ALTERNATING BRAIDPLAIN AND LACUSTRINE DEPOSITION IN A STRIKE-SLIP SETTING - THE PENNSYLVANIAN BOSS POINT FORMATION OF THE CUMBERLAND BASIN,MARITIME CANADA

The Boss Point Formation was deposited in the Cumberland Basin of sout heastern New Brunswick and northern Nova Scotia during the early Penns ylvanian. The basin is bounded to the northwest and south by strike-sl ip faults, active during the Pennsylvanian. The Boss Point comprises t wo main facies associations: sandy braidplain (BFA) and muddy lacustri ne (LFA), the abrupt alternation of which allows 16 depositional megac ycles to be defined. The LFA forms units less-than-or-equal-to 50 m th ick that rest sharply but nonerosively on braidplain sandstone. In som e 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 th e braidplain. The BFA comprises multistory channel sandbodies less-tha n-or-equal-to 90 m thick, separated from the underlying LFA by sixth-o rder surfaces with up to 15 m of erosional relief. The erosion surface s are commonly overlain by spectacular mudstone breccias and boulders that reflect deep erosion and rotational slumping into channels cut in to, and sometimes right through, lacustrine deposits. BFA packages are internally partitioned by fifth-order erosion surfaces into individua l channel-fill units, commonly about 5 m thick. The BFA is dominated b y trough-cross-bedded fine to lower medium sandstone. Conglomerate, pe bbly sandstone, and ripple-laminated and planar-laminated sandstone ar e minor components. Rare mud-filled abandoned channels suggest a mean width and depth of > 160 m and 4.8 m, respectively. The dominance of t rough cross-bedding suggests relatively deep channels, whose banks may have been maintained by a dense vegetation cover. Repeated channel av ulsion and flood-related erosion resulted in a rock record dominated b y deep-channel and flood-stage deposits; bar-top facies were rarely pr eserved. Channel dimensions, lateral consistency of facies, paucity of muddy channel plugs, and regionally consistent paleoflow suggest depo sition 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 southwe st. 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 tecto nic juxtaposition involving ?several tens of kilometers of sinistral o ffset.