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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 69 (1985)

Issue: 9. (September)

First Page: 1429

Last Page: 1429

Title: Subsurface Lignite Occurrence in Wilcox Group, Northeast Louisiana and Northwest Mississippi: ABSTRACT

Author(s): Mark D. Purcell, George F. Hart, Charles G. Groat

Article Type: Meeting abstract

Abstract:

An investigation of lignite occurrence in the Wilcox Group of northeast Louisiana and northwest Mississippi revealed high lignite concentrations associated with a highly constructive elongated delta in the lowermost Wilcox Group. Bar-finger sandstone and distributary channel sandstone facies of an elongated delta lobe were recognized trending north-south through the central portion of the study area by sandstone percent maps, net sandstone isopach maps, and characteristic SP curves on electric well logs. Lignite isopleth maps identified principal areas of lignite occurrence adjacent to the elongate delta. Fewer, but thicker seams were found in the northern regions, where a maximum of four seams >= 5 ft thick were identified. To the south, a greater concentration of thi ner seams occur. The distribution is attributed to a change in the position on the deltaic plain. Numerous, thin lignite beds in the southern region are indicative of lower delta-plain environments, where bifurcating distributaries, crevasse splays, and marine inundation inhibited thicker peat development. The presence of thicker, but fewer lignites to the north supports a transitional to upper delta-plain environment.

Mapping of individual sandstone beds revealed dip-oriented, bifurcating, fanlike geometries indicative of a lower to transitional delta plain. Capping lignite seams are blanket type, having areal extents of up to several hundred square miles and thicknesses ranging from 2 to 20 ft. Thicker seam development occurs along the delta-lobe margins, extending landward into adjacent interdistributary basins. The large extent of the blanket seams is in part attributed to (1) lignite beds capping channel-fill deposits, and (2) distributary channel and point-bar sandstones directly overlying lignite beds, with no truncation evident. Cross sections illustrate subsidence of the lignite beneath the overlying channel deposits.

High-quality lignite originates in transitional to upper delta-plain environments. Optimal areas for high-quality lignites in the Wilcox Group should be the extreme northeastern and northwestern portions of the study area, and farther north into central Tensas and Franklin Parishes, where thicker, transitional to upper delta-plain paleoenvironments should exist. Minimum subsurface depth to the highly lignitic, lowermost Wilcox Group ranges from approximately 5,000 ft (1,510 m) in the northwestern region to 7,000 ft (2,114 m) along the extreme southern boundary.

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