Paleocene Coal Deposits of the Wilcox Group, Central Texas
Published:January 01, 2011
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Robert W. Hook, Peter D. Warwick, John R. SanFilipo, Adam C. Schultz, Douglas J. Nichols, Sharon M. Swanson, 2011. "Paleocene Coal Deposits of the Wilcox Group, Central Texas", Geologic Assessment of Coal in the Gulf of Mexico Coastal Plain, Peter D. Warwick, Alexander K. Karlsen, Matthew Merrill, Brett J. Valentine
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Coal deposits in the Wilcox Group of central Texas have been regarded as the richest coal resources in the Gulf Coastal Plain. Although minable coal beds appear to be less numerous and generally higher in sulfur content (1 percent average, as-received basis; table 1) than Wilcox coal deposits in the Northeast Texas and Louisiana Sabine assessment areas (0.5 and 0.6 percent sulfur, respectively; table 1), net coal thickness in coal zones in central Texas is up to 32 ft thick and more persistent along strike (up to 15 mi) at or near the surface than coals of any other Gulf Coast assessment area. The rank of the coal beds in central Texas is generally lignite (table 1), but some coal ranks as great as subbituminousC have been reported (Mukhopadhyay, 1989). The outcrop of the Wilcox Group in central Texas strikesnortheast, extends for approximately 140 mi between the Trinity and Colorado Rivers, and covers parts of Bastrop, Falls, Freestone, Lee, Leon, Limestone, Milam, Navarro, Robertson, andWilliamson Counties (Figure 1). Three formations, in ascending order, the Hooper, Simsboro, and Calvert Bluff, are recognized in central Texas (Figure 2). The Wilcox Group is underlain conformably by the Midway Group, a mudstone-dominated marine sequence, and is overlain and scoured locally by the Carrizo Sand, a fluvial unit at the base of the Claiborne Group.
Coal deposits in the Wilcox Group of central Texas have been regarded as the richest coal resources in the Gulf Coastal Plain. Although minable coal beds appear to be less numerous and generally higher in sulfur content (1 percent average, as-received basis; table 1) than Wilcox coal deposits in the Northeast Texas and Louisiana Sabine assessment areas (0.5 and 0.6 percent sulfur, respectively; table 1), net coal thickness in coal zones in central Texas is up to 32 ft thick and more persistent along strike (up to 15 mi) at or near the surface than coals of any other Gulf Coast assessment area. The rank of the coal beds in central Texas is generally lignite (table 1), but some coal ranks as great as subbituminous C have been reported (Mukhopadhyay, 1989). The outcrop of the Wilcox Group in central Texas strikes northeast, extends for approximately 140 mi between the Trinity and Colorado Rivers, and covers parts of Bastrop, Falls, Freestone, Lee, Leon, Limestone, Milam, Navarro, Robertson, and Williamson Counties (Figure 1). Three formations, in ascending order, the Hooper, Sims-boro, and Calvert Bluff, are recognized in central Texas (Figure 2). The Wilcox Group is underlain conformably by the Midway Group, a mudstone-dominated marine sequence, and is overlain and scoured locally by the Carrizo Sand, a fluvial unit at the base of the Claiborne Group.
Within central Texas, the thickness of the Wilcox Group ranges from less than 1000 ft in the northeast to more than 3500 ft in the southwest (Ayers and Lewis, 1985). This thickening is accompanied by an increase in basinward southeasterly dip, from less than 1 degree in the northeast to approximately 2 degrees in the southwest, which accounts in part for the southwestward narrowing of the Wilcox outcrop belt from approximately 17 mi in Freestone County to 12 mi in Bastrop County (Figure 1). In the northern portion of central Texas, salt-dome tectonics have resulted in local thickness and structural anomalies in the Wilcox Group. The Mexia Fault Zone (Figure 3), a narrow gra-ben system that trends north-northeastward obliquely across the Wilcox from Bastrop to Limestone Counties, also is responsible for local thickness and structural variations within the Wilcox Group.
