Identification of systems tracts in low-angle carbonate ramps: examples from the Upper Jurassic of the Iberian Chain (Spain)

Abstract

The Upper Jurassic of the Iberian Chain in northeast Spain was selected for the study and identification of sequence boundaries and systems tracts in low-relief carbonate ramp systems. The study area provides a special opportunity for sequence stratigraphic analysis because the outcropping Upper Jurassic carbonate ramps are relatively undeformed, well-exposed from proximal to distal areas and have an established ammonite zonation. Two sequences are recognized. Sequence A, uppermost Callovian to uppermost Oxfordian in age (up to 60 m thick in the proximal areas and between 15 and 5 m thick in the middle and distal areas), and Sequence B, Kimmeridgian to lowermost Tithonian (from 80 to 160 m in thickness across the basin). The evolution of the first sequence is related to regional relative sea-level changes, i.e., tectono-eustatic changes superimposed on the sedimentary basin. On the other hand, the larger accommodation created during the second sequence is regarded as controlled mainly by steady subsidence. Sequence boundaries indicative of subaerial exposure show as unconformities with stratigraphic hiatuses in the proximal areas of the basin. The lowstand systems tract in Sequence A is represented by a decimetre-thick reworked level, consisting of several ammonite biozones. In Sequence B, this systems tract consists of a thick, wedge-shaped, marly unit. In both sequences, the transgressive surface overlaying these lowstand deposits is generally planar and marks a sharp boundary between the restricted and open marine fossils. However, in Sequence B, in the middle part of the basin, the transgressive surface can be absent; instead a transitional change to open marine facies exists. The transgressive systems tracts show sedimentary condensation at least in the distal areas of the basin with mostly ammonites and benthic fossil wackestones. The transgressive unit is generally overlain by a hard ground surface that corresponds to a maximum flooding surface. Finally, in the upper part of both sequences, highstand systems tracts are represented by progradational siliciclastic and shallow carbonate complexes in the proximal areas of the basin. In Sequence A, sedimentation is condensed in the distal areas with glauconite marl levels present. The descriptions of sedimentary features of the systems tracts of these two sequences, developed in a low-angle carbonate ramp, are suggested to be used as tools for the recognition of systems tracts patterns within sequences in comparable depositional settings.