Odale-Articles-Merewether

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IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

MEREWETHER (Possible Impact Structure)

The Merewether (possible) impact crater, Labrador, Canada.
The Merewether (possible) impact crater, Labrador, Canada.
  • Age: <870 years* Dimensions (three structures)
    • Prime: Diameter: 198.12 Metres, Depth: 47.244 Metres;
    • North: Diameter: >46 Metres, Depth: >24 Metres;
    • South: Diameter: >15 Metres, Depth: unknown.
  • Location: Labrador, Canada N 58° 02’ W 64° 02’
  • Dating Method: Estimated from sediment analysis – see text – (Meen 1957)
Possible link to the Younger Dryas Extinction

The Merewether structure is located north of the tree-line in Labrador approximately 93 kilometres south-west of the Saglek Fiord. I initiated my study of the Merewether structure based on an article in the February 1975 issue of the Journal of the Royal Astronomical Society of Canada. The authors classified this structure as “Probable Meteorite Crater”, an almost perfectly circular steep-walled crater, 0.2 km in diameter, in ground moraine (Grieve 1975). Meen found no evidence of meteoritic material or brecciation at this site and there has been no reported scientific investigation since. It remains on the list of probable impact sites because of its striking, crater-like form.

In the summer of 2005 I had the opportunity to over-fly the structure on one of my aerial exploration trips to the northern part of Labrador. I noted during my short stay over the structure that there are actually three small craters formed in a line (suggesting a crater "string"?). Merewether has been on the “list of suspected impact craters” for at least 50 years but it remains an enigma. There are no known natural geological causes to explain the formation of the structures and irrefutable evidence to indicate an impact event has not been discovered to this date (April 2006).


The Merewether (possible) impact crater - visible under the wing, looking north.
This image looking north in the area surrounding Merewether was taken from approximately 1000’ above ground and shows just how inconspicuous the small water filled crater is. The structure is the small greenish pond in the center of the picture, just under the wingtip. It was first seen during a routine flight over Labrador in 1943 by Arthur F. Merewether, the Chief Meteorologist for American Airlines. It was later named Merewether Crater and Merewether Lake. In the summer of 1954, Dr. V.B. Meen, of the Royal Ontario Museum, headed a ground study to the area. The Merewether structure closely resembled the Pingualuit crater in shape and his expedition was initiated to document any impact evidence. During the short time allowed with the season, Dr. Meen documented a substantial amount of geologic data (quoted in this article). To my knowledge there has not been any other ground research performed here.
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 Merewether (possible) impact crater - north west.

A generalized geologic sketch of the area by Dr. Meen (left) illustrates that the “grain” of the glacial moraine plateau forms north-south ridges. The moraine consists of coarse boulders bound together with sands and clays. The two smaller bowl shaped structures on each side of the prime structure are indicated by numbers 6 (north) & 14 (south). The south-west facing image (right) highlights the “three craters” of the Merewether structure that Dr. Meen sketched. The two smaller structures are positioned on each side and in a roughly north-south line through the prime structure. The water’s green colour is due to suspended silt (rock flour) which reflects the incident light before the longer wave-lengths have been absorbed by the water.

Topographic map of the Merewether (possible) impact crater with contours of the bottom of Merewether Lake. The elevation datum was transferred by Paulin altimeter from Saglek Fiord. Elevation of water surface 2308 feet. (G.M. Stanley and F. Rigler).
The Merewether (possible) impact crater - north west corner.
This topographic map of the prime Merewether structure illustrates its almost perfect circular bowl shape formed in ground moraine. The structure walls slope uniformly at an average of 35° to about 100 feet (30.48 metres) depth and then flatten out to form an oval basin. It is not known whether the structure bottom reaches close to or penetrates bedrock. The bottom of the structure is composed of large blocks except at the centre where some sediment has accumulated. The sediment contains very little organic matter but consists almost entirely of alternating bands of silty clay and sand which may represent annual fluctuations in the type of material carried into the lake from its drainage basin. The age of the structure is estimated based on the following assumptions:

- the banding of the sediments was caused by annual fluctuations in the deposition, - the rate of deposition has decreased or remained constant since the structure was formed, and - the rock basin of the lake was originally conical.

Since there were 30 varves in one foot (0.3048 metre) of sediment and the maximum depth of sediment was estimated to be less than 29 feet (8.8392 metres), the maximum age of the structure would be less than 870 years.

This image of the prime structure (right), looking to the east, further illustrates its almost circular shape and steep walls. There is no bedrock uptilted to form a rim. Around the north, east and south the rim merges with the adjacent hills. On the west the rim is definitely higher than the adjacent terrain. The “long” diameter of its slight elliptical shape is at right angles to the past directional movement of the glaciers.

Topographic map of the Merewether Craterlet(possible) impact crater (lake surface shaded) with contours of the bottom of Lake 6 (F. Rigler and L.I. Cowan).
The topographic map of the north Merewether structure (left) illustrates its distorted circular shape. It is not known whether the structure bottom reaches close to or penetrates bedrock. The north structure is at least 24 metres deep from the top of its banks to the bottom of the bowl. This is an unnatural depth for so small a pond.


