Chilhowee Group

The lower Cambrian (Terreneuvian?) Chilhowee Group (fig. 3) lies unconformably above Mesoproterozoic basement rocks in the Birmingham graben and the downthrown block to its southwest but is most likely absent on adjacent horst blocks. Kidd and Neathery (1976) indicate that the Chilhowee Group thins dramatically and pinches out between northwestern Georgia and the Black Warrior basin in north-central Alabama. In northeastern Alabama, the Chilhowee Group includes four formations (in ascending order): the Cochran, Nichols, Wilson Ridge, and Weisner Formations. It is unclear whether these four formations as described by Mack (1980) in northeastern Alabama extend as far south as our study area. For this reason, on cross section N–N′ and figure 3 we show the Chilhowee Group as a single undivided unit with a total thickness of about 2,500 ft (762 m), based on the sum of the thicknesses of the Cochran, Nichols, Wilson Ridge, and Weisner Formations (233, 410, 1,394, and 492 ft [71, 125, 425 and 150 m], respectively, from Mack [1980]) in the Birmingham graben and the upper level of the downthrown block to its southwest (below drill holes 204, 206, 207 and 208). In the lower level of the downthrown block (below drill holes 202 and 203, and between drill holes 204 and 205), we show Chilhowee Group fill from the basement rocks up to the overlying Shady Dolomite with thicknesses ranging from 2,500 to 4,400 ft (762 to 1,341 m).

The lower Cambrian (Terreneuvian?) metamorphosed (lower greenschist facies) clastic sediments of the Kahatchee Mountain Group, the lowest stratigraphic unit in the Talladega slate belt in Shelby, Chilton, Coosa, Talladega, and Clay Counties, Alabama (fig. 9), are considered the lateral equivalent of the Chilhowee Group, with thicknesses greater than 2,000 m (6,562 ft) (Tull and others, 1988; Johnson and Tull, 2002). The Talladega slate belt is a thrust sheet that was translated more than 62 mi (100 km) on the Talladega-Cartersville fault from a location southeast of the study area to its present location (Johnson and Tull, 2002). Rocks of the Talladega slate belt contain lower greenschist facies metamorphic mineral assemblages representing possible maximum temperatures between 300 and 400 degrees Celsius (572 to 752 degrees Fahrenheit) (Johnson and Tull, 2002).

Shady Dolomite

The lower Cambrian (Terreneuvian? to Series 2?) Shady Dolomite (fig. 3) lies conformably above the Chilhowee Group. It occurs in the Birmingham graben and the downthrown block to its southwest but is absent on the adjacent horst blocks to the northwest. As with the underlying Chilhowee Group, Kidd and Neathery (1976) indicate that the Shady Dolomite thins and pinches out between northwestern Georgia and the No. 1 F.W. Smith well (drill hole 112) in Cullman County, Alabama (fig. 10). It is composed of limestone and dolomite, with the limestone being bluish to yellowish gray, thick bedded, and partially medium crystalline, and the dolomite being light to dark gray, finely to coarsely crystalline, argillaceous to sandy, siliceous, and massively bedded to laminated. It ranges from 0 ft thick in Cherokee, Etowah, St. Clair, Shelby, and Bibb Counties, Alabama, to over 1,000 ft (305 m) thick in Clay, Cleburne, Talladega, Coosa, and Chilton Counties, Alabama (fig. 10) and it crops out in the eastern VRPA in northeastern Alabama, in central Shelby County, Alabama (Butts, 1940). Outcrop exposures are generally of poor quality exhibiting residual lithologies like sandy clay, limonite, chert, and finely crystalline quartz veins. The Shady Dolomite represents the earliest part of a regionally extensive carbonate bank facies.

The lower Cambrian (Terreneuvian? to Series 2?) Jumbo Dolomite of the Sylacauga Marble Group, which lies conformably above the Kahatchee Mountain Group in the Talladega slate belt in Shelby, Talladega, and Coosa Counties, Alabama (fig. 9) is considered to be equivalent to the Shady Dolomite (Tull and others, 1988; Johnson and Tull, 2002). The Jumbo Dolomite has an average thickness of about 300 m (984 ft), but it can range as high as 600 m (1,969 ft) thick (Johnson and Tull, 2002).

Figure 10 shows the Shady Dolomite (and [or] Jumbo Dolomite) increasing in thickness southeastward from 0 ft in northwestern Bibb, Shelby, St. Clair, Etowah, and Cherokee Counties to greater than 1,000 ft (305 m) in Chilton, Coosa, Talladega, Clay, and Cleburne Counties northeast of the study area. We used thicknesses from 23-point locations in Alabama, Georgia, and Tennessee from 18 publications to locate the isopach lines on the figure 10 map (see fig. 10 caption for the list of point locations and source publications). Southwestward extensions of isopach lines in fig. 10 suggest that the Shady Dolomite may be present in parts of Bibb, Perry, Hale, and Greene Counties, but the precise locations of these isopach lines beneath the Cretaceous onlap is unknown (fig. 10). The Shady Dolomite has not been identified in drill holes shown on cross section N–N'. On cross section N–N′, we show the thickness of the Shady Dolomite ranging from 0 ft (about one mile [1.6 km] northeast of drill hole 206) to about 500 ft (152 m) at locations southwest of drill hole 205 based on the proximity of the 500-ft isopach to drill holes 201 through 205 in fig. 10.

Rome Formation

The lower Cambrian (Series 2) Rome Formation (figs. 2 and 3) lies conformably above the Shady Dolomite in the Birmingham graben and the downthrown block to its southwest. No drill holes reached the Rome Formation in place above the Chilhowee Group formations and Shady Dolomite, but a partial section was encountered in the Rock Creek School thrust sheet at 1,361 ft below kelley bushing (BKB) in drill hole 205. Thomas and others (2001) determined an age for the Rome Formation of between 520 and 510 Ma (Series 2) based on Strontium isotope ratios from anhydrite in the No. 1 John Goodson 9-7 well (drill hole 205 on cross section N–N′) and the No. 1 Alabama Property well in Shelby County, Alabama (point 17 in fig. 10). Kidd and Neathery (1976) indicate that the Rome Formation thins between northwestern Georgia and the Black Warrior basin in north-central Alabama, where it lies unconformably above Precambrian crystalline basement rocks; further to the northwest they show it pinching out entirely in the Interior Highland Rim province of Giles County in southern Tennessee. It consists of red to grayish-green shale that is laminated to thin bedded; interbedded with red, yellow, and brown siltstone and sandstone; light-gray calcareous sandstone; and minor amounts of limestone, dolomite, and anhydrite. The Rome Formation can be found in the VRPA, eastern Appalachian Plateau province, and Black Warrior basin in Alabama. The thickness of the Rome Formation ranges from 290 ft (88 m) in the Black Warrior basin and Appalachian Plateau provinces to a possible maximum thickness of 4,000 ft (1,219 m) in the VRPA.

On cross section N–N′, we show the Rome Formation (not including the mushwad part of the formation) with a thickness of about 1,050 ft (320 m) in the Birmingham graben and the downthrown block to its southwest, and the same thickness in the hanging wall above the Rock Creek School fault based on Osborne and others (1998).

The GSA sample description log for drill hole 205 describes 2,490 ft (759 m) of Rome Formation strata between depths of 150 and 2,640 ft (46 and 805 m) in drill hole 205, but the upper part of this unit is probably Conasauga Formation. Therefore, we placed an arbitrary contact between the Rome and Conasauga Formations at a depth of 1,590 ft (485 m), assigning the lower 1,050 ft (320 m) to the Rome Formation and the upper 1,440 ft (439 m) to the Conasauga Formation.

