Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.
Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Stratigraphy, sedimentology and depositional environments of the Upper Jurassic Morrison Formation, Colorado Front Range
Abstract
The Upper Jurassic Morrison Formation intermittently crops out along the Colorado Front Range. Morrison Formation sediments are predominantly composed of gray mudrock intercalated with minor lithographic limestone and variegated fluvial deposits. Lacustrine sediments are volumetrically more important than previously reported. The uppermost Morrison consists of greenish-gray and variegated tuff. I have used the lowest stratigraphic occurrence of the massive tuff bed as a lithostratigraphic datum for facies correlation. Identification of this tuff bed allows the first regional correlation of Morrison rocks outside the Colorado Plateau. Charophytes and ostracodes within Morrison lacustrine deposits indicate Morrison lakes were shallow, fresh bodies of water. Ubiquitous evidence of subaerial exposure within lacustrine sediments suggests lakes experienced frequent water level fluctuations. Previous studies on symmetrical ripples in New Mexico and southern Colorado indicate lakes may have been tens of miles across. Previous geologic studies in the northern Denver Basin have determined that movement along basement fault blocks has controlled sedimentation patterns in rocks adjacent to the Morrison Formation. Thick Morrison sections above pre-Morrison paleo-highs suggests syndepositional subsidence has also affected Morrison sedimentation patterns. Morrison Formation contacts are commonly ambiguous. Where Morrison lacustrine deposits overlie marine deposits the contact is clearly disconformable. Where Morrison fluvial deposits overlie Ralston Creek fluvial deposits the contact is enigmatic, and I have mapped both units in these areas as Morrison undifferentiated. The upper Morrison contact with the Dakota Group is also difficult to delineate. Previous studies have suggested this contact represents a 30 Ma hiatus. Because the depositional environments and lithologies of the two formations are similar, there is no sharp contact, rather only a conformable sequence with minor disconformities. Thin Upper Jurassic sections onlapping Precambrian rocks suggest the pre-Morrison depositional surface was not flat. As much as 100 feet of topographic relief may have been present between Precambrian highs and adjacent depositional basins.
Subject Area
Geology
Recommended Citation
Johnson, Jeffrey Steven, "Stratigraphy, sedimentology and depositional environments of the Upper Jurassic Morrison Formation, Colorado Front Range" (1991). ETD collection for University of Nebraska-Lincoln. AAI9121923.
https://digitalcommons.unl.edu/dissertations/AAI9121923