Fault-block rotation controlling the distribution of fluvial sediments; a quantitative test on a Lower Pennsylvanian (Carboniferous) cyclothem succession

Abstract

Depositional models of axial fluvial systems in half-grabens predict that the fluvial-sandstone percentage increases towards the down-thrown side of a fault, because channel systems tend to migrate to the area of maximum subsidence. This is at the expense of mudstone, but floodplain deposition occurs near faults occasionally. The models assume gradual, transverse tilting and no external base-level change, and their applicability to cases involving tectonics and/or sea-level change may therefore be restricted. Here a quantitative analysis is presented on a subsurface dataset from an Upper Pennsylvanian cyclothem succession, which formed under conditions of differential subsidence and fluctuating sea level. The studied interval is wedge-shaped and shows a systematic thickness increase from 165 to 245 m, controlled by syndepositional fault-block tilting. It comprises three depositional units, bounded by coal groups. These units display an upward change from wedge-shaped (75-120 m) to tabular (42-55 m). Despite their variable thickness the units contain almost equal amounts of ~45 m of floodplain deposits, plus ~5 m of encased channel sandstones, in all boreholes. Where units are thicker, the remaining thickness comprises fluvial-braidplain sandstone. This arrangement indicates that the units represent equal time periods, during which background subsidence allowed the deposition of thin channel sands and overbank muds on a level floodplain. Occasional tilting produced additional accommodation space, which was completely filled by sand-dominated braided systems. The temporary cessation of floodplain-mud deposition suggests that aggradation of the river system could not keep up with floodplain tilting. In addition, bypass of floodplain fines may have been promoted by a basin parallel tilting component. It is shown that (i) cases in which the standard models fully apply, and (ii) cases in which differential subsidence is too strong or too abrupt, can be distinguished by analysing cross-plots of cumulative-sandstone and cumulative mudstone thickness.