Expression and modelling of stratigraphic sequence distortion

Abstract

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 (“spatial distortion” D), or (ii) superposition of two frequencies of stratigraphic cyclicity (“cycle superposition distortion” D′). 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. (ii) The sedimentation rate can modulate the amount of distortion related to subsidence by amplifying, limiting, compensating or even inverting the temporal offset. (iii) Spatial variations in sedimentation rate alone may also induce changes in the timing of trend inversion: the onset of progradation may take place sooner and the onset of retrogradation may be delayed in the area showing the fastest sedimentation rate. (iv) The amount of distortion depends not only on the sedimentation and subsidence rates, but also on the maximum rate of sea-level change causing the depositional sequence, that is, it depends on the period and amplitude of the sequence. The faster the sea-level change (i.e. the higher the frequency and the larger the amplitude), the weaker is the distortion produced. (v) In terms of resulting time lag, numerical analysis shows that the distortion of high frequency depositional sequences (genetic units) is negligible. These sequences can therefore be safely used to correlate time lines across the studied area, whereas use of lower frequency sequences with significant spatial distortion could lead to significant errors in correlation. We use these results to interpret the distortion observed in the case study, in terms of temporal and spatial variations of subsidence and sedimentation rates. In this case, complex temporal and spatial variations in subsidence and sedimentation rates lead to variations of the distortion of stacks of genetic unit. This distortion produced difference in timing of the onset and duration of the inversion of trend within prograding and retrograding half-cycles.