The Rome Formation was probably deposited in marine-shelf environments, and along with the underlying Chilhowee Group and Shady Dolomite, filled the Birmingham graben and the downthrown block to its southwest during the early Cambrian. Later, during the Alleghanian orogeny a basal decollement formed in the Rome and (or) Conasauga Formations and was involved in the deformation of the ductile shales that formed the contorted mass of shale that is the Bessemer mushwad in the northeastern half of the study area (Surles, 2007; Thomas and Pashin, 2011). At the northeastern end of cross section N–N′, we show the Bessemer mushwad between depths of 10,000 and 16,900 ft (3,048 and 5,151 m) based on cross section I of Thomas and Pashin (2011) where it intersects cross section N–N′. The Bessemer mushwad thins southwestward and is absent on cross section 20 of Surles (2007), which intersects cross section N–N′ at approximately mile 54 (fig. 6); we show the mushwad completely closing at mile 55.

The lower Cambrian (Series 2?) Fayetteville Phyllite of the Sylacauga Marble Group, which lies conformably above the Jumbo Dolomite in the Talladega slate belt in Shelby, Chilton, Coosa, Talladega, and Clay Counties, Alabama (fig. 9), is considered to be the lateral equivalent of the Rome Formation (Tull and others, 1988; Johnson and Tull, 2002), with a thickness of 80 to 290 m (262 to 951 ft) (Johnson and Tull, 2002).

Conasauga Formation

The middle to upper Cambrian (Series 2? to Furongian) Conasauga Formation (figs. 2 and 3) is generally thought to lie conformably above the Rome Formation, but as mentioned previously, Read and Repetski (2012) interpret an unconformity at the top of the Rome Formation. The Conasauga Formation consists of limestone, dolomite, and shale in varying amounts. In the western VRPA, Conasauga Formation limestone is bluish gray, fine grained, thin bedded, and partially argillaceous. In the eastern VRPA, the lower part of the formation is composed of shales and mudstones with limestone and rare siltstone interbeds. The shales and mudstones are dark greenish gray, dusky yellow, or pale olive, and the limestone interbeds are medium to dark gray, thin to medium bedded, micritic, argillaceous, and locally oolitic or oncolitic. The upper part of the formation at the northeastern end of the eastern VRPA (northeast of the study area) is composed of dolostone that is light to dark gray and medium to thick bedded that is laterally equivalent to the Brierfield, Ketona, and Bibb Dolomites at the southwestern end of the eastern VRPA. The upper part of the Conasauga may contain minor amounts of white, light-bluish-gray, or light-brownish-gray chert, that may be cryptocrystalline or oolitic. Locally, the Conasauga can have very fossiliferous intervals. The Conasauga Formation crops out in the VRPA and ranges from 500 ft (152 m) thick in Talladega County, Alabama, to about 2,600 ft (792 m) thick elsewhere. Thomas and Bayona (2005) estimated that the Conasauga Formation may have been up to 6,000 ft (1,829 m) thick in the Birmingham graben before orogenic uplift, erosion, and thrust faulting (Surles, 2007).

Over most of cross section N–N′, we show the Conasauga Formation with a thickness of about 1,500 ft (457 m) based on Butts (1940), but southwest of the Bibb-Scott thrust fault we show the Conasauga Formation with a thickness of greater than 2,020 ft (616 m) based on a partial, faulted section from the GSA sample description log for drill hole 202. This abnormally thick section of the Conasauga Formation shown in the hanging wall of the Bibb-Scott thrust fault is an unresolved problem. Perhaps some of the 2,020 ft (616 m) of the Conasauga Formation in drill hole 202 could be the lower part of the overlying Ketona Dolomite. If there was 1,500 ft (457 m) of Ketona Dolomite southwest of the fault as we show northeast of the fault (the 360 ft [110 m] of strata assigned to the Ketona Dolomite by the GSA sample description log, plus the upper 1,140 ft [347 m] of the Conasauga Formation), that would leave only 880 ft (268 m) of Conasauga in drill hole 202. On cross section N–N′, we show a dashed line and query (?) at the possible location of the contact of the Conasauga Formation and Ketona Dolomite with a 1,500 ft (457 m) thick Ketona Dolomite section as described above. Additional well and seismic data could help to better understand the configuration of thrust faults in this area.

As mentioned previously, we assigned the upper 1,440 ft of strata in the Rock Creek School thrust sheet in drill hole 205 to the Conasauga Formation above an arbitrarily placed contact with the underlying Rome Formation.

The Conasauga Formation was deposited in a marine-shelf environment. In the northeastern half of the study area, a basal decollement formed during the Alleghanian orogeny in the Rome and (or) Conasauga Formations and was involved in the formation of the Bessemer mushwad (Surles, 2007; Thomas and Pashin, 2011). We show the Bessemer mushwad at the northeastern end of cross section N–N′ below the Jones Valley thrust fault and above the basal decollement where it transitions into the upward ramping Blue Creek thrust fault. At the northeastern end of cross section N–N′ where it intersects cross section I of Thomas and Pashin (2011), the Bessemer mushwad is present between depths of about 10,000 and 16,900 ft (3,048 and 5,151 m). To the southwest, the mushwad terminates before mile 54 of cross section N–N′ and cross section 20 of Surles (2007). Another mushwad associated with this decollement may be present at depth at the southwestern end of cross section N–N′ as described earlier in the section “Thin-Skinned Structures and Mushwads.” Our inclusion of this mushwad is presented here only as a possible structural feature and should not be taken as demonstrable interpretation at this time.

The middle to upper Cambrian (Series 2? to Furongian) Shelvin Rock Church Formation of the Sylacauga Marble Group (lower greenschist facies), which lies conformably above the Fayetteville Phyllite in the Talladega slate belt east and north of the study area in Shelby, Chilton, Coosa, Talladega, and Clay Counties, Alabama, (fig. 9) is considered to be the lateral equivalent of the Conasauga Formation, and ranges from 350 to 1,000 m (1,148 to 3,280 ft) thick (Tull and others, 1988; Johnson and Tull, 2002).

Brierfield, Ketona, and Bibb Dolomites

At the southwestern end of the western VRPA, the Conasauga Formation (figs. 2 and 3) is generally thought to be conformably overlain by the upper Cambrian (Dresbachian to Franconian) Ketona Dolomite, which is laterally equivalent to (in ascending order) the Brierfield, Ketona, and Bibb Dolomites of the southwestern end of the eastern VRPA, but, as mentioned previously, Read and Repetski (2012) interpret an unconformity at the top of the Conasauga Formation. The Brierfield, Ketona, and Bibb Dolomites are in turn laterally equivalent to the upper Cambrian (Dresbachian to Franconian) part of the Conasauga Formation at the northeastern end of the eastern VRPA. Sternbach and Friedman (1984) suggest that the Brierfield, Ketona, and Bibb Dolomites are merely laterally equivalent facies of the upper Conasauga Formation rather than separate formations.

Knox Group

The upper Cambrian (Franconian to Trempealeauan) to Lower Ordovician (Ibexian [Canadian]) Knox Group lies conformably above the Ketona and Bibb Dolomites in the western and eastern parts of the VRPA, respectively. In ascending order, the Knox Group includes the following formations: the Copper Ridge Dolomite, the Chepultepec Dolomite, the Longview Limestone, and the Newala Limestone. These four formations are mappable in much of the VRPA, but it is difficult to identify them in the western VRPA, thus, the term “Knox Group undivided” is often employed there. As previously described, these Knox Group formations along with the underlying Brierfield, Ketona, and Bibb Dolomites comprise the “stiff” competent rocks that were thrust over the underlying contorted, ductile, Cambrian shales of the Bessemer mushwad. Drill hole 202 is the only well on cross section N–N′ that penetrates each of the formations comprising the Knox Group strata, and total Knox Group thickness in drill hole 202 is 5,160 ft (1,573 m).

