Pomona Fault

Part of the Ste. Genevieve Fault Zone

Location

Southwestern Jackson County (J-5)

References

Desborough 1958, 1961a, b, Pickard 1963, Satterfield 1965, Johnson 1970, Nelson and Lumm 1985

Description

The Pomona Fault was mapped from outcrops in bedrock uplands by Desborough (1958), Pickard (1963), and Satterfield (1965). The fault was traced farther west by means of subsurface data collected from beneath the Mississippi River floodplain (Nelson and Lumm 1985). The Pomona Fault strikes parallel with and lies 3 to 5 miles (5-8 km) northeast of the Ste. Genevieve Fault Zone. The Pomona Fault strikes about N60°W across the uplands and curves to due west across the floodplain.

Desborough (1958), Pickard (1963), and Satterfield (1965) all indicated that the Pomona Fault displaces the lower Pennsylvanian and upper Chesterian bedrock at the surface with the northeast side downthrown as much as 200 feet (60 m). Nelson and Lumm (1985) observed that Pennsylvanian rocks along the Pomona Fault are folded and heavily jointed but not displaced. Pennsylvanian rocks define a northeast-facing monocline with the flank dipping at 7° to 15°. Both outcrops and well data indicate, however, that underlying Mississippian rocks are faulted. Well records in T10S, R3W, suggest several hundred feet of displacement down to the north on Chesterian rocks. These findings also suggest that a major episode of faulting took place after Chesterian deposition and before the first Pennsylvanian rocks were deposited. Subsequent post-early Pennsylvanian movements along the Pomona Fault were sufficient to bend but not shear Pennsylvanian rocks.