First Paleogene selachifauna of the middle American-Caribbean Antillean region, La Mesa de Copoya, west-central Chiapas, Mexico—Geologic setting

Abstract

The area lies between 16º35’-16º45’ N Lat. and 93º00’-93º10’ W Long; and consists of 323.3 km2 of moderately rugged terrain set between 380-1,240 mamsl, formed by these units: Sierra Madre Limestone (Middle Cretaceous, light olive gray, thick bedded biomicrite, 800 m thick occurring in the southwest), Angostura Formation (Late Cretaceous, light gray, medium bedded, sparsely fossiliferous biomicrite to biomicrudite and calcarenite, 300-350 m. thick, cropping out both in the southwest and northeast), Soyaló Formation (Paleocene, gray, shaly, thin bedded, arkosic-phyllarenitic, clayey siltstone, 150 m. thick; it is exposed in the outer margins of the lowlands that surround La Mesa de Copoya, which is the outstanding physiographic feature of the area), El Bosque Formation (Early Eocene, grayish red, medium to thick bedded, phyllarenitic sandstone and breccioid conglomerate, 100 m. thick, occurring out in the southern lowlands.), and San Juan Formation (Middle Eocene, marly -quartz phyllarenitic- biomicrite to biosparrudite with biostromic zones, intercalated with biomicrudite-supported, coarse grained sand-granule-to-gravel, quartz phyllarenitic sandstone to conglomerate; the unit becomes less limy upward (upper third), is 450 – 500 m. thick, makes up both La Mesa and the slope around it, and bears the name-sake selachifauna. Quaternary deposits complete the sequence.

The Pre-Quaternary units are broadly folded into a NW-SE trending syncline, disrupted by faults. The area largely records Cretaceous-Paleogene marine sedimentation -punctuated by Early Eocene continental deposition- that ceased sometime during the Tertiary, probably by Late Miocene time, because of a change in the tectonic regime that led to folding, fracturing/faulting and regional uplift. Subsequent erosion nearly removed the Tertiary units, the remainder is preserved in the syncline nucleus.