Post-Knox Unconformity

As previously described, the regionally extensive post-Knox unconformity is shown above the Newala Limestone in figure 3 and extends as deep as the Chepultepec Dolomite in the northeastern half of cross section N–N' below drill holes 205 through 208. The unconformity spans from the end of the Early Ordovician (Ibexian [Canadian]) through the Middle Ordovician (Whiterockian and Chazyan), and possibly as late as the Late Ordovician (Blackriveran) at the southwestern end of the VRPA (Raymond and others, 1988; Raymond, 1991a) (fig. 3). The amount of erosion was greatest at the southwestern end of the western VRPA with truncation of up to hundreds of feet of the Newala Limestone, Longview Limestone, and Chepultepec Dolomite (Raymond, 1991a). Paleokarst depressions in the Newala, Longview, and Chepultepec are locally filled with the overlying Attalla Chert Conglomerate Member at the base of the Chickamauga Limestone (Raymond, 1991a). Dwyer and Repetski (2012) describe extensive secondary porosity development (including solution pipes), solution-collapse breccias, and erosional truncation of dolomites below the post-Knox unconformity. Fritz and others (2012) describe “paleokarst collapse breccias occurring 3,000 ft (914 m) below the top of the Knox,” suggesting that some breccias may have formed along other unconformities within the Knox Group.

Unfortunately, the post-Knox unconformity is not easily recognized in the subsurface where the Attalla Chert Conglomerate Member of the Chickamauga Limestone is missing, thin, or without a significant thickness of erosional residuum that is detectable on wireline logs. The apparent absence of the Attalla Chert Conglomerate Member in the drill holes of N–N′ may indicate that the period of subaerial erosion was too short to create a detectable unconformity using conventional wireline log tools. Well logs examined for this study displayed no clearly identifiable interval or erosional surface, and the GSA sample description logs for drill holes 202, 204, and 205 showed no obvious lithologic markers to indicate the presence of an unconformity.

The post-Knox unconformity has been explained by some authors as the result of the collision of a volcanic arc with the east coast of Laurentia (Mussman, 1982; Read and Repetski, 2012), but Hatcher and Repetski (2007) propose that it may reflect a global sea-level drop that took place during the Middle Ordovician (Whiterockian). As previously mentioned in the Structural Framework section of this report, Bayona and Thomas (2003) describe inversion of the Birmingham graben boundary faults resulting in subaerial exposure and erosion of Knox Group carbonates during the Middle Ordovician Taconic orogeny.

A regional unconformity overlies the Gantts Quarry Formation of the Sylacauga Marble Group, and erodes as deep as the Kahatchee Mountain Group in the Talladega slate belt east and north of the study area in Shelby, Chilton, Coosa, Talladega, and Clay Counties, Alabama (fig. 9). It is considered to be the lateral equivalent of the post-Knox unconformity (Tull and others, 1988; Johnson and Tull, 2002).

Chickamauga Limestone

In the western VRPA, the Middle to Upper Ordovician (Chazyan to Shermanian) Chickamauga Limestone (figs. 2 and 3) lies unconformably on top of the Knox Group which was truncated by the post-Knox unconformity as previously described (Raymond and others, 1988; Raymond, 1991a). The Chickamauga Limestone is composed of medium- to dark-gray limestone that is thick to thin bedded, sometimes argillaceous or silty, micritic to medium grained, with locally abundant fossil horizons and thin dolomite beds near the top and bottom of the unit. It crops out in the western VRPA, and its thickness ranges from 260 to 1,100 ft (79 to 335 m). On cross section N–N′, we show the Chickamauga Limestone with measured thicknesses (from GSA sample description logs and [or] gamma-ray logs) of 443, 476, 557, and 1,040 ft (135, 145, 170, and 317 m) in drill holes 201, 202, 204, and 205, respectively. The GSA sample description log for drill hole 205 shows the Chickamauga Limestone lying directly above the Chepultepec Dolomite, without Longview or Newala Limestones intervening. For drill hole 208, we interpret the limestone shown on the mud log at depths between 6,520 and 7,170 ft (1,987 and 2,585 m) as the Chickamauga Limestone, but it is possible that the lower part includes Newala and Longview Limestones because they are difficult to differentiate. At the northeastern end of the cross section, we show the Chickamauga Limestone with an estimated thickness of about 600 ft (183 m). Locally, at the base of the Chickamauga Limestone, the Attalla Chert Conglomerate Member fills paleokarst depressions in the Knox Group as previously described (Raymond and others, 1988; Raymond, 1991a). The Attalla member is a conglomerate composed of subangular to rounded pebbles, cobbles, and boulders of chert and occasionally dolomite or quartzite, within a matrix of chert, quartz sand, and clay. It sometimes grades into a medium- to coarse-grained sandstone and may feature greenish-gray and red shale lenses or beds at its base. The thickness of the Attalla Chert Conglomerate Member ranges from 0 to 110 ft (0 to 34 m) in the western VRPA.

Henderson (1991) discussed the “Snow zone,” a zone of high porosity in the Middle to Upper Ordovician Stones River Group (laterally equivalent to the Chickamauga Limestone) that was associated with production of a small amount of oil and gas from the Magnolia Petroleum No. A-1 J.B. Snow well in Monroe County, Mississippi (fig. 10). The Anderman-Smith No. 4 Gilmer well (currently named Gilmer 17-12 No. 1) in Lamar County, Alabama, also produced a small amount of oil and gas from “Snow zone” strata (Henderson, 1991). Henderson (1991) interprets the “Snow zone” as a zone of brecciation created as the result of a widespread regression and karstification event in the early Middle Ordovician. Similar brecciated dolomite zones may be recognizable in density porosity logs from the Chickamauga Limestone in drill holes 202, 204, and 205, but lacking detailed correlations, we cannot be certain they are equivalent to the “Snow zone” of northeastern Mississippi. Fritz and others (2012) state that a “lack of markers makes it difficult to distinguish the upper Knox dolomites from the Snow dolomite” [lower Stones River Group in the Black Warrior basin, which is equivalent to the Chickamauga Limestone].

In drill hole 205 at a depth of 8,767 ft (2,672 m), a gamma-ray spike exceeding 100 API (American Petroleum Institute) units may indicate the presence of the T-4 bentonite, which is identified by the GSA sample description log. A similar gamma-ray spike is found in drill hole 208 at a depth of 6,978 ft (2,127 m). The T-4 bentonite (also known as the Millbrig or Mud Cave bentonite) has been used to correlate Upper Ordovician strata across much of North America (Huff and Kolata, 1990; Haynes, 1994; Bergstrom and others, 2004). Using graptolite zones, the Millbrig bentonite has been placed in the earliest Chatfieldian (Rocklandian) Stage (Huff and Kolata, 1990; Haynes, 1994; Bergstrom and others, 2004). Unfortunately, the mud log for drill hole 208 does not describe bentonitic clays within the limestone unit.

