INTERPRETATION OF THE LATE CRETACEOUS POLYGONAL FAULT SYSTEM AT ALIDA, SASKATCHEWAN

Siliceous Late Cretaceous sediments of the Western Interior Seaway of North America are host to a previously unreported polygonal fault system (PFS). The North American PFS (NAPFS) is an expansive network of fractures and faults developed in these fine-grained sediments. Interpretation of seismic data from southeast Saskatchewan has identified faulted sedimentary strata with a polygonal planform geometry of aggregated fault traces. The faults have spacing, throws, and stratigraphic tiers that are consistent with other PFS.

Interpretation of a 10 mile² 3-D seismic dataset from Alida, Saskatchewan provides some insight into fault timing and geometry for the NAPFS. Here, faulting of the Pierre Shale occurred in two stages. The lower PFS zone is basal Campanian, starts at ~500 m depth, and has arcuate-shaped listric normal faults that have an average throw of ~10 m and a fault density of ~10 faults/mile². The upper PFS zone (upper Campanian) has similar fault throws and areal densities. However, the upper PFS zone has markedly different planform geometry to the coalesced fault traces. Interpretation of this difference is ongoing, with the interpretation of the seismic data volumes aided by ~80 wellbores within the survey outline.

Implications for a shallow PFS are numerous. Water wells have been drilled in the area to depth of the PFS. Open or closed faults could affect water deliverability. Surface faults have been erroneously attributed to deeper zones. Some of the wellbores have encountered fault traces; these wellbores should help in the interpretation for PFS in other areas. A better understanding of the NAPFS in this area should result in improved models for these points.