Alternating Braidplain and Lacustrine Deposition in a Strike-Slip Setting: The Pennsylvanian Boss Point Formation of the Cumberland Basin, Maritime Canada

Abstract

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 fades 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 that rest sharply but nonerosively 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 t e braidplain. The BFA comprises multistory channel sandbodies 90 m thick, separated from the underlying LFA by sixth-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 fifth-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 sandstone. Conglomerate, pebbly sandstone, and ripple-laminated and planar-laminated sandstone are minor components. Rare mud-filled abandoned channels suggest a mean width and de th of > 160 m and 4.8 m, respectively. The dominance of trough cross-bedding suggests relatively deep channels, whose banks 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; bartop 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 northwest, but much of the sand is likely to have been sourced far to the southwest. The Boss Point falls into two sharply disjunct paleoflow domains, one directed to the north and northeast and the other to the east and southeast, 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 kilometers of sinistral offset.