Episodic Growth in Waulsortian Mounds: Lake Valley Formation, New Mexico; and Pekisko Formation, West Central Alberta

Kent C. Kirkby, Dave Hunt, J. A. Toni Simo

Waulsortian mounds form an integral part of many Lower Carboniferous reservoirs, yet much of their character and growth remain enigmatic. Exposed Lake Valley mounds and subsurface Pekisko mounds grew episodically. Long periods of hiatus and erosion separated intervals of mound growth. These intervening hiatal 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.

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 also influence reservoir development and partitioning.

Mound growth patterns may reflect relative sea-level change, or basin anoxia. Growth phase boundaries truncate underlying strata and are locally overlain by breccias that may reflect relative sea level fall. Alternatively, regional bioturbation patterns in both basins record episodic anoxia, and fluctuations in the anoxic levels of stratified basins could have curtailed mound growth. Global ocean anoxia occurred during the Upper Devonian/Lower Carboniferous transition, and in many basins a tendency towards anoxia continued through the Tournasian.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995