Abstract:
Two significant caves in Glacier National Park are developed in Middle Proterozoic carbonate rocks. One lies within two large-scale duplex structures resting on the Lewis Thrust. The other is in the hinterland region of one of the duplexes. Both of the caves are aligned along bedding planes, joints, and faults. Poia Lake Cave has large segments that, in part, are aligned along low-angle thrust faults. Both Poia Lake Cave and Zoo Cave have uptrending, dead-end passages developed above the main passage along near-vertical normal faults. In Poia Lake Cave, three small maze sections also lie above the main passage. My previous speleogenic model involving a semi-confined aquifer, with mixing zones along faults and fracture zones now seems unlikely because the strata would be unable to simultaneously confine the aquifer and allow descending water to mix along fracture zones and faults. A second model involving a deep-looping system, while more feasible, also seems unlikely due to the short flow length of postulated cave passages. Recent studies suggest cave development occurred under confined aquifer conditions whereby long-traveled deep water ascends from an artesian aquifer near the Lewis Thrust. The aquifer developed after the hinterland region of the Lewis Thrust was uplifted during the Laramide Orogeny. It remained active until the system was disrupted by late Pleistocene glacial erosion. Since the original phreatic development of the caves, they have been subjected to some collapse, vadose entrenchment, and deposition of clastic sediment including rounded cobbles and glacial varves.