Offshore to deep marine sedimentation in the Río Mayer Formation, Austral Basin, Patagonia Argentina.

RICHIANO S.,

Mendoza, Argentina

Congreso; 18 International Sedimentological Congress; 2010

IAS

In the southernmost part of Patagonia the Austral Basin is developed between Jurassic and Tertiary times. The initiation of this basin was related to an extensional phase during the Late Jurassic whit the accumulation of a thick sin-rift sequence (El Quemado Complex). On top of this complex, shallow marine deposits of the Springhill Formation accumulated between the Tithonian and Berriasian related to an initial transgressive even. Nut it is not until the Berriasian that a major transgression took place with the accumulation of the black shales of the Río Mayer Formation (Berriasian to Albian), which is the first episode of offshore to deep marine sedimentation in the basin and mark the transition to a post-rift stage for this basin. The study area is located between Lago Argentino and Lago Viedma in Santa Cruz Province, more specifically in the Seccional Río Guanaco (S 49º 57’ 11’’; W 72º 04’ 56’’) of Los Glaciares National Park. During two field trips seven detailed sedimentary sections (1:100) of the Río Mayer Formation were collected. Preliminary palaeoenvironmental interpretations were performed, allowing the division of the 350 m thick unit in the study area in three sections. The lower section is composed mainly by laminated black shales with both tabular and concretional marls levels. Trace fossils are not recorded in this section, but ammonites, belemnites and bivalves are frequent. It has been interpreted as accumulated in a distal platform, in anoxic conditions. The middle section is characterized by bioturbated black marls and shales, with well preserved Zoophycos ichnofacies, suggesting a slope depositional environment. Body fossils are frequent, especially belemnites (Belemnopsis sp. - Valanginian to Hauterivian-). The upper section is marked by massive black shales with intercalations of very fine- to fine-grained sandstones and less frequent conglomerates. In this section were identified both debris flows and distal low density turbidity current deposits. Zoophycus ichnofacies and Ophiomorpha rudis ichnosubfacies were identified in association with molds of petrified wood containing Teredolites isp. The presence of turbulent currents and large trace fossils suggest a more oxygenated conditions. In summary, above the upper Jurassic rifts deposits and/or the littoral sediments that are filling the half-grabens, the Río Mayer Formation is developed in response to a regional transgression. This took place in the Berriasian, and the result was the deposition of the lower section of Río Mayer Formation. The anoxic conditions produced well laminated black shales without bioturbation. Then as a consequence of continuum transgression the middle section was deposited during ca. Valanginian – Hauterivian when the terrigenus sediment input to the basin was minimum and marls condition was established. A very well preserve Zoophycus ichnofacies and the few appearances of fine sandstones are the evidences to think that the environment was at least disoxic. At the top of this section a maximum flooding surface is recorded by a glauconitic level probably developed in the upper Hauterivian. After that, the development of two deltaic successions, first the Río Belgrano Formation (Barremian) and then the Piedra Clavada Formation (Albian) was produced at north of the basin. The very frequent intercalation of sandstone in the upper section could proceed from these two littoral units. Evidence for this interpretation is the same age of Piedra Clavada and Río Mayer Formations (from paleontological evidence) and the presence of very similar petrified wood containing Teredolites isp. in both units.