Brecciated Textures Research Articles

Geological evidence concerning compressional tectonics in the eastern mediterranean

Data from the 1978 “Cobblestone Project” Deep Tow surveys and associated intensive sampling programs have been combined with published data to address the question: “Are the Mediterranean- and Calabrian Ridge convergent plate margin accretionary prisms?”. A dome-shaped feature on the crest of the Mediterranean Ridge was found to be made of sticky and watery mud breccia of Early Cretaceous age. Its circular plan view, concentric surface deformation, old but uniform-aged clasts and matrix, diachronous Quaternary veneer, brecciated texture and plastic matrix suggest a piercement structure, but the total lack of evaporitic or dolomitic lithologies, the thinness of both Messinian evaporites and Plio-Quaternary cover in the observation area and the great age and presumably great depth of the Cretaceous mother bed, rule out simple salt diapirism. We suggest instead that the diapir rose from the overpressured zone at the base of a deep seated overthrust, where grinding and injection of water shattered a low permeability shale layer and reduced its density and effective viscosity. The perimeters of the Calabrian- and Mediterranean Ridge are marked by an abrupt transition (“deformation front”) from flat abyssal plains to ridge-and-trough morphology. The deformation front apparently migrates outward with time and is the site of uplift and folding of abyssal plain sediments: A search for analogous “deformation fronts” in better understood convergent plate margins revealed a very close analogy with the toe of the Curacao- and Barbados Ridge which are interpreted as classic accretionary wedges. The analogy is further supported in the Calabrian Ridge case by the observation of an arcward-dipping reflector, which may be the detachment surface, beneath a thickening wedge of Messinian evaporites. Extension of the analogy requires that the outcrop of the subduction zone or the site of initiation of decollement be located external to the Mediterranean Ridge and not in the Hellenic Trough. Our surveys showed that (except possibly along the recently deformed outermost perimeter of the deformation front) the surface of both ridges is extensively modified by the secondary processes of diapirism, dissolution, mass-wasting and downhill gliding. Thus the fine-scale surface morphology alone, as revealed by Seabeam, Deep-Tow or GLORIA, may be a poor or misleading indicator of deep crustal processes in this region.

Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance

ABSTRACTRhizoliths are defined as organosedimentary structures resulting in the preservation of roots of higher plants, or remains thereof, in mineral matter. They are abundant and characteristic features of Quaternary terrestrial carbonates (calcretes and aeolianites) from coastal regions of the western Mediterranean. Field and petrographic observations indicate that five basic types of rhizoliths can be recognized: (1) root moulds, which are tubular voids that outline positions of former, now decayed roots; (2) root casts, which are sediment‐ and/or cement‐filled root moulds; (3) root tubules, which are cemented cylinders around root moulds; (4) rhizocretions s.s., which are pedodiagenetic mineral accumulations (here low magnesian calcite) around living or dead plant roots; and (5) root petrifactions, which are mineral impregnations or mineral replacements of organic matter whereby anatomical features of roots have been preserved partially or totally. Apart from rhizoliths themselves, roots of higher plants are responsible for the formation of numerous and characteristic features of pedogenetically affected terrestrial carbonates. Plant roots are responsible for, or contribute to, the formation of alveolar textures, in situ brecciation (rhizobrecciation) textures, horizontal sheet cracks, vertically elongate glaebules (concretionary soil structures) and micritization (rhizomicritization) within terrestrial carbonates. Rhizoliths, together with the above features, are products of pedodiagenesis. More significantly, rhizoliths and related features are indicators of palaeosols and hence of subaerial vadose environments in ancient (post‐Silurian) successions.

Brecciation textures and tepee structures in Quaternary calcrete (caliche) profiles from eastern Spain: The plant factor in their formation

AbstractField studies of brecciation textures and tepee structures in Quaternary calcretes from eastern Spain reveal that vegetation plays a fundamental role in their formation, alteration and destruction. Biophysical and biochemical activities of higher plant systems produce brecciation textures and some tepee structures. Brecciation of indurated calcrete profiles form rhizo‐breccias which may be used as indicators of terrestrial conditions in ancient successions.

Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, northeastern Kentucky

ABSTRACTSequences of laminated limestones found within thin Carboniferous carbonate strata of northeastern Kentucky were studied to determine their origin and palaeo‐environmental significance. These laminated zones are strikingly similar to Holocene and Pleistocene surficial calcareous crusts (caliche) profiles that occur in various parts of the world.Carboniferous laminated carbonates are associated with shallow marine carbonate units, palaeokarst, and overlying palaeosol zones. A typical laminated profile ranges in thickness from 1 to 2 m and contains brecciated, light olive‐grey to brown micrite that lacks distinctive bedding. Structures and textures common in most profiles include: (1) calcareous and silicious laminae (laminae form diffuse, alternating light and dark bands that generally parallel bedding but often fill fractures and vugs within the rocks); (2) particles (allochems, and micrite and microspar fragments) coated by brown microcrystalline calcite; (3) brecciated texture; (4) circular to elliptical fossil moulds (occur in sinuous patterns and fill fractures within the rocks); (5) large and small scale fracture patterns.Subaerial weathering and vadose diagenesis of carbonate mud banks or islands is suggested as a mechanism for the formation of these Carboniferous calcareous crust profiles. These ‘crusts’ formed by a combination of solution (karsting), brecciation, and soil development that transformed an exposed marine biomicrite (‘host’ rock) into a porous subsoil rubble. Laminated ‘crusts’ and coated particles developed as the result of dissolution and reprecipitation of CaCO3 and SiO3 from the soil and carbonate bedrock.Carboniferous laminated carbonates in northeastern Kentucky are often referred to as ‘algal limestones’ because of their superficial similarity to some modern and ancient algal structures. This study, however, reveals numerous characteristics that can only be explained by diagenesis in a subaerial environment.

Cornstone (Fossil Caliche): Its Origin, Stratigraphic, and Sedimentological Importance in the New Red Sandstone, Western Scotland

An examination of more than 200 cornstone (concretionary carbonate) profiles of New Red Sandstone age, in western Scotland, confirms that these carbonate accumulations are analogues to the caliche of semi-arid areas. New evidence of laminated, pisolitic and brecciated textures from the cornstone "caprock" strongly suggests that downward percolating water and non-tectonic fracturing were important during subaerial diagenesis. The cornstones occur preferentially in floodplain deposits, and particularly within the upper parts of alluvial, fining-upwards cyclothems. The occurrence of cornstones in this part of the cyclothems and their persistence through such cyclothemic sequences suggest, contrary to a commonly accepted hypothesis, that climatic controls are important in the formation of such alluvial sequences. The cornstones are stratigraphically more significant than the associated rock types in the monotonous New Red continental sequences. In particular, thick, mature cornstones indicate condensed sequences and are also a useful palaeoclimatic index, such caliche normally forming in semi-arid regions.

Intrusion features of some hypabyssal south African kimberlites

Kimberlite in certain dykes and in the deepest parts of some diatremes show textural and other features which contrast with those in the breccia diatremes. Some hypabyssal kimberlite intrusions show relatively high-temperature contact phenomena including baking of country-rock sediments and sedimentary xenoliths, and contrasting with the brecciated texture of most diatreme-facies kimberlites, in the hypabyssal kimberlites are numerous examples of preferred orientation of inequidimensional minerals (? trachytic flow texture), and rapid mineralogical gradients from the contact towards the dyke centres that may be attributable to flowage differentiation. In the Benfontein sill (Kimberley area) there is well-developed horizontal banding due to gravitational settling, and pseudo-sedimentary structures are also present. The accumulated evidence indicates that kimberlite existed as a relatively hot fluid up to depths of 2–3 km below the land-surface at the time of intrusion; above this level, gas release caused diatreme formation, brecciation and adiabatic cooling. These views are contrary to those of geologists who postulate eruption of kimberlite as a cold breccia directly from the mantle or deep within the crust, but accords with the views of many Russian geologsts who accept the existence of kimberlite magma, the extrusive equivalent of which is the ultrabasic lava meimechite.

Middle and Late Pennsylvanian Flint Clays

ABSTRACT Nearly all literature relating to flint clays of Pennsylvanian age in the eastern United States concerns deposits occurring in the lower part of the Pennsylvanian System. This report describes the geologic occurrence and the physical and mineralogical features of flint clay deposits of middle and late Pennsylvanian age in the eastern United States, many of which have not previously been described as flint clay. The brecciated texture, conchoidal fracture, and variegated color--including shades of yellow, brown, gray, green, and red--of these clays make them distinctive. They are composed primarily of well crystallized kaolinite. The flint clays are often closely associated with coals, and at several localities they occur in channels or lenses where the normal continuity of the coal bed has been interrupted by erosion or nondeposition. The flint clay is believed to have formed by alteration of ordinary fine-grained Pennsylvanian sediments in acid swamps in a manner similar to that recently proposed by Patterson and Hosterman (1960) to explain the formation of early Pennsylvanian flint clay in the Olive Hill district, Kentucky.

