This study describes the lateral variability of stratigraphic sequences under changing conditions of subsidence and/or sediment supply. Changes in sediment supply, subsidence and absolute sea-level result in at least three modes of distortion of a depositional sequence. Our study of a case history from offshore West Africa, together with numerical analysis, provides insights as to how changes in subsidence and sedimentation may be extracted from the stratigraphic record.
We analysed a listric fault/raft system, located on the Congo-Cabonda margin, resulting from gravity-induced extension of a mixed siliciclastic/carbonate platform sequence. Within this system, quantitative analysis was carried out on high-resolution stratigraphic sequences of five wells, as well as on their geometry, on the relative duration of prograding compared to retrograding half-cycles, and on the timing of the inversion of these half-cycle trends.
Parameters were defined to quantify the distortion of depositional sequences that results from either (i)spatial variations in subsidence and sedimentation rates (bspatial distortionQ D), or (ii) superposition of two frequencies of stratigraphic cyclicity (bcycle superposition distortionQ DV). Using a numerical model, we investigate the distortion of depositional sequences showing two superposed scales of cyclicity, in response to spatial and temporal variations of subsidence and sedimentation rate.
We show that:
(i) Spatial variations in subsidence rate can result in modification of the timing of trend inversion: the onset of progradation may be delayed and the onset of retrogradation may occur earlier in the more rapidly subsiding areas.