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A Late Turonian Anoxic Episode in a Portion of the Western Interior Seaway: The Juana Lopez Member of the Mancos Shale in the Southeastern Uinta Basin, Utah and Colorado

Anderson, Donna S.1; Harris, Nicholas B.1; Waanders, Gerald L.2; Fisher, Ryan G.3
1 Dept. Geology & Geol Engr, Colorado School of Mines, Golden, CO.
2 Consulting Palynologist, Encinitas, CA.
3 Norwest Questa Engineering, Golden, CO.

While Cretaceous global oceanic anoxic events (OAE I, II, III) figured prominently in the regional occurrences of organic-rich mudrocks in the western interior Cretaceous seaway, there were important subregional events between OAE episodes during which significant amounts of organic carbon were deposited. One such event occurred during the early late Turonian stage within the lowermost Mancos Shale in the southeastern Uinta basin of Utah and westernmost Colorado.

In this study, lithofacies and biostratigraphic observations were combined into a sequence stratigraphic framework and integrated with organic geochemical analyses to interpret the basal 650 ft (198m) of the Mancos Shale in a study area encompassing a portion of the Book Cliffs of Utah, the southern and eastern Uinta basin, and the western flank of the Douglas Creek Arch of Colorado. A 305-ft (93m) measured section near Westwater, Utah and a 262-ft (80m) measured section near Dinosaur, Colorado yield a sequence stratigraphic framework correlated to nearby wells via gamma ray curves. Geochemical and palynologic analyses from well cuttings provide critical data not discernable by macroscopic observation. Over 300 wells and associated well logs facilitate regional stratigraphic correlation.

The most organic-rich mudrocks in the study exist within a 110-ft (33m) thick interval that correlates to the Juana Lopez (early late Turonian) Member of the Mancos Shale on the eastern San Rafael Swell. The interval, within the 650-ft thick lower Mancos study section, has a highly restricted marine fauna, the highest TOC (>3 wt %) and hydrogen index between 350 and 450 values, and it tracks the Type II kerogen trend. The organic-rich interval was deposited during a 2nd to 3rd-order transgression within a restricted embayment created by coalesced coeval delta-fronts on the landward margin of the embayment and a forebulge uplift on the seaward margin. The subregional anoxic episode was <<500,000 years long and occurred between OAE II (early Turonian) and III (Coniacian). This reconstruction suggests that paleobathymetry can set up locally favorable conditions for organic carbon deposition at times outside the range of the global anoxic events; we infer that at such times, nutrient delivery from land sources combined with restricted bottom water circulation to create a setting in which moderate bioproductivity combined with moderate preservation rates.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009