Seismic features diagnostic of contourite drifts

摘要

The sedimentary construction of oceanic margins ismost often carried out by the combined action of gravitationalprocesses and processes related to bottom (contour) currents. Oneof the major difficulties encountered in the interpretation ofseismic profiles crossing such margins is the differentiation ofthese two types of deposit, especially where they display verycomplicated imbricated geometries. The aim of this paper,therefore, is to derive criteria for the recognition of contourite vs.turbidite deposits, based on the analysis of many seismic profilesfrom both published and unpublished sources. The followingfeatures are the most diagnostic for the recognition of contouritedrifts. At the scale of the basin, four different drift types can bedistinguished according to the morphostructural context, theirgeneral morphology and the hydrodynamic conditions. These are:contourite-sheeted drifts (including abyssal sheets and slope-plastered sheets). elongate-mounded drifts (detached and separatedtypes), channel-related drifts (including lateral and axial patchdrifts and downstream contourite fans), and confined drifts trappedin small, tectonically active basins. At the scale of the drift, threefeatures provide the best diagnostic criteria for recognisingcontourite deposits on seismic profiles: major discontinuities thatcan be traced across the whole drift and represent time linescorresponding to hydrological events, lenticular, convex-upwarddepositional units with a variable geometry, and a specific style ofprogradation-aggradation of these units that is influenced byinteraction of the bottom current with Coriolis force and with themorphology. At the scale of depositional units, the seismofaciesshow a wide variety of reflector characteristics, many of which arevery similar to those observed in turbidite series. Distinctionbetween sediment wave seismofacies deposited by turbiditycurrents and bottom currents still remains ambiguous. 1999Elsevier Science fl.V. All rights reserved.