Interplay of Oceanographic and Paleoclimate Events with Tectonism During Middle to Late Miocene Sedimentation Across the Southwestern USA

Interplay of oceanographic and paleoclimate events with tectonism during middle to late Miocene sedimentation across the southwestern USA

Charles E. Chapin New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA

ABSTRACT INTRODUCTION Miocene (ca. 27–16 Ma) and is refl ected in the unconformities and missing time-stratigraphic Continental sedimentation refl ects a com- The southwestern United States hosts exten- intervals (lacunas) beneath the Ogallala For- plex interplay of tectonics and climate. A sive continental sedimentary deposits of middle mation on the east fl ank of the Southern Rocky 2000-km transect from coastal California to and late Miocene age between the western Great Mountains and the Fence Lake and Bidahochi the western Great Plains documents a major Plains and the coast of California (Fig. 1). The formations on the west fl ank. Cather et al. increase in sedimentation (ca. 16–6 Ma) deposits occur within several physiographic (2007) document ~1230 m of exhumation of the coeval with deposition of the hemipelagic provinces and at variable elevations and tectonic southeastern Colorado Plateau between eolian Monterey Formation along the California settings. Deposition occurred mainly between accumulation of the Chuska erg (ca. 33.5– coast. Basin and Range-style regional exten- 16 and 6 Ma (Fig. 2), coeval with deposition 27 Ma) and deposition of the Bidahochi Forma- sion following elongation of the Pacifi c– of the hemipelagic Monterey Formation (Ingle, tion (16–6 Ma). Similar exhumation (~1500 m) North American transform boundary at 1981; Behl, 1999) along the California coast and timing (ca. 28–16 Ma) were obtained by ca. 17.5 Ma provided fault-bounded basins (Figs. 1 and 2). In several areas, the deposits Flowers et al. (2008) from (U-Th)/He thermo- for accommodation space, but sedimenta- accumulated following long intervals of erosion chronometry. This exhumation began during tion also occurred on unextended erosional and/or nondeposition (Fig. 2). The middle to late the peak of middle Tertiary ignimbrite volca- surfaces of the Great Plains and Colorado Miocene interval of continental sedimentation nism and supports the interpretations of Roy Plateau. Two global climate transitions ended between ca. 6 and 5 Ma, as integration et al. (2004) and Eaton (2008) of magmati- bracket this sedimentary interval. The of drainages brought about widespread incision, cally driven middle Tertiary uplift. The second middle Miocene transition (ca. 17–12 Ma) exhumation, and deposition of contrasting fl u- period of exhumation began in late Miocene records the global change from equatorial to vial deposits of exterior drainages (Eberly and (ca. 7–6 Ma) and is interpreted herein as result- meridional circulation caused by: (1) closing Stanley, 1978; Scarborough, 1989; Spencer et ing from intensifi cation of the North American of the eastern Tethys Seaway (ca. 18 Ma); al., 2001a; Connell, 2004; Mack, 2004; Smith, monsoon and integration of drainage systems (2) opening of the Arctic–North Atlantic 2004, Polyak et al., 2008). that largely ended accumulation of closed-basin connection (ca. 17.5 Ma); (3) growth of Tectonic events affecting middle and late continental deposits. Thus, the two periods of the East Antarctic Ice Sheet (ca. 14 Ma); Miocene sedimentation and erosion across the exhumation illustrated in Figure 2 document and (4) closing of the Indonesian Seaway Southwest were mainly of two types: (1) those both middle Tertiary tectono-magmatic–driven (ca. 12 Ma). Upwelling of cold waters along that resulted in accommodation space for aggra- uplift and late Miocene–Pliocene climatically the California coast, abetted by domina- dation of sedimentary deposits, and (2) those driven exhumation. tion of La Niña phases of El Niño–Southern that changed oceanic and atmospheric circula- The Miocene record of continental sedimen- Oscillation (ENSO), progressively aridifi ed tion, which affected both climate and sedimen- tation and erosion has traditionally been inter- the Southwest as refl ected in sedimentary tation. A correlation chart that summarizes tec- preted in terms of tectonic or epeirogenic uplift and biologic records. The second climate tonic events, oceanic and atmospheric changes, (for example, Trimble, 1980; Steven et al., 1997; transition occurred as opening of the Gulf of and various sedimentation and climatic effects Eaton, 1987, 2008) with little regard to possible California (ca. 6 Ma) intensifi ed the North in a temporal framework is presented as Fig- climatic effects (Molnar and England, 1990; American monsoon, resulting in integration ure 3. The timing of events is based on compila- Molnar, 2004). Since climate is determined of drainages, incision of uplifts, and exhu- tion of published dates of various types from the mainly by coupling of oceanic and atmospheric mation of basin fi lls. The Miocene ended literature. Ages and descriptions of stratigraphic circulation, I burrowed into the oceanographic with the driest climate of the Tertiary (both units are summarized in Appendix 1, with the literature seeking tectonic, oceanographic, and regional and global) accompanied by con- key references. paleoclimate events that had the appropriate version of savanna to steppe or scrub desert, Figures 2 and 3 show two periods of exhu- timing, scale, and location to have infl uenced the spread of C4 grasses, and the greatest mam- mation that affected the Southern Rocky Moun- Miocene sedimentation and erosion history of mal extinction of the Neogene. tains. The fi rst occurred in Oligocene to middle the southwestern USA. Two climate transitions

Geosphere; December 2008; v. 4; no. 6; p. 976–991; doi: 10.1130/GES00171.1; 4 fi gures.

976 For permission to copy, contact [email protected] © 2008 Geological Society of America

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125°W 120° 115° 110° 105° 100° W

GM NE ID 40° N CB SV OG WY 35 BP NP MP 30 25 GB GS D HP SF M 20 35 CR NV

CA UT SL 40 OG M CO KS AZ NM 35° N VR GC R OK B DS LV RGR 45 CP ES O SV RL 30 SB BS SF M V B A LA HP F 50 L S P R Pl W OG ME RG SP T HR BA EP 25 SC ° TX 30 N GOC ME 55

RG 0 500

KM

Figure 1. Shaded relief map of the southwestern United States from the western Great Plains on the right (east) to the Pacifi c Coast of California at far left. Northern Gulf of California and Baja Peninsula at lower left. The Southern Rocky Mountains, Rio Grande rift, and Colorado Plateau make up the high topography at center. Selected middle and upper Miocene terrestrial sedimentary deposits (red) were compiled from numerous sources cited in text and Appendix 1. Miocene basins in the Basin and Range province of Arizona and southwest- ern New Mexico are partly obscured by widespread gravel-covered pediments and Pliocene-Quaternary alluvial deposits. Only basins with published Miocene surface or subsurface stratigraphic data are shown. Exposures in Arizona are from Scarborough (1989) and those in southwestern New Mexico modifi ed from Wilks (2005). Monterey Formation and equivalent Miocene deposits of onshore California are shown in blue (from Graham and Williams, 1985, and Williams, 1988). Blue contour lines show the percentage of annual precipitation pro- vided by the North American Monsoon in July, August, and September for the 26-yr period 1963–1988 (from Higgins et al., 1999). Names of states are abbreviated inside their boundaries. A—Albuquerque, B—Bakersfi eld, B—Bidahochi Formation, BA—Baja California, BP—Browns Park Formation, BS—Barstow (Town and Formation), CB—Circle Bar basin, CP—Colorado Plateau, CR—Colorado River, D—Denver, DS—Dove Spring Formation, EP—El Paso, F—Fence Lake Formation, GB—Great Basin, GC—Grand Canyon, GOC—Gulf of California, GM—Granite Mountains basin, GS—Glenwood Springs, HP—High Plains, HR—Hatch–Rincon basin, LA—Los Angeles, LV—Las Vegas, M—Monterey Formation, ME—Mexico, MP—Middle Park, N—North Park, O—Ocate volcanic fi eld, OG—Ogallala Formation, P—Phoenix, PI—Picacho basin, R—Raton, R—Reserve graben, RG—Rio Grande, RGR—Rio Grande rift, S—Socorro, SB— Santa Barbara, SC—Sonoita Creek basin, SF—San Francisco, SF—Santa Fe, SL—San Luis Basin, SP—San Pedro trough, SV—Saratoga Valley, SV—Shadow Valley basin, T—Tucson, V—Verde Basin, VR—Virgin River depression, W—Winston graben. Base from U.S. Geo- logical Survey Shaded Relief Map, R.E. Harrison, 1969, scale 1:7,500,000. Underlined abbreviations used only to distinguish between localities with similar spelling.

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10 15 20 25 30 Ma

E

N. N.

ARIKAREE GROUP ARIKAREE OGALLALA WHITE R. GRP.

