Source Control Over Calciturbidite Facies Distributions in the Lower Isaac Carbonate,
Windermere Supergroup, Canada
Gammon, Paul R.1, Bill Arnott2
(1) Carleton University, Ottawa, ON (2) University of Ottawa, Ottawa, ON
The informally named lower Isaac carbonate
(LIC) is the only calcareous unit in ~3 km of Neoproterozoic
Windermere Supergroup strata in eastern British Columbia. In general, deepwater
calcareous turbidites (calciturbidites)
are uncommon in most turbidite or limestone systems.
The LIC is a mix of three facies: coarse sand to
gravelly turbidites that form shallow channel fills
dominated by Ta (Bouma A-division)
deposits that have a quartz (~70%) – carbonate (~30%) composition. Channel
fills are generally surrounded by calciturbidite
levee deposits that laterally grade into non-calcareous, Tde
overbank mudstone turbidites.
Proximal levees consist of laminated Tdebc
calcarenites to thick-bedded Tbc
calcilutites that are generally >70% carbonate clasts plus cement. The calciturbidites
display m-scale coarsening and fining-upward trends; between calcarenite and calcilutite lithologies. On the distal levee these fine to siliciclastic mudstone – calcilutite
cycles, with locally well developed bypass surfaces. The origin of the three facies can be ascribed to compositional differences between
grainsize classes in a mixed siliciclastic-carbonate
continental shelf source region. Subsequently, turbidity current dynamics
distributed the grainsize classes into different
depositional environments, and hence facies
compositions. The m-scale cycles are probably an autocyclic
channel-related feature, although their distribution, plus the rarity of
carbonate strata in the Windermere, suggests eustasy
and tectonics ultimately controlled the distribution of grainsizes
and compositions on the shelf. Common photic-zone
carbonate clasts further suggest the LIC calciturbidites may represent late highstand
to early regressive times, when progradation-aggradation
had produced shallow outer shelf waters, but prior to a probable lowstand restriction of this carbonate factory by low
relative sealevel, and an abundant siliciclastic mud flux.