Field Trip Report: Great Valley Sequence And Coast Range Ophiolite

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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…
By using U-Pd dating on the zircon found in plagioclase granite and gabbro interface, the age of the upper middle part of the Coast Range Ophiolite is 162-167 Ma after correcting for lead loss. This is around the same age as the sedimentary rocks in the lunch area (145-166 Ma), which is the oldest in the Great Valley Sequence. Thus, it is logical to infer that the GVS deposited directly on top of the Coast Range Ophiolite as the age difference between the two is small. The age difference, nevertheless, can be explained, because the gabbro portion is not the youngest part in the ophiolite. The parts of the ophiolite that are not exposed to the surface, including the sheeted dikes, pillow basalts and the chert, might be formed within around 2 – 20 million years before the GVS

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