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CSPG Special Publications

Abstract


Devonian of the World: Proceedings of the 2nd International Symposium on the Devonian System — Memoir 14, Volume I: Regional Syntheses, 1988
Pages 161-182
U.S.A.

Early and Middle Devonian Paleogeography of Western United States

J. G. Johnson, Charles A. Sandberg, Forrest G. Poole

Abstract

Lochkovian sedimentary rocks are recognized in accreted terranes in Washington and northern California, in the Roberts Mountains allochthon (RMA), and in both outer-shelf-basin and carbonate- shelf sequences. A delta-Zone transgression is recognized. Late Lochkovian regression produced an unconformity everywhere on the middle and inner shelf.

Pragian and Emsian sedimentary rocks are transgressive (T-R cycles la, lb) with a late Emsian unconformity at the top. Quartz sand marks thi hiatus and the one between Lochkovian and Pragian. Early Devonian sedimentation was most widespread in the Pragian and is represented by Beartooth Butte Formation outliers on the craton.

Eifelian carbonate rocks were transgressive, but most are essentially coextensive with Lower Devonian rocks. Latest Eifelian transgression, however, produced onlap, flooding the Williston basin. Early and middle Givetian sedimentation followed depositional patterns established in the latest Eifelian, but Taghanic onlap in the late Givetian (T-R cycle IIa) changed middle- and inner-shelf lithotopes from peritidal to open marine.

On the carbonate platform, collapse breccias, interpreted as resulting from dissolution of cyclically deposited evaporites, occur in T-R cycles Id, If and lower IIa, but evaporites are not evident in outcrop.

Although all Lower and Middle Devonian stages probably are represented in accreted terranes and the RMA, their depotectonic histories differ greatly from the epicontinental sequences.


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