Crackle Breccia
Gaynelle Alnutt
Crackle breccia is a type of breccia where the clasts have been separated by planes of rupture, but have experienced little or no displacement.[1] The individual clasts in crackle breccia must not have experienced more than 10 average rotation.[2]
Crackle breccia in the Paleozoic carbonates exposed at Buffington Pockets, Nevada in the hanging wall of the Muddy Mountain Thrust. Specific environment of brecciation is unknown although Brock & Engelder (GSA Bulletin 1977) suggest both tectonic and sedimentary origins. It is classified as crackle breccia since the bedding is broken but not disrupted (near-horizontal colour banding). A strand of chaotic breccia cuts across the photo from upper left center toward lower right. This appears to be a matrix-rich shear which deforms and disaggregates earlier crackle breccias. Posted by Christie.
In the lower left of the above photo, the black, fractured to brecciated rock is resting on a rock slab or outcrop of gray breccia. (Trust me, breccia is everywhere on this particular slope!)
The above rock, and the next, speak a little to the question of what were the rock holes seen in a previous post of mine. The light brown veinlets of silica and iron oxide stand out with respect to the somewhat weathered out black marble and marble fragments. These irregularly shaped to rounded-looking fragments may or may not have been rotated as in most breccias.
The above breccia is composed of subangular to rounded gray marble fragments, partly weathered out from the brown, iron-oxide-rich silica matrix and silica veinlets. At first, I thought the light brownish gray fragments, which are particularly shiny in the sun, were composed of pyrite or marcasite, but no, they are marble. There are a few square holes completely weathered out in the silica matrix where cubic pyrite crystals used to live. This breccia could be a tectonic or hydrothermal breccia, or some combination thereof - but, again, an outcrop rather than a loose rock on a scrabbly slope would be more helpful in determining the origin of the breccia, and so would a little microscope work.
Cortez Hills Breccia Zone (CHBZ) is a breccia-related Carlin-type ore deposit located
in northeastern Nevada. While most breccias associated with Carlin-type mineralization
can be genetically tied to tectonic, sedimentary, or dissolution processes
via their geometry or textural relationships, the breccia body that hosts the bulk of
CHBZmineralization is not so easily categorized. This breccia body is conical in shape
with an elliptical cross section and crosscuts stratigraphy. It has gradational margins
with the surrounding country rock, and large (>10 m) blocks of country rock are present
within the breccia body itself. Zonation within the breccia body consists of a higher
energy polymictic portion in the center of the pipe grading outward to rotated breccia
and a peripheral shell of crackle breccia. Alteration occurred pre-, syn-, and
post-brecciation, including extensive deep post-breccia oxidation. High grade gold
mineralization is spatially associated with the central polymictic portion of the breccia,
and gold grade drops in the surrounding rotated and crackle breccias. This paper
describes the breccia body and its apparent spatial relationship to mineralization and
temporal relationship to alteration, and outlines how breccia architecture was integrated
into logging and underground face mapping in the production environment.
Figure 4. From Woodcock and Mort (2008). a) Thin section examples of crackle breccia, mosaic breccia, and chaotic breccia from the Dent Fault Zone, NW England. b) Subdivision of fault breccias by percentage of large clasts
Figure 9. Conceptual model of breccia generation on Zuccale Fault by fluidization from Smith et al., (2008). (a) Precursors to fluidization: the Zuccale fault possesses a strongly foliated fault core which acts as a low-permeability seal to CO2-bearing fluids migrating within the footwall. The fault core is underlain by a high-angle footwall fault. Fluids infiltrate pre-existing frictional breccias, leading to dissolution and a loss of cohesion. (b) Fluidization: periodic slip along high-angle footwall faults leads to focused and rapid fluid flow, causing fluidization of clasts within the frictional breccias. The fluid pulse spreads laterally as it encounters the fault core. Ponding of fluids, and deformation of the boundary, may occur during continued input of fluids. (c) Hydrofracturing: critical fluid overpressure leads to embrittlement within the core of the Zuccale fault, allowing fluids to drain from footwall to hanging wall. The fractures undergo healing processes returning to a low-permeability nature, allowing the fault-valve cycle to repeat.
