Upper Jurassic Ramp Carbonate and Associated Evaporite, Neuquen Province, Argentina

Bruce H. Nickelsen, Debra A. Merrill

The Oxfordian La Manga Limestone (10-65 m) and overlying Auquilco Gypsum (315 m maximum thickness) crop out along the west flank of the Neuquen basin, Neuquen Province, Argentina (36°40°S lat.). The contact with the underlying Lotena Sandstone is gradational, and both formations are cut by the Late Jurassic Araucanian angular unconformity. Seven lithofacies have been identified within sections measured through the entire interval along the northeast to southwest-trending, 30-km long Sierra de la Vaca Muerta ridge (38°30^prime-39°S). Moving from basin margin to the basin center (southwest to northeast) along the ridge, these are: bioturbated siliciclastic mudstone and very fine-grained sandstone with carbonate packstone interbeds (13.5 m max thickness); coral and red algae floatstone and grainstone (9.5 m); ooid and peloid grainstone (50 m); peloidal wackestone (15 m); fine calcareous sandstone (38 m); Auquilco Formation gypsum (300 m); and Auquilco Formation grainstone (15 m).

The La Manga Limestone is interpreted as a temperate ramp carbonate that developed over the Lotena Formation siliciclastic shelf. Interpretations of lithofacies from southwest to northeast are: behind-barrier subtidal lagoon with washovers; coral and red algae biostromes; ooid and peloid sand shoals; downslope wackestone and packstone mud mounds; and deep-water carbonate turbidites. A minor regression separates La Manga and Auquilco Formations. Lithofacies of the Auquilco Formation indicate a shallowing-up sequence comprised of initially deep (hundreds of meters) subaqueous evaporite deposition followed by shallow, subtidal carbonate peloidal and shell fragment grainstones and evaporites. Thickness of the subaqueous evaporite gives an order of magnitude estimation of Auquilco basin de ths of a few hundred meters at most. The Neuquen basin has an intermediate proportion of carbonate in comparison to relatively carbonate-poor basins to the south and carbonate-rich basins to the north.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.