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Keweenawan Supergroup
The Keweenawan Supergroup is a supergroup of volcanic and sedimentary rocks that fill the Midcontinent Rift System in the U.S. states of Michigan, Wisconsin, and Minnesota. It is about thick and it formed about 1.1 billion years ago. Stratigraphy Members of the Keweenawan Supergroup are exposed at the surface only in the Lake Superior region, particularly at the perimeter of the Midcontinent Rift System. To the southeast and southwest, they are covered by sedimentary rocks of Paleozoic age. At its thickest, the supergroup consists of about of volcanic rocks overlain by about of sedimentary rocks.Nicholson, p. 505. The supergroup consists of the following members: Magnetism The Powder Mill Group is reversely polarized except for intervals at the base and top of the upper Kallander Creek Volcanics.Nicholson, pp. 507–8. The Portage Lake Volcanics and all younger formations have normal magnetic polarity.Nicholson, p. 508. References Bibliography *{{cite journal, last1=Nichols ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geological Unit
A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features (facies) that characterize it. Units must be ''mappable'' and ''distinct'' from one another, but the contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when the sandstone component exceeds 75%". Lithostratigraphic units Sequences of sedimentary and volcanic rocks are subdivided the basis of their shared or associated lithology. Formally identified lithostratigraphic units are structured in a hierarchy of lithostratigraphic rank, higher rank units generally comprising two or more units of lower rank. Going from smaller to larger in rank, the main lithostratigraphic ranks are Bed, Member, Formation, Group and Supergroup. Formal names of lithostratigraphic units are assigned by geological surveys. Units of formation or hi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |