New K-Ar age determinations from syntectonic deposits (Oligocene and Miocene), in southern Nevada and northwest Arizona D.G. Carpenter and J.G. Carpenter Isochron/West, Bulletin of Isotopic Geochronology, v. 55, pp. 10-12 Downloaded from: https://geoinfo.nmt.edu/publications/periodicals/isochronwest/home.cfml?Issue=55 Isochron/West was published at irregular intervals from 1971 to 1996. The journal was patterned after the journal Radiocarbon and covered isotopic age-dating (except carbon-14) on rocks and minerals from the Western Hemisphere. Initially, the geographic scope of papers was restricted to the western half of the United States, but was later expanded. The journal was sponsored and staffed by the New Mexico Bureau of Mines (now Geology) & Mineral Resources and the Nevada Bureau of Mines & Geology. All back-issue papers are available for free: https://geoinfo.nmt.edu/publications/periodicals/isochronwest This page is intentionally left blank to maintain order of facing pages. 10 NEW K-Ar AGE DETERMINATIONS FROM SYNTECTONIC DEPOSITS (OLIGOCENE AND MIOCENE), IN SOUTHERN NEVADA AND NORTHWEST ARIZONA DANIEL G. CARPENTER Department of Geology, Oregon State University, Corvaiiis, OR 97331-5506 JAMES G. CARPENTER Present address: Mobii Exploration and Producing U.S. inc., 1225 17th Street, Denver, CO 80202 Four new K-Ar age determinations were obtained on plagioclase, rare K-feldspar, quartz and biotite shards, and Cenozoic syntectonic deposits that are important to struc rare lithic fragments in a partially devitrified groundmass. tural and stratigraphic investigations in the southern These beds Me up section from the basal limestone and Nevada, southwest Utah, and northwest Arizona region. polymictic conglomerate beds of the formation. The radiometric ages were obtained from biotite contained Greater than 7,600 m of low-density Cenozoic beds in tuffaceous beds. The Horse Spring-Cottonwood Wash were deposited at the Virgin Valley basin depocenter and Muddy Creek formations are composed of lacustrine based on seismic, gravity, and a synthetic seismic well limestone, clastic, evaporite, volcaniclastic, and volcanic tie (D. Carpenter, 1988; 1989; J. Carpenter, 1989; beds. Thick asymmetric sequences of these syntectonic Carpenter and others, 1989, fig. 1). Seismic reflection beds were deposited in basin-range extensional basins interpretations of the crustal structure demonstrate tilted from Oligocene to Recent time. The basins lie in the horst blocks separated from half-graben blocks by high- downdropped hanging wall blocks of high-angle normal angle normal faults (Carpenter and others, 1 989, fig. 1, fault systems. Some Cenozoic beds were deposited on D Carpenter, 1989; J. Carpenter, 1989). The seismic topographically low portions of range blocks, where the data image a fanning upward reflector geometry for K-Ar samples were collected, and represent condensed and Cenozoic beds that correspond to the beds that were run incomplete stratigraphic records relative to the thick for K-Ar age determinations. The fanning upward geometry basinal sequences. resulted from the syndepositional relation of the syntec tonic beds with high-angle (60 degree dipping) normal faults. Consequently, this indicates that tl^ high-angle INTRODUCTION normal faulting in the area initiated in the Oligocene (or