Tonto Group

Facies analyses of the Tonto Group indicate deposition in a variety of fluvial, nearshore, and shallow shelf environments. Braided stream and intertidal-to shallow-subtidal deposits of the Tapeats Sandstone grade seaward into a complex array of shallow shelf sands and muds of the Bright Angel Shale. Shelf sedimentation was influenced by both tidal and storm currents. Sand ridges, sand waves, and broad areas where fine-grained siliciclastics were deposited from suspension settling following storms and during fair-weather periods characterized the shelf. Farther offshore, carbonate islands dotted the shelf. Here, the carbonate buildups were characterized by intertidal and possible supratidal zones separated by deeper water areas where tidal currents were active and where finer-grained carbonate sediments were deposited. A number of transgressions and regressions resulted not only in the intertonguing of the formations of the Tonto Group but also in the vertical juxtaposition of facies belts that probably were not laterally adjacent. -from Authors