 2005 resulted in the discovery, unconformity that separates it from the cuss their significance for the understan- in the same horizon, of more material. The overlying López de Bertodano Formation ding of paleoenvironmental settings. In ad- detailed mapping of the interpreted tracks (Pirrie et al. 1997). In this redefined strati- dition, we report putative dinosaur foot- was done with a measuring tape and a com- graphy of the studied area, the uppermost prints recorded in the López de Bertodano pass. part of the Snow Hill Island Formation Formation, Snow Hill Island. includes the informal Rotularia Unit 1, and GEOLOGICAL AND the basal López de Bertodano Formation METHODS STRATIGRAPHICAL the Rotularia Units 2 to 5 of Macellari FRAMEWORK (1988). Absolute dating based on 87Sr/86Sr This paper is based mainly on the prelimi- chronostratigraphy suggests an early Maas- nary results of field studies conducted in In the James Ross archipelago, the Upper trichtian age for both stratigraphic units in southwestern Seymour and northeastern Cretaceous Marambio Group is about 3 km the studied area (Crame et al. 2004), thus Snow Hill islands by three of the present thick and consists of highly fossiliferous supporting previous data based on ammo- authors in 2004 and 2005 (EBO, JJP, and marine, shelf deposits originated in a back- nite biostratigraphy (cf. Crame et al. 2004, DRM). The study of sedimentary facies is arc basin located to the east of the Antarc- Olivero and Medina 2000, and references based on four detailed sedimentary logs of tic Peninsula (cf. Pirrie et al. 1997, Olivero therein). the basal López de Bertodano Formation in and Medina 2000, and references therein). Snow Hill and Seymour islands (Fig. 2). In the ice-free areas of northeastern Snow SEDIMENTOLOGY OF THE Details of sedimentary structures and bed- Hill and southwestern Seymour islands LOPEZ DE BERTODANO ding geometry are mostly visible on fresh (Figs. 1, 2), the lower Maastrichtian part of FORMATION exposures at the sea cliff; consequently the Group is well exposed (Crame et al. much of the effort in field research was res- 2004), and includes the upper part of the The basal unconformity tricted to this area. The unconformity bet- Snow Hill Island Formation and the basal ween the Snow Hill Island and López de part of the López de Bertodano Formation According to Pirrie et al. (1997), in Snow Bertodano formations, as well as the erosi- (Macellari 1988, Pirrie et al. 1997). These Hill and Seymour islands the base of the ve basal surfaces of large channels were two units are characterized by monotonous, López de Bertodano Formation rests on an mapped by walking along the recognized friable sandy siltstones and mudstones, with important unconformity, which erosively surfaces and recording their geographic abundant concretionary horizons and occa- cuts more than 60 m of the underlying position with a GPS navigator. The recor- sional intercalation of indurate very fine- Snow Hill Island Formation. The results of ded points were then plotted on published grained sandstone beds. Due to this homo- our field mapping support this conclusion, topographic maps of the area (Brecher and geneous appearance, all these fine-grained but we found that the unconformity is located at a lower stratigraphic level, within the upper 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 the sedimentary bodies stratigraphically located either above or below the main unconformity (cf. Olivero 1998), and recognition of a major sequence boundary requires proper distinction of erosive surfaces generated by autocyclic or allocyclic processes (Fig. 2b). In our interpretation, the main sequence boundary is coincident with an irregular, high-relief erosive surface locally marked by a basal bed with abundant pebbles of reddish concretions, rounded sandstones, and fossils reworked from the underlying Figure 1: Location map. Snow Hill Island Formation, and isolated Depositional settings of the basal López de Bertodano Formation, Maastrichtian, Antarctica 523

