SEDIMENTARY, PETROGRAPHIC, AND GEOCHEMICAL EVIDENCE FOR BENTHIC MICROBIAL MATS AND A REFINED MUDSTONE STRATIGRAPHY FOR THE NEOPROTEROZOIC KWAGUNT FORMATION, CHUAR GROUP, GRAND CANYON

Integrated study of the sedimentology, stratigraphy, and geochemistry of the mudstones of the Chuar Group reveals many important details of the break-up of Rodinia, the rise of early macroscopic eukaryotes, and conditions leading up to the first Neoproterozoic Snowball Earth episodes.

Mudstones of the Kwagunt Formation (~500 m thick), Chuar Group, accumulated in an intracratonic, marine rift basin on the western edge of Laurentia. They display a wide range of sedimentary and geochemical characteristics that suggest that organic carbon burial in Kwagunt Formation was mediated by benthic microbial mats. Pseudo-cross laminated structures, carbonaceous lenses, crinkly, silty, anastomosing and discontinuous laminations occur in deeper-water intervals, and fenestral laminations, roll-up features and pustular surfaces, indicative of mat desiccation, are more common in shallower regions. The occurrence of even-over-odd preferences in n-alkane distributions and possible monomethyl-alkane series support the interpretation of theses sedimentary structures as benthic microbial mats with associated sulfide-oxidizing bacteria and accord with previous observations of quaternary-branched-diethylalkanes and monomethyl-alkane series by other researchers.

Detailed mudstone stratigraphy displays two modes in the relation between total organic carbon (TOC) and silt content. The bulk of the stratigraphic section (~425 m) is TOC-poor (0-5 %TOC) and displays a positive relation between silt content and TOC. Two TOC-rich (> 5 %TOC) intervals within the Walcott Member, the uppermost member of the Kwagunt formation, demonstrate an inverse relation where silt content is to proportional to 1/%TOC. These two trends are attributed to variations in basin evolution, sediment distribution and dilution, along with increased primary production and more reducing conditions that enhance carbon burial. Greater %TOC associated with increased silt delivery may be the result of increased riverine nutrient delivery or chemocline rise during wetter conditions. The two TOC-rich intervals within the Walcott Member are likely a result of basin deepening and expansion that traps silt nearshore which fosters the deposition TOC-rich black shales ( >5% TOC) by limiting siliclastic dilution.