SITES 991/992/993
Short holes were drilled at three sites on the crest and flanks of the Cape Fear Diapir (Fig. 1 & 2). The diapir is one of about 20 large structures that originate from deep within the sediments of the Carolina Trough and penetrate through the Carolina Continental Rise. Although the core material of these diapirs is unknown, many investigators believe they are salt cored. The crest of the Cape Fear Diapir breeches the seafloor within the scar of the giant Cape Fear Slide. Although the Cape Fear Diapir occurs within a region where a BSR is present, the continuity of the BSR is lost on the flanks of the diapir. The objectives of Sites 991, 992, and 993 were to investigate the effects of diapir intrusion and large-scale sediment failure on the regional gas hydrate field and on the transport of fluid and gas through the sedimentary sequence. A second major objective was to establish the nature of the core material of the diapir.
Lithologic variations, physical property changes, and nannofossil biostratigraphy indicate that a discontinuous Neogene sediment section was recovered at Hole 991A (Fig. 3). The uppermost unit (I) (0-2.05 mbsf) consists of greenish gray nannofossil silty clay with a sharp, irregular contact at its base. Unit II (2.05-47.66 mbsf) is composed of firmer gray nannofossil silty clays. The top (2.05-12.67 mbsf) and bottom (18.37-47.66 mbsf) of this unit are characterized by zones of steeply dipping, discordant, and truncated beds and laminae of variegated colors; beds deformed by flowage, folding and faulting; and mud clasts of various sizes, shapes, and colors. However, the middle (12.67-18.37 mbsf) of this unit is apparently undeformed. Unit II is interpreted as a large slide block. A long hiatus, correlative with earliest Pleistocene to late early Pliocene times was detected within Unit II. Unit III (47.66-57.16 mbsf) is predominantly undeformed dark gray nannofossil-rich clay. Sediment at the bottom of the hole is late Miocene in age (CN9b Zone).
Two major lithologic units were recognized in Hole 992A (Fig. 3). The top unit (0-9.1 mbsf) is composed of strongly deformed gray nannofossil silty clay beds, which are similar in lithology and structural style to Unit II in Hole 991A. The unit appears to represent a mass-transport deposit that was emplaced by slumping. The bottom unit (9.1-50.75 mbsf) is a homogeneous olive gray diatom-rich nannofossil clay, which appears to be undeformed. At Hole 992A, most of the uppermost Pleistocene sequence is missing. Another long hiatus corresponding to early Pleistocene to late Miocene times was identified, and the oldest sediment recovered was middle late Miocene in age (CN9a Zone). The lithologies did not reveal the composition of the underlying diapir.
Hole 993A contains one major unit, from 0 to 47.27 mbsf, that is predominantly gray nannofossil clay (Fig. 3). The material is generally homogeneous greenish to olive gray nannofossil clay and silty clay. The only exception is a short interval from 4.43 to 4.9 mbsf that contains an indurated carbonate and overlies a carbonate silty clay (4.43-4.9 mbsf). The entire sequence is of early late Miocene age (Zone CN7).
At all three sites, the methane concentrations increased with depth, ranging from 5 to 29,000 ppm at Site 991, from 2 to 81 ppm at Site 992, and from 6 to 18,000 at Site 993. The methane-to-ethane ratios of most samples indicate a microbial origin for the methane.
Detailed pore-water profiles from Holes 991A and 993A, both of which are located on the flanks of the diapir, show that chloride concentrations increase at a rate of 2 millimolar per meter (mM/m) (Fig. 4). However, in Hole 992A, located on the crest of the diapir, the gradient is significantly less (about 0.8 mM/m). No significant trend in the Na+/Cl- ratio exists. There are several possible explanations for the greater pore-water chloride content at Sites 991 and 993 relative to Site 992. High dissolved chloride content may indicate that the core of the diapir is composed of evaporitic salts. The variations in chloride concentration of the interstitial waters analyzed from these sites may have been modified by fluid-circulation patterns around the diapir. The variations in the profiles also may indicate that major slumps have truncated the sedimentary sections on the flanks of the diapir more recently than those on the crest of the diapir. Thus, the present pore-water profiles on the flanks are still steeper than the pore-water gradients on the crest of the diapir. Alternatively, the variations in interstitial water chloride contents could be produced by ion exclusion associated with gas hydrate formation on the flanks of the diapir. Seismic reflection profiles indicate a strong BSR in the sediments surrounding the diapir. Sites 991 and 993, on the flanks of the diapir, are closer to the saline waters that are generated as a consequence of gas hydrate formation. Conclusions about the cause of the high pore-water chloride content await shore-based analytical work.
Dissolved sulfate content in interstitial-water samples from Site 991A decreases linearly with increasing depth, declining to negligible concentrations at ~40 mbsf. At this depth, there is a corresponding alkalinity maximum in the interstitial-water samples. Ammonium contents increase linearly with depth, passing through the base of the sulfate reduction zone without inflection. Active anaerobic methane oxidation is suggested by these profiles.
Both PCS runs at Hole 991B were only partly successful. Core 164-991B-1P was pressurized at ~2500 psi, about 70% of that expected for hydrostatic pressure at the coring depth, but contained only water. Core 164-991B-2P was pressurized at 63 psi but contained 1.04 m of homogenous nannofossil-bearing clay.
Magnetic intensities are very weak throughout the sedimentary sequence at all three sites, and no consistent magnetostratigraphy can be recognized. Rock-magnetic analysis (saturation isothermal remanent magnetization [IRM], back-field IRM, and partial anhysteretic remanent magnetization [ARM] acquisition) indicate the presence of two magnetic carriers. This paleomagnetic and rock magnetic behavior probably is caused by dissolution of single-domain magnetite and reduction to magnetic iron sulfides. Rock magnetism, bulk magnetic susceptibility, and magnetic remanence all confirm the distinction between lithostratigraphic Units I and II at both Holes 991A and 992A. Unit I is characterized by the presence of substantial proportions of single-domain magnetite. However, single-domain magnetite is absent in Unit II and appears to have been replaced by magnetic sulfides, resulting in the loss of magnetostratigraphy. The break is sudden, collaborating the break interpreted in the depositional record. Anisotropy of susceptibility indicates a strongly foliated fabric in Unit II, consistent with overconsolidation at this depth.
In summary, the sediments recovered at Sites 991, 992, and 993 are typical Neogene continental rise deposits and do not directly indicate the composition of the underlying diapir. The sedimentary sequence has numerous stratigraphic gaps. Pre-Quaternary sediments occur between 0.5 and 30 mbsf at all three sites, indicating that the upper Quaternary section has been substantially truncated. Pervasive soft-sediment deformation was observed within the Pleistocene to Pliocene sequences from Sites 991 and 992, especially near the tops of both sections, suggesting that these unconformities resulted from vigorous mass-transport processes associated with sediment failures (e.g., slumping, sliding, and debris flows). The sediments near the surface tend to be over consolidated for their current burial depths. The relative absence of Holocene and late Quaternary age materials in a region where the Holocene and late Pleistocene sedimentation rates are known to exceed 20 cm/k.y. (Paull et al., 1996) indicates that the most recent deformation occurred within the late Quaternary and Holocene. The paucity of recent sediments around the diapir suggests that the diapir is still active.