The first year of a three-year Mississippi Mineral Resources Institute project to map the surface and subsurface terrigenous clastic depositional systems and lignite units of Mississippi has been completed. Data from 620 oil- and water-well electric logs, 65 sample logs, and 35 surface exposures have been used 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 Wilcox-Meridian vertical stratigraphic 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 northwester Mississippi the highest 200 ft (61 m) of the Wilcox is composed of massive sandstone and is genetically related to the overlying Meridian Sandstone (Claiborne Group). This is the Meridian--upper Wilcox aquifer system (basal part of the Memphis aquifer) of hydrologists. Hence, a fourth Wilcox subdivision, a massive upper Wilcox coarse-grained fluvial unit, can be delineated for the northernmost counties of the study area.

Sandstone-body geometry and lignite distribution in the Wilcox-Meridian systems are indicated by net sandstone isolith, net sandstone percent, thickness of most

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massive sandstone unit, and lignite isopleth maps. Dendritic, contributary sandstone belts of low to moderate sinuosity comprise the dominant geometry displayed by these terrigenous clastic rock bodies, even with the basal Wilcox progradational interval. Nine distinct sandstone belts between 6 and 10 mi (9.6 and 16 km) wide are noted in the lower Wilcox: seven of these same belts occur in the upper Wilcox. The Meridian has a much higher overall sandstone content than the Wilcox units, averaging 70%, but the 90% sandstone contour line highlights 8 sandstone belts occupying roughly the same positions as maximum sand belts in the lower Wilcox.

A bifurcating, distributary net sandstone pattern characteristic of upper delta plain facies is noted for the basal Wilcox, lower Wilcox, and upper Wilcox stratigraphic intervals of southern Bolivar, Yazoo, Sharkey, Issaquena, and Washington Counties. Lignite isopleth maxima of 17 and 18 lignites, respectively, for the lower and upper Wilcox of central Washington County lend further support for this interpretation. The principal distal deltaic sand bodies of the Wilcox high-constructional Holly Springs delta system, however, have been mapped by other researchers 75 to 125 mi (120 to 200 km) 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.

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