HOWELL CREEK STRUCTURE, British Columbia, Canada
by: Charles O’Dale
- Type: Simple
- Location: N 49° 13’ W 114° 38’
- Age Ma: ~90-97a – CRETACEOUS
- Diameter: ~10 km
a This age assumes that the impact occurred after the intrusion of the 97 Mya Howell Creek syenites but preceded deposition of the 90 Mya Cardium sands. (Maine, 2003).
ABSTRACT The Howell Creek structure (HCS) exposes an area of Upper Cretaceous sandstones and shales surrounded by Paleozoic limestones in the southern Canadian Rocky Mountains. Since its discovery, the structure has become widely known as a tectonic window in which the usual relationships between overthrust and subthrust strata have been modified by later faulting. Recent road works and a forest fire have created new exposures which show that the Howell Creek structure is an ancient rock slide, emplaced during or shortly after the phase of extension faulting that followed the early Tertiary deformation of the Front Ranges of the Rocky Mountains.(Peter B. Jones, 1997).
ABSTRACT The assumption that the Upper Cretaceous strata in the Howell Creek area occur as a window in the Lewis Thrust presents structural difficulties which are not easily reconciled. The writer presents an alternative hypothesis suggesting that the strata between Howell Creek and the Flathead Fault are an allochthonous block of a supra-Lewis Thrust now preserved in a graben. The evidence to support this is mainly stratigraphic. The hypothesis depicts a structural evolution which conforms to the deformation seen elsewhere in the Rocky Mountains, modified by tensional faulting as observed in the Basin and Range area.(D. H. Oswald, 1964).
An Idea of an Impact
Jones found that contrary to the continuous stratigraphic column expected for a tectonic window, the CIGOL well had shown that the HCS was not stratigraphically continuous (Figure left). Rather an unexpected unconformity was encountered when the well passed directly from 90 MYa Cretaceous Cardium sandstones into 325 MYa Mississippian Rundle Group limestone. This unconformity represents a geochronological gap of nearly 235 MY in the strata making up the HCS. This is equivalent to approximately 1,400 m of missing rock based on equivalent sections from the Shell MacDonald d-30-H located southeast of the CIGOL 10E; Jones suggests up to 3,050 m of equivalent section could be represented within this unconformity.
HYPOTHESIS: the Howell Creek Structure (HCS) was not a window at all, rather it was originally a marine or lacustrine trough on the order of 458m deep in which an 8 cubic kilometer Paleogene landslide was deposited. The scattered Proterozoic outliers within the structure were actually megabreccia blocks which are remnants of this slide. Of the preliminary work that Jones completed in synthesizing this new model of genesis for the HCS he left a number of key questions to be answered: “what was the origin of the basin into which the slide came to rest?”
The HCS “may be” the footprint of a complex marine impact crater that was originally on the order of 10 km in diameter. Dating of the impact is still uncertain though it appears to be of Cretaceous age. This age assumes that the impact occurred after the intrusion of the 97 Mya Howell Creek syenites but preceded deposition of the 90 Mya Cardium sands. The unusual exposure of the complex was due largely to excavation and sliding initiated by this event. The truncation of the structure occurred when the Harvey Fault roughly bisected the basin and uplifted the eastern quadrants on the footwall of the fault. The removal of this segment of the structure from the protective basin in which the western segment remained exposed it to erosion and its ultimate loss from the geological record. The wasting of the eastern quadrants of the structure may have contributed essential detrital materials to the generation of the Kishenehn-like breccia/conglomerates found in the Eastern Outlier of the structure (Jones, 1977). As these rocks appear to be isolated to the HCS they may provide important mineralogical clues either for or against this model and as a result will receive intense scrutiny. The ongoing deformation and shaping of the structure could easily have triggered sliding due to recurring rim collapse as the Laramide Orogen continued to mold and shape the structure into the truncated, ellipsoidal form we see today. As with any hypothesis the challenges and questions abound and so they should. It is a place to start though on this journey of discovery. And by following the map that has been preserved within the rocks, the folds, the minerals, and the many mysteries yet to be unmasked to add a little more to the understanding of this remarkable mystery in the mountains, the Howell Creek Structure. (Darren P. Maine, CET, 2003)
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Grieve R.A.F., Robertson P.B., IMPACT STRUCTURES IN CANADA, the Journal of the Royal Astronomical Society of Canada, February 1975
Peter B. Jones, The Howell Creek Structure — A Paleogene Rock Slide in the Southern Canadian Rocky Mountains. Bulletin of Canadian Petroleum Geology Vol. 25 (1977), No. 4. (August), Pages 868-881
Darren P. Maine, CET. Howell Creek Structure? 2003. Canadian Rockhound Vol 7, No. 2
D. H. Oswald, The Howell Creek Structure. Bulletin of Canadian Petroleum Geology Special Guide Book Issue: Flathead Valley Vol. 12 (1964), No. 2S. (August), Pages 363-377
Price, R.A., “Fernie Map-Area, East Half, Alberta and British Columbia 82G E-1/2”, Paper 61-24, Geological Survey of Canada, 1962