鄂尔多斯盆地南部奥陶纪末端变陡缓坡沉积模式

鄂尔多斯盆地中央隆起带以南地区,奥陶纪时南有秦岭深水海槽,北为中央隆起南延段,其间地势平缓平距可达100 km,为一末端变陡的碳酸盐缓坡。当处于高海平面时期,由于气候温暖潮湿,大气降水丰沛而且与广海连通性好,海水循环亦好。这时的整个缓坡环境多发育泥晶灰岩、生屑灰岩、砂屑灰岩、粉屑灰岩、球粒-团粒灰岩,间有生物礁、风暴岩等沉积。当处于低海平面和极低海平面时期,气候越来越炎热干旱,雨量减少,蒸发量大于降水量,海平面降低,缓坡与广海的连通受阻、海水循环受限,这时区内总体上以发育泥晶白云岩、粉晶白云岩为主,少量颗粒泥粉晶白云岩、泥粉晶颗粒白云岩、风暴岩等沉积,潮缘及浅缓坡的局部洼凼内因CaSO4甚至NaCl过饱和,也可发育含膏质白云岩以至盐岩。缓坡以南的斜坡区则发育重力流成因的砾屑灰岩和巨角砾灰岩以及浊积岩等沉积。

SEDIMENTARY MODEL OF DISTALLY STEEPENING RAMP OF ORDOVICIAN IN SOUTHERN ORDOS BASIN

Geologically, the southern area of the central doming of or-dos basin is called Weibei flexural fold. Its width is about 100km from south to north. The Qinling through was in the southern ordos basin. The area was s distally steepening carbon-ate ramp in Ordovician. In the time of high sea level, the cli-mate was moist and warm, and the seawater could communicate with open seawater. In this period, the main sediments were micite, some bioclast limestone, calcarenite, calcisilte lime-stone, reef et al. In the time of low sea level, climate was be-coming more and more scorching and dry. It was difficult for the communication of seawater on the ramp with open sea. In this period, the main sediments were mud-sized crystalline dolo-stone, silt-sized crystalline dolostone, some grain mud-silt-sized crystalline dolostone, mud-silt-sized crystalline grain dolosone,strom rock et al.Because of oversaturated CaSO4or NaCl in some hollows,there were some gypsiferous dolostone and salinerock deposited on the peritidal are and shallow ramp. On the slope area out the south-ramp, there were some calcirudite and turbidite deposited.