Chronostratigraphic and Architectural Analysis of a Modern Point Bar, Powder River, Montana

Abstract

Logistical challenges and the dynamic nature of fluvial systems, make it difficult to study modern point bar deposits over formative time periods. Seasonal and annual changes in precipitation can greatly influence the rate at which deposition is recorded. The lack of accurate sediment-package dating makes it difficult to compare sedimentation rates to actual chronostratigraphic events such as floods. This study combines photogrammetry, mapped surface migration, a survey of sediment elevation change, a trench, and water discharge rates to develop a more complete understanding of how a point bar forms on an annual scale. Powder River in Montana has little influence from engineering, offers a unique opportunity to study a seasonally exposed point bar and how its internal architecture and surface features form through time.

The study area is along Powder River between Moorhead and Broadus, Montana, USA. Powder River is a northward flowing, meandering river that originates in the Bighorn Mountains in northeast Wyoming, USA and is a tributary to the Yellowstone River. The focus of this study, is a point bar that formed as the result of the neck cut-off during a 50-year flood in 1978. Sediment elevation surveys were conducted annually, with a few exceptions, at the centimeter scale to determine sediment elevation change and the building and erosion of the point bar. These surveys are applied to the architectural-element analysis of the sediment packages within the point bar to compare time with sediment deposition.

This study reconstructs the growth of the point bar, its discrete accretionary architecture at the scale of years, and determines the inter-relation between annual flooding and bar accretion. The sediment survey timeline shows that on average the river builds one accretionary body per annual flood cycle. On occasion, the river builds multiple bodies during the year or can take several years to build one accretion set. Change in the accretion set building period is attributed to changes in river flow. Continual change of deposit direction, grain size distribution, erosion, and reshaping of the bar surface between accretion events leads to fragmentation of the point bar body, vastly different from the textbook model of a point bar. This detailed study of how a modern point bar forms lends insight into the fragmentation of fluvial hydrocarbon reservoir bodies.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018