Late Paleogene Emergence of a North American Loess Plateau Majie Fan1*, Ran Feng2, John W
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https://doi.org/10.1130/G47102.1 Manuscript received 22 May 2019 Revised manuscript received 15 October 2019 Manuscript accepted 21 November 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 3 January 2020 Late Paleogene emergence of a North American loess plateau Majie Fan1*, Ran Feng2, John W. Geissman3,4 and Christopher J. Poulsen5 1 University of Texas at Arlington, Department of Earth an Environmental Sciences, 500 Yates Street, Arlington, Texas 76019, USA 2 University of Connecticut, Center for Integrative Geosciences, 354 Mansfield Road, unit 1045, Storrs, Connecticut 06269, USA 3 University of Texas at Dallas, Department of Geosciences, ROC 21, 800 West Campbell Road, Richardson, Texas 75080, USA 4 University of New Mexico, Department of Earth and Planetary Sciences, MSC03 2040, Albuquerque, New Mexico 87131, USA 5 University of Michigan, Department of Earth and Environmental Sciences, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA ABSTRACT logic characterization of three records (sites LTG, The relative roles of tectonics and global climate in forming the hydroclimate for wide- EF, LG) where Oligocene loess has been previous- spread eolian deposition remain controversial. Oligocene loess has been previously docu- ly identified in the western United States (Fig. 1). mented in the interior of western United States, but its spatiotemporal pattern and causes Our study presents several sets of data that suggest remain undetermined. Through new stratigraphic record documentation and data compila- loess deposition initiated earlier, and was more tion, we reveal the time transgressive occurrence of loess beginning in the latest Eocene in broadly distributed, than previously thought. We the central Rocky Mountains, that expands eastward to the Great Plains across the Eocene- use a series of climate model simulations that test Oligocene transition (EOT). Our climate simulations show that moderate uplift of the south- scenarios of global cooling, sea-level drop, and ern North America Cordillera initiated drying in the Cordilleran hinterland and immediate topographic change to investigate the roles of tec- foreland, forming a potential dust source and sink, and global cooling at the EOT expanded tonics and global climate on eolian deposition. the drying and eolian deposition eastward by causing retreat of the North American Monsoon. Therefore, the eolian deposition reflects continental aridification induced both by regional GEOLOGIC BACKGROUND tectonism and global climate change during the late Paleogene. We studied the late Eocene–Oligocene White River Formation and White River Group in the INTRODUCTION global climate by altering the radiation budget and central Rockies and the adjacent Great Plains The roles of global climate change and tec- nutrient cycling (e.g., Carslaw et al., 2010). There- (Fig. 1). The Rockies are an extensive region of tonics in driving eolian deposition and associ- fore, the timing of deposition, spatiotemporal pat- high mountains and intermontane basins on the ated continental aridification remain debated. tern, and characteristics of the loess have profound east side of the North American Cordillera. The The best-known example is the Chinese Loess influences on late Paleogene ecosystem evolution high-elevation, high-relief (1.5–4 km) topogra- Plateau in central Asia. Loess deposition in the and the feedback of dust in global cooling. phy gradually diminishes eastward toward the region initiated in the late Eocene or during the Previous studies of eolian deposits in the Great Plains, at ∼1.0 km elevation (Fig. 1). The Eocene-Oligocene transition (EOT) (Licht et al., western United States have placed the initia- region is arid to semi-arid, with annual precipita- 2014; Sun and Windley, 2015), concurrent with tion of deposition in the early Oligocene in the tion amounts increasing eastward from ∼15 cm global cooling, retreat of the Paratethys epiconti- central Rocky Mountains (Rockies) and at the to ∼60 cm (Bryson and Hare, 1974). Precipita- nental sea, and uplift of the Tibetan Plateau, and EOT on the Colorado Plateau (Evanoff et al., tion across the Great Plains is from summer and these changes have been variously attributed as the 1992; Cather et al., 2008). Evanoff et al. (1992) fall moisture mainly originating from the Gulf of causes of loess deposition (e.g., Ramstein et al., suggested an eastward younging trend based on Mexico, from winter storms in the Arctic and the 1997; Guo et al., 2002; Dupont-Nivet et al., 2007; fossil ages and attributed this to global cooling. Pacific Ocean, and from recycled continental vapor Wang et al., 2008). Oligocene eolian deposits have However, there is considerable evidence that the (Zhu et al., 2018). These moisture contributions previously been documented in the western Unit- southern North American Cordillera experienced may have varied as a function of surface topogra- ed States (Evanoff et al., 1992; LaGarry, 