Paleogeography and sedimentology of fluvial point bars, chute-fills and oxbow-fills in the lower Liard River, NWT.
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1992
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Abstract
The sedimentology and geometry of chute and oxbow channel-fills deposited by meandering rivers are poorly understood. In the ancient fluvial rocks, oxbow-fill deposits are conceptually important stratigraphic traps for small oil and gas prospects and reservoirs in the subsurface. This concept advocates the notion that clay-filled oxbow lakes provide a lateral seal which contains sandy point bar deposits. Why some oxbow traps contain hydrocarbons while others don't has concerned exploration geologists. This study is an attempt to help answer this question by describing the sedimentology and morphology of point bars, chute channel-fills and oxbow-fills in Holocene meandering channel deposits. Terraces up to 18 m high, flanking most reaches of the modern Liard and South Nahanni rivers, NWT, consist of Holocene (5750 BP± 60; GSC-4103) meandering river sediments. Fresh cutbanks have exposed the internal sedimentary anatomy of the river channel: point bar, chute channel-fill and oxbow-fill deposits. The surficial features are so distinct that scroll bar patterns, abandoned channels and oxbow lakes can be easily identified and mapped. The direct association between surface morphology of the floodplain and fluvial stratigraphy, provides a 3-dimensional perspective of a meandering river system and its channel deposits. Aerial photography indicates that the terrace was formerly a floodplain deposited by a meandering river. Exposed sandy point bar deposits are representative of a high energy range of deposition for sandy meandering rivers. The Liard point bar succession was found to be similar to those described in the literature. The chute channelfills, small isolated channel-fills (8 m thick by less than 400 m wide), were dominated by rhythmic-bedded sand and mud. Oxbow-fills were found to have a complex sedimentology contrary to the simplistic all-mud facies presented in most text books. Oxbow-fills have extensive cross-sectional areas, at least 16 m thick and greater than 400 m in width, and are composed of at least 65% thin bedded, fine grained sand. Rhythmic bedding of sand and mud is common. The laterally extensive sand interbeds within the oxbow-fill may not provide impervious stratigraphic traps when deeply buried.
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Bibliography: p. 101-107.
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Citation
Piet, L. J. (1992). Paleogeography and sedimentology of fluvial point bars, chute-fills and oxbow-fills in the lower Liard River, NWT. (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/19106