Sedimentology of the Judith River Formation in the Milk River Valley and the Little Rocky Mountains, Montana
Date
1990-08
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ORCID
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Masters
Abstract
The Judith River Formation along the west bank of the
Milk River and in the Little Rocky Mountains areas of
Montana attains a maximum thickness of 116m and consists of
sandstone, siltstone, shale or mudstone, and carbonaceous
shale. It conformably overlies the marine shale of the
Claggett and underlies the marine shale of the Bearpaw
Formations. The formation is readily divisible into a lower
and upper member,and detailed sedimentologic study has
allowed division of the formation into three major
sedimentary facies.
The Judith River Formation in the study area forms one
coarsening-upwards sedimentary sequence that represents a
typical prograding deltaic sequence. The lower facies has
been interpreted as deposits of a shallow marine (prodelta
and delta-front) environment; whereas the middle facies
consists of deposits of a lower delta-plain (including:
tide-influenced channels, distributary channels,
interdistributary bays, and marshes) environment. The upper
facies represents deposits of meandering and braided
streams, in an upper delta-plain or alluvial plain. The
prograding sequence records the evolution of four types of paleochannels from tide-influenced channels at the base, to
meandering distributary channels, to large meandering
channels, and ultimately to braided channels. Paleocurrent
data indicate that the streams flowed generally towards the
east, probably draining the Cordilleran highlands to the
west. Ammonite index fossils found indicate that the Judith
River Formation was deposited during the Upper Campanian
substage, over a time-span of 4 MY.
The paleoshoreline of the Claggett seaway in the study
area extended generally along a north-south trend, as
suggested by paleocurrent directions and orientation of
ripple crests in the coastal sediments. It is evident from
the study that the Claggett seaway was affected by a low-range
tide and was periodically influenced by storms. The
withdrawal of the Claggett sea from the study area was
relatively rapid, and the subsequent transgression of the
Bearpaw sea was probably also rapid.
Comparison made between the Judith River Formation
sediments in the study area and their equivalent in the
Canadian plains, shows that both are remarkably similar, and
hence the name "Judith River Formation" should be used in
the two regions to unify the nomenclature. Comparison was
also made between the Judith River Formation and the Horseshoe Canyon Formation (Edmonton Group); and it was
concluded that sedimentary sequences deposited along the
coastal plains of the regressive Claggett and Bearpaw
seaways were much alike.
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Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geological Sciences