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| Figure 2. Downhole logging data obtained from Site U1346: Shirshov Massif. |
Site U1346: Shirshov Massif
Site U1346 was situated at the north edge of the Shirshov Massif summit in northern Shatsky Rise (Figure 1), where acoustic basement is nearly flat, implying a subaerially eroded summit platform (Sager et al., 1999). A single hole was drilled (Hole U1346A), penetrating 191.8 m below the seafloor, with 139.2 m of sedimentary cover and 52.6 m of igneous rock.
Downhole logging data obtained from Hole U1346A included natural and spectral gamma ray, density, photoelectric factor (PEF), and electrical resistivity measurements from three depths of investigation (Figure 2).
Interpretations of gamma ray and electrical resistivity downhole logs were used to identify 14 logging units in Hole U1346A with three in the section covered by the bottom-hole assembly (BHA), four in the sedimentary sequences in the open hole interval, and seven in the basaltic basement. The sedimentary sequence shows several prominent gamma ray anomalies associated with uranium enrichment. The most prominent anomaly is found at the sediment/basement interface and may be indicative of focused hydrothermal fluid flow. Shallower anomalies recorded through the BHA may represent oceanic anoxic events (OAE's) previously interpreted in this area. Electrical resistivity measurements in the basaltic basement show four distinctive massive zones characterized by higher resistivity values, which may represent individual thick lava flows. Relatively high potassium content in the basement section also suggests a high degree of hydrothermal alteration.
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3. Downhole logging data obtained from Site U1347: TAMU Massif.
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Figure 4. Formation MicroScanner (FMS) images from Site U1347.
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Site U1347: TAMU Massif
Site U1347 was situated on the upper flank, east of the summit of TAMU Massif on southern Shatsky Rise (Figure 1). The site location was chosen at a spot where sediments are thin and the "layered basement" signature seen elsewhere on southern Tamu Massif was also thin. Drilling in a single hole (U1347A) penetrated 317.5 m below the seafloor, including a 157.6 m sedimentary section and a 159.9 m igneous section.
Downhole logging data obtained from Hole U1347A included natural and spectral gamma ray, density, neutron porosity, photoelectric factor, and electrical resistivity measurements from three depths of investigation (Figure 3)
Formation MicroScanner (FMS) images show zones of distinctive pillow lavas, zones with high fracture density, and intervals that seem to represent massive lava flows (Figure 4).
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Figure 5. Downhole logging data obtained from Hole U1348A.
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Figure 6. Formation MicroScanner (FMS) images for Hole U1348.
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Site U1348: TAMU Massif
Site U1348 was located on the north flank of TAMU Massif (Figure 1). The drilling target was the upper part of a basement high where sediments are thin. A single hole (U1348A) was drilled at the site, with 324.1 m of penetration below the seafloor that included a thick sequence (~120 m stratigraphically) of volcaniclastic sediments underlying shallow water calcareous sandstones, greenish clays, nannofossil ooze, and chert.
Downhole logging data obtained from Hole U1348A included natural and spectral gamma ray, density, photoelectric factor, compressional-wave velocity, and electrical resistivity measurements from three depths of investigation (Figure 5).
Interpretations of gamma ray and electrical resistivity downhole logs were used to identify a total of 15 logging units in Hole U1348A with one in the section covered by the BHA, five in the sedimentary sequences in the open hole interval, and nine in the volcaniclastic section. Electrical resistivity measurements show distinctive higher resistivity zones that likely represent less altered intervals, interspersed with low resistivity zones that mark sediment interbeds and more altered sequences. Natural gamma-ray measurements show several intervals of higher readings that indicate interbedded sediments and higher alteration. These intervals also display higher potassium, uranium, and thorium values.
Formation MicroScanner (FMS) images show zones with distinct horizontal layering, dipping beds, and vesicular or brecciated intervals. Preliminary structural analyses of dipping beds show features striking northeast–southwest and dipping mostly 20-30° to the southeast (Figure 6).
Site U1349: Ori Massif
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7. Downhole logging data obtained from Hole U1349.
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Figure 8. Formation MicroScanner (FMS) images from Hole U1349.
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Site U1349 was located at the summit of ORI Massif (Figure 1) on a flat-topped basement ridge that seems to have been shaped by sea level erosion. Hole U1349A was the only hole drilled at the site. It penetrated 250.4 m beneath the seafloor, through 165.1 m of sediment and 85.3 m of igneous basement.
Downhole logging data obtained from Hole U1349A included natural and spectral gamma ray, density, and electrical resistivity measurements from three depths of investigation (Figure 7).
Interpretations of gamma ray and electrical resistivity downhole logs were used to identify a total of 19 logging units in Hole U1349A with one in the section covered by the BHA, five in the sedimentary sequences in the open hole interval, and thirteen in the basaltic basement section. Electrical resistivity measurements show distinctive higher resistivity zones that likely represent less altered intervals, interspersed with low resistivity zones that mark more altered sequences. Natural gamma-ray measurements show a large peak just below the sediment-basement interface that may indicate a zone of concentrated hydrothermal alteration. This interval also displays very high uranium values and a smaller peak in potassium values.
Formation MicroScanner (FMS) images show zones with highly fractured intervals, potential veins, and vesicular or brecciated intervals (Figure 8).
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Figure 9.
Downhole logging data obtained from Hole U1350.
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Figure 10. Gamma ray measurements from Hole U1350.
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Site U1350: Ori Massif
Logging operations in Hole U1350A consisted of two attempts to deploy one tool string and took place in deteriorating weather with initial ship heave conditions of ~ 2 m, which gradually changed to ~ 4 m peak-to-peak heave and wind gusts of up to 56 knots.
Deteriorating weather conditions, time constraints, large fluctuations in surface tension, and a failure of the head tension sensor (Figure 9) contributed to aborting logging operations in this site.
The down log inside the BHA recorded approximately 27.7 meters of the shallow sediments (Figure 10).
The gamma ray measurements in the shallow sediments show an anomaly from seafloor to approximately 25 m WMSF. The contributions to this anomaly are mainly an increase in thorium and a smaller contribution from uranium.
