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The Wilcox and Jackson Groups are the most important lignite-bearing geologic units in Texas. Depositional systems interpreted for these units in east Texas are fluvial and deltaic, respectively. Assessment of the geologic characteristics of a lignite deposit in both the Jackson and the Wilcox Groups indicates positive correlations with the regional depositional models. Regional evaluations of the Jackson Group associate high-sand areas with lignite occurrence. In the Wilcox Group, lignites are associated with low-sand areas.
A Jackson Group deposit in east Texas reflects two distinct
processes of lower delta plain deposition. Thin, discontinuous lignite seams apparently formed in small interdistributary areas, which were commonly inundated by sediment during overbank flooding and crevassing. Thick coal seams, deposited on sand platforms, are laterally continuous and represent lignite deposition during periods of delta lobe abandonment. A change in position on the delta plain from stratigraphically older to younger seams is reflected in both seam characteristics and comparisons of average heating values.
A Wilcox deposit in east Texas shows most of the characteristics of an alluvial plain setting. The individual seams are lenticular; the thickest lignite occurs in the center of the lignite bodies but decreases abruptly along the margins. Adjacent to the lignite bodies are channellike barren areas that are filled with either mud or sand. Channels are normally parallel to the individual lignite bodies. Large, irregular and circular mud-filled areas completely surround some of the lignite seams. Overall quality of the lignites in this environment is found to be variable, but generally low in ash and high in heating value.
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