Nivel condensado con estromatolitos pelágicos en el Cretácico de la Sierra de Estepa (Subbético Externo, prov. de Sevilla)

Abstract

The stratigraphy of a condensed bed essentially composed of pelagic stromatolites and micrite with planktonic foraminifera have been set. The thickness of the bed is 25 cm, and its boundaries are two main discontinuities. The basal discontinuity is intravalanginian, and the upper one is an early-Santonian paraconformity that pass laterally into a discontinuity. Both surfaces, are laterally superposed. The studied condensed bed is the only existing record of the upper Valanginian-early Santonian interval in most of the Sierra de Estepa (External Subbetic). In the formation of this condensed bed three different stages can be distinguished: 1) Growth of pelagic stromatolites, with various morphologies, probably in relation with life activity of bacteria and a very low sedimentation rate; this phase took place probably during the Late Aptian-Middle Cenomanian; 2) Phase of erosion, submarine exposure, glauconitization and emersion with vadose cementation which occurred during Middle Cenomanian, and 3) New phase of very reduced sedimentation with the deposition of pelagic micrite, developed during Late Turonian-very Early Santonian (?) times. The two depositional stages during which the condensed bed was configurated, may correspond to each depositional sequences (or supersequences), separated by a middle Cenomanian discontinuity. In a pelagic swell paleogeographical setting, with a particular oceanographic regime dominated by an intense bottom currents circulation which swept off the pelagic swell, the condensed section would be the whole record of the depositional sequence, instead of a part of it.