N. PLAINS OGALLALA

S. S.

LACUNA OGALLALA Ages of forma- ojave Desert. and fornia on the west (left) to ROCKS PERMIAN S. PLAINS OGALLALA MESOZOIC inty in age control. The age ranges of inty in age control.

op. Missing chronostratigraphic inter- op. Missing chronostratigraphic MULTI FMS FMS MULTI AND RIFT EARLY ROCKS BASINS VOLCANIC N. & C. RIO

GRANDE RIFT MULTI FMS FMS MULTI AND RIFT EARLY ROCKS BASINS VOLCANIC

SOUTHERN

NEW MEXICO

FM

LACUNA BIDAHOCHI FM and ROCKS

PERMIAN

BIDAHOCHI MESOZOIC

FM

LACUNA FENCE LAKE LAKE FENCE to FENCE ROCKS LAKE FM

MESOZOIC OLIGOCENE

MULTI FMS FMS MULTI ARIZONA TERRANES DETACHMENT

SOUTHERN INTEGRATION OF DRAINAGE ca. 6–3 Ma INTEGRATION FMS MULTI and LAKE MEAD LACUNA MEMBER RAINBOW ROCKS GARDENS

PERMIAN

MESOZOIC

SHADOW V. SHADOW

BARSTOW DOVE SPG. DOVE DETACHMENT TERRANES DETACHMENT

DESERT MOJAVE

FM MONTEREY FM PLATE BASINS BOUNDARY MONTEREY W ~2000 km 0 5 vals (lacunas) are shown in diagonally ruled pattern. Dashed lines above and below labeled stratigraphic units indicate uncerta vals (lacunas) are capitalized and listed below the columns. unconformities are tions of wide lateral extent underlying regional western Great Plains on the east (right). Formation names or areas, in the case of multiple formations, are listed across the t listed across in the case of multiple formations, are areas, Plains on the east (right). Formation names or western Great Arizona and the M southern shown for and partly overlapped with Basin Range extension are detachment terranes that preceded Figure 2. Correlation chart of Miocene continental and coastal sedimentary sequences along a 2000-km transect from coastal Cali chart of Miocene continental and coastal sedimentary sequences along a 2000-km transect from 2. Correlation Figure 10 15 20 25 30 Ma

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MIOCENE PLIOCENE PLST. AGE 16 14 12 10 8 6 4 2 Ma

HEMING. BARSTOVIAN CLARENDONIAN HEMPHILLIAN BLANCAN IRV. RA. NALMA PACIFIC – NORTH AMERICAN TRANSFORM BOUNDARY TECTONIC REGIONAL EXTENSION – HALF GRABENS AND FOOTWALL UPLIFT

TETHYS CLOSED CLOSURE OF INDONESIAN SEAWAY EVENTS

FRAM STRAIT OPENED GULF OF CALIFORNIA

UPWELLING ALONG CALIFORNIA CURRENT OCEAN EAST ANTARCTIC ICE SHEET – GLOBAL COOLING ATMOS- WESTERN PACIFIC WARM POOL–ENSO PHERE

WEST ANTARCTIC ICE SHEET EFFECTS

NORTH AMERICAN MONSOON

MONTEREY FORMATION COASTAL EFFECTS OGALLALA GROUP – NORTHERN PLAINS

OGALLALA FM – S. PLAINS

BIDAHOCHI FORMATION (AZ) CONTIN- ENTAL FENCE LAKE FORMATION (NM)

S. WYOMING–N. COLORADO INCISION

AND NORTHERN and CENTRAL RIO GRANDE RIFT

SOUTHERN NEW MEXICO EXHUMATION SOUTHERN ARIZONA

LAKE MEAD AREA EFFECTS MOJAVE DESERT

MIXED WOODLAND and C3 GRASSLANDS C4 STEPPE and DESERT

HYPSODONT UNGULATES

PIEDMOUNT–FLUVIAL SYSTEMS

RIO GRANDE

LOWER COLORADO RIVER

16 14 12 10 8 6 4 2 AGE Ma

Figure 3. Chart showing chronology of middle Miocene and younger tectonic events and their oceanographic, climatic, and sedi- mentologic effects. Antarctic and Arctic glaciations are included because of their signifi cance to the hemispheric thermal gradient and global oceanic and atmospheric circulation. The age ranges for events and effects are based on a compilation of published data cited in the text and Appendix 1. Boundaries of the North American land mammal “ages” (NALMA) are after Tedford et al. (2004) and Woodburne (2004). Abbreviations: AZ—Arizona; NM—New Mexico; ENSO—El Niño–Southern Oscillation; PLST.— Pleistocene; IRV—Irvingtonian; RA—Rancholabrean.

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stand out: (1) the middle Miocene global cli- ern Great Plains and in the basins of Wyoming thick sequences of anhydrite, halite, and gypsum mate transition (ca. 16.8–12 Ma) as defi ned by and northern Colorado generally overlie eolian (Eberly and Stanley, 1978; Scarborough and Shevenell and Kennett (2004), to which I add and fl uvial deposits of Oligocene–lower Mio- Peirce, 1978). Eolian sands and loess are com- effective closing of the Indonesian Seaway to cene age with variable, relatively minor, uncon- mon components of the sedimentary record. The surface and thermocline waters by 12 Ma (van formities between them (Swinehart et al., 1985; western Great Plains and southeastern Colo- Andel et al., 1975; Leinen, 1979; Keller and Tedford et al., 1987, 2004; Larson and Evanoff, rado Plateau experienced major erosional strip- Barron, 1983; Kennett et al., 1985; Romine 1998; MacFadden and Hunt, 1998). The reasons ping that cut deeply into underlying formations and Lombari, 1985; Kuhnt et al., 2004; Cloos for this contrast with more southerly depos- (Fig. 2) with removal of enormous volumes of et al., 2005) with its effect on ocean circulation its are the enormous quantities of volcanic ash sediment from the region. The abrupt change and initiation of the ENSO climate system; and blown northeastward from volcanic centers of from erosion to aggradation suggests that the (2) intensifi cation of the North American mon- Eocene to early Miocene age in the Great Basin changing climate had become so arid by 12 Ma soon by opening of the Gulf of California at and western Snake River Plains (Larson and that there was no longer adequate stream fl ow to ca. 6.4 Ma (Oskin and Stock, 2003). Evanoff, 1998) and greater erosion to the south carry erosional detritus from the region. Thus, discussed above. the Ogallala Formation on the western Great MIDDLE TO LATE MIOCENE The beginning of regional Basin and Range– Plains is essentially a runoff deposit consisting CONTINENTAL SEDIMENTATION style extension in early middle Miocene of superposed paleovalley cut-and-fi ll deposits (ca. 17–15 Ma, Dickinson, 2002; McQuarrie and (Fig. 4) that, together with eolian contributions, The age ranges of nine middle to upper Mio- Wernicke, 2005) provided both fault-bounded formed a widespread, relatively thin, blanket cene continental sedimentary formations, or basins for depocenters and uplifted basin mar- proximal to the Southern Rocky Mountains. groups of geographically related formations, gins for source areas. Earlier detachment-style The Gulf of Mexico is the terminal depocen- are shown diagrammatically on Figure 2 and deformation generally formed shallower, more ter for sediment eroded from the Great Plains summarized in Appendix 1. The timing of the complex supradetachment basins in southern and east fl ank of the Southern Rocky Moun- beginning and end of deposition across a west- Arizona and the Mojave Desert during late Oli- tains; it should refl ect the Neogene tectonic and to-east transect of ~2000 km from California to gocene and early Miocene (see for example, climatic history. However, multiple fans, growth the western Great Plains is remarkably similar. Spencer and Reynolds, 1991). Some overlap in faults, salt tectonics, sediment bypass, and sea For several of the sedimentary units, deposition both time and space exists between these two level fl uctuations make paleogeographic analy- began in middle Miocene after many millions of styles of extension (Fig. 2). The major half- ses diffi cult. Nevertheless, a few observations years of erosion and exhumation that removed graben basins of the Rio Grande rift overlie from the massive compilation of Galloway et older Cenozoic and Mesozoic formations. For poorly defi ned basins in some areas that are al. (2000) are pertinent here. The Oligocene example, the Ogallala Formation on the southern generally dominated by volcaniclastic deposits is recorded as an immense, long-lived infl ux Great Plains of Texas and New Mexico overlies (Chapin and Cather, 1994; Smith, 2004; Con- of recycled sedimentary and volcanic rocks, a gentle, southeast-sloping surface carved across nell, 2004; Mack, 2004). These early-rift basins and reworked ash into the Frio and Vicksburg rocks of Late Cretaceous to Permian age (Haw- formed (ca. 36–16 Ma) while erosion was strip- depocenters in the northwest and western Gulf ley, 1993; Gustavson, 1996). Vast quantities of ping the eastern and western fl anks of the rift Basin. The early Miocene interval (25–18 Ma) volcanic ash that must have been deposited on (Fig. 2); thus, they may represent localized records the maximum extent of the sandy basin the southern plains during the Eocene-Oligocene stretching during the late Eocene-Oligocene fl oor apron and the fi rst major sediment infl ux ignimbrite fl are-up (Chapin et al., 2004) of New magmatically induced uplift proposed by Roy et to the central Gulf, perhaps refl ecting the pre- Mexico, Arizona, and northern Mexico were al. (2004) and Eaton (2008). An alternative view Ogallala erosion from the fl anks of the Southern also eroded from the southern plains. The time is presented by Ingersoll (2001). Rocky Mountains. The middle Miocene inter- interval represented by the basal unconformity While subsidence of Basin and Range–style val (15.6–12 Ma) shows a marked reduction in of the Ogallala Formation is labeled a lacuna on half-graben basins was a major factor in the sediment infl ux to the western Gulf, which may Figure 2. It represents a specifi c chronostrati- roughly synchronous onset of continental sedi- refl ect both a lull in volcanism and sediment graphic interval between the age of the under- mentation, it was not the only factor. The depos- trapping in subsiding half-graben basins of the lying formations and the ca. 12 Ma age of the its on unextended terrain of the Great Plains Rio Grande rift and southern Basin and Range. basal Ogallala Formation (the lacuna diminishes (Fig. 1) and the Colorado Plateau blanket gently The middle Miocene interval also records the to the north). Major unconformities are also sloping surfaces that lack subsidence features onset of an energetic deep Gulf current by the present beneath the Bidahochi and Fence Lake except in local areas of evaporite dissolution fi rst deposition of contourite drift deposits in the formations on the west side of the Rio Grande (Hawley, 1993; Gustavson, 1996). Could a major western Gulf and erosional truncation along the rift–Southern Rocky Mountains (Fig. 1). These change in climate be the other factor? Middle to Florida carbonate ramp (Mullins et al., 1987). lacunas refl ect uplift of the Southern Rocky late Miocene aridity in the southwestern USA is The late Miocene interval (12–6.4 Ma) records Mountains during the Eocene-Oligocene peak evidenced by several sedimentologic and bio- the fi nal decline of the western and northwest- of ignimbrite volcanism, as suggested by Roy et logic indicators. Caliche horizons are common ern sediment dispersal systems and the shift of al. (2004) and Cather et al. (2007). Beginning in in sedimentary deposits throughout the region fl uvial infl ux to the Mississippi delta system the middle Miocene, sedimentation began across (Appendix 1; Holliday, 1987; Hawley, 1993) and of the central Gulf. The latest Miocene–early the eroded fl anks of the Southern Rocky Moun- became increasingly abundant in late Miocene. Pliocene interval (6.4–4.2 Ma) is dominated by tains approximately concurrent with deposition Internally drained basins containing alkaline the Mississippi delta system as refl ected in a of other middle Miocene continental deposits and saline lakes and playas left evaporite depos- broad bulge of the central Gulf margin. The Rio elsewhere in the southwestern USA. its from Wyoming to California (Eberly and Grande had not yet reached the Gulf, and much In contrast to the southern Great Plains, mid- Stanley, 1978; Flanagan and Montagne, 1993). of the drainage from southwestern USA fl owed dle Miocene continental deposits on the north- Several basins in southern Arizona accumulated southwestward to the Gulf of California.

