CHAPTER 4

Geology and depositional setting of the lower Calvert Bluff Formation (Wilcox Group) in the Calvert Mine area, east-central Texas

By Mark Middleton¹ and James A. Luppens²

¹Walnut Creek Mining Co., Calvert Mine, P.O. Box H, Bremond, TX 76629
²Phillips Coal Co., 2929 North Central Expressway, Richardson, TX 75080

ABSTRACT

The Calvert Mine area lies within the western Gulf Coastal Plain section of the Gulf Coast basin of the south-central United States. Five major and two minor lignite seams comprise the recoverable reserve within the Calvert Mine. The average lignite seam thickness ranges from 0.8 to 1.7 m. These seams occupy approximately 85 m of stratigraphic section within the lower Calvert Bluff Formation (Paleocene - Eocene, Wilcox Group). Other deposits found in the Calvert Mine area include the Simsboro Formation (Paleocene, Wilcox Group), which is dominated by fluvial sandstone, and Quaternary terrace and floodplain deposits. Faulting associated with the Mexia Fault System has offset the lignite-bearing intervals from a few meters to more than 100 m.

INTRODUCTION

The geology of the Calvert Mine (CM) has been extensively studied. Substantial information was gathered during numerous exploration drilling programs since 1974. The majority of the mine area data base was collected under the direction of Phillips Coal Company. These data were the fundamental tool in characterizing substrate composition, geometry, and structure. The data were analyzed in numerous investigations by Phillips Coal Company, Espey, Huston & Associates, Inc., R.W. Harden & Associates, Morrison-Knudsen Company, Inc, and Walnut Creek Mining Company. Outside sources, mainly published works of the University of Texas at Austin, Bureau of Economic Geology, were utilized to characterize regional geologic conditions.

REGIONAL GEOLOGY

Physiography

The CM area lies within the western Gulf Coastal Plain section of the Gulf Coast basin (fig. 1). The Gulf Coastal Plain is characterized by gently rolling to hilly topography. The physiography closely follows the outcropping substrate: the sands are generally resistant to erosion and form topographic highs, whereas mud (clay/silt) deposits are more easily eroded and are expressed as valleys (Henry and Basciano, 1979). The major sand formations of the Claiborne and Wilcox Groups (fig. 2) form low irregular escarpments or cuestas of rounded, steep-sided hills that rise as much as 30 m above adjacent areas. The outcrops of the Reklaw Formation and muddy parts of the Wilcox develop a low linear terrain parallel to, and between, the major sand formations of the Claiborne and Wilcox Groups (figs. 2, and 3). The mud-rich Midway Group crops out as a subdued gently rolling and featureless topography (Henry and Basciano, 1979; Barnes, 1974). Major rivers that flow to the southeast form broad flat valleys.

The principal escarpments of the CM region are the Balcones-Whiterock Escarpment and the Nacogdoches Escarpment (fig. 1). The Balcones-Whiterock Escarpment occurs along the Balcones and the Mexia-Talco Fault Systems and bounds the West Gulf Coastal Plain. The Nacogdoches Escarpment trends northeast-southwest in Robertson and Leon Counties and results from the outcrop of the resistant indurated ironstone of the Weches Formation and the Claiborne Group (Fisher and others, 1965).

Stratigraphy

Midway Group. The Paleocene Midway Group unconformably overlies the calcareous marls and shales of the Cretaceous Navarro Group.

The top of the Midway Group is about 365 m below the base of the Calvert Bluff (Ayers and Lewis, 1985) and crops out 12 km to the northwest of the CM mine area (figs. 2, and 3) in adjoining Falls and Robertson Counties. There the Midway crops out primarily within a north-northeastern trending graben of the Mexia Fault zone (fig. 1). The Midway Group is composed of highly glauconitic sands and sandy clays. The upper Midway consists of interbedded marls and glauconitic limestone (Barnes, 1974). The top of the Midway grades into the overlying mudstones and sands of the lower Wilcox Group (Barnes, 1974).