The Songwe scarp carbonatite and associated feldspathization in the Mbeya Range, Tanganyika

The carbonatite and associated rocks described occur on the Mbeya mountain range, which is at the junction of the eastern and western rift valleys in south-west Tanganyika. The main occurrence of carbonatite is a dyke-like body, possibly in discontinuous lenses, with an approximately linear outcrop trending nw-se over a distance of some 12 miles at the base of the Songwe scarp. This scarp bounds the Mbeya Range and delimits the southern end of the Rukwa trough. The carbonatite was emplaced in a zone of structural weakness along which there was probably rift-valley faulting before the introduction of the carbonatite and along which there has since been considerable faultmovement. The rift-faulting here follows the north-westerly trend of the crystalline basement rocks. Typically the carbonatite is ankeritic, fine-grained, and not flow-banded. Commonly there is a fragmentary or brecciated texture, the fragmentary material consi sting largely of feldspathized and carbonated country-rock. Minor amounts of niobium, uranium, and thorium are present but the location of these elements is uncertain and rare accessory minerals have not been detected. Feldspathization by the introduction of potassium is strongly developed in the immediate vicinity of the carbonatite and also sporadically elsewhere on the Mbeya Range; both quartzofeldspathic cataclastic schists and coarsegrained mica-garnet schists have been converted to aggregates of potassium feldspar. Such feldspathization is particularly well displayed in the vicinity of Itete on the north-east flank of the Mbeya Range, where elongated belts of feldspathized country-rock illustrate the strong control exercised over their formation by the regional schistosity. Injection and brecciation of these feldspathized schists by carbonatite veins is common. It is suggested that the Songwe scarp carbonatite is a high-level representative of the crystallization of a parent carbonatite at depth; the unusual form is due to the strong structural grain of the basement schists of the area.

Petrogenetic Significance of Some Feldspars from the Judith Mountains, Montana

Most of the alkali-rich, lime-poor porphyritic rocks of the Judith Mountains, Montana, contain phenocrysts of a single, optically homogeneous, alkali feldspar (cryptoperthite). A few rocks that are chemically similar to those contain two alkali feldspars; these feldspars are not the result of exsolution or replacement but are orthomagmatic. Textual relations suggest that in some magmas there was a reaction relation between the two feldspars and in others a eutectic relation. Vesicular and intrusive brecciated textures and alkali metasomatism of sedimentary inclusions indicate that the Judith Mountains rocks known to contain two alkali feldspars crystallized from wet magmas; a high pressure of water vapor and probably other volatiles is believed to have been the dominant factor in reducing the temperature of initial crystallization of the alkali feldspars to the reaction and eutectic points. Rocks containing plagioclase phenocrysts of two different compositions may possibly result either from fractionation by mantling or from the mixing of magmas.

On the paragenesis of ore minerals in pyrrhotite deposits

Writer is studying the properties of pyrrhotite deposits in Japan, and in this report, it is especialy dealt with the paragenesis of ore minerals from following mines: Motoyama mine, Okayama prefecture. Tsuda mine, Okayama prefecture. Otani mine, Hyogo prefecture. Futaba mine, Hyogo prefecture. Komori mine, Kyoto prefecture. Okura mine, Niigata prefecture. The main results obtained, are as follows:- (1) Ore minerals, observed on examing ores from these mines, are commonly pyrrhotite, arsenopyrite, chalcopyrite sphalerite and galena. (2) Writer could find little pyrite in most case, and found magnetite in the ore from Motoyama mine. (3) Magnetite in the ore from Motoyama mine, shows rosette texture or relict texture with sphalerite. (4) Writer found a little pyrite, taking an idiomorphic form in the ore from Otani mine. (5) Pyrrhotite, chalcopyrite and sphalerite show generally mutual boundary texture and sometimes serrate texture. (6) Arsenopyrite and pyrite show brecciated texture or veined texture with chalcopyrite, sphalerite and pyrrhotite. (7) In the ore from Okura mine, writer found chromite in pyrhotite, taking idiomorphic form and cubanite which may be formed by unmixing. (8) Chalcopyrite emulsion in sphalerite, can be said to be formed by unmixing. (9) The sequence of deposition in thses mines, shows the same tendency as follows: (1) Quartz, calcite (2) Skarns (hedenbergite, lievrite, garnet) (3) Oxide minerals (magnetite, chromite) (4) 1 st sulphide minerals (arseno pyrite, pyrite) (5)2 nd sulphide minerals (sphalerite, pyrrhotite, chalcopyrite, galena, cubanite) (6) Quartz, calcite.