ANCIENT METEOR CRATER IN NORTHERN ALBERTA

HOTCHKISS STRUCTURE

by: Charles O’Dale

• Type: Complex
• Age (ma): 120-330a – MISSISSIPPIAN
• Diameter: 3.9 km
• Location: N 58° 49′ W 118° 16′ (approximate)

a Determining the timing of this event is poorly constrained stratigraphically since the Gething-Debolt unconformity marks a nearly 200 million-year gap in the geological record in this area.

Uniquely identifying this structure as an impact structure is a difficult task since
there are no wells penetrating the disturbed rocks of the structure.

The Hotchkiss structure is  indicated by the small red circle on the Alberta map,  in the north west near the Chinchaga River.

The Hotchkiss structure,  located near the Chinchaga river, is an anomalous feature that has been observed on seismic data. The structure  bears many of the diagnostic features of a complex impact crater. The structure is 3.9 km across and is buried approximately 1000-m below the surface. There are few wells in the area. Also in the area are a number of kimberlite pipes that are of considerable economic interest to local mining companies. The presence of these pipes, however, complicates the interpretation of this feature as an impact structure.

A combination of wind and weather prevented me from an overflight of the Hotchkiss structure (red circle). The routes we flew in GOZooM in our efforts to see the structure location are indicated here. In the area, we did manage to overfly Carswell, Pilot Lake and Gow craters.

The Hotchkiss structure exhibits many features, including evidence for a central uplift, large-scale faulting at the rim and in the central uplift, a breccia infill, and a continuation of the disturbance to depths in excess of 1500-m below the top of the feature. The structure also obeys many of the scaling relationships relating to impact features. At the time of formation between 120 and 330 million years ago,  the original size of this structure is estimated to have been 3.9 km in diameter by 480 m in depth. At the end of the modification state, the transient cavity had a diameter of 2.26 km and was about 630 m deep. (Mazur 1999)

Simplified stratigraphic column for the Hotchkiss area. Subsequent to its formation, the Hotchkiss structure experienced a large amount of erosion. The Gething-Debolt unconformity marks this period of erosion during which an estimated 500 m of the structure was eroded. (Mazur et al 1998)

Uniquely identifying Hotchkiss as an impact structure is a difficult task since there are no wells penetrating the disturbed rocks of the structure.

Mazur 1999

The Hotchkiss structure in northwestern Alberta shows many of the morphological characteristics of a complex impact crater.

Shown here are faults  and a continuous surface representing the general shape of the structure as interpreted on seismic line 2 (Mazur 1999).

2-D seismic data interprets the current extent of the feature and its preerosional dimensions. The current size of the area of disturbance is 3.5 km across and 400 m thick. Using scaling relations, the Hotchkiss structure is estimated to have been 4.5 km in diameter and 500 m deep at the time of formation between 120 and 330 million years ago (Mazur, Stewart and Hildebrand, 1999).

This is typical northern Alberta geology around the buried Hotchkiss structure.

Virginia Falls north of the Hotchkiss structure.

References

[see – METEORITE]
Mazur M.J. The Seismic Characterization of Meteorite Impact Structures, Department of Geology and Geophysics, Calgary Alberta (1999)

Michael J. Mazur and Robert R. Stewart, Interpreting the Hotchkiss structure: A possible meteorite impact feature in northwestern Alberta CREWES Research Report — Volume 10 (1998)