Figure 2: a) Geological sketch of the NE Snow Hill and SW Seymour islands. b) Sedimen- tary log of the Snow Hill Island and the López de Ber- todano formations, Section 3. pebbles of slates and rhyolites derived from no need to invoke a major extensional fault in thickness of individual laminae or beds older rocks cropping out along the Antarc- along the Picnic Passage (Fig. 2a). that could be arranged in vertically cyclical tic Peninsula (Fig. 3). This bed rests above a patterns. Generally, a dense mottling produ- slightly angular unconformity that erosively Sedimentary facies and architecture ced by bioturbation resulted in the loss of cuts more than 100 m of the underlying the original stratification; in less bioturba- Snow Hill Island Formation, being the The dominant sedimentary facies of the ted areas the sandstone laminae or beds lowest erosional relief located near the López de Bertodano Formation are muds- preserve delicate current ripples with mud Picnic Passage (Figs. 2a, 4a). tone-dominated heterolithics with inclined drapes. Herringbone and flaser bedding are The position of the unconformity is appro- bed sets, lenticular sandstone-dominated rare. Paleocurrent measurements indicate ximately coincident with the level detected heterolithics, and fossiliferous conglomera- that major flows were directed to the east, by Pirrie et al. (1997) in the upper Snow Hill tes. Detailed features of individual sedi- with minor flow reversals (Figs. 4a, 5a-d). Island Formation, which marks a major mentary facies and architecture are best The heterolithic mudstones record thick compositional break in the palynomorph seen in fresh exposures at the sea cliffs. In- stratigraphic packages with variable propor- assemblages. The provenance levels of the land, the combination of low-cuesta topo- tions of sand-mud ratios, but they are diffi- upper three palynological samples from this graphy and the friable character of these cult to map because of the poor quality of Formation (figure 8 of Pirrie et al. 1997) are fine-grained sediments prevent detailed the outcrops. Nonetheless, based on a large now reassigned to the basal López de Ber- observations. Accordingly, these individual number of grain-size analyses Macellari todano Formation. Furthermore, the loca- facies are grouped for ease of description in (1988) was able to show an intertonguing of tion of the unconformity proposed in this a single facies association dominated by silt-rich, clay-rich, and sand-rich units in paper indicates that the outcrop belt of the heterolithic mudstones. Seymour Island that he distinguished as the Snow Hill Island Formation is laterally con- The most abundant facies consists of regu- informal units U2, U3, and U4, respectively. tinuous along Snow Hill and Seymour lar alternations of thinly stratified, millime- The bedding attitude, particularly in the islands. As this fact eliminates the proposed ter- to decimeter-scale, dark-gray mudsto- units U3 and U4 and their equivalents in lateral offset of the outcrop belt between nes and pale-gray sandy siltstones to very Snow Hill Island, are also highly variable, these two islands (Pirrie et al. 1997), there is fine-grained sandstones showing variations but two repetitive motifs are evident. In one 524 E. B. OLIVERO, J. J. PONCE, C. A. MARSICANO, AND D. R. MARTINIONI

Figure 3: a) Panoramic view of the main unconformity (sb) between the Snow Hill Island and the López de Bertodano formations at Snow Hill Island 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 underlying Snow Hill Island Formation (c), and rounded schist pebble derived from the Antarctic Peninsula (d). motif, lenticular packages of 1- to 3-m ked concretions and shell fragments. The tes are relatively abundant in several strati- thick, are characterized by low dipping to largest channel-form sandstones, about 2- graphic horizons within the heterolithic subhorizontal strata, which define low-an- km wide and up to 50-m thick, are prefe- mudstones. In the informal unit U2, abun- gle, offlapping bed sets with inclined hete- rentially located near the Picnic Passage dant specimens of the bivalves Nordenskjol- rolithic stratification that can be followed (Figs. 2, 4a). Because of the dense ichnofa- dia, Eselaevitrigonia, Oistotrigonia, and Pinna laterally for 50- 70-meters. The other motif bric that has destroyed most original physi- are frequently preserved in life position. records large, sometimes deeply incised, cal structures, it is difficult to discern the Relatively large forms of the ammonite concave upward erosion surfaces bounding bedding style of these channel-form sands- Maorites tuberculatus, together with M. densi- channel-shaped heterolithic packages, up to tones, but appears to be dominantly accre- costatus, Grossouvrites gemmatus, and Diplomo- 50 m thick (Fig. 4). In the mid and upper tional. ceras lambi, indeterminate gastropods, echi- parts of unit U4 on Seymour Island these Just above the basal unconformity of the noid spines, and