Late Cretaceous Layered Cherts in the Pacific Analyzed Using Downhole FMS Logs

Details Late Cretaceous Layered Cherts in the Pacific Analyzed Using Downhole FMS Logs Late Cretaceous Layered Cherts in the Pacific Analyzed Using Downhole FMS Logs

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmpp41c0846w&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #PP41C-0846

Astronomy and Astrophysics

Astronomy

0915 Downhole Methods, 3022 Marine Sediments: Processes And Transport, 4267 Paleoceanography, 9355 Pacific Ocean, 9609 Mesozoic

Scientific paper

ODP and DSDP drilling of sediments that contain chert has yielded only limited core recovery. The low core recovery results from the contrast in hardness between the chert and the host sediment (often ooze or chalk), because coring is optimized for either hard or soft sediments. During ODP Leg 198 to Shatsky Rise, Hole 1207B was logged using downhole geophysical tools, including the Formation Micro-Scanner (FMS), providing an alternative method of investigating such sequences. The FMS gives a resistivity image of the borehole wall with a resolution of about 1 cm, and the chert layers stand out as resistive bands in the image. At this site, the chert layers occur on average every 83 cm, with an average thickness of about 9 cm. The cherts are typically layered rather than nodular, suggesting that they formed at horizons that were originally rich in silica, and that diagenesis enhanced this depositional signal. The FMS image logs cover the Cenomanian to Campanian, and the chert layers are most abundant in the Turonian. When the time intervals between chert layers are plotted as a histogram, the intervals cluster around the Milankovitch astronomical periods. Spectral analysis of the FMS data gives peaks in the general range of the astronomical frequencies. We will discuss the various paleoceanographic conditions that could have led to the observed short and long term patterns of chert formation.

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