BRENT IMPACT CRATER

BRENT IMPACT CRATER

by: Charles O’Dale

  • Type: Simple
  • Age (ma): 396 ± 20a – DEVONIAN
  • Diameter: 3.8 km
  • Location: Ontario, Canada. N 46° 05′ W 78° 29′
  • Shock Metamorphism: Shatter cones and PDF in quartz and feldspar.

Impactor type:

  • Impactor type: (2025 update) –  identification of the Brent impacting body as an L-chondrite (Palme et al., 1978)
  • IA or IIIC non‐magmatic iron (Claeys 2010)
  • Ordinary chondrite; type L or LL – siderophile elements (PGE, Ni, Au) (Tangle, Hecht 2006).

a Dating Method: K-Ar studies on the coarsely crystalline melt rocks using post 1977 decay constants (J. Whitehead).

Characterization of the alteration present at the Brent impact structure, revealed at least the presence of a chloritization, Au‐depletion and K‐enrichment process in the melt‐fragment breccias.

Based on a multi‐signature approach by combining the moderately and highly siderophile elements, a precise meteorite classification into the IA non‐magmatic irons is possible. While the Ni/Cr, Co/Cr and Pd/Ir ratios point to a LL or L ordinary chondrite, the Ni/Co, other platinum group elements (PGE) and combined siderophile ratios do not. Based on a linear and magnified PGE pattern that is assumed to be representative for the impact meteorite, the IA or IIIC non‐magmatic irons are the only possibility. When all siderophile ratios are taken into account, the IA is by far the best fitting group.

The meteorite type is inferred as an L or LL chondrite from analysis of the impact melt samples for siderophile trace elements and for a Ni-Cr correlation (Palme et al., 1981).

Table of Contents

(I have added links to various chapters for ease of navigation)

  1. Introduction

  2. Geomorphology

  3. Aerial Exploration

  4. 2003 Ground Exploration – #1 Crater Floor & Rim

  5. 2006 Ground Exploration – #2 Breccia Search & Shattered Rock Wall Discovery

  6. 2007 Ground Exploration – #3 Canoe Exploration & Breccia Discovery

  7. 2025 Ground Exploration – #4 Shattered Rock Wall Detailed Analysis

  8. Side Notes

  9. References

1. INTRODUCTION

A search for meteorite impact sites in Canada was initiated following the discovery and interpretation of the Pingualuit Meteorite Crater as an impact site (Meen 1950). On the strength of Meen’s discovery, Beals, the Dominion Astronomer for Canada, instituted a crater research program at the Dominion Observatory, which included a systematic search of aerial photographs (Grieve 1975). This led to the confirmation of the Holleford structure as an impact site. As the Observatory program became known, others reported unusual, circular topographic features in Canada such as Brent and Clearwater.

This high altitude image of the Brent crater shows a vague circular area with two bordering lakes.
Early in 1951, Mr. John A. Roberts was looking over some of the high altitude aerial photos, similar to the above image that his aviation company had taken for the Government of Canada. He noticed that Gilmour and Tecumseh Lakes form a semicircle in a circular feature straddling the boundary of Algonquin Park north of the village of Brent on Cedar Lake. After consultation with the Dominion Astronomer for Canada and the Geological Survey of Canada, investigations were initiated at the site. Over the next ten seasons topographical, geophysical and geological investigations (including diamond drilling of 12 holes into the crater) were performed. Greater than 5,000 metres of drill core were recovered. Brent aerial image courtesy of Earth Impact Database, UNB.

General Area: South of the Ottawa River in the Canadian Shield in an area of rolling hills. The area has been glaciated. The target rocks are crystalline. Specific Features: Brent crater has been heavily eroded and is partially in-filled by post-crater sediments. There are two lakes within the crater, forming a semi-circular hoof-print shape. The remnants of the crater form a 3 km circular depression 60 m deep. The interior of the crater is noticeably smoother than the surrounding terrain and the structure clearly cuts across pre-existing folds and tectonic trends in the crystalline bedrock.

