On Intrusive, Tuff-like, Igneous Rocks and Breccias in Ireland
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For several years past it has been known to us that fragmental igneous rocks exist in different parts of Ireland, which, though they resemble tuffs, and in certain cases have been described as volcanic rocks, cannot be regarded as ejectamenta, on account of their character and mode of occurrence in the field. Of those which have come more especially under our notice, we may at the outset briefly mention a few particulars, to introduce our subject, before describing in detail the sections exposed in the South-east of Ireland which afford the chief evidence upon which our views of such rocks are based. In the Explanatory Memoir (1888) accompanying Sheet 24 of the Geological Survey Map of Ireland, pp. 34 & 35, certain breccias occurring to the east of Lough Easke, in Donegal, are described as ‘agglomerates,’ though not in the sense of their having been at any time considered volcanic rocks. In parts, these masses might better be described as crush-breccias, as they, in such cases, follow lines of dislocation. In parts, however, they consist of partly-fused, broken-up, felspathic mica-schist or ‘gneiss,’ and they merge with felsite-dykes. Sometimes they occur dispersedly in sporadic masses in the mica-schist; and north-east of Lough Easke the breccia forms a wide band adjoining the granite, suggesting the conclusion that its formation may be attributed to the earth-stresses which immediately preceded, or in a sense accompanied, the intrusion of the Barnesmore granitic mass. Rocks similar to these occur in the district of Forkhill, in ArmaghKeywords:
Breccia
For several years past it has been known to us that fragmental igneous rocks exist in different parts of Ireland, which, though they resemble tuffs, and in certain cases have been described as volcanic rocks, cannot be regarded as ejectamenta, on account of their character and mode of occurrence in the field. Of those which have come more especially under our notice, we may at the outset briefly mention a few particulars, to introduce our subject, before describing in detail the sections exposed in the South-east of Ireland which afford the chief evidence upon which our views of such rocks are based. In the Explanatory Memoir (1888) accompanying Sheet 24 of the Geological Survey Map of Ireland, pp. 34 & 35, certain breccias occurring to the east of Lough Easke, in Donegal, are described as ‘agglomerates,’ though not in the sense of their having been at any time considered volcanic rocks. In parts, these masses might better be described as crush-breccias, as they, in such cases, follow lines of dislocation. In parts, however, they consist of partly-fused, broken-up, felspathic mica-schist or ‘gneiss,’ and they merge with felsite-dykes. Sometimes they occur dispersedly in sporadic masses in the mica-schist; and north-east of Lough Easke the breccia forms a wide band adjoining the granite, suggesting the conclusion that its formation may be attributed to the earth-stresses which immediately preceded, or in a sense accompanied, the intrusion of the Barnesmore granitic mass. Rocks similar to these occur in the district of Forkhill, in Armagh
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Marine breccias of Jurassic to Early Cretaceous ages are present in the Breccia Nappe of the French Prealps. Breccia types Ia and Ib are restricted to the lower part of the sequence in the Lower Shale, Lower Breccia, and Upper Shale formations. Type Ia breccias occur in beds from a few centimeters to tens of meters thick. They contain clasts up to more than 1 m in diameter, and are sometimes graded. Sole markings occur but are not common. Tops of some beds have large scale cross-stratification or parallel bedding, usually in granule-pebble grade material. Individual beds are of limited lateral extent--of the order of 1-2 km along the depositional strike and in places up to 7-8 km across it. The breccias have a clast framework and interstitial material is usually pebble or granule size. There is a continuous spectrum, with change in relative proportions of gravel and sand, from the breccias to pebbly turbidite sandstones. Type Ib is much less common than Ia. It has clasts of the same composition and size but it is characterized End_Page 342------------------------------ by more than 50% sand-grade matrix. Beds are less than 200 cm thick, parallel-sided, and have an uppermost sand-silt layer with convolute lamination. Type Ia is interpreted as a deposit from mass flow of coarse granular debris where internal shear was extensive enough to allow development of grading. For the transport of Type Ib beds, a slide mechanism is favored. End_of_Article - Last_Page 343------------
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Breccias with highly disordered internal structure are widespread in southern California, especially in non-marine parts of the Tertiary and Quaternary sections. Nearly all of them occur as tongue-like or otherwise tabular masses and as erosional remnants of such masses, and they range from a few feet to several miles in maximum exposed dimension. At different localities they have been variously referred to as megabreccias, chaotic breccias, rubble breccias, cyclopean breccias, monolithologic breccias, and as slide, slump, mudflow, debrisflow, or fault breccias. The most characteristic features of these rocks are the following. 1. The clasts are angular to sub-angular, and pebble-size to cyclopean; most of the very large ones are fractured to severely shattered. 2. The abundance ratios of clasts to matrices are very high; the matrices are themselves predominantly clastic. 3. Sorting is poor to good, and in general can not be attributed to the process of breccia formation per se. 4. Stratification is crudely developed or absent. 5. Many of the breccia masses are essentially monolithologic and commonly intertongue with other monolithologic masses of similar or contrasting lithologic character, or with breccia masses that consist of heterogeneous clasts. 6. The abundance of rock types among the clasts is directly related to the cliff-forming characteristics of the same rocks where they are exposed in place in the same regions. Most of the breccia masses are underlain and overlain by fanglomerates and other sedimentary rocks with clasts that are lithologically similar to those in the breccia masses but generally more rounded. The masses have sharp lower margins and sharp to gradational upper margins, and most of them butt against or interfinger with various kinds of sedimentary rocks. No lower margins have been traced into major faults; instead most of the masses conform with the structure of the underlying rocks. Nearly all of the breccias occur near zones of major faulting or flank areas of major uplift. Many of the breccia masses are demonstrably of sedimentary rather than tectonic origin, and most of the others seem best interpreted in this way. They evidently were formed under conditions that permitted rapid mass migration of rock debris, in some areas for distance measured in miles. Debris flows, derived from localized source areas, are thought to account satisfactorily for most occurrences. This specialized type of sedimentation may well have been more widespread in both space and time than has been recognized heretofore. End_of_Article - Last_Page 355------------
