Field Trip Report : Great Valley Sequence And Coast Range Ophiolite
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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 as muscovite is present, and the texture suggests that the source is not far from the Moreno Formation. The shale beds found in the west side of the road should be stratigraphically older, because the load structure found, when heavy sand was deposited on top of shale, indicates an inverted gravitational layering. The shale was formed in abyssal environment and it can be a facie of a turbidite sequence because of the graded bedding found. The formation is dipping 60 degrees to the east, and the uplift of the Franciscan Complex might have caused this dip.
The Panoche Formation consists of matrix-supported conglomerates interbedded with sandstones. By dating ammonite fossils, we know that the Panoche Formation (Mid-Cretaceous) is older…