DIGGING INTO BONEBEDS: A QUANTITATIVE METHOD FOR ANALYSIS OF DINOSAUR DEPOSITS USING QEMSCAN, KAIPAROWITS FORMATION, UTAH

Two chasmosaurine ceratopsid dinosaur bonebeds in the Upper Campanian Kaiparowits Formation of southern Utah were compared to discern how fossil preservation varies in relation to depositional and biostratigraphic factors. This study illustrates a new methodology using quantitative sedimentologic and taphonomic data to analyze sedimentary facies of bonebeds. The fossil assemblages contain disarticulated elements that accumulated in a lag deposit at the base of a sandy river channel and articulated elements and skeletons in a fine-grained sandy siltstone crevasse splay deposit. Lithologic samples were analyzed using QEMSCAN, a rapid SEM-EDS system that provides high resolution spatial mapping and numeric 2D mineralogical information from abundant point count data to characterize mineral types, proportions and associations in the host rock. Modal mineralogy in the lower part of the bonebeds suggests a similar suite of 5 dominant minerals totaling >95% of the sampled material: overall mineral abundance is dominated by feldspar and quartz (45-79 wt.%) followed by muscovite, calcite and dolomite (19-50 wt.%). One notable pattern is the greater relative abundance of feldspar over quartz for the bone layer in the crevasse splay deposit, while higher values in the channel lag are at the quarry base and above the bone unit. This presumably reflects more immature composition and less distance of transport in the crevasse splay. Average values of muscovite in sandstones are usually less than 3%, however in this study they range from 2-14% and suggests higher amounts of metamorphic and volcanic minerals were brought in by ground water from nearby sources. High values of calcite (>30 wt.%) were recovered from below the bone layer in the channel lag and above the bone layer in the crevasse splay, suggesting higher porosity values and early cementation from fluid flow in these beds. Only one fine-grained sandstone unit from each site has higher levels of dolomite: the channel bone layer and the unit above the bones in the crevasse splay. This suggests greater open pore space, as well as alteration of other minerals. The information gained from QEMSCAN aids in understanding the key components of the behavior of the source material with the aim of reconstructing the regional paleoenvironment of chasmosaurine dinosaurs.