GLAUCONITIC GRAINS IN TERRIGENOUS MARINE PALEOENVIRONMENTS REFLECT SEA LEVEL DYNAMICS

Glauconitic grains occur in a variety of near shore, terrigenous, dynamic depositional environments throughout the Phanerozoic. Glauconitic minerals originate on a quiet shelf in a fully marine, low energy, environment at water depths greater than 50 m, under particular Eh, pH and temperature conditions, when sea level was at a highstand during the sediment-starved phase of a transgressive episode. The source substrate for glauconitic minerals commonly includes abundant fecal pellets, which provide microenvironments of iron reduction and then become glauconitized on the sea floor over an interval up to 10⁵ to 10⁶ years.

This study documents relict glauconitic grains that were reworked in place probably with little transport, mixed with terrigenous quartz, and incorporated into primary sedimentary structures (e.g., ripplemarks and crossbeds) and biogenic structures (e.g., burrows). Once formed, glauconitic grains are chemically stable in the marine environment, and although initially soft, they persist through changing energy conditions and sediment transport. A pelleted substrate that became glauconitized during a transgressive phase was subjected to shallowing conditions of shoreline regression. Trace fossils characteristic of near shore, high energy environments may contain glauconitic pellets that originated under different conditions when the water was deeper. This genetic interpretation is consistent with observations and analyses of glauconitic deposits of different ages and geologic settings, including the Cambrian Lion Mountain Member of central Texas, Upper Cambrian Reno Member of southern Wisconsin, Jurassic Curtis Member of northern Utah, Cretaceous (Campanian) Shannon Sandstone of central Wyoming, and Miocene of theTarragona Basin northeastern Spain.