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South North Elevation 5100 ft G (1555 m) F B D 4900 F F G F E B B E C 4700 A Agate Ash 4500 Present land Surface 21.3 Ma Niobrara River at Agate 4300 Volcanic Ash Beds 4100 ft (1250 m)

G Snake Creek Fm. (Ash Hollow Fm) Early Clarendonian and Hemphillian (ca.11.5– 9.5 Ma) Ogallala F Olcott Fm. Early and Medial Barstovian (ca.16.0– 15.0 Ma) Group E Sheep Creek Fm. Late Hemingfordian (ca.16.9– 16.0 Ma) D Box Butte Fm. Late Hemingfordian (ca.17.6– 17.0 Ma) C Runningwater Fm. Late Medial Hemingfordian (ca.18.9– 17.5 Ma)

Arikaree B Marsland (Upper Harrison) Early Hemingfordian ? Group A Harrison Fm. Late Arikareean (ca. 23–20 Ma)

Figure 4. Diagrammatic north-south cross section of Ogallala paleovalleys between Scotts Bluff, Nebraska, on the North Platte River and Agate on the Niobrara River (modifi ed from Skinner et al., 1977, Fig. 12). The section is based on detailed mapping and paleonto- logical work by the above authors in a 52 km2 (20 mi2) area and wider reconnaissance. See Figure 3 for age ranges of North American land mammal “ages” (NALMA). The equivalent numerical ages in Ma were measured on Figure 6.2 of Tedford et al. (2004) and are only approximations. Note that the basal Ogallala Runningwater paleovalley cut almost as deep as the present Niobrara River at Agate. The Snake Creek beds have since been correlated with the widespread Ash Hollow Formation that comprises the upper Ogal- lala Group in Nebraska (Tedford et al., 1987). More than 7 m.y. of geologic time is represented in ~170 m of net thickness Ogallala Group deposited during several cut-and-fi ll cycles. Fm.—Formation.

MIDDLE MIOCENE TECTONIC AND the Mojave Desert, coeval with rotation of the rapid extension of the central Basin and Range OCEANOGRAPHIC EVENTS western Transverse Ranges (Crouch and Suppe, province (Fig. 1) between the Colorado Plateau 1993; Nicholson et al., 1994; Miller, 2002; and the Sierra Nevada block (Dickinson, 2002; For about ten million years following initial Dickinson, 2002). These episodes of large- McQuarrie and Wernicke, 2005) and acceler- contact between the Pacifi c and North American magnitude, “ductile” extension are represented ated extension in the Rio Grande rift east of the plates opposite southern California or northern on Figure 2 as detachment terranes that precede Colorado Plateau (Chapin and Cather, 1994). Baja at ca. 28 Ma, the southwestern margin and partially overlap with the widespread mid- Widespread half-graben development provided of North America underwent a very complex dle Miocene Basin and Range extension. both accommodation space for aggradation of structural evolution (Atwater, 1989; Nicholson In late early Miocene (ca. 17.5 Ma), the continental sediments and uplifted basin mar- et al., 1994). The Farallon plate fragmented into partially subducted Monterey and Arguello gins for source areas. Numerous continental- several partially subducted microplates, and the microplates were captured by the Pacifi c plate, margin basins (Figs. 1 and 2) formed along the Pacifi c–North American transform boundary greatly elongating the Pacifi c–North American new Pacifi c–North American plate boundary, grew amidst a welter of rotating blocks, numer- transform boundary as the Rivera triple junc- including the basin-and-ridge submarine topog- ous strike-slip faults, slab windows, volcanism, tion jumped south past the California border- raphy of the southern California borderlands and large-magnitude regional extension (Crouch lands block (Nicholson et al., 1994; Atwater and (Ingle, 1981; Dickinson, 2002). and Suppe, 1993; Nicholson et al., 1994; Atwa- Stock, 1998; Miller, 2002; Dickinson, 2002). Late early Miocene (ca. 19–17 Ma) also ter and Stock, 1998; Dickinson, 2002). Begin- An 800-km segment of the North American saw the opening and closing of seaways ning ca. 24–22 Ma, detachment faulting and tec- plate margin was then free to expand westward between ocean basins that resulted in major tonic exhumation affected the inner California behind the northwest-moving Pacifi c plate effects on oceanic circulation, global climate, borderlands, Los Angeles Basin, Colorado River (Dickinson, 2002; McQuarrie and Wernicke, and sedimentary environments. Research by extensional corridor, southwestern Arizona, and 2005). Space for plate-margin expansion led to Vogt (1972), Schnitker (1980), and Wright

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and Miller (1996) indicates that subsidence of wet climates in both central Europe (Boehme, ation of ocean basins and marine circulation, are the Greenland-Scotland ridge permitted over- 2003) and the western USA (Retallack, 2007); required to explain the middle Miocene climatic fl ow from the Arctic Ocean and Norwegian- (7) increases in mean annual temperatures in optimum and subsequent Antarctic glaciation. Greenland Sea into the North Atlantic, produc- both Europe (Boehme, 2003) and western North The second group of hypotheses focuses on ing a major fl ux of Northern Component Water America (Wolfe, 1994) beginning ca. 20–18 Ma, tectonic opening and closing of seaways between (proto–North Atlantic Deep Water) between as shown by paleobotanical records, followed by ocean basins and the consequent changes in ca. 20 and 15 Ma (Wright and Miller, 1996). decreases at ca. 13 Ma; and (8) a transition from ocean circulation. The cumulative effects of clos- Coring of the Lomonosov Ridge in the central wet-based alpine glaciers to cold-based alpine ing of the eastern Tethys Seaway (ca. 18 Ma), Arctic Ocean by the Integrated Ocean Drilling glaciers in the Olympus Range of Antarctica opening of the Arctic–North Atlantic connection Program in 2004 (Jakobsson et al., 2007) pro- before 13.94 Ma as determined by mapping of through opening of Fram Strait (18.2–17.5 Ma), vided sedimentologic evidence and detailed age glacial deposits and 40Ar/39Ar dating of interbed- and subsidence of the Greenland-Scotland ridge control for the initiation of bidirectional fl ow ded volcanic ash layers (Lewis et al., 2007). This (ca. 20–15 Ma), increased upwelling along oce- between the Arctic and North Atlantic oceans brackets the middle Miocene thermal maximum anic boundary currents as indicated by initiation through Fram Strait between Greenland and in the Olympus Range between ca. 17 Ma (off- of organic-rich sedimentation along both the Svalbard. Jakobsson et al. (2007) report that shore sediment records, Hambrey et al., 2002) Pacifi c (Monterey Formation, ca. 18–16 Ma), and the transition began at 18.2 Ma, was completed and 13.94 Ma. The change to cold-based glacia- Atlantic coasts of the United States (phosphorite by 17.5 Ma, with exchange strengthening by tion is estimated to involve a drop in mean annual deposits, ca. 18–13 Ma). The global distribution of 13.7 Ma when Fram Strait began to open at temperature of 25–30 °C (Lewis et al., 2007). warmth during the middle Miocene climatic opti- great depths (present sill depth is >2000 m with The above list is not comprehensive but is mum (ca. 17–15 Ma) strongly suggests a major a width of 7400 km, Jakobsson et al., 2007). At intended to put a “face” and some constraints increase in thermohaline circulation. Schnitker about the same time (ca. 18 Ma), the eastern on a rather enigmatic climate interval of the (1980) proposed that early North Atlantic Deep Tethys Seaway between Africa and Eurasia was Miocene. The unusual warmth was the open- Water resulted from Norwegian Sea Overfl ow closed as evidenced by exchange of a diverse ing phase of the middle Miocene global climate Water, which then traveled down the length of fauna across a land bridge (Keller and Barron, transition (16.8–11 Ma) as defi ned by Shevenell the Atlantic Ocean and upwelled as an intermedi- 1983; Boehme, 2003; Prothero, 2006). Clos- and Kennett (2004). It was followed immedi- ate water mass into the circum-Antarctic system. ing of the eastern Tethys Seaway diverted part ately (Shevenell and Kennett, 2004) by growth The large volume of relatively warm and saline of the latitudinal, semi-equatorial global ocean of the East Antarctic Ice Sheet between ca. 15 North Atlantic Deep Water injected into the cold circulation, active since the Cretaceous (Haq, and 13 Ma with peak growth between ca. 14.2 circum-Antarctic environment contained heat 1981), and partially isolated the Mediterranean and 13.8 Ma (Lear et al., 2000; Shevenell et al., that could be converted to latent heat by evapora- Sea. Saline outfl ow waters of the Mediterranean 2004; Shevenell and Kennett, 2004; Holbourn tion (Schnitker, 1980). The resultant high evapo- fl owed northward in the Atlantic and became an et al., 2005). Hypotheses to explain the middle ration rates supplied moisture to the Antarctic important factor in generation of Northern Com- Miocene climate transition fall into two groups Continent that had been precooled since its isola- ponent Water (Reid, 1979; Price et al., 1993). with the Monterey Formation central to both. tion from the warmth of low-latitude oceans by Opening of the Arctic–North Atlantic con- The fi rst group includes the Monterey hypothesis formation of the Antarctic Circumpolar Current nection and closing of the eastern Tethys had a of Vincent and Berger (1985), which posits that at ca. 25–23 Ma (Lyle et al., 2007). dramatic effect on global climate, as refl ected burial of organic carbon in marginal marine sedi- Another plate-tectonic event that contributed in the middle Miocene climatic optimum. ments caused global cooling by drawing down to Monterey deposition and the middle Miocene

