ASSESSING THE PROVENANCE OF CAMPANIAN VERTEBRATE-BEARING CARBONATES FROM THE EGG MOUNTAIN LOCALITY

Egg Mountain is a late Campanian (~75 Ma) fossil-bearing locality from the Two Medicine Formation of Montana. It is known for its abundant, and in some cases exceptionally well preserved dinosaur nests and mammal remains. These vertebrate remains are preserved in a sequence of blue-grey, strongly carbonate cemented mudstones. The origin of these mudstones has been debated: original interpretations suggested they were lacustrine in nature, but more recent data support a sub-aerial origin, with potential diagenetic overprinting

Using a combination of sedimentological, petrographic and isotopic analyses we have better constrained the nature of a 2m stratigraphic section of the upper Egg Mountain carbonates. The section shows a sequence of alternating mudstone horizons differentiated primarily by their degree of carbonate cementation. The stable isotope geochemistry of carbonates from terrestrial settings is controlled by a number of factors, including climate, biotic effects, and source water chemistry, that interact in complex manners. These primary geochemical signals can also be diagenetically modified post-burial. Carbon and oxygen isotope ratios were measured from samples from each horizon using a carbonate mass spectrometer. The average δ¹⁸O for the Egg Mountain carbonates is -14.85, with a range from -14.26 to -16.17, with the observed uniform, negative values suggesting likely diagenesis from meteoric water. Carbon isotopic ratios are, however, less susceptible to alteration through such diagenesis, so may preserve a primary environmental signal. Average δ¹³C for the samples is -7.91, with a range from -6.34 to -8.61. These values are markedly more negative than those usually associated with lacustrine systems from environments similar to the Two Medicine, but similar to those found in soil carbonates deposited in arid environments. In combination with the lack of sedimentological and petrographic indicators of lacustrine deposition, we interpret the Egg Mountain carbonates as primary pedogenic, but overprinted by diagenesis after burial. This interpretation of the depositional environment of Egg Mountain necessitates the reinterpretation of the palaeoecological information preserved in its diverse fossil assemblage to reflect a subaerial genesis.