Expression and Modeling of Stratigraphic Sequence Distortion

Robin, Cecile¹, Delphine Rouby¹, Didier Granjeon², François Guillocheau³, Pascal Allemand⁴ (1) University of Rennes, Rennes, France (2) Institut Français du Pétrole, Rueil Malmaison, France (3) Géosciences Rennes, Université de Rennes-1, Rennes, France (4) University of Lyon, Villeurbanne, France

We study the lateral variability of stratigraphic sequences under changing conditions of subsidence and/or sediment supply. We carried out a quantitative analysis on high-resolu-tion stratigraphic sequences of a listric fault/raft system. We defined parameters quantifying the distortion of depositional sequences resulting from either (i) spatial variations in subsi­dence and sedimentation rates (“spatial distortion” D), or (ii) superposition of two frequen­cies of stratigraphic cyclicity (“embedding 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. (i) Spatial variations in subsidence rate modify the timing of trend inversion. (ii) Sedimentation rate modulates the amount of distortion related to subsidence. (iii) Spatial variations in sedimentation rate alone also modify the timing of trend inversion. (iv) The dis­tortion amount also depends on the period and amplitude of the sequence (v) The amount of distortion of short wavelength depositional sequences (genetic units) is negligible. These sequences can therefore be safely used to correlate time lines across the studied area, whereas use of the longer ones with significant spatial distortion could lead to significant errors in correlation. We use these results to interpret distortions observed in the case study: complex temporal and spatial variations in subsidence and sedimentation rates lead to variations of the distortion within stacks of genetic units. This distortion produces differ­ence in the timing of the onset and duration of the inversion of trend within prograding and retrograding half cycles.