ROLLING STONES AND DINO BONES: ONCOIDS OF THE CEDAR MOUNTAIN FORMATION, LOWER CRETACEOUS, UTAH

Oncoids of the Lower Cretaceous Cedar Mountain Formation (CMF) in the San Rafael Swell, Utah, yield important clues to the depositional environment of this complex formation. Overlying the Morrison Formation, the CMF was deposited in a terrestrial environment through mostly fluvial processes. However, the details are lacking for the interval below the variegated mudstone of the upper Ruby Ranch Member.

Fieldwork in summer, 2003, in the Woodside Anticline northwest of Green River, Utah, yielded abundant deposits of oncoids in the basal CMF, overlying coarse cobble conglomerate of the Buckhorn Member. Oncolite crops out in discontinuous meter-thick beds that comprise the base of fluvial-progradational sequences that fine upward from conglomerate (bearing oncoids) to low-angle, medium sorted sandstone. In some areas, the base of the progradational sequence is a grey wackestone with abundant ostracodes and other lacustrine fossils. Individual sequences vary from 4 to 8 meters thick. Disarticulated dinosaur bones and wood fragments are present in the conglomerate and form the nuclei of larger oncoids. Some dinosaur bones retain fragile vertebral processes so a local origin is suspected. In two areas, exposure of the oncoids led to development of extensive caliche beachrock.

The oncoids are light grey, subround to oblate and mimic the external shape of the nucleus. Size of the oncoids is quite variable, ranging from 1 cm to over 20 cm diameter. Nuclei are composed of chert, sandstone, conglomerate, wood, and dinosaur bone. Within an oncolite, up to 70% of the grains are coated and random oncoids are found within the lower overlying conglomerate. Oncoid laminae are composed of grey micrite and range from 0.3 to 2 mm thick (average 0.97 mm). Rare laminae are composed of palisade crystal cements. Laminae are either flat-lying or comprise ministromatolites 2-6 mm wide (average 4.2 mm). Fine siliciclastics infill between columns. Possible algae were seen in two thin-sections and the micrite resembles other presumably microbial micrite from throughout the fossil record.

The occurrence and distribution of oncolite helps to demonstrate an ephemeral lacustrine-braided fluvial system within the CMF, marking a transition from the braided fluvial Buckhorm Conglomerate to the palustrine and meandering fluvial Ruby Ranch Member.