SUDBURY IMPACT STRUCTURE – GEOMORPHOLOGY

SUDBURY IMPACT STRUCTURE GEOMORPHOLOGY

The Sudbury Igneous Complex (SIC) adjacent to the Wanapetei Impact Crater.

The original circular crater, some 200 km in diameter, has been squeezed by Earth forces and eroded, so that only a much smaller remnant of it remains today (Ontario Geological Survey).
Diagrammatic representation along a northwest southeast profile of the possible sequence of events forming the Sudbury structure.
Geological map of the study region, simplified from Geological Map 2543 of the Ontario Geological Survey (1991) From Halls 2009. “Sudbury Igneous Complex (SIC) and Whitewater Group (WG). Archean granite and gneiss (AG), Archean greenstone (AV), Levack Gneiss (LG), Agnico Lake intrusion (AL), Huronian Supergroup (HS), Nipissing diabase (N), Grenville Province (GP), Murray fault (MF),Grenville Front (GF), Benny Deformation Zone (BDZ) and Flack Lake fault (FL). The dykes of three swarms, the 2450 Ma Matachewan, 2170 Ma Biscotasing and 1230 Ma Sudbury, are shown respectively as blue green and red lines. Faults are shown as black dashed line” (Halls, 2009).
The Sudbury Igneous Complex (SIC) geomorphology.
The Sudbury Igneous Complex (SIC) adjacent to the Wanapetei Impact Crater.
When the crater is viewed from the altitude of the International Space Station, only the SIC within the Sudbury Structure (highlighted by the oval in the landsat image) is identifiable as being related to an impact event. The structure is located in central Ontario, north of Georgian Bay and north-west of Lake Nipissing. The city of Sudbury is located to the south-east of the SIC.

Formation of the original crater

The Sudbury Structure is situated within a unique Geotectonic setting in northeastern Ontario, being sandwiched between:

  • the Archean-age (>2.5 billion-year-old) Superior Geologic or Structural Province, situated to west and north of the structure, and;
  • the Proterozoic-age (>1.9 billion-year-old) Southern Geologic or Structural Province Huronian Supergroup, deformed by the (1.9 billion –year-old) Penokean orogeny , and situated to west, south and east of the Sudbury Structure.
  • The boundary of the Proterozoic-age (~1 billion-year-old) Grenville Geologic Province presently lies approximately 10 km to southeast of the SIC. The Grenville orogeny occurred 800 million years after the Sudbury Crater was formed. The SW-NE trend of the Grenville Front Structural Zone, which delineates the northernmost margin of the Grenville Structural Province, is roughly parallel to the long-axis of the SIC.
Illustrated in this sketch (courtesy of James E. Mungall) is a view of the sequence of events that may have produced the current structural relations between the SIC and the Huronian outliers (Huronian sediments were deposited between 2450 and 2219 Ma on the subsiding margin of the Superior craton).

The transgressive nature of the passive margin produced a sequence which onlapped and thinned progressively toward the northwest. The Blezardian orogeny caused the formation of basement-cored tight folds in the metasediments, which were peneplained and submerged by 1850 Ma. At 1850 Ma a large impactor created a transient crater at least 100 km in diameter and 30 km deep somewhere in the vicinity of the current SIC. Within about ten minutes of the impact, the crater had rebounded and collapsed into its final form. Inward collapse of the transient crater walls was accomplished along detachment surfaces, now preserved as anastomosing networks of pseudotachylite-filled faults (Sudbury Breccia) tens of km in length. Lateral collapse and structural uplift in the center worked together to form a crater approximately 200 km in diameter. The South Range Shear Zone (SRSZ) line on the sketch is the transition from pristine North Range to deformed South Range of the SIC and occurs over a distance of less than 20 km.

Two Segments of the Crater

The Sudbury Structure is interpreted to represent the tectonized and deeply eroded remnant of a multi-ring or peak-ring impact basin (Stoffler et al). Approximately 4 km of erosion over the eons has obliterated the crater rim. Tectonism has possibly deformed the original crater into an ellipse. The subsequent metamorphism in the structure is tied to tectonic activity such as collision of continents and folding and thrusting up of crustal rocks. A zone of deformation (shatter cones and rock metamorphism) has been documented to 74 km from the SIC.

Geological map of the study area created by Ames et. al, (2005). This map highlights the
Foy offset dyke and some of the large faults in the area.

This geologic schematic of the Sudbury structure illustrates the present day remnant of the Sudbury Meteorite Crater comprising of:

  • the surrounding brecciated footwall rocks of both the Superior and southern Structural Geologic Provinces extending up to 100 km away from the present-day position of the Sudbury Igneous Complex (SIC);
  • the Sudbury Igneous Complex (that formed as a result of impact-triggered magmatism, or deep crustal melting); and
  • the Sudbury Basin within the SIC, comprising rocks of the Whitewater Group (found only in the interior of the SIC). The Whitewater Group consists of the Onaping, Onwatin and Chelmsford Formations (J.E. Mungall).

1. The Sudbury Igneous Complex (SIC)

Thick sheets of melted rocks line the bottom of many large meteor craters. Some of these impact melts are derived from the release of kinetic energy at impactor contact that is converted to heat. Also, rocks lying kilometers deep within Earth are often on the verge of melting but are prevented from doing so by the immense pressure from the weight of the material lying above them. A large impactor would blast away this weight, releasing the pressure on the buried rocks and causing the underlying minerals to melt.

The impact melts may not fully cool for hundreds of thousands of years. In the meantime, water from the environment and the heat from the newly exposed rocks can combine to form hydrothermal systems in the heavily fractured rocks in and around the crater. Scientists believe such warm mineral-rich venues could have played a role in the early development of life on Earth. (Science News: 3/9/02, p. 147) Evidence of the hydrothermal systems is documented in my ground tour.

The SIC is this type of large melt sheet produced from crustal melting resulting from a cosmic impact. The target rocks, which remained within the crater after the impact, ponded to form a sub horizontal sheet of magma and differentiated as it cooled. It is currently exposed as an elliptical 60 x 30 km, 2.5 km thick remnant of the original impact melt sheet and consists, from bottom to top, of inclusion-rich, in places ore-bearing, quartz diorite sub layer, norite, quartz gabbro, and granophyre layers, and, within the target rocks surrounding the SIC, the quartz dioritic offset dikes.

RADARSAT radar image of the Sudbury (left) and Lake Wanapitei (right). The close proximity of these two impact structures is strictly coincidence. The Wanapitei crater occurred over 1.8 billion years after the Sudbury impact.

2. The Whitewater Group

The SIC is overlain by the 1.8 km thick Onaping Formation. It consists of impact melt breccia, suevite and reworked suevite from:

Fall-back (collapse of the original crater) and Fall-out (impact debris) forming a 2 km post impact sediment over the SIC melt rock; and,

Wash-in – post impact sediment (the impact happened in a shallow sea).

The rock fragments in the breccias of the Onaping Formation are from the impact target Archean and Proterozoic rocks of the Superior and Huronian Provinces of the Canadian Shield (Brunton).

The Onaping Formation is covered by 600 metres of argillites and minor exhalative carbonates and cherts of the Onwatin formation. This formation occurred during a period of quiescence after the impact basin formation.

The end of this quiet period was signaled by the abrupt appearance of the 850 metre-thick siliciclastic turbidites (sedimentary deposits settled out of muddy water carrying particles of widely varying grade size caused by turbidity currents) of the Chelmsford Formation (Rousell, 1972, 1984), which have been interpreted as a flysch apron deposited in the foredeep ahead of an advancing late Penokean mountain front (Young et al. 2001).

 

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