by: Charles O’Dale
The scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near the Earth’s surface. The following example illustrates the shortcoming of using geomorphology in crater identification:
Alsever Lake compared to Brent impact crater
Alsever Lake – NON-IMPACT
Brent Crater – IMPACT
Alsever Lake (image LEFT) is located at the southern boundary of Algonquin Park. It is similar in appearance to the Brent impact crater (image RIGHT) with its two distinct bodies of water forming a circular pattern. Alsever has a central “land mass” like Brent and what appears to be circular outline. This is best viewed on some topographical maps.
2. CONFIRMED IMPACT
Carswell structure – geomorphology (courtesy of the University of New Brunswick).
Carswell structure – Interior (pink) = GRANITE; Inner ring (yellow) = DISTURBED ATHABASCA FORMATION; Outer ring (green) = CARSWELL FORMATION (dolomite) – (Sawatzky).
Fault zone and differing geology under the Charlevoix impact structure. I superimposed a schematic of the geologic faulting under the Charlevoix crater onto this aeronautical chart of the crater area. A line drawn west to east on the chart corresponds to the physical position of the schematic illustrating the fault zone and differing geology under the crater.
Charlevoix impact structure Digital Elevation Model with Earthquake Epicentres
Reflection seismic cross-section of Chicxulub along Chicx-A and -A1 (Bell et al. Forthcoming). The post-impact Tertiary sediments are clearly identifiable as high-frequency reflections from 0 to ~1 sec two-way travel time (TWTT). A topographic peak ring, with draped sediments, is identifiable on the floor of Chicxulub and separates the central basin from a surrounding annular trough. (GRIEVE et al 2003)
Structural contour map on the pennsylvanian Tensleep Formation (modified from Stone 1999). Contour interval is 200 ft (60 m). Base level datum is mean sea level. The trace of the outer rim fault zone just below the Tr-J unconformity is marked by the largest dashed circle. The smallest dashed circle near the center of the map is the annular fault zone that encircles the central peak. Within this annular peak-ring fault zone (the area marked by a gray stippled pattern), the coutoured tensleep horizon is missing (by ejection and/or erosion) and Amsden Formation rocks are uplifted and truncated at the Tr-J unconformity.
Structural cross-section B-B’ restored by flattening on the Sundance datum to remove the effects of Laramide deformation. Formation symbols are identified in the figure to the left. (Stone et al 2003). Comparison of the “standard” stratigraphic column on the Casper Arch and the section found in the central peak of the Cloud creek impact structure. Stratigraphic uplift (SU) of the Madison Formation in the central peak area above its normal elevation outside the crater is ~520 m. At the Tr-J unconformity, there is progressive onlap of Sundance members over and around the central peak (Stone et al 2003).
Topography and bathymetry of Deep Bay. Also indicated are location of drill holes and a geologic cross-section based on these drill holes (bottom). Innes et al (1964). PDF in quartz grains and feldspar were recovered from the drill site DOM 66-1 (Dence et al 1968. .
At Deep Bay the circular fracture zone marks approximately the outer limit of the fractured zone and may be interpreted as a hinge line about which the granitic rocks forming the rim have been uplifted. Indeed the rocks within this circle, have the general appearance of having been uplifted and shattered into huge blocks without having undergone much horizontal movement. (Innes et al 1964).
The 8-km diameter Des Plaines Structure exhibits complex faulting and shock features such as percussion fractures and strain lamellae, as well as a few shatter cones. The center of the crater lies under Big Bend Lake on the Des Plaines River. Seismic reflection data suggest that there are numerous other faults within the bedrock of Cook County. Courtesy of United States Meteorite Impact Craters.
The (WISCAH), evaluation presents evidence that the black mat stratum at Glover Bluff is the depositional result of an ET event due to its clear association with ejecta strata. Additional geological analysis is recommended at Glover Bluff to establish the exact timing, and to determine the relationship, if any, to known phenomena. The impact origin was confirmed in 1983, by the discovery of shatter cones (Read, 1983).
Profile of the Holleford Crater as reconstructed from drill-hole and surface observations. It will be seen that the original crater surface dips nearly 800 feet below plain level, while the zone of fractured rock extends to an estimated depth of about 2,400 feet. The estimate of breccia depth at the centre depends on theoretical considerations advanced by J.A. Rothenberg.
Looking North from the South East rim, note the “steep scarps”.
This false-color image shows a green ring depression that surrounds a central peak. The ring depression contains the Manicouagan Reservoir.
3-D perspective of the Maple Creek (White Valley) structure. Both the Belly River (RED) and Mississippian (BLUE) horizons are depicted. The general morphology of the complex crater can be seen in the Belly River horizon. (Westbroek et al 1995).
