Dramatic Increase in Late Cenozoic Alpine Erosion Rates Recorded by Cave Sediment in the Southern Rocky Mountains

Earth and Planetary Science Letters 297 (2010) 505–511

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Earth and Planetary Science Letters

journal homepage: www.elsevier.com/locate/epsl

Dramatic increase in late Cenozoic alpine erosion rates recorded by cave sediment in the southern Rocky Mountains

Kurt A. Refsnider

Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, 1560 30th St UCB 450, Boulder, Colorado 80309, United States article info abstract

Article history: Apparent increases in sedimentation rates during the past 5 Ma have been inferred at sites around the globe Received 25 May 2010 to document increased terrestrial erosion rates, but direct erosion rate records spanning this period are Received in revised form 30 June 2010 sparse. Modern and paleo-erosion rates for a small alpine catchment (3108 m above sea level) in the Accepted 2 July 2010 Southern Rocky Mountains are measured using the cosmogenic radionuclides (CRNs) 10Be and 26Al in cave Available online 24 July 2010 sediment, bedrock on the overlying landscape surface, and coarse bedload in a modern ﬂuvial drainage. The Editor: T.M. Harrison unique setting of the Marble Mountain cave system allows the inherited erosion rates to be interpreted as basin-averaged erosion rates, resulting in the ﬁrst CRN-based erosion rate record from the Rocky Mountains Keywords: spanning 5 Myr. Pliocene erosion rates, derived from the oldest cave sample (4.9±0.4 Ma), for the landscape − erosion rate above the cave are 4.9±1.1 m Myr 1. Mid Pleistocene erosion rates are nearly an order of magnitude higher cosmogenic nuclide (33.1±2.7 to 41.3±3.9 m Myr−1), and modern erosion rates are similar; due to the effects of snow cave shielding, these erosion rate estimates are likely higher than actual rates by 10–15%. The most likely Rocky Mountains explanation for this dramatic increase in erosion rates, which likely occurred shortly before 1.2 Ma, is an Pliocene and Pleistocene climate increase in the effectiveness of periglacial weathering processes at high elevations related to a cooler and periglacial weathering wetter climate during the Pleistocene, providing support for the hypothesis that changes in late Cenozoic climate are responsible for increased continental erosion. © 2010 Elsevier B.V. All rights reserved.

1. Introduction from a small, non-glacial alpine catchment (0.25 km2 averaging 3108 m above sea level; asl) in the otherwise glacially-sculpted The nature of modern landscapes reflects the cumulative effects of Sangre de Cristo (SdC) Range of southern Colorado (Fig. 1A) using tectonic, erosional, and sedimentary processes, but determining the cosmogenic radionuclides (CRNs). Paleo-erosion rates are recorded by contribution of each individual component from the geologic record is detrital sediment washed into a cave system situated below this rarely straightforward. Changes in erosion rates during the late catchment, and modern erosion rates are determined from surface Cenozoic have been documented in or inferred from a broad range of bedrock and stream sediment within the catchment. The ability to settings and archives around the globe (Hay et al., 1988; Zhang et al., interpret the cave sediment as reflecting basin-averaged paleo- 2001; Molnar, 2004; Schaller et al., 2004; Balco and Stone, 2005; erosion rates yields the first CRN-based late Cenozoic terrestrial Schuster et al., 2005; Häuselmann et al., 2007). However, the erosion rate record from the Rocky Mountains. interpretation of many records is complicated by changes in sediment budgets, basin accommodation space, sediment sources, and process- 2. Marble Mountain cave system es with long recurrence intervals (Sadler, 1981; Schumer and Jerolmack, 2009), so confidently identifying variations in erosion Marble Mountain (4043 m asl), located in the northern SdC Range, rates and attributing such variations to specific processes presents a holds a cave network with some of the highest solution caves in North serious challenge. America, ranging in elevation from ~3500 to 3700 m above sea level Whether or not surface erosion rates increased during the late (Fig. 1B). The SdC Range, situated along the eastern margin of the Rio Cenozoic in the Southern Rocky Mountains remains an open question. Grande Rift, was exhumed beginning in the late Oligocene (Lindsey et Taking advantage of a unique geologic setting insulated from many of al., 1986). The range is predominantly composed of Paleozoic sedimentation-related complications mentioned above, I present sedimentary rocks. Carbonate strata (Pennsylvanian–Permian SdC modern and paleo-erosion rates spanning the past 5 Myr derived Formation) only crop out on the east slope of Marble Mountain above tree line (3400 m asl locally), and above this limestone, the upper 200 m of the mountain are composed of arkosic sandstone and conglomerate (Fig. 2; Johnson et al., 1987). Deep glacial valleys have E-mail address: [email protected]. been carved along the north and west flanks of Marble Mountain

Fig. 1. Location map and cross sections of the study area. (A) Marble Mountain is located in the Sangre de Cristo Range and above the Wet Mountain Valley. The location of Spanish Cave is denoted with the black circle. Cross sections shown in (B) are indicated by the solid and dashed lines. The approximate extents of glaciers in the South Colony and Sand Creek drainages during the Last Glacial Maximum are shown in dark grey (Refsnider et al., 2009). The polygon indicates the view shown in Fig. 2. The square in the inset shows the area in (A), and the line between the study area and Westcliffe denotes the location of Promontory Divide. (B) Cross sections through Marble Mountain showing Spanish Cave (grey lines), cave sediment (open circles) and surface (ﬁlled circles) sampling sites.

(Refsnider et al., 2009), but there is no evidence that the eastern side truncated by lowering of the landscape surface (Davis, 1960; Wilson of the mountain has ever been glaciated, and the caves are not and Withrow, 2000). situated in a favorable location to have been affected by glacier Detrital sediment is found in several isolated locations within the meltwater streams. cave system and contains predominantly limestone fragments, limey The largest cave in Marble Mountain is Spanish Cave (N2000 m of clay, and rounded feldspar and quartz clasts. The silicic minerals must passage; Fig. 2), and there are at least nine other small caves b300 m be derived from surficial erosion and inwash of the overlying arkosic in length. Passages are generally only a few meters across, with the sandstone and conglomerate. The area of the modern watershed from largest rooms rarely exceeding 5 m across. The majority of the which surficial material sourced for Spanish Cave is approximately passages in Spanish Cave exhibit classic keyhole geometry with an 0.25 km, 75% of which is above the limestone–sandstone contact (Fig. 2). upper phreatic tube that was subsequently incised by vadose flow as the water table dropped, resulting in sinuous canyons up to 25 m in 3. Methods depth. The upper 100 m of Spanish Cave, however, have a characteristically different morphology with fewer distinct phreatic Quartz-bearing sediment samples were collected from three sites tubes and considerably steeper passages (Figs. 1Band2), though within the Marble Mountain cave system, as well as two sites on the some of the highest passages in the cave system are phreatic tubes surface above the caves (Figs. 1B and 2, Table 1). The Sand Crawl (Davis, 1960). Passages in the smaller caves are predominantly sample is from the floor of a horizontal phreatic tube at the boundary phreatic tubes with limited vadose entrenchment. Most of the caves between the more vertical upper reaches of Spanish Cave and the are linked and once formed part of a larger system that has been more horizontal phreatic tubes of the lower part of the cave. A sample Download English Version: https://daneshyari.com/en/article/4678379

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