Reports: UR853255-UR8: Magnetostratigraphy of the Lodo Formation and Kinematics of the Vallecitos Syncline, California
Christopher J. Pluhar, PhD, California State University, Fresno
The marine Lodo Formation was deposited on the continental slope during the late Paleocene and early Eocene epochs and consequently records the Paleocene-Eocene boundary, a time of benthic extinction. Abundant evidence shows that this boundary and extinction coincide with major greenhouse gas emissions to the atmosphere and climatic warming (Zachos et al., 2005 and the references therein), called the Paleocene-Eocene Thermal Maximum (PETM). The grant proposal outlined three main objectives: 1) test the hypothesis proposed by Lee and Kodama (2009) that a paleomagnetic normal polarity subchron occurred coincident with the PETM, 2) produce a magnetostratigraphy of the Lodo Formation type section, 3) sample multiple localities of the Lodo Formation along the Vallecitos Syncline to assess timing of the curvature of the fold axis. In year one, we completed objective 1 (and thus we will not repeat that content). During year two, we made significant progress towards objectives 2 and 3. In year three we made further progress on objective 2. During year four we conducted major work on tasks 2 and 3, though the sediments did not yield quality data. At present we have
Objective 1 – Test for normal subchron at the PETM. This part of the project was completed during project years 1 and 2. We found no evidence at the Lodo formation type section to support Lee and Kodama’s (2009) hypothesis of a normal polarity subchron coinciding with the PETM hyperthermal event.
Objective 2 - Magnetostratigraphy of the Lodo Formation. During project year 4, we collected and analyzed 43 additional paleomagnetic samples from the Lodo Formation type section. This brings the total number of samples from the type section to >200. We conducted stepwise thermal demagnetization experiments on these samples up to 580°C (the blocking temperature of magnetite), or beyond in many cases, and completed final demagnetization from some samples collected during the previous year. At this point we have sampled the entire Lodo Formation type section, but magnetization is poorly recorded in the middle and upper parts of the section. The portion that does yield quality data exhibits geomagnetic polarity chrons of mid-to-upper C24r, all of C24n and probably lower C23, as well as calcareous nanoplankton zones NP10, NP11, and lower NP12. The sampled section thus records ~53-56 Ma, including one major hiatus at a sandy zone and one possible fault break in the section. Stratigraphically above that, we have yet to establish age control in the current study. In project year 4 we continued sampling and analysis for grain-size variation as supplemental data to support our age model.
Objective 3 - Assess Timing of Curvature of the Vallecitos Syncline. During project year four we collected 23 new samples from two new localities in order to assess vertical-axis rotation of these localities. Neither of these localities yielded quality data. At this point we have sampled six localities for this part of the project, the number that we originally proposed. It is our best understanding that post-depositional oxidation and bedding parallel shear has disrupted much/all of the magnetization originally acquired at the time of deposition/consolidation. Oxidation is concentrated in the sandy lithologies but occurs in the clays also, while bedding-parallel shear is (probably) concentrated in the clayey lithologies, where it is interbedded with sandstones. Thus, in most parts of the Vallecitos Syncline, neither lithology yields good results. Only the Lodo Gulch type section has yielded high quality results so far, and there, only part of the section did so.
Lee, Y., and Kodama, K., 2009, A possible link between the geomagnetic field and catastrophic climate at the Paleocene-Eocene thermal maximum: Geology, v. 37, no. 11, p. 1047.