Depositional Systems and Lignite Prospecting Models--Wilcox Group and Meridian Sandstone (Eocene), Northern Mississippi: ABSTRACT

Abstract

ABSTRACT The first year of a three year Mississippi Mineral Resources Institute project to map the surface and subsurface terrigenous clastic depositional systems and lignite seams of the Eocene Wilcox-Meridian stratigraphic interval in Mississippi has been completed. Data from 620 oil and water well electric logs, 65 sample logs, and 35 surface exposures have been utilized to determine the distribution of the principal sandstone bodies in the northern third of the state. Evidence from eight regional cross sections indicates that the combined Wilcox and Meridian study interval can be subdivided into a minimum of four distinct units: (1) a basal Wilcox progradational interval, (2) a lower Wilcox fluvial-deltaic unit, (3) an upper Wilcox fine-grained fluvial unit, and (4) a Meridian coarse-grained fluvial unit. In extreme northwestern Mississippi the highest 200 feet of the Wilcox is made up of massive sandstone and is genetically related to the overlying Meridian Sandstone (Claiborne Group). This is the Meridian-Upper Wilcox Aquifer System of hydrologists. Hence, in the northern-most counties of the study area, the upper Wilcox and Meridian become a single, very massive, coarse-grained fluvial unit. Sandstone-body geometry and lignite distribution within the Wilcox-Meridian interval have been evaluated using net-sandstone isolith, net sandstone percent, thickness of most massive sandstone unit, and lignite isopleth maps. Vertical curve shapes for individual wells furnish additional information for interpreting specific environments of deposition. Dendritic, contributory sandstone belts of low to moderate sinuosity comprise the dominant geometry displayed by these rock bodies, even for the basal Wilcox progradational interval. Nine distinct sandstone belts 6 to 10 miles wide are noted in the lower Wilcox; seven of these same belts are also present in the upper Wilcox. The Meridian has a much higher proportion of sandstone, averaging approximately 70 percent, but the 90 percent sandstone contour line highlights 8 sandstone belts occupying roughly the same trends as the sandstone maxima for the lower Wilcox. A down-dip bifurcating (distributary) net-sandstone pattern characteristic of the upper delta-plain facies has been mapped in the basal Wilcox, lower Wilcox, and upper Wilcox units in southern Bolivar, Yazoo, Sharkey, Issaquena, and Washington Counties. Lignite isopleth maxima in central Washington County of 17 and 18 for the lower and upper Wilcox lend further support for identifying the proximal delta plain. The principal distal deltaic sandstone bodies of the Wilcox high-constructional Holly Springs Delta System, however, have been mapped by other researchers as being 75 to 125 miles downdip from this area. Lignite prospecting in the Wilcox-Meridian section of North Mississippi can be greatly aided through an understanding of the geographic and stratigraphic distribution of the major depositional systems. Regarding fluvial systems, organic debris preferentially accumulates with fine-grained sediment. Significant lignite deposits most likely formed in overbank deposits away from active stream channels. As a result, commercially exploitable seams associated with ancient fluvial systems should be sought in interstream basins of fine-grained meanderbelt systems where values of lower than 50% sandstone occur. Such areas are present in the lower Wilcox of Calhoun, Webster, and Choctaw Counties and in the upper Wilcox as far north as Marshall County. Coarse-grained meanderbelt and incised valley-fill fluvial systems represent much less satisfactory settings for lignite formation, inasmuch as most of the fine-grained material is transported downdip without being deposited. Consequently, the Meridian coarse-grained meanderbelt system throughout North Mississippi is barren of lignite. High-constructive deltas furnish optimum conditions for the accumulation of organic material. In North Mississippi, the thickest individual seams and the greatest number of seams occur at the junction of the alluvial plain and proximal delta plain. This junction is the position where channels first bifurcate to exhibit a distributary pattern and is well demonstrated by lower and upper Wilcox sandstone bodies in Washington, Sharkey, and Yazoo Counties. End_Page 283------------------------