Abstract

The Boss Point Formation was deposited in the Cumberland Basin of southeastern New Brunswick and northern Nova Scotia during the early Pennsylvanian. The basin is bounded to the northwest and south by strike-slip faults, active during the Pennsylvanian. The Boss Point comprises two main facies associations: sandy braidplain (BFA) and muddy lacustrine (LFA), the abrupt alternation of which allows 16 depositional megacycles to be defined. The LFA forms units 50 m thick which rest sharply but non-erosively on braidplain sandstone. In some instances the contact can be traced for over 12 km perpendicular to paleoflow. Commonly dunes and ripples are preserved in relief beneath lacustrine mudstone, implying rapid but very low-energy flooding of the braidplain. The BFA comprises multi-storey channel sandbodies 90 m thick, separated from the underlying LFA by 6th order surfaces with up to 15 m of erosional relief. The erosion surfaces are commonly overlain by spectacular mudstone breccias and boulders that reflect deep erosion and rotational slumping into channels cut into, and sometimes right through, lacustrine deposits. BFA packages are internally partitioned by 5th order erosion surfaces into individual channel-fill units, commonly about 5 m thick. The BFA is dominated by trough cross-bedded fine- to lower medium-grained sandstone. conglomerate, pebbly sandstone, ripple and plane-laminated sandstone are minor components. Rare mud-filled abandoned channels suggest a mean width and depth of 160 m and 4.8 m respectively. The dominance of trough crossbedding suggests relatively deep channels, the banks of which may have been maintained by a dense vegetation cover. Repeated channel avulsion and flood-related erosion resulted in a rock-record dominated by deep channel and flood-stage deposits - bar top facies were rarely preserved. channel dimensions, lateral consistency of facies, paucity of muddy channel plugs and regionally consistent paleoflow suggest deposition on a braidplain. The distribution of conglomerates indicates a local gravel source area in the Caledonia Highlands to the NW, but much of the sand is likely to have been sourced far to the SW. The boss Point falls into two sharply disjunct paleoflow domains, one directed to the N and NE and the other to the E and SE, separated by the Harvey-Hopewell fault zone. The absence of an adequate watershed between the two domains strongly suggests tectonic juxtaposition involving several tens of km of dextral offset.

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