ICHNOLOGICAL ASSESSMENT OF DEPOSITIONAL ENVIRONMENTS IN THE CRETACEOUS DAKOTA GROUP, CAÑON CITY, COLORADO

The Dakota Group (DG) of Colorado (CO) has been well studied sedimentologically, however, the ichnology has been not fully analyzed. This project assessed the physicochemical parameters of these deposits using ichnology, as they record the initial flooding of the Western Interior Seaway (WIS). The DG in Cañon City, CO, is divided into the Lytle (LF), Plainview (PF), Glencairn (GF) and Muddy (MF) formations. The LF, a trough cross-bedded sandstone, contains Skolithos, and Rhizoliths on upper surfaces. The PF was deposited in fluvial, paralic, and subtidal settings, recording the initial flooding of the WIS in this area. The lower PF beds consist of deltaic and lagoonal deposits containing abundant plant debris, paleosols, and traces, including Lockeia. Influence of marine conditions is marked by the appearance of Teredolites in a bed that laterally contains Asthenopodichnium. The upper PF transition from subtidal to upper shoreface deposits is highly bioturbated with Ophiomorpha, Diplocraterion, and Chondrities, among others, grading from proximal to distal Skolithos ichnofacies. Above the PF, the GF is composed of six upward-coarsening parasequences: basal deeper-water shales shallowing upward through lower shoreface, and subtidal environments to tabular beach sandstones. The shales contain Helminthopsis and Chondrities and are consistent with Nereites ichnofacies, while the middle muddy sandstones containing Thalassinoides and Teichichnus, among others, is a Cruziana ichnofacies. The upper portion of each parasequence grades into highly bioturbated Skolithos ichnofacies. The uppermost formation, MF, is composed here of the Lower Channel Sandstone (LCSM) and Upper Transitional members (UTM). The LCSM is a progradational sequence from Cruziana to Skolithos ichnofacies topped by fluvial-shoreface deposits with plant material and wood debris containing Paleoscotylus. The UTM contains very fine sandstone, sandy shale, and shale. The uppermost shale is a Zoophycos ichnofacies consisting mostly of Rhizocorallium and Zoophycos, whereas the lower sandier beds are consistent with previous Cruziana ichnofacies-dominated beds. We have integrated trace fossil abundance and distribution with sedimentologic attributes to more accurately interpret EODs, and depositional histories in this area.