serpulids are also relatively heterolithic packages present multiple ero- López de Bertodano Formation in Snow abundant. A more depauperate fauna is sion surfaces, resulting in complex cut and Hill Island an unusual conglomerate facies present in unit U3, where only serpulids fill structures (Olivero 1998) characterized is locally developed. This facies, exposed in and echinoid spines are abundant throug- by a dominantly aggradational bedding style discontinuous patches of reduced extend hout its thickness (Fig. 5e). However, gas- (Fig. 4b). Other channel-shaped fills, mainly (2- to 5-m), consists of a hard cemented tropods, oysters, and ammonites could be composed of thicker very fine-grained concentration (20- to 50-cm thick) of peb- abundant in certain horizons located at the sandstone beds irregularly interbedded with ble- to cobble-sized reworked concretions base of the offlapping inclined-bed sets. thinner mudstone lenses, form isolated and sandstone fragments, derived from the Unit U4 records a similar pattern, but with bodies encased in mudstone-rich heteroli- Snow Hill Island Formation, occasional increased abundance of M. tuberculatus, thic beds (Fig. 4c). The basal contact of pebbles of acidic volcanics and schists, and Nordenskjoldia, Pinna, and cirripeds (cf. Ma- these channel-shaped fills is an erosive sur- abundant shell fragments (Figs. 3b-d). cellari 1988, Olivero 1998. Olivero and Me- face associated with a lag deposit of rewor- In Seymour Island marine fossil invertebra- dina, 2000, and Crame et al. 2004). Depositional settings of the basal López de Bertodano Formation, Maastrichtian, Antarctica 525

Figure 4: a) Along strike correlation of sections 1 to 4 showing the high-relief sequence boundary, the architecture of sedimentary facies, and the distribution of channels of different scale and fossil-rich horizons in the basal López de Bertodano Formation. Note the concentration of largest channels and fossil horizons near the Picnic Passage, between sections 1 and 3. b) Sedimentary fillings and internal erosion surfaces (es) of the large channel indicated in Section 1, Seymour Island. Height of the cliff is about 50 m. c) Medium-scale, muddy-filled channel with inclined heterolithic stratification (ihs), Section 4, Snow Hill Island. Bedding is gently dipping to the north. d) Small- scale channel filled with offlapping packages (inclined heterolithic stratification). The basal erosion surface (es) and some offlapping beds are indicated.

In northeastern Snow Hill Island, marine base of the López de Bertodano Formation mudrich beds of the López de Bertodano invertebrates are restricted to localized stra- records abundant specimens of corals, oys- Formation in Seymour and northeastern tigraphic horizons and are only abundant in ters, and the bivalve Thyasira townsendi Whi- Snow Hill islands the dominant trace fossils lenticular lag deposits. The basal lags of the te. Limited horizons within the mud-rich are Phycosiphon, Nereites missouriensis, Paradic- two large, isolated channel-form sandstones heterolithic beds, which are dominant to tyodora (cf. Olivero et al. 2004), and "Terebelli- bear abundant specimens of gastropods, bi- the southwest of Snow Hill Island, could re- na", whereas Neonereites biserialis, Patagonich- valves, and ammonites; the latter dominated cord abundant serpulids and echinoid spines. nus stratiforme, and Ophiomorpha dominate by Maorites tuberculatus. The discontinuous The trace fossils distribution has a similar the sandier beds. Bioturbation is scarce or basal lag resting on the unconformity at the pattern to that of the megafauna. In the absent in the clay-rich heterolithics exposed 526 E. B. OLIVERO, J. J. PONCE, C. A. MARSICANO, AND D. R. MARTINIONI

Figure 5: Main sedimentary facies and fossils of the López de Bertodano Formation. a) General view of rhythmic alternation of sandy (light-gray, arrowhead) and muddy (dark-gray) mudstone couplets. Field notebook (encir- cled) for scale. b) Detailed view of a sandy siltstone bed (arrowhead) with ripple cross-lamination and mud drapes. c) Current ripple cross- lamination in fine- grained sandstones. d) Fully bioturbated siltstone and mudstone couplets. The burrows of Paradictyodora isp. (arrow) cross-cut the mottled background, full relief, vertical view. e) Mudstone bed with abundant specimens of Rotularia sp. f) Neonereites biserialis (ne), upper bedding plane view. g) Scolicia isp. (sc), full relief, upper view. h) Phycosiphon isp. (phy), full relief, upper view.