Nearly all thick coal beds of central Texas occur within the lowermost part of the Calvert Bluff Formation (Figure 2). Five major surface mines (Figure 1) have been developed in coals of the Calvert Bluff Formation in central Texas, and, from the late 1880s to the middle 1940s, underground mines also exploited thick coal beds in the Calvert Bluff Formation. Coal beds of the Hooper Formation have not been commercially developed and are generally thinner and more discontinuous thanCal-vert Bluff coal beds. Likewise, coal deposits within the Simsboro Formation are only local in extent in the shal-low subsurface.
Since 1955, the Bureau of Economic Geology, University of Texas at Austin (BEG), has reported estimates of coal resources in central Texas (Perkins and Lonsdale, 1955; Fisher, 1963; Kaiser, 1974; Kaiser et al., 1980; Tewalt et al., 1983; Ayers and Lewis, 1985; Ayers and Kaiser, 1986; Tewalt and Jackson, 1991). During the middle 1960s through the late 1980s, the BEG published geologic interpretations, geochemical and pet-rographic data on coal, mine-specific reports, and hy-drologic and environmental reports related to Wilcox coal resources in central Texas (Fisher and McGowen, 1967; Kaiser, 1974, 1976, 1978a, b; Henry, 1976; Henry et al., 1976; Kaiser et al., 1978,1980; Henry and Kastning, 1978; Henry and Basciano, 1979; Dutton, 1985, 1986; Ayers et al., 1986; Jones, 1986; Macpherson, 1986a, b; Tewalt, 1986; Tewalt et al., 1986; Ayers, 1986, 1989a, b; Mukhopadhyay, 1986, 1989).
Studies by workers not affiliated with the BEG include several unpublished theses that address mine-specific hydrogeologic factors, reclamation, or both (Lentz, 1975; Schneider, 1977; Pepper, 1980; Pollock, 1982; Green, 1984). Other reports based upon data collected in central Texas Wilcox mines include sedimen-tologic, stratigraphic, petrographic, and geochemical data (Breyer, 1987; Palmquist, 1987; English, 1988; Mid-dleton and Luppens, 1995; Crowley et al., 1997). Pal-ynologic studies indicate that all of the Calvert Bluff coal deposits analyzed to date are late Paleocene in age (Atlee et al., 1968; Elsik, 1968,1978,1986; Nichols, 1970; Nichols and Traverse, 1971).
The present summary and the assessment chapter (Warwick et al., 2011) are based upon the above works, drillhole records (geophysical logs, descriptions of cut-tings and cores) and cross sections from permits of surface mines regulated by the Railroad Commission of Texas (RCT), coal-quality data contained in the same permits, records from the U.S. Geological Survey National (USGS) Coal Resource Data System (USGS NCRDS), proprietary coal-exploration data from industry sources, and field work, including sampling of active surface mines, undertaken by the USGS since 1994. Approximately 1800 drillhole records were used to calculate resources for the central Texas study area (Warwick et al., 2011).
History of Coal Mining
Commercial mining of Wilcox coal deposits in central Texas began in the 1880s. Three districts, the Rockdale in Milam County, the Evansville-Bear Grass area in Leon County, and the Bastrop-Sayersville area in Bas-trop County, emerged as underground mining centers during the 1910s (Figure 1). Other shallow shaft and slope mines operated in Anderson County near the Palestine salt dome, in southern Freestone County near the present-day Jewett mine, in north-central Lee County, and in western Robertson County near the active Calvert Mine (Dumble, 1892; Phillips, 1902; Phillips and Worrell, 1911, 1913; Taylor, 1911; White and Thiessen, 1913; Broman, 1915; Schoch, 1918; Gentry, 1921; Bullock, 1924; Plummer, 1932; Sellards, 1934; Crow in Stenzel, 1938; Harris, 1941; Stenzel et al., 1948; Dietrich and Lonsdale, 1958; Fisher, 1963,1965; Betancourt, 1977).
The Rockdale area of Milam County was the most prolific of the central Texas underground mining dis-tricts. At least 27 companies operated shaft or slope mines (Figure 4) from 1890 to the 1930s in lower to mid-dle Calvert Bluff coal deposits in south-central Milam County (Rockdale abandoned mines, Figure 1). Nearly all the shafts were less than 100 ft in depth, and the maximum reported coal bed thickness was 8 ft. Sur-face mining began in the same area as early as 1918 and continued intermittently until 1950. Aluminum Company of America's (ALCOA) present Sandow mine in southern Milam County (Figure 1) started in 1954 to supply fuel to an onsite power plant that furnishes electricity for refining aluminum. From 1976 to 2007, the cumulative production of the Sandow mine was 150,966,982 short tons (Railroad Commission of Texas, 2009). In 2005, the Three Oaks mine opened in an area adjacent to the Sandow mine; by 2007, cumulative production totaled 13,672,715 short tons (Railroad Commission of Texas, 2009).