The study area lies at the juncture between Nevada, earlier) and has continued to the Recent. Utah, and Arizona (fig. 1) Several deformational episodes In the North Muddy Mountains, on the west, and the have affected the area, including Late Proterozoic rifting Vi gin Mountains, on the east of the east-tilted Virgin (Moore, 1972; J. Carpenter, 1989), Cretaceous decolle- Valley half-graben, Cenozoic formations were deposited ment style folding and thrusting (Longwell, 1949; Hirectiv on Cretaceous, Jurassic, and older rocks and Armstrong, 1968; Carpenter and Carpenter, 1987) Creta document angular ^n the^North ceous-Eocene? basement-involved folding and reverse croman^ faulting (Reber, 1951, 1952; Moore, 1972; Hintze, Muddy Mountains, beds . 1987) are SanostoneSandstone (Carpenter^ and Carpenter,Cenozoic Horselaa/) Spring are 1986; Carpenter and others, 1989), and Cenozoic basin- unconforma y cenozoic beds range between range extension (Longwell, 1928; Longwell and others, Formation (fig. ^ m age (Tschanz, 1960; 1965; Olmore, 1971; Stewart, 1971; D. Carpenter, 21.3 ± ®-:5„^2^ Ekren and others, 1977; Carpenter and 1988, 1989; J. Carpenter, 1989; Carpenter and others, Anderson, 1972, uorse Spring-Cottonwood Wash 1989). others, 1989). x^-fpably overlain by the Miocene- sequence tinco . pormation in the Muddy and CENOZOIC STRATIGRAPHY, STRUCTURE. Quaternary Muddy Muddy Creek Formation is the AND TIMING OF NORMAL FAULTING The Cenozoic rocks in the study area are into two primary sequences: 1) the Oligocene discussion and conclusions Horse Spring-Cottonwood Wash sequence, and 2)t wotprminations, for the Horse Spring cene to Quaternary Muddy Creek sequence. A seque New K-Ar age a coupled with fanning a relatively conformable succession of genetically re Cottonwood Wasn the thick Cenozoic basinal beds bounded by unconformities or their upward reflector onset of crustal formities (Mitchum, 1977). Kowallis and Everett sequence suggest an degree dips) normal fault describe, in detail, the Muddy Creek Formation ' ^tension, and h.g^® '%88, 1989; J. Carpenter, pret the depositional environment. We discuss the ® _P"*^®~^^^®riwood Wash sequence for which we r9T9rSrp» Obtained nevi, aae determinations. acknowledgements ta^ed^n«=,^w®'i"® cryptalgalaminate iimestone beds con- Mormon Horse Spring Formation in the orted by Mobil 01! Corporation stone heci«s nl* u® ^si'psoter, 1986)is similar to lime- This work was supp chevron USA, Inc. (San «on in the vfr!: Cottonwood Wash Forma- (Denver, Colorado Affiliate; ± A .O m V \ f?'" 'yj®or»tains. An age determination (24.3 Ramon, California). of Geological Sciences at tioninth4 o* Cottonwood Wash Forma- Faculty at the Dep ^ Oregon) and geologists the formation is the first obtained on Oregon State Chevron USA, Inc. are appre- Three age V®'®® (Carpenter and others, 1989). at Mobil Oil Corpora I related research. 16.6 ± 0 7 ZZT Q ° ciated for ^ uy Krueger Enterprises Geochron Horse Sprino Form ^®'® ®6tained from tuff beds in the Analyses were performed oy ^ ^ Samplepie iithologieslithninn-.^ are vitric tuffsNorth with Muddy phenocrysts Mountains, of Laboratory, Cambridge, Massachusetts. IISOCHRON/WEST, no. 56, April 1990] 11 \ f 36®-- i 20 25 miles OREGON STATE UNIVERSITY OAHttL «. CARPCMTER flt JAMtS A. CARPtMTtR SEISMC LINEC r^iflTESYCOURTESY oor SEiSOATA ^SERVICES INC I—H 1 1 h—I WASATCH HtKOCLlME PROORA* 0 5 10 15 20 25 kilometers figure 1. Physiographic map showing fault-bounded horsts (ranges) and grabens (basins) and maior cultural features. [ISOCHRON/WEST, no. 55, April 1990] 12 SAMPLE DESCRIPTIONS Carpenter, D. G.