Updip To Downdip Variations In Turbidite Facies, Lower Pennsylvanian Jackfork Group, South-Central Arkansas, U.S.A.

Slatt, R.S., Colorado School of Mines, Golden, Co. 80401, C.G. Stone, Arkansas Geological Commission, Little Rock , AR 72204, and Paul Weimer, University of Colorado, Boulder, Co. 80309

Abstract

Field work during the past decade has allowed us to develop a stratigraphic framework for the Jackfork Group in the Little Rock-DeGray Lake area of south-central Arkansas. The stratigraphic occurrence of several excellent outcrops have been placed withi n the 7,000 ft lower-middle-upper Jackfork succession, and a spectrum of facies has been identified.

Contemporaneous slope canyon/channel-fill facies of the upper Jackfork occur at Big Rock Quarry and at nearby Jeffrey's Quarry and Highway 9. The facies consists of fine-grained, poorly sorted, muddy sandstone interbedded with debris flow and mud turbidite beds. Sandstones are typically laterally discontinuous and exhibit scoured and/or slumped upper surfaces which are overlain by mudstone drape. The entire canyon/channel-fill complex is approximately 8 miles wide and up to 460 ft thick in the Little Rock area.

Slope channel-fill facies of the upper Jackfork occurs at Pinnacle Mountain State Park and McCain Mall near Little Rock, and at Caddo Valley roadcut and Hollywood quarry, near DeGray Lake. The facies consists of massive to parallel bedded, thick- to thin-bedded sandstones, sometimes with scoured bases. The sandstones are fine-grained, relatively clean and matrix-poor. Lateral continuity of these beds is variable. Conglomeratic channel-fill sandstones occur at DeGray Lake spillway/intake and at nearby Murray quarry.

Basin floor sheet sandstones of the upper Jackfork occur at DeGray Lake spillway/intake, Caddo Valley roadcut, and possibly at Murfreesboro quarry. Sandstone packages, comprising both layered and amalgamated sheet sandstones, can be traced for at least one mile, and in one case, for a minimum distance of 8 miles. These sandstones are also fine grained and contain variable amounts of matrix. Sheet sandstones and channel sandstones comprise the lower Jackfork along Highway 7 near DeGray Lake.

Channel-levee strata comprise much of the middle Jackfork in the DeGray Lake area. A deep (5-15 ft) weathering profile occurs in these steeply inclined beds beneath a major angular unconformity, which is overlain by gently dipping Cretaceous strata. This zone of unusual weathering, which likely formed in Triassic to early Late Cretaceous time (post-Ouachita orogeny), has resulted in uncemented Jackfork strata and provides an excellent outcrop example of secondary porosity development beneath an unconformity.

Thick vegetation and the presence of several large thrust faults creates difficulty in long distance correlation of major stratal surfaces and in developing a sequence stratigraphic framework. However, in the DeGray Lake spillway area, three potential depositional sequence boundaries have been identified, two within the upper Jackfork and one at the base of the lower Jackfork. Though somewhat complicated by thrust faults, a probable hiatus occurs near the top of the Jackfork at Murray quarry, which has significant relief over several miles; it apparently cuts out part of the underlying spillway strata to the west. All of the outcrops and their characteristics, as well as composite and detailed stratigraphic sections, will be displayed in the complete article.