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, 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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.