UPPER JURASSIC MORRISON FORMATION CLAMS ON THE HALF SHELL, CENTRAL MONTANA

A gray-green, illitic mudstone bed containing freshwater bivalves was discovered in the Upper Jurassic Morrison Formation of central Montana. The 20-cm thick bed lies 19 m above the marine Swift Formation. Five bivalve species have been identified from the assemblage-Unio felchi, U. mammillaris, U. nucalis, U. stewardi, Vetulonaia whitei-plus an unidentified genus. Thick and thin shelled ecophenotypes are represented, indicating lotic and lentic environments. In addition to the bivalves, gastropods (Viviparus, Tropidina), ostracods (Alicenula, Candona, ?Cetacella, Theriosynoecum), and charophytes (Aclistochara, Mesochara, Porochara) are represented. Fish bones and piscivorous fish teeth were found. The occurrence of Viviparus places the deposit in sediments equivalent to the Brushy Basin Member. The occurrence of the ostracods C. morrisonenis, T. wyomingense and the charophyte P. minima assign the deposit to Morrison Formation Biozone 3.

Paired bivalve specimens are crushed due to post-burial compaction. Unpaired valves are commonly well-preserved with some over 13 cm in length. Unbroken single valves are typically oriented convex; however, some are concave or vertically oriented. Shell fragments are also incorporated in the deposit. Preserved amorphous organic material is abundant and implies rapid burial. The allochthonous assemblage is representative of a crevasse splay flood event. There are no associated fluvial sandstone beds exposed in the outcrop. The formation mudstone/sandstone ratio and morphology of the sandstone beds indicates an anastomosing floodplain environment. Shells are poorly sorted and have no preferred apex orientation. Valve imbrication shows a northwest paleoflow direction. The largest shells were used to approximate an entrainment velocity of 0.65 m/s.

Ten shells were thin-sectioned to observe growth bands. Thin closure lines demarcate various growth bands. Correlation of band widths is observed among the specimens signifying a communal response to environmental disruptions. Thin closure lines indicate short periods of adverse conditions, such as turbidity due to seasonal storms. The varied fauna and flora fossil assemblage is evidence of a salubrious perennial environment that experienced negligible seasonal variation, punctuated by occasional storms.