Internal Fabric Research Articles

Finite strain and progressive deformation in models of diapiric structures

The distribution of strain within and around gravitationally produced diapiric structures was studied through the use of experimental models which were deformed in a large-capacity centrifuge. A new method of model construction was developed which is equivalent to building the model of initially square 1-mm elements. After deformation the elements assume shapes which approximate parallelograms and their finite strains can easily be calculated. If several initially identical models are deformed to different extents, the finite strain states of an element in each of the models define points on the deformation path of that element. The deformation path can be used to make estimates of the nature of the internal fabric which would be expected in the equivalent element of the natural structure. This method was applied to the study of the finite strain in diapiric ridges. The models demonstrate that the highest strain is always in the region above the diapir. Within the diapir initial vertical stretching is followed by vertical flattening. Large portions of the structure can be seen to suffer what would in natural examples be called polyphase deformation, even though all of the deformation was due to a single buoyant overturn of unstable density stratification. The strain patterns within the models support the contention that in salt diapirs the buoyant salt has a lower viscosity than the overlying sediments, but that in mantled gneiss domes the reverse is true.

Deformation and Metamorphism of the Franciscan Subduction Zone Complex Northwest of Pacheco Pass, California

The Franciscan Complex northwest of Pacheco Pass, California, includes three fault-bounded units, each characterized by a different deformational style and suite of metamorphic mineral assemblages. Structurally highest is jadeitic pyroxene-bearing metagraywacke semischist. The areally extensive Garzas tectonic melange separates the semischist from the structurally lowest pumpellyite-bearing Orestimba metagraywacke. The Garzas melange is representative of Franciscan melanges in general. These mappable bodies have an internal fabric dominated by penetrative, mesoscopic shear fractures and contain tectonic inclusions of all sizes immersed in a pervasively sheared, generally fine-grained matrix. The shear fractures record brittle deformation of consolidated rock bodies. Many melanges contain exotic inclusions, clearly not derived from adjacent units, and metamorphic mineral assemblages and textures indicate that they were metamorphosed before being tectonically mixed with more voluminous, generally lower grade, metagraywacke inclusions. The structural units are grossly sheetlike in external form and are separated from one another by gently to steeply dipping major faults. Unlike low-angle thrusts in imbricated Cordilleran terranes, the faults do not systematically repeat or offset a normal stratigraphic sequence but rather juxtapose rock units that bear no apparent stratigraphic, deformational, or metamorphic relation to one another. The structural units were separately deformed and metamorphosed under a variety of conditions prior to their tectonic juxtaposition during late Mesozoic continental margin subduction. Field and petrographic evidence permit, but do not prove, the hypothesis that both the semischist and exotic, high-grade melange inclusions once were more deeply buried and have been emplaced upward into their present anomalously shallow structural positions.

Distribution of Concretions from Selected Soils of the Inner Bluegrass Region of Kentucky

AbstractTotal iron and manganese was determined on concretions and soil samples by X‐ray emission. Soils studied included Maury, Mercer, and Lawrence, which are well‐drained, moderately well‐drained and somewhat poorly‐drained, respectively. Iron‐Manganese concretions were observed in all horizons, with a maximum of 46% by weight in a Maury C horizon. The Fe/Mn ratios decreased with increasing concretion size. This decrease was largely due to increased Mn content associated with the larger size concretions. Further analyses for composition showed that quartz and clay minerals (vermiculite, montmorillonite, kaolinite, and mica) account for most of the remaining fractions. Traces of titanium and phosphorus were also found. Microscopic studies of the concretions indicated two types of internal fabric; (i) undifferentiated (structureless), and (ii) differentiated or concentric structure. Concretions from the somewhat poorly drained soils are characterized by undifferentiated fabric and diffuse external boundaries. In contrast the concretions from the well‐drained soils are usually spherical, with sharp external boundaries and differentiated fabric.

Submarine Channel and Fan Deposits, Silurian of Central Wales, United Kingdom: ABSTRACT

