Abstract

From Late Lutetian times through to the Oligocene, the Jaca Basin evolved as a piggyback basin transported to the south by the San Felices thrust sheet. During the Oligocene, emergence, imbrication, and erosion of the frontal parts of this thrust sheet created the External Sierras and re-organized the pre-existing drainage systems. We describe here new magnetostratigraphic dates for stratigraphic sections within both the Jaca and northern Ebro basins. The resultant chronologies provide a temporal framework for quantifying and synthesizing the mid-Tertiary depositional and structural evolution of the western Pyrenees.

Introduction

The Jaca Basin represents the westernmost structurally partitioned basin of the South Pyrenean foreland (Fig. 1). Previous studies (Puigdefabregas, 1975) have delineated the detachment of the basin as a sole thrust propagated beneath it during late Eocene times, after which it survived as a long-lived piggyback basin accumulating detrital sediments at least until the mid Oligocene. During this interval, the External Sierra emerged as an important structural and topographic range defining the southern margin of the Jaca Basin. Because most of the piggyback strata, as well as those fronting the External Sierra in the northern Ebro Basin, were deposited subaerially and have only yielded a sparse faunal record, previous temporal controls on the evolution of the basin and bounding ranges have been imprecise. We describe here the results of recent magnetostratigraphic studies within the Jaca Basin and the northern Ebro Basin.