上扬子克拉通南缘中泥盆统--石炭系高频层序及复合海平面变化

上扬子克拉通南缘中泥盆统-石炭系地层高频层序可以划分为三个级别,并与层序地层学术语可以对比:六级层序-韵律层;五级层序-准层序;四级层序-准层序组。不同级别高频层序的形成受米兰柯维奇驱动力引起的具不同周期和频率的复合海平面变化控制。克拉通边缘沉积环境中,低频海平面变化的上升阶段形成以淹没节拍为主的高频层序,而在停滞至下降阶段形成以暴露节拍为主的高频层序。所以,通过对不同特征的高频层序及垂向叠加的分析,可以求解低频海平面的变化。

The Middle Devonian to the Carboniferous High-frequency Sequences and Composite Sea-Level Changes on the Southern Margin of the Upper Yangtze Craton Basin

Sedimentary suite, which deposited from littoral to neritic environments and wer e mainly composed of carbonate rocks, developed on the southern margin of the up per Yangtze craton basin during the middle Devonian Epoch to the Carboniferous P eriod. This set of suite, which was the products of transgression and regression processes of plaeo marine water with the condition of stable tectonics, recorde d the changing processes of plaeo sea-level precisely. The study of sea-level change is helpful to the division of sequence、the research of sequence stacking patterns and the foundation of sequence time-space pattern through the analysi s of high-frequency sequences. The middle Devonian series to Carboniferous system strata high-frequency sequ ences in the study area may be distinguished from three orders corresponding to the terminology of sequence stratigraphy. Sixth-order sequence is corrsponding to the cyclothems, the fifth-order sequence to the parasequence and the fourth -order sequence to the parasequence set. Composite sea-level changes with th e different frequences and cycles which are caused by the Milankovitch cycles co ntrol the formation of high-frequency sequences of the different orders. Sixt h-order sequence is equal to the meter-scale cyclic sequence. Its genetic mech anism is punctuated-aggradational allocyclic mechanism which is related to high -frequence cyclic sea-level changes resulted from the Milankovitch cycle. Th e criteria for recognition of sixth-order sequence include the rock types,bou ndary surface nature and sedimentary environments. In the study area there are 1 116 sixth-order sequences, belonging to three types including carbonate rock type,clastic rock type and hybrid sediment type. All these sixth-order sequen ces are represented by progradational sequence consisting of an upward thickenin g, shallowing and coarsening rocks, and bounded by puncturated-deeping bounda ry or eroding surfaces. The upward stacks of two or more sixth-order sequence s form fifth-order sequence, whose bottom or top boundary surfaces are more cl ear eroding surfaces or abrupt lithofacies change surfaces, and whose number is 434 in the study area. The upward stacks of four or less fifth-order sequences inform fourth-order sequence, whose number is 134 in the study area. The development of high-frequency sequences are controled by the changes of sea-level,provenance,environment,weather,tectonic subsidence. Sixth-order, fifth-order and fourth-order sequences are the products of high-frequency and short cycle sea-level changes control ed by low-frequency and long cycle se a-level changes. So,through the analysis of the high-frequency sequences with the different characteristics and cycle stacking patterns, the low-frequency and long cycle sea-level changes may be explained. In the craton basin margin environment, during the rising stages of low-frequency sea-level changes, th e high frequency sequences are characterized by the "drowning band", while by th e "exposing band" in the standstill and fall stages.