(1 989) Geology of the North Muddy Mountains, Clark County, Nevada and regional structural synthesis —Fold- The decay constants used for the age determinations thrust and basin-range structure In southern Nevada, are: Xjj = 4.962 x lO-'o/yr; = 10-"»/yr; K'K'/K = southwest Utah, and northwest Arizona: Oregon State Univer 1.193 X lO'-'g/g. sity, MS thesis. Carpenter, D. G., and Carpenter, J. A. (1987) New K-Ar ages 1. 34981-1 K-Ar from the Baseline Sandstone (Cenomanlan), North Muddy Light tan biotite-bearing vitric tuff from the Cotton- Mountains, Clark County, Nevada: IsochronAIVest, no. 49, wood Wash Formation (36°36'20"N, 113°57'29''W; p. 3. swy4 swy* NWA S18,T37N,R15W; Cane Springs Carpenter, D. G., Carpenter, J. A., Bradley, M. H., Franz, U. A., quad., Mohave Co., AZ). Analytical data: K = 6.630, and Reber, S. J.(1989) Comment-On the origin of Isostatic and 6.578%; •Ar"® = 0.01114, and 0.01126 ppm; rebound In the footwalls of low-angle normal faults by B. P. Wernlcke and G. A. Axen: Geology, v. 17, p. 774. •Ar^o/EAr"® = 0.618 and 0.611. Carpenter, J. A.(1986) Deposltlonal and diagenetic history of the (biotite concentrate) 24.3 ± 1.0 m.y. Rainbow Gardens Member of the Tertiary Horse Spring Forma tion, Clark County, Nevada: Society of Economic Paleon 2. 35014-1 K-Ar tologists and Mineralogists, abstracts with programs, v. 3, White biotite-bearing vitric tuff from previously un mapped Horse Spring Formation {36°30'60"N, (1989) Structure of the Southern Mormon Mountains, 114°28'38''W; NW'A SE'A NE)4 S27,T16S,R67E; Clark County, Nevada and regional structural synthesis: Fold- Overton quad., Clark Co., NV). Analytical data: K = thrust and basin-range structure In southern Nevada, 5.414, and 6.361%; 'Ar"® = 0.006298, and Luthwest Utah, and northwest Arizona: Oregon State Univer- 0.006171 ppm; *Ar''®/EAr''® = 0.644, and 0.445. Ekr^n^k'^B® oS",P. P.. Sargent, K. A., and Dixon, G. L. (1 977) (biotite concentrate) 16.6 ± 0.7 m.y. Geologic map of Tertiary rocks, Lincoln County, Nevada: U.S. 3. 35053-2 K-Ar Geological Survey Map structure of the Beaver Dam White biotite-bearing vitric tuff from previously un- Hintze, L. F.(1986) 9 ijtah* Utah Geological Association Mountains, southwestern Utan. uian vjo » Horse Spring Formation (36®37'30"N, Publication 15, p. 1. q u (1986) Sedimentary en- »»,Weiser 33'50"W; Ridge quad., NEy4 NEy4Clark NE'ACo., S23,T15S,R66E;NV). Analytical data: K = 4.997, and 4.869%; •Ar"® = 0.005113, 0.005374, and 0.004837 ppm; *Ar*®/EAr''® = Longer 0.268, 0.168, and 0.271. (biotite concentrate) 14.9 ± 0.7 m.y. '^^''^'"(1941; StrilTcture' of'the northern Muddy Mountain area, Nevada: GSA Bulletin, v. R., and Roberts, R. J. 4. 35099-1 K-Ar White biotite-bearing vitric tuff from previously un ^°7lT6^)Geolog;a^Tner^^^^^^^^ mapped Horse Spring Formation (36°32'37"N, MltcTum'R'r(19i?;'s:^mlc stratigraphy and global changes 114035'37"W; SEy4 SWy4 810,T1 6S,R66E; Weiser Ridge quad., Clark Co., NV). Analytical data: K = of sea level: AAPG Memoir ' • gnd Beaverdam Moun- 6.497, and 6.491%; *Ar'^o = 0.006516 and Moore, R.