CAMBRIAN-ORDOVICIAN STRATA IN WASHINGTON-IDAHO-MONTANA INDICATE SEDIMENTATION ON AN ACTIVELY EXTENDING PASSIVE MARGIN

Lower Cambrian to Lower Ordovician passive margins sediments were deposited across northeastern Washington, northern Idaho and western Montana. Lower Cambrian strata record the Sauk transgression onto Laurentia and are restricted to northeastern Washington. Middle Cambrian to Lower Ordovician units were deposited across a much broader area and record the establishment of a western algal shoal complex that restricted water circulation in an intra-shelf basin that formed between the shoal and the craton toward the east. Pre-existing basement structures (i.e., Montania and the Lemhi Arch) controlled, in part, the location of the algal-shoal complexes and siliciclastic sediment input from the west. The size and shape of the algal-shoal complex influenced sediment distribution, salinity, dolomitization, current flow, faunal distribution, evaporation, and tidal range.

The Sauk sequence sediments comprise multiple grand cycles (1-10 my duration) that are regionally correlative. Each grand cycle consists of a basal shale and sandstone unit deposited as sea level transgressed onto the craton that are gradationally overlain by carbonate deposited as sea level continued to rise and extensive carbonate platforms developed. The carbonate tops of some grand cycles are unconformities on the craton, recording rapid sea level drop and erosion. Just below these unconformities, mass extinctions or “biomere boundaries” interrupt the evolution of shallow-shelf faunas during deteriorating environmental conditions.

North of Metaline, WA the Cambrian shoreline trended east-west and transitioned into the Kicking Horse rimmed shelf of southern British Columbia. This change in shoreline orientation represents underlying basement control along active faults (e.g. Lund, 2008). The change from a narrow rimmed shelf margin to a broader, ramp geometry with landward intrashelf basins records a fundamental change in the style of extension, related to development on upper or lower plate extensional margins. The narrow rimmed shelf margin likely reflects development on an upper plate margin whereas the broader ramp with intra-shelf basin records a lower plate margin. The development of a landward intrashelf basin indicates continental extension was still active during the Middle Cambrian-Early Ordovician.