Latest Proterozoic and Cambrian Rocks of the Western United States—an Overview
Latest Proterozoic and Cambrian Rocks of the Western United States—an Overview
复制标题
美国西部最新元古代和寒武纪岩石——概述
DOI：
--
复制
发表时间：
1991
期刊：
影响因子：
--
通讯作者：
J. H. Stewart
中科院分区：
文献类型：
--
作者：
J. H. Stewart
美国西部的新元古代和寒武纪岩石可分为三个垂直叠加的地层单元：（1）杂岩和火山岩序列（新元古代），（2）陆源碎屑序列（新元古代和下寒武纪），（3）碳酸盐和页岩序列（最上部的下寒武纪至上寒武纪）。混晶岩和火山岩序列在美国西部不连续地出露，由混晶岩、千枚岩或泥质岩、砂岩、砾岩、碳酸盐岩和拉斑玄武岩组成。根据华盛顿东北部的同位素测年，杂岩和火山岩序列的年龄约为770 Ma，但根据微体化石的不确定年代，其中一部分可能为650 Ma或更年轻。杂岩和火山岩序列的地层被解释为冰川和深水至浅水沉积的产物。杂屑岩和火山岩序列通常存在于局部深槽中。部分为同沉积伸展断裂作用形成。陆源碎屑序列由一个向西增厚的石英岩和砾岩楔形体组成，间层有粉砂岩、千枚岩、泥质岩和局部厚的碳酸盐岩单元。陆源碎屑序列是美国西部最古老的存款，清楚地显示了一个区域分布模式类似的下，中古生代地层的科迪勒拉migeocline。这一序列从东部的零边缘到西部的6,000多米处变厚。寒武系的基底就在这一序列中。陆源碎屑序列的地层沉积在各种环境中，包括辫状河系统，浅水潮汐为主的陆架系统，潮下陆架系统，潮下风暴为主的系统。最上层的下寒武统到上寒武统的碳酸盐和页岩序列覆盖了美国西部的大部分三角形台地和浅地槽地区，由碳酸盐岩和不同数量的页岩、石英岩或砂岩组成。页岩通常与碳酸盐岩互层，但局部形成厚层单元，除页岩外几乎不含页岩。在犹他州西部，碳酸盐岩和页岩序列厚达2，600 m，在犹他州西北部，可能厚达5，100 m。它主要由浅水潮下带和潮缘沉积物组成，但也包括堆积在冒地斜（House Range海湾）内盆地的深水页岩和碳酸盐地层，以及西部露头区的深水页岩和碳酸盐地层，这些地层被解释为大陆斜坡和斜坡底部碳酸盐海底扇沉积物。构造孤立的寒武纪岩石位于大陆边缘序列的西部。这些岩石包括与内华达州北方塞拉山脉和加州的克拉马斯山脉的蛇绿岩组合有关的同位素定年的晚元古代和早寒武纪侵入岩。此外，在内华达州中部的罗伯茨山外来岩中还发现了中、上寒武统岩石。这些外来岩中最重要的是假定上寒武世时代的和谐组，由页岩和浊流长石砂岩组成，沉积为海底扇。晚元古代和寒武纪的构造事件塑造了北美西缘，并导致了古生代科迪勒拉微地斜的发展。边缘可能是由于长期裂谷，或者可能是两个不同的时间裂谷，开始之前和晚元古代杂岩和火山序列的沉积过程中，并达到高潮之前，也许继续在最新的元古代和下寒武纪陆源碎屑序列的沉积。
Latest Proterozoic and Cambrian rocks in the western United States are divided into three vertically stacked stratigraphic units (1) a diamictite and volcanic succession (Late Proterozoic), (2) a terrigenous detrital succession (latest Proterozoic and Lower Cambrian), and (3) a carbonate and shale succession (uppermost Lower Cambrian through Upper Cambrian). The diamictite and volcanic succession is exposed discontinuously in the western United States and consists of diamictite, phyllite or argillite, sandstone, conglomerate, carbonate rock, and tholeiitic basalt. Based on isotopic dating in northeastern Washington, the diamictite and volcanic succession is as old as approximately 770 Ma, but part of it could be as young as 650 Ma, or younger, on the basis of uncertain dating of microfossils. Strata of the diamictite and volcanic succession are interpreted as the products of glacial and deepto shallow-water deposition. The diamictite and volcanic succession commonly is present in locally deep troughs that were, in. part, formed by syndepositional extensional faulting. The terrigenous detrital succession consists of a westwardthickening wedge of quartzite and conglomerate, interstratified with siltstone, phyllite, argillite, and locally thick carbonate units. The terrigenous detrital succession is the oldest deposit in the western United States that clearly shows a regional distribution pattern similar to that of lower and middle Paleozoic strata of the Cordilleran miogeocline. This succession thickens from a zero edge on the east to over 6,000 m on the west. The base of the Cambrian System lies within this succession. The strata of the terrigenous detrital succession were deposited in a variety of environments including braided fluvial systems, shallow-water tidally dominated shelf systems, subtidal shelf systems, and subtidal storm-dominated systems. The uppermost Lower Cambrian through Upper Cambrian carbonate and shale succession covered most of the cratonal platform and miogeoclinal areas of the western United States and consists of carbonate rocks and varying amounts of shale and quartzite or sandstone. The shale commonly is interstratified with the carbonate rocks but locally forms thick units containing little but shale. The carbonate and shale succession is as thick as 2,600 m in western Utah and perhaps as thick as 5,100 m in northwestern Utah. It consists mostly of shallow subtidal and peritidal deposits, but it also includes deep-water shale and carbonate strata that accumulated in basins within the miogeocline (the House Range embayment) as well as deepwater shale and carbonate strata in western outcrop areas that have been interpreted as continental-slope and base-of-slope carbonate submarine fan deposits. Tectonically isolated Cambrian rocks lie west of the continental margin successions. These rocks include isotopically dated Late Proterozoic and Early Cambrian intrusive rocks associated with ophiolitic assemblages in the northern Sierra Nevada and Klamath Mountains of California. In addition, Middle and Upper Cambrian rocks are present in the Roberts Mountains allochthon in central Nevada. The most important of these allochthonous rocks is the Harmony Formation of presumed Upper Cambrian age that consists of shale and turbiditic arkosic sandstone deposited as a submarine fan. Tectonic events in the Late Proterozoic and Cambrian shaped the western margin of North America and led to the development of the Paleozoic Cordilleran miogeocline. The margin probably resulted from protracted rifting, or perhaps two distinct times of rifting, starting prior to and during deposition of the Late Proterozoic diamictite and volcanic succession and culminating prior to, and perhaps continuing during, deposition of the latest Proterozoic and Lower Cambrian terrigenous detrital succession.