ROCK MAGNETIC CYCLOSTRATIGRAPHY AND MAGNETIC POLARITY STRATIGRAPHY OF THE QUATERNARY BLACKWATER DRAW FORMATION, WEST TEXAS

The Blackwater Draw Formation is a collection of Quaternary aged (ca. 1.4 Ma-Holocene) loess-paleosol couplets that mantles the Southern High Plains (SHP) in the Texas Panhandle. The Blackwater Draw Formation (~10 m thick) was sampled at high resolution (2.5-5 cm intervals), and the rock magnetic property data are used to test for the presence of an internally consistent rock magnetic cyclostratigraphy. The magnetic parameters include bulk magnetic susceptibility (χ, median: 1.56 * 10-4 SI volume), anhysteretic remanent magnetization (ARM, median: 0.1612 A/m), and isothermal remanent magnetization (IRM, median: 2.5367 A/m) intensity, which allow for the determination of two common environmental magnetic ratios (ARM/χ and ARM/IRM, medians: 1.051 *103 and 0.068 respectively) that are often used to approximate magnetic grain size. Bulk magnetic susceptibility vs temperature measurements demonstrate that the principal magnetic minerals are magnetite/maghemite and minor amounts of hematite. Moreover, the rock magnetic parameters for the formation all show cyclic variations that coincide with individual loess-paleosol couplets. The sinuous behavior of the data is most evident in the magnetic parameter ratios, and we infer that changes in rock magnetic parameters correlate with climate events. In addition to revealing climate variations, all of the rock magnetic proxies show a noticeable change in periodicity. The ARM, IRM and χ values in the uppermost section exhibit a much higher amplitude and lower frequency than the bottom part of the section (with the opposite being true for parameter ratios). An example of this change in periodicity is shown in the χ data, which range from 2.5*10-4 to 1*10-4 SI volume section in the bottom section and then ranges from 2.4 * 10-4 to 2.1 *10-4 SI volume in the top section. This change is interpreted to represent the Mid-Pleistocene Transition (MPT) (1.25 to 0.70 Ma), a time interval when the frequency of Earth’s climate cycles shifted from a 41 Ka dominated cycle in the lower Pleistocene to a 100 Ka cycle in the later Pleistocene.