AUTOCYCLIC PERTURBATIONS OF ORBITALLY FORCED SIGNALS IN THE SEDIMENTARY RECORD

Small-scale cyclicity in stratigraphic sequences (1 mm-10 m) is often attributed to regular orbital signals. Commonly, however, these signal s are overprinted by other signals. A dynamic forward model for elasti c sedimentation in foreland basins suggests that this overprint can be caused by slope instability, stress variations in the lithosphere, an d autocyclicity induced by climate change. Spectral analyses of predic ted sedimentation rates with various external controls demonstrate the occurrence of autocyclic fluctuations in sediment flux on time scales smaller than the time scale of the variation of the original, trigger ing, external control. This feature probably represents the response o f the sedimentary system to external variations, depending on the eige nfrequencies of this system. Because the eigenfrequencies are determin ed by the basin geometry, changing through time, a chaotic frequency d istribution of sedimentation rates is expected. This hypothesis explai ns the commonly observed absence of a significant number of frequencie s of the orbital spectrum in a large number of sediment sequences.