222-2 Ubiquity of Parasequence-Level Shoreface-Incised Channels in the Blackhawk Formation to Lower Castlegate Sandstone Stratigraphic Interval, Campanian Book Cliffs, Utah: Sequence Stratigraphic Implications and Significance

Session: Delta Evolution from Rivers to the Shelf: Past, Present and Future Perspectives for Society (Posters)

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There are 39 parasequences in the Blackhawk Formation to Lower Castlegate Sandstone stratigraphic interval with 33 of 39 (85%) cut by estuarine-tidal-fluvial shoreface-incised channels (SI-CHs).  Most SI-CHs have similar facies, facies associations, multi-story stacking patterns (2 to 5 stories), channel dimensions, and depth of incision.  Heterolithic-filled SI-CHs are the largest.  Notable examples of SI-CHs occur within the Desert (D) Member to Lower Castlegate (C) Sandstone interval at the D4, D5, D6, D9, D10, D11, C2, C3, and C4 parasequence levels.  The thickest multi-story SI-CH is 30.1 m (C3).

Conventional sequence stratigraphic models focus solely on the estuarine-tidal-fluvial channel-fill successions at the top of progradational parasequence sets, interpreting these as sequence boundary (SB) based incised valley-fill (IVF) deposits.  Previous studies have interpreted 7 of 33 SI-CHs (21%) in the Blackhawk-Castlegate interval as SB-based estuarine-tidal-fluvial IVFs.  As such, the majority of SI-CHs in the Blackhawk-Castlegate interval (i.e., 26 of 33; 79%) are disregarded from a sequence stratigraphic perspective, despite their similar facies, facies associations, multi-story stacking patterns, dimensions (i.e., width, depth), and depth of incision.  SI-CHs occur at various levels within aggradational, progradational, and retrogradational parasequence sets and therefore are independent of stratigraphic level.

Parasequence-level shoreface-incised estuarine-tidal-fluvial channel-fill successions are the norm rather than the exception in the Campanian Book Cliffs strata.  Multiple autocyclic scenarios are plausible for their genesis including estuarine-tidal channel incision, backwater scour during river flood events, and/or tidal inlet incision.  All of these channel types can deeply incise into the underlying shallow marine facies belt.  An allocyclic SB-IVF origin is also possible but it would require additional evidence such as a regionally correlatable SB and a basinward shift of facies (e.g., detached lowstand sandstones). 

The sequence stratigraphic implications and significance are as follows: (1) Sequence stratigraphic models should be revised to include the SI-CH as a basic element of a parasequence irrespective of stratigraphic level.  (2) Most SI-CHs are not IVFs, nor are they exclusive to the top of a progradational parasequence set.  (3) Careful application of the SB-IVF model is required.  IVFs are the exception rather than the rule in the Campanian Book Cliffs.  The ubiquity and similarity of SI-CHs at most parasequence levels in the Blackhawk-Castlegate interval casts significant doubt on the model-driven SB-IVF interpretation for some but not all SI-CHs.  

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5779

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