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Reports: UR853255-UR8: Magnetostratigraphy of the Lodo Formation and Kinematics of the Vallecitos Syncline, California

Christopher J. Pluhar, PhD, California State University, Fresno

This report details findings during year one 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 of this grant we have made significant progress on the first two objectives.

Did a normal polarity subchron occur around the time of the PETM?  Paleomagnetic reversals are globally synchronous events within two to ten thousand years.  Thus, a paleomagnetic subchron at the PETM would be of major significance.  It would permit global synchronization of PETM records, allowing an examination of time lags or synchroneity of the climate, extinction, and other datasets.  However, our results strongly argue against the existence of a paleomagnetic subchron at the PETM like that proposed by Lee & Kodama (2009).  We have collected two parallel magnetostratigraphic records across the PETM and find little evidence of the proposed normal subchron. The PETM is identifiable in the Lodo formation as a distinctive brown-red claystone in an otherwise green, calcareous, silty clay (Figure 1).  At a gully near and parallel to the Lodo Gulch type locality of Lodo Formation, John et al. (2008) demonstrate the carbon isotope excursion of the PETM starting at the base of the clay.  One of our magnetostratigraphic sections (collected before the funding period and called locality BG) exactly coincides with the carbon isotope record of John et al. (2008) and our other magnetostratigraphy (collected during this study and called locality LG) spans the PETM within Lodo Gulch proper.  Our sampling density across these zones is at the sub-meter and sub-decimeter level, respectively (Figure 1).  Due to the high sedimentation rate at LG and BG localities, our sampling density should be more than adequate to capture polarity structure such as that reported in Lee & Kodama (2009), but our samples do not record a normal polarity zone (Figure 2).  Single samples at each locality appear to suggest a normal polarity, but these are low quality samples and are very likely to instead represent poor paleomagnetic recording.  We tentatively conclude that the normal subchron proposed by Lee & Kodama (2009) is spurious.  This is not surprising, as a normal subchron has never been found at the PETM before.  Nonetheless, we will conduct rock magnetic studies to further test our conclusions.

Magnetostratigraphy of the Lodo Formation  We have analyzed about 90 paleomagnetic samples spanning the Lodo Formation.  These data are of variable quality, often not settling on a stable primary paleomagnetic direction.  This is especially true in the upper portions of the Lodo Gulch type section.  We are exploring the reason for problematic samples, including tectonic disruption of these massive featureless clays, secondary magnetization, and near-surface creep causing sample disruption.  At present, as a result of this complication, we are not at a stage to present a magnetostratigraphic interpretation of the Lodo Formation type section.

References Cited

John, C., Bohaty, S., Zachos, J., Sluijs, A., Gibbs, S., Brinkhuis, H., and Bralower, T., 2008, North American continental margin records of the Paleocene–Eocene thermal maximum: Implications for global carbon and hydrological cycling: Paleoceanography, v. 23, p: PA2217, doi, v. 10.

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.

Zachos, J. C., Ršhl, U., Schellenberg, S. A., Sluijs, A., Hodell, D. A., Kelly, D. C., Thomas, E., Nicolo, M., Raffi, I., and Lourens, L. J., 2005, Rapid acidification of the ocean during the Paleocene-Eocene thermal maximum: Science, v. 308, no. 5728, p. 1611-1615.