Although less numerous than those of the Rockdale area, mines in northwestern Leon County (Evansville -Bear Grass mines, Figure 1) are among the most well-documented historic mines of the Texas Gulf Coastal Plain (Crow in Stenzel, 1938). From 1907 to 1930, seven shafts from 30 to 110 ft in depth penetrated the up-permost part of the Calvert Bluff Formation. Mined coal bed thicknesses ranged from 7 to 11 ft. Some of these old mine workings have been encountered in the present-day Jewett mine (Figure 1), which was opened in western Leon and southern Freestone Counties for mine-mouth fuel in 1985 by the Northwestern Resources Company (Palmquist, 1987). The Jewett mine is now operated by Texas Westmoreland Coal Com-pany and cumulative production through 2007 totaled 167,693,510 short tons (Railroad Commission of Texas, 2009).
Approximately 30 underground mines were developed in Bastrop County between 1886 and the middle 1940s. At one time, 14 mines were active along the railroad in western Bastrop County. Three coal beds were mined within a 100 ft stratigraphic interval in the lower Calvert Bluff Formation (Dietrich and Lonsdale, 1958). Coal bed thicknesses ranged from 2.5 to 12 ft; a mudstone parting up to 1.5 ft thick occurred in the lowermost bed, which also was the thickest bed. From 1984 to 1993, the Lower Colorado River Authority operated the Powell Bend mine, a major surface mine in the Bastrop-Sayersville district. By 2007, cumulative production from this mine totaled 1,569,875 short tons (Railroad Commission of Texas, 2009).
In 1971, the Texas Utilities Mining Company opened the Big Brown mine in northern Freestone County to supply mine-mouth fuel for power generation. This marked the beginning of the modern era of large-scale surface mining for electrical power in the Texas Gulf Coastal Plain. In order to manage increased amounts of overburden and interburden, Texas Utilities began using a bucketwheel excavator and a cross-pit conveyor system at Big Brown in 1985. Luminant Mining Company now operates the Big Brown mine, and cumulative production at the mine from 1976 to 2007 was 160,688,997 short tons (Railroad Commission of Texas, 2009). The Railroad Commission of Texas (2009) also reports that Luminant Mining Company recently opened the Twin Oak and Kosse mines in the central Texas assessment area, and in 2007, cumulative production from these mines was 197,042 short tons.
The Calvert mine (Figure 1) is located inwesternRob-ertsonCounty (Figure 1). The mine was opened in 1989 by the Walnut Creek Mining Company in conjunction with Phillips Coal Company. Cumulative production from the Calvert mine through 2007 totaled 32,643,244 short tons (Railroad Commission of Texas, 2009).
Most of the Wilcox Group in the outcrop and subsurface of the central Texas assessment area was deposited in the Houston Embayment along the northwestern margin of the Tertiary Gulf Coast (Figure 3). The northeastern part of the central Texas area adjoins the southern end of the East Texas Basin, and the south-western margin of the area is formed by the San Marcos Arch, a southeastward-trending feature that separates the Houston and Rio Grande Embayments. Within the Houston Embayment, more than 18,000 ft of Mesozoic and lowermost Tertiary rocks underlie the Wilcox Group (Salvador, 1991). The lowermost part of the sedimentary sequence in the Houston Embayment includes thick (5000 to 7000 ft) Middle Jurassic salt deposits (Jackson and Seni, 1984; Galloway et al., 1991). Following salt deposition, marine conditions prevailed in the Houston Embayment until the middle to late Paleocene when progradational deltaic systems began to introduce terrigenous clastics into the region.
