Abstract

ROSALES-DOMINGUEZ, María del Carmen; GRAJALES-NISHIMURA, José Manuel  and  CARRENO, Ana Luisa. Planctonic foraminifera in clastic dikes injected into the Miocene sequence at the western edge of the Salina del Istmo Basin. Rev. mex. cienc. geol [online]. 2005, vol.22, n.3, pp.419-428. ISSN 2007-2902.

Miocene siliciclastic rocks of the western margin of the margin of the Salina del Istmo Basin are characterized by a monotonous succesion of marl, sand and sandstone beds, with a few interbedded tuffaceous horizons. In four localicities around Sayula, southern Veracruz State, this sedimentary succesion 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 sandy material, the marly clasts embedded in the dykes, and the intruded beds allow the interference 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 foshi peripheroronda, G. Mayeri, Catapsydrax dissmisilis 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 900m thick stratigraphic column. Because of the geologic and tecnotic 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.

Keywords : foraminifera; clastic dykes; Oligocene; Miocene; Salina del Istmo basin; Mexico.

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