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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 the formative water depth can be determined using two different approaches.  First, throughout most of the Lahontan basin, two volcanic ashes (Wono and Trego Hot Springs) were found deposited within deltaic or beach sediments.  This allows the depth of the lake to be known at locations where we find those same volcanic ashes at lower elevations.  Sedimentary features associated with the tephras were documented at these locations, the results of which are in press in Quaternary Research.  Second, incision along the lower Walker River exposes interbedded deltaic and other lacustrine deposits where offshore sedimentary units can be traced to their 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, the results of which were published in 2007.  Analyses of the relations of sedimentologic features to their formative water depths is ongoing and will allow a predictive tool to be developed which can be used in similar, closed, underfilled, clastic-dominated, rapidly fluctuating lacustrine basins.  Results from this synthesis will be published separately.

A parallel phase of this project is documenting 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.  The location chosen for this comparison and companion study is Mohawk Valley in northeastern California .  This basin 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 shallower and smaller.  However, there are some locations where wave ripples exist near related deltaic sediments in proglacial lake deposits, which will allow independent determinations of formative water depths and a test of predictors developed in the Lahontan basin.

Present interpretations suggest the Mohawk basin was incised prior to repeated damming by large-scale landslides and likely affected by large amounts of sediment sourced from the many glacial streams in this relatively small basin.  The resulting changes in base level caused extended periods of aggradation represented by glacial till, glacio-fluvial, fluvio-deltaic sediments, 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.  In order to reconstruct the progression of cutting and filling that occurred within this basin, the discrete stratigraphic sections are being 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.

Prior work has identified thirty-two tephras from six different sites that spanned from ~740 ka to ~7 ka (Andrei Sarna-Wojcicki, pers. comm., 2007).  The present study has added substantially to the number of data points and stratigraphic sections that are being tied together to document the stratigraphic architecture of this basin.  Seven additional tephra beds from five additional sites have now been analyzed and identified.  Of these samples, six were identified as the Rockland ash bed (~ 575 ka, Lamphere et al., 2004), totaling seven data points that are being used to reconstruct the valley floor at that point in time.  One sample has been identified as the Summer Lake tephra bed LL (~160 ka, Negrini et al., 1994), for a total of two data points within the valley at that time.  In addition, thirteen additional samples have been submitted from twelve additional sites, the results of which should be available in October 2009.  Twenty more tephras have been collected and we are actively seeking funding for their analysis as well. 

Mapping of these Quaternary deposits and geomorphic surfaces, in combination with the use of relative dating techniques (geomorphology, soil development, etc), has complemented information gleaned from the detailed stratigraphic sections.  Analysis of data from this mapping is not yet complete because the current field season has not ended.  Initial interpretations from the mapping suggest that more of these deposits are related to glacial cycles than previously thought.  In particular, a large outwash fan with multiple buried soils has been located and documented.  This outwash fan and its soil development provide independent evidence from that gained from the stratigraphic sections that also shows: (1) multiple periods of cutting and filling of this valley and (2) the antiquity of these deposits.

Delving into the details and intricacies of sedimentary features, at a variety of scales, has greatly enhanced my ability to correctly reconstruct lake-level histories. Commonly, paleo-lake research focuses more on the geomorphology of a basin (shorelines, deltas, etc.), or on core records, than on the details of outcrops within the basin. Although often discontinuous and isolated from one another, lacustrine outcrops offer a wealth of lake level and other paleo-environmental information. The research conducted under this grant is allowing myself and others to better exploit and interpret these records.

The student came to this program with a strong background of soils and geomorphology and needed to learn more about stratigraphy and sedimentology to allow her to be a more successful Quaternary geologist.  Through working on this project, she has learned new skills that will enhance her scientific contributions.  Additionally, past and future presentations will help her learn how to better synthesize information and effectively present it to the scientific community.