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Abstract
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Chapter 7
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Sequence , Parasequence,
and
Intraparasequence Architecture
of the Grassy Member, Blackhawk Formation, Book Cliffs, Utah, U.S.A.
Ciarán J. O'Byrne
Amoco Production Company
Houston, Texas, U.S.A.
Stephen Flint
Department of Earth Sciences
University of Liverpool
Liverpool, United Kingdom
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ABSTRACT
The upper Campanian Grassy Member of the
Blackhawk Formation, Mesa Verde Group, Utah, consists of two high-frequency,
unconformity-bounded sequences within the upper part of the Blackhawk Formation
highstand sequence set. Incised valley-fill deposits overlie the sequence
boundaries and are laterally separated by subaerially exposed interfluve
areas with minor pod-like channel tributaries. There is indirect evidence
of an emerging structural high which controlled the position of incised
valleys and associated interfluves. Evidence for shelf erosion and subaerial
exposure together with a significant basinward shift in facies tracts is
indicative of Type 1 sequence boundaries. Within the Grassy Member, the
degree of shelf erosion in outcrop appears far less significant than that
associated with type 1 sequence boundaries described from the stratigraphically
higher Desert Member and Castlegate Formation. There appears to be a hierarchy
of unconformities bounding sequences within the Blackhawk Formation recording
progressively larger basinward shifts of facies tracts and greater amounts
of shelf incision with time. Similarly, several orders of stratal surfaces
define the internal architecture of each sequence and constituent parasequences.
Valley-fill deposits consist of multistory
fluvial-estuarine complexes representing lowstand and early transgressive
systems tracts. Transgressive facies preserved between the subaerially
exposed interfluve and maximum flooding surface usually consist of a downdip
heavily bioturbated lag deposit correlatable updip with the top of a coal
or swamp facies. The internal architecture of Grassy shoreface parasequences
consists of stacked bedset shingles. The lateral shift in facies tracts
across bedset
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