Summary:
The salt at Cote Blanche is a sedimentary deposit. The deposit only extends to inside the boundary of the salt dome.
Approximately 170 million years ago, shallow seas began to infiltrate Louisiana and the Gulf Coast. These shallow seas had restricted circulation from other larger bodies of water. This caused large amounts of salt and other evaporates to deposit, creating large salt deposits named the Louann Formation. After this cycle of evaporitic deposition, the Gulf of Mexico was flooded by a much larger open sea. Due to the relative buoyancy of the Louann salt, the salt pushed upward, through the overlying strata. The upwelling of the salt created the diapers, or salt domes throughout the Gulf Coast region.
The Cote Blanche salt dome is one the five ‘island salt domes’ in the Gulf Coast region. The five island domes form a northwest-southeast trend approximately 60 miles long, and each dome consists of one or more salt diapirs which have risen upward relative to 3 to 20 km of overlying sediment from their original horizontal position in the Permian-Jurassic Louann Formation.
According to the Southeastern Geological Survey Guidebook, thicknesses of the Louann Salt range from 1000 meters in the East Texas and Northern Louisiana salt basins, 1200 to 1500 meters for the Mississippi salt basin and coastal belt from southeast Texas to southern Louisiana (including the Cote Blanche area), and up to 3000-4000 meters in the Texas-Louisiana continental slope area. Up to 2000 meters of rock salt may be in the southern area in the Gulf of Campeche. The stratigraphic position of the “mother salt” layer of the Louann is presently as much as 18 km deep (60,000 feet; 11.2 miles), although it is assumed that no salt is left at that level (having long since been mobilized and evacuated in concert with sedimentation), and no salt was actually that deep (the original position having subsided with sediment loading) (Kupfer et al. 1995).
The Louann Salt is composed primarily of medium- to coarsely-crystalline, translucent light-medium gray, to opaque white, halite (up to ~98%), with lesser anhydrite (up to 10%;), and minor sylvite. The darker gray bands of halite are generally richer in anhydrite (Kupfer et al., 1995). Pyrite, quartz, and dolomite occur in trace amounts. Carnalite (KMgCl3 . 6H2O) and various Borate Group minerals have also been reported to form the Louann (Gann et al., 1987; Dockery and Thompson, 2016). On average, halite crystals are between 5-15 mm (generally 0.5 to 1cm) in length, but some are larger (“pegmatitic salt”). Halite crystals are usually interlocked, equigranular, and slightly elongated. Recrystallization is common. Larger crystals are usually associated with moisture, bubble-like inclusions of methane gas, or clastic sediment. A common rock salt texture at Cote Blanche (but rare in the other Five Island Domes) is poikiloblastic salt (a descriptor normally used in metamorphic rocks), in which small salt crystals are embedded in larger crystals (metacrysts). The anhydrite (CaSO4) occurs as small, disseminated, euhedral crystals. Sylvite (KCl/“potash”) is typically red or pink color. Some salt contains interstitial brines (connate water, trapped in the salt during its formation) which evaporate upon exposure, leaving residual iron oxide stains (limonite), coloring the salt yellow or red (and not to be confused with the pink-red color of sylvite). Halite stalactites may form in areas where the brines drip (Kupfer et al., 1995).
The Cote Blanche salt dome consists primarily of translucent to white salt interbedded with discontinuous bands of dark grey salt varying in thickness from several centimeters to over a meter. Banding at Cote Blanche, as in most other salt domes in the region, is primarily vertical to near vertical at a mine scale. Bands are interpreted to represent original bedding in evaporitic horizons, which have subsequently been deformed from horizontal into their current sub vertical orientations. At the scale of individual pillars, contorted banding shows that locally the salt has been tightly and complexly folded.
The Cote Blanche dome is somewhat unusual in that it does not have a cap rock (likely a function of local hydrogeologic conditions). In addition to the dark bands described in the previous section, the dome also contains thin beds of a reddish-brown Aeolian sandstone, as well as local zones of material known as ‘anomalous’ salt (i.e., material that is unusually coarse, discolored, friable, hard, or contains gas pockets). Salt mined from the dome is variable enough in terms of grain size, calcium (Ca) /magnesium (Mg) content, etc., that it is divided into three broad categories: chemical-grade salt, highway salt, and specialty (high-magnesium) salt.
In contrast to the other four major island salt domes in the USA, there is no underground evidence of an internal boundary zone at the Cote Blanche Mine that would indicate the convergence of two or more salt plumes.
As in all of the other Louisiana five island salt domes, the internal structure is nearly vertical, although locally salt beds have been observed to roll over to nearly 45° from horizontal. Mapping shows internal structure to be extremely complex. The alternating bands of light and dark salt are considered to be original bedding from the evaporate sequence. Sediment inclusions occur mainly in the form of an interbedded sand, as opposed to the massive incorporation of elastic and organic debris in anomalous zones.
These sandy beds are considered original bedding for several reasons. First, they follow closely the trend of salt layering in occurrences throughout the mine; second, they maintain a fairly constant thickness (1 to 6 ft); third, the contact between the sand and the adjacent salt is very sharp; and fourth, the sand has a fairly consistent mineralogic composition. This sand unit is one of the few marker beds in the complexly folded salt strata. Another marker bed is a black, carbonaceous silt clay member.
The sandstone areas described above can form areas of disturbance or low salt quality, which can prove problematic during mining and therefore Compass completed a seismic survey during 2016 to improve the confidence in the modelling of such units on the 1500-foot level. Initial results have been received but the current investigation is on-going at the time of reporting. Based on these marker beds and structural mapping, it appears that although local folding is complex, the axis of a large fold runs from the northwest to the center of the mine.