Upper Jurassic (Lower Kimmeridgian-Olvido) carbonate strata from the La Popa Basin diapirs, NE Mexico

Resumen

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