Paleoenvironmental Patterns in Middle to Lower Upper Cambrian Interval in Eastern Great Basin

Abstract

In the eastern Great Basin region during the Middle Cambrian and early Late Cambrian a north-trending, near-sea-level shoal separated a deeper water open shelf on the west from a shallow shelf-lagoon. Regression in the late Middle Cambrian (unit K) led to a flooding of terrigenous sand across the continental shelf and the emergence of islands flanked by supratidal and intertidal zones on the shoal. At times preceding and following this event, a lacework of tidal algal mud banks and shallow subtidal basins occupied the shoal. Hydrologic conditions determined the morphology of the stromatolites on the mudbank. Normal supratidal processes were responsible for dolomitization along island shorelines, but the biogenic concentration of magnesium in algal mats, in conjunction with magnesium-enriched brines, accounted for the dolomite in the stromatolites associated with the tidal mudbanks. Dolomitization in the subtidal basins and shelf-lagoon was controlled by bedding and mottled structures of physical and biogenic origin, and was more pervasive in the lagoon. Unit K and its correlatives across the shelf are believed to be time-parallel in that there is little time differential along unit boundaries. On the shoal other algal boundstone units also may be time-parallel and serve as a basis for establishing temporal relations in the eastern Great Basin. Comparison of tidal-subtidal cyclic sequences in the Cambrian of North America and elsewhere may permit the recognition of eustatic and/or epeirogenic events useful in establishing temporal relations on a continental or larger scale.