A PALEOENVIRONMENT ANALYSIS OF ONCOIDS FROM THE LOWER CAMBRIAN CHAMBLESS LIMESTONE OF THE MARBLE MOUNTAINS, CA

The Lower Cambrian Chambless Limestone Formation is located in the Marble Mountains of the Mojave Desert in eastern California, and is well known for containing, large, well-developed oncoids, which are spherical to oblate microbial allochems that form from the precipitation of calcium carbonate during algal photosynthesis. Previous research conducted on the oncoids from the Chambless Limestone by Foster (2011) and Unal and Zinsmeister (2007) have noted that the oncoids are porostromate and contain Girvanella. Foster (2011) and Unal and Zinsmeister (2007) proposed that the oncoids found within the Chambless Limestone formed under quiet conditions based on: 1) the large size of the oncoids, which average 2-4 cm in diameter, including the largest examples found near the base of the Chambless Limestone that were likely disturbed only during storm events; 2) the micritic nature of the matix; and, 3) the presence of archaeocyaths, which indicate water depths of 20 m to 50 m (Hill, 1964; Foster, 2011). Closer analysis of the oncoids and surrounding sedimentary rocks of the Chambless Limestone provide an opportunity to test the hypothesis of Foster (2011) and Unal and Zinsmeister (2007) that the oncoids indeed formed under quiet conditions. A detailed stratigraphic section of the Chambless Limestone was measured, and the unit was broken up into smaller subunits based on lithological changes. Samples were removed from each subunit that will be slabbed and examined in thin sections. Initial results indicate that larger and oblate oncoids are found in the upper and lower portions of the Chambless Limestone, while smaller and round oncoids are found in the central portion of the formation. Further study of collected samples at the macroscopic and petrographic scale will examine both the oncoids and surrounding rocks to precisely determine the depositional setting, and will also seek to better describe the oncoids, including defining their shape, determining their nuclei, and examining their cortices to establish if the layers that comprise their cortices are even or irregular. Overall, this study hopes to gain a better understanding of the conditions that led to the growth of oncoids, and how microbialites fit in to early metazoan ecosystems.