Fahar Al-Rabeei (fahar@fs1.ge.man.ac.uk), Dave Hunt (dhunt@fs1.ge.man.ac.uk), Stuart Hardy (shardy@fs1.ge.man.ac.uk) & Rob Gawthorpe (rgawthorpe@fs1.ge.man.ac.uk)
Department of Earth Sciences, University of Manchester, Oxford Rd., Manchester, M13 9PL, UK
The earliest deposits of syn-rift basin-fills are often characterized by mixed carbonate-clastic sedimentation (e.g. Holocene of the Red Sea Rift province; Miocene, Red Sea; Miocene, SE Spain; Carboniferous of Arctic Canada, Norway and Russia). In the modern and ancient it is clear that in the early stages of rift basin development carbonates thrive in areas where rates of clastic sediment supply are high and would normally be expected to inhibit their production. However, although a well-known facies association, the controls on the development and sustainment of such mixed depositional systems are poorly understood. This study uses the results of a recently developed 3D numerical model to examine the response of mixed carbonate-clastic depositional systems to variations in the amplitudes, rates, and wavelengths of sea-level changes,and in the rates of tectonic subsidence, clastic sediment supply, and fluvial incision.
The 3D numerical models of carbonate-clastic interaction presented simulate deposition within a 16x14 km grid with a cell size of 40 x 40 m (grid and cell size are variable and user-defined to suit the scale of study). Clastic sediments can be introduced at any location at user-defined rates and follow a random walk/steepest descent pathway from source to shoreline. The clastic input in the model runs is differentiated into a coarse bedload fraction and a mud/siltstone suspension fraction. The coarse bedload sediment is deposited immediately at the shoreline whereas the fine fraction is dispersed as a plume, the path of which can be skewed to represent the effect of currents. Subaerial sediment transportpaths (fluvial channels) incise to stay in grade, while slope failure is modelled using a non-linear (threshold) diffusion approach. Carbonate sedimentation is modelled using a simple depth-dependent production curve that is more sensitive to the rate of fine-grained suspended clastic deposition than that of the bedload. The numerical experiments examine the influence of single and multiple sources of clastic sedimentation on carbonate production when introduced into ramp and fault bounded half-graben settings with hinged differential tectonic subsidence. The rates of processes and the length-scale of facies transitions are derived from the literature and from the study of satellite and aerial photographs of Holocene systems in the Red Sea.
The model experiments provide important new insights that reveal why mixed carbonate-clastic systems tend to dominate the early stages of rifting, whereas in later stages of extensional basin development the carbonate factory appears significantly less tolerant of clastic input, restricting carbonate sedimentation to tectonic highs and areas lacking significant clastic input. The model results indicate that the key factors controlling the nature of the carbonate-clastic interaction include; i) rates of fluvial incision, ii) rates, amplitudes and wavelength of relative sea-level change, iii) coastal plain width, iv) rate of sediment supply vs. accommodation development. Contrary to intuition, and in contrast to previous studies, the model's results indicate that in the early phases of rift basin evolution times of relative sea-level fall and lowstand are associated with the most areally widespread carbonate deposition. This relationship is most pronounced during times of high-amplitude glacioeustatic sea-level changes and/or high rates of fluvial incision. However, this situation is reversed as either i) the basin fills and coastal plains widen, ii) during greenhouse intervals, or iii) when fluvial incision rates are low. These results help explain the dominance of abruptly mixed carbonate-clastic systems in earliest phases of rift basins in times characterized by a high-amplitude glacioeustatic signal (e.g. late Carboniferous, late Tertiary).
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