October Meeting: A chronology of research on the Castle Rock Conglomerate – Steve Keller, CGS

Date: Thursday, October 19

Times:

• 5:30 – 6:30 pm: Social Hour and Dinner
• 6:30 – 7:30 pm: Society Business and Presentation
• 7:30 – 7:45 pm: Raffle to raise money for student attendance!

Talk abstract:

The late Eocene Castle Rock Conglomerate occurs in the southwest part of the Denver Basin, mostly in Douglas and Elbert Counties, and is the youngest Cenozoic unit in that area. The formation exists as a swath of unconnected erosional remnants, trending northwest to southeast approximately from Sedalia to Calhan. The swath is ~65 km long and is ~3 km wide in the northwest and ~10 km wide in the southeast. The formation is well exposed and topographically prominent, forming flat mesas, steep cliffs, and narrow canyons. It is a fluvial unit deposited by a wide braided-stream system, and depositional features such as large-scale cross bedding, massive bedding, large angular tuff blocks,
other clast lithologies, incised channels, fining-upward sequences, and fossil logs are readily observable. Brontothere fossils are present but
rare. Because the conglomerate is geologically and scenically striking it has interested geologists since the late 1860s. Because of improved access to the unit over the last 60 years (in Castlewood Canyon State Park and in county and municipal open spaces), it 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 century and a half, including recent work. The talk also will treat recent research on the formation’s diagenesis. There are two major diagenetic
cements: an older opal cement forming coatings of fairly uniform thickness around the grains and precipitating before any grain compaction; and a younger chalcedony cement partially filling the pores between the grains and precipitating after opal cementation ceased. The cementation causes the conglomerate to be well indurated and resistant to erosion. The cements were derived, at least in part, from the Wall Mountain Tuff, which was incised by the conglomerate and clasts of which are common in the formation.

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