SITE 1043

Lithologic objectives included description of the complete section through the wedge, décollement, and underthrust units, and determination of whether material was missing at the top of the underthrust section. One lithologic unit was defined above the underthrust section at Site 1043, Unit T1 (Fig. 10). Below the décollement, U1B through U3A of the Cocos Plate reference section were recognized. Unit T1 (Holocene? to early Pliocene; 0-150.57 mbsf) consists mainly of thick intervals of clay and silty clay interbedded with relatively thin intervals of matrix-supported breccia. The breccias consist of firm lumps of claystone, siltstone, and ooze embedded in a clayey to sandy matrix. Thin layers and small pods of volcanic ash, and minor interbeds of silt and sand, appear throughout Unit T1. Minor clasts of Pliocene and Miocene limestone and calcareous ooze suggest that older calcareous units contributed debris to the unit. Units U1B, U2, and U3A at Site 1043 are similar to the units defined at the reference site, Site 1039. Three structural domains were defined at Site 1043. Domain I (0-141.3) is the less-deformed part of the wedge toe, and coincides with all but the lowermost part of Unit T1. Domain II (141.3-150.57 mbsf) is the more intensely deformed décollement zone at the base of Unit T1, and Domain III represents the underthrust section of Units U1B-U3A, which are only slightly deformed.

Within the Site 1043 wedge sediments, reversals in age-depth distribution can be used to define the occurrence of thrust faults. A foraminifer marker with an age of 1.77 Ma, the C2n onset at 125 mbsf, and the occurrence of a diatom marker with an age less than 0.62 Ma below that interval, indicate that an age inversion occurs between 125 and 130 mbsf, arguing strongly for the existence of a thrust fault. Another possible age-depth inversion defined by nannofossil and foraminifer datums and the Brunhes/Matuyama (B/M) magnetic polarity boundary matches an interval of low bulk density and high porosity at 25-30 mbsf in the LWD logs of Hole 1043B. Below the décollement, age-depth relationships can be calculated using a combination of biostratigraphic and paleomagnetic information. In Unit U1 (150-194 mbsf), age-depth rates range from 52 to 40 m/m.y., within Unit U2 (194-263 mbsf) the rate is 15 m/m.y., and within Unit U3 (263 mbsf to total depth [TD]) the rate is less than 12 m/m.y. Comparing the depth of the B/M boundary at this site (180 mbsf, 30 m below the décollement) with the depth of this polarity transition at Site 1039 (59 mbsf) indicates that the portion of lithostratigraphic Unit U1 above the B/M boundary has been thinned approximately 30 m by some combination of faulting and compaction at this site.

Geochemistry objectives included determining the pathways and potential sources of pore fluids through the deformed wedge, décollement, and underthrust section. Two regions showed narrow zones of very low density on the LWD logs. One was 74-77 mbsf and the other was in the décollement zone (148-157 mbsf). Each of these depths is associated with major anomalies in Cl, Si, K, PO4, and salinity. These anomalies are zones of relative freshening of the pore waters. The source of the fresher water could be from dehydration of clay minerals or input from meteoric water. Gas hydrate effects could influence some of the anomalies but cannot explain the entire effect. Post-cruise analyses will be required to distinguish these alternatives. Sulfate decreases rapidly with depth in the upper 15-20 mbsf, beneath which it is completely reduced through the décollement. Immediately beneath the décollement, sulfate jumps to relatively high levels and continues to approach the value of seawater with depth in the hole. Alkalinity behaves opposite to that of sulfate, increasing rapidly in the top 15 mbsf, staying high until the décollement, then jumping to lower values below the décollement and gradually decreasing with depth in the underthrust section. Alkalinity approaches seawater values at 270 mbsf. Our geochemistry objectives have been well satisfied at Site 1043.

Physical properties measurements, including downhole observations, were central to the objectives of Site 1043. Significant changes in thickness and reduction in density were noted between correlative units at Sites 1039 and 1040. Site 1043 is located at a position intermediate in terms of potential dewatering. Bulk density and porosity in the wedge section here show a complex pattern, including a number of intervals where the consolidation state decreases somewhat with depth. Some of these may be related to increased fracturing, which affects the downhole logs more than core measurements. The most prominent negative excursion of bulk density down to 1.1 g/cm3 from the baseline (1.8 g/cm3) occurs in the interval 72-75 mbsf, which is correlated to the interval characterized by a geochemical anomaly. A broad negative excursion down to 1.3 g/cm3 occurs in the interval 88-107 mbsf. All negative excursions in the upper 150 mbsf are correlated with the relatively large stand-off of up to 7.5 cm indicated by the differential caliper log. At the décollement zone itself, there is a steep gradient in physical properties, possibly indicating some shear-enhanced compaction in addition to the dewatering of the underthrust section produced by the weight of the prism. The trends through the underthrust section match those in the reference section (Site 1039), but density, P-wave velocity, and resistivity are higher, whereas porosity is somewhat lower. A distinct increase in density occurs at 285 mbsf, which coincides with the top of nannofossil ooze in Hole 1039B. It is apparent that only the upper part of the underthrust section (150-190 mbsf) has dewatered significantly, and there is an interval below this (190-250 mbsf) where the physical properties suggest a modest reversal in the consolidation state.

In summary, drilling at Site 1043 attained all of our objectives. We both cored and logged complete sections through the décollement, acquiring excellent quality core and data. Geochemistry, as it has all through the leg, provided detailed signals of the fluid behavior with depth, and the fluid pathways required by the geochemistry coincide with zones of very low density seen with the logs and with the structural geology of the cores.

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