Location
35.721750,
-93.688083
Site name
Shop Branch
Type
Bluff exposure along hillside ravine in Boston Mountains
Age/Formation/Unit (in stratigraphic order)
Features of interest
Vertical section of 25 ft. Unconformable contact of Bloyd
and Atoka formations is well exposed at base of basal Atoka sandstone (“Orr/Kelly”
pay zone of Arkoma basin).
Lithology present
Shale, sandstone, conglomerate
State/County
AR / Johnson County
Access
Accessible
off Johnson County Road 33/5051, approximately four miles north of Arkansas
Highway 215, in NE/4, NW/4, Section 7, T12N, R25W, Ozark
National Forest.
Support material available
Illustrated geology note (text, photos, site map, references)
Site contributed by
M. R. Shinn, 6/19/16. Site
visit by Shinn 11/11/13. Section was
originally described by Corbin (1979, UArk MS Thesis).
 |
|
Figure 1. Shop Branch site map
|
 |
|
Figure 2.
Basal Atoka exposure on east side of
Shop Branch ravine. Massive,
cross-bedded, coarse-grained sandstone, with a 4-6”
shale pebble conglomerate at the base of the unit (Corbin, 1979). Exposed thickness: 22 feet of sandstone and
conglomerate, overlying 3’ of upper Bloyd shale. Basal Atoka sandstone at Shop Branch is composed
of low-stand fluvial beds deposited above the post-Morrowan unconformity. Unit is correlated with the “Foster” channels
at the base of the basal Atokan Spiro sandstone in eastern Oklahoma (Lumsden,
1971) and with the “Orr/Kelly/Barton A” pay zones of the Arkansas Arkoma,
downthrown to the Cass Fault, immediately south of Shop Branch.
|
 |
|
Figure 3.
Unconformable contact of Atoka Formation
with Upper Kessler (Bloyd) shale. This photo
was taken at base of the bluff exposure in Figure 2.
|
 |
| Figure 4. Close-up of unconformable contact between Atoka Formation and gray, fissile shale of the upper Kessler Member of the Bloyd Formation. Tip of knife blade is at base of shale pebble conglomerate and marks the Atoka/Morrow sequence boundary. |
No comments:
Post a Comment