Facies architecture and geometry of a prograding carbonate ramp during the early stages of foreland basin evolution: Lower Eocene sequences, Sierra del Cadí, SE Pyrenees, Spain

Abstract

Abstract This paper describes the Early Eocene sedimentological evolution of the Sierra del Cadí carbonate platform during the early development of the southeastern Pyrenean foreland basin, Spain. The platform developed as a south-facing ramp system located either across the southern margin of a growth anticline within the newly formed basin or along the axial margin of the basin. Sequences I and II both comprise basal transgressive systems tracts (TSTs) dominated by a thick succession of outer ramp lime mudstones, shallowing upwards into a highstand systems tract (HST) of mid- and inner ramp bioclastic shoals, sand banks and lagoonal deposits. In Sequence III the TST comprises a thick, wedge-shaped unit of east to west backstepping, inner and mid-ramp Nummulites shoals and lagoonal deposits, progressively overlain by a thick succession of outer ramp lime mudstones and marls of the HST. No lowstand systems tracts are recognized. Sequence boundaries correspond to flooding surfaces which grade basinwards into correlative conformities, interpreted to represent the onset of high tectonic activity in the orogenic wedge and resulting high flexural subsidence across the foreland and drowning of inner ramp regions. TSTs represent continued high flexural subsidence creating accommodation space infilled by high rates of outer ramp sediment accumulation. In Sequences II and III, TSTs show a pronounced west to east thickening, interpreted to represent the effect of variable subsidence across the margins of an uplifting growth anticline within the otherwise flexurally subsiding foreland. Progradational successions that characterize HSTs developed during periods of reduced tectonic subsidence, both within the foreland and in the orogenic belt, allowing the growth and expansion of carbonate ramp facies belts.