Heat Flow and Hydrologic Characteristics at the AND-1B borehole, ANDRILL McMurdo Ice Shelf Project, Antarctica

Abstract

TheAntarctic Drilling Program (ANDRILL) successfully drilled and cored a borehole, AND-1B, beneath the McMurdo Ice Shelf and into a fl exural moat basin that surrounds Ross Island. Total drilling depth reached 1285 m below seafl oor (mbsf) with 98 percent core recovery for the detailed study of glacier dynamics. With the goal of obtaining complementary information regarding heat fl ow and permeability, which is vital to understanding the nature of marine hydrogeologic systems, a succession of three temperature logs was recorded over a fi veday span to monitor the gradual thermal recovery toward equilibrium conditions. These data were extrapolated to true, undisturbed temperatures, and they defi ne a linear geothermal gradient of 76.7 K/km from the seafl oor to 647 mbsf. Bulk thermal conductivities of the sedimentary rocks were derived from empirical mixing models and density measurements performed on core, and an average value of 1.5 W/mK ± 10 percent was determined. The corresponding estimate of heat fl ow at this site is 115 mW/m 2 . This value is relatively high but is consistent with other elevated heat-fl ow data associated with the Erebus Volcanic Province. Information regarding the origin and frequency of pathways for subsurface fl uid fl ow is gleaned from drillers' records, complementary geophysical logs, and core descriptions. Only two prominent permeable zones are identifi ed and these correspond to two markedly different features within the rift basin; one is a distinct lithostratigraphic subunit consisting of a thin lava fl ow and the other is a heavily fractured interval within a single thick subunit.

Kinematics of the Neogene Terror Rift: Constraints from calcite twinning strains in the ANDRILL McMurdo Ice Shelf (AND-1B) core, Victoria Land Basin, Antarctica

“…Thick twins and bent twins tend to be more common in the folded vein sample suite. Overall, twin thicknesses (thin and thick), twin morphologies, and twin densities (e.g., 5 twins/mm to 71 twins/mm in ALL) suggest that twinning generally occurred at temperatures <200 °C (Burkhard, 1993;Ferrill, 1991;Ferrill et al, 2004), consistent with AND-1B borehole logging studies that indicate a 76.7 °C/km contemporary temperature gradient at the drill site (Morin et al, 2010). Differential stresses inferred from average twin densities range from 68 to 265 MPa for the raw data set (ALL) (Rowe and Rutter, 1990), but these values are likely unreliable overestimates because the twin density technique is not applicable to low-temperature (<200 °C) deformation (Burkhard, 1993;Ferrill, 1998).…”

“…Northwest-southeast extension is subparallel to the west-northwest (~288° azimuth) deviation of the AND-1B borehole from vertical (from 700 to 1024 mbsf), which might refl ect updip prodding of the drill bit by a population of fractures with a dominant southeast dip direction within this section of the borehole (Morin and Wilkens, 2005;Morin et al, 2010). Northwest-southeast extension is also subparallel to the northwest-southeast extension direction implied by the elongation of the ca.…”

Lithostratigraphy from downhole logs in Hole AND-1B, Antarctica

“…In this way, downhole logs provide important support for the visual stratigraphic logs and contribute to the overall understanding of the paleoenvironment at the AND-1B drill site. There is a 1-m-thick washed out fracture or fault at the top of this interval, dipping 60° to the north (Morin et al, 2010). Lithologically, high natural gamma radiation and low magnetic susceptibility indicate that claystone composes most of this interval.…”

“…1), logging instruments were lowered down into the hole to take in situ measurements of the petrophysical properties of the surrounding rocks. The resulting downhole logs include natural gamma radiation (NGR), magnetic susceptibility, electrical resistivity, borehole diameter, borehole fl uid temperature, and acoustic images of the borehole wall (Morin et al, 2007(Morin et al, , 2010. Elsewhere in the Ross Sea area, similar downhole data have been used to better understand lithostratigraphy, faults and structure, and heat fl ow (ANDRILL [Antarctic Drilling Project] Southern McMurdo Sound Site AND-2A; Wonik et al, 2008, and Cape Roberts Sites CRP-2 and CRP-3; Bücker et al, 2000Bücker et al, , 2001Claps et al, 2000;Brink et al, 2000) Downhole log data are widely used in industry and in other scientifi c drilling programs, such as the Ocean Drilling Program (Goldberg, 1997).…”

‘Late Neogene chronostratigraphy and depositional environments on the Antarctic Margin: New results from the ANDRILL McMurdo Ice Shelf Project’

“…Initial characterisation included visual core description, sedimentology and stratigraphy (Krissek et al, 2007), paleomagnetism (Wilson et al, 2007b), paleontology (Scherer et al, 2007) and petrology (Pompilio et al, 2007). Down-hole logging was undertaken on the HQ and NQ gauge hole following completion of coring including Vertical Seismic Profiling to enable correlation of the borehole with seismic reflection surveys (Morin et al, 2007;Naish et al, 2007b;Williams et al, 2012).…”