Stratigraphy and sedimentology of the Athabasca Group in the Net Lake-Maybelle River area, northeastern Alberta

The stratigraphy and sedimentology of the unconformity-bounded Fair Point, Manitou Falls and Lazenby Lake formations of the Athabasca Group were examined in the Maybelle River-Net Lake area of northeastern Alberta. The lowermost Fair Point Formation is subdivided into two members: the lower and upper Fair Point Formation. The lower Fair Point Formation consists of locally occurring, massive to planar-laminated siltstone units, interbedded with and overlain by matrix- and clast-supported pebble conglomerate and pebbly sandstone. It is conformably overlain by massive and crudely crossbedded, coarse-grained sandstone of the upper Fair Point Formation. Deposition of the Fair Point Formation commenced with shallow lacustrine sedimentation in fault-controlled paleovalleys that were subsequently infilled with locally derived pebble conglomerate. These valleys are overlain by progradational alluvial-plain deposits. The overlying Manitou Falls Formation consists of a basal, crossbedded, medium- to coarse-grained sandstone unit and an upper, ripple-laminated to crossbedded, medium-grained sandstone unit. These are assigned to the Manitou Falls 'c' and 'd' members, respectively. Deposition of the Manitou Falls Formation occurred in relatively low velocity, perennial, braided river system tracts that are traceable across the basin. The uppermost Lazenby Lake Formation is incompletely preserved within the study area. This unit consists of massive and ripple-laminated, pebble-bearing sandstone with convolute bedding. Initial sedimentation of the Fair Point Formation was fault controlled and probably affected by extensional tectonics related to regional thermal activity that postdated the Trans-Hudson Orogeny. The sedimentological characteristics and distribution of the overlying Manitou Falls and Lazenby Lake formations suggest a significant change in the regional tectonic regime. The exact mechanism is unclear, but may be related to late-stage post-Trans-Hudson Orogeny intrusions or the effects of dynamic loading combined with regional, thermally induced subsidence.