HR: 0800h
AN: PP11B-1460    [Abstracts]
TI: Milankovitch Cyclicity: A Tool for Determining the Stratigraphic Evolution of the Late Neogene Offshore Central California Margin
AU: * Hill, P S
EM: phill@ess.ucla.edu
AF: University of California, Los Angeles, Department of Earth and Space Sciences, Los Angeles, CA 90095-1567 United States
AU: Behl, R J
EM: behl@csulb.edu
AF: California State University, Long Beach, Department of Geological Sciences, Long Beach, CA 90840-3902 United States
AB: Latest Miocene/Pliocene stratigraphy of the offshore central California margin records Milankovitch cyclicities on eccentricity through precessional time scales. Dating and correlation of Milankovitch cycles in the natural gamma ray logs of sediments from Ocean Drilling Program (ODP) Site 1016, 150 km offshore, to more proximal offshore oil wells provide new, higher-resolution sedimentation rates, refined mass accumulation rates of calcium carbonate and total organic carbon (TOC), identification of condensed sections, a suggested refinement of the ages of select Late Miocene radiolaria, as well as an overview of stratigraphic trends and geography along the paleomargin. This is the first application of astrochronology to dating the fine-grained, homogeneous sediments of the Late Neogene Monterey and Sisquoc formations of the petroliferous Santa Maria Basin. The relatively well-dated ODP Hole 1016A was used to create a high-resolution dating and correlation template for more proximal wells in the basin. Further applications and refinements of this template will aid in increasing knowledge of the architecture and stratigraphic history of the basin. The success of this project suggests that astrochronology is a valuable approach for correlating and dating formational and facies changes, and their lateral development across the basins of the California margin. A distinctive pattern in the gamma ray template of OPD Hole 1016A, dated from 5.2 to 4.8 Ma, was also found in eleven studied oil wells of the offshore Santa Maria Basin, permitting the determination of areal trends in sedimentation rates. Sedimentation rates increase towards the paleoshoreline, from 8 cm/ky 150 km from shore, to about 50 cm/ky in the medial wells, to about 90 cm/ky at the most proximal well. The rates also increase from NW to SE in the oil wells. The present depth of the 5.2 Ma horizon also increases toward the paleoshore, from about 245 m 150 km from shore, to about 1500 m in the medial wells to as much as 2600 m in the most proximal well. The depths also increase from NW to SE. Sedimentation rates at ODP Site 1016A, in general, decreased during the Late Neogene. From a high of 12-14 cm/ky circa 5.8 Ma, the sedimentation rate decreased to 2 to 3 cm/ky circa 2.6 Ma, with intermediate maxima of 11-12 cm/ky circa 4.9 to 5.1 Ma and 9 to 10 cm/ky circa 4.4 to 4.6 Ma. In spite of decreased linear sedimentation rates, the mass accumulation rate for calcium carbonate has generally increased from 5.8 to 2.6 Ma with a major peak at about 4.4 to 4.6 Ma. The mass accumulation rate for total organic carbon (TOC) shows a major peak at about 5.6 to 5.7 Ma, then a gradual increase to a second maximum about 4.3 to 4.4 Ma, a fairly sharp decrease from 4.3 to 4.0 Ma, and then a low, somewhat undulating rate to 2.6 Ma. The new higher-resolution MARs from this study show greater values before 3.5 Ma than previous works using earlier age scales. According to the new age scale, the dates suggested for the Miocene radiolarian stratigraphic markers at ODP Site 1016A are about 1.5 million years younger than the originally assigned ages.
DE: 4910 Astronomical forcing
DE: 4946 Milankovitch theory
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005