toward the southwest of Snow Hill Island, by a raised rim of convoluted sediment. but sandier beds locally record abundant Tetrapod footprints These depressions have been found within Scolicia, Neonereites biserialis, and Teichichnus one of the offlapping packages with incli- isp. Large "Rosselia"-like traces are relatively In Snow Hill Island, one thin, very fine- ned heterolithic stratification (Figs. 2, 3a, abundant at the base of the offlapping, grained, well-cemented sandstone bed be- 4a), consisting of thin, indurated sandsto- inclined-bed sets (Figs. 5f-h). ars decimeter-scale depressions surrounded nes and thick, massive, friable mudstones, Depositional settings of the basal López de Bertodano Formation, Maastrichtian, Antarctica 527

during impression. The two remaining trackways apparently correspond to qua- drupeds but in only one of them this con- dition is certain. It includes approximately four manus-pes sets where the pedal prints (approximately 30 cm in length) are larger than those of the manus (approximately 20 cm in length) and partially overprint the posterior half of the former (Figs. 6, 7). DISCUSSION AND CONCLUSIONS

As previously mentioned, the interpretation of the depositional settings of the basal Figure 6: a) General view of the track-bearing surface; the arrow indicates the position of the footprints in b. b) Detailed view of a natural mold of manus-pes pair surrounded by raised rims part of the López de Bertodano Formation of convoluted sediment produced by compression of the substrate by the animal; scale bar in cm. in Seymour and Snow Hill islands based on paleoecological analyses is controversial. produced by differential weathering in non- The depauperate invertebrate macrofauna uniformly cemented beds. Instead, close dominated by the serpulid genus Rotularia inspection of the internal structure shows Defrance and echinoid spines, typical of that the sandy laminae bent downward near the basal López de Bertodano Formation, the center and it conforms a raised rim of was interpreted to reflect shallow marine convoluted sediments toward the margin of depositional settings, with deltaic or estuari- the depressions (Fig. 6). Because the inter- ne influence (Macellari 1988). On the con- nal laminae were deformed before cemen- trary, Crame et al. (2004) interpreted that the tation of the sandstone bed, deformation lowest, mud-dominated part of the López by modern cryogenic processes, such as de Bertodano Formation, with sparse ma- frost heaving, are ruled-out. Most of the crofauna, actually represents a deeper-wa- depressions are irregularly distributed, but ter, outer shelf setting. The later authors in one case up to seven of them were alig- considered that the whole formation repre- ned in almost a straight line (Fig. 7). The sents a large-scale shallowing-upwards bedding plane distribution, the associated trend, with most taxa occurring in nearsho- deformation pattern, and their relative size re, shallow-shelf settings by the latest Ma- suggest that these depressions could be astrichtian, i.e. in the upper part of the unit. best interpreted as poorly preserved dino- The homogeneous character of these de- saur tracks and trackways. The putative posits causes difficulty in the analysis of the footprints are preserved as natural molds sedimentary facies. Nevertheless, the stratal and the raised rim of convoluted sediment geometry and heterolithic mudstones, with was probably produced by the compression sedimentary structures typical for tidalites of the substrate by the animals (Fig. 6). (Figs. 5 a-c), suggest shallow-water, tide-in- The assemblage includes many isolated fluenced depositional settings. The basal