河流沉积占优势地层中高频层序地层——以贵州盘县西部龙潭组为例

贵州西部龙潭组主要含有6种沉积相组合:即浅海沉积、细粒滨岸平原沉积、溢岸沉积、小型河道砂体、叠置河道砂体和煤层。多层叠置砂体一般10-25m厚,2-10km宽,常含海绿石,切入下伏的三角洲平原、滨岸平原和浅海沉积中,被认为是下切谷充填。在龙潭组中共识别出广泛发育的10个层序界面,由此所限定的层序大致相当于4级旋回层序。在这些层序中,准层序或准层序组识别不出,然能识别体系域,层序几乎全由海进体系域(TST)和高位体系域(HST)组成,低位体系域(LST)发育不好。在垂向上,它们又可叠置成3级复合层序,并由低位、海进和高位层序组组成。在低位层序组中,河道下切常冲刷掉下伏层序的全部HST和部分TST,致使其与下伏层序的下切谷充填重合。在海进层序组中,下切作用最弱,具最小砂/泥比值,下切谷充填侧向孤立。高位层序组是低位和海进层序的过渡类型,下切谷充填也趋于孤立。

HIGH-FREQUENCY SEQUENCE STRATIGRAPHY IN FLUVIAL-DOMINATED STRATA: A CASE FROM LONGTAN FORMATIONOF UPPER PERMIAN,PANXIAN, WESTERN GUIZHOU

The Longtan Formation of Upper Permian in the westem Guizhou, China, contains six major facies associations. These facies associations are shallow marine shales, fine-grained coastal plain deposits, overbook deposits, distibutary channel sandbodies, muld-storey staked sandbodies and coals. The muld-storey stacked sandbodies are gcnerally 10-25m thick and 2-10km wide, and contdn authigcnic glauconites. They are commonly incised into the underlying delta plain and coastal plain deposits, and occasionally into the shallow maine deposits. These sandbody complexes are interpreted as incised valley fills, their bases are thus. regarded as sequence boundary unconfomides. It is relatively difficult but possible to identify interfluvial sequence boundades through careful examination of the paleosol development. The interfluvial sequence boundaries are commonly characterized by carbonaceous shales, underclay, roodets (in situ tree stumps locally), slickensides and/or slight bleaching mottles, in which oolitic and pisolitic siderites occur. In the Longtan Formation, ten regional sequence boundaries are identified and the sequences bounded by these boundary unconformities therefore correspond to the 4th-order sequences. Parasequences and parasequence sets are not identifiable in the sequences. Nevertheless, it is possible to identify system tracts. Sequences almost consist of transgressive and highstand system tracts by least developed lowstand deposits. They are vertically stacked into transgressive, highstand and lowstand sequences sets, which constitute the third-order composite sequences. In the lowstand sequence set, channel incision commonly removed almost all the HST and partial TST of the underlying sequences, leading the incised valley fills to amalgmate underlying ones. Within transgressive sequence set. incision was at a minimum and inciscd valley fills tended to armnge discretely with minimum sand: shalc radios. The highstand sequence set was transitional between the lowstand and transgressive sequence set. Incised valley fills also tended to stack discretely.