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doi:10.2204/iodp.proc.319.104.2010

Lithology

We defined three logging units at Site C0010 on the basis of LWD/MWD measurements (gamma ray and bit resistivity) and guided by previous logging and coring results at nearby Site C0004 (Expedition 314 Scientists, 2009b; Expedition 316 Scientists, 2009). We interpret logging Unit I (0–182.8 m LSF) as slope sediments deposited on top of an underlying thrust wedge. This unit generally correlates with Site C0004 lithologic Unit I. It is subdivided into Subunit IA (0–161.5 m LSF), composed of hemipelagic mud and minor turbidite interbeds, and Subunit IB (161.5–182.8 m LSF) composed of mass transport deposits probably reworked from the thrust wedge. Logging Unit II (182.8–407.0 m LSF) represents accreted sediments that are part of a thrust wedge and correlate with Site C0004 lithologic Units II and III. However, the log character of sediments in the thrust wedge at Site C0010 differs from that at Site C0004, most notably in that sediments appear to be more clay rich at Site C0010. Our interpretation of logging Unit III (407.0–555.1 m LSF) corroborates the idea of slope deposits being overridden by the megasplay fault, suggested at Site C0004, which are lithologically similar to the slope sediments of Subunit IA above and correlate to Site C0004 lithologic Unit IV.

Log characterization and lithologic interpretation

Three logging units are defined at Site C0010 (Figs. F7, F8). Seismic reflection profiles indicate that similar lithologic sequences may occur at Sites C0004 and C0010 (Moore et al., 2009). Although in detail the sites are not identical in seismic reflection data, observations from Site C0004 (Expedition 314 Scientists, 2009b; Expedition 316 Scientists, 2009) provide valuable constraints on lithologic variations at Site C0010. Data acquired during Expeditions 314 and 316 at Site C0004 show that lithologic changes are best defined based on gamma ray measurements, whereas changes in resistivity values may vary independently from lithology and show some variations related to porosity (Fig. F8).

Unit I (0–182.8 m LSF)

Logging Unit I is divided into Subunits IA and IB. Subunit IA (0–161.5 m LSF) is characterized by a gradual increase in gamma ray (from 50 to 85 gAPI [American Petroleum Institute]) with depth (Fig. F7). Bit resistivity values increase from 0.6 to 0.8 Ωm within the same depth interval. We define the Subunit IA/IB boundary based on a sharp decrease of gamma ray values at ~161.5 m LSF. Subunit IB (161.5–182.8 m LSF) is characterized by lower gamma ray values (<65 gAPI) than those found in the base of logging Subunit IA (>80 gAPI) (Fig. F7). Bit resistivity values decrease slightly at this depth from 0.7 to 0.65 Ωm. The base of logging Subunit IB is defined by a sharp increase in bit resistivity. Gamma ray values in Subunit IB are dissimilar to those in Subunit IA above but similar to those in Unit II below.

Unit II (182.8–407.0 m LSF)

Logging Unit II is characterized by large-amplitude variations in gamma ray values (65–120 gAPI), with an overall increasing trend and higher values than units above and below. Six ~7–15 m thick intervals of low gamma ray (<70 gAPI) are observed within this unit (Fig. F7). Bit resistivity values are also more variable (0.9–1.7 Ωm) than those in logging Unit I (0.6–0.8 Ωm) and indicate six conductive intervals that correspond to the intervals of low gamma ray values. It is also noteworthy that the magnitude of variations in both gamma ray and bit resistivity (e.g., 373–386 m LSF) seen in logging Run 1 are reduced during relogging of the hole (Run 2) (Fig. F7). The Unit II/III boundary is based on the sharp decrease in both gamma ray and bit resistivity values. This boundary also coincides with the lower boundary of the thrust wedge (See "Log-Seismic integration" and "Structural geology").

Unit III (407.0–555.1 m LSF)

Logging Unit III is characterized by constant gamma ray values (70–75 gAPI) (Fig. F7). Bit resistivity also remains relatively constant in Unit III, ranging from 0.8 to 0.9 Ωm. The low gamma ray readings (<40 gAPI) between 513–515 and 522–530 m LSF correlate well with the depths of low bit resistivity readings (<0.7 Ωm) (Fig. F7).

Lithologic interpretation

Interpretations of seismic reflection data (Moore et al., 2009) and results from Site C0004 (Expedition 314 Scientists, 2009b; Expedition 316 Scientists, 2009) combined with log data from Site C0010 suggest that three distinct lithologic packages occur at Site C0010, comprising from top to bottom: slope deposits, thrust wedge, and overridden slope deposits. Below, we provide a lithologic interpretation of Site C0010 logging units based on these combined data sets (Fig. F8).

Site C0010 logging Subunit IA displays gamma ray and bit resistivity patterns similar to Site C0004 Unit I (Fig. F8). We interpret this unit as slope sediments deposited on top of logging Unit II (thrust wedge). Based on gamma ray values, slope sediments in the 70–158 m LSF depth interval are characterized by higher clay mineral content than similar sediments at Site C0004 and likely represent hemipelagic mud with minor thin, fine-grained turbidite interbeds. We interpret Site C0010 Subunit IB as mass transport deposits composed of material reworked from upslope Unit II equivalent material, based on (1) similar gamma ray and bit resistivity measurements at Sites C0004 (~78–96.2 m LSF) and C0010 (161.5–182.8 m LSF), (2) comparison with the cored interval at Site C0004 that includes brecciated material probably reworked from the thrust wedge, and (3) seismic data that suggest Site C0010 Subunit IB includes deformed slope sediments overlying the thrust wedge (Moore et al., 2009) (Fig. F8).

We interpret logging Unit II as a thrust wedge that is part of the accretionary prism, correlating with Site C0004 lithologic Units II and III (Fig. F8). The thrust wedge at Site C0010 differs from that at Site C0004 in that it has higher gamma ray values. This may indicate that Unit II is mostly composed of pelagic mud or poorly lithified pelagic mudstone with a higher clay mineral content, distinct from the hemipelagic mud observed at Site C0004 (Expedition 316 Scientists, 2009). Higher resistivity in Site C0010 logging Unit II relative to logging Units I and III and Site C0004 lithologic Units II and III (equivalent to Unit II at Site C0010) may indicate increased compaction, or alternatively, increased tortuosity associated with higher clay content (see "Physical properties"). Local decreases in gamma ray and bit resistivity values may reflect (1) interbeds of hemipelagic silty mud within the overall compacted pelagic mud, (2) breakouts or fracture zones (see "Structural geology"), or (3) artifacts related to acquisition of the data as indicated by differences in gamma ray values between Runs 1 and 2 (Fig. F8).

Logging Unit III is composed of slope sediments overridden by the thrust wedge and correlates with Site C0004 lithologic Unit IV (Fig. F8). On the basis of the LWD data and coring results from Site C0004, we interpret Unit III as hemipelagic mud with minor turbidite interbeds and rare volcanic ash layers. Coarser grained turbidite beds may occur at 513–515 and 522–530 m LSF where gamma ray values decrease markedly.

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