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45825-AC8
Quantifying Paleo-Water Depth Estimates from Lacustrine Sedimentary Features

Kenneth D. Adams, Desert Research Institute

The Lahontan basin in the western U.S. is a large underfilled basin that was repeatedly filled by a succession of large lakes throughout the Quaternary. The sedimentary record in this basin is being utilized to develop a model relating sedimentary features to their formative water depths. This is being accomplished by making direct observations of sedimentary features in locations where their formative water depth can be determined. This is possible from two different perspectives in the Lahontan basin. First, throughout most of the basin two volcanic ashes (Wono and Trego Hot Springs) were found deposited within beach sediments in this lake basin, or essentially at zero water depth. These same two tephras are found in many other locations in different lacustrine facies, which allows the paleowater depth in which they were deposited to be directly estimated. Sedimentary features were documented at these locations, the results of which will be presented in a manuscript that will be submitted this year. Second, the Walker River has incised into the Walker lakebed as lake level has lowered since AD 1900 and has exposed a long, continuous deltaic section sediments. Along this nearly 20 km stretch of incision, offshore sedimentary units can be traced to the related shoreline allowing water depth to be determined. Sedimentary features were documented in relation to water depth and used to interpret the sedimentary architecture and lacustrine history during the past ~4000 years, these results were published in Adams (2007). Analyses of the relations of these documented sedimentary features and their independently determined formative water depths is ongoing and allowing for a predictive tool to be developed which can be used in similar, closed, underfilled, clastic-dominated, rapidly fluctuating lacustrine basins.

The next phase of this project is to document the sedimentary architecture of a different type of sedimentary basin for comparison to what is known about the Lahontan basin and to test if the predictive tool developed within the Lahontan basin can be applied elsewhere. Ms. Joanna Redwine is undertaking this project for her Ph.D. project. The location chosen for this comparison and companion study is Mohawk Valley in Northeastern California, which is in an intermontane setting now drained by the Middle fork of the Feather River (MFFR). It is also clastic-dominated and underfilled, but has repeatedly changed from a closed to an open basin. The lacustrine systems within Mohawk Valley vary drastically from those that have existed in the Lahontan basin. The lakes were much more shallow and smaller. However, there are some locations where wave ripples exist near related beach deposits, which will allow independent determination of formative water depth and a test of predictors developed in the Lahontan basin.

Present interpretations suggest the Mohawk basin was incised prior to repeated damming by glacial ice, till, and large-scale landslides. The resulting changes in base level caused long-lived periods of aggradation represented by glacial till, low-gradient fluvial deposits, and lesser lacustrine deposits. Intervening periods of cutting removed some of that sediment, most recently by confined, deeply incised fluvial systems. The result is a large valley with an enormous amount of well exposed alluvial fill composed of ~350 meters of till, glacio-fluvial, and fluvio-deltaic sediments. In order to reconstruct the progression of cutting and filling that occurred within this basin, the discrete stratigraphic sections will be tied together using the many volcanic tephras deposited within these sediments. These tephra deposits are widespread throughout the valley and cover the area adjacent to the axial stream, MFFR, along the valley margins, and within the tributaries. Once these collected tephras are analyzed and identified, the valley fill can then be reconstructed at multiple points in time (~780 ka to ~7 ka) enabling documentation of the stratigraphic architecture of this basin. This research will be part of Ms. Redwine’s Ph.D. dissertation and will also result in one or more journal publications.

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