This relatively brief interval (ca. 17–14 Ma) is atmospheric CO2. Hodell and Woodruff (1994) climate transition was progressive restriction of characterized by: (1) a conspicuous decrease noted the temporal coincidence of eruption of the the Indonesian Seaway by northward movement (ca. 17–14 Ma) in δ18O in the deep-sea, oxygen- Columbia River fl ood basalts (ca. 17–14.5 Ma) of the Australian Continent and growth of the isotope curve of Zachos et al. (2001); (2) a some- with the middle Miocene climatic optimum and Indonesian archipelago. The best evidence for δ13 what broader increase in C (ca. 17–13 Ma) on proposed that volcanic emissions of CO2 com- timing is development of the equatorial under- the deep-sea, carbon-isotope curve of Zachos et pensated for Monterey-type burial of organic current system (ca. 12 Ma) that returns warm al. (2001); (3) the beginning (ca. 18–16 Ma) of matter, thus explaining the 3-m.y. lag between water to the eastern Pacifi c from the pileup of strong upwelling of cold, nutrient-rich waters the beginning of Monterey deposition and surface waters against the Indonesian blockage along the coast of California that began aggra- growth of the East Antarctic Ice Sheet. (van Andel et al., 1975; Leinen, 1979; Keller and dation of the diatomaceous Monterey Forma- The Monterey hypothesis and modifi cations Barron, 1983; Romine and Lombari, 1985; Ken- tion (Ingle, 1981; Pisciotto and Garrison, 1981; thereof seemed very logical, but research on nett et al., 1985). Also tectonic studies by Cloos

Keller and Barron, 1983); (4) partly contempora- Miocene atmospheric CO2 levels using various et al. (2005) indicate that the Central Range

neous deposition of upwelling-related phospho- proxies indicated that CO2 concentrations were orogeny created the mountainous spine of New rite deposits (ca. 19–13 Ma) along the Atlantic low and relatively invariant. The authors, prox- Guinea by ca. 12 Ma. Equatorial Pacifi c waters

Coast from Florida through North Carolina as ies, and CO2 levels are: Pagani et al. (1999), were diverted into the North and South Pacifi c the proto–Gulf Stream fl owed around bathymet- carbon isotopic analyses of alkenones in marine gyres, thus strengthening the California and ric highs on the continental margin (Riggs, 1984; algae and carbonates from surface-dwelling Peru boundary currents that bring cold, nutrient- Compton et al., 1990; Riggs et al., 1997); (5) foraminifera, 260–190 parts per million by rich waters toward the equator. The Coriolis migration of warm-water molluscan taxa as far volume (ppmv); Pearson and Palmer (2000), effect, which defl ects winds and surface waters north as Kamchatka and Alaska (62°N) between boron-isotope ratios of planktonic foraminifer to the right in the Northern Hemisphere and to 15.9 and 14.9 Ma (Oleinik and Marincovich, shells, <500 ppmv; and Royer et al. (2001), leaf the left in the Southern Hemisphere, produced 2002); (6) formation of lateritic weathering stomatal indices, 300–450 ppmv. These authors offshore fl ow via the Ekman effect and inten- products and paleosols characteristic of warm, suggest that other factors, such as tectonic alter- sifi ed upwelling of cold subthermocline waters

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along the coasts of California and Peru (Ingle, an 1100-km-long, NNW-trending conduit a broad strike valley that contains basalt fl ows 1981; Soutar et al., 1981). from the tropical Pacifi c to southwestern North from 7.4 Ma to Quaternary in age (Stroud, 1997; In addition to strengthening gyral circulation America (Hales, 1972). Such a long fetch over Olmsted and McIntosh, 2004) that directly over- in the Pacifi c, closing of the Indonesian Seaway very warm water greatly increased advection of lie Cretaceous and older rocks. The Ogallala For- provided the basis for the ENSO system. ENSO water vapor by southerly winds with resultant mation was largely eroded from the Raton area results from an anomalously warm body of sur- intensifi cation of the North American monsoon (Fig. 1) before 7 Ma. In the Ocate volcanic fi eld, face water that moves back and forth along the (Adams and Comrie, 1997; Hunt and Elders, 80 km to the south (Fig. 1) basalt fl ows dated equatorial belt in the Pacifi c, depending on the 2001). Today, the North American monsoon by 40Ar/39Ar at 6.6–6.1 Ma (Olmsted and McIn- strength of the easterly trade winds and modifying provides 25%–50% of the annual precipitation tosh, 2004) decline in elevation from 2800 m to effects of ephemeral bursts of westerly winds and in the region from the western Great Plains to 1950 m over a distance of 60 km on a southeast- eastward-propagating Kelvin waves (see reviews western Arizona, as shown by the contours on facing slope. This relationship was previously by Cane [1986] and Wang and Picaut [2004]). Figure 1 (Higgins et al., 1999). There is now interpreted as evidence of uplift of the Rocky Restriction of surface and thermocline waters by general agreement that the bulk of monsoon Mountains relative to the High Plains (O’Neill the rise of the Indonesian archipelago (ca. 12 Ma) moisture is advected at low levels from the east- and Mehnert, 1988). However, fi eld relation- caused the buildup of a vast pool of warm sur- ern tropical Pacifi c and the Gulf of California ships indicate that the basalts fl owed downslope face waters in the western Pacifi c that previously (Douglas et al., 1993; Higgins et al., 1999; Hunt into the strike valley of the Canadian River after would have been driven into the Indian Ocean by and Elders, 2001; Mitchell et al., 2002; Bordoni erosion removed the Ogallala Formation. Drain- the easterly trade winds. The resultant Western et al., 2004). These studies have shown that mon- age integration in northeastern New Mexico was Pacifi c Warm Pool (WPWP) is the largest single soon rainfall does not occur prior to the onset similar in timing to other areas of the Southwest, expanse of anomalously warm open-ocean water of Gulf of California sea-surface temperatures as shown on Figure 2; incision in the Raton and on Earth, covering an area larger than the conti- (SSTs) exceeding 26 °C and that the incremental Ocate areas of as much as 750–1000 m occurred nental United States and containing waters 2 to advance of SSTs >26 °C up the mainland coast between 8 and 6 Ma (Chapin, 2002; Olmsted and 5 °C warmer than other equatorial waters (Yan et of Mexico appears necessary for northward McIntosh, 2004). Similar strike valleys eroded al., 1992). When the trade winds weaken, waters advance of the monsoon. Additional moisture in Paleogene and Cretaceous formations parallel of the WPWP fl ow back to the central and eastern is carried overland at higher levels in the tropo- the east fl ank of the Southern Rocky Mountains Pacifi c producing the El Niño, or warm phase of sphere from the Gulf of Mexico (Meehl, 1992) in Colorado and contain the major cities of the ENSO, which reoccurs on a 2- to 7-yr time frame by clockwise circulation around a high-pressure Front Range urban corridor. (Wang and Picaut, 2004). system centered over Texas. Monsoonal fl ow is Aggradation of most middle to upper Mio- ENSO is an important factor in the regional frequently interrupted or enhanced during the cene sedimentary formations ended in lat- climate of the Southwest for several reasons. summer by passage of high- and low-pressure est Miocene time (Fig. 2), as formerly closed Most importantly, the relatively cold eastern systems across the southwestern United States. basins were integrated into regional drainages. Pacifi c promotes atmospheric high pressure that Additional late Neogene uplift of the Southern The oldest known record of the Rio Grande is defl ects the subtropical jet stream toward the Rocky Mountains and Colorado Plateau (Trim- an axial river gravel in the Santa Domingo basin Pacifi c Northwest during the cold (La Niña) and ble, 1980; McMillan et al., 2002; Steven et al., between Albuquerque and Santa Fe, which near-neutral phases (~80% of the 1948–1993 1997; Eaton, 2008) would also have increased contains rhyolitic pumice dated by 40Ar/39Ar at period; U.S. National Oceanic and Atmospheric monsoonal circulation by providing a more 6.9–6.8 Ma (Smith et al., 2001). By ca. 5 Ma, Administration (NOAA) National Weather Ser- elevated heat source. The consequent increase the Rio Grande had integrated several basins vice Climate Prediction Center (CPC) web site, in the regional land–sea temperature contrast to the south by basin fi lling and overtopping 2005), leaving the Southwest relatively dry. In would facilitate convection and drawing in of (Connell, 2004) and was emptying into paleo- contrast, during El Niño phases, the subtropi- low-level moisture from the Gulf of California lake Cabeza de Baca in the Hueco Basin near El cal jet is steered directly across the southern (Meehl, 1992; Bordoni et al., 2004). Paso, ~500 km to the south (Mack, 2004). tier of states, resulting in heavy snow packs in The lower Colorado River is younger than the higher terrain and widespread winter rains, LATE MIOCENE EXHUMATION AND 6 Ma at the mouth of the Grand Canyon, as both of which affect the continental sedimentary DRAINAGE INTEGRATION evidenced by the 5.97 ± 0.07 Ma 40Ar/39Ar age record. The bimodal southwestern winter, arid (biotite) of a tuff bed near the top of the Hualapai most of the time, but with episodic extremes of Thunderstorms generated by monsoonal Limestone (Spencer et al., 2001a). The Colorado moisture and runoff, was an important contrib- moisture (Mann and Meltzer, 2007; Tucker et River arrived at the head of the early Pliocene uting factor in aggradation of middle Miocene al., 2006) dramatically increased erosion of the Gulf of California by ca. 5 Ma, as evidenced by continental deposits. semiarid Southern Rocky Mountains–western 40Ar/39Ar ages of 5.51 ± 0.13 Ma (sanidine) of a Great Plains area in latest Miocene. Incision of tuff bed underlying the Bouse Formation mid- LATE MIOCENE TECTONIC AND strike valleys along the east fl ank of the South- way to the Gulf (House et al., 2005) and 5.01 OCEANOGRAPHIC EVENTS ern Rocky Mountains beheaded the Ogallala ± 0.09 Ma (glass, minimum age) on a tuff bed depositional systems from sources of sedi- in the Bouse Formation near the Gulf (Spencer A change in direction of the Pacifi c plate to ment in the mountains. The western edge of the et al., 2001a). The Bouse Formation consists of a more northerly trend between ca. 8 and 6 Ma remaining Ogallala outcrop belt retreated rapidly lacustrine and fl uvial sedimentary deposits of the (Cox and Engebretson, 1985; Atwater and Stock, 10–140 km to the east (Fig. 1), where it now caps Colorado River (Spencer et al., 2001a; Spencer 1998) resulted in capture of the Baja Peninsula local drainage divides as high as 250 m above and Pearthree, 2001). How the Colorado River by the Pacifi c plate and opening of the Gulf the bottoms of strike valleys. In northeastern integrated the 500-km stretch from the Grand of California by ca. 6.4 Ma (Oskin and Stock, New Mexico, the upper Canadian River parallels Canyon to the Gulf of California has been a 2003). The resultant marine incursion provided the Rocky Mountains–High Plains boundary in subject of controversy. Headward erosion and