Figure 7: Map of the probable trackways iden- prints randomly distributed, of highly varia- strata of the López de Bertodano Forma- tified on the track-bearing surface; the circle ble sizes (from 20 to 60 cm), thus sugges- tion are characterized by a marked lateral indicates the manus-pes pair in Figure 6 b. ting a trampling surface. Three trackways variability originally interpreted as an inter- with abundant specimens of the serpulid were identified probably representing both tonguing of clayey- and sandy-mudstone Rotularia (Austrorotularia) fallax (Wilckens). bipedal (one) and quadrupedal (two) pro- bodies, with their main axis aligned parallel The circular to oval depressions are preser- gressions (Fig. 7). Seven steps compose a to the paleoshore, i.e. in a northern direc- ved in a single horizon within a thin, hard- long trackway of a biped, nearly crossing tion (Macellari 1988). However, the lateral cemented very fine-grained sandstone bed the entire surface from north to south. variability of these strata appears to reflect (3 to 5 cm thick) interbedded with friable, Each print is slightly anteroposteriorly elon- the lenticular filling of large channels, up to clay-rich mudstones. The rounded depres- gated (approximately 60 cm in length); di- 1 km in width and 50 m in thickness, with sions, at least 3 to 4 cm deep, are not sim- gits are not distinguishable probably related their main axis oriented normally to the ple holes on the bedding surface, as usually to the high water content of the sediment paleoshore (Olivero 1998). Moreover, these 528 E. B. OLIVERO, J. J. PONCE, C. A. MARSICANO, AND D. R. MARTINIONI

lenticular bodies are bounded by large ero- cesses in different paleogeographic located on the lateral sides of a mud-dominated sive surfaces; present ample evidence of tions. The offlapping packages, 1- to 3-m estuary. Larger channels, with a higher complex cut and fill structures; and are thick, with inclined heterolithic stratifica- sand-grain content and bounded by exten- composed of rhythmical alternations of tion probably represent the lateral accretion sive erosive surfaces, probably represent sandy- and clayey-mudstone couplets, with of point bars adjacent to migrating tidal major subtidal inlets, located on a more ripple cross lamination, mud drapes, and channels. The large, heterolithic channel central position in the estuary. The relati- flaser bedding (Figs. 2b, 4b, 5a-c). Conse- fills, up to 50-m thick, with complex cut vely higher abundance of marine fossils in quently, these lenticular bodies are best and fill structures and an overall aggradatio- southwestern Seymour Island, compared to interpreted as the sedimentary filling of nal filling pattern, probably reflect fluctua- stratigraphically equivalent beds in Snow large, tide-influenced estuarine channels (cf. ting sea-level changes and accompanying Hill Island is consistent with this scenario. Olivero 1998). Several of these major chan- erosion and deposition by large, subtidal Thus, the main paleoenvironmental settings nels are indicated in the accompanying map channels (cf. Clifton 1994, Eberth 1996). In are interpreted as a relatively large, tide-do- (Figs. 2a, 4a). In addition to the above men- Seymour and NE Snow Hill islands these minated estuary or embayment. The stu- tioned features, these large channels are two motifs alternate at different stratigra- died outcrops of Snow Hill Island were also associated with variable bedding attitu- phic levels. As both types of channel fills probably located on the mud-dominated des, with strike orientations almost perpen- bear relatively abundant marine fossils and intertidal flats, along the margin of the