Lenoir Limestone

In the eastern VRPA, the Middle Ordovician (Whiterockian to Chazyan) Lenoir Limestone (figs. 2 and 3), a lateral equivalent of the Chickamauga Limestone, lies unconformably on top of the Knox Group, which was truncated by the post-Knox unconformity as previously described (Raymond and others, 1988; Raymond, 1991a). It is composed of dark-gray limestone that is very argillaceous, medium to thick bedded, evenly bedded, finely crystalline, and fossiliferous, with thin irregular clay partings and fine-grained impurities that remain as anastomosing bands after weathering. Locally in Alabama, at the base of the Lenoir Limestone is the Middle Ordovician (Whiterockian) Mosheim Limestone Member (not shown on cross section N–N'), which is a medium- to dark-gray lime mudstone that is thick bedded to massive with abundant fenestrae. Its thickness ranges from 0 to more than 200 ft (61 m) in the eastern VRPA. Below the Mosheim Member the post-Knox unconformity spans only about 10 million years (Whiterockian) (Raymond, 1991a). On cross section N–N', we show the Lenoir Limestone above the Newala Limestone in the Helena thrust sheet at the surface between miles 54 and 55 of the cross section based on its presence on the geologic map of Alabama (Szabo and others, 1988) (fig. 2). We interpret that the Helena thrust sheet transported the Lenoir Limestone and several other formations into the study area from the southeast in the eastern VRPA. The stratigraphic position of the Lenoir Limestone is shown in figure 3.

Athens Shale

The Middle to Upper Ordovician (Chazyan to Porterfieldian) Athens Shale overlies the Lenoir Limestone (figs. 2 and 3). It is a black, graptolitic, fissile shale that contains dark-gray to black limestone interbeds in its lower part. The Athens Shale ranges from 0 to 300 ft (0 to 91 m) thick in outcrop and is found mainly at the southwestern end of the eastern VRPA. Based on the geologic map of Alabama (Szabo and others, 1988) we show the Athens Shale above the Lenoir Limestone in the Helena thrust sheet at the surface between miles 54 and 55 of cross section N–N′ (fig. 2). We interpret that the Helena thrust sheet transported the Athens Shale and several other formations such as the underlying Lenoir Limestone into the study area from the southeast in the eastern VRPA. The stratigraphic position of the Athens Shale is shown in figure 3.

Sequatchie Formation

At the southwestern end of the western VRPA (our study area location), the Upper Ordovician (Edenian) Sequatchie Formation (fig. 3) conformably overlies the Chickamauga Limestone. It is composed of reddish-gray, green, and yellow mudstone that is calcareous and laminated with limestone that is medium to coarsely crystalline and fossiliferous. In places, it is glauconitic, arenaceous, and silty, and may include yellowish-gray and greenish-brown shale that is thinly bedded and calcareous, or may include argillaceous limestone. Thickness ranges from 0 to 200 ft (61 m) in the western VRPA. GSA sample description logs for drill holes 202 and 204 describe shales above the Chickamauga Limestone that are 560 and 390 ft (171 and 119 m) thick, respectively, that may be of Late Ordovician age, but GSA sample description logs do not identify these strata as the Sequatchie Formation. The GSA sample description log for drill hole 205 shows no Ordovician strata above the Chickamauga Limestone. We do not show the Sequatchie Formation on cross section N–N', even though it is known to exist in the western VRPA. It is possible that the lower part of the overlying Silurian Red Mountain Formation in drill hole 205 may include the Sequatchie Formation, but the GSA did not identify it as such. We have followed Pashin and others (2011) who consider these strata to be Silurian, and do not include the Sequatchie Formation in our cross section.

Red Mountain Formation

At the southwestern end of the VRPA, the regional Cherokee unconformity at the Ordovician-Silurian boundary (Swezey, 2002) above the Sequatchie Formation is overlain by the Silurian (Llandovery, Wenlock, and Pridoli [with a Ludlow hiatus]) Red Mountain Formation (Ferrill, 1984; Berdan and others, 1986; Chowns and Rindsberg, 2015) (fig. 3). The Red Mountain Formation contains six members that include four iron-ore beds that were important to the steel industry of the late 19th to middle 20th century in Bessemer and Birmingham, Alabama (Chowns and Rindsberg, 2015). The formation consists of interbedded sandstones, shales, and limestones, some containing large amounts of hematite (Chowns and Rindsberg, 2015). Note that the two upper members of the Red Mountain Formation (the Rocky Row and Sparks Gap Members) were originally mapped as the lower part of the overlying Devonian Frog Mountain Sandstone but were reassigned to the Red Mountain Formation based on paleontology (Ferrill, 1984; Berdan and others, 1986). The Sparks Gap Member is mainly limestone and thickens to about 98.4 ft (30 m) at the southwestern end of the VRPA.

The GSA describes “Silurian undifferentiated” strata in their sample description logs for drill holes 202 and 204. The GSA sample description log for drill hole 202 shows the top of the “Silurian undifferentiated” strata at a depth of 6,330 ft (1,929 m), whereas Pashin and others (2011) show it at 6,577 ft (2,005 m) (see cross section B–B′ on plate 1 of Pashin and others, 2011). On cross section N–N′ in drill hole 202, we follow Pashin and others (2011) and place the top of the Red Mountain Formation at 6,577 ft (2,005 m). In drill hole 204, the GSA sample description log shows the top of “Silurian undifferentiated” strata at 10,359 ft (3,157 m), whereas Pashin and others (2011) show it at 10,300 ft (3,139 m). On cross section N–N′ in drill hole 204, we follow the GSA sample description log and place the top of the Red Mountain Formation at 10,359 ft (3,157 m).

The GSA sample description log identifies Silurian strata in drill hole 205 as the “Red Mountain Formation” with its top at 8,070 ft (2,460 m) (with intentional use of quotation marks), which probably indicates that the limestone-dominated lithology is considered unusual for the Red Mountain Formation, or possibly indicates that the unit may include the underlying Sequatchie Formation and the overlying Frog Mountain Sandstone. Pashin and others (2011) show the Red Mountain Formation as limestone and dolomite in drill hole 205 with its top at about 8,050 ft (2,454 m) (see cross section B–B′ on plate 1 of Pashin and others, 2011). On cross section N–N′ in drill hole 205, we follow the GSA sample description log and place the top of the Red Mountain Formation at 8,070 ft (2,460 m).

In drill hole 208, Pashin and others (2011) show the Red Mountain Formation as carbonate strata above and below a middle shaley unit. On cross section N–N′ in drill hole 208, we follow Pashin and others (2011) with the 202 ft (62 m) of Red Mountain Formation between 6,318 and 6,520 ft (1,926 to 1,987 m). Tim Chowns (University of West Georgia, written communication, 2022) measured 312.3 ft (95 m) of Red Mountain Formation in a roadside outcrop at Tannehill State Park, Alabama, about 5 miles (8 km) northwest of the northeastern end of cross section N–N′.

