DIFFERENTIAL PRESERVATION OF PLANT FOSSILS IN FLUVIO-DELTAIC ENVIRONMENTS FROM THE BLACKHAWK FORMATION, UTAH

The Blackhawk Formation of Utah consists of Campanian-age sedimentary rocks that were deposited in fluvio-deltaic environments. Many of these rocks contain plant fossils of varying degrees of preservation and coal beds. From field analysis of Blackhawk sections containing plant fossil material near Electric Lake and in Strait, Ferron and Salinas Canyons eight different fluvio-deltaic depositional environments were recognized. The beds that make up these eight environments were assessed for the amount, type and preservation of plant material that they contained and analyzed for total organic carbon (TOC) content. Using field and grain size data, beds were ranked in terms of their energy of deposition on a scale of 1-5. Beds with high proportions of clay grains were assigned to the lowest energy environments and given a ranking of 1. Beds with high proportions of sand were assigned to the highest energy environments and given a ranking of 5. Beds deposited in higher energy environments (ranked 4-5), such as proximal splays and distributary channels, contained the best-preserved leaf fossils. Beds deposited in lower energy environments (ranked 1-3), such as swamps and distal crevasse splays, had high TOC contents (up to 82%) and contained coal beds and leaf mats but they did not preserve distinct, whole leaf fossils. In lower energy environments from the Blackhawk plant material was not mechanically degraded but it was biodegraded or built up mats, facilitated by slow rates of sedimentation. Although rapid deposition in higher energy environments quickly covers plant material, preventing biodegradation and formation of leaf mats, the material is often mechanically degraded. However, in the high-energy fluvio-deltaic environments from the Blackhawk plant material was not mechanically degraded prior to burial. This suggests that transportation of plant material in these environments was minimal or that it was not vigorous enough to cause mechanical damage.