es- dicular to the regional, NNE bedding strike are intensively bioturbated, they are inter- tuary, whereas those of the Seymour Island within the basin. These variable bedding preted as subtidal channels, probably repre- represent subtidal settings located toward attitudes, which commonly delineate the senting a network of channels of different the central to outer parts of the estuary. channel margins, reflect the aggradational sizes. The major unconformity at the base of the character of the sedimentary filling, with In southwestern Snow Hill Island, the mud- López de Bertodano Formation was pro- successive beds onlapping the channel mar- rich beds with inclined heterolithic stratifi- bably associated with subaerial erosion, as gins and dipping toward their axis. The cation are poorly fossiliferous and, except indicated by the erosional downcutting of variable bedding strike in part also reflects for a few levels, they lack an intense biotur- more than 100 m of sediments of the un- minor scale, offlapping packages of hetero- bated ichnofabric. In addition, the topmost derlying Snow Hill Island Formation. Con- lithic mudstones, interpreted as lateral ac- bed of one of the offlapping packages ap- sequently, the inferred estuarine settings are cretion deposits, i.e. inclined heterolithic pears disturbed by putative dinosaur foot- totally consistent with the beginning of a stratification, migrating in a direction orien- prints (Fig. 6). These features strongly sug- new transgressive sedimentary cycle repre- ted normally to the axis of minor channels gest that the offlapping packages represent sented by the López de Bertodano For- (Fig. 4c). the lateral accretion of point bars adjacent mation. The facies association of the López de Ber- to intertidal creeks, probably located on the Even though the dinosaur groups represen- todano Formation is interpreted to repre- fringes of a mud-dominated estuary or em- ted by the ichnites can not be assessed, they sent sedimentation in tidal-dominated bayment. support the presence of a diverse dinosaur channels of various scales and paleogeogra- The footprints represent additional eviden- fauna during the latest Cretaceous in the phic positions within an embayment or ce for very shallow water settings, with tem- continent. Non-avian dinosaur remains in estuary. Sedimentary controls by tidal pro- porary subaerial exposure, in the basal Ló- Antarctica are only known from the Lower cesses are suggested by the dominance of pez de Bertodano Formation. This eviden- Jurassic of the Transantarctic Mountains vertically stacked packages with inclined he- ce and the associated sedimentary features (Hammer and Hickerson, 1994) and the terolithic stratification; current ripple cross- strongly suggest that the interpreted track- Upper Cretaceous of the James Ross archi- lamination with abundant mud drapes; evi- bearing site represent an intertidal mud flat, pelago. Particularly, the Upper Cretaceous dence of paleoflow reversals; and the com- dissected by shallow, small tidal creeks. In record is rather scarce and fragmentary. The plex, channel-shaped geometry of sedi- this scenario, the footprints were made on a first dinosaur material reported from An- mentary bodies containing marine fossils. thin wet sand layer covered by soupy muds, tarctica was a single individual of a nodo- No clear examples of tidal cyclicity were probably located on the margins of the ti- saurid ankylosaur from shallow marine recorded, however several thinly laminated, dal creek. The poor preservation of these deposits of the late Campanian Santa Marta interbedded mudstone/sandstone couplets imperfect tracks is probably original and Formation, exposed