Sedimentology of very thick calcarenite‐marlstone beds in a flysch succession, southwestern Pyrenees

Abstract

ABSTRACTSingle beds of up to 41 m thick are exposed for 16 km along the strike in an Eocene flysch, Spanish Pyrenees. These mega‐beds consist of a lower calcarenite (up to 25 m) and an upper marlstone (up to 16 m). Their volume is minimally of the order of 1 km3. The mega‐beds are underlain by slump sheets which in places exceed 100 m in thickness.The calcarenites show erosional sole markings, no internal amalgamation, a graded texture expressed by matrix percent, coarsest quartz grains, and coarsest foram tests. The marlstones are burrowed from their top, they are graded as expressed by matrix percent, coarsest quartz grains, and carbonate percent. Their grading continues the upward size decline in the underlying calcarenites, and their thickness and carbonate percent vary with those of the underlying calcarenites.Single calcarenite‐marlstone beds are interpreted as deposited by turbidity currents. The great thickness and other uncommon features (e.g. consistent association with an underlying slump sheet, distal thickening, locally repetitive grading, compositional inhomogeneity) can be accounted for by (1) widespread slumping initiating voluminous turbidity currents, (2) concurrence of tributary turbidity currents to deposit a single mega‐bed, and (3) ponding of the turbidity currents behind what may have been a local, palaeobasin floor high. Slumping and simultaneous turbidity currents were probably triggered by earthquakes of great magnitude. The basin floor high may have formed by basement faulting.The mega‐beds do not occupy a particular niche in a facies sequence and their great thickness does not reflect a particular environment of deposition. Instead, they more likely reflect the seismic regime (periodic earthquakes of great magnitude) and tectonic style (block faulting) of the flysch basin.