Environmental controls on the evolution of alluvial fans in Buena Vista Valley, North Central Nevada, during late Quaternary time

Abstract

Alluvial fans in Buena Vista Valley, north-central Nevada, formed during at least four time periods during the late Quaternary as evidenced by four temporally and spatially constrained fan deposits: Qf1, Qf2, Qf3, and Qf4, from oldest to youngest. The stratigraphic relation between Qf3 and remnant beach ridge deposits of pluvial Lake Lahontan, the latter of which forms a paleoclimatic proxy for this region, indicate that climate and climatic change are primary controls on the evolution of fans. Qf3 deposition commenced during or immediately following the highstand of Lake Lahontan, between 13.5 and 12.5 ka, and continued as lake levels plummeted to levels below the sill elevation of the Buena Vista subbasin. We interpret Qf3 to be the result of a dynamic interplay between hillslope vegetation, soils, and hydrology as they are conditioned by climate and climatic change. Accelerated pedogenesis, because of aerosolic silts and clays from exposed lake sediments, coupled with decreased density of vegetation reduced infiltration capacity of the hillslope soils in interscrub areas. In contrast to fans in more arid climates, the increased runoff was not associated with increased coarse-grained sediment yield and major fan aggradation. Discharge from sediment-deficient basins entrenched older, proximal fan deposits and transported the sediment downfan to form secondary fans. The process–response of Qf3 to climatic change in north-central Nevada is especially illustrative of the impact of climate on landscapes in that: (1) it results from changes in basin sediment and/or water yield that occur in less than 500 years, and (2) it differs from responses in arid climates to the south and more semiarid climates to the north. The magnitude of vegetation change, including type and density of vegetation and the relative proportion of interscrub area, is particularly critical to water and sediment yield. Unit Qf2, an early late Pleistocene deposit, may also have been deposited during a transition from a wetter to drier climate. The synchronism of aggradation, stability, and entrenchment on all fans along the perimeter of Buena Vista Valley in the late Quaternary is further support for the dominance of climatic control on fan process; on the other hand, the impact of other extrinsic variables on fan process, including tectonism, base level, and basin lithology, all of which vary along the perimeter, is negligible.

Introduction

Alluvial fan stratigraphy, sedimentology, and geomorphology are governed by a hierarchy of environmental factors, including climatic regimes and their variation, rates and styles of tectonic activity, lithology and geomorphology of the upland source area, and the nature of contiguous environments Bull, 1977, Bull, 1991, Wells et al., 1987, Wells et al., 1997. These factors, particularly, climate and tectonics, force a dynamic interplay between energy and mass availability in the source area and mass transfer to the alluvial fan. This interplay is further complicated by its variability over time and space, perhaps best illustrated by the role climate plays in the aggradation of Quaternary alluvial fans. In presently arid climates, sediment mantles hillslopes during wetter and/or cooler regimes, but is transferred to the fan during transitions to drier and/or warmer, arid regimes (e.g., Wells et al., 1987, Bull, 1991). In contrast, in basins presently located in semiarid climates, sediment is transferred to the fan during wetter and/or cooler regimes (e.g., Pierce and Scott, 1982, Ritter et al., 1993). Although climate is used in the preceding illustration, disagreement exists as to whether climate or tectonics is the primary regulator of mass availability in the source area and mass transfer to the alluvial fan (e.g., DeCelles et al., 1991, Ritter et al., 1995). We believe this disagreement reflects, in part, the historical development of geomorphic inquiry as it relates to alluvial fans as well as discipline-based differences in time and space scales of fan studies.

Early studies of Quaternary alluvial fans were conducted in tectonically active areas (e.g., Eckis, 1928). Most were descriptive in nature and emphasized the role of faulting in initiating erosion in the source area and the aggradation of fans (e.g., Davis, 1905, Blissenbach, 1954). Later, more quantitative studies related tectonism to fan morphology, including segmented radial fan profiles Beaty, 1961, Bull, 1964a, incision of the fan-head Denny, 1967, Hooke, 1967, and the development of complex fans Denny, 1967, Bull, 1977. More recently, tectonism has been considered a first-order control on the creation of accommodation space and energy for the fan system (Fraser and DeCelles, 1992). Alluvial architecture DeCelles et al., 1991, Fraser and DeCelles, 1992 and facies assemblages Blair and McPherson, 1994a, Blair and McPherson, 1994b of alluvial fans have also been described in terms of stages of the evolution of drainage basins following uplift. These more recent studies address fan deposition on time and space-scales appropriate to basin evolution.

