BRECCIA

BRECCIA

by: Charles O’Dale

  1. introduction;
  2. breccia dike;
  3. breccia impact;
  4. breccia monomict;
  5. breccia polymict;
  6. breccia bunte;
  7. stishovite;
  8. suevite;
  9. conglomerate breccia;
  10. specific crater breccia.

1. INTRODUCTION

BRECCIA – from a Latin word meaning “broken” or from Italian indicating both “loose gravel and stone made by cemented gravel”.
A clastic sedimentary rock composed of angular clasts in a consolidated matrix of finer-grained rock fragments, glass, or crystallized melt.. Breccias can be produced in several geologic processes: tectonic breccia, volcanic breccia (eruption breccia, vent breccia), sedimentary breccia (e.g., rock fall breccia), collapse breccia (e.g., in karst areas). Breccias may be distinguished depending on the origin of the clasts, monomictic (monogenetic, monolithologic) and polymictic (polygenetic, polylithologic).

Breccias are extremely common in the central uplift, in crater-fill deposits, and in the ejecta blanket of meteorite impact craters.

Dimict breccias or dilithologic breccias are made up of only two lithologies. The term is usually applied to a common type of rock collected on the Apollo 16 mission that consists of anorthosite (light color) and mafic (dark, iron rich) crystallized impact melt in a mutually intrusive textural relationship.

Most brecciated lunar meteorites are regolith breccias.

2. BRECCIA DIKE

A dike in the common geological sense is a mostly tabular body of different materials (minerals, rocks, ores) cross-cutting the host rock. In impact structures, breccia dikes have played an important role in the understanding of the impact cratering process. It is generally suggested that for the most part the breccia dikes are formed in the excavation stage by injection of brecciated material into the walls and the floor of the expanding excavation cavity. Later formations of breccia dikes in the modification stage incorporating earlier formed ones may lead to generations of breccia dikes.

Schematic radial cross-section through one-half of a simple impact structure, showing locations of different impact-produced lithologies. Curved lines show isobars of shock pressures (in GPa) produced in the basement rocks by the impact.

3. BRECCIA IMPACT

The rocks at an impact target site are melted, shattered, and mixed during the impact explosion. When the site finally settles and cools, a new composite rock, impact breccia in bodies tens to hundreds of meters in size, is the result.

The petrographic and geochemical study of actual rocks from the potential impact structure will bring final confirmation of the presence of an impact structure. In case of a structure that is not exposed on the surface, drill-core samples are essential. Good materials for the recognition of an impact origin are various types of breccia and melt rocks. These rocks often carry unambiguous evidence for the impact origin of a structure in the form of shocked mineral and lithic clasts or a contamination from the extraterrestrial projectile.

Lithologies showing these unique diagnostic shock effects, formed at pressures ≥10 GPa, tend to be restricted to two locations:

  1. crater-fill materials (suevites, melt breccias, and fragmental impact breccias) deposited in the crater; and
  2. brecciated basement rocks, often containing shatter cones, near the center of the structure.
in situ Impact Breccia within the Manicouagan Impact crater. An impact-melt breccia can be regarded as in igneous rock because it formed from the cooling of a melt.

4. BRECCIA MONOMICT

Monomict breccia will contain clasts of identical lithology and origin. If the dislocation metamorphism is impact-related, the produced cataclasite may be termed a monomict impact breccia.

Cataclasite is a type of cataclastic rock that is formed by fracturing and comminution during faulting. It is normally cohesive and non-foliated, consisting of angular clasts in a finer-grained matrix.

Monomict impact breccia in the Manicouagan Impact Crater. In this image are original blocks of country rock within the impact melted country rock forming impact breccia.

5. BRECCIA POLYMICT

Polymict breccia is a general term that encompasses all breccias that aren’t either monomict or dimict. Types of polymict breccias are glassy melt breccias, impact-melt breccias, granulitic breccias, regolith breccias, and fragmental breccias. Each of these breccia types has a different texture because the set of conditions that formed them differed.

In situ polymict breccia on Patterson Island east within the Slate Islands impact structure. Polymict Breccias. The dominant type of breccia on the Slate Islands is polymict breccia and this typically occurs as veins and dikes varying in width from ~5 cm to 5 m. The matrix is typically fine grained and grey in colour. It contains a wide variety of angular to sub-rounded clasts of different lithologies, ranging in size from <1 mm to 10s of centimeters. Some of the clasts can also be seen to contain shatter cones. (Kerrigan et al, 2014)

6. BRECCIA BUNTE

Multicolored breccia; local name.  Impact ejecta deposit of the Ries impact structure (Germany).

Suevite overlying Bunte Breccia at Aumühle quarry Photo: G. Osinski, University of Western Ontario

At the Ries Crater in the image, there are two varieties of impact metamorphism. A grey well-consolidated suevite overlying a coarser bunte breccia. The suevite contains numerous pieces of black deformed glass and altered basement rocks held in a fine matrix of altered glass. Bunte breccia is largely a turbulent mixture of deformed sedimentary rocks. The suevite is formed from the basement below 700 m, and the bunte breccia from the overlying sedimentary rocks. (Dence 2005; “Half a Century of Searching for Impact Craters in Canada” – RASC presentation.)

