Channel-fill Deposits Formed by Aggradation in Deeply Scoured, Superimposed Distributaries of the Lower Kootenai Formation (Cretaceous)
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ABSTRACT Three large channels of the lower Kootenai Formation are exposed in the walls of the Missouri River valley east of Great Falls, Montana. Although the channels occur in two different stratigraphic units, they have several features in common. Each channel is contained within crevasse and bay-fill sequences, but the contacts between channel-fill deposits and laterally adjacent strata are erosional. The channels have a broad U-shape, range up to 300 m wide and 35 m deep, and exhibit a distinctive style of fill. Channel-filling occurred in increments by accretion from the bottom up and sides in, to form a concave layering which is more or less symmetrical about the axis of each channel. Lithology of the fill of each channel is quite different, however, and ranges from mudstone, to interbed ed sandstone and mudstone, to sandstone. The channels are interpreted as superimposed distributaries formed by avulsion when the locus of sedimentation shifted from one lobe to another. The lithology of the channel-fill deposits appears to be a function of the abandonment rate. A mudstone-filled channel results where abandonment is rapid, as is the case with upstream diversion of a trunk river system. Sandstone and mixed sandstone-mudstone fills predominate where a distributary is progressively abandoned, for example, where the discharge is diverted into an alternately favored distributary. Superimposed channels are difficult to map in the subsurface by geologic means. They cut across the trend of adjacent facies, and so their presence cannot be predicted from analysis of the containing strata.Keywords:
Aggradation
For the past 15 years, paleomagnetic studies on various rock types have consistently shown southern and Baja California to have been located at 10/sup 0/-17/sup 0/ lower latitude, relative to cratonal North America, than it is today. Similar studies on the Salinian block and in southwestern Oregon also indicate substantially lower latitudes for the deposition of Upper Cretaceous rocks. In seeming contradiction, apparent correlations across the Gulf of California plate boundary relate Cambrian(.) to Triassic stratal rocks of Sonora and the Great Basin to their contemporaries in Baja California, and Jurassic and Cretaceous arc rocks in the peninsula to those in mainland Mexico. Therefore, relative movement along the San Andreas system seems limited to approximately 300 km in a right-lateral direction since the Miocene. A possible accommodation to both sets of evidence places the Baja Peninsula near its present position relative to cratonal North America until about the Middle Jurassic, when it began moving relatively southeastward. This left-lateral motion placed it about 11/sup 0/ southeast by the Aptian-Albian and 17/sup 0/ southeast by the Campanian-Maestrichtian. The Late Jurassic to Late Cretaceous arcs trended southeast through Sonora, Sinaloa, and Jalisco, and then down the length of peninsular California. The right-lateral returnmore » of the peninsula began during the Late Cretaceous. The fault systems for the return motion cannot lie west of the Gulf of California, and thus, neither can the earlier left-lateral fault.« less
Peninsula
North American Plate
Aptian
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Recent studies of the Redding Formation of California provided new information concerning the extent of Cretaceous deposits underlying the southern Modoc Plateau. Understanding the stratigraphy and depositional environments of the Cretaceous rocks in this region is important for both paleogeographic reconstruction and hydrocarbon exploration. The Redding Formation is an approximately 1600-m thick clastic sequence that can be divided into five lithologic members. Biostratigraphic data indicate that the members of the Redding Formation are time-transgressive. The lowest three members were deposited during the middle to late Turonian, whereas the upper two accumulated during the Coniacian to santonian. The disconformity in the section developed during the latest Turonian to early Coniacian. Deposition in the Redding region was restricted to shelf environments and may have been controlled partly by euastatic sea level rise and fall. Initial transgression was directed northward and eastward with turonian strata accumulating across the basin. After the early coniacian hiatus, maximum marine inundation occurred briefly during the Santonian. Then late Santonian conglomerates and sandstones of the highest member prograded rapidly across the basin from the north, and shoaling apparently followed shortly thereafter. The southern limit of these late Santonian conglomerates appears to be the Tuscan Springs region where theymore » interfinger with deep shelf mudstones of the Chico Formation. These mudstones are considered to reflect an eastward swing of the Santonian shoreline around the northern Sierra Nevada. Thus, by the late Santonian, deposition had ceased in the Redding region but continued in a narrow trough to the south and southeast. The observed stratigraphy suggests that a thick sequence of Upper Cretaceous clastics beneath the southern Modoc Plateau is unlikely.« less
Marine transgression
Lithology
Cenomanian
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Correlation of facies from hydrocarbon-bearing continental and transitional marine sandstones to time-equivalent high-energy shelf-margin carbonates provide insight into hydrocarbon habitats of the Baltimore Canyon basin. These facies occur within a thick (> 10,000 ft) prograded wedge of shelf sediments in this passive margin basin. Wells drilled to test structural closures in shallow-water ( 5000 ft) drilled off the continental shelf edge to test large structural closures along the downdip termination of the Upper Jurassic/Lower Cretaceous carbonate shelf edge encountered no significant hydrocarbon shows. Reservoir rocks in these wells consist of (1) oolite grainstone which was deposited within a shoal-water complex located at the Aptian shelfmore » edge, and (2) coral-stromatoporoid grainstone and boundstone which formed an aggraded shelf-margin complex located at the Kimmeridgian through Berriasian shelf edge. Structural closures with reservoir and top seals are present in both updip and downdip trends. The absence of hydrocarbon shows in downdip carbonate reservoirs suggests a lack of source rocks available to charge objectives at the shelf margin.« less
Margin (machine learning)
Passive margin
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Peninsula
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