Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 503–513 www.elsevier.com/locate/palaeo Ages and significance of glacial and mass movement deposits on the west side of Boulder Mountain, Utah, USA ⁎ David W. Marchetti a, ,1, Thure E. Cerling a, John C. Dohrenwend b, William Gallin c a Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA b Southwest Satellite Imaging, PO Box 1467, Moab, UT 84532, USA c Department of Geology, Carleton College, Northfield, MN 55057, USA Received 19 April 2006; received in revised form 20 April 2007; accepted 14 May 2007 Abstract Using air photos, satellite images, and field observations we mapped Quaternary glacial and mass movement deposits on the west side of Boulder Mountain in south-central Utah, USA. Prominent glacial moraines were deposited by outlet glaciers that emanated from an ice cap that existed atop Boulder Mountain. Cosmogenic 3He exposure ages of these deposits range from 20.2± 1.5 to 22.5±2.5 ka and indicate maximum ice advance during the global last glacial maximum (LGM). Three 3He exposure ages of boulders from the Pine Creek slump deposit indicate that the slumping likely occurred at or before ∼125 ka. Eight 3He exposure ages of boulders from the Miller Creek Potholes debris flow deposit range from 20.2±2.0 to 50.4±3.0 ka; five of those boulders yielded ages in the range of 26–33 ka and suggest emplacement of the debris flow deposit during that time. Both of these mass movement features cover prominent fault traces related to the Thousand Lakes fault but do not appear to be offset, suggesting that major faulting has not occurred since ∼125 kyr ago in this area. © 2007 Elsevier B.V. All rights reserved. Keywords: Cosmogenic isotopes; Pinedale glaciation; Mass movement; Colorado Plateau; Utah 1. Introduction parts of many studies and maps, yet none has inves- tigated these deposits in detail and their implications for The west side of Boulder Mountain has a variety of landscape development and paleoclimate in the Fremont geomorphic features that provide insight into the River drainage basin. Quaternary landscape development of both Boulder The first work describing the west side of Boulder Mountain and the Awapa Plateau (Fig. 1). Glacial and Mountain is Dutton's (1890) monograph on the High mass movement features and deposits, and Late Tertiary Plateaus of Utah. He recognized both the faulting to Quaternary faults, are well exposed along the west associated with Basin and Range extension that isolates side of the mountain. These features have been ancillary Boulder Mountain from the Awapa Plateau and evidence of past glaciation on the mountaintop. Gould (1939) described the erosive glacial evidence on the mountain- ⁎ Corresponding author. Tel.: +1 801 581 4271; fax: +1 801 581 7065. E-mail address: [email protected] (D.W. Marchetti). top as well as some of the depositional features of the ice 1 Current address: Geology Program, Western State College of cap that spilled off the mountain during Pleistocene time. Colorado, Gunnison, CO 81231, USA. The main published work on the Quaternary features on 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2007.05.016 504 D.W. Marchetti et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 503–513 Fig. 1. Grayscale satellite image of the western Fremont River drainage basin and the larger field area in south-central Utah, USA. FLP=Fish Lake Plateau; GP=Geyser Peak; HHP=Hen Hole Peak; TLM=Thousand Lake Mountain; BM=Boulder Mountain; TLF=Thousand Lake Fault; PF=Paunsagunt Fault. the west side of Boulder Mountain is Flint and Denny delineate the west side of the mountain. Small-scale (1958). They describe, map, and correlate most of the mapping (1:250,000) by Williams and Hackman (1971) glacigenic and mass movement features around Boulder also includes many of these faults, some of the surficial Mountain. They then assigned ages to the various features, and more complete mapping and descriptions deposits based on soil development, degree of boulder of the volcanic units that cap Boulder Mountain, and deposit weathering, and relative position. Smith Aquarius Plateau, and Awapa Plateau. et al. (1963) provide a more detailed geologic map of the In this paper we report field-based mapping and entire Capitol Reef area, but appear to use most of Flint cosmogenic 3He exposure age results on selected glacial and Denny's surficial units and age assignments in the and mass movement deposits from the west side of areas around Boulder Mountain. The mapping by Smith Boulder Mountain. We interpret the ages of these et al. (1963) includes many of the late Tertiary faults that deposits in light of previously published maps of the D.W. Marchetti et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 503–513 