UNUSUAL, FOSSILIFEROUS CONCRETIONS FROM THE LOWER JURASSIC MOENAVE FORMATION OF ST. GEORGE, UTAH, USA: IMPLICATIONS FOR ANCIENT FISH MASS MORTALITIES

Two unusual types of fossiliferous concretions from the Lower Jurassic Whitmore Point Member of the Moenave Formation provide information about the environmental conditions and paleobiology of an ancient lake. Both types of concretions contain abundant semionotid fish scales, ostracod carapaces, apparent rip-up clasts, and conspicuous hematite. The concretions have markedly different morphologies and were found in two nearly adjacent stratigraphic layers. Roughly cylindrical, elongate concretions were found in parallel and regularly spaced rows, and irregularly shaped and distributed concretions were discovered only a few centimeters above. 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. Ostracod carapaces within each type of concretion have been altered and match the chemistry of the surrounding groundmass.

We propose a model for the formation of the Moenave concretions based on characteristics of the specimens, comparisons with other concretions, and studies of modern lake systems. In this scenario, mass fish mortalities provided copious quantities of disaggregated fish material, which adhered to microbial mats. The microbial mats and embedded fish debris were rolled and shaped into elongate and regularly spaced configurations by strong, unidirectional water flow, forming the cylindrical masses that were subsequently mineralized. In contrast, multidirectional water flow formed the microbial mats and fish debris into the flattened masses of variable sizes and shapes that became the irregular concretions. In both types of concretions, multiple phases of mineral precipitation occurred. Hematite precipitated around and within the fish skeletal material and ostracod carapaces. Authigenic minerals precipitated between the fish scales and other inclusions, and a hematite cement rim precipitated around the external surfaces of the concretions. The specific set of circumstances required to form the Moenave concretions are presumably why comparable structures are rare or non-existent.