Carbonate sedimentation on inactive fan-delta lobes: response to sea-level changes, Sant Llorenç del Munt fan-delta complex, NE Spain

Abstract

Abstract The Tertiary Ebro Basin is a part of the South Pyrenean foreland basin, and is bounded to the southeast by the Catalan Coastal Range. This feature developed as a major sinistral strike–slip system during Alpine deformation, which led to uplift of the southeastern basement. The Eocene Sant Llorenc del Munt fan-delta complex with its complex transgressive–regressive stratigraphy evolved on the down-thrown side of this fault system. This work presents an analysis of a ca. 20-m thick sedimentary succession occurring across the transition between the transgressive and the regressive parts of a 50–200,000 year sequence. During the study time interval, there was low siliciclastic influx and relatively extensive carbonate platform (covering an area of approximately 12 km2) developed over the middle and distal portions of the Sant Llorenc del Munt fan-delta complex. The carbonate production was dominated by benthic foraminifera (mainly nummulites but also Discocyclina), and coralline algae. The nummulites formed platform shoals (mixed with sand-sized siliciclastics) and buildups, with sediments rich in Discocyclinae deposited in the depressions between them. The coralline algae grew mainly on top of coarse siliciclastic deposits, forming algal ridges and bioherms. Limestone units have been identified on five different stratigraphic levels within the studied interval, and the limestones are separated by marls and sandy marls. These marl deposits probably represent partial suppression of carbonate production, due to an increase in the influx of sand-sized siliciclastics. This interbedding between limestone and marls makes it possible to subdivide the carbonate succession into five high-order transgressive–regressive sequences. The limestone units dominate the transgressive phases, and the marls the regressive phases of these sequences. The base of each sequence is found in a progressively more landward position, indicating the transgressive regime during early deposition. In the topmost sequence, however, the transition from the transgressive into the regressive part is represented by a basinward shift of patch-reef development, representing the onset of an overall regressive depositional regime (the Guruguru regressive unit).