The ratio of shear strength to overburden pressure is a measure of the consolidation state of sediments. If the ratio is 0.25, the sediment is normally consolidated for its burial depth. The sediment is defined as underconsolidated if the ratio is <0.25 and overconsolidated if the ratio is >0.25. Sediments in the upper 5–10 mbsf often show what is called apparent overconsolidation (AOC; i.e., the ratio of shear strength to overburden is >0.25, often with values as high as 10). This abnormally high ratio can often be an artifact if the sediments have not been carefully handled or if the overburden pressure is incorrectly calculated (e.g., using a wrong mudline definition). The ratio of shear strength to overburden is also strongly affected by biogenic components in the sediment, clay mineralogy, age, sedimentation rate, erosion, and cementation (e.g., Silva and Jordan, 1984; Nacci et al., 1974; Skempton, 1970; Francisca et al., 2005).
We observed AOC at Sites 1244, 1245, and 1246 within the upper 10–40 mbsf (Fig. F12A, F12B, F12C). However, at Site 1251 no such AOC was observed (Fig. F12D).
Since we can exclude incorrect mudline definition and improper sediment treatment, the observed AOC is believed to be a real property of the sediments.
AOC is only observed at Sites 1244, 1245, and 1246, and there is no obvious correlation with the occurrence of carbonate cementation (increase in shear strength), biogenic opal (decrease in shear strength), and grain-size distribution within the upper 40 mbsf (see "Lithostratigraphy" sections in the Sites 1244, 1245, and 1246 chapters in the Leg 204 Initial Results volume; Tréhu, Bohrmann, Rack, Torres, et al., 2003). AOC at Site 1246 is about twice that at Sites 1244 and 1245. However, all three sites with AOC are located at the flank of the southern Hydrate Ridge anticline and therefore have most likely been shut off from recent and more rapid sedimentation by turbidites. Furthermore, these three sites were probably affected by erosion as part of the uplift of the anticline. The removal of overburden pressure by erosion in combination with a slow pelagic sedimentation rate can explain the observed AOC.
No AOC is observed at Site 1251. This site is located in a basin east of the anticline of southern Hydrate Ridge and is probably prone to a large supply of sediments through turbidite sedimentation and almost no erosion. Site 1251 has the highest sedimentation rate (160 cm/k.y. in the upper 100 mbsf) of all of the sites analyzed during Leg 204. This site is also characterized by an increased concentration of biogenic opal in the upper 20 mbsf, which also decreases shear strength and, therefore, AOC.