Wilcox Group. The Wilcox Group con-formably overlies the Midway Group (Fisher and McGowen, 1967). The Wilcox is divided into three formations in east Texas: from the lowest unit to the highest these are the Hooper Formation, the Simsboro Formation and the Calvert Bluff Formation. Kaiser and others (1980) report that the Wilcox represents a major progradational phase in the Eocene and estimate the Wilcox Group to be 365 to 1,060 m thick in east-central Texas.

The Hooper Formation conformably overlies the Midway marine muds and ranges in thickness from 120 to 300 m. The Hooper is described regionally as containing mudstone and sandstone and locally as containing glauconite in the lowermost part (Barnes 1970). Ayers and Lewis (1985) describe the Hooper as an upward-coarsening sequence that records a succession from prodelta through distributary channel fill, delta plain mudstone, and lignite. They report that the Hooper documents initial progradation of Wilcox fluvial-deltaic systems into the Houston area.

The Simsboro Formation conformably overlies the Hooper Formation (Kaiser and others, 1980). The outcrop belt of the Simsboro extends from east of Waco to a little east of Austin (fig. 1). The thickness of the Simsboro ranges from 60 to 240 m in east-central Texas. Lithologically, the Simsboro consists of fine to coarse-grained, kaolinitic sand, thinly laminated sand/silt, clay conglomerate, and homogeneous clay and silty clay (Bammel, 1979).

The Calvert Bluff Formation conformably overlies the Simsboro. It is the principal lignite-bearing formation in the Texas Gulf Coast with a thickness ranging from 150 to 600 m in east central Texas (Kaiser and others, 1980). The Calvert Bluff crops out in a broad 8 to 19 km belt trending northeast-southwest in east central Texas. Lithologically at outcrop, the Calvert Bluff is primarily composed of mud with various amounts of sand and lignite. Ferruginous concretions occur irregularly throughout the formation. Locally, the uppermost part is glauconitic (Barnes, 1974).

Claiborne Group. The Claiborne Group unconformably overlies the Wilcox Group (fig. 2). It crops out 5 to 8 km southeast and downdip of the CM mine area and occupies a 5 to 8 km wide band trending northeast-southwest. Regionally at outcrop, the Claiborne is made up of seven distinguishable formations that manifest a cyclic depositional pattern of nearshore marine and non-marine sediments consisting primarily of sandstones, conglomerates, clays and shales.

Quaternary Deposits. Terrace and floodplain deposits of Pleistocene and Holocene age regionally occur along the rivers and their tributaries in east Texas. Holocene alluvial deposits resulted from active floodplain and point bar deposition. Pleistocene terrace deposits are remnants of fluvial deposition (Barnes, 1974).

Structure

The CM lies within the East Texas Basin (fig. 1) of the Gulf Coast Basin, an extensive structural and depositional province consisting of a broad gulfward dipping homocline. The predominant structural feature in the CM area is the Mexia Fault Zone. This fault zone consists of a series of strike-parallel normal faults forming narrow grabens paralleling the buried Ouachita Mountains. The Mexia Fault Zone trends north-northeast from east of Austin to east of Dallas (fig. 1). The major displacement is upward to the southeast, basinward from the Llano Uplift in central Texas, with a graben development toward the uplift. Faults in the Mexia zone are steep, with measured displacement at the Lower Cretaceous level ranging from a few meters upward to 300 m (Zink, 1957).

The Mexia graben reportedly formed as a pull-apart structure between stable strata and strata which gravitationally slid over a décollement zone of weak Louann Salt of Jurassic age (Jackson, 1982). This is considered to be a major finding since the faults were previously reported to have originated from subsidence associated with flexure at the basin margins. The Mexia faults were active from the Jurassic until at least the Eocene which are the youngest strata offset. Quaternary terraces across the faults have not been offset, indicating that faulting ended during the Tertiary (Jackson, 1982).

The CM area lies within the southwestern extremity of the East Texas Basin (fig. 1). The principal structure of the basin consists of regular basinward dips in the east, west and north and a low rim in the south along the Angelina Flexure (Jackson, 1982). Figure 1 illustrates the CM area in relationship to the regional tectonic setting of the Eastern Texas Basin. The figure further illustrates that the predominant structural feature in the CM area is a series of high-angle normal faults associated with the Mexia Fault Zone. Due east of the CM area is the west-southwestern extension of the Angelina Flexure. Jackson (1982) defines the Angelina Flexure as a hinge line that is generally monoclinal at its ends and anticlinal in the middle.

