Geological Survey Professional Paper Volume 345-348 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. Excerpt: ...versus unconformities and other criteria in making stratigraphic correlations over broad regions. Longwell (1950, p. 414-415) considers the entire sequence just referred to as of Cambrian age, although he acknowledges that the lower beds in the sequence do nor contain known fossils. He regards the record of essentially continuous deposition as the significant feature on which to base the age assignment. In Death Valley (Noble, 1934, p. 173-174; McCulloh, 1954, p. 13-15) rocks reputed to be Precambrian include a highly metamorphosed assemblage and in addition comparatively unmetamorphosed beds as much as 11,000 feet thick. Although the two appear generally separated by a marked unconformity, in places none is recognized. The relatively metamorphosed beds include limestone, shale, sandstone, and conglomerate, intruded by sills and dikes of basic igneous rock. The limestone contains algal remains. Noble says these strata are "as a whole roughly equivalent to the Algonkian Grand Canyon series (Uhkar and Chuar groups)" and are separated from overlying beds of Early Cambrian age by an angular unconformity. TEXAS AND VICINITY Precambrian rocks crop out in various isolated localities in Texas. Those near Van Horn have recently been thoroughly discussed (King and Flawn, 1953. p. 18-20, 27-34, 39, 41-43, 45-49, 51-67, 73-97,125-133). Here the Carrizo Mountain group consists of metamorphic, presumably relatively old, rocks. The Allamore formation and Hazel formation (formerly grouped in the Millican formation) are below the Van Horn sandstone, now regarded as Precambrian (?). The Allamore consists of interbedded cherty limestone, phyllite, and volcanic rocks. Some of the limestone contains structures that may be stromatolites. The Hazel formation has a thick cong...