Sedimentology and Dispersal pattern of a Cretaceous Flysch Sequence, Patagonian Andes, Southern Chile

Abstract

The Upper Cretactous Cerro Toro Formation is a flysch-like sequence overlain transitionally by molasse lithologic types and including conformable detached masses of coarse detritus emplaced in part as submarine mudflows. Sediment was dispersed in a pattern that reflects both a general eastward slope, indicated by apparent gravity-controlled structures, and the action of the dominant south-flowing currents. Axial trends and overturn directions of synsedimentary folds, trends of slide channels, overturn directions of flame structures, some clast imbrication, and one example of giant flutings on conglomerate bed soles suggest west-to-east movement of material. However, most current structures, including flute casts, current-ripple and convolute lamination, linear channels fo med by current scour, rare large-scale cross-bedding, and some clast imbrication, show north-to-south flow. Continuous graded bedding in the sequence is rare, and grain orientations show that the sandstone beds themselves were deposited or redeposited by currents with the same orientation as those that cut basal flutings. Current structures, both internal and on bed soles, and perfection of graded bedding are inversely related. The conglomeratic mudflow deposits are noteworthy for a great variety of textural types, including pebbly mudstone, conglomerate with a dispersed framework and mud matrix, and other conglomerate with a sandy matrix or intact framework. The conglomerate beds are definite lateral equivalents of zones of failure up to 100 feet thick which include large synsedimentary contortions indicating mainly west-to-east slumping. The features of the zones indicate that they repres nt sea-floor deformation induced by the catastrophic introduction of the conglomeratic mudflows into the flysch environment. Geographic distribution, bed-thickness changes, and provenance of sandstone and conglomerate indicate original transverse sediment supply (normal or oblique to tectonic trends) into the flysch environment. In contrast, nearly all current structures indicate longitudinal distribution (parallel with tectonic trends). The deflection of gravity-controlled turbidity currents by the axial plunge of a geosynclinal trough could be indicated. However, the marked discordance between current and apparent slope directions over a wide area, the results of recent oceanographic research, and a general consideration of the paleogeography of flysch deposits with longitudinal paleocurrents suggest that an alternate working hypothesis be considered: downslope lateral supply by gravity-controlled mechani ms, including turbidity currents, sandflow, mudflow, or sliding, into a regime of marine bottom currents sufficiently powerful to distribute detritus and produce sedimentary structures. Sedimentary structures and paleontological evidence indicate that the dominant longitudinal currents in this example operated in both deep- and shallow-marine environments. Positive interpretation of either source or slope direction from current structures in flysch and flysch-like sequences is unwarranted without considerable supporting evidence.