Abstract The Snow Hill Island Formation (SHIF; late Campanian – early Maastrichtian) crops out in the northeast of the Antarctic Peninsula and constitutes the basal part of the late Campanian-early Maastrichtian sedimentary succession of the James Ross Basin (NG Sequence). Its major exposures occur at the James Ross and Vega islands. Several fossil-bearing localities have been identified in the SHIF providing a valuable fauna of invertebrates and vertebrates, and flora. Our study focuses on the vertebrate fauna recovered at Gamma and Cape Lamb members of the SHIF. The marine vertebrate assemblages include chondrichthyans, actinopterygians, and marine reptiles (elasmosaurid plesiosaurs and mosasaurs). A diverse terrestrial vertebrate assemblage has been reported being characterized by dinosaurs (sauropod, elasmarian ornithopods, nodosaurid ankylosaur, and a paravian theropod), pterosaurs and birds. Most SHIF dinosaurs share close affinities with penecontemporaneous taxa from southern South America, indicating that at least some continental vertebrates could disperse between southern South America and Antarctica during the Late Cretaceous. The Snow Hill Island Formation provides the most diverse Late Cretaceous marine and continental faunas from Antarctica. The present study summarizes previous and new vertebrate findings with the best actualized stratigraphical framework, providing a more complete fauna association and analyzing further perspectives.

The La Meseta Formation crops out in Seymour/Marambio Island, Weddell Sea, northeast of the Antarctic Peninsula and contains one of the world's most diverse assemblages of Weddellian marine/coastal vertebrates of Early Eocene (Ypresian) age. The La Meseta Formation is composed of poorly consolidated, marine sandstones and siltstones which were deposited in a coastal, deltaic and/or estuarine environment. It includes marine invertebrates and vertebrates as well as terrestrial vertebrates and plants. The highly fossiliferous basal horizon (Cucullaea shell bed, Telm 4 of Sadler 1988) of the Cucullaea I Allomember is a laterally extensive shell bed with sandy matrix. The fish remains, including 35 species from 26 families, of the Ypresian Cucullaea bed represent one of the most abundant and diverse fossil vertebrate faunas yet recorded in southern latitudes. Stratigraphic distribution and phylogenetic relationships of the Weddellian sphenisciforms are consistent with a first radiation of this group in the Early Eocene. The first inquestionable archaeocete from Antarctica is recorded in this unit and is referred to a new taxon

La Formación La Meseta aflora en la Isla Seymour/Marambio, Mar de Weddell, noreste de la Península Antártica y contiene una de las asociaciones de vertebrados costeros/marinos de edad Eoceno temprano (Ypresiano) más diversa que se conoce a nivel mundial. Esta unidad está compuesta por areniscas marinas pobremente consolidadas las cuales fueron depositadas en ambientes costeros, deltaicos y/o estuarinos. Esta incluye invertebrados y vertebrados marinos así como plantas y vertebrados terrestres. El horizonte basal (el banco de Cucullaea, Telm 4) del Alomiembro Cucullaea I es lateralmente extensor y altamente fosilífero. Los restos de peces del banco de Cucullaea (Ypresiano) incluyen 35 especies con 26 familias y representa una de las más abundantes y diversas fauna de vertebrados fósiles registradas en latitudes altas. La distribución estratigráfica y las relaciones filogenéticas de los pingüinos fósiles (Sphenisciformes) son consistentes con la primera radiación de este grupo en el Eoceno temprano. El primer incuestionable Archaeoceti de Antártida es registrado en esta unidad y es referido un nuevo taxón

