Investigations of the Late Eocene Castle Rock Conglomerate, 1869 to the Present, Including Recent Research on its Diagenesis
Steve Keller and Mark Longman
Colorado Geological Survey (CGS) and Denver Museum of Nature and Science
Abstract
The Castle Rock Conglomerate CRC) occurs in the Colorado Piedmont, specifically in Douglas and Elbert Counties, and is the uppermost and youngest Cenozoic unit found in that region. The formation covers a relatively small, northwest-to-southeast-trending area. It is well exposed and topographically prominent, forming flat mesas, steep cliffs, and narrow canyons. The conglomerate is a fluvial unit deposited by a wide braided-stream system. Depositional features such as large-scale cross bedding, large angular blocks of tuff, a variety of lithologies in other clasts, incised channels, fining-upward sequences, and fossil logs are readily observable. Because the conglomerate is both geologically and scenically striking it has interested geologists since the late 1860s.
Improved access to the unit over the last 60 years (e.g., in Castlewood Canyon State Park and in county and municipal open spaces), has increasingly attracted educators, students, and the public. This talk will present a chronology of geologic investigations (description, nomenclature, mapping, and paleocurrent studies) in the unit, and summarize deposition, geologic history, and age as presented by various investigators over the last 150 years, including recent work.
One puzzle that has long gone unstudied is “Why is the CRC so resistant to weathering that it forms these magnificent buttes and canyons?” Petrographic study of some of the finest-grained sandstones in the unit reveal ubiquitous amorphous silica (opal) cements followed in many places by radial-fibrous length fast chalcedony. The opal cements may be either isopachous (indicating precipitation below the water table) or pendant (above the water table), which indicates very early opal precipitation at very shallow, near-surface conditions. The source of the silica is the slightly older Wall Mountain Tuff into which many CRC channels incised. Despite its relatively young age, the hardness and extent of these silica cements account for the CRC being so well exposed in most areas where it is present.
Bio
Steve received a B.A. (1972) and an M.S. degree (1974) in geology from the State University of New York and a Professional Master’s degree (1992) in hydrogeology from the Colorado School of Mines (CSM). From 1975 through 2013 he was employed in geologic mapping, minerals exploration and consulting, the Yucca Mountain Project, environmental site investigations, and long-term groundwater monitoring. This work was in many parts of the U.S. and in several foreign countries. From 2007 to 2013 he was a senior associate with Behre Dolbear Minerals Advisors. His association with the Colorado Geological Survey (CGS) began in 2006. With Matthew Morgan (present CGS Director), Steve completed a comprehensive paleocurrent study of the Castle Rock Conglomerate and, with other CGS personnel, contributed to CGS minerals investigations. Beginning in 2016 he has been the lead geologic mapper for seven 7.5’ quadrangles in the northern Colorado Piedmont (mainly late Quaternary alluvial and eolian deposits). Since 2013 he has given a Van Tuyl lecture at CSM, given two Geological Society of America (GSA) conference talks and been coauthor on two others, led a GSA field trip in the Castle Rock Conglomerate, and served as field trip co-chair for the GSA 2016 conference in Denver.
Biosketch for Mark Longman
Mark received his B.A. degree from Albion College (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 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. Mark then joined the Denver Museum of Nature and Science as a Research Associate and continues to work on various projects with the Museum including his recent work on the Castle Rock Conglomerate. He specializes in the description of cores, outcrops, and petrographic thin sections with a focus on integrating sedimentology and petrology to interpret depositional environments and diagenesis.