Ribbon sandstones in the Pennsylvanian Waddens Cove Formation, Sydney Basin, Atlantic Canada: the influence of siliceous duricrusts on channel-body geometry



Channelized bodies up to 9 m thick in the Pennsylvanian Waddens Cove Formation show low width: thickness ratios (8:1 to 14:1), up to four vertically stacked storeys, and deep incision into associated floodplain strata. The presence of lateral accretion sets up to 6 m thick indicates that the migration of bank-attached bars was a major channel-filling process. The progressive rise in elevation of the basal part of the accretion surfaces where the sets adjoin the steeply inclined sides of the channel bodies shows that the channel floor aggraded progressively as the bars approached the channel sides. In planform, the river system probably showed a low-sinuosity, incised valley within which the channel followed a more sinuous and partially confined course.

The alluvium surrounding the channel bodies includes sandstone sheets attributed to crevasse splay and levee accumulation. The upper strata of many sheet's and some channel bodies contain siliceous duricrusts (ganisters) which formed by pedogenic lithification of sandy material commencing shortly after the landforms became inactive. The channel bodies contain duricrust-bearing slumpblocks and show stepped margins over duricrusts. The stepped nature of the banks suggests that duricrusts were an important factor in confining the reach, and we suggest that channel-body geometry was influenced, although not necessarily controlled, by the presence of the lithified layers.

Alternation in the Waddens Cove Formation of grey, coal-bearing zones with red, duricrust-rich zones is attributed to water-table fluctuation in response to base-level changes. The low width: thickness ratios and aggradation mode'of the channel bodies may in part reflect channel incision and subsequent aggradation resulting from these allocyclic events.