MANICOUAGAN IMPACT STRUCTURE – MORPHOLOGY

MANICOUAGAN IMPACT STRUCTURE – GEOMORPHOLOGY

This false-color image shows a green ring depression that surrounds a central peak. The ring depression contains the Manicouagan Reservoir. A fracture halo, which extends out to -150 km from the center, was first noted on Skylab photography. This halo is best developed in the west and south.

The morphological elements of the Manicouagan structure are based on topographical expression:

  1. outer circumferential depression – ~150-km outer diameter;
  2. outer disturbed zone – ~150 km diameter;
  3. inner fractured zone – ~100 km outer diameter;
  4. annular moat – ~65 km outer diameter;
  5. inner plateau – ~55 km outer diameter;
  6. central region – ~25 km outer diameter. (Grieve, Head 1983)
The most striking morphologic element is the -70 km diameter annular depression filled by the waters of the Lac Manicouagan reservoir. The annular depression is interpreted as the glacially overdeepend expression of the interior contact between the crater floor and inner blocks of the original rim . In the middle of the lake is a dissected plateau capped by -200 m of impact melt rocks and a series of uplifted peaks -5 km north of the center.

At the point of impact, the country rocks were instantaneously evaporated/melted/shattered by the energy released leaving a 200 to 600 cubic kilometre sheet of impact melt directly on basement rocks. The target rock in the vicinity of the structure is Grenville age amphibolite to granulite facies quartz and feldspar gneiss, with local anorthosites, metagabbro and metasediments overlain by Ordovician limestones, dolomites, slates and sandstones. The force of the impact exhumed and liquefied these target rocks down to as deep as 9 kilometres. The original crater became a melting pot for relatively young rocks at the surface and for much of the older minerals originally buried kilometres below the site of the impact. The heat released was so intense that it took between 1,600 and 5,000 years before the melted rocks cooled. Changes in these impactite textures toward the interior of the crater progressively increased in proportion of superheated melt and decreased in fraction of cold fragmented country rock material (Simonds 1976).

Bouguer gravity map of the Manicouagan Impact. (2009 Kord Ernstson)
This map represents the Manicouagan Impact structure before the Manicouagan multiple-arch buttress dam was constructed. The dam filled the annular moat to its present depth, creating a circular reservoir for hydro-electric power.

 

Outer Circumferential Depression, Outer Disturbed Zone and Inner Fractured Zone

 The magnitude of fracturing of the country rocks in the Manicouagan structure increased towards the centre of the crater, the point of maximum shock effect. The fragmentation increased to where the energy from the impact caused the rocks to melt. These melted rocks remain today as the central peak of the crater (the island in the image). The “smaller” fragmented rocks surrounding the “melt rock” central peak were easily evacuated by glaciation and erosion. The annular moat around the Manicouagan central peak is what remains after the country rocks that experienced “maximum” fracturing were removed. This circular moat is an impact indicator.

Peripheral Trough (annular moat)

The water filled circular annular moat that is prominent in space images is only one third of the size of the original crater. The water in the annular moat fills a ring where impact-brecciated rock was eroded away by glaciation. Before flooding of the reservoir, isolated outcrops of tilted and deformed limestone, siltstone and shale were found on the inner edges of the moat (Murtaugh, 1975). This rock formation (RIGHT) is found at the extreme eastern portion of the annular moat on one of the small islands. Note the rock structure is breccia free gneiss. The central peak of the structure is visible over 10 km in the distance.

Inner Plateau

The Inner Plateau of the Manicouagan structure is bounded by the annular moat, overlain by melt sheet, underlain by shocked basement rock (Orphal, Schultz 1978). We found a “lunar landscape” here containing various breccia types. The astronauts exploring the moon found that impact-melt breccias, similar to what we found here, were the most common rock types at the Apollo highland sites (Apollos 14, 15, 16 and 17) (Haskin 1998). We documented impact breccias formed by similar and very different country rocks like those found on the moon!

Central Region

The Central Region of the Manicouagan Structure is a complex zone of uplifted, shocked and metamorphosed basement rocks with small tabular bodies of impact melt and pseudotachylite veins (Orphal, Schultz 1978). Recent U-Pb zircon dating of the impact melt gave an age of 214 ± 1 million years.

The Copernicus Crater on the moon. The annular moat of the Manicouagan impact crater would completely fit inside Copernicus (Courtesy NASA – A12).

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