Geometría y facies del margen progradante de una plataforma carbonatada carbonífera (Unidad de Picos de Europa, Zona Cantábrica)

Resumen

The spectacular seismic-scale stratal patterns observed on aerial photograhs in the near vertical bedded carbonate successions of Las Llacerias (Picos de Europa Region, northwestern Spain), allow the identification of two discrete stages of development in an Upper Carboniferous platform. The first one, represented by the Bashkirian Valdeteja Fm., corresponds to a small sedimentation rate phase, during which the slope, toe-of-slope and the basin facies deposition of the platforms, located to the north or south, took place. In the second phase, that crops out as the Moscovian Picos de Europa and the Kasimovian Puentellés Fms., the geometric relations of stratal patterns show a clinoformal succession that prograded westward up to 4 km in length, with a slope relief of 450 m. The slope angles range from 15º to 35°. Nearly planar and tangential clinoforms are developed. Calibration with other outcrops has permitted to distinguish three members in the Picos de Europa Formation: (a) Lower Member, is 140 to 250 m thick, well stratified and formed by basin (marly shales, shales, espiculitic and calcarenitic limestones) and toe of slope deposits (alternating intraclastic, skeletal and oolitic calciriuditic and calcarenitic grainstone and packstone, and espiculitic); (b) Middle Member, is from 350 to 450 m thick, made up by slope deposits with bundles of medium to thick-bedded margin derived breccia beds; and (c) Upper Member is 150 m thick, made up by outer platform lithofacies associations, constituted by thick, well bedded skeletal packstones and grainstones. The Puentellés Formation represents inner platform environments composed by skeletal and marly wackestones, with interbedded calcarenitic storm deposits and small algae mud mounds. Both phases of platform development have been separated by a stage (part of Vereisky) of siliciclastic supply from the adjacent westward basins. Terrigenous pollution reduced the carbonate production at the western end of the platform. Relative sea-level changes caused by regional tectonics are suggested to be the main responsible for the depositional architecture of the platform.