CLEAVAGE DEVELOPMENT WITHIN A FOLDED SEQUENCE OF JACKFORK SANDSTONE, EASTERN OUACHITA MOUNTAINS, ARKANSAS

Structural analysis of a new exposure of folded and cleaved Jackfork Sandstone was conducted to evaluate the mechanisms and timing of cleavage development with respect to folds in the area. The exposure is located on the northern edge of the Benton uplift within the Maumelle zone (frontal imbricated zone) of the eastern Ouachita Mountains. The Maumelle zone is comprised of the Pennsylvanian Jackfork Sandstone, which includes thin to massive-bedded, fine to course-grained quartzitic sandstone interbedded with shale. The large-scale structure of the Maumelle zone includes antiforms exhibiting pervasively disrupted strata and sandstone blocks embedded in less competent shale layers, and synclines with slight to moderate disruption of beds.

The exposure of Jackfork Sandstone studied is located in western Little Rock, Arkansas south of Highway 10 and west of I-430. Structurally, the exposure is immediately south of the Big Rock syncline, a major east-west trending, south-vergent fold with north-dipping cleavage that in places transects the Big Rock Syncline. Structures observed at the exposure studied include a series of mesoscopic folds, slaty cleavage, pencil cleavage, and minor faults within a sequence of thin-bedded sandstone and shale. Joints and quartz veins are present in massive sandstone beds at the south end of the exposure. Fold style is variable with folds ranging from angular to rounded and from tight to open. The folds are south vergent as axial surfaces dip steeply to gently to the north. Cleavage is variably developed throughout the exposure. It dips steeply to gently to the north and is most strongly developed within the cores of tight folds where it exhibits a convergent fan geometry. At other locations within the exposure, cleavage appears to transect fold limbs. These two cleavage geometries suggest a complex structural history with cleavage forming both during and following fold development.