ABSTRACT

There has been some speculation on the existence of Waulsortian-type mounds in the Mississippian of the Williston Basin but, to date, little evidence has been found to make any positive identification. Location and recognition of Waulsortian-type mounds in the subsurface can be based on the geometry and composition of outcropping mounds of Mississippian to Permian ages in various parts of the Rocky Mountain region, as well as on their paleogeographic and paleotectonic settings.

Waulsortian-type mounds in outcrop are found in slope or toe-of-slope positions, as individual mounds (150 m across by 100 m high) or as mound complexes (1500 m across by 50 m high), and containing lime mudstone and skeletal remains. The Mississippian mounds are flanked by dipping beds of crinoidal grainstones and packstones. Sites of growth of the mounds appear to have been paleobathymetric positive elements on unstable crustal blocks.

Mounds in the Mississippian of the Williston Basin should be expected on positive elements created by repeated basement block paleotectonic or syn-depositional salt solution, the most likely localities being in the vicinity of the NACP conductor zone or proximal to the Nelson River gravity anomaly.

Two cores from the Souris Valley beds of extreme southeastern Saskatchewan contain lithologies that can be interpreted as representing mound or associated strata. One core contains unstratified lime mudstones with crinoidal debris in a geological setting in which surrounding cored wells reveal thinly bedded typical slope lithologies. The second core consists of steeply dipping crinoidal grainstones and packstones. Neither core features well developed reservoir rock, but Waulsortian-type mounds yield oil elsewhere. The reservoir potential of mounds in the Williston Basin depends on porosity enhancement by either dolomitization or exhumation of primary porosity by solution of pore-filling cements.