Geometry and Evolution of the Palisades Reef Complex, Silurian of Iowa
Geometry and Evolution of the Palisades Reef Complex, Silurian of Iowa
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爱荷华州志留纪栅栏礁群的几何形状和演化
DOI：
10.1306/74d71f0f-2b21-11d7-8648000102c1865d
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发表时间：
1970
影响因子：
2
通讯作者：
M. Philcox
中科院分区：
文献类型：
--
作者：
M. Philcox
摘要 早期将高尔组细分为勒克莱尔（Le Claire）（礁）和阿纳莫萨（Anamosa）（礁间）是不充分且具有误导性的，因为岩性多种多样，需要更详细和精确的术语；而且因为阿纳莫萨礁的很大一部分比勒克莱尔礁晚。 Palisades 地区为了解爱荷华州其他地方的相关系提供了初步模型，但并未代表所有 Gower 岩性。栅栏礁复合体是不对称的，北侧和西侧有弧形边界，沉积物颗粒尺寸和总厚度向东南方向普遍减小。这可能反映了来自西北的盛行风。复合体的第一生长阶段以海百合-腔肠动物相为代表，由大量海百合丘组成，这些丘在边缘（迎风）区分化最为强烈；海百合主区较大，但较简单；在背风区较小或不存在。边缘区土丘本身是不对称的，仅在其迎风面有一层晚期腔肠动物床毯。这些土丘并不是 Lowenstam（1950）意义上的抗波礁石。第二个生长阶段在边缘区以“Brady 相”为代表。主要由层状致密白云石组成，但也包括厚厚的棘球岩层。这些相在土丘侧面形成大型楔形床，据信源自早期海百合土丘顶部的平台。 Brady 相在岩性和几何上分级为几乎完全由层状白云石 (Anamosa) 组成的层。在背风区，第二生长阶段仅以阿纳莫萨白云岩为代表。礁外岩性已暴露，但难以关联。准同生洞穴沉积物表明，该复合体在其开发第一阶段结束时就暴露于地下。
ABSTRACT Earlier sub-division of the Gower Formation into Le Claire (reef) and Anamosa (inter-reef) is inadequate and misleading, because the wide variety of lithologies demands a more elaborate and precise terminology; and because a large proportion of the Anamosa is later than the Le Claire reefs. The Palisades area provides an initial model for understanding facies relations elsewhere in Iowa, but not all Gower lithologies are represented. The Palisades Reef Complex is asymmetrical, having an arcuate boundary on the north and west sides, and a general decrease in sediment grain-size and total thickness towards the south-east. This presumably reflects a prevailing wind from the north-west. The first growth phase of the complex, represented by the crinoid-coelenterate facies, consists of a large number of crinoid mounds, which are most highly differentiated in the Marginal (windward) Zone; large, but simpler in the Main Crinoid Zone; and small or absent in the Leeward Zone. The Marginal Zone mounds are themselves asymmetrical, having a late-stage blanket of Coelenterate Beds on their windward sides only. The mounds are not wave-resistant reefs in Lowenstam's (1950) sense. The second growth phase is represented in the Marginal Zone by the "Brady facies." This consists mainly of laminated dense dolomite, but includes thick beds of rhynchonellids. The facies forms large-scale wedge-beds on the mound flanks, and is believed to have been derived from a platform on top of the earlier crinoid mounds. The Brady facies grades lithologically and geometrically into beds consisting almost entirely of laminated dolomite (Anamosa). In the Leeward Zone the second growth phase is represented solely by Anamosa dolomites. Off-reef lithologies are exposed, but difficult to correlate. Pene-contemporaneous cave deposits suggest sub-aerial exposure of the complex at about the end of the first phase of its development.