Donald E. Owen, 1991. "Road Log, Day Five: Notes on Selected Aspects of the Sequence Stratigraphy of the San Juan Mountains, Colorado", Sequence Stratigraphy Applications to Shelf Sandstone Reservoirs: Outcrop to Subsurface Examples, J. C. Van Wagoner, C. R. Jones, D. R. Taylor, D. Nummedal, D. C. Jennette, G. W. Riley
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OBJECTIVES: The scale of sequence recognition shifts to major first-order sequences, which may be called Sloss sequences or megasequences. The Kaskaskia-Absaroka sequence boundary is seen at STOP ONE and the Zuni-Tejas sequence boundary is seen at STOP THREE. A higher order sequence boundary within the Absaroka sequence that has been enhanced by local tectonic activity producing an angular unconformity is seen at STOP TWO. An objective today is to emphasize the effects of recurrent movement of basement faults on stratigraphy and sequence boundaries. These effects may be seen on outcrop in the San Juan Mountains, but their effects continue in a more subtle manner in the subsurface of the San Juan Basin, the site of the next two days of stops.
Leaving Grand Junction, drive south to US 50 east. The road log begins at the junction of US 50 and CO 146 approximately eight miles southeast of Grand Junction.
0.0 Junction of US 50 and CO 146 near Whitewater. View of Book Cliffs to north and west, Uncompahgre uplift to west, and Grand Mesa (approximately 10,000 feet in elevation) to east, capped by Pliocene (9 Ma) lava flows. The road between here and Delta is in the lower Mancos Shale. The underlying Dakota Sandstone and Burro Canyon Formation may be seen to the west in the Gunnison River valley and close along the road in places. The rest of the Mancos Shale forms the hills to the east
31.0 Delta. Confluence of Gunnison and Uncompahgre rivers. Continue on US
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The Lower Sego provides an opportunity to study well-exposed, high-frequency sequences and their systems tracts. Criteria for identification of sequence boundaries will presented. Sequences and their boundaries will be contrasted with parasequences and their bounding surfaces. The Upper and Lower Sego contain well-exposed tidal deposits within the lowstand systems tracts of high-frequency sequences. These tidal deposits and their relationship to incised valleys and systems tracts will be examined. The incised valley interpreted to form during relative falls in sea level will be contrasted with distributary channels related to autocyclic mechanisms.
0.0 Leave the parking lot of the Grand Junction Hilton. Grand Junction , Colorado. Turn left onto Horizon Drive. Pass under the 1-70 bridge, Turn left into the entrance ramp for 1-70 west.
0.2 Enter 1-70 heading west toward the Colorado –Utah State line. For the next 20 miles the Interstate will parallel the Colorado River flowing along the west side of the Grand Valley. The Interstate is built on the gray Cretaceous Mancos Shale. To the west of the Colorado River are the red cliffs of the Colorado National Monument. The Monument is operated by the National Park Service. These cliffs are the eastern edge of the Uncomahgre Uplift. As you drive north along the Interstate, the steeply dipping eastern limb of the Uncomahgre is clearly visible. This tight monoclinal fold is the result of horizontal compressional tectonics associated with Laramide deformation (Heyman, 1983). The red rocks in the Mounment include, from stratigraphically oldest to youngest: the Chinle Formation forming the lower, less resistant slops, the Wingate formation forming the massive cliffs up to 400 feet thick , the Kayenta Formation overlain by an unconformity along which the Navajo and Curtis Formations are missing , the Summerville and Entrada Formations, and the Morrison Formation consisting of fluvial sandstones and associated mudstones, within which some of the earliest dinosaur bones in North America were discovered in the late 19th century. The Jurassic Morrison Formation is unconformably overlain by the brown, Cretaceous, coal-bearing Dakota Sandstone. The Dakota caps many of the high mesas within the Mounment and forms well-exposed dip slopes along the Interstate in the vicinity of the exit to Mack, Co..