PALEOECOLOGY AND ENVIRONMENTS OF THE CRETACEOUS SEDIMENTARY BASINS OF PATAGONIA (SOUTHERN ARGENTINA)

Abstract

The evolution of the continental and marine Cretaceous basins of Patagonia and adjacent areas (Chilean marine strip, basins developed around the Malvinas Islands, and the austral extremity of South America) is here considered, taking into account their origin, lithofacies, sequences, fossiliferous content, and paleoclimatic changes. An attempt to separate three stages of evolution, with transitional limits, has been made: 1) Early rift-syn-rift (Mid Jurassic-Barremian): 2) Thermal sink (sag stage), and increased continental sedimentation (Aptian-early Maastrichtian), and 3) The late Cretaceous transgression (late Maastrichtian-Paleocene). The first stage, during the late Triassic-Late Jurassic tectonism, produced extensional faults of regional extent with EW orientation, related to the opening of the southern Atlantic Ocean. Also a marginal basin (Magallanes or Austral basin) was sketched between the active magmatic are and the stable platform with a NNW coast controlled by the Deseado Massif (Valanginian-Aptian). After the Initial Mirano events (Early Barremian) a westward regression occurs. In the San Jorge basin a progressive migration of the lacustrine depocenters to the East, and in the western and southern margins of the Magallanes basin the deposition of deeper slope and pelagic facies, related to ophiolithic magmatism occurred. Between the Initial and Middle Mirano movements (base of the Aptian) another magmatic cycle started on the western magmatic arc, throwing ashes into the San Jorge basin. Simultaneously, erosion of the faulted margins of this basin caused the progradation of fan deltas into the lacustrine sediments (Matasiete Fm.) and in NW Patagonia the sea invaded the Neuquén basin, with the deposition of deep marine anoxic shales, limestones, and submarine fans (Tithonian-Berriasian). This anoxic character is also common in the Malvinas, Magallanes and even the African basins. During the second stage, the Malvinas, North Malvinas and Magallanes basins include retrogradational and aggradational successions deposited in littoral to extensive distal platforms. In the Magallanes basin a progressive regression occurs by the rise of the Proto-Cordillera and the eustatic lowering of the sea level, while its deeper parts were filled by submarine fan deposits. The marine sedimentation continued up to the Coniacian, but coincident regressions occurs by the deforming intrusion of the Andean Batholith during the Main Mirano movements (Albian-Cenomanian). The late Turonian-Coniacian movements (85 My) define the end of this intrusive event. The Main Mirano movements separated the Neuquen basin from the Pacific Ocean and favoured the deposition of red fluvial clastics, evaporites and marine limestones of the Neuquen Group and of three fining-upwards fluvial cycles (Río Limay, Río Neuquen and Rio Colorado, as well (Cenornanian-early Campanian). In the San Jorge basin, the volcanic activity of the arc and subsidence processes (sag phase) allowed the deposition of thick volcaniclastic successions of the Chubut Group (Castillo, Bajo Barreal and Laguna Palacios Fms.), and the Cañadón Asfalto and Los Adobes depocenter were incorporated to the new enlarged basin. The third stage shows a widespread marine transgression related to subsidence accelerated by the Huantraquican tectonism (83 My) and due with the continued separation of the South American and African plates (Late Campanian - Late Maastrichtian-Paleocene). The sea entered into the Colorado, Valdes and Rawson basins, and partially covered the North Patagonian platform, with the deposition of lagoonal, open bays to deep marine sediments related to Allen, Jagüe, Los Alamitos, La Colonia, Huantraico, Paso del Sapo and Lefipan Fms. Later, during the Paleocene, a southward gradual regression occurs. The seawater was warm and oxygenated, allowing the development of pelagic foraminifers, diatoms and carbonate nannofossils, but the presence of a siliceous microfauna indicates an increasing influence of polar currents.

KEY WORDS. Argentina. Patagonia. Cretaceous. Sedimentary basins. Stratigraphy. Paleoenvironments. Paleoecology.