The Characteristics and Origin of Small-scale Sequences in Majiagou Formation(Middle Ordovician)in the L liang-Norhern Shaanxi Area

Shallowing-upward sequences in the Majiagou carbonates in the Luuliang-Northen Shaanxi area consist of a great number of small-scale sequences. The small-scale sequences (1.2-5min thickness) are the fundamental units of the Majiagou sedimentary deposit. The small-scale sequences have obvious boundaries of abrupt change tO deeper facies, and are of internally regressive deposits,containing little or no transgressive deposits. The small-scale sequences are vertically stucked and laterally extensive asymmetry cycles for tens of kilometers (independent of lateral facies). These sequences appear to occur in the depositional facies of most environments.Six types of small-scale sequences Can be recognized, i. e., A: trace-fossil cycles in deeper open sea subtidal facies; B:dilution cycles of hypersaline brine in deeper lagoon factes; C:intertidel to supratidal overprinting of shallow subtidal factes:D: intertidal to supratldal ovetprining of shallow brackish subtidal factes, ofen associated with early dolomitization; E: Sabkha sequences; F:terrestrial overprinting of intertidal factes. The characteristics and distribution of small- scale sequences were formed during high-frequency but low-amplitude sea level variations superimposedon the third order transgressive - cycles. The types of the small-scale sequences depend on the position of high- frequency sea level variations within the third order cycles and the palaeotopography of sedimentary environments. During the high sea level of long-term cycles periodically changed the physical-chemical conditions of water body in deeper open sea subtidal racies. Thus small-scale sequences recorded the benthic and in fauna response to trasgressiveregressive cycles. In shallow peritidal environments during the longer-term sea level fall, highfrequency sea level variations made the ralative sea level reach the critical water depth. Consequently, C. D. E. F small-scale sequences were formed. In local depressions, the highfrequency sea level rise provided the punctuated supplement of sea water for the evaporation and caused the dilution cycles of hypersaline brine which formed B small- scale sequences. Highfrequency small-scale sea level variations seem to be related to Milankovitch climatic cycles caused by the Earth's orbital cycles. identification and correlation of sequence boundaries make it possible to set up a framework of isochronous surfaces, and thus to interpret in derail the palaeographic,sedimentological evolution of the Majiagou carbonate environments and to predict the distribution of oil-gas reservoirs in the sedimentary basin.