Comparison of Sedimentation in the Bavarian Flysch (Cretaceous) and Recent San Diego Trough (California)

Abstract

ABSTRACT The directions of sediment dispersal within the oblong Bavarian deep-water (?) trough (Cretaceous) were inferred from sedimentary structures, grain size variations, and mineral distribution. The paleocurrent directions were remarkably constant and parallel with the trough axis during long time intervals, their azimuths having been reversed from west to east several times. Paleocurrent directions have been verified in part, by the detailed petrographical study of key beds of the 200 m thick Gault-Formation, correlated by R. Hesse (1965) over 115 km. They suggest mainly longitudinal filling from the distal ends of the trough. Some correlated graywacke beds contain up to 4109 m3 of sediment, orders of magnit de larger than the estimated volume of recent submarine slumps that are thought to be comparable. The study of 95 undisturbed box cores from La Jolla Submarine Fan and central San Diego Trough shows that most sand layers which have well-preserved flysch-type sedimentary structures are discontinuous, elongate, lense-shaped bodies (maximum volume 0.25105 m3 per bed). Parallel and current-ripple cross-laminated beds are more abundant than complete graded cycles. The sedimentary and magnetic fabric, morphology, and sediment distribution generally show sand transport downvalley and transverse to the trough axis along the fan-valley, and longitudinal along the axis of the trough. Bottom currents with a pulsating tide-related flow (maximum velocity 10-25 cm/sec) have been measured along the canyon, the fan-valley, nd San Diego Trough. Within the environment of La Jolla Fan and San Diego Trough most of the recent fine-to medium-grained well-sorted sand was probably transported, or at least reworked by tide-related (?), cyclic or periodical low-speed bottom currents, rather than by spasmodic high-velocity turbidity currents. Differences between the facies of La Jolla Fan and San Diego Trough include the sand:mud thickness ratio, maximum grain size, and thickness of sand layers, as well as the dispersal pattern (table 1). The recent environment of San Diego Trough is flysch-like in the sense of the sedimentological definition. Although the general geographic and tectonic setting of the Bavarian trough is different, one can compare the basin morphology of the two environments. The lithofacies of the two flysch troughs, including the characteristic sedimentary structures, rate of deposition, psammite: pelite ratio, and a typical longitudinal dispersal pattern, is roughly similar, whereas the extent and the continuity of sand layers appear to be strikingly different (table 2).