--> Valley Fill Architecture of the Missouri River Valley: Preferential Preservation Creating Classic Sequence Stratigraphic Architecture

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Valley Fill Architecture of the Missouri River Valley: Preferential Preservation Creating Classic Sequence Stratigraphic Architecture

Abstract

Early observations from mapping and dating of fluvial terraces within the Missouri River Valley illustrate a valley architecture similar to common fluvial sequence stratigraphic models. However, opticially stimulated luminescence (OSL) and 14C dating of these surfaces indicate this architecture was created by multiple upstream controlled cut and fill events with preferential preservation of lower channels. The classic fluvial sequence stratigraphic model consists of external forcing causing valley incision and sediment bypass when down-stream anchors are low, followed by channel amalgamation as valleys begin filling, and ending with dispersed floodplain dominated channel sequences when accommodation is high. Mapping of fluvial channels throughout the Missouri River Valley presents a similar architecture which appears to fit the classic down-stream anchor controlled sequence stratigraphic model, but is instead created by preferential preservation of lower channels deposited at the end of incisional events. OSL dating techniques applied to terraces within the valley has presented two major incisional and aggradational cycles since the Last Glacial Maximum. The oldest incisional surface was dated between 16–14 ka BP, around the time of breakdown of the last glacial advance. Around 13–12 ka BP, a meandering channel belt aggraded close to the modern level of the Missouri River, and was followed by incision around 11–10 ka BP. Finally, around 8 ka BP, the Missouri River aggraded to modern levels where it has remained throughout the Holocene. These events were most likely caused by glacially driven upstream controls, and not by down-stream anchor controls. Instead, lower channels are preferentially preserved due to lack of erosion during incision while upper channels are eroded and therefore misrepresented in the record. This work shows that the classic fluvial sequence stratigraphic model, which is commonly interpreted in the rock record, can be produced by multiple upstream-controlled incision/aggradation cycles with preferential preservation of lower channels and does not necessarily have to be a one cycle down-stream anchor controlled sequence. This may be of economic importance in reservoir modeling because applying a classic fluvial sequence stratigraphic model to such a valley will underestimate the heterogeneity of the amalgamated channels at the base of the valley since these have been created by multiple scouring events as opposed to just one sequence.