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Chronology of Miocene–Pliocene Deposits at Split Mountain Gorge, Southern California: a Record of Regional Tectonics and Colorado River Evolution

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Chronology of Miocene–Pliocene Deposits at Split Mountain Gorge, Southern California: a Record of Regional Tectonics and Colorado River Evolution Chronology of Miocene–Pliocene deposits at Split Mountain Gorge, Southern California: A record of regional tectonics and Colorado River evolution Rebecca J. Dorsey* Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA Amy Fluette Department of Geology (ES 236), 516 High Street, Western Washington University, Bellingham, Washington 98225-9080, USA Kristin McDougall Flagstaff Science Center, U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001, USA Bernard A. Housen Department of Geology (ES 236), 516 High Street, Western Washington University, Bellingham, Washington 98225-9080, USA Susanne U. Janecke Department of Geology, 4505 Old Main Hall, Utah State University, Logan, Utah 84322-4505, USA Gary J. Axen Department of Earth & Environmental Sciences, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA Catherine R. Shirvell Department of Earth & Space Sciences, University of California, Los Angeles, California 90095-1567, USA ABSTRACT B ca. 5.3 Ma Late Miocene to early Pliocene deposits at Split Mountain Gorge, . R o California, preserve a record of basinal response to changes in regional d a SAF r o A l tectonics, paleogeography, and evolution of the Colorado River. The o present day C base of the Elephant Trees Formation, magnetostratigraphically 116° 114° ? R. dated as 8.1 ± 0.4 Ma, provides the earliest well-dated record of exten- o S Y JF ad EF sion in the southwestern Salton Trough. The oldest marine sediments SAF North America WSDF are ca. 6.3 Ma. The nearly synchronous timing of marine incursion in Color WSDF R. S Gila the Salton Trough and northern Gulf of California region supports a SMG .T . SD model for localization of Pacifi c–North America plate motion in the Y S.T U.S . Gulf ca. 6 Ma. The fi rst appearance of Colorado River sand at the 32° . Mexico 32° 32° Miocene-Pliocene boundary (5.33 Ma) suggests rapid propagation of the river to the Salton Trough, and supports a lake-spillover hypoth- SF SF esis for initiation of the lower Colorado River. WB P P IT Keywords: California, stratigraphy, tectonics, Miocene, Pliocene, Salton 30° Baja Ca 30° 30° Trough, Colorado River. Pacific Plate liforn ia IT INTRODUCTION The Salton Trough is a large fault-bounded basin that occupies the 0 50 100 km 0 50 100 km Pacifi c–North America plate boundary in Southern California and north- Figure 1. A: Regional tectonic map showing major faults in south- western Mexico (Fig. 1). Late Cenozoic subsidence resulted from combined eastern California and northwestern Mexico, and location of Split slip on the dextral San Andreas fault on the NE and the oblique-normal West Mountain Gorge (SMG, star) in western Salton Trough. B: Recon- Salton detachment fault on the SW (Axen and Fletcher, 1998; Kairouz, struction for 5.3 Ma restores ca. 250 km of dextral offset based on 2005; Shirvell, 2006; Steely, 2006). Age estimates for the southern San data of Oskin and Stock (2003b). EF—Elsinore fault, IT—Isla Tiburón, P—Puertecitos, SAF—San Andreas fault, SF—San Felipe, SJF—San Andreas fault range from 10–13 Ma (Matti and Morton, 1993) to 5–6 Ma Jacinto fault, S.T.—Salton Trough, WB—Wagner basin, WSDF—West (Ingersoll and Rumelhart, 1999). Recent work provides evidence for rapid Salton detachment fault, Y—Yuma. Stipple pattern shows area of localization of plate motion in the Gulf of California ca. 6 Ma (Oskin and subaerial Colorado Delta deposition, observed in the modern setting Stock, 2003a, 2003b), but analysis of total extension suggests that dextral and inferred for 5.3 Ma. shear may have started in the Gulf as early as ca. 12 Ma (Fletcher et al., 2004). Improved dating of crustal deformation is needed to reconstruct the kinematic evolution of the plate boundary and test models for lithospheric timing and signifi cance of regional tectonic and geomorphic transitions rupture in the Gulf of California. in southwestern North America. This paper presents a high-resolution stratigraphic study of late Miocene to early Pliocene sedimentary rocks at Split Mountain Gorge TECTONIC SETTING in the southwestern Salton Trough (Figs. 1 and 2). The timing of earli- Split Mountain Gorge is a narrow canyon incised into late Ceno- est extension in this area was previously not well known (Kerr, 1984; zoic sedimentary rocks near the southwest margin of the Salton Trough Winker, 1987; Winker and Kidwell, 1996; Axen and Fletcher, 1998), and (Fig. 1). These deposits occupy the lower part of a thick Miocene to Pleis- the age of marine transgression has been widely but inaccurately cited tocene sedimentary section that accumulated in a large rift basin (John- as 4.3 Ma on the basis of existing magnetostratigraphy (Johnson et al., son et al., 1983; Kerr, 1984; Winker, 1987; Winker and Kidwell, 1996). 