THE NEOPROTEROZOIC WINDERMERE TURBIDITE SYSTEM, WESTERN CANADA: DISTRIBUTION, TECTONIC SIGNIFICANCE, AND GEOCHEMICAL RESERVOIR

The Windermere Supergroup is a regionally widespread succession of predominantly deep marine sedimentary rocks deposited on the ancestral continental margin of western Canada following its separation from Australia ca. 700 Ma. A vertical succession from post-rift sand-rich basin plain turbidites to mud-rich slope facies occurs over a ca. 7 km thick stratigraphic interval and records the progradation of the continental margin attendant with diminished rates of thermally-driven subsidence. Careful stratigraphic mapping, utilizing a lithologically distinct condensed interval and two carbonate-rich lowstand intervals, has established regional continuity of this depositional system over ca. 4o of latitude. Taken in concert with facies patterns, paleocurrent data, and uniform detrital zircon provenance the Windermere turbidites are interpreted as a longitudinal turbidite system that flowed west-northwest, subparallel to the base of the continental margin. Restoration of published estimates of Mesozoic tectonic shortening, suggest that this system was deposited over an area of at least 160,000 km2, comparable in size to modern passive margin fans such as the Amazon. This makes the Windermere a candidate for one of the largest intact passive margin turbidite sequences preserved in the rock record. The regional distribution and thickness of deep-water mudrocks within the Windermere turbidites has important implications for understanding the geochemical record of Neoproterozoic seawater. These rocks contain the deep water geochemical and isotopic record that complements the existing global geochemical database, derived largely from shallow water oxidized facies. Preliminary geochemical and stable isotopic work indicate locally high total organic carbon (TOC) values (4%) and evidence for gradients (5 permil) in the isotopic composition of carbon in seawater. The high TOC values may be the "smoking gun" for the large fractional burial of OC required to drive the positive isotopic composition of seawater carbon typical of Neoproterozoic carbonates. Gradients in the isotopic composition of inorganic carbon lend support to the idea that post-glacial negative excursions seen worldwide may owe their origin to overturn of stratified oceanic water rather than buffering by mantle degassing.