CRevolution 2: Origin and Evolution of the Colorado River System II themed issue Paleogeomorphology and evolution of the early Colorado River inferred from relationships in Mohave and Cottonwood valleys, Arizona, California, and Nevada Philip A. Pearthree1,* and P. Kyle House2 1Arizona Geological Survey, 416 W. Congress Street, Tucson, Arizona 85701, USA 2U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona 86001, USA ABSTRACT INTRODUCTION relationships to the underlying paleolandscape and to the obvious Colorado River deposits Geologic investigations of late Miocene– Deposits of the latest Miocene or early Plio- that immediately succeeded them. The primary early Pliocene deposits in Mohave and Cotton- cene Bouse Formation and slightly younger, purposes of this paper are to (1) draw upon this wood valleys provide important insights into unequivocal deposits of a through-fl owing Colo- recent work to reconstruct the general character- the early evolution of the lower Colorado rado River provide critical evidence regarding istics of the landscape and depositional systems River system. In the latest Miocene these val- the inception and early evolution of the Colo- that existed in these valleys immediately prior leys were separate depocenters; the fl oor of rado River. Both sets of deposits are exposed to Bouse deposition; (2) describe the charac- Cottonwood Valley was ~200 m higher than throughout the lower Colorado River (LCR) ter and distribution of Bouse deposits in these the fl oor of Mohave Valley. When Colorado valley from the Chocolate Mountains of south- valleys, and how their characteristics vary with River water arrived from the north after eastern California northward to Cottonwood landscape position; (3) describe the Bullhead 5.6 Ma, a shallow lake in Cottonwood Val- Valley in Arizona and Nevada (Fig. 1). The fi ne- alluvium and summarize the stratigraphic and ley spilled into Mohave Valley, and the river grained sediments of the Bouse Formation were geomorphic relationships between Bouse and then fi lled both valleys to ~560 m above deposited on a landscape of bedrock hillslopes, Bullhead deposits; (4) consider the implications sea level (asl) and overtopped the bedrock alluvial fans, and axial valley deposits that was of this evidence for changing regional base level divide at the southern end of Mohave Valley. broadly similar to the one we see today, except through a scenario of lake and river evolution; Sediment-starved water spilling to the south that there is no evidence of a through-fl owing and (5) consider the competing hypotheses gradually eroded the outlet as silici clastic fl uvial system linking basins from north to south explaining Bouse deposition and LCR develop- Bouse deposits filled the lake upstream. prior to Bouse deposition. The interval of Bouse ment in the context of the current constraints on When sediment accumulation reached the deposition was succeeded by accumulation of the magnitude and timing of base-level changes elevation of the lowering outlet, continued a thick sequence of primarily quartz-rich sand in the LCR valley. erosion of the outlet resulted in recycling of and far-traveled rounded gravel deposits, the We use elevations relative to modern sea stored lacustrine sediment into downstream Bullhead alluvium (House et al., 2005, 2008b; level for the vertical positions of various out- basins; depth of erosion of the outlet and Howard et al., 2015). These deposits were clearly crops, paleovalleys, and paleo–water bodies in upstream basins was limited by the water transported and emplaced by the Colorado the modern landscape. This is not intended to levels in downstream basins. The water level River. These two sets of deposits record a dra- exclude the possibility that local deformation in the southern Bouse basin was ~300 m asl matic change in the geomorphology of this area, or regional uplift has changed absolute eleva- (modern elevation) at 4.8 Ma. It must have and they are intimately involved in the initial tions of various outcrops and landscape features drained and been eroded to a level <150 m development of the LCR. after Bouse and Bullhead deposition. By using asl soon after that to allow for deep erosion In this paper we focus on the latest Miocene this convention, we make no assumptions about of bedrock divides and basins upstream, and early Pliocene deposits and paleogeo- whether elevations of deposits and landscape leading to removal of large volumes of Bouse morphology of Mohave (MV) and Cottonwood features throughout the region have changed sediment prior to massive early Pliocene Valleys (CV), before, during, and after the relative to sea level or to each other since 5 Ma. Colo rado River aggradation. Abrupt lower- arrival of Colorado River water and sediment. ing of regional base level due to spilling of a Detailed geologic mapping and reconnais- PREVIOUS WORK southern Bouse lake to the Gulf of California sance investigations in the past 15 yr (Howard could have driven observed upstream river et al., 1999, 2013; Faulds et al., 2004; House The Bouse Formation was fi rst thoroughly incision without uplift. Rapid uplift of the et al., 2004, 2005, 2008a, 2008b; Pearthree and described and defi ned by seminal geohydrology entire region immediately after 4.8 Ma would House, 2005; Spencer et al., 2007; Pearthree, studies conducted in the 1960s (Metzger, 1968; have been required to drive upstream inci- 2007; House and Faulds, 2009; Malmon et al., Metzger et al., 1973; Metzger and Loeltz, 1973). sion if the southern Bouse was an estuary. 