PROVENANCE AND THE EARLY DIAGENETIC ALTERATION OF VOLCANICLASTIC MATERIAL INCORPORATED IN FLUVIAL CHANNEL SANDSTONE DEPOSITS, MORRISON FM. NEAR CAPITOL REEF NATIONAL PARK, UTAH

The Morrison Formation has been studied for over a hundred years, primarily because of its rich dinosaur fauna. In spite of its proximity to a westward volcanic arc, the diagensis of Morrison Fm. volcaniclastic material has received little attention. The study includes a 9 m thick interval within the Brushy Basin Member, which consists of pebble conglomerate towards the base and fine-to medium-grained sandstone towards the top. The conglomerate beds are crudely cross-stratified and rest on an erosive, scoured surface incised into underlying claystone and siltstone paleosols; clast compositions include chert, metaquartzite, vein quartz, and rare fossiliferous and oolitic carbonate lithologies. Sandstone lenses intertongue with conglomerate and grade upward into trough cross-stratified and bioturbated sandstone beds. Overall the sandstone interval is fining-upward. The sandstone body is interpreted as an element of an anastomosing stream system that flowed north and northeasterly from the Elko and Mongollon Highlands, perhaps distal from alluvial fan systems. Both the base of the sandstone/conglomerate body, as well as the top of the underlying truncated paleosol are strongly cemented by silica. Brushy Basin Member ash deposits were apparently reworked and deposited within fluvial channels, which were then diagenetically altered to release soluble silica that accumulated at the boundary of the sandstone/conglomerate interval and the underlying paleosols. The purpose of this study is to test the hypothesis that the silicification is due to the early diagenetic alteration of ash and volcaniclastic material that had fallen into the alluvial fan from the highlands in the west. Petrographic analysis of the underlying paleosols will identify if the ash fall had been directly introduced into the system at the source or in the alluvial fan. A provenance study will also test the hypothesis that the pebbles within the conglomerate unit are from the Elko Highlands. This study demonstrates the importance of detailed studies of volcaniclastic materials in deducing Morrison Fm. provenance and in helping to better define the paleoenvironment and paleoclimate during this part of Jurassic time.