Barnes (1970, 1974) has identified one strike-parallel fault (designated F1) occurring immediately west of the CM area (fig. 2). The fault trends northeast where it dies out north of the CM area in the Simsboro outcrop area. Southwest of the CM area, the fault has been mapped to the Brazos River Valley. Another fault has been identified by Barnes (1970, 1974) as occurring immediately east of the northern portion of the CM area. This fault trending roughly east-west parallels the Angelina Flexure (fig. 1).

Closely spaced exploration drilling in the CM area has identified as many as seventeen additional discontinuous faults. Their orientation and near surface geometry suggest that they are associated with the major Mexia Fault Zone.

Fault F1 offsets the Simsboro Formation outcrop to the northwest and down-throws a section of the Calvert Bluff Formation to the southeast (fig. 2). This fault was identified by Barnes (1970). The fault trends northeast in the western extremity of the CM area and north-northeast in the northern portion of the CM area. The F1 fault which parallels regional strike has down faulted the lower Calvert Bluff section in its present structural position in the western portion of the CM area. Displacement along the faults varies from a few meters to 100 m. Generally all of the faults terminate to the northeast with displacement along those faults increasing to the southwest. The throws of fault F1 are unknown since it is below the base of exploration drill holes.

The structure of the northwestern portion of the CM area is related to unique depositional processes and postdepositional faulting. The western portion of the CM area has progressively steeper dips to the west. Dip angles range from 0.5^o to 3.0^o (9 to 53 m/km) in that area. Figure 4 provides a cross sectional representation of the structural setting of the CM area. At the western end of the cross sections, the lignite seams are terminated where they encounter faults. Underlying the A seam are two thick clay and silt (mud) units that may have formed in abandoned Simsboro channels. The silt unit abruptly grades easterly into sands of the Simsboro. The cross sections illustrate that Calvert Bluff units are draping over underlying thick Simsboro sand development. Using the A seam as a marker bed, it is apparent that the present cross-sectional shape of the Calvert Bluff is not the original one. It is postulated that the weight of overlying Calvert Bluff sand channels caused subsidence of the sands into underlying unconsolidated abandoned channel muds. The resulting lateral differential compaction caused by the deformation of the basal mud units distorted the original depositional shape and is responsible for the progressively westerly dip of the Calvert Bluff units in that direction.

The faults in the CM area impacted the design of lignite mining operations and modified the distribution of reserves by altering the structural setting. The large displacement faults act as reserve boundaries to the mine plan, and affect pit orientation. This necessitated confinement of individual recovery areas within structural fault blocks. The faults also provide hydrological boundaries which have several positive impacts on the mining operations.

GEOLOGIC CHARACTER OF THE CALVERT MINE

PHYSIOGRAPHY OF THE MINE AREA

The Simsboro Formation crops out within the northwestern part of the CM area (fig. 2). Surface features expressed by this formation are primarily rolling hills, with rolling to steep slopes (from 3 to 8%). The substrate consists of friable to loose sands that are locally iron cemented (Henry and Basciano, 1979).

The Calvert Bluff Formation outcrop occupies the majority of the surface area within the CM area (fig. 2). Surface features of the Calvert Bluff are expressed as rolling to steep hills of moderate relief with slopes ranging from 3 to 8% (Henry and Basciano 1979). Slopes of up to 8% probably represent the sandy portion of the upper Calvert Bluff Formation. Environmental maps indicate that the lower Calvert Bluff Formation, including the CM area, trend toward sandy mud prairies of low relief indicating that it contains less sand than the upper Calvert Bluff.

Pleistocene terrace deposits occur above the west bank of the confluence of Big Willow and Walnut Creeks (fig. 2).

Holocene alluvium is observed in nearly all the drainage valleys in the CM area. These deposits have slopes of less than 2% and are present in areas where there is high to moderate flood frequency. Landforms and topography have been largely influenced by erosion caused by Walnut Creek and its tributaries. Topography for the CM area ranges from approximately 90 to 140 m above mean sea level (MSL). The highest elevation is observed along Walnut Creek near the confluence of South Walnut Creek and Walnut Creek. Upland landforms, including Eocene and Pleistocene units, occur at various levels from 100 to 140 m above MSL. The bottomland floodplains (Holocene alluvium) occur at elevations ranging from 90 to 100 m above MSL.

