Sedimentology of the Judith River Formation in the Milk River Valley and the Little Rocky Mountains, Montana
Abdel Malik, Abdel Moneim E.
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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. Note:This thesis contains maps that have been sized to fit the viewing area. Use the zoom in tool to view the maps in detail or to enlarge the text.