Sequence Stratigraphy of the Békés Basin

Abstract

The 6500-m-thick section of Neogene and Quaternary fill in the Békés basin (3900 km²) reflects a normal cycle of rift-related sedimentation that began in late Badenian time (16.5 Ma) when marine waters were shallow (<200 m). By late Sarmatian or early Pannonian time (12−10 Ma), lacustrine conditions prevailed and water depths in the basin increased as a result of basin subsidence rates that greatly exceeded sedimentation rates. The latest history of the basin, from approximately 5.78 Ma, reflects continuously shallowing waters, which resulted from sedimentation rates that were generally higher than basin subsidence rates. By late Pannonian time (4.25 Ma), water depths in the lake were 200–400 m, and continued shoaling culminated in the eventual disappearance of the lake in Pliocene time.

Detailed mapping of sequences, the boundaries of which are attributed to the cyclic shifting of delta lobes, indicates that the Neogene post-rift history of the Békés basin reflects one of the last episodes of filling of the much larger Pannonian Basin. It was not until other subbasins to the northwest, north, and northeast became filled by southward-prograding sediment wedges that the major river systems were able to advance to the area of the Békés basin. Thereafter, rapid deltaic filling of the basin proceeded as major river systems converged and lacustrine sedimentation became confined to a progressively shoaling basin that was rapidly decreasing in areal extent.

During much of the early Pannonian history of the Békés basin, large volumes of clastic sediments transported by rivers were channeled along structural depressions; the depressions or troughs were derived from prerift or early rift topography of the basin. In some of these troughs, large subaqueous fans, consisting chiefly of very fine grained material, developed near the base of the slope. In later stages of basin filling, sand was derived primarily from prograding delta fronts and moved downslope by slumping, sliding, mass flow, and turbidity currents. In this later stage, subaqueous fan systems and channels on the slope were poorly developed.