CRevolution 2: Origin and Evolution of the Colorado River System II themed issue Evidence for a marine incursion along the lower Colorado River corridor Kristin McDougall1 and Adriana Yanet Miranda Martínez2 1Geology, Minerals, Energy, and Geophysics Science Center, U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona 86001, USA 2Posgrado en Ciencias de la Tierra, Instituto de Geología, Universidad Nacional Autónoma de México, México D.F. C.P. 04510, México ABSTRACT INTRODUCTION be found (~74) and examining new samples collected in the past 15 yr (~100). The Bouse Foraminiferal assemblages in the strati- The depositional setting of the Bouse Forma- foraminiferal faunas were compared to forami- graphically lower part of the Bouse Forma- tion, preserved as a series of basins along the niferal faunas in saline lakes and estuaries. tion in the Blythe basin (lower Colorado lower Colorado River corridor, is controversial In addition, the presence of Charophytes and River corridor, western USA) indicate and is variously attributed to a lacustrine origin reworked microfossils was noted. Arguments marine conditions, whereas assemblages in in a series of lakes, or a marine incursion related supporting the presence of a saline lake were the upper part of the Bouse Formation indi- to the proto–Gulf of California, which affected examined in light of the foraminiferal analysis. cate lacustrine conditions and suggest the the Bouse Formation in the Blythe basin. The Flood deposits, which are cited as evidence of presence of a saline lake. Benthic forami- lacustrine origin suggests deposition of the lake deposition, do not occur or have not been niferal assemblages in the lower part of the Bouse Formation in a series of lakes that fi lled found in the Blythe basin; mapping, sedimen- Bouse Formation are similar to lagoonal and spilled from one basin to the next until sys- tary studies, and structural analysis of the area and inner neritic biofacies of the modern tem was integrated and the Colorado River was are inadequate to consider uplift or the lack of Gulf of California. Evidence suggesting a a through-fl owing river (House et al., 2005a, uplift as a viable means of proving deposition change from marine to lacustrine conditions 2005b, 2008; Spencer et al., 2005, 2008). Argu- in a marine or saline lake environment. Many includes the highest occurrence of planktic ments in favor of a lacustrine origin of the Bouse of these studies are currently ongoing. Isotopic foraminifers at an elevation of 123 m above Formation are based on isotopic analyses (Spen- data are frequently used to advocate a lacus- sea level (asl), the change from low diversity cer and Patchett, 1997; Buising, 1990; Poulson trine origin for the Bouse Formation. Although to monospecifi c foraminiferal assemblages and John, 2003; Roskowski et al., 2010; Spen- the Sr data suggest deposition in a lacustrine composed only of Ammonia beccarii (between cer et al., 2013), step-like maximum elevations environment, the data are limited for the older 110 and 126 m asl), an increase in abundance of the Bouse paleolakes suggesting that no uplift or basal Bouse Formation, and in the interval in of A. beccarii specimens (above ~110 m asl), or southward tilting has occurred since deposi- which marine microfossils and high Sr values increased number of deformed tests (above tion of the unit (Spencer et al., 2008, 2013), and overlap there is evidence of mixing of marine ~123 m asl), fi rst appearance of Chara (at sedimentological evidence of fl oodwater infl ux and river water. The high strontium isotope ~85 m asl), lowest occurrence of reworked derived from northern sources immediately values characteristic of the Bouse Formation Cretaceous coccoliths (at ~110 m), a decrease preceding Bouse Formation deposition in the are considerably higher than modern Colorado in strontium isotopic values (between 70 and Mohave and Cottonwood Valleys (House et al., River values, suggesting that additional study 120 m), and δ18O and δ13C values similar to 2008; Spencer et al., 2013). An alternative inter- is needed to understand the source and concen- seawater (between 70 and 100 m asl). Plank- pretation suggests that the Bouse Formation tration of the strontium before the marine or tic foraminifers indicate a late Miocene age accumulated in lakes along the northern part nonmarine signal can be determined. Carbon between 8.1 and 5.3 Ma for the oldest part of of the lower Colorado River corridor, but in the and oxygen isotopic ratios are also limited and the Bouse Formation in the southern part Blythe basin, the formation was initially depos- the marine or nonmarine interpretation of these of the Blythe basin. Benthic and planktic for- ited in a marine environment at the northern end values is reexamined. aminifers correlate with other late Miocene of the proto–Gulf of California (Buising, 