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Abstract This paper describes an unusual occurrence of igneous material as clasts in dyke and pipe breccias associated with late Caledonian minor intrusions. It is shown that the clasts were in a plastic condition when incorporated into the breccia rock. These igneous clasts were derived from magma disrupted at depth and then transported into the fluidized breccia columns where they were mixed with large numbers of clasts derived from the quartzite wall‐rocks. Textures and planar fabrics developed during collapse of the fluidized system are described and shown to be separable from the later compaction associated with extensive pressure solution of the fine matrix. Most Caledonian breccia pipes lack igneous clasts and it is considered that this group of breccias represent the rarely‐preserved boundary zone between active magma and breccia systems.
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Devonian
Conglomerate
Ultramafic rock
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ABSTRACT Pipe-shaped breccia bodies associated with diorite intrusions are composed mainly of angular clasts of local schists with a few transported clasts of quartzite. Plate shaped fragments are commonly oriented to define planar fabrics in the breccias. These features indicate the operation of gas fluidisation within the pipes and both entrainment and expanded bed conditions are inferred. The fabrics result from the collapse of the fluidised suspensions as the gas flow declined. Dilational fracture patterns in the country rock comparable with the stress release patterns found around mine shafts can be matched with the fractures required to produce the angular schist clasts. It is concluded that fracturing and the introduction of fragments into the fluidised breccia system was a continuous process and that the pipe diameter increased progressively with time. Microdiorite sheets and related stock like bodies of diorite cut and metamorphose the breccias. Compaction, hornfelsing and hydrothermal alteration also contributed to breccia formation.
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The Ries Crater, an impact structure of 26 km diameter in south Germany, is the largest terrestrial crater where substantial amounts of ejecta are preserved, on occasion >100 m deep. Further, the target stratigraphy is well known, and it is possible to relate specific clasts and breccia lithologies to initial target depth. As a consequence the continuous deposits of the Ries, also known as Bunte Breccia, may be studied with exceptional field control. We report field observations and laboratory analyses obtained from 560‐m core materials, taken at nine different locations that range from 16 to 37 km in radial distance from the impact center. The objective is to relate the Ries observations to ejection, and to emplacement processes of large‐scale, planetary crater deposits. The observations regarding the modal‐stratigraphic characteristics of the Bunte Breccia may be summarized as follows: only <0.15% (weight) of the total deposit consists of crystalline clasts larger than 1 cm that are derived from depths of >600 m; some 0.7% is composed of Triassic clasts, originating from 300 to 600‐m depths; Lower and Middle Jurassic horizons (approximately 300–150 m) constitute some 2.3%, and Upper Jurassic (0–150 m) makes up some 31.5%. In addition, the Bunte Breccia contains Tertiary materials in the form of >1‐cm clasts (29.1%) and as highly comminuted, fine‐grained “matrix” (<1 cm) accounting for the remaining 36.3%; these Tertiary materials constituted the immediate crater environment, i.e., a substrate, onto which the Ries ejecta were deposited. These substrate materials were thoroughly mixed into the continuous deposits. The ratio of “primary crater ejecta” to local substrate components decreases with increasing radial range. There is, however, no vertical stratification with regard to modal‐stratigraphic composition at any specific location; modal‐stratigraphic composition is highly variable on meter scales; Bunte Breccia appears to be a chaotic mixture resulting from a highly turbulent depositional environment. Also, the orientation of clasts larger than 1 cm is random. Detailed grain size data reveal progressively decreasing grain sizes with increasing radial range of both primary crater ejecta and local substrate materials. In addition, progressive comminution of primary ejecta related to increasing target depth is observed. Components shocked to >10 GPa constitute <0.1% (weight) of the entire deposit, which indicates that Bunte Breccia was emplaced at essentially ambient temperatures. When possible, the above observations are quantified via linear regressions throughout the text. All of these observations are consistent with, if not predicted by, a ballistic emplacement scenario as postulated by Oberbeck and co‐workers: primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate; a mixture of primary and secondary ejecta results and combines into a highly turbulent, ground‐hugging debris surge as the final phase of ejecta emplacement. Total emplacement time for the Bunte Breccia (⪖200 km³) is estimated to be of the order of 5 min only. These findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so‐called Z model, addressing material flow during various stages of crater formation. Qualitative to fair agreement of observations and predictions results. An initial crater radius of 6.5 km, an excavation depth of 1650 m, an excavation volume of 136 km³, and an associated transient cavity volume of aproximately 230 km³ appear to be reasonable estimates. Approximately 170 km³ of material was involved in slumping and restoration of the transient cavity for the above radius and Z=2.7. The modal composition of Ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large, complex planetary craters are predominantly derived from depths as small as one‐hundredth the apparent crater diameter. A number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias.
