层序地层框架下的鲕粒滩形成样式:以北京西郊下苇甸剖面寒武系第三统为例

Forming patterns for the oolitic bank within the sequence-stratigraphic framework: An example from the Cambrian Series 3 at the Xiaweidian section in the Western Suburb of Beijing

华北地台寒武系第三统,大致包括毛庄组、徐庄组、张夏组和崮山组,在其中上部大面积发育鲕粒滩相灰岩,从而不符合标准的威尔逊碳酸盐相带中"台地边缘颗粒滩相"的概念。在北京西郊下苇甸剖面,基于沉积相序列的层序地层划分表明,这些鲕粒滩相地层,常常构成层序的晚期高水位体系域或强迫型海退体系域,表现出层序地层框架下特别的鲕粒滩形成样式,成为相对快速沉降背景下的相对海平面缓慢下降的沉积响应,从而不符合标准的层序地层学模式,因为在标准的层序地层学模式中沉积作用主要发生在相对海平面上升和停滞阶段,而且在海平面下降期将形成一个连续的侵蚀作用不整合面。再者,与海侵作用期的陆棚相钙质泥岩和深缓坡相条带状泥晶灰岩、及其所代表的泥质碳酸盐沉积物主导的浅海陆架型台地相对应,强迫型海退过程之中发育的大面积分布的鲕粒滩,代表着寒武纪类型多样的碳酸盐台地类型中的一种新的碳酸盐台地类型,即鲕粒砂所主导的浅海碳酸盐陆架为特征的一种特别的碳酸盐台地,即浅海陆架被颗粒质的鲕粒沉积物大面积覆盖,向海一侧为缓坡、向陆一侧为潮坪。所以说,北京西郊下苇甸剖面的寒武系第三统,鲕粒滩的层序地层位置所代表的特别的形成样式,以及所蕴含的寒武纪碳酸盐台地的类型,将为更大区域的追索和对比、以及华北地台第三世的古地理重建提供重要的线索和思考途径。

The Cambrian Series 3 in North-China Platform is approximately made up of the Maozhuang, Xuzhuang, Zhangxia and Gushang formations, and the mid- and upper parts of the Series 3 are marked by the large-area development of grainstones of the oolitic-grain bank facies that does not conform to the concept of ＂the platform-margin grain bank＂ in the standard Wilson model of carbonate facies belts. The sequence-stratigraphic division that is on the basis of the sedimentary-facies successions demonstrates that these strata belonging to the oolitic-grain bank facies generally constitute the late-stage high-stand system tract of the forced-regressive system tract of the third-order depositional sequence and express a particular forming pattern that is responded to the relative sea-level fall; further, this forming pattern of the oolitic-grain bank facies does not fit to the standard model of sequence stratigraphy, in which deposition occurs principally during rise and stillstands of relative sea level and a continuous erosional unconformity develops during sea-level fall. Moreover, in contrast with mudstones and banded micrites belonging to the shelf facies that are formed during the transgressive stage, the oolitic-grain bank with the large-area development formed in the forced-regressive process might represent a new type of diversified Cambrian carbonate platforms, i.e. a carbonate platform that is characteristic of following features： the shallow shelf is mantled by grainy oolitic sediment, flanked seaward by a ramp and landward by tidal flats. Therefore, a particular forming pattern of oolitic-grain banks delegated by the sequence-stratigraphic position within the Cambrian Series 3 at the Xiaweidian section in the Western Suburb of Beijing might implicate a new type of diversified Cambrian carbonate platforms, which will provide an important clue and a thinking approach for the regional tracing and correction, as well as the palaeogeographical reconstruction for the Cambrian Series 3 in the North-China Platform.