贺兰奥拉槽早古生代深水重力流体系的沉积特征和充填样式

SEDIMENTARY CHARACTERS OF THE EARLY PALEOZOIC DEEPWATER GRAVITY FLOW SYSTEMS AND BASIN FILLING STYLEIN THE HELAN AULACOGEN, NORTHWEST CHINA

位于鄂尔多斯西缘的贺兰构造带为一中元古代一古生代的奥拉槽。在区内的中寒武和中奥陶统中识别了一套巨厚的深水重力流沉积,其中包括下斜坡滑塌泥石流复合体、浊积扇以及碳酸盐岩斜坡扇裙等沉积类型。主要的相单元包括充填沟道或进入扇面形成的泥石流钙质角砾岩和砾岩、充填辫状水道的多层叠置的砂岩和砂砾岩、上叠扇的砂、泥岩互层以及浊积砂屑或含砾砂屑灰岩等。在中奥陶世该奥拉槽发展成一深水一半深水海槽,沿盆地西侧发育有浊积扇,而东侧仅有碳酸盐岩滑塌扇裙。它们可能是沿深水盆地两侧深大断裂产生的陡坡或水下断崖分布的,代表了早古生代贺兰奥拉槽在强烈沉陷期特定的深水盆地充填。

Helan Mountain structural belt, located in western margin of Ordos basin, is a long lived and deeply-subsiding aulacogen extending from Qilian geosyncline far into Huabei platform. The Middle Cambrian and Early ?Middle Ordovician coarse clastic rocks, more than 3000m thick, have been firstly recognized as deep water gravity flow deposits in the aulacogen. The study has shown that the gravity flow deposits mainly consist of lower slope slump debris flow complex, submarine fans and carbonate slope aprons based on detailed examination of the outcrops. The Middle Ordovician submarine fan system consists predominantly of ( 1 ) lenticular channel-fill and sheet-like calcareous breccias of debris flow deposits formed on inner fan ; (2) lenticular massive sandstones and conglomerates formed as braided channel fill on mid-fan; (3) interbedded turbidite sandstones deposited on suprafan, showing Bouma sequences with graded beddings, flutesand tool marks. They are associated with mudstones containing graptolites. These facies elements generally comprise two types of megasequences coarsening-upward and fining-upward, reflecting respectively the progradation and retreation of the fan systems. The lower slope apron deposits are mainly composed of calcareous breccias and grainstones. Three facies elements have been identified: (A) massive breccia of proximal debris flow deposits; (B) gradded conglomerates locally showing imbrication structures formed from dilute debris (fluided flow) and (C) pebbly grainstone developing large scale flutes, gradded bedding, parallel laminations and cross bedding. They are associated with thinly interbedded finer turbidite grainstones, mudstones and micrites with graptolites. The lithofacies mapping shows that the Ordovician gravity flow deposits in the basin developed along both the western and the eastern marginaJ slopes bounded by north to south trend major faults of the basin, with submarine fan containing siliciclastic turbidite formed along the western side of the aulacogen and only calcareous apron deposits along the eastern one that was adjacent to a broad shallow marine carbonate platform. These deposits represent the distinct facies associations of deep water basin fill formed in intensively-subsiding evolutionary stage of the aulacogen and it is evident that the Helan aulacogen developed into a deep water trough during Ordovician period.