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Abstract

Modern meandering river point bar deposits formed in fluvial and tidally-influenced environments were investigated to explain why large-scale epsilon cross-stratification (ECS) is common in ancient fluvial rocks but appears to be absent in modern deposits. To resolve this problem several modern meandering systems were studied; the Athabasca upper delta plain in northeast Alberta, Canada; the mesotidally-influenced reach of the Willapa River, southwest Washington State, U.S.A.; and the lower Daule and Babahovo Rivers, Ecuador, which have micro- and mesotidally-influenced depositional conditions. As well, exposed point bar facies in 18-m-high cut-banks were examined in the fluvial, lower Liard River (Middle Holocene), NWT, Canada, and tidally-influenced Willapa River (Late Pleistocene), Washington State.

Based on sedimentologic results obtained from these areas, a threefold lithofacies classification of point bar deposits is proposed: (1) fluvial sandy point bar facies, (2) low-energy fluvial and microtidally-influenced (upper estuary) point bar facies, (3) mesotidally-influenced point bar facies deposited in upper and middle estuary settings. The latter two facies are very similar to many reported ancient meandering river point bar rocks. The three lithofacies models are compared with four ancient examples of point bar rocks selected from Alberta, Canada, the Lower Cretaceous middle McMurray Formation (Athabasca Oil Sands), the Upper Cretaceous Judith River and Horseshoe Canyon Formations and the Paleocene Paskapoo Formation.

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