on James Ross Island are arranged in repeated vertically cyclical was controlled by the high water content of (Olivero et al. 1986, Gasparini et al. 1987, patterns (Figs. 4, 5), which are suggestive of the soupy substrate. As indicated by the rim Olivero et al. 1991, Salgado and Gasparini tidal rhythmites (Nio and Yang 1991, Klein of folded sediment around the footprints 2006). Also from James Ross Island, but 1998). (Fig. 6), the slumping back of the surroun- from a different locality and slightly older The bedding architecture, lithology, and ding sediments into the footprint (cf. levels (Coniacian, Hidden Lake Formation), thickness of the fills in the interpreted tidal Lockley 1986, Milàn and Bromley 2006) Molnar et al. (1996) described a tibia attribu- channels are highly variable, but the recor- modified the original track impression. ted to a theropod. Additional Cretaceous ded motifs appear to characterize tidal pro- These small tidal creeks were probably loca- dinosaur specimens are known from Vega Depositional settings of the basal López de Bertodano Formation, Maastrichtian, Antarctica 529

Island and they include skeletal elements of Dalrymple, R.W., Boyd, R., and Zaitlin, B.A. Olivero, E.B. 1998. Large mud-filled channels in a hypsilophodontid (early Maastrichtian, (eds.) Incised-Valley Systems: Origin and Se- the Maastrichtian of the López de Bertodano Snow Hill Island Formation, Hooker et al. dimentary Sequences, SEPM Special Publi- Formation (Seymour Island, Antarctica): 1991) and a single tooth of a hadrosaur cation 51: 321-333. Stratigraphical implications. Revista de la A- (Maastrichtian, López de Bertodano For- Crame, J.A., Francis, J.E, Cantrill, and Pirrie D.J. sociación Geológica Argentina 53: 553-556. mation, Case et al. 2000). Recently, two ad- 2004. Maastrichtian stratigraphy of Antar- Olivero, E.B; Buatois, L.A., and Scasso, R.B. ditional theropod fragments were recovered ctica. Cretaceous Research 25: 411-423. 2004. Paradictyodora antarctica: A new complex from the Maastrichtian of Vega and James Eberth, D.A. 1996. Origin and significance of vertical spreite trace fossil from the Upper Ross islands (J.E. Martin, pers. comm. to mud-filled incised valleys (Upper Cretaceous) Cretaceous-Paleogene of Antarctica and EBO, 2005). Therefore, the combination of in southern Alberta, Canada. Sedimentology Tierra del Fuego. Journal of Paleonto-logy the known dinosaurs body-fossil record 43: 459-477. 78: 783-789. with these new ichnites might suggest a Gasparini, Z.; Olivero; E. B., Rinaldi, C. A., and Olivero, E. B., Gasparini, Z., Rinaldi, C. A., and more extensive distribution of dinosaurs in Scasso, R. A. 1987. Un ankylosaurio (Reptilia, Scasso, R. A. 1991. First record of dinosaurs Antarctica than previously assumed. This Ornithischia) campaniano en el continente from Antarctica (Upper Cretaceous, James circumstance could be reflecting opportu- antártico. 10º Congreso Brasileiro de Paleon- Ross Island): Palaeogeographical implica- nistic dispersal of members of the group at tología, Anais 1: 131-141, Rio de Janeiro. tions. In Thomson, M.R.A., Crame, J.A., the end of the Cretaceous among southern Hammer, W.R. and Hickerson, W.J. 1994. A cres- Thomson, J.W. (eds.) Geological Evolution of Gondwanan areas via Antarctica, as was ted theropod dinosaur from Antarctica. Antarctica: 617-622. Cambridge University previously proposed (Sampson et al. 1998), Science 264: 828-830. Press, Cambridge. probably during periods of more favorable Hooker, J.J., Milner, A.C., and Sequeira, S. 1991. Olivero, E.B. and Medina, F.A. 2000. Patterns of climatic conditions in the region. An ornithopod dinosaur from the Late Cre- Late Cretaceous