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, erosi- ve 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 pro- bably 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-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 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 loca- ted at a lower stratigraphic level, within the upper beds of the Snow Hill Island For- mation as originally mapped by Pirrie et al.