Waulsortian mounds form an integral part of many Lower Carboniferous reservoirs, yet much of their character and growth remain enigmatic. Study of exposed Lake Valley mounds and subsurface Pekisko mounds shows that the mounds grew episodically. Long periods of hiatus and erosion separated intervals of mound growth. These intervening hiatus periods distinguish the mounds' growth phases from other reef subdivisions that form depositional continuums. Each growth phase represents a separate colonization of the buildup's topography and has a distinct geometry and facies composition.
Both mound suites grew in significant depositional energy along the toe-of-slope of carbonate ramps that rimmed sediment-starved basins. Mounds were drowned by continued deepening and prograding wedges of fine-grained basin strata. The greater geographic scale of the Pekisko study resolved lateral differences in stacking patterns of the ramp and basin strata that are difficult to interpret at the outcrop scale.
Pekisko mounds contain anomalously porous and permeable reservoirs that are a combination of preserved primary porosity, fractures, and fabric-selective dissolution. Dissolution selectively removed micrite and certain skeletal components, particularly bryozoans. Secondary porosity occurs in ramp and mound carbonates in a broad belt that parallels the strata's updip erosional truncation. Although dissolution was associated with a major unconformity, it may have resulted from burial fluid migration beneath the unconformity, rather than from meteoric processes.
Recognition of the mounds' episodic growth has implications for mound, basin, and reservoir studies. (1) Growth phases provide a finer temporal framework for the mounds' study than biostratigraphy. (2) Cumulative growth patterns in the mounds form a record of the basin's physical evolution that is not available in the starved intermound section. (3) The growth phases' stratal patterns and depositional facies can influence reservoir development and partitioning. Mound growth patterns may reflect relative sea-level change, or basin anoxia. Regional bioturbation patterns in both basins record episodic anoxia, and fluctuations in the anoxic levels of stratified basins may have curtailed mound growth. Global ocean anoxia occurred during the Upper Devonian/Lower Carboniferous transition. This study suggests that a tendency towards anoxia continued through the Tournasian.