ammonite biogeography in taceous of West Antarctica. Antarctic Science southern high latitudes: the Family Kossma- ACKNOWLEDGEMENTS 3: 331-332. ticeratidae in Antarctica. Cretaceous Research Klein, G.D. 1998. Clastic tidalites- A partial re- 21: 269-279. Logistic support by the Instituto Antártico trospective view. In Alexander, C.R., Davis, Olivero, E.B., Scasso, R.A., and Rinaldi, C.A. Argentino and Fuerza Aérea Argentina are R.A., and Henry, V.J. (eds.) Tidalites: Proces- 1986. Revision of the Marambio Group, Ja- greatly appreciated. A. Sobral helped with ses and Products. SEPM Special Publication mes Ross Island, Antarctica. Contribuciones the 2005 fieldwork. We thank Matthew Ca- 61: 5-14. Científicas del Instituto Antártico Argentino rrano, Rodolfo Coria and James E. Martin Lockley, M. G. 1986. The paleobiological and pa- 331: 1-28. for helpful comments on the dinosaur foot- leoenvironmental importance of dinosaur Pirrie, D., Crame, J.A., Lomas, S.A., and Riding, prints in an early version of this contribu- footprints. Palaios 1: 37-47. J.B. 1997. Late Cretaceous stratigraphy of the tion. This work was partially funded by Macellari, C.E. 1988. Stratigraphy, sedimentolo- Admiralty Sound region, James Ross Basin, PICT 8675 (ANCYPT, Argentina) and PIP gy and paleoecology of Upper Cretace- Antarctica. Cretaceous Research 18: 109-137. 5100 (CONICET, Argentina). Constructive ous/Paleocene shelf-deltaic sediments of Rinaldi, C.A., Massabie, A., Morelli, J., Rosen- suggestions and comments made by the Seymour Island (Antarctic Peninsula). In man, H.L., and del Valle, R. A. 1978. Geolo- reviewers L. A. Spalletti and J. E. Martin Feldmann, R.M., Woodburne, M.O. (eds.) gía de la isla Vicecomodoro Marambio. Con- help to improve the study. Geology and Palaeontology of Seymour tribuciones Científicas del Instituto Antártico Island, Antarctic Peninsula. Geological So- Argentino 217: 1-44. WORKS CITED IN THE TEXT ciety of America, Memoir 169: 25-53. Salgado, L. and Gasparini, Z. 2006. Reappraisal Milàn, J. and Bromley, R.G. 2006. True tracks, of an ankylosaurian dinosaur from the Upper Brecher, H.H. and Tope, R.W. 1988. Topogra- undertracks and eroded tracks, experimental Cretaceous of James Ross Island (Antar- phic map of Seymour Island. In Feldmann, work with tetrapod tracks in laboratory and ctica). Geodiversitas 28: 119-135. R.M., Woodburne, M.O. (eds.) Geology and field. Palaeogeography, Palaeoclimatology, Sampson, S.D., Witmer, L.M., Forster, C.A., Paleontology of Seymour Island, Antarctic Palaeoecology 231: 253-264. Krause, D.W., Connor, P. O., Dodson, P., and Peninsula. Geological Society of America, Molnar, R.E., Lopez-Angriman, A., and Gaspa- Ravoavy, F. 1998. Predatory dinosaur remain Memoir 169: 17-19. rini, Z. 1996. An Antarctic Cretaceous thero- from Madagascar: implications for the Cre- Case, J., Martin, J., Chaney, D., Reguero, M, Ma- pod. Memoirs of the Queensland Museum taceous biogeography of Gondwana. Science renssi, S., and Santillana, S. 2000. The first 39: 669-674. 280: 1048-1051. duck-billed dinosaur (Family Hadrosauridae) Nio, S.D. and Yang, C. 1991. Diagnostic attribu- US Geological Survey. 1995. Topographic map from Antarctica. Journal of Vertebrate Pa- tes of clastic tidal deposits: a review. In of Seymour Island, Antarctica. Reston, Vir- leontology 20: 612-614. Smith, D.G., Reinson, G.E., Zaitlin, B.A., and ginia. Clifton, H.E. 1994. Preservation of transgressi- Rahmani, R.A. (eds.) Clastic Tidal Sedimen- ve and highstand late Pleistocene valley-fill/ tology. Canadian Society Petroleum Geolo- Recibido: 1 de septiembre, 2006 estuary deposits, Willapa Bay, Washington. In gists, Memoir 16: 3-27. Aceptado: 21 de mayo, 2007