In-situ petrophysical properties of volcanic rocks from Detroit Seamount (ODP-Leg 197, Site 1203A)

During Leg 197 an extensive logging program was performed on Site 1203A after drilling was completed on Detroit Seamount to a total depth of 914.6 mbsf. Logging comprised measurements with the ODP/LDEO standard logging tool strings triple combo and FMS/sonic (see: http://www.ldeo.columbia.edu/BRG/ODP/). Logging data imply a classification of the drilled lithology into five groups: three types of sedimentary rocks and two types of basaltic rocks. Non-volcanic sediments, determined down to a depth of 457 mbsf, exhibit constant and low gamma ray values (<15 gAPI). A strong increase in p-wave velocity (3.12 \pm1.15 km/s) between 388.2-393.5 mbsf correlates with the 388-mbsf reflector in seismic profiles. A gradual increase in density from 1.3 g/ccm to 1.8 g/ccm, and a decrease in porosity from 92.7 % to 69 % with depth record expected consolidation and compaction effects. FMS pictures and standard logging tool data mark the sediment - basement boundary at 464.5 mbsf. A transition zone between 464.5-468.5 mbsf reveals high porosity/low density, high gamma-ray/low resistivity data, which is probably caused by alteration. The basement is characterised by an interlayered succession of volcanoclastic rocks and basaltic lava flows. Lava flows of type B-I are marked by a bimodal data distribution of resistivity (68.80 +/-2.41 ohmm) and porosity (21.71 +/-8.27 %) with maximum values for resistivity at 32 ohmm and 100 ohmm, and for porosity at 11 % and 19.5 %. Usually, basaltic lava exhibits low gamma-ray values as observed for rocks of the B-I flow type (18.75 +/-7.32 gAPI). Instead, lava flows of type B-II show higher gamma-ray data of 28 gAPI (\pm5.92). This flow type occurs in the lower basement part between 761-920 mbsf and could be target of an increased alteration. It reveals higher porosity (36.56 +/-10.03 %), caused by a high vesicularity, and lower density (2.33 +/-0.18 g/ccm) values than the B-I flows (density = 2.60 \pm0.16 g/ccm). Resistivity and porosity data are characterised by an unimodal distribution. Volcanoclastics of type V-I and V-II are limited to the basement and combine different types of volcanic sediments, e.g. basaltic tuffs, hyaloclastic breccia, and volcanoclastic silt-, and sandstones. V-I volcanoclastics occur in the upper borehole part (492-518 mbsf) and consist of basaltic tuffs. With increasing depth these rocks are alternated by V-II type volcanoclastic sandstones with minor tuff and breccia content. They show a high porosity of 73.53 % in combination with large porosity variations (+/-17.04 %), and a low resistivity (2.65 +/-1.74 ohmm). The high gamma-ray activity of (36.48 +/-12.08 gAPI) can be attributed to a higher content of clay and/or altered material. In contrast, V-I rocks are characterised by a lower porosity (60.35 +/-6.34 %), a higher resistivity (11.74 +/-1.81 ohmm), and a lower gamma-ray (18.26 +/-4.18 gAPI), which are similar to those of B-I massive units. Both rock types exhibit density values of 1.9 - 2.0 g/ccm. Lithological boundaries, as indicated by core lithology, might be target to depth shifting by a log derived lithology. The part between 467 - 628 mbsf seems to be more influenced by depth changes of lithological boundaries then deeper parts of the borehole. Logging data reveal higher amounts of volcanoclastics than implied by core lithology. The large amounts of volcanoclastics, and the occurrence of high porosity lava flows in great depth, which was also observed in HSDP-2, are unexpected on the basis of recent volcano models. These effects indicate strong similarities between the two volcano areas.