STRATIGRAPHY OF THE MINE AREA

The units that are disturbed by mining include the alluvium, terrace deposits and the lower one-third of the Calvert Bluff Formation. The Simsboro Formation, which will not be mined, underlies the Calvert Bluff and is considered the first significant aquifer beneath the mine. Figure 4 illustrates the stratigraphic setting of the CM area. Figure 2 identifies the location of each cross section.

Simsboro Formation. The Simsboro For-mation generally strikes northeast-southwest and ranges from 70 to 170 m in thickness in the CM area. It is composed of a wide range of lithologies characterized by white, moderately well-sorted, fine-grained to typically medium- to coarse-grained sand. The Simsboro is locally indurated in a white kaolinitic clay matrix or sandy mudstone-boulder conglomerate. The upper Simsboro contains thin lenses of medium to dark-gray mudstone (Kohls, 1963; Barnes, 1970).

The Simsboro Formation is reported to be part of the Mt. Pleasant fluvial system, the main feeder of the Rockdale delta system (Fisher and McGowen, 1967). Ayers and Lewis (1985) later combined the two systems by calling them the Rockdale fluvial deltaic system. Kohls (1967) recognized five subenvironments in the Simsboro. They are the point bar, scour pool, chute channel overbank and abandoned channel facies.

The point bar facies is the most widely distributed deposit within the Simsboro comprising more than 60% of the entire formation. These point bar sands are fine- to coarse-grained, rich in quartz (Kohls, 1967).

Laterally and vertically associated with the point bars is the abandoned channel facies. The facies consists of persistent beds of fine-grained homogeneous clay and silty clay (Bammel, 1979) and is easily recognized by low gamma-resistivity breaks on geophysical logs. The Simsboro is identified in the CM area subsurface and in the region on the basis of its high resistivity response on geophysical logs when compared to the Calvert Bluff (Kaiser and others, 1986). Typically, the Simsboro sand exhibits a highly resistive blocky signature averaging 11 m beneath the basal A seam in the CM area. This response signature is indicative of the point bar facies. Locally, thick mud sequences underlie the A seam indicating over-bank or abandoned channel facies. The overbank facies is typically associated with the upper Simsboro -Calvert Bluff Formation contact (Bammel, 1979). It is difficult to differentiate the Simsboro - Calvert Bluff contact when these thick mud sequences occupy the upper Simsboro section.

Calvert Bluff Formation. The most striking characteristics of the Calvert Bluff Formation within the mining blocks are the cyclical deposition, lateral persistence of the lithologic units (including the lignite seams) and the overall sand-poor nature of the overburden. Vertical repetition of these characteristics results in a fairly straightforward depositional setting facilitating comprehensive analysis of the geological as well as the geotechnical, geochemical and geohydrological baseline conditions.

The University of Texas at Austin, Bureau of Economic Geology, conducted numerous studies in the 1970's and 1980's concerning the occurrence of lignite in the Calvert Bluff Formation of Texas. The objective was to establish exploration models by relating lignite occurrence and sand geometry. Kaiser (1974) and more recently Ayers and Lewis (1985) constructed regional lithofacies maps emphasizing sand geometry. The maps showed that near outcrop straight or slightly meandering multistory sand bodies are dip-oriented. They report that these sands are paleofluvial channel complexes that encase extensive interchannel floodbasins that are mud-rich with abundant lignite. Ayers and Lewis (1985) report that exploitable lignite seams occur primarily in low-sand floodbasin areas. These studies conclude that regionally the distribution of Calvert Bluff lignites is facies controlled by depositional framework elements (channel sands).