Mapping and detailed sedimentological study of a group of geosynclinal formations of Llandoverian age (Lower Silurian) around Rhayader, central Wales, have been undertaken jointly by the writer and M. A. Woollands (University College, London). These formations occur near the southeastern margin of the Welsh trough and consist essentially of bodies of sandstone and conglomerate, lenticular through distances of several kilometers, and enclosed within a framework of graptolite-bearing argillaceous sediments. Marked lateral and vertical variations in the geometry, internal features, and transport patterns of the coarser intercalations may be demonstrated and are attributed to differences in the mechanisms and sites of deposition. On the southeast, an argillaceous facies with a few thin, laterally derived tractional sandstone beds and calcareous bands, displaying slumps, is interpreted to represent a slope environment. This facies contains a few longitudinally derived distal turbidite units which appear in more profusion on the northwest in an argillaceous facies of inferred basin-plain origin. Mantling the pelagic sediments are broadly lenticular bodies of coarse sandstone interpreted as proximal turbidites. Sole marks in these sandstone beds present a complex but essentially longitudinal transport pattern but ripple marks and other depositional features indicate modification of the northeast-southwest longitudinal pattern by lateral tractional currents (from the southeast). Such proximal turbidites grade both laterally and vertically into more distal turbidites and into coarse granule-gritstone and boulder conglomerate. The rudites occupy successive, steep-sided channels excavated in the pelagic basin floor and slope sediments, and display features characteristic of fluxoturbidite associations. The orientation of channels, sole marks, ripples, and pebble imbrication consistently indicates current movement toward the north-northwest whereas the internal features and fabric of the coarse sediments suggest emplacement by a sand-avalanche End_Page 535------------------------------ mechanism in which shearing processes were dominant. The rudites are interpreted as a complex of submarine channel fills, excavated in base-of-slope sediments and possibly representing the lower ends of small submarine canyons. The more extensive proximal turbidite bodies may then be regarded as fan accumulations fronting the canyon mouths and feathering out into the distal representatives. One of the most interesting features of this sequence is the abruptness of the change from channel rudite through proximal facies to distal turbidites, which in several localities may be shown to occur within less than 8 km down-current. The short-lived and unusual character of the turbidity currents in this example may be linked to the clean, mud-deficient nature of the original supply of gravel and sand. End_of_Article - Last_Page 536------------

Regional metamorphism of pelitic rocks around Kandra, Singhbhum, Bihar

A suite of pelitic rocks around Kandra, Singhbhum District, Bihar, displays a metamorphic gradient registered by the index minerals chlorite, biotite, garnet, staurolite and sillimanite in a Barrovian sequence. Metamorphism was by and large coeval with folding movements, and correlating the internal fabric of minerals and deformational characters, a regular sequence of the index minerals is derived. It is argued that the chronological order by itself is not sufficient to prove that metamorphism was progressive in time. Among the index minerals, garnet appears to have formed by the reaction chlorite+biotitea+quartz ⇌ garnet+biotiteb+H2O. For the origin of sillimanite, a new reaction, 3 staurolite+muscovite+quartz=7 sillimanite+biotite+3H2O, is suggested on the basis of significant textural features. Textural and petrological indications regarding the formation of staurolite are in discordance. Staurolite was either derived from the biotite zone phases, or should be taken to have formed, against textural evidences, from chloritoids of the garnet zone. Graphical analysis of the assemblages by Thompson's ‘AFM’ projection reveals that chlorite and staurolite are excess phases owing to retrogression and incomplete reaction. Shifting of apices of triangular fields and intersection of garnet-biotite tie lines within a zone can be satisfactorily explained in terms of extra components CaO and MnO or their ratios. It is pointed out that if MgO/(MgO + FeO) between two phases show a linear relation, their tie lines will be concurrent on the AF side of the projection, the point of concurrence reflecting equilibrium and temperature of recrystallisation.

GLAEBULES: THEIR DEFINITION, CLASSIFICATION AND INTERPRETATION

SummaryA broad group of pedological features has been recognized by their occurrence as three‐dimensional entities embedded in the matrix of soil materials. They have been named glaebules and are divided into sub‐groups on their internal fabric, mineralogy, distinctness as a unit, and shape. These features include nodules, concretions, and septaria which have all been recognized by the petrologist, and pedodes, papules, and glaebular haloes which are newly recognized features. An attempt is made to interpret the origin of these features in terms of processes of formation and inheritance versus formation in situ. This is based on interpretation of the characteristics used for their classification.

Structure variation within two red-brown earth profiles

Stereomicroscopic and thin-section studies of structure and fabric features of two red-brown earth profiles from the Riverine Plain of south eastern Australia are assessed in relation to depositional layering and soil formation as suggested by field evidence. Internal fabric in relation to mode of occurrence suggests that some of these features are inherited from, and characteristic of, the original depositional layer whilst others have been formed or accumulated during soil formation subsequent to the cessation of deposition. Trends in the degree of development of these features with increasing depth suggest that the Hanwood profile consists of five depositional layers with evidence for a depositional time break, accompanied by soil formation, at the top of the fourth layer below the present surface. The Deniboota profile includes three depositional layers with evidence for a time break at the top of the second layer below the present surface.

