please help with geology asap, thanks!!! 1. The Marcellus Formation, which is fa
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Question
please help with geology asap, thanks!!!
1. The Marcellus Formation, which is famous for providing gas when fracked, is a shale formation in the Eastern US that is interpreted to have formed in deep water. In contrast, the Queestone Formation of Ontario, Canada, is a shale formation that is interpreted to have formed in shallow, almost supratidal waters. How do geologists know that one shale formed in deep water and the other in shallow water? What features can be seen in the rock suggesting the environment of deposition?
2.How would you recognize an ancient alluvial fan deposit in the geologic record?
Explanation / Answer
1) Determination of the relative level of water under which the sedimentary formation was formed, is known as paleobathymetry. In the process of paleobathymetry, water depth is determined from the different signatures present in the rock formations.
These signatures include:
Sedimentary structures:
Sedimentary structures provide good indication of the water depth. Mud cracks, rainprints are some of the sedimentary structures formed at shallow water depths, whereas upper flow regime plane beds, planar beds indicate a considerable depth of the water at the site of deposition.
Fossils:
Fossils are the best indicator of the depth of water at the site of deposition. Mainly microfossils are used for the purpose. Benthic Foraminifera are very sensible to the water depth. They mainly flourish at the bottom waters. So, the relative abundance of these benthic foraminifera in the rock formation indicate that the rock was formed at a considerable water depth. Their population is dependent on various factors(temperature, salinity, sunlight). So, different forms of these benthic foraminifera occur at different depths in the waters.
From these evidences geologists infer the depth of formation of different sedimentary rocks.
2) Alluvial fan deposits are recognized by the following characters:
Formation and preservation of the alluvial fans require considerable amount of tectonic uplift of the surface, so the areas of upliftment is the best possible location for the alluvial fans.
These deposits are triangular in map view.
These deposits are mainly composed of unsorted sandstone, conglomerates.
These deposits form coarsening upward sequence, as the fan extends out into the valley.
Regardless of the size of the fan, these deposits show indications of flow over them.
These deposits consists of unsorted sediments, and angular fragments.
Fossils are very rare in these deposits but red beds are very common.
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