Deformation of thick Jurassic salts induced by sediment loading produced numerous syndepositional tectonic features in central Texas. In the northern part of the area, salt diapirs and anticlines influenced local sedimentationpatternsinoverlying UpperJurassic through lower Tertiary units (Jackson and Seni, 1983, 1984). The Elkhart Fault Zone, a graben structure that continues eastward from Anderson County to join the Mt. Enterprise Fault Zone of east Texas (Figure 3), initially formed over a salt anticline. The Elkhart—Mt. Enterprise Fault System was active from the Cretaceous through the Quaternary (Collins et al., 1980; Jackson, 1982). Regionally, the updip limit of Middle Jurassic salt deposits coincides with the Mexia-Talco Fault System, a narrow (generally less than 12 mi wide) peripheral graben complex of en echelon faults that originated with basinward salt migration in the Late Jurassic (Ewing, 1991). Syndepositional movement along this system influenced sedimentation patterns and strat-igraphic thicknesses through the Eocene (Ayers and Lewis, 1985).
Wilcox Group Depositional Settings
Most of the siliciclastic sediments of the Wilcox Group in central Texas were derived from a northwestern source area, possibly as distant as the Rocky Mountain foreland region (Bammel, 1979; Winker, 1982; Ayers and Lewis, 1985). Some sediment that was shed from the Ouachita Mountains in nearby Oklahoma and Arkansas (Figure 1) was channeled southward through the East Texas Basin and deposited in the Houston Em-bayment (Harris, 1962; Kohls, 1967) (Figure 3).
The Hooper Formation is the lower 400 to 1000 ft of the Wilcox Group in central Texas (Figure 5). Known mainly from oil and gas data, the formation is a transitional sequence between the fully marine Midway Group below and the predominantly fluvial Simsboro Formation above (Figure 5). Most geophysical logs show that the Hooper consists of multiple coarsening-upward sequences indicative of prodeltaic or inter-distributary to distributary deposits. Associated sandstones have been mapped as linear to curvilinear in the shallow subsurface and grade downdip into distributary facies (Fisher and McGowen, 1967; Ayers and Lewis, 1985). In the updip portion of the subsurface, the upper 120 to 350 ft of the Hooper Formation is dominated by fluvial deposits that contain massive sandstone units in Lee, Burleson, and Milam Counties (Figure 1; McGowen and Garner, 1970; Ayers and Lewis, 1985). Locally thick (more than 5 ft), laterally discontinuous coal beds also occur in the uppermost part of the formation.
Throughout most of the central Texas area, the Simsboro Formation is a thick (up to 800 ft) sandstone-dominated interval (Figure 5). The greatest amounts of sandstone occur in south-central Milam County, where stacked, multistorysequences of medium to coarse-grained sandstone represent bed-load to mixed-load fluvial sys-tems (McGowen and Garner, 1970; Bammel, 1979; Ayers and Lewis, 1985). Similar sandstone-dominated facies extend along strike in the shallow subsurface for nearly 75 mi from Leon County in the northeast to Bastrop County in the southwest. Downdip in the deep subsurface, these fluvial tracts bifurcate and merge with deltaic facies. The Simsboro Formation is not recognized formally north of the Trinity River (Figure 1) because the interval contains less sandstone and its lower and upper contacts usuallyare indistinct ingeophysical logs. Likewise, in southeastern Bastrop County, the character of the Simsboro Formation changes to one of mud-stone and sandstone, interpreted as strandplain deposits by Fisher and McGowen (1967), and does not constitute a separate unit of the Wilcox Group. Coal beds within the Simsboro Formation are thin and laterally discontinuous.
The Calvert Bluff Formation ranges from 500 to 2000 ft in thickness and consists of mudstones, sandstones, and coals that were deposited in alluvial to deltaic en-vironments (Figure 5). Intertidal to shallow marine de-posits have been distinguished in the lower and up-permost parts of the formation on the basis of marine invertebrates, glauconitic sandstone beds, and wave-ripple lamination (Plummer, 1932; Harris, 1962; Jones, 1986; Breyer, 1987). Mudstone-dominated, coarsening-upward deposits grade laterally into interbedded sequences of mudstone and sandstone and into sandstone-dominated channel facies. Stacked or multistory sandstone deposits up to 100 ft thick represent major fluvial or distributary channels.
The Wilcox Group increases in thickness across the Mexia Fault Zone in the southern part of the central Texas assessment area (Ayers and Lewis, 1985) (Figure 3). In the northeast part of the area, near the Palestine salt dome (Figure 1), Wilcox channels were deflected awayfrom active salt domes and into adjacent salt-withdrawal depressions, which contain sandstone-dominated deposits up to 40 percent thicker than adjoining areas that were not influenced by salt tectonics (Seni and Jackson, 1983).
