Syndepositional shallow-water precipitation of glauconitic minerals

Numerous studies have demonstrated that glauconitic minerals predominantly form in water depths of mid-shelf to upper slope in modern oceans. These areas tend to have slow sedimentation rates, another commonly cited requisite for glauconitic mineral precipitation. Cambro-Ordovician strata from the southwestern US are rich in glauconitic minerals. Stratigraphic, sedimentological, and petrographic constraints indicate that the glauconitic minerals are autochthonous. In marked contrast to the modem environments of deposition, these Cambro-Ordovician strata formed under very shallow-water to tidal-flat conditions. The trough cross-stratified deposits of the most glauconitic mineral-rich accumulations (glaucarenites) indicate a high energy environment and probably a normal to high rate of sedimentation. The presence of fibroradiated rims of glauconitic minerals on glauconitic mineral pellets, echinoderm fragments, and quartz grains demonstrates that the Cambro-Ordovician glauconitic minerals precipitated on or in close proximity to the sea floor and prior to calcite precipitation. Consequently, glauconitic minerals must have formed under markedly different conditions in the lower Paleozoic than they do today. Thus, the occurrence of glauconitic minerals in the rock record cannot be used a priori as an environmental indicator of either mid-shelf and deeper water and/or a slow rate of sedimentation. (C) 2000 Elsevier Science B.V. All rights reserved.