Possible Non-Turbidite Origin of Deep-Sea Sands in Cretaceous Flysch (Bavarian Alps, Germany) and Recent San Diego Trough (California): ABSTRACT

Abstract

The Bavarian-Austrian flysch between the Rhine River and Vienna consists of Cretaceous clastic rocks, about 1,500 m. thick, deposited in an east-west-trending trough more than 500 km. long, 20-40 km. wide, and more than 200 m. deep. The directions of sediment supply, as inferred by sedimentary structures, grain-size variations, and heavy- and light-mineral distributions, were remarkably constant and parallel with the trough axis during long time intervals. Filling took place from both ends of the trough. The source area was at the west (Switzerland) during the Early Cretaceous, at the east (Bohemian Massif) during Turonian time, and at the west again during the Late Cretaceous. Within this flysch sequence the quartz-graywacke of the 200-m.-thick Gault Formation of Early Cretaceous (Albian) age was deposited as a continuous blanket. Single beds can be traced continuously for as much as 115 km. toward the western source area. Correlation was accomplished by comparison of the distinctive sequence, thickness, and petrography of individual beds (fingerprint-type identification). Grain-size, thickness, and content of unstable minerals in correlated sandstone beds increase slightly toward the source. Observations in the Bavarian flysch not easily explained by the hypothesis of avalanche-type, turbidity currents include the following. 1. Sedimentary structures indicating breaks in sedimentation within graded beds or fluctuations in current velocity and type of material (concentrations of heavy-mineral layers in the upper part of graded beds; repeated grading within millimeter-thick laminae). 2. The consistency of bed thicknesses and current directions, as well as heavy- and light-mineral associations of individual beds through a distance of 115 km. The enormous amount of sand contained in one single bed (2-10 km.3) is more easily explained as grain-by-grain deposition from constant bottom currents with perennially fluctuating velocities. Study of a large number of oriented, undisturbed box cores from the central San Diego trough and the La Jolla submarine fan shows that most of the sand layers have well-preserved sedimentary structures very similar to those observed in the Bavarian flysch. However, because of the absence of distinctive compositional differences in the mineral associations, individual sand layers could not yet be correlated through End_Page 632------------------------------ longer distances within the San Diego trough. Parallel and current-ripple cross-lamination, as well as imbricated mud pebbles, reworked from the underlying old clay, are more abundant than distinct grading. Study of grain fabrics (imbrication) by the use of magnetic susceptibility anisotropy and evaluation of cross-lamination foresets generally show downslope direction of sediment transport, confined to the canyon-fan valley system. Current measurements and observations from deep submersible vehicles indicate that bottom currents capable of transporting medium to coarse sand have a pulsating (tide-related?) flow (maximum velocity, 10-25 cm./sec.) both up and down La Jolla Canyon. These data suggest the probably most of the sand in water depths up to 1,100 m. was transported, or at lea t reworked, by ordinary tractive bottom currents (or diluted steady suspension currents), rather than by occasional spasmodic turbidity currents. End_of_Article - Last_Page 633------------