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The Paleocene/Eocene Colton Formation: A Fluvial-Dominated Lacustrine Deltaic System, Roan Cliffs, Utah
Exposures of the Colton Formation from Soldier Summit to Sunnyside, Utah, represent a classic progradational deltaic wedge that built into ancient Lake Flagstaff/Lake Uinta from the southeast. Deltaic facies change from the mudstone-dominated lower delta plain to the more sand-rich upper delta plain/lower alluvial plain from west to east over a distance of more than 40 mi (65 km).
In Emma Park and Whitmore Park the Colton Formation displays a variety of fluvial deltaic subenvironments within the progradational sequence. Subenvironments associated with this low gradient delta plain include interdistributary bay and mud flat mudstones, overbank and splay deposits, distributary mouth bars, and fluvial channel sandstone deposits. Along strike from west to east (Soldier Summit to Whitmore Park), the middle Colton section becomes progressively more sand-dominated. Sandstone percent within the section varies from less than 10 percent in Emma Park to greater than 50 percent in Whitmore Park. The section becomes even more sand-dominated near Sunnyside with the sandstone percentage reaching as high as 70 percent. Sandstone percent can be used to subdivide fluvial systems into suspended-load (low sandstone percent), mixed-load (intermediate sandstone percent), or bed-load (high sandstone percent).
Fluvial sandstone bodies are composed of either multilateral sandstone sets or a series of multistory to multilateral channel-fill sandstones or both. Multilateral sandstone sets signify lateral accretion of point bar deposits from meandering fluvial systems. Multilateral sandstone sets are more abundant in suspended-load fluvial systems and are rarely found in bed-load systems. Grain and sand body size are smaller in the suspended-load channel systems than in mixed- or bed-load channel systems. Suspended-load fluvial systems dominate the lower delta plain and represent small distributary channels. Mixed-load channel systems contain both multilateral point bar sets and multistory channel sandstones. Mixed-load fluvial channels represent both meandering and low sinuosity fluvial systems located on the intermediate and upper delta plain. Bed-load fluvial systems contain only multistory stacked channel-fill sandstones. The lack of multilateral accretion sets in bed-load fluvial systems suggests that rivers were sinusoidal geometrically and not meandering. Channel-fill sandstones interpenetrate one another three dimensionally. Bed-load fluvial systems represent the upper delta plain and the lower alluvial plain.
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