Sedimentary facies and sequential architecture of tide-influenced alluvial deposits: an example from the middle Eocene Capella formation, South-Central Pyrenees, Spain

Abstract

The sediments investigated consist of a thick sequence of clastic deposits of middle Eocene age, the Capella Formation. At the time of deposition the sedimentary basin was tectonically active. Tectonic influence in the sedimentary sequence is recognized from angular unconformities, synsedimentary faults and vertical as well as lateral variations in the sedimentary facies and thickness distribution. The most conspicuous structural feature is the synsedimentary Lascuarre fault system, which largely controlled the sedimentation of the Capella Formation. Three unconformity-bounded basin infilling units or tectonosedimentary units are recognized. The boundaries of the tectonosedimentary units do not coincide with those of previously defined lithostratigraphic formations. Sedimentation took place in and near a shallow bay, which was submitted to intertidal conditions. Sediment was supplied either by one or two alluvial depositional systems. Coeval depositional systems are distinguished by their facies association and paleocurrent direction. Different process-related types of sediment bodies are recognized on the basis of the external geometry and internal organization: channel deposits, depositional lobes, sheet deposits and composite forms. A further differentiation of these groups is based on similar criteria. These types are incorporated into the architectural analysis (i.e., mainly an evaluation in time and space). The interaction between sediment rate, tectonics and sea level fluctuation is discussed based on the architectural analysis. As a result of the variations in the ratio between the rate of relative sea level rise and the rate of sedimentation, repeated oscillations in the depth of the basin occurred. Within the youngest tectonosedimentary unit stationary episodes, regressive episodes, tectonic-related deepening episodes and transgressive episodes are recognized. The overall trend is transgressive.