Middle Cambrian of Grand Canyon: Milankovitch-Scale Climate/Sea-Level Cycles Driving Repetitive Silicic-Clastic, Carbonate, Biochemical-Mineralization, and Biological Community Cycles

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 sandstone (Tapeats) to interbedded fine sandstone and shale (Bright Angel Shale) to thin-bedded limestone (Muav). Within are several scales of cycles including 1–4 meter thick, Milankovitch-driven cycles of decreasing shale upwards each representing sea-level inundation, shallowing sedimentation, and emergence. Each cycle changes and slightly thickens 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 had phosphatic, hematitic, skeletal, and/or carbonate-cement precipitation, reflecting temporally varied aquatic chemistry during prolonged quasi-exposure.

Interpretation of these resultant cycles, sedimentary facies patterns, and diagenesis necessitates understanding (a) the dominant role of wind removal/dispersal of immature, suspension-transported fine sand, silt, and clay from the pre-vegetation continent and epeiric-sea surface during exposure; (b) continual input of freshwater, sediment, nutrients, and fine organics along the epeiric sea’s landward margin and spreading out into the epeiric sea or onto its surface; (c) marine-water input and renewal from the west and spreading across the epeiric sea; (d) a setting a bit south of the equator with warmth, trade winds, and storm events; and (e) repeated, small sea-level fluctuations that drove sea level in and out across this shallow, low-relief epeiric-sea surface.