Delle Phosphatic Member: An anomalous phosphatic interval in the Mississippian (Osagean-Meramecian) shelf sequence of central Utah

Phosphatic rocks are a distinctive part of the Lower and lower Upper Mississippian Delle Phosphatic Member of the Woodman Formation at the type locality of the member in the Lakeside Mountains, northwestern Utah. There, as elsewhere in central and western Utah, the Delle is composed of a relatively thin, anomalous depositional interval characterized by phosphorite, siliceous microfossils, secondary chert, distinctive dense lime mudstone, and argillaceous rocks stratigraphically sandwiched within a thick sequence of subtidal and peritidal Mississippian shelf carbonate and craton-derived siliciclastic rocks. Strongly reducing in-sediment conditions evidently prevailed at the onset of Delle deposition, yet sedimentologic features in the Delle such as endolithic-algal borings in pisolitic phosphorite and features in the rocks above and below the Delle such as tidally laminated fecal-pellet limestones and exclusively tractional sedimentary structures indicate shallow, shelf depositional environments. These rocks do not support the concept of a deep, sediment-starved depositional basin, the "Deseret deep starved basin" of previous authors. Rather, the nature of the Delle suggests incursion far onto the broad Early Mississippian shelf of nutrient-rich, organically productive waters possibly produced by upwelling in the coeval Antler foreland trough in central Nevada. This interpretation places the Early Mississippian shelf margin to the west of the Lakeside Mountains—probably west of Utah—rather than in central Utah, as required by the "Deseret deep starved basin" model.