Carbonate ramp drowning caused by flexural subsidence: The South Pyrenean middle Eocene foreland basin

Abstract

A detailed sedimentological and magnetostratigraphic analysis of three selected sections of the Eocene carbonate platforms of the South Pyrenean Jaca-Pamplona foreland basin has been carried out in the Aragonese External Sierras (Huesca, Spain). The stratigraphic record spans from the middle-late Lutetian (Chron 20r, SBZ15) to the Bartonian (Chron 18r, SBZ17). The Lutetian-Bartonian transition was characterized by a major paleogeographic change, from a turbiditic foredeep with associated peripheral carbonate ramps to deltaic sedimentation. In most of the outcrops of the External Sierras, the peripheral carbonate platforms (Guara Formation) are overlained by prodelta and outer ramp marls (Arguis Formation). The drowning unconformity between these two formations in the outcrops located in the westernmost tip of the External Sierras is documented herein for the first time. In this area, the Guara Formation is overlain by glauconitic limestones deposited in a carbonate ramp setting. Between these units a characteristic unconformity is observed, with hard-ground formation and local evidence of subaerial exposure in the shallowest domains of the study area. The abundant glauconite found on top of this unconformity records a starvation period in the basin, followed by the development of a new carbonate ramp system during Bartonian times (Santo Domingo Member of the Arguis Formation). Magnetostratigraphic and sedimentological data show that tectonic tilting occurred in latest Lutetian times, prior to the drowning process. This suggests the influence of the flexural subsidence in the cratonic margin of the foreland as a result of the advance of a basement thrust in the hinterland. Nevertheless, the correlation of the uppermost Lutetian drowning unconformity with a basinwide regional sea level rise, which may have further been global, suggests an interplay between tectonic and eustatic processes in the drowning of the late Lutetian South Pyrenean carbonate ramps. In this work we point out a narrowing of the carbonate ramp, together with a relative sea level rise, as the processes responsible for the drowning of the Upper Guara Mb carbonate ramp. These processes could be responsible for the development of drowning unconformities in cratonic margins of other foreland basins. As carbonate ramps have a high sediment production potential, the narrowing of the carbonate factory could provide the conditions for the drowning of the carbonate ramp system in a subsequent sea level rise.