The stratigraphic and depositional framework of the Calvert Bluff Formation in the CM area can be divided into two distinct settings. Mining will occur in blocks that are situated in nonframework, mud-rich interchannel floodbasin deposits. The interchannel floodbasin deposits are composed of laterally persistent, upward coarsening sequences that are capped by thick continuous lignite seams. A typical coarsening upward sequence is composed of thin bedded clays grading upward into silts which grade upward into thin silty sands or sandy silts. The coarsening upward sequence is often overlain by an extensive lignite seam. Adjacent to the mining blocks are framework multistory channel sands that truncate the laterally equivalent floodbasin deposits. The channel sands are generally thick (>12 m) fining-upward deposits.

Five major and two minor lignite seams comprise the recoverable reserve within the CM. These seams occupy approximately 85 m of stratigraphic section within the lower Calvert Bluff Formation. Ayers and Lewis (1985) report that Calvert Bluff lignites that are thickest and most continuous occur in the muddy interval just above the Simsboro. The average interburden thicknesses represent the average relative stratigraphic interval between each lignite seam, which vary throughout the CM area depending on their proximity to the framework source channel sands. The intervals often thicken approaching a channel sand. Approximately 75% of the recoverable reserve is associated with the thicker A and E seams. They are continuous and laterally persist throughout the majority of the CM area. They significantly control burden to coal ratios, mining depths and boundaries.

Laterally associated with the A and E seams are two thick multistory channel sands that interrupt the major lignite bearing sequence. Channel deposits exist primarily on the periphery of the mining blocks. The channel sands essentially encase the adjacent lignite-bearing mining blocks or floodbasin deposits. They provided the sediment source into the adjacent floodbasins.

The A seam is stratigraphically the lowest mineable major seam in the CM area (fig. 4). It is the thickest and most areally extensive lignite seam. The A seam overlies the Simsboro sands by an average of 11 m and commonly rests on muddy overbank deposits. The A₂ seam is a minor seam associated with the major A seam. It occurs as a lower split of the A seam in the northern CM area and is absent in the extreme southern CM area. The interval between the A₂ and A seams varies from less than 0.15 m in the northern CM area to 10 m in the south west CM area.

The E seam is second only to the A seam in terms of both thickness and areal extent. The E₂ seam, like the A₂ seam, represents a minor split associated with the overlying major E seam. The E seam is thinner in those areas where the E₂ seam separates from the overlying E seam. The stratigraphic interval between the A and E-E₂ seams varies from a minimum of 11 m near the western boundaries of the CM area to a maximum of 35 m near the northeastern CM area. Cyclic sequences of floodbasin deposits characterize the interval between the A and E seams in the mine blocks. As many as three distinct coarsening-upward floodbasin sequences occupy the A to E seam strati-graphic interval.

The F seam lies between 3 and 20 m above the E seam (fig. 4). Nonframework muddy overbank deposits characterize the F to E seam interval.

The G seam lies about 8 m above the F seam (fig. 4). This interval does not vary significantly in the CM area. Typically the material between G and F seams is composed of a single sequence of upward-coarsening floodbasin material.

The J seam will be the uppermost lignite recovered in the CM area (fig. 4). It crops out in the southern portions of the CM area. Within the CM area, the distance between the J and G seams ranges between 20 and 27 m. As many as three distinct coarsening upward floodbasin sequences characterize the interval between J and G seams.

Quaternary Deposits. Pleistocene terrace deposits occupy the higher elevations that border some of the present day major streams in the CM area. The terrace deposits represent abandoned floodplains that were dissected by the downcutting of a stream or river. The composition of the terrace deposits range from light gray, fine- to coarse-grained sand interbedded with yellowish-brown clays and silts.

Floodplain alluvium is Holocene in age and in the CM area rests on truncated surfaces of Wilcox and Quaternary terrace units. Alluvial deposits are found in the drainage valleys of Walnut and South Walnut Creeks, Big Willow and Bee Branches (Henry and Basciano, 1979). The composition of the alluvium ranges from fine- to coarse-grained, reddish-tan to dark-gray and brown quartz sand to silt, and red-brown to brown clay (Cronin and Wilson, 1967, Barnes, 1974). The alluvial composition reflects the residual products of the surrounding Calvert Bluff and Simsboro Formations. Generally, the floodplain alluvium fines upward in a sequence of gravel, sand, silt, sandy clay and clay. The thickness of these deposits does not exceed 6 m.

