STRUCTURAL CONTROL ON CAVE DEVELOPMENT IN A FOLD-THRUST BELT

Clarksville, NY area caves are situated near the northwestern terminus of the Hudson Valley foreland fold-thrust belt where a series of thrust faults ramp upward cutting up-section through mechanically rigid Devonian formations (Schoharie and Onondaga) from bedding-parallel detachment faults likely originating within underlying comparatively ductile shale and sandstone formations (Devonian Esopus Fm and/or Ordovician Indian Ladder Beds and Schenectady Fm). Cross-strike megascopic fault-related folds have first-order wavelengths and amplitudes ranging from 570 m to 1,074 m and up to 18 m, respectively. Cave development has preferentially occurred along imbricate thrust faults.

Caves afford 3D examination of the internal geometry of fault zones and fault bend deformation structures. Fault-related features in area caves (Clarksville, Onesquethaw, SHED, End of Gulch, Trap) include sheared beds, extension veins, blocky calcite spar (to 70⁺ cm in Onesquethaw Cave), slickensides, chevron and kink folds, and bed duplication. Cross sections of these structural features may be observed in surface outcrops throughout the area but especially well in a gorge south and cross strike with Clarksville Cave. Along strike (N8°E), fault-bend folds were traced for 1.7 km.

In Clarksville Cave, a thrust ramp cuts the Schoharie and Onondaga Fms, thrusting the upper pyritic and locally silicified Schoharie Fm within the overlying Onondaga Fm. Cross strike, this fault ascends at approximately 4°, sharply increases to about 27° in the southeastern portion of the cave, and then continues up section from 12° to 19° with a minimum hanging wall displacement of 110 m. The northern Ward’s portion of the cave and its infeeders have developed along the uppermost portion of this fault ramp. South of the Ward’s cave section, the cave stream and floodwaters descend down the steep face of the fault ramp, resulting in significant change in conduit cross sectional shape. An epiphreatic conduit superimposed along the strike of the fault ramp records the hydraulic inefficiency of the cave’s outlet during times of major subglacial meltwater invasion. A separate imbricate thrust with associated chevron folds ascends westward through the Lake Room at about 65°. During waning glacial stages, it served as a major water and sediment infiltration pathway.