Abstract

VEGA, Francisco J.  and  LAWTON, Timothy F.. Upper Jurassic (Lower Kimmeridgian-Olvido) carbonate strata from the La Popa Basin diapirs, NE Mexico. Bol. Soc. Geol. Mex [online]. 2011, vol.63, n.2, pp.313-321. ISSN 1405-3322.

Carbonate strata that occur as blocks encased in gypsum of three different salt diapirs in La Popa Basin contain a single invertebrate fauna indicating a Late Jurassic (Kimmeridgian) age for all sampled fossiliferous blocks. The most biostratigraphically significant species, present in all diapirs, is the gryphaeid oyster Nanogyra virgula. Other species identified in the blocks include the bivalves Astarte gracilicostula, Cercomya durangensis, Myophorella sp., Grammatodon sp. and the nautiloid Cymatoceras? sp. Geologic relations of the entire assemblage of carbonate blocks, including nodular gypsum interbedded with the fossiliferous beds, salt casts in micrite of the blocks, random structural position of blocks within the diapiric evaporite, absence of blocks younger than Early Kimmeridgian in the diapirs, and regional stratigraphic relations, suggest that the carbonate strata were originally interbedded with, or directly overlie, evaporite strata. This inference is corroborated by the presence of Nanogyra virgula (previously reported as Exogyra cf. E. susplicifera) in carbonate strata directly overlying massive gypsum at Potrero Minas Viejas, adjacent to La Popa Basin. Although previously correlated with the Zuloaga Limestone, the fauna in the carbonate blocks of the diapir instead indicate a correlation with carbonate and evaporite strata of the Olvido Formation, and by extension, with siliciclastic strata of La Caja and La Casita formations. The new biostratigraphic data thus indicate that evaporite deposition persisted into the Kimmeridgian and, because the fossiliferous beds are inferred to have overlain a very thick section of halite (>2 km), it is suggested that evaporite deposition was continuous with only brief interludes of carbonate deposition from Callovian to early Kimmeridgian time in La Popa Basin.

Keywords : Late Jurassic; bivalves; evaporites; La Popa Basin; NE Mexico.

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