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CHAPTER IV

ENVIRONMENTAL

CONSEQUENCES

[blocks in formation]

Geologic constraints are adverse geologic features or conditions that, due to their low risk, can be eliminated or reduced to an acceptable level through conventional engineering practices (Keer and Cardinell, 1981). Geologic constraints present within the proposed sale area include filled channels, erosion of bottom sediments, sand waves, faults present below the unconsolidated sedimentary section, and gassy sediments. These constraints are routinely located and identified as a result of the site-specific geophysical surveys required under the OCS Operating Orders.

Sediment filled channels

Buried stream channels cut by water flowing out onto the MidAtlantic Shelf,
Slope, and Rise during periods of lower sea level in Pleistocene times, are
common within the proposed sale area. Extensive amounts of sediment were
deposited within these channels. Transgression of the sea in later periods
of geologic time resulted in the reworking of the sediments originally depos-
ited and subsequent burial of the channels.

Channel fill material generally shows unusual amounts of internal variability in sorting, texture, porosity, and load-bearing capacity because it was deposited in a high-energy environment (Carpenter and McCarthy, 1980). Grain sizes of sediment usually present within these channels range from clay to boulders. The mechanical properties of channel fill can differ markedly from bordering sediments, resulting in differential settling of structures that straddle the boundary between the two sediment types. The variation in grain sizes present can also present problems in terms of drilling mud circulation and emplacement of surface casing (Keer and Cardinell, 1981).

The major buried stream channels are noted on Visual No. 3. Smaller buried channels that have not been mapped may also be present. Site specific geophysical analysis prior to any drilling within a leased block will aid in delineation of any buried channels. Subsequent engineering accomadations during drilling activities will adequately mitigate this constraint.

Sand waves and bottom sediment erosion and scour

The presence of sand waves in proximity to an offshore structure are a problem because of the inherent mobility of the sand particles within the bedform. Sand may pile up against a supporting member of a structure and weaken it, or move away and expose it, leaving it unsupported.

Sand waves are not extensive within the proposed sale area. These bedforms appears to be present only near the heads of Wilmington and Lindenkohl Canyons. Visual No. 3 shows the approximate locations of these features.

Knebel and Folger (1976) initially reported the existence of sand waves near the head of Wilmington Canyon. These features are spaced from 100 to 650 m (328 to 2,133 ft) apart and have a relief of 2 to 9 m (7 to 30 ft). Twichell (1979), however, concluded that these sand waves are relict, static features, inconsistent with the present hydraulic regime. No net migration of these features appears to be taking place.

A long sinuous field of sand waves has also been observed near the head of Lindenkohl Canyon (Hall and Ensminger, 1979). This field is approximately 11 km long and 15 km wide. It is not known if these features are mobile or static.

Although sand ridges commonly are found throughout the shelf region of the proposed sale area, they are not considered to be a constraint to offshore operations. Sand ridges are generally assumed to be static features (Hall and Ensminger, 1979).

Erosion or scour of sediment around an offshore structure would pose a similar constraint to that of sand waves, but on a somewhat smaller scale (Keer and Cardinell, 1981). These conditions do not appear to be particularly prevalent within most of the proposed sale area. There are some specific zones, however, where erosional conditions may be present.

Knebel (1979) identified a zone of irregular bottom topography landward of the shelf-slope break and just south of Hudson Canyon (see Visual No. 3). topography is composed of a series of depressions having variable spacings (less than 100 m to 2 km), depths (1 to 10 m), outlines, and bottom configurations. The depressions are largest near the canyon and decrease in size southwestward. These configurations give the area a "jagged" appearance in profile, which contrasts sharply with the surrounding shelf which tends to be relatively smooth and featureless. The absence of a surficial sand sheet within this area (which is commonly found throughout the remaining portions of the Mid-Atlantic Shelf) may define a zone of non-deposition and erosion. Drilling operations that have thus far taken place within this zone (from leases awarded during OCS Sale Nos. 40 and 49) have not, however, experienced any problems in terms of erosion or scour.

Knebel (1977) also identified a major sea-floor escarpment, the Tiger Scarp, in the central portion of the Mid-Atlantic Shelf. This scarp has been noted as a potential surface of instability (see Visual No. 3). The escarpment has a declivity of 0.5°, is approximately 0.4 to 2.4 km wide, and is roughly defined by the 50 m isobath. Knebel (1977) presents evidence for modern sand movement over and along the scarp: 1) sub-bottom reflectors in seismic records that are conformable with the scarp: 2) the presence of a subdued ridge, which tops the western half of the scarp (the ridge resembles a natural levee); 3) the relatively young age of shells collected from cores obtained at shallow depths beneath the scarp.

Other than the specific zones mentioned above, erosion and scour would seem to pose little or no problem to offshore operations within the remainder of the proposed sale area. Some degree of sediment movement, ripple marks, etc., have been noted on the Mid-Atlantic outer shelf (Butman et al., 1982; Slater et al., 1982), and slope in both the intercanyon and canyon areas (Slater et al., 1982; Shepard et al., 1979). However, the observations and measurements reported indicate that the constraint posed is negligible.

Gassy sediments

Gassy sediments are surface or near-surface sediments which contain enough gas, either in the bubble phase or dissolved, to be detectable as acoustically turbid zones or blankouts on high-resolution seismic reflection data (Keer and Cardinell, 1981).

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