Upper Devonian buildup-margin and slope development in the southern Canadian Rocky Mountains

Abstract

The geometries of Upper Devonian (mid-Frasnian) carbonate buildup margins of the Peechee Member (Southesk Formation) in the southern Canadian Rocky Mountains reflect the influence of rising sea level punctuated by stillstands. Most Peechee buildup margins in the Cordillera retrograded, whereas some aggraded or prograded in response to these sea-level changes and variations in the relative rates of adjacent basin-filling sedimentation. Peechee-margin accumulation and adjacent basin deposition is best explained by a reciprocal model of sedimentation. Thick tongues (tens of meters) of stromatoporoid rudstones and bioclastic grainstones aggraded vertically or retreated during periods of sea-level rise. As sea level decelerated, carbonate production was restricted and outbuilding occurred. Basinal (onlapping) sedimentation of the Perdrix Formation muds increased during the ensuing stillstand of sea level. Subsequently, a renewed episode of relative sea-level rise caused build-up-margin retrogradation. This pattern of sedimentation, driven by episodic fluctuations of relative sea level, occurred repeatedly during Peechee time. A broad-scale shallowing-upward trend culminated in a progressive increase up-section in the abundance of submarine erosion surfaces in the eastern Main Ranges and paleocaliche horizons in the Front Ranges. Southwesterly paleocurrents are inferred to have influenced Peechee buildup-margin geometries and adjacent basin sedimentation. Retrograded (windward) margins formed in response to active currents 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 the windward buildup margins to the northwest and southeast are vertically aggrading, whereas the coeval Peechee buildup margin to the southwest prograded ∼2 km. Onlapping dysaerobic waters during Upper Cairn and Peechee time inhibited the accumulation of forereef skeletal organisms and served as an additional control on buildup-margin geometry.