陆相伸展盆地强伸展期沉积格架与断块翘倾分析——以冀中坳陷廊固凹陷沙河街组三段中亚段为例

DEPOSITIONAL FRAMEWORK AND FAULT-BLOCK TILTING IN STRONG EXTENSIONAL STAGE OF CONTINENTAL BASIN:AN EXAMPLE FROM THE MIDDLE PART OF 3rd MEMBER, SHAHEJIE FORMATION IN THE LANGGU DEPRESSION, CENTRAL HEBEI BASIN

根据地质、地球物理等资料研究了冀中坳陷廊固凹陷强伸展期 (沙河街组三段中亚段 )的沉积格架与断块翘倾运动。作为一个翘倾断块 (半地堑 )沉积区 ,廊固凹陷沙三段中亚段沉积充填以湖相细碎屑岩石组合为主 ,可分 :1 )砾岩—泥岩互层型 ,其中砾岩有碳酸盐岩砾和硅质砾等两种亚类 ;2 )砂岩—泥岩互层型 ;3)泥岩夹砂岩型。古沉积环境重建显示沙三段中亚段沉积期的廊固凹陷是一个窄而深的湖泊 ,发育断层陡崖、轴部和缓坡带等沉积相域以及冲积扇、近岸水下扇、扇三角洲、湖泊和浊积扇等沉积体系 ,物源主要来自大兴凸起。断块翘倾几何分析表明 ,廊固凹陷在沙三段中亚段沉积期的沉降过程具有书斜式断块特征 ,最大沉降值至少为 1 40 2m。廊固凹陷西部扇的演化过程与沉降、沉积物供给和大兴断层倾角等因素有关。

The depositional framework and fault block tilting of the Langgu Depression, Central Hebei Basin, during strong extensional stage (sedimentary period of the middle part of 3rd Member, Shahejie Formation) was studied based on geological and geophysical data. The sediments filling the Langgu Depression, a half graben, consisted predominately of fine grained clastic rocks deposited in a lacustrine system. Three lithological associations were distinguished as follows: 1) conglomerate, deposited in an alluvial fan or submarine fan consisted of carbonatite or quartzite gravel, respectively, alternated with mudstone deposited in a deep offshore lacustrine environment; 2) sandstone interbedded mudstone; and 3) mudstone alternated with sandstone. The paleoenvironmental reconstruction indicated that the Langgu Depression was occupied by a narrow, deep lake during sedimentary period of the middle part of 3rd Member, Shahejie Formation. Three sedimentary facies suites were recognized: the hanging wall, the axial system and the fault scarp derived from alluvial fan, submarine fan, fan delta, lacustrine and turbidite. The sediments were originated mainly from the Daxing uplifting fault block. The geometric analysis of fault block showed the subsidence of the Langgu Depression characterized by domino shaped block faulting. During sedimentary period of the middle part of 3rd Member, Shahejie Formation, the maximal subsidence of the Langgu Depression was 1 402 m. Finally, evolution of fan bodies in the western part of Langgu Depression were possibly controlled by factors as precipitation rate, sediment supply and fault dip, etc.