ABSTRACT: Patterns of Sedimentation in Rivers Disturbed by Active Tectonics

JORGENSEN, DAVID W., FRANK G. ETHRIDGE,* and STANLEY A. SCHUMM, Colorado State University, Fort Collins, CO

Localized active tectonics may control stream morphology and lateral migration, which are the principal controls on geometry and continuity of alluvial deposits. Uplift and subsidence affecting channel lengths approximately 100 times channel width cause enhanced or diminished sediment transport, which in turn affects channel morphology. Decreased channel efficiency and rapid downstream fining occurs in reaches undergoing subsidence or in reaches adjacent to zone of uplift. In contrast, less sediment storage and increased grain size occur in zones of uplift. Lateral stability of channels depends in part on sediment load: suspended-load channels are more stable and less sinuous in flat, depositional reaches, whereas bedload channels are more sinuous and laterally active. In the latter c se, sediment deposition combined with rapid lateral migration and downstream fining, create a system that is particularly suited to the selective deposition of coarse-grained sediment. The Humboldt River in Nevada provides an excellent example of the effects of active tectonics on an alluvial river. In a zone of tectonically induced subsidence, this mixed-load stream is characterized by a rapidly migrating channel, large downstream-migrating bars with avalanche faces, and coarse-grained bar surfaces. Upstream and downstream from this zone deposition occurs as smaller bank-attached bars, and the channel is more stable. Based on geomorphic evidence and a review of published paleodrainage maps, active tectonics probably played a critical role in localizing coarse-grained sediments and hence many hydrocarbon reservoirs in ancient fluvial deposits.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)