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capture of an inferred northwestward-fl owing increasing aridity and openness of the landscape 5 m.y. (Donnelly, 1982; Janecek and Rea, 1983; upper Colorado was proposed by Luchitta (1979, (Fortelius et al., 2006). Hay, 1988; Molnar, 2004). Recent research on 1989). More recently, top-down river integra- The late Miocene climate change occurred the late Miocene–early Pliocene (ca. 8–4 Ma) tion by progressive spillover from formerly between ca. 7 and 5 Ma, as opening of the Gulf of biogenic bloom (Hermoyian and Owen, 2001; closed basins was proposed by Spencer and California intensifi ed the North American mon- Diester-Haass et al., 2004) in the World Ocean, Pearthree (2001), Meek and Douglass (2001), soon. This latest Miocene climate change was also known as the late Miocene carbon shift Scarborough (2001), and House et al. (2005). part of a global climate change, often referred to (LMCS) (Tedford and Kelly, 2004), indicates These recent papers present well- reasoned argu- as the terminal Miocene event, or the Messinian, a widespread increase in nutrient fl ux, perhaps ments for downstream extension by basin fi lling after the European stage during which dramatic due to the combined effects of aridifi cation and and overtopping. The similarity in timing of the changes occurred in the Mediterranean area (see increased monsoonal runoff. opening of the Gulf of California with the end summaries by Cita and McKenzie [1986] and of aggradation of most middle to late Miocene Hodell et al. [1986]). The latest Miocene was the DISCUSSION AND CONCLUSIONS continental sedimentary deposits (Fig. 2) and driest part of the Tertiary (Axelrod, 1981; Webb the top-down integration of the Rio Grande and and Opdyke, 1995) and was marked by major The middle Miocene global climate transi- Colorado rivers suggests that the monsoonal cli- changes in both fl ora and fauna. Grasses under- tion, defi ned by Shevenell and Kennett (2004) mate was likely responsible for major changes in went a global transition from the C3 to C4 pho- as the interval ca. 16.8–12 Ma (also reported as hydrologic regimes in the southwestern USA. tosynthetic pathway that began earlier in tropical 11 Ma) was a multiphase revolution in Ceno- regions (ca. 8–7.5 Ma) and expanded into sub- zoic climate. The transition was from warmer THE BIOLOGIC RECORD OF CLIMATE tropical and temperate regions at higher latitudes and wetter earlier to cooler and drier later. It CHANGE between ca. 7 and 4 Ma (Cerling et al., 1997). started with the unusual warmth of the Miocene C4 grasses have a competitive advantage under Climatic Optimum (MCO, ca. 17–14 Ma), fol- Sedimentation and erosion history can conditions of aridity, higher temperatures, low lowed immediately (Shevenell and Kennett,

indicate climate change, but proof rests with atmospheric CO2, and strong seasonality with 2004) by atmospheric and deep-ocean cooling, changes in the biologic response to changing a summer growing season (Cerling et al., 1997; culminating in peak growth of the East Antarctic environmental conditions. The middle Miocene Tipple and Pagani, 2007). Today, C4 grasses Ice Sheet (EAIS) between ca. 14.0 and 13.8 Ma climate transition was superposed on a long- dominate the Great Plains from Mexico to the (Shevenell et al., 2004; Holbourn et al., 2005). term cooling and drying trend that began at Canadian border, north of which C3 grasses The interval 13.8–12 Ma recorded continued ca. 50 Ma, following the early Eocene climatic predominate (Wang et al., 1994). Timing of the growth of ice on Antarctica with deep-water cir- optimum (Wing, 1998; Zachos et al., 2001). C3–C4 transition in southwestern USA has been culation in the Southwest Pacifi c dominated by Between ca. 20 and 15 Ma, forested habitats documented by changes in carbon- isotope ratios Southern Component Water and a strong infl ux gave way to mixed woodland-grassland mosa- measured in fossil horse teeth (Wang et al., of Pacifi c Deep Water (Hall et al., 2003; Shev- ics (Fig. 3), and ungulate (hoofed) herbivores 1994; Latorre et al., 1997; Cerling et al., 1997) enell and Kennett, 2004). I suggest that effective underwent an impressive diversifi cation, dur- and in pedogenic carbonate (Quade et al., 1989; closure of the Indonesian Seaway by ca. 12 Ma ing which many species evolved high-crowned Retallack, 1997, 2001). (Keller and Barron, 1983; Kennett et al., 1985; (hypsodont) teeth adapted for grazing and/or The latest Miocene and early Pliocene also Cloos et al., 2005) is an important third phase of gritty vegetation (Webb, 1983; MacFadden, witnessed the greatest land-mammal extinc- the middle Miocene climate transition. 1992, 2005; Webb and Opdyke, 1995; Janis et tion of the Neogene, during which 62 genera Progressive restriction and effective closure al., 2002). In addition to hypsodont teeth, sev- (35 of which were large mammals) disappeared of the Indonesian Seaway as Australia moved eral ungulate lineages (equids, rhinoceroses, from North America (Webb, 1984; Webb and northward and a complex volcanic archipelago oreodonts, and camels) also developed long Opdyke, 1995). The stable assemblage of the was constructed above subduction zones in the legs and large size, which traditionally has been middle Miocene (Clarendonian) chronofauna convergence zone with Asia produced major interpreted as evidence that savanna or grass- evolved the greatest diversity of land mammals changes in ocean circulation. Most notable were land vegetation spread during this time (Webb of the entire Cenozoic and was the peak of equid strengthening of Pacifi c gyral circulation and and Opdyke, 1995; Jacobs et al., 1999; Strom- (horse) diversity, with as many as 13 contem- associated upwelling, generation of the Western berg, 2006). However, the assumption that poraneous genera and 15 species (MacFadden, Pacifi c Warm Pool as easterly trade winds piled tooth crown height in equids was an evolution- 1992). This explosive radiation of browsing and up warm surface waters against the Australian- ary response to open, grass-dominated habitats grazing ungulates occurred as forests were con- Indonesian blockage, initiation of the Equatorial has been questioned by Stromberg et al. (2007) verted to mixed woodlands and grasslands under undercurrent system that returns thermocline based on studies of fossilized plant opal (phyto- increasingly cool and dry conditions (MacFad- waters to the eastern Pacifi c from the buildup in liths) that indicate a 4-m.y. lag between spread den, 1992). After ten million years of stability, the West, and the El Niño–Southern Oscillation of grass-dominated habitats in early Miocene the Clarendonian chronofauna was decimated at (ENSO) system of coupled oceanic and atmo- (ca. 22 Ma) and development of full hypso- the end of the Miocene, as the higher produc- spheric effects as warm waters of the Western donty. Research by Fortelius et al. (2006) sug- tivity savanna mosaic was replaced by exten- Pacifi c Warm Pool fl ow back to the central and gests a strong relationship between local mean sive, largely treeless, arid steppes, such as the eastern Pacifi c when trade winds weaken. Dating hypsodonty and local mean annual precipitation southern Great Plains (MacFadden, 1992; Webb more precise than middle Miocene (Kennett et in modern mammal communities. Hypsodonty and Opdyke, 1995), and by scrub deserts to the al., 1985) has been diffi cult, but paleontological is not restricted to grass eaters today, and not southwest. The late Miocene aridifi cation is also and paleomagnetic dating of sharply increased all modern grazers are hypsodont (Fortelius and refl ected in an increased terrigenous component biogenic sedimentation along the equatorial Solounias, 2000). Many factors favor hypso- in marine hemipelagic sediments beginning upwelling zone in the eastern Pacifi c (van Andel donty, and virtually all increase in effect with ca. 15 Ma and increasing sharply in the past et al., 1975; Leinen, 1979; Keller and Barron,

