GEOMETRY OF MAFIC SILLS IN THE MESOPROTEROZOIC BELT-PURCELL BASIN, NORTHERN ROCKY MOUNTAINS

Mafic sills are widespread in the Mesoproterozoic Belt-Purcell basin of the northern Rocky Mountains. We analyzed the geometry of the sills on a palinspastic map that removes the effects of Jurassic through Paleocene thrusting and rotation and later Cenozoic extension. There appear to be four major sets of sills. The earlier sills appear to conform to the basic geometry of the evolving basin. The first set, U-Pb dated at 1469 Ma and 1455 Ma (Sears et al., 1998), intruded the lower Belt-Purcell Supergroup during sedimentation, and is associated with syn-sedimentary faulting, sediment boiling, mud diapirism, and black-smoker SEDEX deposition. These sills form smooth seismic reflectors that occupy approximately constant stratigraphic depths for more than 300 km of strike-length along the Purcell anticlinorium. The second set appears to be regionally bowl-shaped. The deepest levels occupy the lower parts of the Belt-Purcell Supergroup along the axis of the basin. These sills rise systematically outward towards both the northeast and southwest across the width of the basin and may have erupted to feed the 1443 Ma Purcell/Nichol Creek lavas of Glacier/Waterton National Park and vicinity (U-Pb dated by Evans et al., 2000). This may indicate a feeder conduit along the central, spreading axis of the basin. The third set, U-Pb dated to 1370 Ma (Doughty and Chamberlain, 1996), includes a large layered mafic intrusion exposed along the Salmon River, and suggests that the main rift axis shifted to the south. A fourth set is of uncertain form and extent, but intrudes the younger part of the supergroup (Garnet Range Formation) and is cut by the sub-Cambrian unconformity. It is clearly younger than the Purcell lava, and may correlate with the Salmon River complex. Conversely, it may link with the Irene volcanics to the northwest, and 750-780 Ma mafic dikes to the southeast in Wyoming (Harlan et al., 1997) and manifest Neoproterozoic Windermere rifting.