Deposition of the Tapeats Sandstone (Cambrian) in central Arizona
Grain size, bedding thickness, dispersion of cross-stratification azimuths, and assemblages of sedimentary structures and trace fossils vary across central Arizona; they form the basis for recognizing six facies (A through F) in the Tapeats Sandstone. Five of these (A through E), present in western central Arizona, are marine deposits containing the trace fossil Corophioides; several intertidal environments are represented. The association of large-scale cross-bedding (50 to 300 cm) that is characterized by compound cross-stratification, numerous reactivation surfaces, and herringbone patterns is typical of facies A and generally typical of the finer-grained, thinner-bedded facies B. The sedimentary structures and polymodal distribution of foreset azimuths common to facies A and B probably formed on intertidal sand bars during emergence and late-stage tidal runoff. Facies C consists of well-sorted sandstone, gently cross stratified or with continuous parallel stratification, and foresets tangential to the lower bedding surface. This facies generally occurs where the gradient of the depositional surface increases; it apparently was deposited on a beach by shoaling waves. Facies D and, to a lesser extent, the coarser-grained facies E are sandstones with trough cross-stratification, fining-upward cycles, abundant intercalated thin shale and sandstone, rare flaser bedding, and local bipolar distribution of foreset azimuths. Both facies are tidal flat deposits; facies D was probably produced by meandering tidal channels, whereas facies E was likely produced by migration of braided tidal channels. The sixth facies (F), present in eastern central Arizona, is an arkosic small-pebble conglomerate that lacks trace fossils; low dispersion of foreset azimuths and large-scale (1 to 11-m wide) cut-and-fill structure are typical. Facies F was deposited by bedload streams that transported coarse, poorly sorted sand and gravel westward to the intertidal flats.
|Deposition of the Tapeats Sandstone (Cambrian) in central Arizona
|USGS Publications Warehouse
|Geology, Minerals, Energy, and Geophysics Science Center