Unusual, fossiliferous concretions from the lower Jurassic Moenave formation in St. George, Utah, USA: Implications for ancient fish mass mortalities

摘要

Two types of unusual concretions with similar contents but markedly different shapes and distributions were found in close stratigraphic proximity within the Whitmore Point Member of the Moenave Formation. Roughly cylindrical, elongate concretions were found in parallel and regularly spaced rows, and a layer of irregularly shaped and distributed fossiliferous concretions was discovered only a few centimeters above the cylindrical concretions. Both sets of concretions contain abundant hematite as well as enameloid fish scales. In addition, the concretions contain numerous ostracod carapaces and what appear to be rip-up clasts. Microprobe and Raman analyses of representative concretion samples reveal that the cylindrical concretions have a groundmass largely composed of silica while the irregular concretions have a groundmass largely composed of dolomite, and the ostracods within each type of concretion have been altered and match the chemistry of the surrounding groundmass. Evidence of multiple cement precipitation events is present within each concretion. These unusual concretions suggest mass fish mortality events in the large lake that occupied the St. George area in the early Jurassic.