The role of climate in the formation of alluvial fan has generally been considered from two perspectives; the first treats the landform as a function of the climate in which it occurs, whereas the second focuses on the paleoclimatic history of the landform. Kochel and Johnson's (1984; Table 1) review of the literature of alluvial fans in various climatic regimes provides a summary of the differences existing in fan morphology, sedimentology, depositional processes, and facies assemblages. This approach is particularly valid in settings where most or all of the surface of the fan is activated by events of moderate to high recurrence (100's of years to seasonal), such as those that occur in humid-alpine climates by catastrophic floods (e.g., Blair, 1987), humid-temperate climates by debris flows (e.g., Wells and Harvey, 1987), and humid-glacial climates by seasonal meltwater (e.g., Boothroyd and Ashley, 1975). The latter historical approach is based on earlier studies which suggested that climatic changes would induce changes in mass availability and mass transfer processes (e.g., Blissenbach, 1954, Bull, 1964b, Lustig, 1965, Bull and Schick, 1979). Such changes would necessarily be widespread and characterized by synchronous periods of aggradation and entrenchment for all fans in a region of uniform environmental factors. With regionally correlative geomorphic surfaces and either absolute-age or relative-age control on those surfaces, more recent fan studies have demonstrated a temporal link between the aggradation of fans and past climates or climatic changes (e.g., Williams, 1970, Wasson, 1977, Pierce and Scott, 1982, Wells et al., 1987, Nemec and Postma, 1993, Ritter et al., 1993, Ritter et al., 1995).

The other three environmental factors affecting alluvial fans, lithology and geomorphology of the source area and the nature of contiguous environments, are generally believed to be subordinate factors. Lithology and geomorphology of the upland source area control various aspects of fan morphology, including size and slope of the fan (e.g., Bull, 1962, Bull, 1977, Hooke and Rohrer, 1977). The drainage basin, mountain front, adjacent fan surfaces, and either axial streams or basins along the distal margins of the fan comprise the contiguous environments of the alluvial fan. Of these, the distal environment of the fan, particularly, as it controls base level of the fan, has received the most attention. Entrenchment of fans and the resultant production of secondary fans have been attributed to lowering the base level (e.g., Eckis, 1928, Blissenbach, 1954, Carryer, 1966, Ryder, 1971). Base-level lowering along the distal fan margin locally oversteepens the fan-channel slope, causing the fan channel to entrench in an upfan direction.

This study evaluates the relative influences of the environmental factors on the evolution of alluvial fans in Buena Vista Valley, north-central Nevada (Fig. 1). We selected this basin because the presently arid climatic regime of the basin had been wetter and/or cooler during the late Pleistocene. Rates and styles of tectonic activity, source area lithology, and distal-fan base level vary along the perimeter of the basin. In addition, the well-exposed stratigraphic relations between the alluvial fan deposits, shoreline features and deposits associated with paleolakes, and faults provide at least a relative temporal framework for evaluating the relationship between fan process, climatic change, and tectonic activity. The objectives of this study include: (1) delineating the surficial stratigraphy of the alluvial fan deposits; (2) relating the stratigraphy of alluvial fans to geomorphic features and deposits associated with the climatic and tectonic history of the basin; (3) establishing a temporal framework for the stratigraphy and comparing it to that developed regionally by Hawley and Wilson (1965); and, (4) examining the driving forces of aggradation and entrenchment on alluvial fans in Buena Vista Valley. If a causative link indeed exists between the activity of Quaternary alluvial fans and either tectonics or climate, this study has important implications for using alluvial fans in reconnaissance studies as a first-order indicator for either tectonic activity or climatic change.