GEOLOGIC HAZARDS

The CM area experiences a low level of hazard from geologic processes. There are no sensitive or unique geological features or potential hazards posed by geological conditions that would preclude the operation of a lignite mine in the CM area.

Slope stability of the CM area is high, reflecting low topographic slope angles. The area occurs in a broad, low-relief upland away from major drainages. This broad, low relief minimizes the concentration of runoff and reduces the potential of high erosion. There are no steep bluffs subject to landsliding or slumping.

It is unlikely that any of the faults in the CM area pose seismic risk. No surface deformation or scarps are evident and no man-made structures are known to be offset along the faults. Jackson (1982) reports that normal fault displacements of East Texas faults ensure that stresses are neutralized by tensile fracture at low stresses because the tensile strength of materials is generally much lower than their compressive strength.

REFERENCES CITED

Ayers, W.B., Jr., and Lewis, A.H., 1985, The Wilcox Group and Carrizo Sand (Paleogene) in east-central Texas: depositional systems and deep-basin lignite: The University of Texas at Austin, Bureau of Economic Geology Special Publication, 19 p., 30 plates.

Bammel, B.H., 1979, Stratigraphy of the Simsboro Formation, East-Central Texas: Baylor University, Baylor Geological Studies Bulletin No. 37.

Barnes, V.E., 1970, Waco Sheet: The University of Texas at Austin, Bureau of Economic Geology, Geologic Atlas of Texas, 1 sheet, scale 1:250,000.

Barnes, V.E., 1974, Austin Sheet: The University of Texas at Austin, Bureau of Economic Geology, Geologic Atlas of Texas, 1 sheet, scale 1:250,000.

Cronin, J.G., and Wilson, C.A., 1967, Ground Water in the Flood-plain Alluvium of the Brazos River, Whitney Dam to Vicinity of Richmond, Texas: Texas Water Development Board Report 41.

Fisher, W.L., Chelf, C.R., Shelby, C.A., Garner, L.E., Owens, D.E., and Schofield, D.A., 1965, Rock and Mineral Resources of East Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 54., 439 p.

Fisher, W.L., McGowen, J.H., 1967, Depositional Systems in the Wilcox Group of Texas and Their Relationship to Occurrence of Oil and Gas: Gulf Coast Association of Geological Societies Transactions, v. 17, p. 105-1125.

Henry, C.D., and Basciano, J.M, 1979, Environmental Geology of the Wilcox Group Lignite Belt, East Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 98, 28 p.

Jackson, M.P.A., 1982, Fault Tectonics of the East Texas Basin: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 82-4, 31 p.

Kaiser, W. R., 1974, Texas lignite: Near surface and deep basin resources: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations No. 79, 7O p.

Kaiser, W.R., Ayers, W.B., Jr., and La Brie, L.W., 1980, Lignite Resources in Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 104, 52 p.

Kaiser, W.R., Ambrose, M.L., Ayers, W.B., Jr., Blanchard, P.E., Collins, G.F., Fogg, G.E., Gower, D.L., Ho, C.L., Holland, C.S., Jackson, M.L.W., Jones, C.M., Lewis, A.H., Macpherson, G.L., Mahan, C.A., Mullin, A.H., Prouty, D.A., Tewalt, S.J., and Tweedy, S.W., 1986, Geology and ground-water hydrology of deep-basin lignite in the Wilcox Group of East Texas: The University of Texas at Austin, Bureau of Economic Geology, Special Publication, 182 p.

Kohls, D.W., 1963, Simsboro and adjacent formations between Brazos and Trinity Rivers, Texas - lithology and clay mineralogy: Gulf Coast Association of Geological Societies Transactions, v. 13, p. 111-117.

Kohls, D.W., 1967. Petrology of the Simsboro Formation of northeast central Texas: Journal of Sedimentary Petrology, v. 37. no. 1, p. 184-204.

Murray, G.E., 1961, Geology of the Atlantic and Gulf Coastal Province of North America: New York, Harper & Brothers, 692 p.

Zink, E.R., 1957, Resume of the Lower Cretaceous of south Texas: Gulf Coast Association of Geological Societies Transactions, v. 8, p. 12-22.

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