The Red Mountain Formation was deposited in shallow-marine shelf and shoreface environments (Chowns and Rindsberg, 2015). According to Raymond and others (1988), thickening of the unit to the northeast appears to indicate that the source of the clastic sediments was the same Taconic highlands that sourced the Ordovician clastics. However, thickening of Silurian sediments in the Greene-Hale-Perry-Bibb depositional margin should not be taken as an indication of a sediment source to the southwest. On cross section N–N′ in drill holes 202 and 204, we show the Red Mountain Formation with thicknesses of 447 and 614 ft (138 and 189 m), respectively. Formation tops for drill holes 202 and 204 are based on Pashin and others (2011) and GSA sample description logs, respectively. Formation bottoms for both drill holes are based on gamma-ray logs. Pashin and others (2011) depict the Red Mountain Formation as having thicknesses of about 450 and 700 ft (137 and 213 m) in drill holes 202 and 204, respectively, in the Greene-Hale-Perry-Bibb depositional margin. For drill hole 205 to the northeast, we identify 230 ft (70 m) of the Red Mountain Formation based on the GSA sample description log while Pashin and others (2011) show only about 130 ft. Pashin and others (2011) show the presence of up to six radioactive shale units in drill holes 202 through 204 (labeled and shown as green horizontal lines to the right of gamma-ray curves for these drill holes on cross section N–N'). Tim Chowns (University of West Georgia, written communication, 2022), Ben Byerly (Geological Survey of Alabama, written communication, 2022), and the authors of this report doubt that there are black shales in the Red Mountain Formation in this part of Alabama. The intervals of the Red Mountain Formation as observed on outcrop in Alabama and Georgia were probably not deposited in environments conducive for anoxic black shale deposition. Perhaps the contact between the Red Mountain Formation and the overlying Frog Mountain Sandstone should be placed below the lowest black shales. However, another possibility is that the depositional environment of the Red Mountain Formation was more anoxic along the Greene-Hale-Perry-Bibb depositional margin than in locations to the north and west where these black shales are not found in the formation. More information about potential oil and gas in black shales of the Greene-Hale-Perry-Bibb depositional margin is contained in the last paragraph in the Frog Mountain Sandstone section of this report.

Frog Mountain Sandstone (Alone and Included in Undivided Devonian Formations)

The Lower to Middle Devonian (Ulsterian to Erian) Frog Mountain Sandstone (fig. 3) was deposited on an unnamed upper Silurian (Cayugan) unconformity above the Silurian and Ordovician rocks previously described in the VRPA. It is composed of gray to brown weathered sandstone that is fine to coarse grained, argillaceous, and medium bedded; gray subarkosic sandstone that is medium to coarse grained, silica-cemented, medium bedded, and cherty, with rare fossiliferous pods; and gray chert (Berdan and others, 1986). Note that the lower part of the Frog Mountain Formation as originally defined was reassigned to the Red Mountain Formation based on paleontology (Ferrill, 1984; Berdan and others, 1986). The Frog Mountain Formation is present in the VRPA and its thickness ranges from 0 to 213 ft (0 to 65 m) (Raymond and others, 1988). Tim Chowns (University of West Georgia, written communication, 2022) measured 12.9 ft (3.9 m) of Armuchee Chert in a roadside outcrop at Tannehill State Park, Alabama, about 5 miles (8 km) northwest of the northeastern end of cross section N–N′. In drill hole 205, the GSA sample description log identifies 60 ft (18 m) of argillaceous cherty limestone between 8,010 and 8,070 ft (2,442 and 2,460 m) as the Frog Mountain Sandstone. From drill hole 201 to mile 44 on cross section N–N′ and in figure 3, we show the Frog Mountain Sandstone and the Chattanooga Shale together in a unit called “undivided Devonian formations” ranging from a maximum of 1,535 ft (468 m) in drill hole 202 to a minimum of 200 ft (61 m) at mile 44. Northeast of mile 44 on cross section N–N′, we show the Frog Mountain Sandstone and the Chattanooga Shale as individual units separated by an unconformity. Unlike the Ordovician and Silurian sediments previously discussed, the clastics of the Frog Mountain Sandstone probably came from a source southeast of the study area, which may indicate a shift in the location of the orogenic highlands southward during the Acadian orogeny.

Pashin and others (2011) show the Chattanooga Shale and Frog Mountain Sandstone in drill hole 208 with thicknesses of about 10 and 20 ft (3.0 and 6.1 m), respectively (see cross section B–B' on plate 1 of Pashin and others, 2011). In drill hole 205, the Chattanooga Shale and Frog Mountain Sandstone are about 40 and 60 ft (12.2 and 18.3 m) thick, respectively (see cross section B–B' on plate 1 of Pashin and others, 2011). Southwest of drill hole 205, Pashin and others (2011) show these two formations greatly increasing in thickness as a single undivided unit of Devonian strata in the Greene-Hale-Perry-Bibb depositional margin ranging between about 1,150 and 1,550 ft (351 and 472 m) thick in drill holes 204 and 202, respectively (on cross section N–N′, we show them as 1,159 and 1,535 ft, based on gamma-ray logs). It is not clear on the Pashin and others (2011) cross section which parts of this undivided unit are equivalent to the Chattanooga Shale and the Frog Mountain Sandstone. In drill hole 204, the lower one-third of the unit, which is mostly gray shale and limestone, may be equivalent to the Frog Mountain Sandstone, the middle black-shale-rich one-third may be equivalent to the Chattanooga Shale, and the upper gray-shale-rich one-third may be an unnamed overlying formation that is not present in drill hole 205 (see cross section B–B' on plate 1 of Pashin and others, 2011). In drill holes 202 and 203, the lower one-third contains black shale as well as gray shale and limestone. An alternative interpretation is that black shale units lying immediately below the Fort Payne Chert in drill holes 202, 203, and 204 (20, 28, and about 10 ft [6.1, 8.5, and 3.0 m] thick, respectively) are Chattanooga Shale, and undivided Devonian strata that lie below are partially to wholly equivalent to the Frog Mountain Sandstone. Up to 24 radioactive shale units are present in Devonian strata in drill holes 202 through 204. On cross section N–N', we show these radioactive shales as green horizontal lines to the right of the gamma-ray curves for these drill holes.

Black shales in the Silurian and Devonian strata of the Greene-Hale-Perry-Bibb depositional margin have been of interest as potential natural gas exploration targets (Pashin and others, 2011). Multiple gas shows were detected in the Bayne-Etheridge 36-9 No. 1 well (drill hole 203 on cross section N–N') in undivided Silurian and Devonian formations at depths between 8,400 ft (2,560 m) and the top of the Chickamauga Limestone at 9,345 ft (2,848 m) (Pashin and others, 2011). In addition to drill hole 203, completions in this interval were attempted in the nearby Caldwell 19-15 No. 1 well (drilled horizontally in Greene County) and the Krout 10-14 No. 1 well in Bibb County (Ben Byerly, Geological Survey of Alabama, written communication, 2022) (fig. 1). Approximate isopach thicknesses of undivided Silurian-Devonian strata in the depositional margin are shown in figure 1 (based on Ben Byerly, Geological Survey of Alabama, written communication, 2022). The depth of burial of the sediments within the depositional margin seems favorable for maturation of kerogen and shale gas generation, but uneconomic production results between 2006 and 2011 discouraged continued exploration (Ben Byerly, Geological Survey of Alabama, written communication, 2022).

In drill hole 208, we identify 30 ft (9 m) of strata from 6,288 to 6,318 ft (1,917 to 1,926 m) as “undivided Devonian formations” following Pashin and others (2011). The Frog Mountain Sandstone is shown as very thin to negligible in geologic maps of the Woodstock and McCalla, Alabama, quadrangles to the southeast and northwest of drill hole 208 (Irvin and Osborne, 2000, and Osborne and others, 2006, respectively). In drill hole 205, we picked formation tops of the Chattanooga Shale and Frog Mountain Sandstone from the gamma-ray log at 7,940 and 7,980 ft (2,420 and 2,432 m), respectively. For more information about this topic, see the discussions in the Red Mountain Formation and Chattanooga Shale sections of this report.

