Print this pageThe regional distribution pattern for the pre-Mount Simon Sandstone (late Cambrian) sedimentary rock sequence, here referred to as the Middle Run Formation, in an area centered about Indiana is interpreted to represent a series of linked sub-basins that formed immediately proximal to the Grenville orogen. Potential field analysis, utilizing relationships of signal intensity to sediment thickness, yields a complex pattern of small localized accumulations of thick sediment interpreted to be depocenters controlled predominantly by preexisting and syndepositional basement faulting. Distribution patterns were primarily derived from the observed linear relationship between gravitational and magnetic intensity and thickness established in a series of structural cross sections that depict sediment thickness as constrained by seismic data. The correlation coefficient in the observed relationship approached 0.7. Assumed density values for these sediments used in the cross sections are consistent with densities for these rocks as measured in boreholes using density geophysical logging.
Calculated thicknesses were found to exceed 15,000 feet in some localities while rapidly decreasing to near zero over basement highs. The overall regional pattern in both thickness and areal extent is supportive of a complex basin history that may have initiated as a dominantly tensional system of linked grabens and half-grabens, and evolved to become a fragmented foreland basin system where individual sediment accumulations were controlled by transtensional or oblique slip faults associated with the closing phases of the compressive Grenville collisional tectonism.
AAPG Search and Discovery Article #90926©1999 AAPG Eastern Section Meeting, Indianapolis, Indiana