SIMILAR SEQUENCE STRATIGRAPHY OF THE NEOPROTEROZOIC WINDERMERE SUPERGROUP IN CENTRAL IDAHO AND THE TRICORNER OF WASHINGTON, BRITISH COLUMBIA, AND NORTHERN IDAHO

The Neoproterozoic Windermere Supergroup of central ID and the Tricorner of WA, BC, northern ID is discontinuous due to younger unconformities and Mesozoic thrust faults and intrusions. The Windermere is in amphibolite facies in central ID and in greenschist in the Tricorner. The presence and similarity of unconformity-bounded sequences (UBS) is under-appreciated because of remote mountainous terrain, km-thick formations, and lithostratigraphic synonyms in the Tricorner. Former models in the Tricorner emphasize conformable formations and facies changes due to syndepositional faulting.

Wheeler-diagrams, by using the most continuous succession in a region as a guide, help to identify missing stratigraphic intervals (unconformities) in seemingly conformable successions elsewhere. Such missing intervals in one or more locations in Washington reveal seven UBSs. In the following Wheeler-diagram, in which absent intervals also imply unconformities, the seven UBSs and their probable correlatives in central ID are:

Formations in WA	Composition	Formations (& Members) in ID
Three Sisters	feldspathic quartzite	Umbrella Butte
absent	pelite & carbonate	Missouri Ridge
absent	quartzite & conglomerate	Moores Lake
upper Monk	pelite & carbonate	Goldman Cut
lower Monk	pelite & diamictite	Moores Station
absent	rhyolite	Edwardsburg (Hogback Rhy.)
upper Irene	volcaniclastic	Edwardsburg (Placer Creek)
lower Irene	metabasalt	Edwardsburg (Golden Cup)
upper Toby	matrix-rich diamictite	Edwardsburg (Wind Riv. Mea.)
lower Toby	matrix-poor diamictite	absent
absent	impure carbonate	Anchor Meadows
absent	feldspathic quartzite	Square Mountain
absent	pelite	Plummer Point

Anchor Meadows, Square Mountain, and Plummer Point may be pre-, or early Windermere. Three Sisters and conformably overlying Lower Cambrian Addy Quartzite and Maitlen Phyllite are Sauk I of Sloss's North American UBSs as is Umbrella Butte.

The correlations suggest great former lateral extents of the UBSs (like Sloss's Phanerozoic UBSs) and, thus, that the constituent formations were not deposited in separate fault-bounded basins. The multiple unconformities demonstrate multiple periods of sedimentation (and stacked depositional basins) along the former trailing margin of Laurentia.