Planctonic foraminifera in clastic dikes injected into the Miocene sequence at the western edge of the Salina del Istmo Basin

Abstract

Miocene siliciclastic rocks of the western margin of the Salina del Ismo Basin are characterized by a monotonous succession of marl, sand and sandstone beds, with a few interbedded tuffaceous horizons. In four localities around Sayula, Southern Veracruz State, this sedimentary ssuccession is intruded by 35 to 110 cm wide sandy clastic dykes that contain marly clasts. Microfossils in this type of intrusion are reported here for the first time in this part of southern Mexico. Detailed studies of planktonic foraminifera found within the sand material, the marly clasts embedded in the dykes, and the intruded beds allow the inference that the intrusive event was injected from lower stratigraphic levels up to the surface. Considering the Oligocene age of the microfossils found in the clasts (e.g., Globorotalia opima opima, Globigerina gortanii and G. ciperoensis s.l., among others), and the Miocene age of the intruded beds containing Globorotalia fohsi peropheroronda, G. mayeri, Catapsydrax dissimilis and C. stainforthi, as well as the known thicknesses of Oligocene and Miocene rocks in the subsurface, it is interpreted that the clastic injections intruded an approximately 900 m thick stratigraphic column. Because of the geologic and tectonic setting at the locality, the intrusive event is attributed to a rapid loss of pressure in the sandy unconsolidated material, induced by tectonic processes, and its subsequent mobilization along planes of weakness.