黔南地区早、中泥盆世沉积演化的动力机制

Dynamics of Sedimentary Evolution for the Early and Middle Devonian, Southern Guizhou

泥盆纪时，黔南地区为一相对稳定的台地，早泥盆世晚期，海水开始漫漫其上．初始发育陆源碎屑沉积体系，中泥盆世发育陆源碎屑～碳酸盐混合体系．空间配置有下列几种类型：滨岸障壁～泻湖～河流体系，碳酸盐缓坡～滨岸障壁～泻湖体系，镶边型碳酸盐台地～泻湖三角洲（潮坪）体系，碳酸盐缓坡～三角洲体系。基底断裂限定了台地和台间沟的延限范围和演化进程，这两种不同沉积背景的沉积演化旋回可能主要受海平面变化控制。

Southern Guizhou region, a relative stable platform adjoined to an intrashelf trough (or basin) southward, was flooded by the rapid sea level rising in South China in the late Early Devonian (Middle Emsian).Terrigenous depositional systems (i. e. fluvial, barrier-lagoon systems) were developed in the late Early Devonian, and hybrid terrigenous/carbonate (rimmed platform or ramp) depositional systems with grand mixed cycles (or sequences) in the Middle Devonian. Spatial configurations(or distribution ) of depositional systems are formed in different sedimentary conditions (i. e. eustusy, tectonism,source supplies), they consist of (1) barrier-lagoon-fluvial complex systems, reflecting an early basin fill stage with abundant source supplies at a relative low sea level, (2) mixed carbonate ramp-siliciclastic barrier-lagoon complex systems formed after rapid sea level rising, (3) mixed rimmed carbonate platform-siliciclastic delta (or tidal) complex systems developed often in late sea level lowstands with a relative rapid subsidence rate at platform margin, and (4) mixed carbonate ramp-siliciclastic delta systems established after rapid sea level rising as (2). In southern Guizhou, the NW-SE extensional faulting zone determines the range of two absolutely different dePOSitional patterns of platform and intraplatform (or intrashelf) trough (or basin), and establishes the peleogeographic framework of sedimentary evolution.Six third-order depositional sequences including two terrigenous siliciclastic sequences in the late Early Devonian and four mixed cabonate-siliciclastic sequences in the Middle Devonian were recognized based on the sedimentary studies. The formation of the sequences is controlled largely by sea level changes.The terrigenous depeitional systems develop in the early basin-fill stage with abundant source supplies. The shoreline and fluvial besinward Ptogradation characterizes the lowstand of sea level. Withdrawal of the sea level below the depostiional break initially leads to the shelf exposure, fluvial incision, and formation of lowstand fans and wedges (progradational shoreline or delta deposits).The rapid sea level rising causes the shoreline to recede landwards swiftly and the semi-pelagic black shale to blanket the trailing edge of the shelf. During the highstand of sea level, the shoreline deposits that aggradationally stack in the early stage gradually change into the progradational coastal or deltaic plain deposits subsequently as the sea level begins to fall.In mixed carbonate-siliciclastic depositional systems, the sedimentary response to changes in relative sea level indicates that basinward transporting of elastics predominates the lowstand of sea level. The elastic materials are conveyed through the incised valley to the intrashelf trough (or basin), forming the lowstand fan and lowstand wedge. Sometimes, organic reefs are developed during the late lowstand (V, VI), and terminated by the rapid sea level rising. The rapid sea level rising leads the shoreline to retrograde quickly and causes intrashelf trough deposits (black shale,nodular limotone etc.) to onlap updip the depositional break. Continual sea level rising above the shelf (or platform) increases the accommodation space, which allows the resumption of carbonate production, hence, the carbonate deposits dominate over the early highstands. However, the siliciclastic systems on the proximal shelf begin to rejuvenate and to prograde basinwards during the late highstands when the accommodation space reduces.