7. STISHOVITE (IMPACT)

Stishovite is an extremely rare, hard, dense tetragonal mineral form (polymorph) of silicon dioxide with a mass density of 4.287 g/cm3 forming only from the impact of a meteorite through the metamorphism of Quartz at extremely high temperatures. Until recently, the only known occurrences of stishovite in nature formed at the very high shock pressures (>100 kbar or 10 GPa) and temperatures (> 1200 °C) present during hypervelocity meteorite impact into quartz-bearing rock. It is very rare on the Earth’s surface, however, it may be a predominant form of silicon dioxide in the Earth, especially in the lower mantle. Occurs only in microscopic platy grains (crystals that are small, flat, and appear flaky).

It is interesting to note that Stishovite is scientifically classified as an oxide and not as a silicate, even though it is polymorphous with Quartz. This is because its crystal structure is identical to some of the oxide minerals, such as Rutile. In fact, Stishovite is isomorphous with Rutile. Stishovite is named after Sergei Mikhailovich Stishov (b. 1937), a crystallographer in the Academy of Sciences, Moscow, who synthesized Stishovite prior to its discovery (Minerals.net).

8. SUEVITE (IMPACT)

A polymict impact fallback breccia composed of fragments more or less shocked, and melt clasts in a clastic matrix.  Glass can make up more than half of the volume of a suevite. The minerals in the rock fragments within suevites (also called suevitic breccias) commonly display shock-metamorphic effects. Suevite was named after a rock found at Ries crater in southern Germany.

Defined as an impact-derived, polymict breccia containing a mixture of shocked and unshocked, lithic and melt fragments and generally considered to possess clastic matrices [von Engelhardt, W. and Graup, G. 1984. Geologische Rundschau 73:2:447–481.].

Suevite is a rock consisting partly of melted material, typically forming a breccia containing glass and crystal or lithic fragments, formed during an impact event. It forms part of a group of rock types and structures that are known as impactites.
Lake Wanapetei suevite.
The suevite samples illustrated in the above images were found on the south shore of Lake Wanapitei crater. It was apparently scooped up from the lake bed by glacial activity and deposited in places along the southern shore of the lake.

9. CONGLOMERATE BRECCIA

A clastic rock made of particles larger than 2 mm in diameter is either a conglomerate or breccia. A conglomerate has rounded clasts while a breccia has angular clasts. Since water transport rapidly rounds large clasts, breccias normally indicate minimal transport. They commonly form as rock-fall and debris flow deposits along cliffs, and underground along faults or where caves collapse. Because water is ubiquitous on the Earth’s surface, conglomerates are far more common than sedimentary breccias.

Conglomerate breccia found within the Pingualuit Impact Crater.

10. SPECIFIC CRATER BRECCIA

BRENT

Impact melt is the “greyish” material between and cementing the country rock fragments. K-Ar dating of the recrystallized melt-bearing breccia gave ages of 310-365 Ma (Shafiquallah et al., 1968). Geochemical analyses show that the “melt” rocks are in fact melted target rock with ~1% contamination by chondritic material (Grieve, 2006).

 

GLOVER BLUFF

Polymictic impact breccia; Glover Bluff impact structure (Wisconsin, USA). Structure diameter 8 km, Cambrian age, or younger (ERNSTSON CLAUDIN IMPACT STRUCTURES – METEORITE CRATERS).

 

ILE ROULEAU

Impact breccia from the Ile Rouleau structure illustrating country rock clasts (fragments of geological loose material).

 

MANICOUAGAN

The Manicouagan impact structure – breccia on the inner plateau of the central peak island – documents “uniformly white” shattered country rocks imbedded in a fine grained matrix impact melt. This breccia outcrop is found in an inlet, cut into the central peak of the impact structure, known as Memory Bay.
Autochthonous impact breccia in the Manicouagan impact crater on the inner plateau of the central peak island at location #2. Note the different types of rock fragments forming the breccia within the fine grained matrix impact melt. Also note the white margin around the large breccia fragment. This white margin is a heat affected zone. The matrix material was hot enough during the formation of this breccia to produce a recrystallized band around the clast , a Heat Affected Zone, but there was not sufficient heat that flowed into the clast to melt it (Dr. Lynn B. Lundberg, PhD).

 

NEWPORTE

Macroscopic view of Newporte core sample – granitic frag-mental breccia D9462.0 (from Duerre 43-5 core) showing one quartz-nich angular granitic fragment (bright area top centre) with other darker granitic fragments in a ark, fine-grained, clast-rich matrix.(After Koeberl and Reimold, 1995)

 

SLATE ISLANDS

In situ polymict breccia on Patterson Island east within the Slate Islands impact structure.

 

SUDBURY

The Sudbury impact structure – grey Whitewater breccia.
The Sudbury impact structure – darker Whitewater breccia.

 

SKELETON LAKE

These are the breccia samples I had recovered from Opal Island within the Skeleton Lake structure. The breccia deposit on Opal Island stands in stark contrast to the surrounding target rock in the Central Gneiss Belt.