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Orbital Forced Cyclicity in the Depositional Environment in the Cape Basin as Depicted in Seismic Data

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Weigelt, E. and Uenzelmann-Neben, G. (2006): Orbital Forced Cyclicity in the Depositional Environment in the Cape Basin as Depicted in Seismic Data , American Geophysical Union, Fall Meeting .
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Abstract:

Cyclicity in depositional sequences is shown in numerous studies and related to changes in sedimentary composition due to orbital controlled climate variations. Based on an integrated study of borehole and seismic data we present a seismostratigraphic model for the Cape Basin, and quest for the impact of climate changes to the seismic reflection pattern.A well-pronounced succession of continuous high-amplitude reflectors characterises the upper acoustic units off Southwest Africa as revealed in the records. Spectral analyses over age-transformed seismic traces were performed. For the northern and middle Cape Basin the results suggest a temporal regularity in the variation of reflector strength according to Milankovitch cycles since the Early Pliocene. Periods of 100 kyr/cycle between 0-2 Ma correspond to eccentricity modulations. From 2-3 Ma shorter periods close to obliquity cycles prevail, whereas in even older units periodicity increases again to 100 kyr/cycle. As an origin for the cyclicity of high-amplitude reflectors we propose orbital driven climate variations, which modulate the sedimentary properties and with it the seismic reflectivity. For the southern Cape Basin however, we cannot observe such a distinct cyclicity of reflectors. In this region a much lower sedimentation rate probably prevents the resolution of a fine scale reflector sequence. Additionally, the deposition sequence seemed to be disturbed by mass movements as indicated in numerous slump scarps recorded on the seismic lines.

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