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Fluvial Architectures in Active Rift Settings

Abstract

The geometry and sedimentary architectures of fluvial systems developing in active rift-basin settings are directly influenced by the ongoing subsidence, which affects fluvial channel-belt distributions, sizes and shapes, mode of belt migration, and mechanisms of accumulation. Such behaviors give rise to predictable variations in the stacking of sediment geobodies preserved in syn-rift continental deposits that store hydrocarbons.

This research analyzes variability in geometries of fluvial meanders and channel belts with respect to proximity to active faults that have given rise to a variety of types of basin extension and subsidence. Satellite imagery analysis of 10 major rivers contained in eight depocenters of the East African Rift System are compared with numerical data on depositional geometries from deposits in the rock record. Research objectives are: (i) to quantitatively detail the impact of active faulting on drainage catchment geometries, and on fluvial channel-belt and meander point-bar morphometric features; (ii) to use the collected meander- and channel belt-geometry data to predict zones of preferential fluvial reservoir development in rift settings; (iii) to compare collected data to geometric data of fluvial deposits from a variety of accommodation settings and link depositional processes to arrangement of fluvial sediment geobodies in the rock record for various rift-basin types.

Data were collected to characterize (i) the total area and position of each river’s drainage catchments relative to the active rift, (ii) width variations of the channel belt along river reaches, (iii) size and shape variations of point-bar sandbodies along river reaches. These data are directly comparable to literature- and fieldwork-derived datasets. All the information is considered within an analogue database to enable quantitative comparisons between fluvial sedimentary units in different structural regimes.

The following results were determined: (i) both point-bar size and channel-belt width increase downstream towards subsidence loci and display a non-linear relationship with respect to distance downstream, (ii) both point-bar size and channel-belt width of fluvial systems with axial drainage catchments are higher than transverse catchments, (iii) channel belt width and sinuosity decrease rapidly with proximity to intra-basin lakes. A comparatively straighter channel is formed on the basin floor, decreasing the frequency and size of sandbodies.