FACIES ARCHITECTURE IN TRANSGRESSIVE SYSTEMS TRACTS AROUND CLASTIC CHANNELS THROUGH CARBONATE BANKS: EXAMPLES FROM THE CAMBRO-ORDOVICIAN OF THE CHAMPLAIN VALLEY (VT & NY)

The Late Cambrian to Middle Ordovician eustatic lowstands in the Champlain Valley are marked by sandstone lenses occupying channels incised into the pre-lowstand carbonate shelf. Seven major eustatic events, all with similar transgressive systems tract facies architecture, are recognized from this stratigraphic association.

In all cases, the channels are mostly filled with siliciclastics, with nearly featureless sand along the axial base grading upward and outward to ripple-marked/finely cross-laminated/burrowed sand. The contact with the carbonates is marked by a zone (generally >10 m thick) of interlayered carbonate and siliciclastic sands. Significant siliciclastics extend out beyond the edges of the channel only along the inner shelf.

The earliest post-lowstand carbonates developed adjacent to the ends of the lenses commonly have characteristics of shelf edge facies (e.g., oolites, bioherms, etc. - these systems predate the arrival of reef-building bryozoans and corals). Significant floating quartz sand/silt grains accompanied by pyrite, with interspersed thin layers and laminae of quartz sand/silt and pyrite (interpreted as channel sediment displaced by storm surges), give these carbonates a distinctive brown crust in outcrop. Coeval carbonates at any significant distance from the sand lenses are normal mid-shelf carbonates.

The bounding carbonate facies migrate inward over the top of a sand lens, the top boundary of the sand lens generally extending only a very little distance, if at all, above the lip of the channel, so stratigraphic thicknesses of overlying carbonate units are not recognized as being thinned (subsequent deformation has further obscured minor stratigraphic thickness variations of these units). Once carbonate has capped the siliciclastics, the carbonates rapidly transition to normal mid-shelf carbonates.