Figure 14. Schematic model of shear zone evolution during layer-parallel shear along the Talhof fault. (a) Formation of distinct cross-joints at high angles to the pre-existing bedding/foliation planes. (b) Formation of joint-bounded slices, rotation of slices, reactivation of joints as shears with antithetic displacement, and formation of secondary joints at the tips and internal parts of slices. Widening of the fault zone is inhibited by external compressive stresses at high angles to the shear zone boundary, stylolites are formed at low angles to the shear zone boundary, perpendicular to maximum principal stress orientation. (c) Kinking, fracturing and disintegration of slices by bookshelf rotation, developing into a cataclastic shear zone at advanced stages of displacement. (d) Cementation of disintegrated slices and subsequent formation of new high-angle joints. (e) Second cycle of brecciation. The newly formed fragments consist of both slice fragments and fragments of sparitic cement. Effective normal stress acts perpendicular to the externally imposed general shear direction. Effective normal stress acts parallel to the externally imposed general shear direction. Hausegger et al. (2010)
Detailed petrographic and petrophysical analyses were performed on a core section of the Nicor #4 Bowman Well, Arkoma Basin, Oklahoma. The studied interval represents the Cambro-Ordovician Arbuckle Group. Petrographic analyses of core samples distinguish six lithofacies. They are as follows from the bottom to the top of the studied core: (1) Quartzose dolostone, (2) conglomeritic dolostone, (3) brecciated dolostone, (3a) crackle breccia, (3b) dissolution-collapse breccia, (4) oolitic dolostone, (5) stylolitic dolostone, and (6) stromatolitic dolostone. Diagenetic evolution has affected the petrophysical properties of these different lithofacies. The diagenetic history of the selected section records early to late dolomitization, stylolitization, dedolomitization (calcification), and silicification. Petrophysical analyses employing mercury porosimetric techniques determine three distinct petrofacies. These petrofacies are as follow: (1) low porosity and high recovery efficiency, (2) low porosity and low recovery efficiency, and (3) high porosity and high recovery efficiency. Based on petrophysical analyses, the best reservoir candidate is the crackle breccia petrofacies because it has intermediate porosity and high recovery efficiency values, plus this lithofacies is bounded by strata with low porosity and recovery efficiency values (impermeable layer).
The Arbuckle Group carbonates were uplifted and subaerially exposed, which, led to the development of an unconformity surface. Consequently, karstification developed and modified various petrophysical properties of the core.
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Abstract: The Yemi breccia developed and is distributed within the Paleozoic carbonate rock (Maggol Formation) in the central part of the Taebaeksan Basin, South Korea. Explanation for the genesis of the Yemi breccia has been controversial. We investigated the petrological and mineralogical properties of the breccia and the matrix materials at 60 outcrops. The Yemi breccia is divided into crackle, mosaic, and chaotic breccias based on morphology. In addition, these are divided into blackish, reddish, grayish, and white to pinkish matrix breccias according to the materials of the matrix. Quartz, calcite, pyrite, hematite (after pyrite), and minor epidote, chlorite, and opaque materials mainly comprise the matrix materials. The pyrite grains from the Yemi breccia can be divided into two types based on the mineral texture: diagenetic and hydrothermal. We analyzed the chemistry of pyrite and hematite (after pyrite) from the Yemi breccia with an electron probe X-ray microanalyzer (EPMA). Invisible gold was detected within the pyrite grains by EPMA and disseminated micron-sized isolated gold particles were discovered by backscattered electron (BSE) images. The texture of Au-bearing pyrite and gold particles in the Yemi breccia is especially well matched with pyrite and gold from the Shuiyindong Carlin-type hydrothermal gold deposits, China. Therefore, we suggest an important role of hydrothermal fluid in karstification within the Paleozoic carbonate rock. Keywords: Yemi breccia; matrix materials; pyrite; invisible gold; isolated gold particles; hydrothermal fluid; karstification
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