The trough and central uplift are imaged by seismic data (Westbroek, 1997).
The Maple Creek (White Valley) structure in southwestern Saskatchewan has been interpreted as a <75 Ma old impact feature. This structure has many of the morphological characteristics of a complex impact crater. The structure is interpreted to have a diameter of about 7 km with an annular trough and a raised central uplift.
The I-94 drill project results confirmed the seismic interpretation of an impact crater. Rocks penetrated by the 1,646-m well can be subdivided into three sequences (Friedenreich 1988)
Structure maps generated from the interpreted basement horizon in Newporte Crater. The evident concave shape and uplifted rim are visible. A vertical exaggeration of 2.5 was utilized. Image from Forsman et al (1996).
Topographic documentation of the area around the Pingualuit Crater.
Seismic intersect image, Red Wing Creek Field. Image courtesy of Roger Barton and True Oil.
Rock Elm structure, Wisconsin (French et al. 2004)
Topographical Map of the Slate Islands Impact Structure.
St. Martin impact structure – preserved with 100 m of Jurassic sediment cover.
Geologic schematic of the Sudbury impact structure (courtesy of F. Brunton).
Isopach of the brecciated Mississippian rim facies at Viewfield oil pool (Donofrio 1981)
Cross-section of Viewfield structure (Donofrio 1981).
Lake Wanapetei topographic (2003).
The 2.44 km diameter 110 m deep West Hawk Impact Crater (the deepest lake in Manitoba) is enclosed and completely submerged within West Hawk Lake (Ogilvie 1984). The depth of crater is indicated in feet, courtesy of Freshwater Institute, Department of Fisheries and Oceans, 2001 (in Boyd et al., 2002).
Subbottom acoustic profile across the center of West Hawk Lake basin, showing bedding in upper *20 m of the sedimentary sequence (H. Thorleifson 1993, personal communication, unpublished); lake is 3.8 km across and water depth is 111 m.
Whitecourt Crater – perspective view from SE. This image is derived by Light Detection And Ranging (LiDAR) technology. (Department of Earth and Atmospheric Sciences, University of Alberta)
Cross sections of the ejecta blanket along 038° and 110° with a reference figure showing the location of the sections. Approximate distribution of the ejecta blanket and the main soil pit and auger hole site locations are also provided. (Kofman et al – Meteoritics & Planetary Science 2010)
3. SUSPECTED IMPACT
Ground penetrating radar survey and interpretation of the Bloody Creek structure. Ground-penetrating radar traverses were obtained when the site was frozen in winter. They confirm the crater morphology of the structure, and the distinction of the shallow levels from the undisturbed bedrock. (Spooner et al 2009)
Image showing the remnants of a crater that UAlberta researchers theorize was left by a massive meteorite strike sometime in the last 70 million years. Colour variation shows metres above sea level.
A. 200 kHz single-beam bathymetry] B. Residual magnetic field. C. N-S chirp seismic profile (Suttaket al, 2013)
The Corossol Crater is a complex crater ~4 km in diameter with a central uplift, a prominent moat, and multiple, low-relief ridges. Quebec, Canada.
(after McCabe, 1982)
Howell Creek Structure Geological map. (BC Ministry of Energy and Mines, 2001).
Seismic data in the James River 3-D volume. Several interpreted horizons are shown. The upper horizon corresponds to the top of the Cambrian, the middle horizon corresponds to the Cambrian ‘event’, and the lower horizon corresponds to the Precambrian. Faulting in the James River dataset is divided between shallow rim faults and deep central and rim faults.
Generalized geologic and topographic map of the Merewether (possible) impact crater and vicinity. Traced from vertical photograph USAF T.P.12, run #22-209.
The layers of sedimentary rock that covered the crater sagged, stretching the rock at the rim causing differencial cracking. The fractures (joints) developed above the rims of the crater were eroded during the last ice age. The glacial erosion of the joints which formed about the crater rim formed the course of the Esopus and Woodland Creeks.
Bathymetry documentation of Skeleton Lake illustrates a suggestive “crater like” form on the lake bottom.
The Victoria Island structure is characterized by a concentric, annular, terraced rim and trough surrounding a structurally uplifted central peak. Contour lines of equal thickness over an area (Isopach map) of interval between upper Nortonville Shale marker and lower Domengine Formation marker, showing series of concentric circular ridges and troughs, together with positions of several major, curvilinear normal faults that surround the structure and cut the lower part of the isopached interval.
Contour lines of equal thickness over an area (Isopach map) of potential crater infill, between upper Nortonville Shale marker and base Nortonville Shale/top Domengine Formation marker.
4. PROPOSED IMPACT