Detailed Architecture of Proximal Basin-Floor Lobe Deposits: Neoproterozoic Upper Kaza Group, British Columbia, Canada
An expansive panel of well-exposed (periglacial) strata of the Upper Kaza Group allows for a detailed study of the architecture of proximal basin-floor lobe deposits. The mapped interval is 800 meters wide, 150 meters thick and consists of 4 recurring stratal elements: bypass unit, thin-bedded turbidites, amalgamated sheet sandstone, and channelized sheet sandstone. All stratal units extend laterally across the study area.
The bypass unit is up to 15 m thick with a deeply eroded base that incises up to 12 m into underlying stratigraphy. The deepest part of the incision is infilled with laterally discontinuous, up to 1 m thick coarse sandstone beds with common intraclasts. The upper part is dominated by thin-bedded upper division turbidites (Tde). The thin-bedded turbidite unit is 10-15 m thick and comprises thin-bedded Tde turbidites interbedded with rare laterally discontinuous sandy Tbc turbidites and breccias. Bedding contacts are largely non-erosional, although shallow scours up to 15 cm deep occur locally. The thick amalgamated sheet sandstone is up to 25 m thick and bounded sharply on its base and top. Internally, it is composed of amalgamated sandstone beds up to 3 m thick separated by irregular scour contacts. Laterally over much of the study area strata change little, but near the eastern limit, and over a distance of about 30 m, amalgamated sandstone transition into graded Ta beds up to a meter thick and separated by thin Tcd or Td turbidites. Lastly, channelized sheet sandstones are 10-25 m thick with shallowly scoured basal contacts at outcrop scale (1-3 m deep over 800 m), often with dune cross-stratified sandstone above the contact. Channelized sheets are composed mostly of Ta or Tab beds up to 1 m thick separated by irregular erosive contacts and discontinuous Tcd or Td turbidites. Like the amalgamated sheet, some channelized sheets become more amalgamated laterally.
Strata of the Upper Kaza Group are interpreted to represent proximal basin-floor distributary lobe deposits. The deeply incised bypass unit, although confined to a single occurrence, represents the main sediment conduit at the headward end of a distributive lobe. Channelized sheets accumulated in the distributary channel network that fed a slightly more downflow terminal lobe (amalgamated sheet sandstone). Horizons of the thin-bedded turbidites between channelized sheets indicate local distributary channel avulsion in this highly depositional sedimentary system.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009