The Brent Impact Crater/Structure adjacent to Lake Nipissing Ontario (courtesy GOOGLE)

On the Google Earth images, note the small white square indicators. These are position reports from my SPOT personal locator beacon.

Brent Impact Crater/Structure (courtesy GOOGLE)

After impact, Ordovician seas deposited a thick layer of sediment over the crater. The crater was thus insulated from erosion until the last glaciers removed the sedimentary covering. The total amount of erosion undergone in the Brent crater area is estimated to be ~400 m (Grieve and Cintala, 1981). When the last glacier entered the crater over the north rim and reached the floor of the crater, it apparently spread out and gouged into the sedimentary rock along the northeastern and northwestern edges of the floor a little more deeply than elsewhere. The resultant slight depressions in the floor were filled with water when the final glacier retreated 11,000 years ago, creating the two lakes Tecumseh and Gilmour. The glacier also sculpted the area between the two lakes with a “ripple” superimposed on the landscape.

If this impact happened today, every tree in Algonquin Park would be flattened and covered with ejecta, Ottawa would experience a major earthquake and the most of the windows in the city’s buildings would be blown out!

2.GEOMORPHOLOGY

The topographical, geophysical and geological investigations carried out at the crater have documented the contents in the bowl shaped depression as (from the top down):

  • >250 metres of sedimentary fill (deposited after the impact in the Devonian period) – limestone, dolostone, sandstone, siltstone, shale and gypsum;
  • ~600 metres of brecciated zone;
  • ~20 metres of melt zone;
  • ~50 metres of fractured crystalline basement over the bedrock, and;
  • Bedrock, 1065 metres under the surface of the center of the crater floor, consisting of Precambrian crystalline igneous-metamorphic basement complex mainly of gneiss of granodioritic composition of the Grenville structural province (Grieve, 1978).

EARTH SCIENCE PICTURE OF THE DAY – BRENT – BRECCIA LENS

 

Paleogeography of Baltica and neighboring cratons at the time of the increased cosmic bombardment following the ~470 Ma asteroid breakup event illustrating the resulting known craters (red dots). Light blue color represents areas of shallow epicontinental seas, and dark blue areas of deep ocean. This distribution may, however, have varied somewhat due to periodical transgressions and regressions of the sea. The timeline documents the related meteorite falls (black dot and line).
The red dot represents the approximate area of the Brent impact 396 million years ago in the late Silurian or early Devonian Period.

At the time of this impact in the late Silurian or early Devonian, the most advanced creatures present on earth were marine crustaceans called trilobites, Europe and America are just about to collide, Ontario was approximately at the equator and plant life is just beginning to appear on land. The Brent impact crater is located within the northern boundary of Algonquin Park 75 km east of Lake Nipissing. It was named the “Brent crater” because of its proximity to the village of Brent, a divisional point on the Canadian National Railway’s transcontinental line. It is the largest known terrestrial crater with a simple, bowl-shaped form and perhaps the best known and possibly the most thoroughly studied fossil meteorite crater in the world.

Bouguer gravity anomaly over Gilmour and Tecumseh lakes region. The crater lies at the center – Modified from Millman et al, 1960.

A gravity anomaly at the Brent Crater produced by the sediments and fragmented rocks in the crater reinforces the meteoritic origin of this crater similar to other structures (seeWest Hawk and Wanapitei) that have been identified as impact events by similar gravity anomalies. It is interesting to note that in this gravity map that was published in 1960 the magnetic north had an indicated west declination (variation) of 10° 05’ W. Today in 2012 it is 12° 00’ W. The change is due to the drift of the magnetic north pole over the past 52 years (Chavez 1986).

