A Paleogene ichnological record from the Wilcox Formation: Ophiomorpha and Venericardia (Venicor) zapatai in the Burgos Basin, northern Mexico

The present paper address the occurrence of Venericardia shell concentrations distributed over hundreds of meters along terrigenous horizons as a typical sedimentary facies of the Wilcox Formation in the Burgos Basin. These extensive deposits overlie Paleogene sandstones so common around the municipality of China, Nuevo Leon, that they are almost overlooked. This report documents for the first time the ichnological assemblage and shell accumulations of the Paleogene in the Burgos Basin. Ophiomorpha cf. nodosa and Ophiomorpha isp. from Skolithos ichnofacies are the main identified trace fossils; the probable paleoenvironment is a near-shore shallow sea under high-energy conditions related to coastal bars. The analyzed taphonomic features suggest that Venericardia shell concentrated or accumulated as bioevents resulting from intermitent tempestites in the proximal zone. These accumulations are composed of the species Venericardia (Venicor) zapatai organized as regular and linear shell arrangements contained within a frail sedimentary matrix from which they are easily-extracted.