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1983), and recent tectonic studies of the Indo- saw the spread of C4 grasses, as outlined in the Depocenter Type. East-trending paleovalleys up nesian archipelago (Kuhnt et al., 2004; Cloos et previous section, and the conversion of savanna to 24 km wide and 100 m deep; complex cut-and- fi ll structures with reworking of earlier paleovalley al., 2005) indicate effective closure of the Indo- habitats to largely treeless, arid steppe, such as fi lls; deposits merge to form widespread blanket by nesian Seaway had occurred by ca. 12 Ma. the southern Great Plains (MacFadden, 1992; ca. 12 Ma (Tedford et al., 2004). The middle Miocene climate transition Webb and Opdyke, 1995), or to scrub deserts Lithology. Interbedded sandstones, siltstones, con- (ca. 17–12 Ma) was a pivotal period, both to the southwest. The effects of these degraded glomerates, lenses of volcanic ash, minor pond depos- its, clasts of Rocky Mountain provenance. globally and in the geologic evolution of the environments were recorded in the greatest land Thickness. 30 to 245 m in W. Nebraska, average southwestern United States. Ocean circulation mammal extinction of the Neogene in North 180 m. changed from largely equatorial to strongly America (Webb, 1984; Webb and Opdyke, Climatic Indicators. Caliche horizons, volcanic meridional (Hay, 1988) with closing of the 1995). These dramatic changes in the South- ashfall beds, episodic cutting and fi lling, numerous eastern Tethys (ca. 18 Ma, Hsü et al., 1977; west occurred during a global climate change internal unconformities, braided streams. Supercrop. Pliocene Broadwater Formation, aver- Prothero, 2006) and Indonesian (ca. 12 Ma) sea- often referred to as the terminal Miocene event, age 15 m (maximum 90 m) of fl uvial sand and gravel ways (Kennett et al., 1985; Romine and Lom- or the Messinian, after the European stage dur- following 1.5-m.y. hiatus. bari, 1985; Cloos et al., 2005) and opening of ing which major changes occurred in southern Subcrop. Volcanic ash-rich White River (32– bidirectional fl ow between the Arctic and North Europe, including desiccation of the Mediterra- 29 Ma) and Arikaree (29–19 Ma) groups; Cretaceous Pierre Shale. Atlantic oceans (ca. 17.5 Ma, Wright and Miller, nean Sea. As summarized by Cita and McKen- Basal Lacuna. Minor (0.5 m.y.). 1996; Jakobsson et al., 2007). Strong thermoha- zie (1986) and Hodell et al. (1986), the increased Age at Base. Circa 19 Ma (early Hemingfordian, line and gyral circulation enhanced by a steeper cooling and drying during the terminal Miocene Tedford et al., 2004); 16.1 ± 3.7 Ma fi ssion-track zircon pole-to-equator thermal gradient, imposed by event was probably caused by increased Antarc- age on volcanic ash 50 m above base (Izett, 1975). Age at Top. growth of the East Antarctic Ice Sheet (ca. 14.2– tic glaciation as refl ected in growth of the West Circa 5 Ma (late Hemphillian, Tedford et al., 2004); 4.6 ± 1.0 Ma fi ssion-track zircon age on 13.8 Ma, Shevenell and Kennett, 2004; Hol- Antarctic Ice Sheet, beginning ca. 7 Ma, and a ash in uppermost Ogallala (Izett, 1975). bourn et al., 2005) brought ocean circulation 50- to 70-m drop in sea level with extensive ero- Key References. Skinner et al. (1977); Diffendal to near the modern state. Upwelling of cold, sion of continental margins. Thus, the regional (1982); Swinehart et al. (1985); Swinehart and Dif- nutrient-rich waters along boundary currents cooling and drying trend in the southwestern fendal (1990). in middle latitudes (~20°–40°) produced major USA reinforced a similar global trend. Ogallala Formation, Southern High Plains hemipelagic deposits of phosphorite and sili- Considering the long-running controversy ceous, organic-rich sediments like the Monterey over the relative effects of tectonism and cli- Geographic Extent. S.E. Colorado, S. Kansas, W. Formation (Cook and McElhinny, 1979; Sum- mate (e.g., Epis and Chapin, 1975; Trimble, Oklahoma, W. Texas, E. New Mexico. merhayes, 1981). Regional extension following 1980; Molnar and England, 1990; Mears, 1993; Depocenter Type. SE-trending paleovalleys that major elongation of the Pacifi c–North American Gregory and Chase, 1994; Steven et al., 1997; headed on the Southern Rocky Mountains and were transform boundary at ca. 17.5 Ma (Atwater and McMillan et al., 2002; Eaton, 1987, 2008), how incised 60–150 m into the pre-Ogallala erosion sur- face; blanket deposit formed as valleys were fi lled and Stock, 1998; Dickinson, 2002) resulted in forma- do you sort out which is dominant? For the interfl uves covered. tion of fault-bounded basins from the California deposits in extensional basins, tectonism was Lithology. Gravelly and sandy braided-stream coast to the Rio Grande rift. The sedimentary obviously important, but climate played a role deposits interbedded with eolian sands and loess-like response to these tectonic, oceanographic, and in aridifying the landscape, increasing vulner- deposits; extensive caprock of calcrete deposits. Thickness. 30–210 m on S. High Plains but locally climatic changes was deposition of the hemipe- ability of uplands to erosion, and producing 360–580 m in solution-subsidence depressions above lagic Monterey Formation along the California interbedded evaporites. For the coeval deposits Permian evaporite deposits. coast beginning ca. 18–16 Ma (Fig. 2), and con- on unextended plains, climate was obviously Climatic Indicators. Eolian sands and loess, calcic current deposition of a suite of continental sedi- important in changing from erosional stripping paleosols, local playa facies, calcrete caprock; fossil ments in extensional basins and on unextended to aggradation in paleovalleys lacking suffi cient fl ora and fauna indicate savanna-like environment. Supercrop. Pliocene Blanco Formation (3.5–2 Ma); areas of the western Great Plains and Colorado stream fl ow to fl ush sediment from the area. But eolian Plio-Pleistocene Gatuna and Blackwater Draw Plateau. Progressive aridifi cation of the South- tectonism also played a role through modifying formations. west caused by the combined effects of strong climate by changing ocean circulation and by Subcrop. Progressively older formations from coastal upwelling and La Niña domination of increasing topographic relief through isostatic north to south; Cretaceous to Permian rocks beneath major erosion surface. the El Niño–Southern Oscillation system trans- uplift of fl anks of the Rio Grande rift (Chapin Pre-Ogallala Neogene Erosion. Post–28 Ma Capi- formed forested habitats into mixed woodland- and Cather, 1994). For the latest Miocene inte- tan granite pluton (Allen and McLemore, 1991), pre– grassland mosaics, or savannas, within which gration of drainages, incision of uplifts, and 12 Ma basal Ogallala (Kelley and Chapin, 1995). the ungulate population experienced dramatic exhumation of basin fi lls, climate was obviously Age at Base. 12 Ma based on Clarendonian verte- growth and diversifi cation (Janis et al., 2002; important, but it was tectonism that opened the brate fossils (Gustavson, 1996; Tedford et al., 2004); 13.0 ± 0.6 Ma based on K-Ar date (glass) on volcanic Prothero, 2006; Stromberg, 2006). Gulf of California and intensifi ed the monsoon. ash near Orla, Texas, initially mapped as Gatuna For- The second major Miocene climate transi- The answer is not one or the other but rather the mation (Powers and Holt, 1993). tion to affect the southwestern USA occurred complex interplay of both tectonics and climate Age at Top. 6–7 Ma based on late Miocene jaw of between ca. 7 and 5 Ma, as opening of the Gulf on scales from regional to global. fossil proboscidean (Morgan and Lucas, 2001) and 6–7 Ma 40Ar/39Ar ages of basalt fl ows in Raton and of California (Oskin and Stock, 2003) intensi- Ocate areas of New Mexico (Stroud, 1997; Olmsted and fi ed the North American monsoon. The erosional APPENDIX 1: FORMATION SUMMARIES McIntosh, 2004); 6.6 ± 0.8 Ma fi ssion-track zircon age response to the energized monsoonal climate FOR FIGURE 2 on ash overlying Hemphillian vertebrate fossils at Cof- was to integrate drainages and effectively termi- fee Ranch, Texas (Izett, 1975); 4.5 Ma NALMA (North Ogallala Group, Northern High Plains nate in many areas the middle to late Miocene American land mammal “ages”) (Tedford et al., 2004). Key References. Holliday (1987); Winkler (1987); deposition of continental sediments in closed Geographic Extent. W. Nebraska, S.E. Wyoming, Gustavson and Winkler (1988); Hawley (1993); basins. The latest Miocene (ca. 7–5 Ma) also S. South Dakota, E. Colorado, W. Kansas. Gustavson (1996).

