Sequence Stratigraphy of the Upper Cretaceous Sego Sandstone Member Reveals Spatio-Temporal Changes In Depositional Processes, Northwest Colorado, U.S.A

Abstract

The Upper Cretaceous Sego Sandstone Member of the Mesaverde Group has been extensively studied in the Book Cliffs area of Utah and Colorado, and has been the focus of stratigraphic reconstruction aimed at developing an understanding of the evolution of the Western Cretaceous Interior Seaway. The Sego Sandstone Member was deposited in a marginal marine, tide-influenced environment of the Cretaceous Seaway. This study documents the sequence stratigraphy of the Sego Sandstone Member in northwestern Colorado, just north of Rangely, and compares and contrasts it with equivalent strata in the Book Cliffs area in Utah. The Sego Sandstone Member in the study area contains three sequences characterized by progradational and aggradational stacking patterns. The stratigraphically lowest sequence consists of a prograding, tide-influenced delta overlain by marine mudstones, which represents a retrogradation and flooding surface. The second sequence is composed of multiple parasequences and consists of an incised valley filled with stacked tidal bars which then pass into a largely aggradational stacking pattern, composed of barrier-island deposits with back-barrier, flood-tidal-delta deposits, and wavedominated-shoreface deposits. The third sequence is a broad, tide-dominated distributary-mouth system with a sharp, incisional basal contact. The three sequence boundaries documented in northwestern Colorado are consistent with the three main sequence boundaries identified in the Book Cliffs. However, whereas barrier islands and flood-tidal deltas are characteristic of the Sego Sandstone Member in northwestern Colorado, similar deposits are not as prevalent in the Book Cliffs of Utah, suggesting different depositional processes and paleogeography. Tidal and fluvio-deltaic processes are the dominant controls on deposition of the Sego Sandstone Member north of Rangely, Colorado. The transition from a tide-dominated fluvio-deltaic system to a mixed wave-tide-influenced coastline indicates a fundamental change in processes and depositional environment in the upper part of Sequence 2. Such change from a prograding fluvio-deltaic system to a more passive tide-modified coastline is not observed in the Book Cliffs, and may be the result either of large scale transgression or of relocation of the river system through large-scale avulsion, which is not observed in the Book Cliffs. Our study shows significant stratigraphic variability between rocks exposed in the Book Cliffs versus time-equivalent rocks exposed in northwestern Colorado in the Upper Cretaceous, which has implications for the regional basin architecture and stratigraphic correlations.

Fluvial deposition during transition from flexural to dynamic subsidence in the Cordilleran foreland basin: Ericson Formation, Western Wyoming, USA

“…This interpretation is consistent with independent structural geological interpretations suggesting that the Uinta uplift was rising beginning in the Late Cretaceous (Bradley, ; Gregson and Erslev, ). Southeastward palaeocurrent indicators from the Campanian Sego Sandstone exposed south of the Uinta Mountains (Willis & Gabel, , ; Painter et al ., ) are also consistent with late Campanian Uinta uplift, and southward palaeocurrent indicators from the Currant Creek Formation indicate that deformation of the Uinta uplift was well underway by Maastrichtian time (Isby & Picard, ).…”

Shelf-edge trajectories and stratal stacking patterns: Their sequence-stratigraphic significance and relation to styles of deep-water sedimentation and amount of deep-water sandstone

“…Changes in regional process regimes and deep-water sedimentation styles may result in significant changes in stratal stacking patterns that can be tied to specific types of shelf-edge trajectories (e.g., Yoshida et al, 2007;Catuneanu et al, 2009Catuneanu et al, , 2011Dixon et al, 2012a, b;Painter et al, 2013). Recognition of shelf-edge growth styles and their relation to process regime changes are useful in predicting volumes of coeval sandstone on the slope and basin floor (Dixon et al, 2012a, b).…”

Modern to Ancient Barrier Island Dimensional Comparisons: Implications for Analog Selection and Paleomorphodynamics

“…It is not feasible to measure strictly the subaerial portion of ancient barrier island deposits because the water line moves through time and deposits are potentially subject to removal during ravinement. Additionally, the foreshore is rarely specifically designated in literature examples (Allen and Johnson, 2011;Kieft et al, 2011;Painter et al, 2013). Inversely, subaqueous width measurements for the modern are not feasible at a global scale because they are inhibited by the scarcity of available data and the difficulty in defining, constraining, and documenting the subsurface back-barrier and shoreface boundary without ground penetrating radar or seismic data (Jol et al, 1996;Daly et al, 2002;Wernette et al, 2018).…”