Shallow Marine and Meso-tidal Paleo-environments of the Late Danian Salamanca Formation, Chubut Province, Argentina

ARI IGLESIAS; EMILY COMER; RUDY SLINGERLAND; MARCELO KRAUSE; ARI IGLESIAS; H GRAHAM; WILF PETER

Huston, Texas

Congreso; AAPG Annual Meeting; 2011

American Association of Petroleum Geologist

The Late Danian Salamanca Formation contains reservoir sandsalong the northern flank of the Golfo San Jorge Basin of Patagonia, the second highest petroleum-producing basin in Argentina with an estimated 473 million m3 of oil and 70,300 million m3 of gas (Rodriguez, 2001). The objective of this research is to better define the facies, depositional environments, and history of the Salamanca Fm. in the Chubut province using data from its southwestern outcrop belt and 80 wireline logs.In February 2010, twenty stratigraphic sections were measured along the outcrop belt at Palacio de los Loros, Ormaechea Park, and Rancho Grande.The Salamanca Fm. in the study region consists of seven facies: 1) transgressive sands, 2) wispy-bedded clays, 3) heterolithic cross-bedded sands,4) white cross-bedded sands, 5) accretion-bedded silts, and 6) transitionalsilty clays. The Salamanca is overlain by blackmottled mudstonesreferred to by earlier works as theBanco Negro.The base of the Salamanca Fm. rests on an unconformity representing the gap between Cretaceous and Late Danian sediments and defining both a sequence boundary and marine flooding surface.Lower sections of the Salamancacontain abundant glauconite and fossils characteristic ofa marine shelf environment. These facies transition upwards to bi-directional trough cross-bedded sands interspersed with flaser bedded sandy silts indicative of meso-tidal currents with dominant orientations of ESE (340-360 degrees) and SW (240 degrees).The upper unitof the Salamanca Fm.gradationallytransitionsfrom sand to silts containing abundant, well-preserved macro-flora indicative of a diverse lowland forest. Overlying these are black muds in the Banco NegroInferior of the Late Paleocene Rio Chico Formation, interpreted as a series of stacked gleyed paleosols in a continental fluvial setting. Two-dimensional hydrodynamic modeling of tidal currents and ranges in the proto-San Jorge embayment produce tidal wave amplitudes of 1-2 meters, consistent with the hypothesis that the Salamanca was deposited in an open estuary that amplified Paleocene Atlantic tides to meso-tidal range.Total organic carbon analyses taken from different facies within the Salamanca Fm. yield TOC values ranging from (values), showing strong correlation to macroflora localities. Synthesis of facies relationships, wireline log character, hydraulic modeling, macroflora, and geochemistry define a detailed Paleocene environment for the Salamanca.