Limnogeology and chemostratigraphy of carbonates and organic carbon from the Cedar Mountain Formation (CMF), eastern Utah

Montgomery, Elizabeth H.
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The Cedar Mountain Formation (CMF) is the oldest preserved Cretaceous continental deposit in the United States. This is the first study of the "Lake Carpenter" site which is a lacustrine sequence within the Ruby Ranch Member of the CMF near Moab, Utah. The study objective is to make paleoclimatic interpretations using mineralogy, lithology, and stable isotope analysis from a 30 m sequence. Furthermore, chemostratigraphy is used to constrain the age of the CMF in this locality. Maximum ages of detrital zircons at 2 and 5 m indicate the base of Lake Carpenter occurs near the Aptian-Albian boundary and continues into the Albian. Carbon isotope values of sedimentary organic carbon range from 21.1 to 32.3 /. Isotope values of carbonates range from 12.0 to +8.8 / for δ 13 Ccarb and from 8.9 to +1.8 / for δ18 O carb . The carbon isotopic values covary, signifying a hydrologically-closed lake. Below 7 m in the sequence, δ13 Ccarb is enriched due to removal of 12 C during photosynthesis and burial of isotopically light carbon, causing DIC enrichment. Above 7 m depletion in δ13 Ccarb accompanies lithological changes to organic-poor rocks. d18Ocarb values below 7 m are enriched due to evaporation, which is supported by dolomite occurrence. Dolomite is prevalent in upper sections, but evaporative enrichment of δ18 Ocarb is concealed by isotopically lighter high altitude runoff from the Sevier Thrust-Belt. Geochemical and mineralogy data support the MCDT placement at 1 m in the Lake Carpenter sequence.

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Cedar Mountain Formation, dolomite, lacustrine, Mill Canyon Dinosaur Tracksite, Ruby Ranch Member, stable isotopes