The Codell: A Thin, Laterally Extensive Sandstone in the Denver Basin
Presented by:
Mark Longman
Discussion Starts at 12:00 (MT)
Webinar
MARK LONGMAN, RICHARD BOTTJER, GINNY GENT, and JAMES HAGADORN
Department of Earth Sciences, Denver Museum of Nature & Science
ABSTRACT
Building on the studies of many others and integrating new regional stratigraphic, petrographic, and sedimentologic data we interpret the depositional processes and environments of the mid-Turonian Codell Sandstone across the Denver Basin. Here it is predominantly a thin (<40-ft) succession of highly burrowed coarse siltstone to very fine sandstone. Grain sizes generally coarsen up-section and to the northwest, and are dominated by quartz and chert grains. We hypothesize that these grains were reworked from a major deltaic source in northwestern Wyoming, and transported into the Denver Basin area by waxing and waning south-flowing currents on the nearly flat floor of the Western Interior Seaway, which we call a marine “shelf.” These shelf currents distributed the Codell across an area of more than 100,000 mi2 in eastern Colorado, southeastern Wyoming, western Kansas, and much of Nebraska, where they were further reworked and mixed by a combination of biological and physical processes. Although Codell-equivalent strata extend across parts of Wyoming and the Dakotas, and east to Minnesota, this presentation focuses on the Denver Basin with its many excellent wireline logs and cores.
Factors offering insight into the depositional environments of the Codell include:
1. It is thin (<80 ft; mostly <40 ft) across most of the Denver Basin except in part of the central basin where it has been removed by erosion. Some would call its geometry “blanketlike."
2. Bioturbation is abundant and pervasive in most of the Codell, but decreases toward the northwest where both grain size and inferred current energy during deposition increase.
3. Despite appearing poorly sorted, the Codell consists of homogenous very small grains that are well sorted and mostly of coarse silt to upper very fine sand size (40-100 microns).
4. The apparent poor sorting is due to a “textural inversion” created by bioturbation mixing up to 25% detrital clays, mainly illite-smectite & illite-mica, into the well-sorted silt and sand grains.
5. The sand grains are mineralogically mature with quartz, chert, and some feldspars, but with few rock fragments other than chert and no unstable minerals (e.g., hornblende, augite).
6. Its detrital zircon population is atypical with predominantly “young” 98-93 Ma zircon crystals derived from weathering of units such as the Arrow Creek, Clay Spur, “X,” and other bentonites.
7. It has an anomalously high phosphorous content compared to adjacent strata. The phosphorous is in abraded bone and teeth fragments suggesting a reworked marine vertebrate fauna.
8. Other than sparse oyster and Inoceramus shell fragments, the Codell is remarkably free of calcareous fossils, particularly intact ones, when compared with the adjacent intervals. This complicates its biostratigraphic dating and documenting the duration of its deposition, which we believe to be quite short, probably less than 0.2 Ma (<than the P. hyatti ammonite zone).
Conclusion: The Codell is a reworked shallow marine shelf deposit covering >100,000 mi2.
Mark received a B.A. degree from Albion College in Michigan in 1972 followed by a Ph.D. in Geology from the University of Texas at Austin in 1976. He then joined the research lab of Cities Service Company in Tulsa, Oklahoma for 5 years before moving to Denver in 1981 to work for Coastal Oil and Gas Company as an exploration geologist mainly in the Williston Basin. From 1984 to 2006, he was a consulting geologist before joining QEP Resources, where he worked as their “Rock Expert” until 2018 when QEP restructured and no longer needed his skills as a sedimentologist and petrographer. Mark enjoys looking at cores, cuttings, and petrographic thin sections with a focus on integrating sedimentology and petrology with reservoir studies and petrophysics. He has worked extensively on the Cretaceous rocks and reservoirs in the Rocky Mountain region.
Mark is currently a Research Associate at the Denver Museum of Nature and Science and a part-time consulting geologist. He has published more than 40 papers in journals ranging from RMAG’s The Mountain Geologist to the AAPG Bulletin and in 2014 co-edited AAPG Memoir 107 on Pinedale Field. In 2016 he received AAPG’s Distinguished Service Award.
Click the Dunes to Register Now!
Webinars are free to attend
Please submit reservations by 10:00 a.m (MT) the day before the talk.
Email questions to information@rmssepm.org