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Abstract
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Chapter 13
*
 Sequence Stratigraphy
of Hilight Field, Powder River Basin, Wyoming, U.S.A.: Unconformity Control
on Muddy Thicknesses and Distributions
A. D. Donovan
Exxon Production Research
Co.
Houston, Texas, U.S.A.
*
ABSTRACT
Detailed stratal correlations of the Skull
Creek, Muddy, Shell Cheek, and Mowry formations in the vicinity of Hilight
field in the south-central Powder River basin, Wyoming, reveal that the
Muddy Sandstone in this region is an unconformity-bounded stratigraphic
unit (depositional sequence). This depositional sequence represents an
erosional remnant of what were originally more widespread incised-valley
and younger (highstand) shoreline deposits.
The unconformity at the base of the Muddy
Sandstone (green sequence boundary) is characterized by incision of underlying
Skull Creek strata and onlap of overlying Muddy strata. In core, this boundary
also corresponds to an abrupt basinward shift in facies (coastal plain
on distal offshore deposits) and, locally, rooted horizons.
The unconformity at the top of the Muddy
Sandstone (red sequence boundary) is characterized by truncation of and
incision into underlying Muddy strata, onlap of overlying Shell Creek strata,
and angular discordance. In core, this sequence boundary coincides with
a marine flooding surface that is locally underlain by root structures.
Within the "Muddy" depositional sequence
(green to red sequence boundaries) three informal units, nonmarine (oldest),
lower paralic, and upper paralic, can be defined. These units are separated
by two regional well-log markers (M1 and M2) that, in core, correspond
to bentonites that overlie interpreted marine flooding surfaces. The basal
nonmarine unit of the Muddy: (1) onlaps topography carved into the underlying
Skull Creek Shale by the green sequence boundary, (2) contains only nonmarine
deposits in core, and (3) is overlain by a flooding surface and the M2
marker. This stratigraphic interval is interpreted as the lowstand systems
tract (incised-valley fill) of the green sequence . The overlying lower
paralic and upper paralic units of the Muddy contain shoreline and coastal-plain
deposits in core and
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