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The Geomorphology, Eolian Activity, and Petrology of the Winnemucca Dune Complex, Humboldt County, Nevada USA
AuthorPepe, Nathaniel E.
Geological Sciences and Engineering
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The objective of this research was to determine the size, shape, activity of dunes, petrological characteristics, and provenance of sand in the Winnemucca Dune Complex (WDC). Methods and procedures included the extraction of weather records from meteorological stations, generating surficial landform maps, measuring dune advancement from historical aerial imagery, and field sampling of sand for laboratory inspection of grain size and mineralogical composition. Grain size parameters and textural classification of dune sand were determined using a Laser Granulometer and GRADISTAT v.8 (Blott & Pye 2001). The mineralogical composition and physical classification of dune sand was analyzed using fine powder X-ray Diffractometry and stained standard thin sections. Results were plotted on ternary diagrams with Quartz-Feldspar-Lithic (Folk 1974) and Quartz-Alkali feldspar-Plagioclase (Streckeisen 1976, 1978) overlays. Measurements from surficial landform maps estimate wind-blown deposits are distributed on 472.2 square kilometers of terrain. Active dunes are universally dominated by unique configurations of intermediate shaped barchan and parabolic dunes. For the purpose of this study these features were termed as barchanbolic. WDC is primarily covered by 6 crescentic complexes, 1 large sand sheet, and discontinuous sets of compound barchanbolic-parabolic dune fields. The crescentic complexes are composed of closely spaced barchanoidal and transverse ridges with occasional star dunes. Between the complexes are repetitive sequences of compound and individual barchanbolic-parabolic dunes that laterally radiate towards the bounding perimeter of WDC. Sand sheets, ramps, climbing, descending, cliff-top, and lee dunes are also present along mountain crests and hillsides. Sand sheets (56.3 square kilometers) and active dunes (162 square kilometers) extend across 218.3 square kilometers which constitutes 46.2% of the wind-blown deposits in WDC. Since the year 1980 sand dunes have been advancing at maximum rates from 1.6 to 6.9 meters per year on an azimuth of 35-130 degrees. Rose diagrams and historical wind records verify the sand dunes reach peak advancement rates during the warm season months of April to the middle of July. During this time of year the strongest winds prevail from west-southwest when the daily maximum wind speed is near 7 meters per second. Measurements of sand dune advancement rates from the years 1980-2012 show eolian activity has spatiotemporally fluctuated within the complex. WDC sand was observed to have distinguishing textural attributes. Sediments from active dunes were mesokurtic, symmetrical, and trended towards moderately well sorted medium sand. Sediments from stable dunes were mesokurtic and trended towards moderately sorted fine sand but varied in skew from symmetrical to fine. Micro-stereoscopic inspection of bulk samples, thin sections, and the QFL ternary diagram revealed that sand traveling down the sediment transport corridor will physically weather from a White to Grey & Very Pale Brown Litharenite into a Very Dark Grey to Light Yellowish Brown & Pale Brown Feldspathic litharenite sand. The QAP ternary analysis and X-ray Diffractometry demonstrated that during the processes of dune stabilization and mineralogical maturation of sand the relative weight percent of total Quartz will increase (20 to 68%) and the percent relative abundance of lithic material will decrease (100 to 45%). Feldspar minerals were plentiful and ranged from 32 to 80 relative weight percent. The mineralogical maturity of sand when interpreted by the ratio of Quartz to Feldspar grades the maturation as low to fractionally intermediate. The QAP ternary diagram demonstrates there are distinct mineralogical differences within the sand and that mixing of sediments from various supply sources have contributed to its composition. Similar to findings from the Mojave Desert (Zimbelman & Williams 2002) the abundance of Feldspar and lack of Quartz enrichment in WDC dune sand may imply the mineralogical maturity is directly inherited from the parent material. The lack of Quartz enrichment also indicates WDC is geologically young and most likely has not endured extended periods of inactivity. Prominent angular to subangular grains in WDC sediments suggest dune sand has not been transported over extremely long distances. Potential sediment supply sources for dune sand may include the Jungo terrane, Comforter Basin Formation, McDermitt-Santa Rose volcanic field, and sedimentary deposits from Lake Lahontan.