Paleomagnetism, geochronology, and possible tectonic rotation of themiddle Miocene Barstow Formation, Mojave Desert, Southern California

Abstract

The Barstow Formation of the central Mojave Desert in the Mud Hillsconsists of 965 m of epiclastic and volcanic sediments deposited in anextensional rift. This widespread unit is of importance because itsmammalian faunas characterize the Barstovian land-mammal age, previousisotopic ages on interbedded tuffs have been used to calibrate middleMiocene time, and it was deposited in a tectonically active region.Volcanic ash samples were collected from the Barstow Formation forisotopic dating using conventional (super 40) K- (super 40) Ar and(super 40) Ar/ (super 39) Ar single-crystal laser-fusion methods.Multiple analyses on minerals from 5 superposed tuffs yielded thefollowing mean (super 40) K- (super 40) Ar ages: Rak Tuff at 16.3 + or- 0.3 m.y., Oreodont Tuff at 15.9 + or - 0.2 m.y., Dated Tuff at 14.8+ or - 0.2 m.y., Hemicyon Tuff at 14.0 + or - 0.1 m.y., and LapilliTuff at 13.4 + or - 0.2 m.y. Similar mean ages were also obtained onthe following tuffs using single-crystal (super 40) Ar/ (super 39) Arlaser-fusion methods: Oreodont Tuff at 15.9 + or - 0.06 m.y., DatedTuff at 14.8 + or - 0.06 m.y., and Hemicyon Tuff at 14.0 + or - 0.09m.y.In order to develop a local magnetostratigraphy, 272 paleomagneticsites were collected from 7 important sections within the Mud Hills.Saturation isothermal-remanence experiments suggested the presence ofeither hematite or magnetite as carriers of remanence. After eitherstepwise alternating-field or thermal demagnetization, characteristicdirections were usually isolated between 30 and 50 mT or 300 and 500degrees C. Positive reversal and fold tests indicate a stableremanence acquired prior to folding. There are 21 magnetic polarityzones interpreted from the continuous part of the Barstow Formation inthe study area. By using the 5 isotopic ages determined frominterbedded tuffs, the local composite polarity pattern is correlatedto the top of chron C5D through the bottom of C5AB on the magneticpolarity time scale, or an age range from 17.6 to 13.4 m.y. Theboundary between Hemingfordian and Barstovian land-mammal ages hereinoccurs 30 m below the Oreodont Tuff (15.8 + or - 0.2 m.y.) within C5Band is taken at about 16.0 m.y. ago. The Barstow formational meandeclination is 347.1 degrees (inc = 47.5 degrees , alpha (sub 95) =6.51 degrees ; Delta D = 7.97 degrees ), suggesting negligible orpossibly slight counterclockwise rotation (R = 4.1 degrees ; Delta R =10.5 degrees ) for this part of the Mojave block during the past 18m.y. A (super 40) Ar/ (super 39) Ar mean age of 19.3 + or - 0.02 m.y.for the Red Tuff, which is near the local base of the BarstowFormation and unconformably underlying our composite section,constrains a minimum age for the beginning of local basin filling withprincipally epiclastic sediments.

11 samples from this reference