4. HOLE 956B: DOWNHOLE FMS MEASUREMENTS IN THE SOUTHERN VOLCANIC APRON OF GRAN CANARIA, CENTRAL ATLANTIC ¹

Nicolas Binard,² Hans-Ulrich Schmincke,² and Mari Sumita ²

ABSTRACT

Hole 956B penetrated a total of 700 m of biogenic and volcaniclastic sediments into the southwestern peripheral volcanic apron of Gran Canaria. Electrical microconductivity measurements, made by the Formation MicroScanner (FMS) tool, were recorded from 701 to 283 mbsf. The cumulative curve and frequency distribution of the FMS data are clearly bimodal, showing two main families: biogenic sediment and volcaniclastics (tuffs and volcanic breccia). As suggested by the cumulative conductivity curve, tuffs and volcanic breccia could not be distinguished from each other as their electrical conductivities are too similar to be reliably differentiated. About 54.9% of the total FMS values represents volcaniclastics comparable to that derived from the core estimate (53.8%). We attribute this to the fact that volcaniclastics are preferentially lost during drilling compared to biogenic sediment. If this estimate is correct, it also suggests that a large portion of unrecovered material is represented by biogenic sediment. Most laminated beds face northwest. The stratigraphic plane azimuths show three major orientations: (1) 270° to 300°, mainly influenced by the southwestern submarine flank of Gran Canaria; (2) 040° and 220°, likely related to sediment mass flows moving downslope from the upper flanks of Gran Canaria; and (3) 340°, which has no satisfactory explanation.

¹ Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998. Proc. ODP, Sci. Results, 157: College Station, TX (Ocean Drilling Program).
² GEOMAR, Wischhofstraße 1-3, D-24148 Kiel, Federal Republic of Germany. Nicolas.Binard@t-online.de