Cambrian of the Grand Canyon--A Reevaluation: ABSTRACT

Abstract

The Grand Canyon Cambrian, previously thought to represent a subtidal transgressive-regressive sequence recording deepening offshore accumulations of marine sandstone, shale, and limestone, is reinterpreted to record shallow-marine, tidal-flat, and fluvial sedimentation on the landward part of a vast cratonic platform marginal to the Cordilleran miogeosyncline. The basal Tapeats Sandstone is dominantly trough-crossbedded, contains no record of organic activity or marine-tracer grains, and displays a low-variance, unimodal paleocurrent trend down the paleoslope. This part of the Tapeats Sandstone records prevegetation, bed-load fluvial sedimentation. Shallow-marine lagoonal deposits dominate the rest of the sequence: burrowed, very thinly interbedded fine sandstones and shales (Bright Angel Shale) and arenaceous or soft-pellet limestones and dolomitic siltstones (Muav Limestone). Within the Bright Angel Shale, many 1-6 m thick units of burrowed, cross-laminated sandstone and glauconitic sandstone record shoaling sedimentation. These sandstones locally are succeeded by hematitic oolite beds which were exposure surfaces. Unburrowed, channeled, flaser-bedded sandstones form an extensive 20 m thick tidal-flat sequence in the western Grand Canyon. Within the Muav Limestone, 1-8 m thick units of dolomitized eocrinoidal biocalcarenite and algal-ball limestone, flat-pebble intraclast beds, and a few stromatolites emphasize the shoaling nature of the carbonate platform. A 20 m thick, dololaminite, End_Page 810------------------------------ tidal-flat unit locally interrupts the marine limestone sequence at the western (more offshore) end of the Grand Canyon. The 250-500 m Cambrian sequence appears to consist of more than 30 laterally persistent sedimentation cycles that are grouped into 5 grand cycles. The basal Tapeats Sandstone and the clastic and carbonate tidal flats are not cyclic. End_of_Article - Last_Page 811------------