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

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 ⁴⁰ K- ⁴⁰ Ar and ⁴⁰ Ar/ ³⁹ Ar single-crystal laser-fusion methods. Multiple analyses on minerals from 5 superposed tuffs yielded the following mean ⁴⁰ K- ⁴⁰ Ar 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 ⁴⁰ Ar/ ³⁹ Ar 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 ⁴⁰ Ar/ ³⁹ Ar 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.