Chattanooga Shale (Alone and Included in Undivided Devonian Formations)

The Middle to Upper Devonian (Chautauquan) Acadian unconformity of Swezey (2002) separates the Frog Mountain Sandstone from the overlying Upper Devonian (Chautauquan) Chattanooga Shale (fig. 3) in the VRPA. The Chattanooga Shale is composed of brownish- to grayish-black shale that is silty and organic rich, with light- to dark-gray, fine-grained sandstone containing pyrite and phosphatic inclusions. Gamma-ray response is typically greater than 150 API (Ben Byerly, Geological Survey of Alabama, written communication, 2022). Its thickness ranges from 0 to 82 ft (0 to 25 m) in Alabama (Raymond and others, 1988). Tim Chowns (University of West Georgia, written communication, 2022) measured 22.6 ft (6.9 m) of Chattanooga Shale in a roadside outcrop at Tannehill State Park, Alabama, about 5 miles (8 km) northwest of the northeastern end of cross section N–N′. As mentioned previously, from drill hole 201 to mile 44 on cross section N–N′ and in figure 3, we show the Frog Mountain Sandstone and the Chattanooga Shale together in a unit called “undivided Devonian formations” ranging from a maximum of 1,535 ft (468 m) in drill hole 202 to a minimum of 200 ft (61 m) at mile 44. Northeast of mile 44 on cross section N–N′ we show the Frog Mountain Sandstone and the Chattanooga Shale as two units separated by an unconformity. The GSA sample description log for drill hole 205 identifies 70 ft (21 m) of shale-dominated rocks at depths from 7,940 to 8,010 ft (2,420 to 2,442 m) as the Chattanooga Shale, however, wireline logs for drill hole 205 indicate the hot gamma-ray signature of the Chattanooga Shale between 7,940 and 7,980 ft (2,420 to 2,432 m).

For drill hole 208, we identify 30 ft (9 m) of strata between depths of 6,288 and 6,318 ft (1,917 and 1,926 m) as “undivided Devonian formations” per Pashin and others (2011) (see cross section B–B′ on plate 1 of Pashin and others, 2011). The top of the undivided Devonian unit in drill hole 208 was placed at a gamma-ray spike that is greater than 150 API at 6,288 ft (1,917 m) that was interpreted by Pashin and others (2011) to be the Chattanooga Shale. We follow this interpretation of Pashin and others (2011) identifying the top of the Chattanooga Shale. Argillaceous cherty limestones in the interval below the Chattanooga Shale are common in the Black Warrior basin; chert content (and porosity) in this interval increases sharply westward into Mississippi where the interval is referred to as “tripolitic chert” (Ben Byerly, Geological Survey of Alabama, written communication, 2022). The Chattanooga Shale is shown as very thin to negligible in geologic maps of the Woodstock and McCalla, Alabama, quadrangles to the southeast and northwest of drill hole 208 (Irvin and Osborne, 2000, and Osborne and others, 2006, respectively).

Wireline logs from the Krout 10-14 No. 1 well (1 mile [1.6 km] southeast of drill hole 205) (fig. 1) indicate the shallowest radioactive Chattanooga Shale between 6,574 and 6,605 ft (2,004 and 2,013 m) (Ben Byerly, Geological Survey of Alabama, written communication, 2022). Slickensides within this interval are identified on the mud log suggesting the possibility of a detachment surface (Ben Byerly, Geological Survey of Alabama, written communication, 2022). Although the potential repetition of the Frog Mountain Sandstone and Chattanooga Shale intervals in imbricate thrust sheets and structural thickening cannot be precluded, the similar thickness of the undivided Devonian formations in the Krout 10-14 No. 1 well and drill holes 201 through 204 is apparent (Ben Byerly, Geological Survey of Alabama, written communication, 2022). The Silurian-Devonian Greene-Hale-Perry-Bibb depositional margin parallels the grain of the Appalachian fold and thrust belt with thickening to the south and southeast (fig. 1) (Ben Byerly, Geological Survey of Alabama, written communication, 2022).

The Chattanooga Shale was deposited in a euxinic basin environment in a cratonic extension of the Acadian foreland basin (Pashin and others, 2011). See the discussions in the Red Mountain Formation and Frog Mountain Sandstone sections of this report for more information about the stratigraphy of the Red Mountain Formation, Frog Mountain Sandstone, and Chattanooga Shale, and the potential for oil and gas in black shales in the Greene-Hale-Perry-Bibb depositional margin.

Maury Shale

The Lower Mississippian (middle Kinderhookian) Maury Shale was deposited on an unnamed Lower Mississippian (lower Kinderhookian) unconformity above the Chattanooga Shale (fig. 3) at the southwestern end of the western VRPA (our study area location). The Maury Shale is composed of greenish-gray to grayish-red thinly laminated shale containing phosphate nodules. It is only 0 to 7 ft (0 to 2 m) thick. The Maury Shale was not recognized in the GSA sample description logs for drill holes 202, 204, and 205, therefore, we do not include it on cross section N–N' or in the figure 3 correlation chart.

Fort Payne Chert and Tuscumbia Limestone

Please note that Raymond and others (1988) show the Mississippian subdivided into a lower and upper series, whereas Cohen and others (2013) recognize a lower, middle, and upper series with the Kinderhookian and lower Osagean in the Lower Mississippian, the upper Osagean, Meramecian, and lower Chesterian in the Middle Mississippian, and the middle to upper Chesterian in the Upper Mississippian. This report follows the practice of the Cohen and others (2013) showing the Lower, Middle, and Upper Mississippian series.

The Lower to Middle Mississippian (Osagean) Fort Payne Chert (figs. 2 and 3) was deposited on an unnamed Lower Mississippian (upper Kinderhookian) unconformity above the Maury Shale at the southwestern end of the western VRPA (our study area location) (fig. 3). The Fort Payne Chert is composed of dark- to light-gray limestone that is finely crystalline to microcrystalline dolomitic and siliceous, with irregularly bedded to nodular smoky chert, dark shale, and light-gray limestone that is coarse, bioclastic, and lenticular. It ranges from 0 to 207 ft (0 to 63 m) thick in the VRPA. The Fort Payne Chert was deposited in a shallow-marine shelf environment during an orogenic lull between the Acadian and Alleghanian orogenies.

The overlying Middle Mississippian (lower Meramecian) Tuscumbia Limestone (figs. 2 and 3) lies conformably above the Fort Payne Chert and is composed of light-gray limestone that is bioclastic, micritic, or oolitic, in individual beds that are generally thicker than 1 ft (0.3 m), sometimes up to 10 ft (3 m) thick and massively crossbedded, very coarsely bioclastic (crinoidal), and sometimes containing light-gray and white chert nodules or concentrically banded “concretionary” chert nodules. It ranges from 0 to 250 ft (0 to 76 m) thick in the VRPA. Like the Fort Payne Chert, the Tuscumbia Limestone was also deposited in a shallow-marine shelf environment during an orogenic lull between the Acadian and Alleghanian orogenies.

The Fort Payne Chert and Tuscumbia Limestone are often shown together as one unit with the name “Fort Payne Chert and Tuscumbia Limestone, undivided.” Well logs in drill hole 202 reveal the top of the Fort Payne Chert and Tuscumbia Limestone undivided at 4,849 ft (1,478 m) and the base at 5,042 ft (1,537 m). The GSA sample description log for drill hole 202 incorrectly does not recognize the Fort Payne or Tuscumbia, showing the Mississippian Floyd Shale lying directly above a unit identified as “Devonian undifferentiated” at 4,740 ft (1,445 m). On cross section N–N′, we show the “Fort Payne Chert and Tuscumbia Limestone, undivided” unit varying from a minimum thickness of 307 ft (94 m) in drill hole 201 to a maximum thickness of 193 ft (59 m) in drill hole 202. In the gamma-ray log for drill hole 208, we identify 183 ft (56 m) of strata between depths 6,105 and 6,288 ft (1,861 and 1,917 m) as “Fort Payne Chert and Tuscumbia Limestone, undivided.” As mentioned previously, a large gamma-ray spike at 6,288 ft (1,917 m) records the presence of the underlying Chattanooga Shale as interpreted by Pashin and others (2011).

