Age and correlation of tephra layers, position of the Matuyama-Brunhes chron boundary, and effects of Bishop Ash eruption on Owens Lake, as determined from drill hole OL-92, Southeast California

Abstract

Tephra layers in the ~323-m-deep Owens lake drill hole OL-92 correlate to tephra layers that have been identified and dated elsewhere in the western United States. Tephra layers identified are the Bishop ash bed (758 ka) at 309.2–298.6 m; the Dibekulewe (ash) bed (ca. 470 ka to ca. 610 ka) at ~224 m; and one of several ash beds in Walker Lake (ca. 60 ka to ca. 80 ka) at ~50.7 m. Other tephra layers, the ages of which are poorly constrained, have also been identified. Age constraints from a sedimentation-rate curve based on dry bulk density and independently derived magnetostratigraphy provide new age constraints to the undated or poorly dated tephra layers: ca. 740 ka for the ash of Thermal Canyon, and ca. 510 ka for the Dibekulewe (ash) bed. Bishop tephra fell into a deep Owens Lake, but the lake shallowed as ash was rapidly reworked by wind and water within the Owens Lake basin. The shallowing of the lake was the result in part to filling with the large volume of ash that was deposited in the basin and then reworked into the lake, but the filling was also an effect of the onset of a moderately warm interstadial period of hemispheric or global extent corresponding to oxygen-isotope stage 19. The lake deepened again as the last several meters of the 10-m-thick, composite ash bed were deposited in the lake. Despite its great thickness, reworking of the light ash must have been rapid. The position of the Matuyama-Brunhes paleomagnetic boundary is estimated to be between 311.4 m and 314.8 m, and most likely between 311.4 m and 312.9 m, in the core, based on (1) the pattern of magnetic inclinations in the Owens Lake core as compared with those at other sites in the region; (2) estimates of the time elapsed between the magnetic reversal and the deposition of the Bishop ash bed; and (3) the probable range of sediment-deposition rates in Owens Lake during this time.