RECONSTRUCTING THE PALEOENVIRONMENT OF THE BRIGHT ANGEL SHALE, GRAND CANYON ARIZONA

The Tonto Group of the Grand Canyon, described by McKee in 1945, has been regarded as a model of the Cambrian transgression onto the North America craton. Deposition progresses from near-shore coarse sands, the Tapeats Sandstone, to finer grained muds, the Bright Angel Shale, followed by carbonate deposition of the Muav Limestone.  Blocks of resistant Precambrian quartzite basement form a basal unconformity of local relief in the eastern Grand Canyon. These Precambrian “islands” are thought to have created restricted embayments throughout the eastern section.  Geochemical analyses of Middle Cambrian samples taken from the Cheops Bay section of the Grand Canyon suggest a different depositional environment for the Bright Angel Shale.  Carbon and strontium isotope data support a large flux of freshwater into the restricted embayment areas.    ⁸⁷Sr/⁸⁶Sr values of 0.7220 indicate strontium derived from a continental source.  d¹³C values are markedly different in restricted embayment samples than open ocean samples.  R-values derived from d¹³C analyses are used to determine the dissolved inorganic carbon [DIC] in the restricted embayment and the open ocean.  A higher value of [DIC]in the restricted embayments is due to increased organic input to these areas.  Glaucony studies also support freshwater influence to the Bright Angel Shale.  The abundance and type of glaucony in the samples is similar in nature to Cambro-Ordovician strata formed under shallow water-tidal flat conditions.