Great Valley Sequence

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Field Trip Report: Great Valley Sequence and Coast Range Ophiolite
The Moreno Formation in the Great Valley Sequence (GVS) is consisting of interbedded sandstones and organic-rich black shales, which were deposited during Upper Cretaceous (74-80 Ma) in near shore and shallow marine environment according to the fossils found. The sandstone in this outcrop is medium-sorted, angular to sub-angular arkosic arenite, with minerals such as quartz, plagioclase, muscovite and altered biotite. 1/4 of the muscovite was dated as 90 Ma old, matching the time of Cathedral Peak Intrusion. The provenance of the sandstone is probably granites. Round concretions on sandstones indicate calcite dissolution. The source of the sandstone can be peraluminous granite
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By dating ammonite fossils, we know that the Panoche Formation (Mid-Cretaceous) is older than the Moreno formation. The conglomerates’ clasts have sizes around 5cm in diameter and they are round to sub-angular. Mineralogy of the clasts includes slightly metamorphosed felsic extrusive igneous rocks (rhyolite to dacite), metamorphic rocks (quartzite) that have high T/P ratios, greywacke, and chert. The source of the clasts might be a continental crust. The Panoche Formation was part of the submarine fan that migrated laterally in the turbidite …show more content…
This observation is reasonable because most deep ocean sediments are clay and mud. Then, the deeper we go in the GVS, there should be more shale. The second trend is the transition of sediment provenance from volcanic rocks to plutonic rocks. Specifically, the provenance of the sandstones from the Moreno Formation is from grantoids and that of the clasts in conglomerates is rhyolite or dacite. It is possible that both the sandstones and conglomerates have sediments transported from the same continental arc, but the arc was getting eroded down through. Its top volcanic rocks were first eroded and were carried to deposit in the submarine fans by turbidity currents as conglomerates, and the lower plutonic part was eroded to form the younger sandstones in the Moreno Formation. Sierran Arc is a plausible candidate for the continental arc that was eroded because of its spatial proximity to the GVS, since the sandstones are not very textually

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