COMPLEX STRUCTURAL HISTORY INDICATED BY CLEAVAGE TRANSECTION OF THE BIG ROCK SYNCLINE, EASTERN OUACHITA MOUNTAINS, ARKANSAS

The Big Rock syncline lies within the Maumelle zone of the northeastern Ouachita Mountains. The Maumelle zone in the study area contains rocks of the Pennsylvanian Jackfork Formation, which consists of interbedded massive- to thin-bedded quartzitic sandstone, silty sandstone, and shale. Structurally, the Maumelle zone is a transition zone between the Benton uplift to the south and the frontal imbricate zone to the north. The general structural style within the Maumelle zone includes mostly south-verging folds, north dipping thrust faults/shear zones, and north-dipping cleavage.

The fold axis of the Big Rock syncline extends over 16 km along the southern margin of the Maumelle zone. It is arcuate in map pattern and trends NE-SW (~65°) at its eastern extent, but curves to a NW-SE (~115°) trend to the west. Where best exposed at I-430 in west Little Rock, the average attitude of the south limb of the syncline is ~110°, 45°-65° N. The attitude of the north limb is ~105°, 55° S. The average attitude of cleavage on the south limb is ~120°, 40° N (consistent with regional cleavage), with slaty cleavage in pelitic rocks dipping more steeply northward than the disjunctive cleavage in coarser-grained lithologies. Slaty cleavage on the north limb is anomalous in that it dips gently (~25°) southward. Refraction of cleavage to a moderate northward dip, however, occurs in coarser lithologies on the north limb. No evidence for more that one regional cleavage has been recognized either in the field or microscopically. Cleavage dips more gently than bedding on both limbs suggesting overturning of strata. However, sedimentary structures indicate that both limbs are upright indicating transection of the fold by cleavage. Anomalous south dipping cleavage has been recognized locally elsewhere in the Maumelle zone on north dipping fold limbs of outcrop-scale folds. The simplest explanation for the unusual cleavage-bedding geometries within the Big Rock syncline involves late stage tightening and limb rotation of the Big Rock syncline during flexural folding following cleavage development.