Abstract: Edgecliff Bioherms - Patterns, Distribution and Basinal Controls

Thomas H. Wolosz

Bioherms of the Middle Devonian Edgecliff Member of the Onondaga Formation are subdivided into two facies - rugosan mounds and thickets, and favositid /crinoidal sand bank deposits. Four types of bioherm, based on variation in development of these two facies, are defined along an on-shore to off-shore transect. Shallow water structures are either small rugosan mounds or ridges. Thicket/bank bioherms are located in shallow but off-shore positions; while large mounds and mound/bank (pinnacle) structures formed furthest from shore. This trend is observed on both the eastern and western sides of the basin, but is asymmetrical across the basin with eastern surface exposures of mound and mound/bank bioherms located roughly 200 km east of the basin axis (eastern subsurface pinna le reefs are unknown) while the western subsurface pinnacles are only 60 km west of the axis. Such asymmetry suggests control by basinal dynamics. Eastern bioherms are rooted along a contact of Edgecliff packestones with underlying calcisilt facies, suggesting initiation of reef growth during a shallowing event. Western bioherms are rooted either on a disconformable surface or in grainstone directly above the disconformity indicating growth during sea-level rise. It is suggested that the absence of large pinnacles along the eastern side of the basinal axis is due to the initial shallowing in the east having been insufficient to trigger reef growth in those areas.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York