From these studies it was theorized that immediately after the meteorite impact the crater was 600 metres deep and its rim was over 100 metres high. But over the eons it was “modified” by Devonion period sedimentary deposits and an estimated 220 metres of vertical erosion (Grieve and Cintala, 1981). The most recent erosion was caused by four or more ice ages, the last of which ended over 11,000 years ago. The gradual addition of the sedimentary layers in the crater tended to compact the under-laid rubble layer causing the bottom of the shallow sea occupying the crater to sink. The sedimentary layer grew in the bottom of the deep basin (crater) and was protected from erosion of downstream running water and glaciers flowing over the crater. This image taken from the south-east illustrates the bowl shape remnant of the crater with the crater floor capped by a 250 metre thick layer of sedimentary rock. If it were not for this layer of sedimentary fill displacing the water, the Brent Crater would resemble the water filled Pingualuit Impact Crater.

 

Old K–Ar Mineral Ages from the Grenville Province, Ontario

M. R. Dence, J. B. Hartung, J. F. Sutter

ABSTRACT – Hornblende-rich concentrates from quartz–feldspar gneisses of the Grenville Province near Brent Ontario, have yielded K–Ar apparent ages of 1570 to 1480 ± 80 m.y., while coexisting biotite- and feldspar-rich separates give ‘normal’ Grenville K–Ar ages near 900 ± 40 m.y. Comparison with the nearest Rb–Sr isochron dates suggests that the indicated hornblende K–Ar age represents a minimum age for time of crystallization of the gneisses in the Brent area and that the younger ages for minerals with lower blocking temperatures indicate a later thermal event in the metamorphic history of the Grenville Province.

The location of the drill holes indicated in the Brent crater.

Brent drill hole # B1-59

GPa
Gigapascal, 1 GPa = 1,000 MPa (Megapascal) = 109 Pascal, the SI unit of pressure. GPa is commonly used in the high-pressure range of shock deformation, 1 GPa = 10 kbar.

 

Geochemistry of the Brent impact structure, Ontario, Canada

Dr. Ph. Claeys, Dr. M. Elburg, Dr. S. Goderis, Dr. Leescommissie, Dr. P. Van den haute,Dr. F. Vanhaecke;

FACULTEIT WETENSCHAPPEN Vakgroep Geologie en Bodemkunde Academiejaar 2010–2011

Abstract

Thirteen impact melt samples and basal suevitic breccias from two drill cores of the 3.8 km diameter Brent impact structure have been analyzed for major and trace elements, including PGE. In selected samples, Os, Cr, W, Sr, and Pb isotope ratios were also measured. This multi-tool approach not only characterizes the projectile responsible for the formation of the structure, but also documents the distribution and nature of the meteoritic component (s) within. The Brent simple crater is an ideal case study: the target consists of mainly gneiss with minor alnoite dykes and the structure was extensively drilled beyond its lenticular melt sheet. Using a Ni-Cr correlation, the Brent structure was previously interpreted as formed by an ordinary chondrite (OC; type L or LL). Although the CI-normalized PGE concentrations of the melt rock show relatively flat patterns, the characteristic deviations from chondrite patterns could only be explained by fractionation of the meteoritic component. In recent years, a previously poorly constrained projectile type was characterized and proposed for the Rochechouart, Saäksjärvi, and Gardnos impact structures. The PGE pattern observed for Brent resembles that of IA and IIIC nonmagmatic iron meteorites (NMI). The moderately siderophile element concentrations were affected by hydrothermal alteration, but the melt zone samples retain Ni/Cr ratios not only consistent with OC, but also with IA and IIIC NMI

The platinum group elements (PGE) comprise platinum (Pt), palladium (Pd), iridium (Ir), osmium (Os), rhodium (Rh) and ruthenium (Ru).

