The Middle Mississippian Delle phosphatic event marks a significant change in depositional conditions in the Devonian-Mississippian Antler foreland basin of North America. Stratigraphic and petrographic studies show that deposition of Delle-event rocks ended a long period of open-marine carbonate sedimentation when facies shifted to phosphorites, cherts, fine-grained siliciclastics, and lime mudstones from highly productive waters that may have been episodically anoxic. This drastic change in lithology is reflected in the chemistry of the two suites of rocks. δ18O values of the Delle-event lime mudstones and phosphatized wackestones-packstones affected by the Delle event are 3-5‰ lighter than underlying and overlying open marine wackestones-packstones. Sr concentrations of Delle-event lime mudstones and open marine wackestone-packstone facies are low, suggesting that both of these lithologies have undergone similar postdiagenetic alteration. The uniform Sr concentrations and variable δ18O data suggest that either the younger Delle-event rocks were exposed to meteoric water during diagenesis or that the relatively light δ18O values represent a primary brackish-water depositional signal. Either interpretation is consistent with sedimentological evidence of the Delle event occurring in a predominantly shallow-water setting. δ13C values of Delle-event lime mudstones are also anomalously light, indicating a high input of organic carbon during their formation. Mn concentrations of the Delle-event lime mudstones and stratigraphic equivalents are highly variable, suggesting fluctuating oxygen concentrations during their deposition and diagenesis. Rare-earth-element data from phosphorites and cherts of the Delle event indicate oxic to suboxic depositional conditions.