Carbonate sedimentation in a starved pull-apart basin, Middle to Late Devonian, southern Guilin, South China

Geological mapping and sedimentological investigations in the Guilin region , South China, have revealed a spindle- to rhomb-shaped basin filled with D evonian shallow- to deepwater carbonates. This Yangshuo Basin is interprete d as a pull-apart basin created through secondary, synthetic strike-slip fa ulting induced by major NNE-SSW-trending, sinistral strike-slip fault zones . These fault zones were initially reactivated along intracontinental basem ent faults in the course of northward migration of the South China continen t. The nearly N-S-trending margins of the Yangshuo Basin, approximately coi nciding with the strike of regional fault zones, were related to the master strike-slip faults, the NW-SE-trending margins were related to parallel, o blique-slip extensional faults, Nine depositional sequences recognized in G ivetian through Frasnian strata can be grouped into three sequence sets (Se quences 1-2, 3-5 and 6-9), reflecting three major phases of basin evolution . During basin nucleation, most basin margins were dominated by stromatoporoi d biostromes and bioherms, upon a low-gradient shelf. Only at the steep, fa ult-controlled, eastern margin were thick stromatoporoid reefs del-eloped. The subsequent progressive offset and pull-apart of the master strike-slip faults during the late Givetian intensified the differential subsidence and produced a spindle-shaped basin. The accelerated subsidence of the basin c entre led to sediment starvation, reduced current circulation and increased environmental stress, leading to the extensive development of microbial bu ildups on platform margins and laminites in the basin centre. Stromatoporoi d reefs only survived along the windward, eastern margin for a short time. The architectures of the basin margins varied from aggradation (or slightly backstepping) in windward positions (eastern and northern margins) to mode rate progradation in leeward positions. A relay ramp was present in the nor th-west corner between the northern oblique fault zone and the proximal par t of the western master fault. In the latest Givetian (corresponding to the top of Sequence 5), a sudden subsidence of the basin induced by further of fset of the strike-slip faults was accompanied by the rapid uplift of surro unding carbonate platforms, causing considerable platform-margin collapse, slope erosion, basin deepening and the demise of the microbialites. Afterwa rds, stromatoporoid reefs were only locally restored on topographic highs a long the windward margin. However, a subsequent, more intense basin subside nce in the early Frasnian (top of Sequence 6), which was accompanied by a f urther sharp uplift of platforms, caused more profound slope erosion and pl atform backstepping. Poor circulation and oxygen-depleted waters in the now much deeper basin centre led to the deposition of chert, with silica suppl ied by hydrothermal fluids through deep-seated faults. Two 'subdeeps' were diagonally arranged in the distal parts of the master faults, and the relay ramp was destroyed. At this time, all basin margins except the western one evolved into erosional types with gullies through which granular platform sediments were transported by gravity flows to the basin. This situation pe rsisted into the latest Frasnian. This case history shows that the carbonate platform architecture and evolut ion in a pull-apart basin were not only strongly controlled bq the tectonic activity, but also influenced by the oceanographic setting (i.e. windward vs. leeward) and environmental factors.