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The AAPG/Datapages Combined Publications Database

Grand Junction Geological Society

Abstract


Paleontology and Geology of the Dinosaur Triangle, 1987
Pages 21-35

Cretaceous Rocks of the Dinosaur Triangle

R. D. Cole

Abstract

Rocks in the Dinosaur Triangle record much of the Early and Late Cretaceous history of western Colorado and eastern Utah. The oldest Cretaceous rocks are probably late Aptian or early Albian in age, whereas the youngest span the Cretaceous-Tertiary boundary. The setting for deposition of Cretaceous strata in the Rocky Mountain region was created in Late Jurassic time during early phases of the Sevier orogeny, which created a north-south zone of compressional folds and overthrusts that extended from Arizona and Nevada through western Utah and Idaho and farther north. The Sevier orogeny was directly responsible for creating the Rocky Mountain geosyncline, the site of a large, shallow epiric sea during much of the Cretaceous. The geosyncline was structurally deepest along a foreland basin immediately adjacent to the Sevier orogenic belt where eventually more than 19,000 ft of nonmarine, marginal-marine, and marine sediment were deposited. Variations in tectonic activity in the orogenic belt, coupled with basin subsidence and changes in eustatic sea level, produced numerous cycles of regressive and transgressive sedimentation.

From a historical perspective, Cretaceous rocks in the Triangle area can be subdivided into 10 major events, triggered either by tectonism or eustatic sea-level fluctuations or a combination of these factors:

1. Nonmarine deposition associated with the initial phase of tectonism in the Sevier orogenic belt (Lower Cretaceous Cedar Mountain and Burro Canyon Formations)

2. A transition from nonmarine to marine deposition in association with initial advance of the Cretaceous sea (Lower/Upper Cretaceous Dakota Sandstone).

3. Inundation of the area by transgression of the sea in response to eustatic sea-level rise, with resultant deposition of marine muds (lower Turonian Tununk Shale Member of Mancos Shale).

4. Retreat of the sea due to a major drop in relative sea level in middle to late Turonian time, resulting in deposition of a coastal sandstone complex in the western Triangle area (Ferron Sandstone Member of Mancos Shale).

5. A rapid transgression of the sea due to eustatic sea-level rise, producing a second episode of marine deposition (late Turonian through late Santonian Blue Gate Shale Member of Mancos Shale).

6. A regression in late Santonian time that produced coastal deposition in the western Triangle area (Emery Sandstone Member of Mancos Shale).

7. A third major transgression of the sea in latest Santonian time that deposited additional marine muds (upper Blue Gate Shale Member of Mancos Shale).

8. A series of four major Campanian thrust episodes in the Sevier orogenic belt that produced large volumes of synorogenic sediment that eventually drove the Cretaceous sea from the area in latest Campanian time. These events created the Star Point Sandstone, Blackhawk Formation, Castlegate Sandstone, and Price River Formation.

9. Transition from Sevier-type to Laramide-type tectonism in Maestrichtian time with the creation of a major unconformity due to uplift of the San Rafael Swell and Douglas Creek Arch.

10. Deposition of lacustrine, fluvial and alluvial-fan strata in the northwest part of the Triangle beginning in late Maestrichtian time and continuing into the Tertiary (North Horn and Currant Creek Formations).


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