Focused Fluid Flow and Cementation in Fault Zones From NanTroSEIZE Site C0002, Expedition 348

Abstract:

A continuation of riser drilling at IODP Site C0002 resulted in deepening of the borehole to >3000 mbsf and intersection of several apparent fault horizons, including a fault zone cored at ~2205 mbsf. To obtain interstitial water chemistry data we attempted the GRIND method (Wheat et al., 1994) on five core samples from 2172 to 2217 mbsf. Only two core samples yielded data comparable to typical values obtained previously in shallower cores from the overlying slope basin. Chlorinity values in these two prism samples averaged 407 mM. If these deeper chlorinity values truly reflect in situ IW chemistry, then substantial freshening in the deep prism by input from mineral dehydration reactions is required. IW chemistry from the deep core samples might be partially controlled by reactions between IW and clay minerals during diagenesis (see companion abstract by Brown et al.), but also by sample retrieval and shipboard processing.

Carbonate veins are present in cores and cuttings. Vein fragments in the cuttings commonly exhibit slickenlines and slickenfibers associated with faulting. The carbonate-carbon data show two relative maxima in CaCO₃ abundance, a first maximum of 7.9 wt% centered at ~1925 mbsf, and a second one of 5.8 wt% at ~2625 mbsf. Aside from these maxima there is a steady decrease of background carbonate abundance from 4 wt% to <2 wt% from 900 to 3000 mbsf, respectively. Preliminary analyses of veins from 11 cuttings samples distributed over a depth range of 1440 to 2720 mbsf give values of δ¹³C from -8.59‰ to -1.45‰ and δ¹⁸O from -10.13‰ to -0.15‰. The highest carbon and oxygen isotope values are concentrated above the cored fault interval. The two lightest carbon isotope values are from samples just above this fault zone, indicating possible input of carbon from light hydrocarbons in this interval. Oxygen isotopes show a general decreasing trend below 2180 mbsf. Inferred paleotemperatures imply carbonate formation at shallower burial depths or a low prism geothermal gradient. Observations of abundant carbonate cements and veins in core section C0002P-348-5R-4, and concentrations of carbonate in discrete intervals of up to 100 m in thickness, indicate that faults serve as fluid conduits, resulting in carbonate cementation. Additional data from the cored fault interval will be presented.