Cenomanian – Coniacian zonation (foraminifers and calcareous algae) in the Guerrero – Morelos basin, southern Mexico

Abstract

A biostratigraphic zonation of the Cenomanian–Coniacian rocks of the Guerrero–Morelos basin (southern Mexico) is proposed. The stratigraphic distribution of 70 species of calcareous algae and benthic and planktonic foraminifers is used to characterize four Zones that in ascending order are: Pseudorhapydionina dubia TRZ (Total Range Zone); Whiteinella archaeocretacea IRZ (Interval Range Zone); Helvetoglobotruncana helvetica TRZ, and Marginotruncana sigali IRZ.

The top of P. dubia (upper Cenomanian) is marked at the last appearance of the marker fossil, which closely corresponds to the last appearance of most miliolid benthic foraminifers. Over most of the area, the transition from shallow–marine limestones up into pelagic facies occurs within the W. archaeocretacea Zone (uppermost Cenomanian–lowermost Turonian). A characteristic of this zone is the scarcity of both benthic and planktonic foraminifers, including the zonal marker. Most large benthic foraminifers disappear in the lower part of this zone. The changes observed within the W. archaeocretacea Zone reflect the successive stages of the platform drowning.

The H. helvetica (lower–middle Turonian) is characterized by the presence the nominal taxon, dicarinellids, praeglobotruncanids, whitenelids and hedbergelids. This zone is recognized in the Mexcala Formation and represents deposition in fully pelagic conditions. Toward the central and eastern part of the area in shallow–open marine facies (Cuautla Formation), this zone is represented by an assemblage characterized by hippuritids, echinoids (crinoids and roveacrinids), gymnocodiacean and udoteacean algae and scarce planktonic foraminifers.

The Marginotruncana sigali (upper Turonian–Coniacian) was defined with the last appearance of H. helvetica, whilst its top was difficult to recognize. Toward the central and eastern part of the area, this zone is represented in shallow–open marine facies (Cuautla Formation) by an assemblage dominated by the hippuritid Vaccinites gosaviensis, solitary corals, gymnocodiacean algae, calcisphaerulids and very scarce planktonic foraminifers.

The Cenomanian–Turonian boundary lies in the lower part of the Cuautla Formation. The appearance of hippuritid mollusks and the diversification of whiteinellids can be used to mark this boundary.