Aspects of Sedimentology and Fluvial Morphology of a Part of the Little Missouri River, North Dakota

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Master of Science (MS)




The Little Missouri River drains 21,500 km in Wyoming, Montana, and the Dakotas- The river is perpetually muddy, although the bulk of its deposits are fine to medium silty sand. The discharge ranges between 0 and 3000 m^/s, with a mean value of approximately 17 m'Vs.

Between Medora, North Dakota and Lake Sakakawea 250 km downstream, the river changes from meandering to braided. Braiding may be a transitory response to the decrease in discharge following the spring flood, although more likely braiding is the result of post-glacial changes in drainage, when the river was diverted eastward from its original course. Diversion, and perhaps continued up-lift of the Little Missouri Badlands, increased the channel slope and induced braiding. Aggradation in the braided portion of the river was probably initiated by impounding Lake Sakakawea.

Sediment mineralogic composition depended on clast size, with coarser sediment dominated by lithic fragments and quartz, and finer sediment characterized by quartz and chert. The distribution of all specie13 of clasts, with the exception of pitted quartz, was size sensitive, in that each size interval had its peculiar mineralogy. The percentage of pitted quartz remained nearly constant because pitted quartz is perhaps the most stable clast type.

Two varieties of quartz were recognized. Rounded, pitted quartz was presumed to have been through several transport and deposition cycles. Clear, glassy, angular quartz was thought to be derived from igneous or volcanic rocks, and to have been exposed to little transport* The proportion of angular quartz to total quartz increased from 0.04 in tre -1 phi size class, to 0.4 in the 4 phi size class.

The ratios of quartz/quartz plus feldspar generally were greater than 0.8, and compare well with quartz/quartz plus feldspar rat'os reported for the Bullion Creek Formation. Similar ratios for o :her strata in the drainage basin are unknown.

The mineralogic composition of the sediments in the Li :tle Missouri River varied in an irregular manner over the stream couise. The variation seems related to Inputs of different sediments by c: e ephemeral tributaries. Therefore I conclude that local source va iations are more important than fluvial processes in determining the lineralogic composition of the Little Missouri River sediments.

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