2009; Pearthree et al., 2009; Howard et al., The studies correlated Bouse outcrops within 2013) have greatly enhanced our understanding and between basins based on similar physi- *[email protected] of the distribution of Bouse deposits and their cal characteristics, including basal carbonate Geosphere; December 2014; v. 10; no. 6; p. 1139–1160; doi:10.1130/GES00988.1; 14 fi gures. Received 1 October 2013 ♦ Revision received 27 June 2014 ♦ Accepted 30 September 2014 ♦ Published online 12 November 2014 For permission to copy, contact [email protected] 1139 © 2014 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/6/1139/3335280/1139.pdf by guest on 24 September 2021 Pearthree and House 116° W 115° W 114° W ! Las Vegas Figure 1. Map showing regional setting of 36° N Mohave and Cottonwood Valleys in the 36° N lower Colorado River Valley. Inferred areas CALIFORNIA BlackBlack of paleolakes and modern water bodies are NEVADA shown with shades of blue. Heavy black lines show locations of inferred paleodams relevant to this paper. Fig. 2 ARIZONA PyramidPyramid ! Bullhead 35° N City 35° N ! Needles Explanaon ! TopockTopock Amboy ! Lake Havasu City River Reservoir AubreyAubrey Salton Sea BlackBlack Paleodivide 34° N Grand, 912 m ! Bouse CALIFORNIA Hualapai, 720 m Vegas, 650 m Blythe ! Coonwood, 400 m Mohave, 560 m Cibola Havasu, 360 m ! ARIZONA Blythe, 330 m Late Miocene to Pliocene Paleolakes Miocene to Late Modern Hydrology Locaon of study area 33° N ChocolateChocolate OR ID WY El Centro UT ! NV CO ! Yuma CA AZ NM MEXICO 50 km MEXICO PKH 116° W 115° W 114° W 1140 Geosphere, December 2014 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/6/1139/3335280/1139.pdf by guest on 24 September 2021 Paleogeomorphology of the early Colorado River (micritic limestone, marl, carbonate-cemented Spencer and Patchett (1997) and Meek and was also recognized (Unit B of Metzger et al., sandstone), tufa deposits, and much thicker Douglas (2001) revived the idea originally sug- 1973; Metzger and Loeltz, 1973). Based on out- clay, silt, and sand deposits of the interbedded gested by Blackwelder (1934) that the Bouse crops and well cuttings, Metzger et al. (1973) unit. Deposits interpreted to be Bouse Formation Formation records a series of lakes sequentially and Metzger and Loeltz (1973) described a range in elevation from ~200 m below sea level to fi lled by a downstream-developing LCR spilling suite of primarily quartz-rich sand and rounded, 330 m above sea level (asl) in the Parker-Blythe- through formerly closed basins, ultimately link- lithologically diverse river gravel in erosional Cibola (PBC) area (Fig. 1) (Metzger et al., 1973; ing to the upper end of the Gulf of California. contact with Bouse deposits; they interpreted Spencer et al., 2013), from ~100 to 370 m asl Strontium isotope ratios determined for Bouse these deposits as the earliest evidence for the in Chemehuevi Valley (Metzger and Loeltz, carbonate deposits throughout their latitudinal existence of the through-fl owing LCR. These 1973; our data), and from ~50 to 560 m asl in and elevation ranges are similar to one another deposits are found along the LCR from the the Mohave-Cottonwood area (Metzger and and to modern Colorado River values, but are mouth of the Grand Canyon to the modern delta; Loeltz, 1973; our data). The constraints on the not consistent with late Miocene marine water they are much coarser than Bouse deposits, and age of Bouse deposits were quite broad; it was (Spencer and Patchett, 1997; Poulson and John, have sedimentological characteristics consistent considered to be late Miocene to Pliocene in age 2003; Roskowski et al., 2010). As additional with subaerial deposition in a major river sys- (Metzger, 1968), ca. 5.5 Ma (Lucchitta, 1979), or Bouse outcrops have been discovered and docu- tem (Metzger and Loeltz, 1973; House et al., between 8 and 3 Ma (Buising, 1990). mented in more detail, it is apparent that their 2005, 2008b; Howard et al., 2015). Thus, Bull- Paleontological and sedimentologic investi- maximum elevations rise to the north in a step- head deposits represent the fi rst defi nitive evi- gations led many researchers to conclude that wise fashion mediated by bedrock divides, a dence of the existence of the continuous LCR all of the Bouse Formation was deposited in an pattern that is consistent with a north to south ending at the Gulf of California.