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Northern and Central Rio Grande rift Lithology. Fluvial sandstones and conglomerates, adequately exposed and basic stratigraphy is known clasts of mafi c to rhyolitic rocks of Mogollon-Datil are included here. These include the Reserve, Alma, Geographic Extent. Central Colorado to central volcanic fi eld, boulders to 1 m, interbedded caliche. Winston, Haynor Ranch, and Rincon Valley basins. New Mexico; San Luis, Santo Domingo, Española, Thickness. 20–100 m. There are many other Neogene basins in southern Albuquerque, Socorro basins. Climatic Indicators. Caliche horizons; 2-m-thick New Mexico (see Fig. 5, Mack, 2004), but they are Depocenter Type. Linked, oppositely tilted, half Stage VI petrocalcic soil caps Fence Lake Formation and largely undissected, little studied, and lack adequate grabens. underlies 6.7 Ma base surge deposits (Love et al., 1994). age control. Lithology. Diverse suite of siliciclastic deposits Supercrop. Basalt fl ows dated by 40Ar/39Ar at 6.8 Depocenter Type. Grabens, half grabens, and irreg- ranging from conglomerates to mudstones but typi- to 6.0 Ma with local basalt insets possibly continu- ular, fault-bounded basins. cally poorly indurated, buff-colored, sandstones and ing until ca. 5 Ma (McIntosh and Cather, 1994); Plio- Lithology. Siliciclastic deposits of alluvial fans and siltstones; also ash beds, mafi c fl ows, dune fi elds, Pleistocene (ca. 4 to <1 Ma) Quemado Formation piedmont slopes, axial-fl uvial and alluvial fl at depos- playa deposits, and caliche horizons. (McIntosh and Cather, 1994). its ranging from conglomerates and sandstones to red, Thickness. As much as 6 km of sedimentary fi ll on Subcrop. Inset against Bearwallow Mountain gypsiferous playa mudstones. Older deposits gener- deep side of larger half grabens. Andesite (ca. 24–26 Ma) and older units; unconform- ally well-lithifi ed volcaniclastic debris; younger strata Climatic Indicators. Closed basins, dune fi elds, ably overlies Eocene Baca and Eager formations and record unroofi ng of older rocks on basin margins. gypsiferous playa deposits, caliche horizons; savanna- upper Cretaceous Moreno Hill Fm. Thickness. Up to 3 km. like fossil vertebrate assemblage similar to Clarendo- Basal Lacuna. Circa 24–14.5 Ma. Climatic Indicators. Alluvial fans, petrocalcic nian assemblage of the Great Plains and transitional to Age at Base. About 14.5 Ma or less, based on fossil soils, gypsiferous playa deposits. that of the Great Basin (Tedford, 1981). mammal bone of a proboscidean whose oldest record Supercrop. Axial-fl uvial and piedmont deposits Stratigraphic Summary. Santa Fe Group— in North America is ca. 14.5 Ma (Lucas and Anderson, related to through-fl owing streams of latest Miocene– sedimentary and volcanic fi ll of Rio Grande rift; lower 1994); fossil bone located ~0.3 m above basal uncon- early Pliocene age; basalt fl ows ranging from 7.1 to Santa Fe Group—deposits of internally drained basins formity of Fence Lake Formation. 4.8 Ma. (ca. 30 to 7–5 Ma); upper Santa Fe Group—axial river Age at Top. Four 40Ar/39Ar ages between 6.80 Subcrop. Oligocene–lower Miocene volcanic rocks and piedmont deposits following integration of drain- and 6.0 Ma on overlying basalt fl ows; one 5.20 Ma (ca. 27–23 Ma) such as Bearwallow Mountain Andes- ages at 7–5 Ma; Pleistocene entrenchment (0.7 Ma) of 40Ar/39Ar age on possibly interbedded basalt (McIn- ite (ca. 27–23 Ma), Thurman Formation (ca. 27 Ma), the Rio Grande Valley—end of Santa Fe Group. tosh and Cather, 1994). Uvas Basaltic Andesite (28–26 Ma). Supercrop. Latest Miocene–Pliocene fl uvial depos- Key References. McLellan et al. (1982); Lucas and Basal Lacuna. Variable, inadequate age control. its of ancestral Rio Grande and bordering piedmont Anderson (1994); McIntosh and Cather (1994). Age at Base. Early to middle Miocene basalt fanglomerates; basalt fl ows. fl ows interbedded in lower basin fi ll and dated at 21.1 Subcrop. Oligocene–early Miocene volcanic and Bidahochi Formation ± 0.05 Ma to 15.2 ± 0.04 Ma (whole-rock K-Ar ages, volcaniclastic units transitional from Oligocene volca- Marvin et al., 1987). nic fi elds to middle Miocene well-defi ned rift basins; Geographic Extent. Approximately 16,000 km2 of Age at Top. Overlying basalt fl ows dated at 6.5, 4.6, lower Popotosa Formation (ca. 26–16 Ma) in structur- southern Colorado Plateau in northeastern Arizona and 5.2, 4.8 Ma, mostly K-Ar whole-rock ages (Seager et ally fragmented basins of Socorro area; unit of Isleta western New Mexico; original extent ~30,000 km2. al., 1984; Marvin et al., 1987). #2 (ca. 36–16 Ma) in Albuquerque basin; volcaniclas- Depocenter Type. Broad, shallow basin incorporat- Key References. Seager et al. (1984); Crews tic aprons of San Juan and Latir volcanic fi elds and ing paleovalleys and upland alluvial plains. (1994); Harrison (1994); Houser (1994); Mack et al. Ortiz porphyry belt in Española Basin, including Los Lithology. In north, mostly fi ne-grained lacus- (1998); Mack (2004). Pinos Formation on west side and Espinaso Formation trine(?) member; middle volcanic member includes on east side; Conejos Formation in Monte Vista graben Hopi Buttes volcanic fi eld; thin beds of distal silicic Fill of Extensional Basins, Southern Arizona of San Luis Basin; Oligocene nonvolcanic sedimen- volcanic ash; lacustrine claystone, siltstone, and marl. tary units in the Española Basin, including the lower In south, increasing fl uvial deposits. Geographic Extent. Southern Basin and Range Nambe Member of the Tesuque Formation and the Thickness. Ranges up to 240 m thick. and Transition Zone. A depth-to-bedrock map of the lower Abiquiu and Picuris formations (Smith, 2004). Climatic Indicators. Eolian sand sheets, calcic Arizona Basin and Range province (Oppenheimer Basal Lacuna. Variable, local, inadequate age control. soils, lacustrine facies with fresh-water fossils, sel- and Sumner, 1981) outlines ~64 basins that contain, Age at Base. Sedimentation began ca. 26 Ma, but enite crystals. or may contain, middle and upper Miocene sedimen- rapid subsidence and accumulation of thick fi lls began Supercrop. Plio-Pleistocene basalt fl ows, Quater- tary fi ll; however, most are undissected and overlain about 18 to 16 Ma (Cather et al., 1994; Connell, 2004; nary alluvium. by alluvial fans and pediment gravels. Figure 1 shows Smith, 2004). Subcrop. Cretaceous to Permian formations portions of 18 basins where outcrops, drill holes, and Age at Top. The transition from closed basins to beneath an erosion surface with up to 60 m of relief. geophysical data provide a basic framework of Mio- through-going Rio Grande drainage began northwest Basal Lacuna. Late Eocene–middle Oligocene cene sedimentation. of Santa Fe, New Mexico, at about 7 Ma as recorded eolian erg of Chuska Sandstone (Cather et al., 2007) Depocenter Type. Grabens, half grabens, structural by axial river deposits underlying volcanic rocks dated completely removed after eruption of 25 Ma mafi c fl ow 40 39 troughs bounded by relatively wide-spaced, steep nor- at 6.93 ± 0.05 Ma ( Ar/ Ar sanidine, McIntosh and and prior to onset of Bidahochi deposition at ca. 16 Ma. mal faults that contrast with the thin-skinned structural 40 39 Quade, 1995); late Hemphillian vertebrate fossils in Age at Base. About 16 Ma based on Ar/ Ar age style of the late Oligocene–early Miocene detachment the San Juan and Rak quarries in the Chamita For- of 15.84 ± 0.05 Ma (geochemical correlation with ash terranes with their relatively shallow supradetachment mation were also dated at 6.75 to 6.93 ± 0.05 Ma bed in Buffalo Canyon), and weighted average age of basins. 40 39 40 39 ( Ar/ Ar, sanidine). The ancestral Rio Grande emp- 15.46 ± 0.36 Ma from two Ar/ Ar (biotite) dates on Lithology. Locally derived clastic sediments inter- tied into the fl uvial-deltaic complex of the Popotosa east point ash bed; ca. 16 Ma onset of sedimentation bedded with basalt fl ows, red-brown lacustrine clay, 40 39 Formation north of Socorro until between 7 and 5 Ma based on above Ar/ Ar ages and strata accumulation thick nonmarine evaporates of halite, anhydrite, and (Connell, 2004), after which by progressive basin rates (Dallegge et al., 2003). gypsum. spillover, it integrated the drainage southward to the El Age at Top. About 6 Ma based on correlation of Thickness. Drill holes and seismic data indicate 40 39 Paso area by 5 Ma (Mack, 2004). ash bed at top of member 5 with 6.62 ± 0.03 Ar/ Ar 200–3000 m of massive evaporites in some basins and Key References. Chapin and Cather (1994); Cather age of Blacktail Creek ash bed and late Hemphillian comparable thicknesses of clastic basin fi ll. et al. (1994); Smith et al. (2001); Connell (2004); (ca. 6–5 Ma, NALMA) age of White Cone fauna Climatic Indicators. Thick, nonmarine evaporites, Koning et al. (2004); Smith (2004). (Baskin, 1979). See Dallegge et al. (2003) for addi- red-brown oxidized lacustrine clays, petrocalcic soils. 40 39 tional Ar/ Ar ages and correlations with dated ash Supercrop. Deposits of external drainage sys- Fence Lake Formation beds in University of Utah database. tems, such as gravels of the Gila and Colorado riv- Key References. Love (1989); Dallegge et al. (2003). ers (ca. 5–6 Ma) and the Pliocene Bouse Formation Geographic Extent. West-central New Mexico, (ca. 5.4 Ma, Spencer et al., 2001a); piedmont alluvial northwest corner of Mogollon-Datil volcanic fi eld and Fill of Extensional Basins, Southern New Mexico fans and pediment gravels; basalt fl ows. bordering Colorado Plateau. Subcrop. Late Oligocene to middle Miocene tilted Depocenter Type. NW-trending paleovalleys and Geographic Extent. South of Socorro to Mexican fault blocks of volcanic rocks (ca. 23–16 Ma), allu- alluvial fans. border, but only basins in which the sedimentary fi ll is vial fan and playa deposits that postdate detachment

