60th Annual Report on Research 2015

This report details findings during year two of this grant, awarded to study the Lodo Formation, a petroleum-bearing unit in the California Coast Ranges.  This marine unit 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 goals two and three.

Magnetostratigraphy of the Lodo Formation.  We have partially analyzed ~50 new samples from the Lodo Formation type section, adding to the ~90 type-section samples already -analyzed in year 1.  The new samples are densely concentrated in a zone directly above last year's PETM segment of the type section.  We densely samples this zone because it appears to span the NP10-11 calcareous nanofossil zone boundary, suggesting that both the C24r-C24n.3n magnetic reversal (53.983 Ma; Ogg, 2012) and the ETM-2 (Eocene Thermal Maximum-2 occurring just before C24r-C24n.3n) hyperthermal event should be contained in the added section.  We are still processing our paleomagnetic samples, having conducted progressive stepwise thermal demagnetization up to 450°C thus far.  The preliminary results exhibit well-behaved demagnetization paths and suggest that we have located C24r-C24n.3n, permitting a detailed search for ETM-2 in this high-resolution section.  This also offers the possibility that the section may also record other proposed Eocene hyperthermals H-2 (right at the C24r-C24n.3n reversal), I-1 and I-2 (during C24n.3n), and ETM-3 (during C24n.1n) that all occur within chron C24n (53.983-52.620 Ma; Galeotti et al., 2010; Ogg, 2012).  In keeping with our proposed work to create a magnetostratigraphy of the Lodo Formation, we will work to establish the magnetostratigraphic framework within which these C24n climate events may be found by collaborators.

Assess Timing of Curvature of the Vallecitos Syncline.  During the recently-completed fiscal year we collected about 70 new samples from three new localities along the Vallecitos Syncline in order to test for vertical axis rotation of sites near the San Andreas fault system as hypothesized by us and Titus et al. (2011a & 2011b).  The new localities are New Idria Road, Griswold Hills, and Paicines.  Sandstone exposure was far better at these localities than clays and shales.  In addition, New Idria Road locality preserved abundant evidence of bedding-plane faulting within the clays.  Thus, we preferentially sampled the sandstones, but these lithologies did not perform very well during stepwise demagnetization.  With demagnetization up to 450°C, for the most part sample magnetizations have still not settled on a primary remanence direction.  We have not yet completed demagnetization of these samples, due to lab access difficulty during summer 2015 (we run our samples at Caltech, since the CSU Fresno magnetometer is not well-suited to friable materials).  We plan to complete sample demagnetization during October 2015 at Occidental College.

References Cited

Galeotti, S., Krishnan, S., Pagani, M., Lanci, L., Gaudio, A., Zachos, J.C. Monechi, S., Morelli, G., Lourens, L., 2010, Orbital chronology of Early Eocene hyperthermals from the Contessa Road section, central Italy, EPSL, v. 290, no. 1-2, pp. 192-200.

Ogg, J.G., 2012, The Geomagnetic Polarity Timescale: The Geologic Timescale 2012, ed: Gradstein, F.M., Ogg, J.G., Schmitz, M.D., and Ogg, G.M., pp. 85-114.

Titus, S.J., Crump, S., McGuire, Z., Horsman, E., Housen, B., 2011, Using vertical axis rotations to characterize off-fault deformation across the San Andreas fault system, central California, Geology, v. 39, no. 8, pp. 711-714, DOI: 10.1130/G31802.1

Titus, S.J., Dyson, M., DeMets, C., Tikoff, B., Rolandone, F., Burgmann, R., 2011, Geologic versus geodetic deformation adjacent to the San Andreas fault, central California, GSA Bulletin, v. 123, no. 5-6, pp. 794-820, DOI: 10.1130/B30150.1