The Merewether (possible) impact crater - looking south.
The image of the north structure (right) looking south with the prime and south structures in the background, illustrates its steep “crater” walls. The west rim of the north structure has been eroded away by the stream course in this valley. The north and south structures are similar in that they are nearly round with very steep encircling banks above the water. Depth sounding of the south structure was not completed.
Magnetic contour map of vicinity of Merewether (J. Vise and L.I. Cowan).
A magnetic survey of the area of the Merewether structure revealed a magnetic low on the western side of the prime structure compared to its immediate surroundings. The differences of magnetic flux on each side of the structure “may” be caused by the remains of the impacting meteorite under a part of the rim.

Irrefutable evidence for a meteorite impact at the Merewether structures is still lacking. Drilling in the craters for evidence of planar deformation features has not been performed and no shattercones, impact melt or meteorite fragments have been found at this site. This may be explained by the existence and movement of glaciers over the structure at the time of impact causing a smoothing of the rims and removal of any fragments of the impactor.

Samples of vegetation were collected at various points and distances about the structure. This dried material was submitted to the Chemistry Division of Science Service, Canada Department of Agriculture, for analysis. The analyses were determined colorimetrically for iron and polarographically for nickel, by Dr. Claude Sirois. The results were inconclusive but pointed to a trend toward higher nickel-iron content in plant material collected in the vicinity of the structure than in that collected further away from it (Gillett 1960).

The geologic study of the Merewether structure has eliminated the following possible causes for the shape aspects of the structure:

  • Volcano or intrusive plug;
  • Collapse due to abstractions beneath;
  • Glacial Kettle; and
  • Pingo.

The following features of the prime structure taken individually would not be conclusive, but together suggest that the Merewether structure may be impact related (meteorite crater remnant):

  • Extreme symmetry;
  • Exceptional depth;
  • Semblance of rim which is highest at the end of the longest diagonal;
  • A slight magnetic anomaly at the west rim;
  • A possible slight increase in the iron-nickel content of nearby vegetation (Gillett 1960);
  • The excellent agreement with Baldwin’s law for explosion craters (Baldwin 1949)*;


*Baldwin (1949) has shown that terrestrial explosion craters all obey two laws which relate rim height, diameter and depth of the craters. There is no appreciable rim at Merewether, hence it is possible to use only one of Baldwin’s equations as follows: D=0.1083d2+0.6917d+0.75 where D=log diameter in feet, d=log depth in feet (using average rim height). Taking the average diameter as 650 feet (198.12 metre) and the depth as 155 feet (47.244 metre) we obtain: D(2.81)=2.79. This is excellent agreement and would be even better if the value of “d” had not been reduced by the presence of sediment in the bottom. If glaciated, then the reduction of rim height also has reduced the value of “d”. The agreement with Baldwin’s formula, therefore, would indicate that this crater is of explosion origin.

Hypothesis by the author: The Merewether structure may have been caused by an impactor that split into at least three parts upon its collision with the earth’s atmosphere. The parts then impacted the earth (at less than their original cosmic velocity) at a position covered by glaciers and penetrated the ice to the underlying moraine material. The glacier removed any trace of the impactor and eroded the craters.

Recent research has indicated that the Merewether structure may be related to the Younger Dryas Extinction.


References

Grieve R.A.F., Robertson P.B., IMPACT STRUCTURES IN CANADA, the Journal of the Royal Astronomical Society of Canada, February 1975

Meen V.B., MEREWETHER CRATER – A POSSIBLE METEORITE CRATER,the Proceedings of the Geological Association Canada 1957 pp 49-67. (unless otherwise noted, the Merewether data quoted above is from this paper).

Ian SPOONER, George STEVENS, Jared MORROW, Peir PUFAHL, Richard GRIEVE, Rob RAESIDE, Jean PILON, Cliff STANLEY, Sandra BARR, and David MCMULLIN, Identification of the Bloody Creek structure, a possible impact crater in southwestern Nova Scotia, Canada. Meteoritics & Planetary Science 44, Nr 8, 1193–1202 (2009)

Haynes, C. V., Younger Dryas ‘‘black mats’’ and the Rancholabrean termination in North America Departments of Anthropology and Geosciences, Arizona, January 23, 2008.

Isabel Israde-Alcántara, James L. Bischoff, Gabriela Domínguez-Vázquez, Hong-Chun Li, Paul S. DeCarli, Ted E. Bunch, James H. Wittke, James C. Weaver, Richard B. Firestone, Allen West, James P. Kennett, Chris Mercer, Sujing Xie, Eric K. Richman, Charles R. Kinzie, and Wendy S. Wolbach, Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis, University of Hawaii, Honolulu, HI, January 31, 2012

Younger Dryas Extinction Impact Related? (Bloody Creek @ 29:30).


Notes

  1. PHOTOGRAPHIC IMAGES: Eric Kujala and Charles O’Dale.


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