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faulting and predate Basin and Range block faulting Key References. Bohannon et al. (1993); Beard Monterey Formation (Spencer et al., 2001b). (1996); Lamb et al. (2005). Basal Lacuna. Variable, regional unconformity Geographic Extent. 1700 km along California developed between ca. 17 and 10 Ma; inadequate age Mojave Desert coast; as far as 300 km seaward and 300 km inland. control to determine missing interval. Depocenter Type. Extensional and oblique-slip Age at Base. Overlies 16.4 Ma andesitic tuff in Geographic Extent. Southeastern California from basins along transform margin; offshore borderlands Exxon No. 1 Yuma Federal drill hole (Yuma basin) Nevada westward through the Baker and Barstow and interior basins. and 14.9 Ma mafi c fl ow in Exxon No. 1 State 74 drill areas and northwest to the El Paso basin. Lithology. Laminated diatomites, diatomaceous hole (Picacho basin) (Eberly and Stanley, 1978); basal Depocenter Types. Basin and Range half grabens and siliceous mudrocks, porcelanites, calcareous and Hickey Formation (16.2 Ma, Leighty, 1998); 15 Ma and sag basins that generally postdate and overprint phosphatic mudrocks, dolostone, limestone. depositional overlap across deformed middle Miocene early Miocene supradetachment basins except in Thickness. Typically 300–500 m on land, locally units (Menges and Pearthree, 1989); middle Miocene Shadow Valley basin where thin-skinned, ductile much thicker and thinner, varies between basins. transition from felsic volcanism and core-complex extension began ca. 13.5 Ma and continued until Climatic Indicators. Marine benthic and plank- extension to high-angle normal faulting and deep ca. 10.5 Ma (Friedmann et al., 1996). tonic foraminifers, diatoms, radiolarians; organic-rich basin formation (Spencer et al., 2001b). Lithology. Barstow Formation––basal fl uvial con- deposits of coasted upwelling. Age at Top. Deep paleochannels cut perpendicular glomerate and sandstone overlain by lacustrine clay- Supercrop. Late Miocene Pismo Formation; Plio- to Mogollon Rim in Verde Valley prior to 5.66 Ma stone, sandstone, and limestone containing tuff beds cene Sisquoc Formation and Fernando Formation. refl ecting lowered base level by opening of Gulf of and grading laterally into coarse fanglomerates; Dove Subcrop. Early Miocene Rincon Shale; middle California prior to 5.5 Ma (Nations et al., 1985); Spring Formation––fl uvial and lacustrine depos- Miocene Obispo Formation. river gravels below 6.0 Ma basalt near Gillespie its with interbedded basalt fl ows, tuff beds, bedded Basal Lacuna. None. Dam indicate external drainage of Gila River began cherts, and caliche and silcrete horizons; Shadow Val- Age at Base. 18.4 Ma at Naples Beach; 16–15 Ma in latest Miocene (Eberly and Stanley, 1978); basin- ley basin––chiefl y alluvial fan and lacustrine facies, in Palos Verdes Hills; 15 Ma in Pismo Basin; 16 Ma fi ll sediments and basalts of Perkinsville Formation including gypsiferous playa deposits, rock-avalanche in San Joaquin Basin; 17.8 Ma at Deep Sea Drilling (6.3–4.6 Ma, Leighty, 1998) deposited in basins of breccias, gravity-glide blocks changing to cobble- Site 575. Transition Zone; upper basin-fi ll units depositionally boulder fanglomerates at ca. 10.5 Ma associated with Age at Top. 6.7 Ma at Naples Beach, 5.5 Ma in overlap basin-bounding faults and bury bedrock pedi- normal faulting of upper plate. Santa Barbara area; 4 Ma, Palos Verdes Hills; 5 Ma, ments fringing adjacent mountain ranges (Menges and Thickness. Barstow Formation, ~1000 m; Dove San Joaquin Basin. Pearthree, 1989). Springs Formation, ~1800 m; Shadow Valley basin, Age Control. Fission-track zircon ages on tuff Key References. Eberly and Stanley (1978); ~3000 m. beds (Obradovich and Naeser, 1981); miscellaneous Menges and Pearthree (1989); Leighty (1998); Spen- Climatic Indicators. Nonmarine limestone, cali- K-Ar ages; magnetostratigraphy (Omarzai, 1992); cer et al. (2001b). che, silcrete, gypsiferous playa deposits, diageneti- Sr-isotope stratigraphy (De Paolo and Finger, 1991); cally altered tuffs in saline lakes, fanglomerates. multidisciplinary biostratigraphic ages. Lake Mead Area Supercrop. Barstow Formation—unconformably Key References. Ingle (1981); Pisciotto and Garri- overlain by Black Mountain Basalt (2.5 Ma) and Qua- son (1981); Behl (1999); Isaacs (1983, 2001); Flower Geographic Extent. Virgin River depression at ternary alluvium; Dove Spring Formation—uncon- and Kennett (1994). junction of southwest corner of Colorado Plateau and formably overlain by Quaternary alluvium; Shadow Basin and Range, including adjacent Grand Wash Valley basin––unconformably overlain by 5.1–4.5 Ma ACKNOWLEDGMENTS trough and Overton Arm basin. basalt fl ows of Cima volcanic fi eld. Depocenter Type. Two deep half grabens (Mormon Subcrop. Barstow Formation—unconformably The author benefi ted greatly from informal reviews and Mesquite basins); pull-apart basin of Overton overlies lower Miocene Pickhandle Formation (23.7– of an earlier version of this paper by S.M. Cather, Arm; Grand Wash structural trough; post-extension 19.3 Ma, Fillmore and Walker, 1996) and lower Mio- W.R. Dickinson, R.F. Diffendal, Jr., G.P. Eaton, and Muddy Creek structural sag. cene Mud Hills Formation (20–19 Ma, Ingersoll et al., S.G. Lucas. I am also grateful to reviews of the pres- Lithology. Interbedded sandstone, conglomerate, 1996); Dove Spring Formation––unconformably over- ent manuscript by Associate Editor Katherine Giles and limestone, gypsum, and tuff beds overlain by red, lies lower to middle Miocene Cudahy Camp Formation reviewers Ray Ingersoll, Richard Langford, and Donald lime-rich sandstone with silicic tuff beds and an upper (Whistler and Burbank, 1992); Shadow Valley strata–– Prothero. The assistance of Lynne Hemenway (word thick sequence of pink siltstones interlayered with unconformably or nonconformably overlie rocks rang- processing), Leo Gabaldon (computer graphics), San- gypsum beds. ing from Proterozoic to middle Miocene in age. dra Licata (interlibrary loans), and Adam Read (elec- Thickness. 2–6 km (Bohannon et al., 1993); Basal Lacuna. Variable, inadequate age control. tronic submittal) are very much appreciated. 5–9 km in Mesquite and Mormon subbasins based on Age at Base. Barstow Formation, ca. 17 Ma based gravity and seismic data (Langenheim et al., 2005); on paleomagnetic and biostratigraphic data (Wood- REFERENCES CITED 1–2 km, Muddy Creek Formation. burne, 1998); Dove Spring Formation, ca. 12.5 Ma Climatic Indicators. Nonmarine limestone, bedded based on geochemical correlation of a tuff about gypsum, red oxidized clastic sediments. 100 m above base with the Cougar Point Tuff dated Adams, D.K., and Comrie, A.C., 1997, The North American 40 39 monsoon: American Meteorological Society Bulletin, Supercrop. Pliocene-Holocene alluvium, river by Ar/ Ar at 12.07 ± 0.04 Ma (Perkins et al., v. 78, p. 2197–2213, doi: 10.1175/1520-0477(1997)07 gravels, playa deposits, calcrete soils, basalt fl ows. 1998) and biostratigraphic estimate of Tedford et al. 8<2197:TNAM>2.0.CO;2. Subcrop. Pre-extension Rainbow Gardens Member (2004); Shadow Valley strata, ca. 13 Ma based on a Allen, M.S., and McLemore, V.T., 1991, The petrology and of Horse Springs Formation deposited on major angu- 13.12 ± 0.26 Ma 40Ar/39Ar age of a volcanic fl ow near petrogenesis of the Capitan pluton, New Mexico: New lar unconformity and overlain by Thumb Member of the base and a 13.4 Ma K-Ar hornblende age of a Mexico Geological Society, 42nd Annual Field Con- Horse Springs Formation (Beard, 1996). hypabyssal sill (Friedmann et al., 1996). ference Guidebook, p. 115–127. Basal Lacuna. 2–6 m.y. lacuna indicated between Age at Top. 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