Comparative Precambrian Stratigraphy and Structure Along the Mid-Continent Rift
Comparative Precambrian Stratigraphy and Structure Along the Mid-Continent Rift
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中大陆裂谷沿线前寒武纪地层和结构的比较
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发表时间：
1986
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通讯作者：
A. B. Dickas
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作者：
A. B. Dickas
中大陆裂谷是一个地质学上确定的构造结构，从堪萨斯向东北一直追踪到苏必利尔湖上级地区。一个相关的手臂已被确定为从苏必利尔湖上级东部向东南延伸到密歇根州的下半岛。该裂谷的时代为前寒武纪（Keweenawan），大约在1.1 b. y. B. P.开始发展。在其大部分范围内，该大陆级裂谷系统埋在中生代地层之下。仅在明尼苏达州中东部、威斯康星州西北部和密歇根州上半岛发现露头。大陆中部裂谷最被接受的构造模型是一个中央地垒，部分被碎屑岩覆盖，以高角度断层为界，两侧是充满碎屑岩的盆地。该模型是结合苏必利尔湖西南部上级地区的实地研究开发的，一般已被明尼苏达州、爱荷华州、内布拉斯加州和堪萨斯采用。一个可比的模型还没有提出相关的手臂延伸到密歇根州南部。在苏必利尔湖上级露头带，最古老的断裂岩石是Keweenawan中期的层状玄武岩。覆盖在这些玄武岩上的是由Oronto（较老）和Bayfield（较年轻）组组成的上Keweenawan厚沉积序列。这些沉积岩几乎完全是碎屑岩，结构从砾岩到页岩（泥岩）不等。岩石的颜色主要是红色和棕色，其次是绿色和白色。Oronto群在矿物学和结构上更不成熟，结构上比Bayfield群更复杂。Oronto组也因其单元之一--Nonesuch地层--而不同，该地层含有有机页岩，这些页岩从密歇根铜矿的地下裂缝和洞穴中排出液态烃。这两个群体的沉积条件与水生大陆环境密切相关。具体的古地理范围从冲积扇和平原，三角洲，泛滥平原，河流，湖泊。苏必利尔湖上级露头带的西南和东南，主要通过在中大陆裂谷趋势内钻到前寒武纪的不到12口重要威尔斯井了解地层关系。对这些威尔斯井内建议的上基韦纳湾红色碎屑沉积岩柱的分析表明，苏必利尔湖上级地层可以在整个中大陆裂谷趋势中临时对比。
The Mid-Continent rift is a geophysically identified tectonic structure that has been traced from Kansas northeastward into the Lake Superior district. A related arm has been identified by gravity as extending from eastern Lake Superior southeastward into the lower peninsula of Michigan. This rift is of Precambrian (Keweenawan) age and began developing approximately 1.1 b.y.B.P. For most of its extent, this continental-class rift system is buried below Phanerozoic strata. Outcrops are found only in east-central Minnesota, northwestern Wisconsin, and the upper peninsula of Michigan. The most accepted tectonic model of the Mid-Continent rift is that of a central horst partly covered by clastic rocks, bounded by high-angle faulting, and flanked by basins filled with clastic rocks. This model was developed in conjunction with field studies in the southwestern Lake Superior area, and generally has been adopted for Minnesota, Iowa, Nebraska, and Kansas. A comparable model has yet to be presented for the related arm extending into southern Michigan. In the Lake Superior outcrop belt, the oldest recognized rifted rocks are layered basalts of middle Keweenawan age. Overlying these basalts is an upper Keweenawan thick sedimentary sequence composed of the Oronto (older) and Bayfield (younger) Groups. These sedimentary rocks are almost entirely clastic with texture ranging from conglomerate to shale (mudstone). Rock colors are principally reds and browns, with secondary shades of green and white. The Oronto Group is more immature mineralogically and texturally, and more complex structurally, than the Bayfield Group. The Oronto Group also is differentiated by one of its units--the Nonesuch Formation--which contains organic shales that bleed liquid hydrocarbons from subsurface fractures and vugs in a Michigan copper mine. Conditions of eposition for both groups are closely allied with an aquatic, continental environment. Specific paleogeography ranged from alluvial fan and plain, to deltaic, flood plain, fluvial, and lacustrine. Southwest and southeast of the Lake Superior outcrop belt, stratigraphic relationships are known principally from fewer than a dozen significant wells drilled into the Precambrian within the Mid-Continent rift trend. Analysis of the proposed upper Keweenawan red clastic sedimentary rock column within these wells suggests that the Lake Superior stratigraphy can be provisionally correlated throughout the Mid-Continent rift trend.