Middle Cambrian of Grand Canyon: Aerosol Dispersal and Accumulation of Fine Silicic-Clastic Sediments Across an Advancing, Broad Epeiric Sea in a Pre-Vegetation World

Within a 5-million-year period in the Middle Cambrian, rising sea level inundated a vast epeiric sea across Nevada and the Grand Canyon portion of Arizona. This inundation and buildup left a westward and upward-building sequence from dominantly bedload-transported riverine sandstone (Tapeats), to suspension-load transported shallow-marine shale (= interbedded fine-grained sandstone, silt, and clay; Bright Angel), to thin-bedded limestone (Muav). Within are several scales of cycles including over 100 1-to-4-meter-thick, Milankovitch-driven cycles, each of decreasing shale upwards and each representing sea-level inundation, shallowing sedimentation, and emergence. Each cycle changes seaward (westward) from shale up to sandstone, shale up to glauconitic sandstone, shale up to rusty brown dolomite (eocrinoidal), shale up to algal-ball limestone, and shale up to layered limestone-dolomite with decreasing shale seaward. A cap to most cycles has phosphatic, hematitic, and/or carbonate-cement precipitation and commonly also medium to coarse quartz sand eastward.

In this pre-land-vegetation world, suspension-transported sediment is removed from terrestrial fluvial environments, emergent cycle cap deposits, and partly to largely removed from the upper part of sandstone and glauconitic sandstone cycles. The upper parts of these cycles have evidence of extremely shallow water and periods of exposure. The cycle caps had prolonged and persistent exposure. Evidence strongly suggests that wind transport was a dominant influence in this removal, although storm-water reworking played a major role in molding the lower (shale) part of each cycle.

Seaward fining and diminishing silicic-clastic silts across the shallow-marine Muav Limestone reflects aerosol transport for some 120-to-200 kilometers west of the riverine coast.