STRUCTURAL TRANSITION: NORTHERN ARKANSAS STRUCTURAL PLATFORM TO ARKOMA BASIN, WEST-CENTRAL ARKANSAS

A horst and graben system, bounded by normal faults and monoclines, characterizes the transition zone between the northwest Arkansas structural platform and the Arkoma basin to the south. The zone trends east-west and lies within the Boston Mountains. The platform is composed of a thin succession of cratonic sedimentary rocks above a Precambrian basement of plutonic and volcanic rocks. Exposures in the Boston Mountains at the southern margin of the platform are composed of sandstone and shale units of early Pennsylvanian age. The entire cratonic succession is less than 3000 feet thick. The Arkoma basin is a peripheral foreland basin associated with the Ouachita orogenic belt to the south. The basin is characterized by asymmetrical sedimentary fill. The succession is thin at the northern margin adjacent to the platform and thickens to over 30,000 feet adjacent to the orogenic belt.

The disturbed zone, that defines the northern border of the basin, ranges from one to three kilometers in width and extends along strike 125 kilometers. Three to four normal faults and a south-dipping monocline characterize this zone. The faults define a horst and graben structure with displacements ranging from 100 to 300 meters. A normal fault, at the southern margin of the platform, dips south at approximately 45o and forms the northern boundary of the graben. The structural displacement along strike is in part accommodated by a monocline. The southern boundary of the graben is formed by a north-dipping normal fault. Another normal fault at the southern margin of the disturbed zone is down-thrown to the south and defines a horst. The horst and graben system of the disturbed transitional zone is separated from a second horst and graben system to the south by approximately 6.4 kilometers.