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Based on laboratory geotechnical tests, the stress history of sediment drift deposits at two Ocean Drilling Program (ODP) sites of Leg 105 is analyzed. Geological analyses of Sites 646 and 647 indicate that sedimentation is primarily controlled by bottom currents having periodic turbidite sequences and no significant hiatuses. Consolidation tests on a limited number of good quality subsamples and other supporting data show that sediments deeper than approximately 5 to at least 150 meters below seafloor (mbsf) are significantly underconsolidated (i.e., these sediments are still consoli­ dating under the existing overburden stresses) and have overconsolidation ratios of less than 0.4 below 50 mbsf. Possible explanations for this underconsolidation include relatively high rates of sedimentation (up to 80 m/m.y.), low permea­ bility layers, an upper zone (~ 5 m) of apparently overconsolidated sediment, high concentrations of siliceous microfos­ sils, and the existence of nonlinear flow behavior at low hydraulic gradients. INTRODUCTION A N D GEOLOGIC SETTING To some extent, sedimentary deposits "remember" stresses borne in the past. Investigation of this phenomenon, referred to as stress history, can yield valuable information pertaining to depositional and post-depositional processes. For example, an overconsolidated sediment is interpreted as having previously sustained an overburden stress greater than that of today. This situation can result by removing the overburden, which is usu­ ally caused by erosion or mass wasting processes. On the other hand, an underconsolidated sediment indicates that the deposit is experiencing excess pore water pressures and has not yet con­ solidated under its present stress. Causes include extremely rapid sedimentation and other phenomena that hinder the normal ex­ pulsion of pore fluids during consolidation. A normally consol­ idated state, which implies complete dissipation of pore water pressures, is associated with steady sedimentation and complete consolidation under the present overburden stress. This research determines the stress history of Pliocene-Pleis­ tocene sediments at two sites recovered during ODP Leg 105. Site 646 (58°12.56'N, 48°22.15'W; water depth of 3451 m) is located on the northern flank of the Eirik Ridge, a drift (con­ tourite) deposit south of Greenland. Site 647 (53° 19.88'N, 45° 15.72'W; water depth of 3862 m) is located on the southern flank of Gloria Drift (Fig. 1). Almost the entire sedimentary sequence at Site 646 was re­ ported as having been deposited or reworked by bottom currents (Srivastava, Arthur, et al., 1987). Although thin layers of turbi­ dites were observed in Subunit IA (0-188 mbsf), sedimentation was described as continuous (Srivastava, Arthur, et al., 1987). Sedimentation rates in the Pliocene-Pleistocene Subunit IA av­ erage 79 m/m.y. Ice-rafted debris is common to abundant in lithologic Unit I (0-236 mbsf). Sediments are terrigenous, with variable amounts of biogenic material, up to 40% to 50% throughout the recovered sequence. The upper 100 m of sediment at Site 647 is part of the Gloria Drift contourite deposit. Turbidite layers are present in Unit I Srivastava, S. P., Arthur, M., Clement, B., et al., 1989. Proc. ODP, Sci. Results, 105: College Station, TX (Ocean Drilling Program). 2 Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882. 3 Department of Ocean Engineering, University of Rhode Island, Kingston, RI 02881. (0-116 mbsf), but deposition processes are dominated by bot­ tom currents. Much of Unit I is bioturbated, and sedimentation rates average 46 m/m.y. (Srivastava, Arthur, et al., 1987). Sedi­ ments consist mainly of fine-grained terrigenous debris, with bi­ ogenic contents in one minor facies that reach 40% to 50%. Icerafted material is important throughout Unit I (Srivastava, Ar­ thur, et al., 1987). Preliminary geological analyses of these holes suggest that Pliocene-Pleistocene sedimentation at both sites is controlled mainly by bottom-current activity and periodic turbidite spill­ over from the Northwest Atlantic Mid-Ocean Channel (NAMOC). Although deposition is somewhat sporadic, the absence of major hiatuses suggests a lack of significant erosion in the Pliocene-Pleistocene sections at either site (Srivastava, Arthur, et al., 1987). As a result, deposits should be normally to slightly overconsolidated. METHODS Whole-round samples having as little disturbance as possible were obtained from the ends of core sections. Specimens were chosen from sections near the center of individual cores to reduce the possibility of flow-in disturbance. Post-sampling changes were minimized by coating specimens with wax and refrigerating them. Samples were then handcarried back to the University of Rhode Island, where they were stored at 4°C, submerged in seawater until testing. Consolidation testing (e.g., Am. Soc. Testing Materials, 1987) gen­ erates a plot of void ratio {e) vs. logarithm effective stress (a') that ex­ hibits a characteristic shape. Certain parameters can be obtained from this curve based on theoretical and empirical considerations (Lambe and Whitman, 1969). Casagrande's method (1936) was applied to the void ratio vs. loga­ rithm effective stress (e log a') curves to estimate the preconsolidation stresses. Overburden vs. depth relationships were developed from bulk density data obtained on board ship (Srivastava, Arthur, et al., 1987). The overconsolidation ratio (OCR) was calculated as the ratio of pre­ consolidation stress to overburden stress. Samples displaying OCRs above 1.2 are considered overconsolidated, whereas those having OCRs of less than about 0.7 are underconsolidated. OCRs that range between 0.7 and 1.2 imply normal consolidation. The compression index (Cc) was determined as the slope of the vir­ gin consolidation curve the portion beyond the "knee," which repre­ sents the compressibility of the material in the field. The coefficient of permeability (hydraulic conductivity) was determined from consolida­ tion data using the square root of time (Taylor, 1948) and the logarithm of time (Casagrande, 1936) methods. Reported permeability values are extrapolated to the void ratio at the preconsolidation stress.

Consolidation and Strength of Pliocene-Pleistocene Sediments from Sites 646 and 647, ODP Leg 105