Transparent heavy mineral suites from forty two samples of the Eocene La Meseta Formation, Marambio (Seymour) Island, Antarctica, were studied in order to obtain new data of heavy mineral detrital modes and to assess the source rocks and tectonic setting of the provenance area. This unit is the youngest formation exposed in the James Ross Basin. It developed at the rear of an active magmatic arc and holds some 6-7 km of Cretaceous-Paleogene clastic sedimentary rocks. The magmatic arc was related to oblique subduction of the Pacific and associated plates below the Antarctic Plate. Calc-alkaline magmatism migrated to the northwest and by Eocene times the arc would has been located some 150 km to the north-northwest of the Marambio area. The La Meseta Formation represents deltaic, estuarine and shallow marine sediments deposited mostly within a NW-SE incised valley cut down into Late Cretaceous and Paleocene sedimentary rocks. The 720 meters thick sedimentary column has been subdivided into six allomembers. Heavy mineral suites are dominated by green-brown hornblende, green-colorless augite-diopside?, garnet, opaques and epidote with minor amounts of hyperstene, zircon, apatite, kyanite, staurolite, zoicite, sphene and tourmaline. They were determined to represent those originally present in the parent rocks namely the metasedimentary Trinity Peninsula Group (TPG) and the calc-alkaline igneous (mainly volcanic) Antarctic Peninsula Volcanic Group (APVG) cropping out along the Antarctic Peninsula. Although garnets would also come from reworked Cretaceous and Paleocene rocks, textural evidences were not able to demonstrate it. Moreover, the unconsolidated nature of most of the rocks of the basin precludes the preservation of sedimentary rock fragments in the light mineral fraction. The relative abundances of MF, MT and GM mineral assemblages (Nechaev & Isphording, 1993) has permitted the characterization of the source area as a convergent continental margin. This is in agreement with most other data from the James Ross Basin. On the other hand, published data of QFL detrital modes from La Meseta Formation showed an upward increase in quartz and feldspar content interpreted as an unroofing trend. However, the heavy mineral suites do not record the same increase in basement participation, but they keep a high proportion of minerals belonging to the MF association. Non opaque heavy minerals from the La Meseta Formation have provided new data not recorded through the study of light minerals. In this particular case, they allowed to challenge earlier hypothesis of a simple uroofing trend and to envisage a more complex scenario with a still active volcanism for the Eocene of the northern Antarctic Peninsula.

Con el fin de establecer las rocas que actuaron como fuente de sedimentos y el régimen tectónico del área de procedencia, se estudiaron las asociaciones de minerales pesados translúcidos de cuarenta y dos muestras de la Formación La Meseta (Eoceno), aflorante en la isla Marambio (Seymour), Antártida. La Formación La Meseta representa la unidad más jóven reconocida para la Cuenca James Ross. Esta cuenca se desarrolló por detrás de un arco magmático acumulando durante el Cretácico y el Paleógeno entre 6 y 7 km de sedimentos. Dicho arco magmático fue generado a partir de la subducción de la Placa Pacífica por debajo de la Placa Antártica. El magmatismo calco-alcalino fue migrando con el tiempo debido a la subducción oblicua y para el Eoceno se habría localizado a unos 150 km al NNW de la isla Marambio. La Formación La Meseta comprende sedimentitas de ambientes deltaicos, estuáricos y de plataforma mareal depositados mayormente dentro de un valle incidido con rumbo NW-SE que ha sido labrado sobre depósitos previos del Cretácico superior y Paleoceno. La asociación de minerales pesados está dominada por hornblenda verde y castaña, augita y/o diópsido incoloro a verde, granate y epidoto, con participación menor de opacos, hipersteno, cianita, estaurolita, titanita, circón, zoicita, turmalina y apatito. Se estableció que estos minerales se hallan presentes en las metasedimentitas del Grupo Trinity Peninsula (TPG) y en rocas ígneas (predominantemente volcánicas) calcoalcalinas del Grupo Volcánico Antarctic Peninsula (APVG) que afloran extensamente en la Península Antártica. Aún cuando los granates podrían provenir del retrabajo de rocas sedimentarias cretácicas y paleocenas, no hay evidencias que permitan demostrarlo. El análisis de la abundancia relativa de las asociaciones minerales MF, MT y GM permitió caracterizar el área de procedencia como un margen continental convergente. Esta asignación es concordante con otros estudios realizados en la Cuenca James Ross. Por otra parte, en estudios previos que involucraron la fracción de minerales livianos, las modas detríticas QFL indican el progresivo descabezamiento del arco magmático. Sin embargo las asociaciones de minerales pesados no muestran un cambio concomitante con esa tendencia y mantienen una alta participación de minerales pertenecientes a la asociación MF. El estudio de los minerales pesados translúcidos ha aportado nueva información que permite confrontar la hipótesis de un descabezamiento simple del arco magmático y sugerir un contexto más complejo con vulcanismo aún activo durante el Eoceno.