Origin of Late Turonian and Coniacian Unconformities in the San Juan Basin
Dag Nummedal, Gregory W. Riley, 1991. "Origin of Late Turonian and Coniacian Unconformities in the San Juan Basin", 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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The objective of this field research conference is to address basic issues in shallow marine sequence stratigraphy. One central issue is the origin of the many discontinuities (unconformities and diastems) we observe: Three possible mechanisms may produce such discontinuities: (1) eustatic sea level fall, (2) tectonic uplift or tilting of the basin, and (3) lateral shifts in depositional environments (the origin of most diastems). Late Turonian and Coniacian rocks of the western San Juan Basin provide an excellent section of rocks in which to address these questions, because we believe that the three major discontinuities present within this part of the stratigraphic column are examples of all these listed modes of origin.
The analysis of the origin of these unconformities is based on: (1) a synthesis of relevant biostratigraphic data (including much not previously published) in order to constrain their ages with the highest possible precision, (2) documentation of parasequence stacking patterns and depositional systems to relate unconformities to their correlative deposits, and (3) subsurface data that demonstrate local tectonic movement.
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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..