505 area and use the ages to constrain the timing of crops are numerous. Many of the bedrock outcrops are Quaternary faulting along the Thousand Lakes fault. sculpted into roche moutonnées or large whaleback features that indicate the dominant directions of ice 2. Study area flow. Mapping of these features indicates that there was a large ice cap on the top of the mountain that had radially Boulder Mountain is located on the western edge of directed ice flow from a central ice dome (or domes) the Colorado Plateau in south-central Utah (Fig. 1). (Gould, 1939; Flint and Denny, 1958; Osborn and Bevis, Boulder Mountain is part of the Aquarius Plateau, which 2001; Marchetti et al., 2005a). Outlet glaciers flowed over is one of the High Plateaus of Utah that characterize the the edge of the summit into several reentrants around the transition between the Colorado Plateau and the Great mountain. Well-developed, sharp-crested lateral and Basin. The Aquarius Plateau is defined as the southern latero-terminal moraine sets are preserved in several of edge of the high-elevation volcanic flows that crop out these reentrant valleys. Cosmogenic 3He exposure ages of to the north of the Escalante River drainage (Fig. 1). The boulders from two prominent moraine sets in the Fish Awapa Plateau is the eastward-tilted, volcanic-capped Creek drainage of Boulder Mountain indicate a maximum landscape between the Paunsagunt fault to the west, the ice advance at ∼23.1±1.3 to 20.0±1.4 ka and a smaller Aquarius Plateau to the south, the Thousand Lake fault and later stand-still or re-advance of ice at ∼16.8±0.5 to to the east, and the Fremont River and Fish Lake 15.2±0.5 ka (Marchetti et al., 2005a). No evidence for a Plateaus to the north. The Awapa Plateau is character- glacial advance older than the last glacial maximum ized by gently dipping, moderately faulted, and only (LGM; ∼21±2 ka; Bartlein et al., 1998) has yet been slightly dissected volcanic and volcaniclastic rock found on or around Boulder Mountain (Waitt, 1997; ranging in age from Oligocene to Pliocene (Williams Marchetti et al., 2005a). and Hackman, 1971; Mattox, 1991, 2001). Down-to-the-west slip on the Thousand Lake fault has 3. Methods raised several patches of volcanic rocks and created Boulder Mountain, Thousand Lake Mountain, Hen Hole We mapped surficial deposits around the west side of and Geyser Peaks (Fig. 1). The Thousand Lake fault is Boulder Mountain using 1:20,000 scale air photos, considered the eastern boundary of the Basin-and-Range/ composite satellite images, and the 7.5 min Government Colorado Plateau transition zone and the western edge of Point, Utah quadrangle (1:24,000) as a base map. We the Colorado Plateau (Wannamaker et al., 2001). Rowley defined mappable units based on the geomorphic et al. (1979) suggest that most of the faulting in the High process of deposition (alluvial, glacial and periglacial, Plateaus of Utah took place at ∼7 Ma, whereas Nelson and mass movement). We used the amount of volcanic (1989) reports field and geochronological evidence near boulder weathering, soil development, and superposi- Geyser Peak (Fig. 1) that suggests that local faulting may tion as relative age indicators. have occurred there after ∼5Ma.Basedonthe Volcanic boulders from the most prominent surficial longitudinal profile of the Fremont River and its features were sampled for cosmogenic 3He exposure age relationship to the Thousand Lake fault zone, Smith et dating. This included eight samples from the Miller al. (1963) estimate that ∼80–100 m of offset has occurred Creek Potholes (MCP) feature, two samples from the on the Thousand Lake fault during the Pleistocene. prominent Miller Creek ridges (MCR), two samples Marchetti et al. (2005b) discuss cosmogenic exposure from the Bone Flat loop feature (BFL), and three ages of volcanic boulders on debris flow fan deposits samples from the Pine Creek slump (PCS) (Table 1; that were offset by the Thousand Lake fault just north of Figs. 1 and 2). the Fremont River (Fig. 1). Their exposure ages suggest All the cosmogenic boulder samples collected for 3He the last offset of the Thousand Lake fault in that area exposure-age dating in this study were from basaltic may have occurred since ∼83 to 213 kyr ago. andesite or latite clasts that contained abundant pyroxene The top of Boulder Mountain is a broad volcanic phenocrysts. Pyroxene quantitatively retains He over plateau ∼180 km2 (∼70 mi2) in area, ranging in elevation million-year timescales (Schäfer et al., 1999), and 3He has from 3323 m (10,900 ft) to 3449 m (11,312 ft) (Fig. 1). the highest production rate of all the routinely measured The mountain is capped with a series of basaltic andesite cosmogenic nuclides (Gosse and Phillips, 2001).