Coal beds of the Calvert Bluff Formation in central Texas are the thickest and most continuous Wilcox coals in the Gulf Coastal Plain. Multiple coal beds occur in coal zones that are flanked by sandstone-dominated channel deposits in the lower, middle, and upper portions of the formation. Of the nine minable coal zones assessed in this study in the central Texas assessment area, cumulative coal thickness of individual coal beds or zones averages 6 ft and ranges up to 33 ft in thickness. In comparison to Wilcox coals in Northeast Texas, Texas Sabine, and Louisiana Sabine assessment areas (table 1), the Calvert Bluff coals have high sulfur values (1 percent thickness-weighted average, as-received basis) and intermediate ash yield and calorific values (13.3 percent thickness-weighted average and 6753 Btu/lb, as-received basis).
At least two lower Calvert Bluff coal zones are mined at the Sandow mine in Milam County, the Calvert mine in Robertson County, and, previously, the Powell Bend mine in Bastrop County, and various underground mines. In the area of the Sandow mine, the coal zones continue for approximately 12 mi along strike and include a main bed that averages from 12 to 15 ft in thickness (Kaiser, 1976; Ayers, 1986). The main bed splits into several benches that pinch out or are cut out by thick channel sandstones. Five coal beds that average from 2.5 to 5.6 ft in thickness are present within an interval of approximately 213 ft on the Calvert mine property (Middleton and Luppens, 1995). The lower two beds are the thickest and most extensive, and both split within the mine area. Underground mines near Rockdale in the lower portion of the Cal-vert Bluff Formation operated in a linear coal zone that was less than a mile in width but several miles in length. The coal bed included splintery to conchoidal zones, reached a maximum thickness of 11 ft, and occupied a depression, which was interpreted as an abandoned estuary or bayou (White and Thiessen, 1913).
Middle and upper Calvert Bluff coal beds are mined at the Big Brown mine in Freestone County. At least four coal beds that average from 5 to 8 ft in thickness have been mined and are laterally continuous for up to 7 mi (Henryet al., 1976). The coals commonly overlie thick coarsening-upward sequences (Lentz, 1975; Breyer, 1987) and are flanked by thick (greater than 100 ft) sandstone deposits. Three individual middle and upper Calvert Bluff coal beds that range from 2.5 to 9 ft in thickness are mined at the Jewett mine in Leon and Freestone counties (Palmquist, 1987).
Although no coal is mined presentlyfrom the Hooper Formation, the uppermost part of the formation contains pods of thick (greater than 5 ft) coal in the relatively shallow subsurface. Oil and gas logs suggest that the Hooper Formation contains more tabular coal deposits in the deep subsurface (Kaiser, 1978b).
Coal zones that consist of two or more minable beds occur in the Calvert Bluff Formation, the uppermost part of the Wilcox Group in central Texas. Although these deposits are less numerous and generally higher in sulfur than Wilcox coals of Northeast Texas and the Louisiana Sabine assessment areas, individual beds are thicker and more continuous along strike at or near the surface. From the late 1880s to the middle 1940s, Calvert Bluff coals were mined underground. Several large surface mines currently exploit the resource as a mine-mouth fuel for power generation. These mines produced a cumulative total of approximately 527.4 million short tons by 2007 (Railroad Commission of Texas, 2009). Most of the minable coal occurs near the base of the Calvert Bluff Formation, and cumulative coal-bed or individual zone thickness averages 6 ft.
Figures & Tables
Geologic Assessment of Coal in the Gulf of Mexico Coastal Plain
This publication is the fifth in a searies of reports by the U.S. Geological Survey on the assessment of the quantity and quality of the nation's coal deposits that potentially can be mined during the next few decades. For eight years (1995-2003), geologic, geochemical, and resource information was collected and compiled for the five major coal-producing regions of the United States. This volume contains the assessment results for the Gulf of Mexico Coastal Plain region. The contents of this volume were compiled mainly during the 8-year period mentioned above. However, every effort has been made to update the references and text to incorporate new work that has been completed since the original compilation period.