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The sedimentary records preserved in lacustrine basins offer a wealth of information on past climate changes, environmental changes, and geologic history, in addition to hosting valuable resources. The Great Basin of the western U.S. contains numerous closed lake basins that held much larger lakes at times during the Pleistocene and Holocene. Several of these basins still contain lakes such as Pyramid, Walker, and Mono lakes. In general, the study of these basins can be separated into two approaches, the first of which is to collect and analyze cores while the second approach focuses on outcrops and landforms. Both of these approaches, however, often emphasize reconstructing lake-level curves by using a variety of proxies including sedimentary features. The ability to more precisely relate sedimentary features to their formative water depths will help scientists to better reconstruct lake-level records and is the goal of this research.

This work is being conducted in the Lahontan basin, located in the western Great Basin, and in the adjacent Mohawk Valley, which is a formerly closed basin that now drains to the Pacific Ocean. The Lahontan basin 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 large 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 the locations of the many other outcrops within the basin where we find those same volcanic ashes. Sedimentary features associated with the tephras were documented at these locations, the results of which were published in Quaternary Research (Adams, 2010). Second, the Walker River, located within a southern sub-basin of the Lahontan basin, exposes a long, continuous deltaic section of Walker Lake sediments. In this location, 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 by Adams (2007).

Work is continuing on more theoretical aspects of this research that include relating the physical characteristics of common wave ripples (spacing, particle size) to wave parameters and water depth that will expand upon and complement the detailed outcrop studies. Integration of the outcrop studies and process modeling will lead to a quantitative model relating sedimentary features to their formative water depths. 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 model developed within the Lahontan basin can be applied elsewhere. Joanna Redwine (Ph.D. candidate) is working in Mohawk Valley in northeastern California for this companion study. Extensive exposures in Mohawk Valley allow detailed documentation of a near complete sedimentary record deposited throughout the past 780 ka. Reconstruction of these deposits through geologic mapping and correlation of tephras documents the spatial extent and character of sediments at multiple points in time. The large number of discrete tephras, covering a broad age range and found throughout the basin have enabled a model, currently being developed, that relates the distribution of sedimentary features in fluvial, lacustrine, and organic rich deposits common and/or diagnostic of intermontane basins. The primary tools used for this project are stratigraphic descriptions, tephrochronology, and geologic mapping. Standard stratigraphic data collected includes particle size, lithology, description and measurement of sedimentary features, and paleocurrent measurements. Through the combined efforts of earlier studies, ten different sections were located with a total of twenty identified tephras ranging in age from ~750 ka to ~7 ka (Sarna-Wojcicki, pers. comm., 2008). The present study has added substantially to the number of data points and stratigraphic sections from throughout the basin that are being tied together. Sixteen additional tephra beds from thirteen additional sites have now been identified and sixty-four stratigraphic sections have been described. Twenty-six more tephras have been collected and await analysis.

Synthesis of all the stratigraphic data with geologic mapping will allow interpretations of depositional environments at discrete points in time throughout the past ~ 780 ka. Mapping of preserved glacial and proglacial landforms complements the stratigraphic studies. Glacial advances represented by preserved surfaces composed of moraines and fluvial outwash deposits grade well above the modern base-level. These glacial deposits overlie a package tens of meters thick that is entirely composed of sand and finer material. This juxtaposition of coarse, poorly sorted, glacial deposits grading above and overlying a fine-grained, thick package of sands supports the interpretation of a Gilbert-type delta on the margin of a proglacial lake. These are the types of relations that will support one another with interpretations of depositional systems and allow construction of maps showing the distribution of various types of sedimentary features and character of sediments representing these depositional systems within this intermontane basin. These data will provide an example of what may be characteristic of these types of basins, perhaps particularly useful to studies based on more limited data such as core studies.

Joanna is learning valuable skills and knowledge through her work in Mohawk Valley and on other projects, in addition to making important contributions to the science. She has presented different aspects of her work at the national meeting of the Geological Society of America in 2009 and in 2010 and is currently working on three different manuscripts and is anticipating graduating with her Ph.D. in December 2011. Conducting this research has greatly contributed to my ability to better interpret the sedimentary records of lacustrine basins. Published papers from this work will hopefully contribute to other scientist’s efforts as well.