Carbonate sedimentation in a starved pull-apart basin,Middle to Late Devonian,southern Guilin,South China
【摘要】：正Geological mapping and sedimentological investigations in the Guilin region,South China, have revealed a spindle- to rhomb-shaped basin filled with Devonian shallow- to deepwater carbonates.This Yangshuo Basin is interpreted as a pull-apart basin created through secondary,synthetic strike-slip faulting induced by major NNE-SSW-trending,sinistral strikeslip fault zones.These fault zones were initially reactivated along intracontinental basement faults in the course of northward migration of the South China continent.The nearly N-S-trending margins of the Yangshuo Basin,approximately coinciding with the strike of regional fault zones,were related to the master strike-slip faults;the NW-SE-trending margins were related to parallel,oblique-slip extensional faults.Nine depositional sequences recognized in Givetian through Frasnian strata can be grouped into three sequence sets(Sequences 1-2,3-5 and 6-9),reflecting three major phases of basin evolution. During basin nucleation,most basin margins were dominated by stromatoporoid biostromes and bioherms,upon a low-gradient shelf.Only at the steep,fault-controlled,eastern margin were thick stromatoporoid reefs developed.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 centre led to sediment starvation,reduced current circulation and increased environmental stress,leading to the extensive development of microbial buildups on platform margins and laminites in the basin centre.Stromatoporoid 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 moderate progradation in leeward positions.A relay ramp was present in the north-west corner between the northern oblique fault zone and the proximal part 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 offset of the strike-slip faults was accompanied by the rapid uplift of surrounding carbonate platforms,causing considerable platform-margin collapse,slope erosion,basin deepening and the demise of the microbialites.Afterwards,stromatoporoid reefs were only locally restored on topographic highs along the windward margin.However,a subsequent,more intense basin subsidence in the early Frasnian(top of Sequence 6),which was accompanied by a further sharp uplift of platforms,caused more profound slope erosion and platform backstepping.Poor circulation and oxygen-depleted waters in the now much deeper basin centre led to the deposition of chert,with silica supplied 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 crosional types with gullies through which granular platform sediments were transported by gravity Hows to the basin.This situation persisted into the latest Frasnian. This case history shows that the carbonate platform architecture and evolution in a pull-apart basin were not only strongly controlled by the tectonic activity,but also influenced by the oceanographic setting(i.e.windward vs.leeward) and environmental factors.