Sinkhole progression at the Weeks Island, Louisiana, Strategic Petroleum Reserve (SPR) site Page: 10 of 20
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fracturing at depths near -61 m (-200 ft). Magorian's mapping of the top-of-salt also showed
that both sinkholes are situated in the center of troughs or valleys (Figure 3) having relief of 15 m
(50 ft). These troughs may reflect boundaries of differential motion of separate segments in the
Anomalous zones, including shear zones, occur frequently in Gulf Coast salt stocks,often
reflecting differential movement of separate lobes or spines of salt and incorporating a variety of
distinctive (anomalous) salt features and/or geologic conditions (Kupfer, 1990; Neal, et al., 1993).
Kupfer believes that three or more of these features should occur in combination to be labelled
anomalous. At Weeks Island the geology has several distinctions at and near the sinkhole(s) that
may indicate it is astride an anomalous zone, even though exploratory drilling did not observe im-
purities in the salt directly.
The southern mine boundary was apparently influenced by the nearby intersection of
problem salt features, specifically gas outbursts, brine seeps, and black salt. The occurrence of
linear gas outbursts experienced during mining does not by itself support an anomalous zone des-
ignation according to Kupfer (Neal, 1993), but it appears that their orientation is a non-random
process. Thoms and Gehle (1995) indicated a zone of black salt, an anomalous feature, was
mapped during mining near the location of the sinkhole. Brine seeps were also mapped near the
subsequent sinkhole location, and although at the time judged not meteoric in character, the brine
chemistry showed some deviation from normal connate analyses (Martinez, 1995).
Thoms and Gehle (1995) suggested that an association of factors may be responsible for-
sinkhole formation at Weeks Island. In addition to the very localized mining-induced stresses ad-
jacent to underground openings that create a disturbed rock zone, there are additional influences
that may work together to localize the initial sinkhole on the southern edge of the mine. Horizon-
tal extension zones result from subsidence over the mine and extend beyond the perimeter, leading
to dilatancy and eventual fracturing over time. Anomalous salt features occur in multiples are
called anomalous zones; they are often more prevalent near the edges of salt stocks and are one
element of susceptible salt zones, which may include a variety of local geologic conditions.
Thoms believes the association and combination of these elements at Weeks Island probably pro-
duced leak prone areas and thus the sinkhole(s).
Rock mechanics modeling in two dimensions by Ehgartner (1993) showed that the areas
near the mine perimeter would be in tension and that dilatant zones and fractures in the top of salt
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Neal, J.T.; Bauer, S.J. & Ehgartner, B.L. Sinkhole progression at the Weeks Island, Louisiana, Strategic Petroleum Reserve (SPR) site, article, November 1, 1995; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc620914/m1/10/: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.