Lithostratigraphy at Site U1364

Site 889

Core and log data from Site 889 showed three major lithostratigraphic units within the uppermost 400 mbsf. Lithostratigraphic Unit I was divided into Subunits IA and IB. Subunit IA (seafloor to 87 mbsf in Hole 889A) comprises mostly clayey silts and silty clays with interbedded thin sand layers. This unit is characterized by subhorizontal to gently dipping beds with little deformation and was interpreted to be turbidites and pelagic sediments deposited in a slope basin. Subunit IB (91–128 mbsf in Hole 889A) is also mainly silty clay but with fewer sand layers. It was interpreted that the sediments in Subunit IB represent a series of sediment gravity flow deposits caused by tectonic uplift at the deformation front and thus a transition between the abyssal plain sedimentation of lithostratigraphic Units II and III to the subsequent deposition of Subunit IA, which represents slope basin sedimentation.

Units II and III consist of mainly clayey silt with a low abundance of sand layers and are distinguished only by an increase in glauconite in Unit III. No structural differences between these two units were observed. They were interpreted as typical abyssal plain sediments that were heavily deformed and fractured during the accretion process. Seismic profiles show that Subunit IB has also suffered deformation, and the boundary between accreted and slope sediments was placed at ~90 mbsf within Unit I.

Site U1327

Five holes were drilled at Site U1327 (Riedel, Collett, Malone, et al., 2006), and the section was divided into three main lithostratigraphic units that differ from those defined at the time of Leg 146. Unit I (0–90 mbsf; age <0.3 Ma) comprises dark greenish gray and dark gray clay and silty clay, often interbedded with silt, clayey silt, sandy silt, sand, and gravel layers. It is characterized by fine-grained detrital sediments (clay and silty clay) with abundant coarse-grained layers as thick as 6 cm that indicate turbidity flow deposition. Some carbonate cement is present. The boundary between Units I and II is marked by a sharp decrease in sand and silt layers and a sharp increase in the abundance of diatoms. The average rate of sedimentation was estimated to be 22 cm/k.y.

Lithostratigraphic Unit II (90–170 mbsf; age >0.3–1.0 Ma) comprises dark greenish gray and dark gray clay; clay with diatoms; and silty clay, silty clay with diatoms, and diatom silty clay locally interbedded with sandy silt and sand layers and lenses. Very few carbonate cements are present. Soupy and mousselike sediment textures correlated with cold sections in the core imaged with an infrared scanner; these were inferred to be the consequence of the dissociation of interstitial gas hydrate during recovery. The average sedimentation rate for Unit II is estimated at ~16 cm/k.y. Intervals with a high ratio of nonmarine versus marine diatoms indicate an increased contribution of terrigeneous detrital sediments from land sources by turbidites. The great abundance of marine diatoms along with spores suggests initial deposition beneath a coastal upwelling environment followed by reworking by turbidity currents.

Lithostratigraphic Unit III (170–300 mbsf; age >1 Ma) comprises dark greenish gray and dark gray silty clay, with some exotic rock clasts. Diatoms are absent immediately below the Unit II/III boundary, although diatoms reappear below 248 mbsf. Unit III is distinguished from Unit II by a higher degree of induration of the sediments compared to lithostratigraphic Unit II. Carbonate cements are rare. The depositional environment for Unit III was interpreted to have been dominated by low-energy turbidity currents in an abyssal plain environment. The biostratigraphically estimated average sedimentation rate in this section is 12 cm/k.y.

Site U1327 logging data

Physical property variations defined by logging at Site U1327 (Fig. F7) do not reflect the lithostratigraphy described above in any obvious way, but they provide valuable contextual information about the section spanned by the Site U1364 ACORK. General trends of increasing bulk density, electrical resistivity, and sonic velocity and decreasing porosity with depth are defined, with porosity ranging from roughly 0.7 at the seafloor (measured on cores) to ~0.50 at 300 mbsf. A major anomaly is seen in the logging-while-drilling (LWD) data between 120 and 145 mbsf. This is inferred to reflect a localized massive hydrate horizon (defined only in Hole U1327A). At 230 mbsf, a decrease in resistivity is seen in the resistivity logs and the resistivity-at-the-bit (RAB) image, as are sharp decreases in density and sonic velocity and an increase in porosity. These changes, and the change in character of the RAB image, are consistent with gas hydrates being present in discrete layers above 230 mbsf and gas being present below. The ACORK screens are distributed roughly symmetrically around this boundary; screens 1 and 2 lie at 74 and 14 m below and screens 3 and 4 lie at 27 and 74 m above the boundary, respectively.