1988, sections in the proto–Gulf of California and 1990). Arguments in favor of the marine origin GEOLOGIC SETTING suggest that the basal Bouse Formation in for the lower part of the Bouse Formation in the the Blythe basin was deposited at the north- Blythe basin are the presence and distribution of The Bouse Formation (Metzger, 1968) ern end of this proto-gulf. After the marine marine fossils. uncon formably overlies a Miocene fanglomer- connection was restricted or eliminated, The purpose of this paper is to document ate and is overlain by the Bullhead alluvium in the Colorado River fl owed into the Blythe the presence of foraminifers in the Bouse the Blythe basin, and is composed of a basal basin, forming a saline lake. This lake sup- Formation of the Blythe basin and determine limestone overlain by silts, clays, sands, and ported a monospecifi c foraminiferal assem- if they existed in a marine, saline lake, or a tufa (Fig. 1). Metzger (1968) designated blage of A. beccarii until the lake spilled into mixed environment. This goal was achieved 233.8 m of sediments penetrated in the U.S. the Salton Trough and the Colorado River by reexamining previously collected micro- Geological Survey test well Mf11684 as the became a through-fl owing river. paleontology slides and residues that could type section for the Bouse Formation (Fig. 2). Geosphere; October 2014; v. 10; no. 5; p. 842–869; doi:10.1130/GES00975.1; 13 fi gures; 12 tables; 1 plate; 1 supplemental fi le. Received 2 August 2013 ♦ Revision received 14 April 2014 ♦ Accepted 11 July 2014 ♦ Published online 29 August 2014 842 For permission to copy, contact [email protected] © 2014 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/5/842/3333907/842.pdf by guest on 26 September 2021 Marine incursion, Colorado River corridor PALEONTOLOGY Generalized Stratigraphy Formations or units Early paleontologic studies suggested that Figure 1. Generalized strati- Recent Alluvium the Bouse Formation was deposited in a marine graphic column and ages, based embayment at the northern end of the Gulf of on studies by Buising (1990, California during the latest Miocene to Pliocene. 1993) with the addition of radio- This interpretation is supported by the presence metric ages that constraint Bullhead Alluvium of benthic and planktic foraminifers (Durham the age of the Bouse Forma- and Allison, 1960; Hamilton, 1960; Metzger, tion. No scale is given due to the Lawlor Tuff 4.83 Ma 1968; Smith, 1960, 1969, 1970; Winterer , 1975; variable thickness of the Bouse Buising, 1988, 1990; McDougall, 2008a), as Formation. Dates in the Osborne well as other invertebrates (Metzger, 1968; Wash strata of Buising (1988) Bouse Formation Taylor, 1983; A. Cohen, 2012, personal com- are from Fugro, Inc. (1975, cited mun.) and vertebrates (Crabtree, 1989). Other in Reynolds et al., 1986) and fossils in the Bouse Formation include diatoms, Buising and Beratan (1993). Tbbf = Bouse basin fill ostracodes, coralline algae, barnacles, mollusks, The presence of the Lawlor Tbbm = Bouse basin margin gastropods, crabs, and fi sh (Table 1). The dis- Tuff and the dates for this tuff tribution of these groups is not addressed in are from Reynolds et al. (2008), this paper and needs additional study, as these Sarna-Wojcicki et al. (2011), tuff age 9.2 + 0.3 Ma groups include marine, brackish, and fresh- and Spencer et al. (2013). basalt age 9.6 + 0.6 Ma water species. Our study focuses primarily on Osborne Wash strata the foraminifers, which are primarily a marine of Buising (1988) group. Nonmarine occurrences of foraminifers are limited and distinctive, and therefore easily recognized. Micropaleontologic samples collected and Two additional reference sections were also the Blythe basin were summarized previously analyzed from various research efforts are designated: (1) outcrops south of Bouse Wash (Metzger et al., 1973; Olmsted et al., 1973; P.J. documented in Tables 2–11. The taxonomy was and east of the Colorado River fl ood plain, Fritts, 1975, unpublished report, provided by evaluated and notes are given in the Supplemen- where 65.5 m is exposed; and (2) outcrops in K. Howard, 2012, personal commun.; Winterer, tal File1. Some of the foraminiferal slides from sections 9 and 16, southeast of Cibola, where 1975; Turak, 2000). The Bouse Formation is outcrops and wells drilled in conjunction with 30.5 m of the basal limestone is exposed identifi ed in the Blythe basin between –173 m the Lower Colorado River Project (LCRP) and (Metzger, 1968). Approximately 46 m of the below sea level (bsl) and 330 m above sea level examined by P.B. Smith (Metzger et al., 1973; reference section south of the Bouse Wash (asl), (Metzger et al., 1973; Buising, 1990; Smith 1970) are available in the U.S. Geologi- was resampled for paleomagnetics and micro- Spencer et al., 2008, 2013). cal Survey Micropaleontologic collection (Flag- fossils (D.