graywacke - teaching hand specimen of an Upper Cretaceous graywacke from the Sites Formation
A graywacke is essentially a dirty sandstone, with fragments of rock, quartz, feldspar and other minerals in a clay matrix that were deposited on the deep sea floor by turbidity flows. The sand grains are not well rounded. The coarse mineral and rock fragments are churned with fine clay particles as a turbidity flow avalanches down the continental slope, creating an unusual sediment where large and small particles are mixed together.
Until turbidity flows and their turbidite deposits were discovered and understood, graywackes, with a mix of large and small sediment fragments, were hard to explain. This graywacke is from the Sites Formation, a member of the Great Valley Sequence and is Upper Cretaceous in age. It was collected at Cache Creek in Yolo County, California.
The sediments of the Great Valley Sequence were deposited in deep water between the ancestral Sierra Nevada and a subduction zone to the west. They were upturned by the collision of the North American and Pacific Plates, and stand as a nearly vertical series of interbedded shales and sandstones. The graywackes represent sediment-rich slurries that flowed off the continental shelf. The interbedded shales represent fine grained sediment that was suspended in the water column and gradually settled out.
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