THE EVOLUTION OF FLUVIAL SYSTEMS IN THE PORT HOOD FORMATION (UPPER CARBONIFEROUS), WESTERN CAPE-BRETON ISLAND, EASTERN CANADA

The sandstones of the Port Hood Formation (Westphalian A) of western C ape Breton Island, Nova Scotia, eastern Canada have previously been in terpreted, mainly from work on the upper part of the stratotype, to be the product of deposition from broad meandering rivers within a conti nental setting. Advances in methodology, namely the development of bou nding surface hierarchies and interpretive architectural element analy sis, allow for a reassessment of fluvial style in the formation as a w hole. This has been accomplished through the detailed lateral profile analysis of coastal outcrops from various stratigraphic levels within the formation, six of which are presented herein. The first profile (P i), is from the base of the Port Hood Formation, and the sandstones ar e interpreted to be the result of ephemeral, very flashy, fluvial disc harge in shallow, poorly confined channels of low sinuosity. The next three profiles, P2, P3, and P4, indicate that larger, perennial, low-s inuosity, braided channel systems, complete with large in-channel macr oforms separated by major (4th order) bounding surfaces, persisted dur ing deposition of overlying lower Port Hood Formation sandstones. Prof iles from higher up in the formation (P5 from the 'middle' and P6 from the upper Port Hood I Formation) again suggest the presence of in-cha nnel bars rather than point-bar deposits, and hence a low-, rather tha n a high-sinuosity fluvial planform is proposed. However, sedimentary structures are by this stage more commonly indicative of deposition in the lower flow regime. Palaeocurrent data collected from throughout t he Port Hood Formation in western Cape Breton Island further indicate that fluvial channels traversed a lower-gradient floodplain by the tim e 'middle' and upper Port Hood Formation strata were deposited. Nevert heless, an overall southeasterly regional palaeoflow persists througho ut the Port Hood Formation. This is more correlative with palaeocurren t directions obtained from Lower Carboniferous strata in the region, a t a time when depocentres were localized in rapidly subsiding half-gra bens in a transtensional tectonic setting, than with data from other U pper Carboniferous strata (mostly of later Westphalian-Stephanian age) , when a prevailing regional northeasterly palaeoflow developed over a large floodplain. Soft-sediment deformation in the fluvial sandstones can be explained by the rapid accretion that took place during basin filling but it might also have been a result of earthquakes that possi bly accompanied graben maintenance.