Estratigrafía y sedimentología de la sucesión Westfaliense del borde sureste de la Cuenca Carbonífera Central (zona cantábrica, N de España)

Abstract

The Westphalian succession of the Central Coal Basin depicts great lateral and vertical facies variability, with frequent terrigenous and carbonate beds in a predominantly mudstone series. The former are commonly found in the lower part (Lena Group) and the latter acompaigned with economic coal seams, in the upon unit (Sama Group). According to these stratigraphie and some structural criterium, the Central Coal Basin has been divided into subbasins or sectors with different litostratigraphic series. From the stratigraphie correlation of the studied localities and the lithology percentage maps, there is an eastward thinning of the carboniferous serie (from 6000 to less than 2000 m) and an increase on limestone content. The facies description and interpretation allows the correlation of several sedimentary environments, from coastal to deep ones: a) The deltaic ones are: coastal alluvial fans, braided plain deltas and fluvial deltas, b) In the shallow sea environments both carbonate and terrigenous shelf are distinguished, c) In the deep sea environments, external platform and a carbonate slope and submarine fans. The studied Bashkirian-Moscovian shows a regressive macrosecuencia, progradational, coarsening upward of alocyclic origin. This can be separated into five mesoscale sequences due to tectonic impulses and to possibility eustatic sea incursions. The cyclothems are related with the progradation and abandon of deltaic lobes of autocyclic or alociclye origin. The cantabrian coal basin is considerated as a foreland basin, where depocentre mobility forwards the east, as a result of the tectonic deformation front. Subsidence curve in différents sectors depict low slopes during Tournaisiense-Viseense, considered preorogenic and higher slopes (sinorogenic) of Namurian-Westphalian age. During Bashkirian times there is a foredeep, a forebulge with local condensed sedimentation at the «tras umbral» basin. Later, these is an uniformation stage, during Moscovian, with shallow sea conditions. Vereisky and Kashirsky are characterized by stacked delta plain episodes, in the proximal sectors, and terrigenous shelf with carbonate shelf episodes in the external parts. During upper Kashirsky there is an irruption of important deltaic systems, braided plain type and most important carbonate shelf deposits are installed at the western border. A new platform expansion takes place during Kashirsky-Podolsky. The sedimentary basin undergoes an important tectonic reduction at Podolsky-Myachkovsky as a result of the orogenic deformation front advance. The irruption of alluvial sediments in the basin prograde over a transitional shallow marine environment giving rise to coastal alluvial fans. Towards the basin interior, they are substituted by braided plain deltas and fluvial deltas alternating with shallow marine episodes either terrigenous or carbonatic dominating in the most external sectors of the basin.