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Abstract

The purpose of this field trip is to examine the sedimentology, sedimentary architecture, stacking patterns, and correlation of fluvial, coastal plain, deltaic, and shoreface to shelf deposits in the low accommodation Desert Member to Castlegate Sandstone stratigraphic interval (Campanian), Book Cliffs, eastern Utah. Traditional sequence stratigraphic models of falling stage deposits will be tested against an alternative sequence stratigraphic model that links the nonmarine to shallow-marine facies belts in both time and space. The trip will focus on the exceptional three-dimensional outcrop exposures in the Thompson Pass to Sagers Canyon region. At least eight sequence stratigraphic rock packages are identified and correlated, and these combine to form two progradational parasequence sets. The lower set comprises Desert Member rock packages 1-7, while the upper set comprises the SM/CC-2 rock package and overlying nonmarine strata of the Castlegate Sandstone. Rock package 7 is bounded above and below by coal-bearing, carbonaceous-rich zones. Correlation of the Upper and Lower Coal zones across the Crescent Canyon to Blaze Canyon region establishes a clear chronostratigraphic link between the nonmarine and shallow-marine strata of the Desert-Castlegate interval. The “fusing/welding” of rock packages 7 and SM/CC-2 with underlying cliff-forming sandstones in the South Face-Central to South Face-East region of Horse Heaven further demonstrates the chronostratigraphic link between the nonmarine and shallow marine.

The alternative sequence stratigraphic interpretation of the Desert-Castlegate interval connects the nonmarine and shallow-marine facies belts in time and space, through correlation of coals, marine flooding surfaces, multi-storey channel complexes, and falling stage shallow-marine successions. “High-frequency” fluvial incision surfaces (sequence boundaries) merge to form a diachronous, lithostratigraphic contact between the nonmarine and shallow-marine facies belts. These contacts were previously defined as the “Desert SB” and “Castlegate SB” by J.C. Van Wagoner and were linked to longer term sea-level falls. Subsequent analysis of the “Desert SB” has revealed an amalgamation of “high-frequency” sequence boundaries that merge and split in a complex manner. Each “high-frequency” surface represents a shorter term portion of the longer term falling sea-level curve, which is equivalent to a parasequence-scale relative fall in sea level. “High-frequency” sequence boundary development, forced regression, and minor coastal plain aggradation occurs during the falling limb of the shorter term sea-level curve, followed by shallow-marine flooding, valley in-fill, coastal plain aggradation, and regional coal deposition during the rising limb of the shorter term sea-level curve. Individual nonmarine to shallow-marine chrono-slabs stack together. Inter-slab incision is rare in shallow-marine sections and where noted is relatively gentle (<3 m incision). In contrast, inter-slab incision is more significant in coastal plain settings due to reduced accommodation under falling stage conditions. The chrono-slab, parasequence-scale model for falling stage deposits in the Desert-Castlegate interval has four facies belts or zones. These record the transition from (i) nonmarine settings with single storey channels and scattered multistorey channel-fill complexes, to (ii) large-scale, multi-storey channel-fill successions (IVFs), to (iii) interbedded large-scale, multi-storey channel-fill successions (IVFs) and sandstone-rich proximal shallow-marine deposits, to (iv) proximal to distal, shoreface to shelf parasequences. This alternative chrono-slab model is likely applicable to falling stage deposits worldwide, especially those within a foreland basin setting. Ongoing research will extend the chrono-slab correlations outside of the study area and examine the relationship between relative sea-level history and chrono-slab thickness, facies belt length, and number of facies belts per chrono-slab.

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