Matrix-rich Sandstones of the Neoproterozoic Windermere Supegroup, B.C., Canada: The Energetic Initiation of the Local Sedimentary System
A common facies observed in Deepwater slope and basin-floor rocks of the Neoproterozoic Windermere Supergroup (British Columbia, Canada) is structureless, coarse-tail graded, medium to coarse sandstone with >10-15% mud matrix content (i.e. matrix rich), which increases abruptly in the upper part of beds. Bed contacts are commonly sharp, flat and loaded. Laterally (~400 m) beds thicken toward the southeast and transition rapidly (10’s m) to strata with significantly less matrix, large (meter scale) tabular clasts and common scoured bases. Further laterally (~400 m) strata continue to thicken, but importantly beds again become tabular, clast size decreases and ripple and planar lamination become common, occurring at the top of the sand-rich basal layer. Matrix-rich beds form laterally continuous units up to a few meters thick, and where not thinned by erosion, show a depositional thinning by as much as 50% over a few 100 m laterally toward the southeast.
The abundance and anomalous size of the tabular, commonly internally stratified clasts implies intense upstream scouring, most probably related to significant erosion within an upflow hydraulic jump. The matrix-rich, poorly-sorted nature of the beds is most probably also related to the erosion within the jump. This quickly charged the flow with fine-grained sediment that likely promoted deposition a short distance downflow. The axis of flow occurs where beds are comparatively thin, are most matrix rich, and the upper mud-rich cap is thickest (~1/2 the bed thickness). Immediately adjacent, distinctively scoured-based, thick, more sand-rich strata with large clasts indicate voluminous deposition in the less dense, intensely turbulent margins of the axial flow. Further laterally, the common occurrence of tractional structures suggests further distance from the axis and accordingly reduced sediment concentration Matrix-rich sandstones described here abruptly overlie a succession of silt/mud thin-bedded turbidites, which then are overlain by an up to 17 m thick, coarse-grained splay deposit. These distinctive strata, therefore, represent the energetic initiation of the local sedimentary system, which most likely relates to an upslope avulsion. Although described here from this single example, matrix-rich units are common in both basin-floor and slope deposits in the Castle Creek study area, but regardless of paleogeographical setting similarly indicate the abrupt activation of the local sedimentary system.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California