J. G. Johnson, E. C. Potter; Silurian (Llandovery) downdropping of the western margin of North America. Geology ; 3 (6): 331–334. doi: https://doi.org/10.1130/0091-7613(1975)3<331:SLDOTW>2.0.CO;2
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The western continental margin of North America lay west of Lower Cambrian cross-bedded quartz sandstone, west of thick sections of Cambrian-Ordovician shelly-facies shelf carbonate rocks, and west of Upper Ordovician–Lower Silurian shallow-water carbonate rocks deposited in central Nevada and southeastern California. During Early Silurian (Llandovery) time, a north-south belt, 64 km or more wide at the edge of the continental shelf, was downdropped to depths as great as 1,000 to 1,500 m, below the deepest shelly-fauna biotopes, and remained there for the rest of Silurian time.
Downdropping of the Silurian continental margin may be attributed to prolonged extension in the interarc basin, some of whose island-arc remnants are now found in the Klamath Mountains of California.