Location
35.777717, -93.587150 (at base of section along roadway)
Site name
Spoke Plant
Type
Exposures along well-maintained gravel county road and in nearby ravines in Boston Mountains. Horizontal length is approximately 2900’. Strata are horizontal.
Age/Formation/Unit (in stratigraphic order)
Features of interest
Vertical section of approximately 170 ft. Section is noteworthy for presence of accessible, well-developed outcrops of both the “Middle Bloyd sandstone” (Parthenon Member) and the Basal Atoka sandstone. The lower beds of both units are massive, medium- to coarse-grained, low-stand fluvial sandstones. At isolated exposures across the Boston Mountains, the identities of these units can easily be confused.
Lithology present
Sandstone, shale
State/County
AR / Madison County
Access
Madison County Road 4845, along east side of Spoke Plant Hollow, Sections 18 and 19, T13N-R24W, Ozark National Forest (Boston Quadrangle), 4.8 miles southeast of Arkansas Highway 16 intersection at Pettigrew.
Support material available
Illustrated geology note (text, photos, site map, stratigraphic section, wireline log from nearby well)
Site contributed by
M. R. Shinn, 7/4/16. Site visits by Shinn in 2013-16
 |
Figure 1. Spoke Plant site map (base from Boston 7.5-minute topographic quadrangle, 40’ CI). Triangle symbols denote locations of outcrop photos in Figures 2-5.
|
 |
Figure 2. Outcrop of “Middle Bloyd sandstone” (Parthenon Member) along Madison 4845, GPS 35.778, -93.587. Freshly exposed road-cut outcrop is 21 feet thick and consists of massive, tan, medium- to coarse-grained, porous sandstone. The unit is a gas pay zone in Johnson and Franklin Counties (“Cannon”, “Brentwood”).
|
 |
Figure 3. Outcrop of “Middle Bloyd sandstone” in ravine, 260’ east of Madison 4845, GPS 35.776230, -93.586156. Outcrop is 24 feet thick.
|
 |
Figure 4. Kessler Member (Bloyd Formation) and Basal Atoka sandstone along Madison 4845, GPS 35.781083, -93.584783. Freshly-exposed outcrop of upper Kessler is 15 feet thick and consists of 5 feet of thick-bedded, upper fine- to coarse-grained, clay-cemented, porous sandstone and 10 feet of overlying shale, with sandstone interbeds. Limestone facies of Kessler is absent here, as in most of eastern Madison County. (Basal Atoka bluff at this spot is on private residential property.)
|
 |
|
Figure 5. Basal Atoka fluvial sandstone in ravine, 700’
east of Madison 4845, GPS 35.776464, -93.584750. Total outcrop is 27 feet thick, massive, cross-bedded,
siliceous, clay-cemented, porous, medium-grained with quartz granules. The unit is a gas pay zone across the eastern
Arkoma Basin (“Spiro”, “Orr”, “Kelly”, “Barton A”).
|
