UPPER DEVONIAN BUILDUP-MARGIN AND SLOPE DEVELOPMENT IN THE SOUTHERN CANADIAN ROCKY-MOUNTAINS

The geometries of Upper Devonian (mid-Frasnian) carbonate buildup marg ins of the peechee Member (Southesk Formation) in the southern Canadia n Rocky Mountains reflect the influence of rising sea level punctuated by stillstands. Most Peechee buildup margins in the Cordillera retrog raded, whereas some aggraded or prograded in response to these sea-lev el changes and variations in the relative rates of adjacent basin-fill ing sedimentation. Peechee-margin accumulation and adjacent basin depo sition is best explained by a reciprocal model of sedimentation. Thick tongues (tens of meters) of stromatoporoid rudstones and bioclastic g rainstones aggraded vertically or retreated during periods of sea-leve l rise. As sea level decelerated, carbonate production was restricted and outbuilding occurred. Basinal (onlapping) sedimentation of the Per drix Formation muds increased during the ensuing stillstand of sea lev el. Subsequently, a renewed episode of relative sea-level rise caused buildup-margin retrogradation. This pattern of sedimentation, driven b y episodic fluctuations of relative sea level, occurred repeatedly dur ing Peechee time. A broad-scale shallowing-upward trend culminated in a progressive increase up-section in the abundance of submarine erosio n surfaces in the eastern Main Ranges and paleocaliche horizons in the Front Ranges. Southwesterly paleocurrents are inferred to have influe nced Peechee buildup-margin geometries and adjacent basin sedimentatio n. Retrograded (windward) margins formed in response to active current s and breaking waves along the northeastern edge of the Fairholme and Southesk-Cairn reef complexes. Leeward buildup margins, protected from the southwest-directed currents, exhibit prograding geometries. These characteristics are exemplified by the Ancient Wall complex, where th e windward buildup margins to the northwest and southeast are vertical ly aggrading, whereas the coeval Peechee buildup margin to the southwe st prograded approximately 2 km. Onlapping dysaerobic waters during Up per Cairn and Peechee time inhibited the accumulation of forereef skel etal organisms and served as an additional control on buildup-margin g eometry.