Summary:
The Red Hills Mine is in the Wilcox Group of Mississippi which is the most prolific lignite-bearing stratum in the state of Mississippi. The formations within the Wilcox Group and the underlying Midway Group consist of sands, silts, clay, and lignite which were deposited during Paleogene time, approximately 66 to 23 million years ago. Deposition occurred in a cyclical manner representing a transition from a transgressive sequence of valley fill, marginal marine strata to predominantly regressive, nonmarine, deltaic strata.
During each depositional geologic sequence, organic material was repeatedly buried by sediment in an ideal, oxygen-free environment. This ideal environment was attributed to the humid, subtropical conditions and high water-table of wetlands present during Paleogene time which prevented plant matter from decaying prior to burial. The oxygen-free environment combined with heat and pressure from continual deposition of overlying sediments allowed for the formation of peat and further mineralization of lignite over time by undergoing a process known as humification and biochemical gelification.
The average thickness of the Wilcox section containing the mineable lignite seams at the Red Hills Mine and surrounding area is approximately 140 feet. Mineable lignite seams may be as thin as 1-foot and typically do not exceed 5-feet in thickness. Five primary lignite seams are targeted for mining.
The local structural geology for the Red Hills Mine follows the regional structure with a northwest-southeast strike dipping to the southwest. The lignite seams are gently undulating due to differential compaction of the underlying sediments.
Gravel Creek Member
The top of the Gravel Creek Member of the Nanafalia Formation (Wilcox Group) lies just below the C-seam and includes a thin sand layer directly beneath the C-seam. The C-seam is currently the lowest lignite seam stratigraphically mined. However, upon further exploration, the B-seam may be mined in the future, particularly in areas with low laying terrain within Mine Area 3.
A basal sand unit, of up to 100 feet in thickness or more, characterizes the Gravel Creek Member. From the limited drill hole data that extends through this member, it appears that these sand units can be fairly widespread, but also may be completely absent. On the geophysical logs and limited cuttings data, the sand unit typically appears to be fairly massive and poorly- to well-sorted, fine- to medium-grained sand. These sand units, along with the sands in the underlying Coal Bluff Formation (Midway Group) are often referred to as the Lower Wilcox aquifer.
Above this basal sand are interbedded silt, clay, sand and lignite (A-seam). Due to the relative depth, anticipated lower quality, and closer proximity to the basal sands, the A-seam is not currently considered a resource targeted for mining.
Grampian Hills Member
Five of the six lignite seams recovered at Red Hills Mine are contained in the Grampian Hills Member of the Nanafalia Formation (Wilcox Group). The formation conformably overlies the Gravel Creek Member and consists of interbedded and interlaminated clays, silts, sands, and lignite. Overall, the section is relatively sand poor.
The clays and silts are typically finely interlaminated. Munsell color varies from dark gray (N 4/ to N 5/) to greenish gray (10BG 5/1). Immediately below the lignite seams, the color is dark grayish brown (10YR 4/2) due to the presence of carbonaceous fragments. These carbonaceous layers often contain lignitized plant roots establishing an autochthonous origin for the peats that formed the lignite seams.
The sands are light gray (2.5Y 7/1) to gray (N 5/) or greenish gray (5GY 5/1). The pale greenish-gray color of many of the sands and silts is a distinctive feature of this unit. Typically, the sands are very-fine-grained, and commonly interbedded with silts and clays. One exception is the tabular sand bed between the D-seam and C-seam. The sand units between the D-seam and G-seam are typically silty, infrequent, lenticular, and probably represent narrow sand channels in the crevasse splay sequences that were penecontemporaneous with peat accumulation. Sand and silty sands compact much less than silts and clays. This phenomenon is probably the chief cause of the gentle structural undulations found in the lignite seams.
Cemented horizons, commonly referred to as “hard streaks”, are also associated with the sand units. These indurated zones are most abundant within the sand channels and the silty, sandy natural levee deposits flanking the channels. The thickness of these hard streaks ranges from less than one foot to about two feet. These zones are cemented with calcite, silica, iron oxide, or siderite, and are suggestive of periods of sub-areal oxidizing conditions during deposition. Modern analogies in the natural levee deposits of the Mississippi Delta have been noted.
The color of the lignite seams ranges from black (N 2.5/) to very dark gray (10Y 3/1). Occasional layers of carbonaceous clay or zones of clay clasts increase the ash content of the lignite. The minimum thickness for recovery is one foot. The maximum thickness of the lignite seams is about eight feet.
Tuscahoma Formation
The Tuscahoma Formation (Wilcox Group) conformably overlies the Grampian Hills Member of the Nanafalia Formation. The basal portion of this formation is the uppermost stratum to be disturbed by mining. The base of the Tuscahoma is marked by a predominantly sandy, often coarse-grained unit with a variable thickness of 10 feet to 110 feet. The variability is due to the occurrence of contemporaneously bedded clay, silt, and lignite. Laterally, these sands grade into finer grained overbank deposits including lignite seams. The overbank facies of the Tuscahoma are essentially identical to the descriptions for the Grampian Hills Member described above.
The H-seam, which is the uppermost seam that will be consistently recovered, lies at the base of the Tuscahoma. Because of its relatively high stratigraphic position, the H-seam is restricted to the upland areas above approximately 450 feet in elevation within the Red Hills Mine. Other lignite seams lying above the H-seam, including the H2-seam and the I-seam, may be encountered on occasion and are mined when seam thickness and quality are sufficient.