First known Terrestrial Impact of a Binary Asteroid from a Main Belt Breakup Event
Jens Ormö, Erik Sturkell, Carl Alwmark & Jay Melosh

ABSTRACT: Approximately 470 million years ago one of the largest cosmic catastrophes occurred in our solar system since the accretion of the planets. A 200-km large asteroid was disrupted by a collision in the Main Asteroid Belt, which spawned fragments into Earth crossing orbits. This had tremendous consequences for the meteorite production and cratering rate during several millions of years following the event. The 7.5-km wide Lockne crater, central Sweden, is known to be a member of this family. We here provide evidence that Lockne and its nearby companion, the 0.7-km diameter, contemporaneous, Målingen crater, formed by the impact of a binary, presumably ‘rubble pile’ asteroid. This newly discovered crater doublet provides a unique reference for impacts by combined, and poorly consolidated projectiles, as well as for the development of binary asteroids.

3 – Aerial Exploration

4 – 2003 Ground Exploration – #1 Crater Floor & Rim

5 – 2006 Ground Exploration – #2 Breccia Search & Shattered Rock Wall Discovery

6 – 2007 Ground Exploration – #3 Canoe Exploration & Breccia Discovery

7 – 2025 Ground Exploration – #4 Shattered Rock Wall

8 – Side notes

9 – References

[see – METEORITE]

Chavez, R.E., An optimisation study of gravity data from the Brent Crater. First Break, Feb. 1986.

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

Grieve, R. A. F., Cintala, M. J., A method for estimating the initial impact conditions of terrestrial cratering events exemplified by its application to Brent Crater, Ontario. Proceedings Lunar and Planetary Science Conference 12th, pp. 1607-1621. 1981.

Grieve, R. A. F., Dence, M. R., Principle characteristics of the impactites at Brent Crater, Ontario, Canada (abstract). Lunar and Planetary Science IX, pp. 416-418. 1978.

Dr. Michael R. Dence and yours truly at a 2012 Sigma Xi Companions in Research meeting, Ottawa. Dr. Dence was prime on impact crater research within the Canadian Shield, 1961-81. He was one of the few indiviuals responsible for transforming terrestrial impact crater research into a respectable and scientific discipline of planetary science.

Grieve, R. A. F., The melt rocks at Brent Crater, Ontario, Canada. Proceedings Lunar and Planetary Science Conference 9th, pp. 2579-2608. 1978.

Grieve, R. A. F., The petro-chemistry of the melt rocks at Brent Crater and their implications for the conditions of impact (abstract). Meteoritics, v. 13, pp. 484-486. 1978.

Grieve, R.A.F.,Robertson P.B., IMPACT STRUCTURES IN CANADA: THEIR RECOGNITION AND CHARACTERISTICS Journal of the Royal Astronomical Society of Canada, V69, 1-21, Feb 1975

Grieve, R.A.F.,Robertson P.B., Shock attenuation at terrestrial impact structures Lunar and Planetary Institute, 1977

Hodgson, John H. 1994, The Heavens Above and the Earth Beneath, A History of the Dominion Observatories – Part 2 1946-1970.

Meen, V.B., CHUBB CRATER – A METEOR(sic) CRATER, Journal of the Royal Astronomical Society of Canada, V44, 169-180, 1950.

Millman, P. A., Liberty, B.A., Clark, J.F., Willmore, P. and Innes,M.J.S., The Brent Crater. Ottawa Dominion Observatory Publication, v. 24, 43 p. 1960.

Palme et al (1978)  New data on meteoritic material at terrestrial impact craters lunar and Planetary science IX p. 856-858 Lunar and Planetary Institute, Houston.

Palme, H., Grieve, R.A.F. and Wolf,R., Identification of the projectile at Brent Crater, and further considerations of projectile types at terrestrial craters. Geochimica et Cosmochimica Acta, v. 45, pp. 2417-2424. 1981.

Shafiqullah, M., Tupper, W.M. and Cole,T.J.S., K-Ar ages on rocks from the crater at Brent, Ontario. Earth and Planetary Science Letters, v. 5, pp. 148-152. 1968.

TAGLE, R. and HECHT, L., Geochemical identification of projectiles in impact rocks. Meteoritics & Planetary Science Volume 41, 26 JAN 2010.

Earth Impact Database