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Hein, James R; Vallier, Tracy L; Allan, Mary Ann (1981): Chert petrology and geochemistry at DSDP Leg 62 Holes [dataset publication series]. PANGAEA, https://doi.org/10.1594/PANGAEA.819712, Supplement to: Hein, JR et al. (1981): Chert petrology and geochemistry, Mid-Pacific Mountains and Hess Rise, Deep Sea Drilling Project Leg 62. In: Thiede, J; Vallier, TL; et al. (eds.), Initial Reports of the Deep Sea Drilling Project (U.S. Govt. Printing Office), 62, 711-748, https://doi.org/10.2973/dsdp.proc.62.129.1981

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Abstract:
Sixty-five chert, porcellanite, and siliceous-chalk samples from Deep Sea Drilling Project Leg 62 were analyzed by petrography, scanning electron microscopy, analysis by energy-dispersive X-rays, X-ray diffraction, X-ray spectroscopy, and semiquantitative emission spectroscopy. Siliceous rocks occur mainly in chalks, but also in pelagic clay and marlstone at Site 464. Overall, chert probably constitutes less than 5% of the sections and occurs in deposits of Eocene to Barremian ages at sub-bottom depths of 10 to 820 meters. Chert nodules and beds are commonly rimmed by quartz porcellanite; opal-CT-rich rocks are minor in Leg 62 sediments 65 to 108 m.y. old and at sub-bottom depths of 65 to 520 meters. Chert ranges from white to black, shades of gray and brown being most common; yellow-brown and red-brown jaspers occur at Site 464.
Seventy-eight percent of the studied cherts contain easily recognizable burrow structures. The youngest chert at Site 463 is a quartz cast of a burrow. Burrow silica maturation is always one step ahead of host-rock silicification. Burrows are commonly loci for initial silicification of the host carbonate. Silicification takes place by volume-f or-volume replacement of carbonate sediment, and more-clay-rich sediment at Site 464. Nannofossils are commonly pseudomorphically replaced by quartz near the edges of chert beds and nodules. Other microfossils, mostly radiolarians and foraminifers, whether in chalk or chert, can be either filled with or replaced by calcite, opal-CT, and (or) quartz.
Chemical micro-environments ultimately control the removal, transport, and precipitation of calcite and silica. Two cherts from Site 465 contain sulfate minerals replaced by quartz. Site 465 was never subaerially exposed after sedimentation began, and the formation of the sulfate minerals and their subsequent replacement probably occurred in the marine environment. Several other cherts with odd textures are described in this paper, including (1) a chert breccia cemented by colloform opal-CT and chalcedony, (2) a transition zone between white porcellanite containing opal-CT and quartz and a burrowed brown chert, consisting of radial aggregates of opal-CT with hollow centers, and (3) a chert that consists of silica-replaced calcite pseudospherules interspersed with streaks and circular masses of dense quartz.
X-ray-diffraction analyses show that when data from all sites are considered there are poorly defined trends indicating that older cherts have better quartz crystallinity than younger ones, and that opal-CT crystallite size increases and opal-CT cf-spacings decrease with depth of occurrence in the sections. In a general way, depth of burial and the presence of calcite promote the ordering in the opal-CT crystal structure which allows its eventual conversion to quartz. Opal-CT in porcellanites converts to quartz after reaching a minimum d-spacing of 4.07 Å. Quartz/opal-CT ratios and quartz crystallinity vary randomly on a fine scale across four chert beds, but quartz crystallinity increases from the edge to the center of a fifth chert bed; this may indicate maturation of the silica.
Twenty-four rocks were analyzed for their major- and minor-element compositions. Many elements in cherts are closely related to major mineral components. The carbonate component is distinguished by high values of CaO, MgO, Mn, Ba, Sr, and (for unknown reasons) Zr. Tuffaceous cherts have high values of K and Al, and commonly Zn, Mo, and Cr. Pure cherts are characterized by high SiO2 and B. High B may be a good indicator of formation of chert in an open marine environment, isolated from volcanic and terrigenous materials.
Project(s):
Deep Sea Drilling Project (DSDP)
Coverage:
Median Latitude: 33.509600 * Median Longitude: 175.670822 * South-bound Latitude: 21.350200 * West-bound Longitude: 173.888800 * North-bound Latitude: 39.860700 * East-bound Longitude: 179.255700
Date/Time Start: 1978-08-02T00:00:00 * Date/Time End: 1978-08-28T00:00:00
Event(s):
62-463 * Latitude: 21.350200 * Longitude: 174.667800 * Date/Time: 1978-08-02T00:00:00 * Elevation: -2525.0 m * Penetration: 822.5 m * Recovery: 305.5 m * Location: North Pacific/SEAMOUNT * Campaign: Leg62 * Basis: Glomar Challenger * Method/Device: Drilling/drill rig (DRILL) * Comment: 92 cores; 820.5 m cored; 0 m drilled; 37.2 % recovery
62-464 * Latitude: 39.860700 * Longitude: 173.888800 * Date/Time: 1978-08-17T00:00:00 * Elevation: -4637.0 m * Penetration: 308.5 m * Recovery: 77.6 m * Location: North Pacific/CONT RISE * Campaign: Leg62 * Basis: Glomar Challenger * Method/Device: Drilling/drill rig (DRILL) * Comment: 34 cores; 308.5 m cored; 0 m drilled; 25.2 % recovery
62-465 * Latitude: 33.820500 * Longitude: 178.919000 * Date/Time: 1978-08-23T00:00:00 * Elevation: -2161.0 m * Penetration: 96 m * Recovery: 44.2 m * Location: North Pacific/CONT RISE * Campaign: Leg62 * Basis: Glomar Challenger * Method/Device: Drilling/drill rig (DRILL) * Comment: 11 cores; 96 m cored; 0 m drilled; 46 % recovery
License:
Creative Commons Attribution 3.0 Unported (CC-BY-3.0)
Size:
5 datasets

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Datasets listed in this publication series

  1. Hein, JR; Vallier, TL; Allan, MA (1981): (Appendix) Occurrence and abundance of chert and porcellanite in cores recovered during DSDP Leg 62. https://doi.org/10.1594/PANGAEA.819706
  2. Hein, JR; Vallier, TL; Allan, MA (1981): (Table 3) Quart/opal-CT ratios and quartz crystallinity for porcellanite-chert beds of DSDP Leg 62 Holes. https://doi.org/10.1594/PANGAEA.819709
  3. Hein, JR; Vallier, TL; Allan, MA (1981): (Table 4) Diagenetic characteristics of radiolarians and foraminifers at DSDP Leg 62 Holes. https://doi.org/10.1594/PANGAEA.819710
  4. Hein, JR; Vallier, TL; Allan, MA (1981): (Table 5) Chemistry of cherts, porcellanites, and siliceous carbonates of DSDP Leg 62 Holes. https://doi.org/10.1594/PANGAEA.819711
  5. Hein, JR; Vallier, TL; Allan, MA (1981): (Table 1) Characteristics of siliceous rocks at DSDP Leg 62 Holes. https://doi.org/10.1594/PANGAEA.819708