Depositional processes, triggering mechanisms and sediment composition of carbonate gravity flow deposits: examples from the Late Cretaceous of the south-central Pyrenees, Spain

Abstract

Cenomanian through Coniacian strata near the town of Sopeira in the south-central Pyrenees (northern Spain) are composed of a variety of autochthonous and allochthonous carbonate slope lithologies that are divided into six depositional sequences based on facies distribution patterns and stratal relationships. The sequences record three major phases of platform margin evolution: rifting, burial, and exhumation. During the first phase (sequences UK-1, UK-2, UK-3, UK-4, and lower UK-5), deposition occurred on the edge of a wrench basin, and a normal fault located beneath the platform margin strongly influenced slope evolution. Background hemipelagic sediments on the slope were commonly redeposited by submarine slumps and slides. More intense reworking resulted in matrix-supported, slope-derived megaconglomerates (debrites). During the Cenomanian and Turonian, seismically triggered debris flows originated at the platform margin, bypassed the upper slope, and were deposited on the lower slope as polymictic, clast-supported, matrix-rich megabreccias. The megabreccias form channelized and sheet-like bodies with erosional basal surfaces. Shallow carbonate environments backstepped during the Late Turonian and Coniacian, but displacement along the fault at this time resulted in the development of a steep submarine scarp and the exposure of Cenomanian and Lower Turonian strata to submarine erosion. Matrix-poor, margin-derived megabreccias form a thick talus pile at the base of the scarp. Some of the breccias were transported into the basin as debris falls, forming sheet-like beds. Marl eventually buried the Coniacian scarp in sequence UK-5, resulting in the second major phase of platform slope evolution. The slope profile at this time was relatively gentle, and redeposited material is less common. In the third phase (sequence UK-6), tectonically induced bankward erosion during the Santonian resulted in a high (greater than 800 m) erosional scarp with a regional east–west trend that was subsequently onlapped by siliciclastic turbidites. Rejuvenation of erosion in the same vicinity suggests that long-term tectonism controlled the position of the slope, rates of erosion, and sediment type on the slope. Sediment gravity flow processes are laterally and temporally related. Submarine slide and slump deposits commonly grade laterally downslope into slope-derived megaconglomerates. Debris flows that originated at the platform margin appear to have initiated slumps, slides, and other debris flows on the slope. Debris fall deposits are commonly capped by coarse, graded, lithoclastic packstones that may represent turbidites generated by the debris falls. Sediment fabric exerted a profound impact on depositional processes, distribution of facies, and morphology of the slope. Fine-grained, mud-rich, lower slope deposits were unstable at even moderate slope angles, and have been extensively redeposited. Redeposition of grain-rich, upper slope facies was triggered by syndepositional seismic activity and upslope migration of instability and erosion. In the presence of mud, the transport mechanisms are typically cohesive debris flows, which were able to carry material onto the lower slope and into the basin. When no mud was available, rock falls and debris falls were the dominant sediment gravity flows, and their deposits are restricted to a position on the hanging wall proximal to the fault.