Paleomagnetism, geochronology, and possible tectonic rotation of the middle Miocene Barstow Formation, Mojave Desert, southern California

Research Article| April 01, 1990 Paleomagnetism, geochronology, and possible tectonic rotation of the middle Miocene Barstow Formation, Mojave Desert, southern California BRUCE J. MAcFADDEN; BRUCE J. MAcFADDEN 1Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar CARL C. SWISHER, III; CARL C. SWISHER, III 2Berkeley Geochronology Center, 2453 Ridge Road, Berkeley, California 94709 Search for other works by this author on: GSW Google Scholar NEIL D. OPDYKE; NEIL D. OPDYKE 3Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar MICHAEL O. WOODBURNE MICHAEL O. WOODBURNE 4Department of Earth Sciences, University of California, Riverside, California 92521 Search for other works by this author on: GSW Google Scholar Author and Article Information BRUCE J. MAcFADDEN 1Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611 CARL C. SWISHER, III 2Berkeley Geochronology Center, 2453 Ridge Road, Berkeley, California 94709 NEIL D. OPDYKE 3Department of Geology, University of Florida, Gainesville, Florida 32611 MICHAEL O. WOODBURNE 4Department of Earth Sciences, University of California, Riverside, California 92521 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1990) 102 (4): 478–493. https://doi.org/10.1130/0016-7606(1990)102<0478:PGAPTR>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation BRUCE J. MAcFADDEN, CARL C. SWISHER, NEIL D. OPDYKE, MICHAEL O. WOODBURNE; Paleomagnetism, geochronology, and possible tectonic rotation of the middle Miocene Barstow Formation, Mojave Desert, southern California. GSA Bulletin 1990;; 102 (4): 478–493. doi: https://doi.org/10.1130/0016-7606(1990)102<0478:PGAPTR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Barstow Formation of the central Mojave Desert in the Mud Hills consists of 965 m of epiclastic and volcanic sediments deposited in an extensional rift. This widespread unit is of importance because its mammalian faunas characterize the Barstovian land-mammal age, previous isotopic ages on interbedded tuffs have been used to calibrate middle Miocene time, and it was deposited in a tectonically active region. Volcanic ash samples were collected from the Barstow Formation for isotopic dating using conventional 40K-40Ar and 40Ar/39Ar single-crystal laser-fusion methods. Multiple analyses on minerals from 5 superposed tuffs yielded the following mean 40K-40Ar ages: Rak Tuff at 16.3 ± 0.3 m.y., Oreodont Tuff at 15.9 ± 0.2 m.y., Dated Tuff at 14.8 ± 0.2 m.y., Hemicyon Tuff at 14.0 ± 0.1 m.y., and Lapilli Tuff at 13.4 ± 0.2 m.y. Similar mean ages were also obtained on the following tuffs using single-crystal 40Ar/39Ar laser-fusion methods: Oreodont Tuff at 15.9 ± 0.06 m.y., Dated Tuff at 14.8 ± 0.06 m.y., and Hemicyon Tuff at 14.0 ± 0.09 m.y.In order to develop a local magnetostratigraphy, 272 paleomagnetic sites were collected from 7 important sections within the Mud Hills. Saturation isothermal-remanence experiments suggested the presence of either hematite or magnetite as carriers of remanence. After either stepwise alternating-field or thermal demagnetization, characteristic directions were usually isolated between 30 and 50 mT or 300 and 500 °C. Positive reversal and fold tests indicate a stable remanence acquired prior to folding. There are 21 magnetic polarity zones interpreted from the continuous part of the Barstow Formation in the study area. By using the 5 isotopic ages determined from interbedded tuffs, the local composite polarity pattern is correlated to the top of chron C5D through the bottom of C5AB on the magnetic polarity time scale, or an age range from 17.6 to 13.4 m.y. The boundary between Hemingfordian and Barstovian land-mammal ages herein occurs 30 m below the Oreodont Tuff (15.8 ± 0.2 m.y.) within C5B and is taken at about 16.0 m.y. ago. The Barstow formational mean declination is 347.1° (inc = 47.5°, α 95 = 6.51°; ΔD = 7.97°), suggesting negligible or possibly slight counterclockwise rotation (R = 4.1°; Δ R = 10.5°) for this part of the Mojave block during the past 18 m.y. A 40Ar/39Ar mean age of 19.3 ± 0.02 m.y. for the Red Tuff, which is near the local base of the Barstow Formation and unconformably underlying our composite section, constrains a minimum age for the beginning of local basin filling with principally epiclastic sediments. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.