Site 1252 (proposed Site HR5a) was drilled in 1039 m of water, ~4.5 km east of the southern summit of Hydrate Ridge. The site is located on the western flank of a secondary anticline that is located east of the crest of Hydrate Ridge. The sediments in the core of the anticline appear to be continuous with the accretionary complex sediments sampled near the base of Site 1244 (~1.5 km to the west). Although there is an anomalously bright BSR at a depth of ~170 mbsf within the core of the anticline, the BSR disappears abruptly at an apparent stratigraphic boundary within the accretionary complex sequence and does not extend beneath Site 1252. Sediments onlapping the anticline from the west can be correlated with sediments sampled at Site 1251.
The principal objectives at Site 1252 were to (1) sample the sediments in the core of the anticline to determine whether they are compositionally and biostratigraphically similar to those at the base of Site 1244; (2) determine the structure of these sediments in order to constrain the mode of growth of the anticline/diapir; (3) determine whether hydrates are present at a site near a very strong BSR but where no BSR is present; and (4) provide age constraints on the geological history recorded by seismic stratigraphy.
One hole was drilled at Site 1252, comprising 28 cores (14 APC and 14 XCB) and sampling sediments to a depth of 260 mbsf. The APCT tool, which was deployed six times to measure in situ temperature, was the only special downhole tool used at this site (Tables T2, T3). This is also the only site where we did not acquire LWD. Wireline logging data were acquired, including one run with the triple combo tool string and one run with the FMS-sonic tool string. This was an alternate site and was the last site drilled during Leg 204.
Drilling at Site 1252 showed evidence for only very limited presence of gas hydrate. Nonetheless, it is a very interesting site in that it showed very clear correlations between lithostratigraphic observations, physical property measurements, and wireline logging results throughout the entire cored sequence. It is the only site to show significant postdepositional carbonate formation beneath the upper tens of meters.
Based on the major and minor lithologies and additional criteria like fabric, physical properties, and microscopic analysis, the sediments recovered at Site 1252 were divided in three lithostratigraphic units. Lithostratigraphic Unit I, subdivided into four subunits, extends from the seafloor to 96.5 mbsf and is dominated by dark greenish gray clay to silty clay ranging in age from 0 to ~0.3 Ma. Subunit IA is characterized by a strong negative density gradient in the MST data, which probably results from the onset of gas exsolution at ~7 mbsf. The lower boundary of Unit I is defined by the same unconformity that was sampled at a depth of 130 mbsf at Site 1251. At both sites, this unconformity is clearly seen in seismic data and is compatible with biostratigraphic data but does not have a striking lithologic signature. At Site 1252, the unconformity is overlain by an apparent debris flow called Subunit ID, the top of which corresponds to a strong anomaly in MS. Subunit ID pinches out just west of Site 1251 and was not sampled there. In contrast, the debris flow that comprises Subunit IB at Site 1251 is not present at Site 1252 (Fig. F8). Subunit IC at both sites is strikingly similar in its lithologic description and its seismic reflection character.
Unit II at Site 1252 is a dark green foraminifer-rich silty clay that is intercalated with lighter-colored fine sand and coarse silt turbidite layers. These thin turbidites result in a nearly continuous zone of high MS. In contrast to Site 1251, where Unit II is ~180 m thick, Unit II at Site 1252 is only ~20 m thick. This is primarily due to the location of this site, where Unit II laps onto the west flank of uplifted accretionary complex sediments. Subunits IB and IC through Unit II are characterized by a normal increase in density and decrease in porosity with depth that is probably caused by sediment compaction.
The boundary between Unit II and Unit III is marked by a 5-m-thick series of glauconite-rich sands, including a 2-cm-thick layer of almost pure glauconite. This is underlain by a layer of authigenic carbonate that required a transition from APC to XCB coring at 125 mbsf. Wireline density, resistivity, and chemical (uranium and potassium) logs all show very high values in a 6-m-thick interval at this depth, consistent with extensive carbonate precipitation forming concretions and cement. The top of Unit III corresponds closely with the top of the uplifted accretionary complex sediments in the core of the anticline/diapir and is referred to as Subunit IIIA (1310–210 mbsf). It is underlain by Subunit IIIB (210–260 mbsf), which is distinguished from Subunit IIIA by an increase in biogenic opal and a decrease in silt. The biostratigraphic age of Unit III is 1.6–>2 Ma.
The density profile of Unit III is unusual. Physical property measurements (MST GRA logs and MAD bulk density) indicate that the density decreases by 0.2 g/cm3 (from 1.8 to 1.6 g/cm3) over a distance of ~25 m below the carbonate-rich zone and is then variable but with an average value of ~1.7 g/cm3. No systematic increase in density is observed until the base of the hole at 260 mbsf. The wireline density logs show similar behavior, with a local increase to nearly 2.0 g/cm3 in the carbonate layers underlain by a nearly constant density of 1.7 g/cm3 with occasional thin (~2 m thick) low-density (~1.5 g/cm3) excursions. This anomalous density profile, similar to but better defined than what was observed in lower part of the section at Sites 1244 and 1251, suggests that density-driven diapirism may be a mechanism contributing to the tectonic evolution of the accretionary prism.
Thermal imaging of cores using the IR cameras indicated only very limited hydrate presence at Site 1252. Two possible hydrate samples were preserved from depths of 83 mbsf (in Subunit ID) and 99 mbsf (in Unit II). The chloride concentration in the pore water and the C1/C2 ratios in void-space gas samples did not show anomalies indicative of dissociated hydrate, unlike what was observed at other sites (see, for example, discussion of Site 1247). In contrast, Subunit IIIA showed several examples of classic moussey and soupy texture in cores recovered from above the base of the GHSZ. A sample of the soupy sediment was taken for postcruise chloride analysis.
The chloride profile shows a general decrease with depth similar to those observed at Sites 1244 and 1251. This decrease has been attributed to diffusion or slow advection of low chloride concentration pore waters generated by dewatering of aluminosilicates deeper in the accretionary complex. A closer look at the chloride profile suggests segments of different slope that can be roughly correlated with lithologic boundaries, suggesting lithologic control on permeability.
The thermal gradient of 0.059°C/m measured at Site 1252 is similar to the gradients of 0.057° and 0.058°C/m measured at Sites 1251 and 1244, respectively, and predicts the base of gas hydrate stability at 170 mbsf, which is consistent with the estimate of 170 mbsf obtained by lateral projection of the BSR observed in the core of the anticline.
Site 1252 provided the best sampling of the older (>1.65 Ma) sediments that comprise the uppermost part of the seismically incoherent facies referred to as the accretionary complex. This zone is characterized by a density inversion relative to the base of the overlying slope basins and an anomalous density vs. depth profile. This observation suggests that gravitational instability should be considered as a possible driving force contributing to the evolution of forearc structure here. The data from this site also reinforce previous observations that the accretionary complex is characterized by low chloride concentration interstitial waters and is relatively permeable. Although this site yielded little direct evidence of gas hydrate, sediment textures and limited IR thermal anomalies indicate that some hydrate was present even though no BSR is observed at this site.