Floyd Shale and Lateral Equivalents

At the southwestern end of the VRPA, the upper Middle to Upper Mississippian (upper Meramecian to middle Chesterian) Floyd Shale (figs. 2 and 3) lies conformably above the Tuscumbia Limestone. It is composed of dark-gray clay shale with siderite nodules interbedded with gray claystone, sandstone, limestone, and chert. It ranges from 0 to about 2,000 ft (607 m) thick. On cross section N–N′, we show the Floyd Shale varying from a maximum complete thickness of 943 ft (287 m) in drill hole 201 to a minimum complete thickness of 485 ft (148 m) in drill hole 208. The Floyd Shale thickens to the southwest, which is the apparent source of sediments, perhaps orogenic highlands associated with the Ouachita thrust belt (Raymond and others, 1988; Surles, 2007). It has been interpreted as northeastward prograding delta front, prodelta, and marine or barrier-bar sediments (Thomas, 1972; Raymond and others, 1988). However, Pashin and others (2011) (see their fig. 5) show an alternative interpretation for the Floyd Shale source area. They suggest two sources for the Floyd Shale and equivalent formation clastics that include (1) from the southwest in Mississippi and (2) from the northeast in Alabama. The Floyd Shale marks the end of the shallow-shelf environment that existed when the Fort Payne Chert and Tuscumbia Limestone were deposited. The Floyd Shale and its black shale facies, the Neal shale (not shown on cross section N–N′), are of interest for their potential as organic-rich shale sources for natural gas production (Pashin and others, 2011).

The Floyd Shale is present throughout the study area, but several Upper Mississippian formations that are laterally equivalent to the Floyd Shale exist to the northeast such as (1) the Middle to Upper Mississippian (Meramecian to Chesterian) Pride Mountain Formation, (2) the Upper Mississippian (Chesterian) Hartselle Sandstone, (3) the Middle to Upper Mississippian (Meramecian to Chesterian) Monteagle Limestone, and (4) the Upper Mississippian (Chesterian) Bangor Limestone (Thomas, 1972; Raymond and others, 1988; Pashin and others, 2011). We do not show any of the lateral equivalents of the Floyd Shale on cross section N–N' because they are not present at the southwestern end of the VRPA.

Parkwood Formation and Pennington Formation

At the southwestern end of the VRPA, the Upper Mississippian to Lower Pennsylvanian (Chesterian to lower Morrowan) Parkwood Formation (figs. 2 and 3) lies conformably on top of the Floyd Shale (Raymond and others, 1988). It is composed of medium- to dark-gray silty clay shale and mudstone interbedded with light- to medium-gray, very fine to fine grained, argillaceous, micaceous, crossbedded, and ripple-marked sandstone, containing siderite nodules, cherty limestone beds, red and grayish-green mudstone, and clayey coal. Thickness ranges from 0 to greater than 2,500 ft (762 m). On cross section N–N′, we show the Parkwood Formation varying from a minimum complete thickness of 510 ft (155 m) in drill hole 208 to a maximum complete thickness of 950 ft (290 m) in drill hole 204. As with the Floyd Shale, the Parkwood Formation thickens to the southwest, which is its apparent sediment source, possibly orogenic highlands associated with the Ouachita thrust belt (Thomas, 1972; Raymond and others, 1988; Surles, 2007). An alternate interpretation suggests that sandstone units in the Parkwood Formation (for example, the Carter sandstone in Alabama) were derived from the northwest (Bearden and Mancini, 1985). Also, Kugler and Pashin (1992) discuss alternate interpretations for the source of the Carter sandstone and other sandstones of the Parkwood Formation. The Parkwood Formation has been interpreted as being deposited in shallow marine bay and distributary front environments (Thomas, 1972).

At the northeastern end of the western VRPA, the Parkwood Formation grades into the laterally equivalent Upper Mississippian (Chesterian) Pennington Formation. We do not show the Pennington Formation on cross section N–N' because it is absent at the southwestern end of the VRPA.

Pottsville Formation

The youngest Paleozoic formation in the study area is the Lower to Middle Pennsylvanian (Morrowan to Atokan; Wanless, 1975) Pottsville Formation (figs. 2 and 3) that conformably overlies the Parkwood and Pennington Formations. The Pottsville Formation is composed of interbedded sandstone, conglomerate, siltstone, shale, and coal. The upper section has the most coal beds; clastic sediments between coal beds exhibit cyclic coarsening upward sequences that start with basal, bioturbated shales containing marine fossils overlain by siltstones and fine-grained sandstones. Total thickness ranges from 0 to more than 9,000 ft (2,743 m). On cross section N–N′, we show the Pottsville Formation varying from maximum partial thickness of 7,605 ft (2,318 m) in drill hole 206 to a minimum partial thickness of about 665 ft (203 m) in drill hole 201 (note that no complete sections of the Pottsville Formation are present along the cross section). At the southwestern end of the VRPA, the source of Pottsville Formation sediments was to the southwest as was the case with the underlying Parkwood Formation. In the Cahaba and Coosa coal fields (fig. 1, index map 2), the Pottsville Formation includes several thick-bedded, conglomeratic, quartzose sandstone members (for example, the basal Shades Sandstone, middle Pine Sandstone, and upper Chestnut Sandstone Members). The Shades Sandstone Member is a thick-bedded, coarse-grained, quartzose sandstone that is conglomeratic at its base. It is present at the base of the Pottsville Formation averaging about 200 ft (61 m) in the Coosa coal field and ranging from 190 to 500 ft (58 to 152 m) in the Cahaba coal field (fig. 1, index map 2). The Pine Sandstone Member grades upward to a flaggy fine-grained sandstone. It lies 200 to 1,000 ft (61 to 305 m) above the basal Shades Sandstone Member. The Pine Sandstone Member increases in thickness from 250 ft (76 m) in the northern Cahaba coal field (index map 1) to 400 ft (122 m) in the southern Cahaba coal field (fig. 1, index map 2), and 200 to 500 ft (61 to 152 m) in the Coosa coal field (index map 2). The Chestnut Sandstone Member lies above the Gould coal group in the Cahaba coal field about 500 to 800 ft (152 to 244 m) above the Pine Sandstone Member. It thins from 200 to 100 ft (61 and 30 m) from south to north in the Cahaba coal field. These quartz arenite sandstones in the Pottsville Formation have been interpreted as barrier island deposits overlain by coal-bearing delta plain deposits in the upper formation (Thomas, 1972). On cross section N–N', we identified the Shades Sandstone Member at the base of the Pottsville Formation by its low radioactivity on gamma-ray well logs. It ranges from 155 ft (37 m) in drill hole 201 to 260 ft (79 m) in drill hole 208. The Pine Sandstone Member may also be represented by intervals of low radioactivity from 250 to 900 ft (91 to 274 m) above the Shades Sandstone Member in the gamma-ray logs of drill holes 201–205 and 208.

As previously described, in the northeastern half of cross section N–N', we determined depths to coal beds in the Pottsville Formation in drill holes 206 and 208 from mud logs, using the generalized section of the Pottsville Formation from the Mineral Resources Map of Bibb County, Alabama (Szabo and others,1976) for identification. We used the geologic map from the same publication to locate and identify coal beds at the surface and to infer the location of coal beds, formation boundaries, and fold structures at depth. Because these coal bed horizons and formation contacts were drawn based on the shape of overlying coal horizons at the surface and not on actual depths determined from well logs or other forms of empirical observation, the actual configuration of subsurface coal beds and formations in this area may be different than the configuration shown on the northeastern half of cross section N–N'.