1983). Data presented here provide precise new controls on the age of The basin formed largely in the upper plate of the oblique, dextral-normal these deposits, allowing us to address long-standing questions about the West Salton detachment fault (Axen and Fletcher, 1998; Kairouz, 2005; Shirvell, 2006; Steely, 2006). The Split Mountain Group includes early to *E-mail: [email protected]. middle Miocene basalts of the Alverson Formation and rift-related alluvial © 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, January January 2007; 2007 v. 35; no. 1; p. 57–60; doi: 10.1130/G23139A.1; 2 fi gures; Data Repository item 2007020. 57 Stratigraphic Paleomagnetics sam- normal reversed polarity Nomenclature polarity Epoch age ples Paleobathymetry 1100 Microfossil Yuha Samples upper middle Mbr Neritic bathyal bathyal 02-4 4.49 04-48 FC05-35 0 150 500 02-3 02-1 02-2 FC05-34 (reworked Cretaceous) 1000 04-54 Nonionella basispinata 04-53 FC05-33 OMZ Elphidium poeyanum Ammonia beccarii 04-52 FC05-32 inflata Valvulineria Nunivak 04-51 04-50 FC05-31 04-55 FC05-30 04-33 Age (Ma) FC05-29 04-66 900 04-35 FC05-28 4.63 Hanzawaia nitidula Mud Bolivina subexcavata 04-72 FC05-27 Hills 04-32 04-71 FC05-26 marine rhythmites 4.80 Nonionella stella Mbr 04-70 FC05-25 04-69 Sidufjall FC05-24 04-68 800 J-1983 04-67 23 DEGUYNOS FORMATION DEGUYNOS 04-47 FC05-22 4.90 04-45 FC05-21 04-31 04-43 04-44 FC05-20 Cibicides mckannai 04-42? 5.00 FC05-19 Planktic Foraminifers 04-40 18 angulosa Trifarina 700 04-38 17 mudstone LOWER PLIOCENE Thvera FC05-16 Bolivina interjunctata 02-32 FC05-15 Globocassidulina globosa 04-30 FC05-14 IMPERIAL GROUP IMPERIAL 04-29 FC05-13 600 5.24 Wind 04-28 FC05-12 turbidites 04-27 02-31 FC05-11 Caves 04-26 04-25 base of Mbr 04-24 FC05-10 LSU 04-23 FC05-9 Amphistegina gibbosa 500 04-22 FC05-8.5 CRS Pliocene 04-21 FC05-8 5.33 02-30 FORMATION Upper Miocene Megabreccia subaq. sturztrom ? 04-20 02-29 FC05-6 FC05-7 (turbidite clast in m-breccia) 5.89 04-19 FC05-5 04-18 Lycium C3An.1n 04-17 FC05-4 400 02-28 FC05-3 marine turbidites (L-suite sst) LATRANIA 04-16 Mbr 6.14 02-27 FC05-2 * 04-15 marine 6.27 04-14 FC05-1 Fish Creek Gypsum pinches out here Split Mt. Sturz- subaerial sturztrom (large rock avalanche) strom C3An.2n 300 04-13 breccia “Red and large debris flow +/- rock avalanche Lower Mega- Grey stzstr.” 6.57 04-12 04-11 04-10 200 04-09 6.94 04-08 EXPLANATION C3Bn 04-06 UPPER MIOCENE 04-05 Species are persistently present 7.09 proximal alluvial fan, 04-07 overall coarsening-up 100 Species are rare or sporadic 04-04 04-03 conglomerate mbr From K. McDougall (unpubl. data) SPLIT MT. GROUP MT. SPLIT ELEPHANT TREES FM TREES ELEPHANT meters 04-02 * 04-01 CRS = Base of Colorado 0 02-24 conformable contact River-derived sandstone lower 02-20-23 red sandstone lower distal alluvial fan and tan sand- (possibly = Red Rock Fm., tan axial braided stream J-1983 = Approximate base of stone mbr base not exposed in Split ? sand- (exposed east of Johnson et al. (1983) section -100 Mtn Gorge) stone Split Mt. Gorge) Cretaceous granitic rocks mbr LSU = Lateral-spread unit (zone of soft-sediment deformation) and pre-Cret. metamorphic rocks thin red pebbly sandstone (possibly Red Rock Fm.) OMZ = Modern oxygen-minimum crystalline basement rock zone (400-1000 m) Figure 2. Measured section at Split Mountain Gorge showing results of stratigraphic, paleomagnetic, and micropaleontologic analysis. Species indicated are benthic foraminifers. m-breccia—megabreccia. 58 GEOLOGY, January 2007 deposits of the Elephant Trees Formation, and is overlain by marine depos- magnetochron (younger than 7.43 Ma). Using a sedimentation rate of 0.2–0.5 its of the Imperial Group. Sediment from the Colorado River accumulated mm/yr (nondecompacted, calculated between 0 and 450 m in Fig. 2) and a fi rst in a marine setting (Imperial Group) and later in a large fl uvial-deltaic maximum age of 7.43 Ma for site 02–23, we extrapolate through 140–190 m system (Palm Spring Group) that prograded into the basin during transla- of sandstone below that level and calculate an age of 8.1 ± 0.4 Ma for the base tion along the San Andreas fault (Winker and Kidwell, 1986, 1996). of the thickest part of the tan sandstone member. The catastrophically emplaced lower megabreccia is overlain by METHODS marine turbidites of the Latrania Formation along a sharp contact that We measured, described, and sampled the section at Split Mountain coincides with pinch-out of the Fish Creek Gypsum (Fig.
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