1998; renewed uplift during the late Eocene–Oligocene phy and climate state in the past (Feng et al., 2013). Cather et al., 2008), presenting a new opportunity (Mix et al., 2011; Chamberlain et al., 2012), post- to test the roles of global climate and tectonism. dating the protracted Jurassic–early Paleogene CHANGES IN DEPOSITIONAL Loess deposition also signals key changes in the crustal shortening and thickening. Mid-Cenozoic ENVIRONMENT regional ecosystem (e.g., Guo et al., 2002; Caves uplift may have forced development of a regional We conducted analyses of lithofacies, grain- et al., 2016), and dust plays a feedback role on rain shadow and caused regional aridification. size, and rock magnetic properties to character- Here, we report a new loess record (site WS) in ize past changes in depositional environment *E-mail: [email protected] the central Rockies, and new detailed sedimento- (see details in the GSA Data Repository1). At 1GSA Data Repository item 2020075, details of sample analysis and model simulation setup, is available online at http://www.geosociety.org/datarepository/2020/, or on request from [email protected]. CITATION: Fan, M., Feng, R., Geissman, J.W., and Poulsen, C.J., 2020, Late Paleogene emergence of a North American loess plateau: Geology, v. 48, p. 273–277, https://doi.org/10.1130/G47102.1 Geological Society of America | GEOLOGY | Volume 48 | Number 3 | www.gsapubs.org 273 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/3/273/4945829/273.pdf by guest on 29 September 2021 Figure 1. (A) Map of the tween the younger massive strata and underly- A 109ºW 107ºW 105ºW 103ºW 101ºW study area in the central ing fluvial rocks are conformable (Figs. 1B and Rocky Mountains and 1C). The fluvial lithofacies has variable mean Buffalo Rapid City adjacent Great Plains, grain size and poor sorting (Fig. 2; Fig. DR1), N 100 km western USA. Red dots represent the four study with grain-size distributions varying between WY SD 43ºN sites in this study: WS unimodal and multimodal (Fig. 3A), which re- Lander Casper in Beaver Divide, LTG in WS LG flect variations in water hydraulic properties, LTG EF NE Flagstaff Rim, and EF near Douglas in Wyoming, and distance of sediment transport, and sediment LG in the Toadstool Geo- mixing (Bui et al., 1989; Sun et al., 2002). R B logic Park in Nebraska. Scanning electron microscope (SEM) observa- Gray areas represent H Cheyenne tions show that fluvial lithofacies quartz grains USA distribution of late Paleo- are angular to subangular, and grain surfaces 41ºN gene sedimentary rocks. Inset: The inferred late contain irregular breakage blocks and broken Mexico GulfCraiCra ofg Paleogene loess plateau cleavage surfaces (Fig. 3B; Fig. DR2). These Mexico Denver is highlighted by the red features are indicative of mechanical break- dashed polygon, based on down of grains in subaqueous conditions (Krin- this study and three other B C studies: B—Benton et al. sley and Doornkamp, 1973). (2015), H—Hembree and The overlying massive deposits have bet- Eolian Hasiotis (2007), R—Rob- ter sorting than the fluvial lithofacies Fig. 2( ), Eolian inson (1963). The Chuska with bimodal grain-size distributions showing erg inferred by Cather Fluvial a major peak at 30–130 m and a minor peak et al. (2008) is shown in μ red polygons, and the at 2–10 μm (Fig. 3A). The bimodal grain-size Fluvial Cordilleran hinterland is distributions are similar to those of the mid- and shown in gray polygon. upper Cenozoic eolian loess deposits in central (B) Field views of the Asia (Sun et al., 2002; Pye and Tsoar, 2009; transition from fluvial to eolian deposition in the WS section. (C) Transition to eolian deposition in the EF section. The Vandenberghe, 2013; Licht et al., 2014; Sun underlying fluvial units contain interbedded sandstone, conglomerate, and mudstone, and and Windley, 2015) and Quaternary loess in the overlying eolian units are massive. GPS locations of the four study sites are provided in the Midwest of the United States (Muhs et al., the Data Repository (see footnote 1). 2008). The major, coarser-grained population is interpreted to have been transported by saltation all four study sites, tan, massive eolian silt- sandstone and pebble conglomerate, recording or near-surface suspension. The minor, finer- stone overlie deposits of interbedded gray to fluvial environments Figs. 1( and 2; Tables DR1 grained population represents the background red mudstone and gray to brown lenticular and DR2 in the Data Repository). Contacts be- supply of suspended particles, fine particles Figure 2. Mean grain-size and bulk magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) intensity data collected from the WS, LTG, EF, LG study sites in the western United States are placed in a chronostratigraphic framework for each section. Compiled maximum depositional ages based on detrital zircon U-Pb geochronology, radiometric ages of ash beds (J, H, G, 5, 7, LWA, SDP), and North American Land Mammal Ages (NALMAs) are labeled along each stratigraphic column and illustrate the eastward diachronous initiation of eolian deposition.