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Abstract
Chapter from: M
64: Sequence Stratigraphy of Foreland Basin Deposits
Edited By
J.C. Van Wagoner and G.T. BertramAuthor:
D.K. Larue Seismic/ Sequence Stratigraphy
Published 1995 as
part of Memoir 64
Copyright © 1995 The American Association of Petroleum
Geologists. All Rights Reserved. |
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Chapter 12
*
Structurally
Aligned, Sediment-Starved Fluvial Valleys Encased in Marine Deposits: Sequence
Boundaries Between the Carlile Shale and Niobrara Formation, Central Powder
River Basin, U.S.A.
D. K. Larue
Exxon Production Research
Co.
Houston, Texas, U.S.A.
*
ABSTRACT
The basal contact of the Niobrara Formation
with the underlying Carlile Shale is a regional unconformity or sequence
boundary which formed about 88.5 Ma. Because the Carlile Shale and Niobrara
Formation both represent offshore to basinal deposits, most previous workers
have suggested that erosion and truncation at the sequence boundary occurred
in a submarine environment.
In the present study, 790 well logs penetrating
the Niobrara-Carlile contact were correlated in the central Powder River
basin, representing an area of 2800 mi2. Field studies established
the character of the Niobrara-Carlile contact in outcrop. Two sequence
boundaries were recognized during log correlation, the older "blue" and
the younger "green." These two sequence boundaries define a sequence between
the Niobrara Formation and Carlile Shale, referred to as the intermediate
unit. The lithology of the intermediate unit is inferred to be shales and
calcareous shales based on mud-logging reports and interpretation of geophysical
logs. The intermediate unit was observed in outcrop in one or two locations
in the Powder River basin area and consists of concretion-bearing microconglomerate.
Isopach maps from the sequence boundaries
to a chronostratigraphic surface inferred to represent a paleohorizontal
datum (a prominent bentonite layer in the Niobrara Formation) were used
to reconstruct the topography of the unconformity. The erosional surfaces
are characterized by mappable highs and lows ("thins" and "thicks," respectively,
on an isopach map): lows are associated with areas where, based on correlation
of cross sections, the underlying Carlile Formation is incised. These topographic
lows characterized by incision of underlying units are interpreted as paleovalleys
that trend approximately NNW, parallel to, and possibly associated with
the Gillette/Keeline structural lineament. The valleys together form what
is interpreted to be a
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