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NASA drill core D was taken about 25 km (2 crater radii) south of the center of the Ries crater in Southern Germany at 10°27'02"E/48°40'44"N. It penetrated 52.3 m of the "Bunte Breccia" continuous deposits of the Ries crater, and 2.7 m of autochthonous Tertiary "Obere Süßwassermolasse" sediments, terminating in Tertiary "Obere Meeres-molasse" sediments at a depth of 62.7 m. The Bunte Breccia consists of lithic fragments derived from the crater cavity (crystalline basement, Triassic and Jurassic sediments) and from local material outside the actual crater rim (Tertiary freshwater and marine sediments). These clasts are embedded in four different types of a fine-grained groundmass made up of sands and clays. More than 4500 clasts > 2 mm in the Bunte Breccia were assigned to their stratigraphic provenance. Clasts 2–28 mm consist predominantly of crater derived material, while clasts 28–200 mm and > 200 mm are dominated by lithologies derived from outside the crater cavity (local material). The population of crater derived clasts of all size fractions is dominated by lithic clasts from the uppermost strati-graphic levels. A comparison of grain size, heavy mineral, carbonate content, and micro-paleontological data of the groundmass with those of locally derived lithic clasts revealed that approximately 90 vol. % of the groundmass of the polymict breccia (grain size < 2 mm) consist of locally derived clastic sediments and only about 10 vol. % are crater derived. The breccia emplacement was a highly turbulent process which involved stripping of the local substrate as well as vertical and horizontal transport and intimate mixing of crater and locally derived materials. The high amount of locally derived material is predicted by the hypothesis of secondary cratering and associated mass wasting and is not compatible with a roll-and-glide mechanism of ejecta transport.
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Abstract The Steen River impact structure (SRIS) formed in mixed target rocks, with Devonian carbonates, shales, and evaporites overlying granitic basement rocks of the Canadian Shield. A detailed study of impact melt phases within a continuous sequence of polymict impact breccia, as intersected by drill core, evaluated the relationship of impact melt to the breccia, identified the target rocks that contributed to the melt, and calculated the amount of melt within the breccia. Impact melt in the SRIS breccia occurs in three main forms (1) as individual matrix‐supported clasts, (2) as rims enveloping granitic clasts, and (3) as layers of agglomerated melt. Major and minor element abundances of large impact melt clasts (>1 mm) are similar to granitic basement, aside from elevated CaO and K 2 O wt% oxides in these melt clasts from incorporation of carbonates and calcareous shales. In contrast, submillimeter‐sized impact melt clasts have a composition derived almost exclusively from melting of shales. The small size of the shale‐derived melt clasts is attributed to increased fragmentation and a wider dispersion due to the volatile‐rich nature of the shale protolith. The wide compositional range of impact‐melted target lithologies documented at the SRIS contradicts breccia clast formation by impact melts that merged into larger bodies but were subsequently disrupted. Our observations are consistent with disruption of impact melt early in its formation history, and the volatile‐rich nature of the target materials likely contributed to this disruption. Bimodal thin section scans provide an estimate of the proportion of impact melt phases in the SRIS breccias (~19 vol%). When compared to similarly sized, mixed‐target impact structures, our results are consistent with the estimated volume of impact melt clasts from Ries, Germany (21 vol%), but are roughly twice that observed at Haughton, Canada (<10 vol%).
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