Oncoids as environmental indicators in the Much Wenlock Limestone Formation of the English Midlands
Oncoids as environmental indicators in the Much Wenlock Limestone Formation of the English Midlands
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DOI：
10.1144/gsjgs.145.1.0117
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发表时间：
1988-01
影响因子：
2.7
通讯作者：
K. Ratcliffe
中科院分区：
文献类型：
--
作者：
K. Ratcliffe
西米德兰兹山脉的大部分文洛克石灰岩地层沉积在陆架中部，可分为三个段；上部和下部采石场石灰岩段被更泥质的球状石灰岩段隔开。球藻主要由含有Rothpletzella和Girvanella的泥晶组构组成，常见于下采石灰岩段。这些结节从直径达5毫米的近球形小体到具有高度不规则和分支的上表面的形状不等，直径可达70毫米。每一种形态代表着不同的沉积环境，这也反映在包裹着钉螺的沉积物中。球状包石中等尺寸的丝状物是通过连续滚动形成的，而包裹在松散堆积的怪石中的较大的分枝状则在波基以下较安静的条件下形成。下采石灰岩段的螺状形态分布表明，相对海平面的微小变化叠加在整个文洛克中晚期海退幕上，在此期间沉积了大量文洛克石灰岩建造。整个西米德兰兹郡地层的一致性表明，海底基本上是平面的，覆盖了大片区域。可以在整个区域内追踪下采石场石灰岩段内螺状物形态的垂直变化，从而能够准确地对比相对海平面的变化。
The Much Wenlock Limestone Formation of the West Midlands was deposited in a mid-shelf setting and is divisible into three members; the Upper and Lower Quarried Limestone members being separated by the more argillaceous Nodular Limestone Member. Oncoids, composed predominantly of micritic fabrics with Rothpletzella and Girvanella, occur commonly in the Lower Quarried Limestone Member. These oncoids vary from subspherical bodies up to 5 mm in diameter to forms with a highly irregular and branched upper surface which reach 70 mm across. Each form is indicative of a different depositional environment, which is also reflected in the sediment enclosing the oncoid. Equidimensional oncoids in peloidal packstones were formed by continuous rolling, whereas the larger, branched forms enclosed in loosely packed wackestones developed in quieter conditions below wave base. The distribution of oncoid morphotypes in the Lower Quarried Limestone Member shows that small variations in relative sea level were superimposed on the overall middle to late Wenlock regressive episode during which the Much Wenlock Limestone Formation was deposited. The uniformity of the formation throughout the West Midlands indicates that the sea floor was essentially planar over a large area. Vertical variation of oncoid morphology within the Lower Quarried Limestone Member can be traced throughout the area, allowing accurate correlation of relative sea-level variations.