Abstract

The South Bar Formation is a 1 km-thick alluvial succession in the Pennsylvanian Morien Group of the Sydney Basin, Nova Scotia that fines upward from conglomerate to pebbly sandstone to sandstone facies assemblages. The conglomerate assemblage comprises mainly massive to horizontally stratified conglomerate and planar cross-stratified sandstone, attributed to deposition in proximal braided rivers confined between bedrock ridges. The sandstone assemblage shows uniformity of facies and paleocurrents across the basin, and is interpreted as a distal braidplain deposit; the pebbly sandstone assemblage is transitional between the other two. Both sandstone assemblages are dominated by trough cross-stratified sandstone formed by dune migration in channel tracts about 4 m deep. This succession differs markedly from Platte River and Icelandic sandy braidplain deposits, which are mainly planar cross-stratified sands formed by broad, shallow flow. The South Bar sandstones are more like South Saskatchewan River deposits, but differ in 1) greater abundance of trough sets and 2) rarity of planar sets and other components of shallow water channel fill. These features are ascribed to 1) inheritance of relatively deep channels from confined upstream reaches, 2) maintenance of deep flow in active tracts by local confinement between temporary banks, and 3) rapid avulsive abandonment of active tracts, which inhibited development of channel-fill sequences. Evidence for local temporary confinement is preserved as in situ and slumped channel-margin mudrock units. Lateral and vertical transitions from the upper South Bar Formation to the mud-rich Waddens Cove and Sydney Mines Formations are ascribed to changes in channel pattern from braided to meandering. The former is attributed to decreased slope, the latter to eustatic rise in sea level.

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