Facies architecture of the upper Calvert Bluff Formation exposed in the highwall of Big Brown Mine, Fairfield, Texas



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Texas A&M University


The facies architecture and geometry of stratigraphic surfaces within a lignite bearing interval of the Paleocene upper Calvert Bluff Formation is mapped on a photomosaic of the 150 ft (50 m) high and 12,000 ft (4km) long ??????C?????? area highwall of Big Brown Mine, near Fairfield, Texas. Observed bedding and facies architecture are interpreted in terms of temporal changes, depositional environments and sequence stratigraphic setting. A three dimensional grid of 89 subsurface logs is correlated to this photomosaic to characterize log response patterns of facies. Six facies are observed: 1) lignite, 2) interdistributary bay mud, 3) prograding delta, 4) delta top mud, 5) distributary channels, and 6) incised valley fill. The six facies were defined by a combination of mapped photomosaic observations and subsurface log correlations. The lignite deposit formed in a low depositional energy, low sediment input, high-organic productivity interchannel basin. Overlying mud records overbank flooding followed by avulsion and progradation of delta deposits. Tidal-flat deposits overlying prograding delta deposits record fluctuating energy conditions on the emerging delta top. Channel deposits cutting into the delta top record lateral channel migration across delta top floodplains. These regressive delta deposits are capped by a local incised sequence boundary overlain by fluvial channel deposits inferred to have allowed sediment to bypass further basinward during lowstand. A sheet of channel deposits capping this highwall exposure records more recent erosion, followed by development of modern soil horizons. The Big Brown Mine highwall exposes a relatively complete high-frequency Paleocene stratigraphic sequence developed in an area landward of the shoreline position during maximum transgression, that progresses upsection from: 1) highstand alluvial flood basin coals, 2) a thin condensed maximum flooding interdistributary shale, 3) a thick succession of regressive deltaic strata, and 4) a high-relief, sequence-bounding erosion surface overlain by a lowstand to transgressive fill of channel deposits. Correlations with regional Wilcox Group stratigraphic studies spanning coeval shoreline and shelf strata indicate that this high-frequency sequence is within the transgressive systems tract of a 3rd order stratigraphic sequence. It appears that high-frequency sequences of sub-regional extent control the complex distribution of coal seams within central Texas.