Normalized strength
profiles from all sites decreased markedly with sub-bottom depth (Fig.
3) in a manner that indicates the sediment is overconsolidated (Su/
'v
> 0.2; Ladd et al., 1977) near the seafloor, but underconsolidated at depth.
This is especially apparent at Sites 994, 995, and 997, because of their greater
drilling depth. However, drilling disturbance, especially in XCB cores, may
artificially reduce the strength ratio (Dadey and Silva, 1989).
Of the three sites transecting the Cape Fear Diapir, Hole 993A has the largest seafloor-normalized strength values and strength, indicating that the shallow sub-bottom sediment is more overconsolidated at that location than the sediment at the other two nearby sites. This is consistent with the location of Hole 993A, which is on the steep flank of the diapir (Paull, Matsumoto, Wallace, et al., 1996) and is therefore more prone to mass wasting or erosion.
An estimate of the thickness of overburden that could have been removed to produce the near-seafloor shear strength profile observed in Hole 993A can be approximated by comparing the uppermost strength value at Hole 993A with the depth at which that strength is reached in the adjacent two sites. Using this method, it is estimated that 10 to 18 m of overburden were removed.
A different technique to
estimate the amount of removal can be made from the Su/'v
ratio. Assuming that Su/'v
equals 0.2 for normally consolidated sediment, ~60-70 m of overburden is
required to develop the shear strength observed at the seafloor at Site 993.
This, however, is probably a gross exaggeration because many deep-sea sediments,
including these from Leg 164, show an apparent overconsolidated layer near the
seafloor (Richards and Hamilton, 1967; Silva and Jordan, 1984). In fact,
different sediments may have different normally consolidated Su/'v
values (Richards and Hamilton, 1967). Furthermore, disturbance induced by XCB
drilling may also influence the normalized strength values (Dadey and Silva,
1989). Care was used to perform the tests only within the more intact biscuits
from XCB cores, rather than in the disturbed infilling material both at sea and
in the laboratory.
Based on the overconsolidation ratios, OCR = P'c/(Wsd), derived from the consolidation tests, all sediment is underconsolidated, OCR < 1.0, except for the sample at 3 mbsf (Table 2; Fig. 4). High overconsolidation ratios have been observed at numerous seafloor locations (Zizza and Silva, 1988) and especially in areas of high organic-rich sediment (Lee et al., 1990). The underconsolidated behavior at Site 995 may have been caused by the high sedimentation rate (up to 400 m/m.y.; Shipboard Scientific Party, 1996) or the presence of gas hydrate, which may have cemented the grains together or filled the voids so that normal compaction processes could not take place in situ. The hydrate in such a sediment would dissociate upon sampling and could produce an underconsolidated sediment. Arguing against hydrate-induced underconsolidation, none of the tested samples showed any signs of having degassed. This undercompaction is also indicated by the relatively constant porosity profiles in the deeper holes (Sites 994, 995, and 997) (Paull, Matsumoto, Wallace, et al., 1996).
Maximum past stresses,
from consolidation tests performed on Hole 995A sediment, typically increase
with sub-bottom depth (Fig. 5).
The excess effective stress, 'e
= P'c
- (Wsd), is a measure of the amount of under- or overconsolidation present in
the sediment. It is sometimes beneficial to use the excess effective stress
instead of OCR because 'e
is not affected by small values of vertical effective stress near the seafloor.
The 'e
values decrease with sub-bottom depth in Hole 995A and may be related to the
amount of excess pore pressure (above hydrostatic) present (Moran et al., 1995).
It is uncertain which of the possible factors contributes the most to the large
degree of underconsolidation at depth. However, sedimentation rate may play a
significant role since the lowest excess effective stress occurs at the same
depth, ~550 mbsf, as the highest sedimentation rate (Shipboard Scientific Party,
1996).
