fanglomerate - teaching hand/display specimen of a consolidated alluvial fan deposit
Fanglomerate is a sedimentary rock with heterogeneous rock fragments (clasts) deposited in an alluvial fan and later cemented. This fanglomerate is weakly cemented and is young, being deposited in the Ridge Basin roughly five million years ago.
The Ridge Basin is a NW-SE trending basin in the Transverse Ranges of Southern California, bounded by the San Gabriel and San Andreas Faults and near the “big bend” in the San Andreas. The San Gabriel fault, forming the west side of the basin, was the boundary between North American and Pacific plates early in basin history. Stretched between the two faults, the basin subsided and filled with an over eight mile thick sequence of marine to non-marine clastic sediments.
At the end of the Miocene and into the early Pliocene, the basin was squeezed and uplifted in a classic example of basin inversion, when the North American – Pacific plate boundary transferred from the San Gabriel Fault to the San Andreas and the basin was compressed between Pacific and North American Plates.
The Hungry Valley Formation is composed of alluvial and braided stream channel deposits roughly 4,500 feet thick, originating at a source in the Mojave Desert that has been displaced relatively to the SE by the San Andreas Fault. This fanglomerate represents the final fill of the Ridge Basin during late Miocene and early Pliocene.
Collected along the west side of I-5 south of the Quail Lake exit south of Gorman in Los Angeles County
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