--> About some exotic limestone lens in the Triassic Zacatecas Formation, Sierra de Salinas

2020 AAPG Hedberg Conference:
Geology and Hydrocarbon Potential of the Circum-Gulf of Mexico Pre-salt Section

Datapages, Inc.Print this page

About some exotic limestone lens in the Triassic Zacatecas Formation, Sierra de Salinas

Abstract

Within the clastic deep-water successions of the Triassic in the Sierra de Salinas are some exotic limestone bodies. The limestones are outcropped in isolated blocks of lenticular, irregular shaped sizes ranging from a few cm up to ca. 1 m long and up to 40 cm thick and are embedded within clay-sandy sequences of the Zacatecas Formation. In hand sample, these limestone lenses appear reddish gray to cream gray and clearly show the presence of macrofauna. The fossils are in large proportions and are clearly oriented. Microfacial analyzes performed on these limestones showed that it is about 90% of components and only a little less than 10% of ground mass. The ground mass is clayey to sandy, distinguished bu isolated, rounded to sub-rounded quartz grains (mainly) and some remains of intraclast-type grains, all of them in sand fraction grain sizes. Interesting is the fact that there is no micritic matrix which suggests that there was no precipitation of CaCO3 under that strictly clastic environment. The calcareous part is composed of an oriented and well packed stacking of bivalve shells of brachiopod type. The brachipods are small with shell lengths between 5 to 10 cm long and very thin (2 to 5 mm). Under the microscope, the typical lamellar microstructure of these organisms is recognized, however there are no punctae or pseudopunctae structures. Some brachiopod shell beds are composed of in-situ assemblages, while others by transported assemblages deposited by currents within depressions of the sedimentary surfaces (Flügel, 2004). Growth lamellae characterizing the 'foliated microstructure' of the shells. The high frequency of life-positioned brachipods in the densely packed well sorted fabric, together with the lenticular shape of the deposit indicates an accretionary buildup, probably on the ridges of some deep barriers of great energy. The importance of studying these calcareous lenses is that in all the locations where the Zacatecas Formation are outcropped there are no limestone rocks except for these calcareous lenses of brachiopods.