The anticline and syncline configurations of coal bed horizons at the surface near drill holes 206 through 207 are our interpretations based on the surface locations of the coal beds along cross section N–N' and the depths to those same coal beds in drill holes 206 and 208. The geologic map from the Mineral Resources Map of Bibb County (Szabo and others, 1976) was also used to determine the locations of the axes of the syncline northeast of drill hole 205 (at mile 54 on cross section N–N′) and the anticline southwest of drill hole 206 (at mile 60). Subsurface coal bed horizons and formation contacts in the northeastern half of the cross section were drawn subparallel to the surface coal-bed horizons above. Northeast of drill hole 207, the formation contacts below the Pottsville Formation were drawn rising to the northeast where they meet drill hole 208 (based on the gamma-ray, mud, and GSA sample description logs for drill hole 208 and cross section B–B′ from plate 1 of Pashin and others [2011]) and cross section I of Thomas and Pashin (2011) (fig. 7) at the northeastern end of cross section N–N′.

Several dozen coal beds in the Cahaba coal field (fig. 1, index map 2) have been described by Culbertson (1964), Shotts (1971), Southern Railway (1972), and Ward (1984). The Gould and Nunnally coal groups, and the Harkness, Big Bone, Woodstock, Gholson, and Lower and Upper Thompson coal beds shown on cross section N–N' were considered important by Southern Railway (1972). Furthermore, Culbertson (1964) described the Woodstock, Gholson, Thompson, and Helena beds as high-quality coals with low sulfur and ash (mineral matter) content. The Nunnally, Harkness, and Montevallo coals were also low in sulfur, but had moderate to high ash content (Culbertson, 1964). Ward (1984) described the Thompson, Coke, and Woodstock coal beds as consistent producers of coking quality coal. Szabo and other (1976) recognize the following 41 coal beds in northeastern Bibb County in the Cahaba coal field (in ascending order): the Earhart, Lower Gould, Upper Gould, Nunnally No. 1, Nunnally No. 2, Nunnally No. 3, Nunnally No. 4, Lower Harkness, Upper Harkness, Martin, Lower Wadsworth, Upper Wadsworth, Clean, Big Bone, Beechtree, Jones, Alice, Atkins, Coke, Sixteen Inch, Woodstock, Gholson, Slatey, Moyle, Little Pittsburgh, Quarry, Smithshop, Lower Thompson, Upper Thompson, Helena, Lower Yeshic, Upper Yeshic, Lower Montevallo, Upper Montevallo, Air Shaft, and Straven No. 1 coal beds, and five anonymous coal beds in the Five-Seam coal group between the Nunnally No. 4 and Lower Harkness coal beds. We show 18 of these coal beds on cross section N–N'. Coal production from the Cahaba coal field varied between 1978 and 1993 (Pashin and others, 1995) with the maximum production of 776,000 short tons in 1980 and a minimum of 18,000 short tons in 1987.

The Pottsville Formation produced 49,623,609 thousand cubic feet (Mcf) (1,405,184 thousand cubic meters [Mcm]) of coalbed gas in the Gurnee coalbed gas field of Bibb and Shelby Counties, Alabama, between April 1990 and December 1991 (Geological Survey of Alabama, 2024). Drill holes 206 and 207 fall within the boundaries of the Gurnee coalbed gas field, but only drill hole 207 was considered part of the field. Drill hole 206 only produced 291 Mcf (8.24 Mcm) of gas in 1990, while drill hole 207 produced 74,088 Mcf (2,098 Mcm) of coalbed gas between March 2009 and June 2022 (mostly between April 2014 and June of 2022) (Geological Survey of Alabama, 2024).

Undivided Lower Cretaceous Formations

Undivided Lower Cretaceous formations (fig. 3) overlie the top of truncated Parkwood and Pottsville strata in the southwestern half of the study area (cross section N–N′). These formations are composed of pink and yellow coarse-grained sand and gravel with gray shale and red to ochre clay. GSA sample description logs for drill holes 202 and 204 record the presence of “undifferentiated” Lower Cretaceous formations, but the unit was not recognized in the sample description log of drill hole 205. On cross section N–N′, we show the undivided Lower Cretaceous formations varying from measured thicknesses of 680 ft (207 m) in drill hole 202 and 210 ft (64 m) in drill hole 204, then pinching out between drill holes 204 and 205.

Tuscaloosa Group

The Upper Cretaceous Tuscaloosa Group (Cenomanian to Coniacian) lies unconformably above truncated strata of undivided Lower Cretaceous formations, the Pottsville Formation, and Cambrian and Ordovician strata of the Helena and Rock Creek School thrust sheets in the southwestern half of cross section N–N'. In the Coastal Plain outcrop belt of Alabama, the Tuscaloosa Group ranges from less than 300 ft (91 m) thick in eastern Alabama to 900 ft (274 m) thick in western Alabama, and includes two formations, the Coker Formation, and the overlying Gordo Formation (Raymond and others, 1988).

McShan Formation

The Upper Cretaceous (Coniacian to Santonian) McShan Formation is an interval of marine sand and clay that rests unconformably on the continental sediments of the Gordo Formation of the Tuscaloosa Group (Monroe and others, 1946). It is mapped at the surface in Hale and Greene Counties within the study area (Monroe and others, 1946) and regionally in the subsurface just northwest of the study area (Eargle, 1946) but is difficult to identify on wireline logs beneath the overlying Eutaw Formation. It is often included with the Eutaw Formation as “Eutaw and McShan Formations, undivided” or “lower, unnamed member of the Eutaw Formation.” It was not recognized by Raymond and others (1988) and is not shown on cross section N–N′ or figure 3.

Eutaw Formation

The Upper Cretaceous (Santonian to Campanian) Eutaw Formation (figs. 2 and 3) lies unconformably above the Gordo Formation of the Tuscaloosa Group in the southwestern half of cross section N–N'. The formation is composed of light-greenish-gray sand that is fine to medium grained, well sorted, micaceous, crossbedded, fossiliferous, and glauconitic, with interbedded greenish-gray silty, micaceous clay, and medium-dark-gray carbonaceous clay. Thickness in the outcrop belt ranges from 100 ft (30 m) in western Alabama to 400 ft (122 m) in eastern Alabama. The upper Tombigbee Sand Member of the Eutaw Formation (not shown on cross section N–N' or fig. 3) is a light-gray sand that is massive to weakly bedded, very glauconitic, somewhat clayey, abundantly burrowed, containing mollusk shells, and layers of calcareous sandstone and sandy chalk; it is present in west-central Alabama. On cross section N–N′, we show the Eutaw Formation with a measured thickness of 430 ft (131 m) in drill hole 202, then rising in elevation to the northeast where it is exposed at the surface between miles 10 and 37. The Eutaw Formation is completely removed by erosion to the northeast of mile 37.

Selma Group

The Upper Cretaceous (Campanian to Maastrichtian) Selma Group lies unconformably above truncated strata of the Eutaw Formation in the southwestern half of cross section N–N'. In the Coastal Plain outcrop belt of central and western Alabama, the Selma Group includes four formations (in ascending order): the Mooreville Chalk, the Demopolis Chalk, the Ripley Formation, and the Prairie Bluff Chalk. The Mooreville Chalk is the only one of these formations that is present in the study area and is the only one discussed in this report.

Quaternary Deposits

The Quaternary deposits in the study area lie unconformably above truncated strata of Cretaceous and Paleozoic formations across the entire length of cross section N–N'. The Quaternary deposits include (in ascending order): Pleistocene terrace deposits, and Holocene alluvial deposits.