Sedimentary Structures and Paleocurrents in the Lower New Red Sandstone, Devonshire, England: ABSTRACT

Permo-Triassic redbeds, consisting of mainly breccias and sandstones exceeding 6,000 feet in thickness, are well exposed in coastal cliffs in South Devonshire. They are disposed in semi-circular or elongate basins or cuvettes, margined by ridges of metamorphic rocks which supplied the bulk of the detritus. The breccias show typical torrential bedding features. Low-angle cross-bedding units, festooned perpendicular to the direction of derivation, are interwoven with generally flat but lenticular beds of breccia and sandstone. Shallow channeling is common, deeper where sandstone is predominant. Mapping of pebble imbrication shows a basically centripetal transportation pattern in the cuvettes, consistent with derivations indicated by cross-bedding, channeling, and fragment composition. Roundness measurements of limestone fragments in the same rocks reveal sub-circular roundness contours increasing in value towards the cuvette centers, and approximately perpendicular to imbrication directions. All features indicate deposition on sub-montane alluvial fans in a semi-arid climate, with converg ng directions of sediment transport in semi-confined cuvettes. Sandstones in the upper part of the sequence show clear eolian cross-bedding, with wedge-shaped units and some festoon cross-beds. Attitude measurement indicates deposition by a uni-directional wind from the SSW. Interbedded breccias show truncation and channeling of dune surfaces. Quicksand injection structures occur in fine-grained silty breccias interbedded with sandstones (distal fan deposits). A saturated sand layer, sealed under an impermeable silty breccia, was mobilized and injected upward through desiccation cracks or other weak spots, forming sand dikes with elongate particles aligned parallel with the walls. Sun-cracked silt layers may show strongly upturned edges on desiccation polygons due to injection (extrusion) of quicksand, which may also be fragmented and dislocated. Annelid burrows occur in some basal breccias, mostly fine-grained. Where elongate fragments constitute the breccia, the burrow filling shows a distinct internal fabric or particle arrangement. Commonly up to one inch in diameter with circular cross section, in coarser breccias they are ovoid and up to 7 inches wide. These large burrows branch, ascend through strata, and avoid large fragments; together with the consistent meniscus fabric, these features indicate organic origin. End_of_Article - Last_Page 535------------

PEDOTUBULES: THEIR DEFINITION, CLASSIFICATION, AND INTERPRETATION

SummaryThe term pedotubules is proposed for a group of pedological features which have a tubular external form and which are distinguished from cutans (Brewer, 1960a) by their complex internal composition and fabric. Pedotubules are classified according to their internal fabric and composition, details of external form, distinctness, and by a comparison of their fabric and composition with that of the horizons of the soil profile. Their general morphology suggests their origin as voids caused by faunal and floral (root) activity which have been filled, or partially filled, with soil material. Since little is known of the details of the effects of faunal activity on soil materials, such interpretations are tentative.

Complex and imitation Tectonites from the Kosciusko Pluton, snowy mountains, New South Wales

Abstract Comparative fabric studies of the Kosciusko granodiorite and its country rocks reveal that apparent similarities in petrology and megascopic fabric can be very deceptive if no proper integration with microfabric investigations is attempted. Thus it may be shown that the granodiorite is locally an imitation‐tectonite”, simulating the tectonic megafabric of the country rocks where frictional drag, due to viscous flow along its walls, was considerable. The isoclinally folded metasediments of the country rock have been subject to various crossed strains with B - B?=B?. Their characteristic cross‐girdle microfabric (due to penecontemporaneous folding and flattening with stretching mainly in b = A? along two sets of equivalent (Ok1) slip planes but also in a = A along the strain‐slip cleavage of the fold fabric and a complementary set of (hO1) planes) is preserved in the internal fabric of a completely granitized xenolith, but no trace of it is encountered in the granitic host‐rocks. Bodily rotation o...

THE CONTROL OF BLOWFLIES INFESTING SLAUGHTER‐HOUSES I. FIELD OBSERVATIONS OF THE HABITS OF BLOWFLIES

The habits of blowflies infesting slaughter‐houses have been studied to determine the conditions under which control measures are most likely to be effective. Flies found in such numbers as to be considered pests of economic importance were: Calliphora erythrocephala (Meig.) and C. vomitoria (L.); Lucilia caesar (L.), L. illustris (Meig.) and L. sericata (Meig.); and Phormia terrae‐novae (R.‐D.). All were found breeding profusely on slaughter‐house refuse.The reactions of adult blowflies to weather and the working conditions at the slaughter‐house are recorded; the three genera react in differing degrees to relatively minor changes in weather conditions, and redistribution of populations caused by these changes is so rapid as to make difficult the accurate assessment of degree of infestation. It was found that blowflies normally do not settle on the internal fabric of slaughter‐house buildings but tend rather to congregate on nearby vegetation. The preferences were studied of adult flies for certain types of meat, for particular parts of carcasses, and for meat exposed to sunlight.Conditions favouring rapid development of larvae were determined and the rates of breeding recorded for the three genera when in natural competition and when breeding independently. The larvae of P. terrae‐novae were found to be predacious on other species and each other. Lucilia sericata was found to oviposit and breed successfully in vegetable matter.Blowflies overwinter as fully grown larvae which pupate only a short while before emerging in the spring.Various methods are discussed whereby the findings of this investigation into the life history and habits of blowflies can be utilized in the control of these pests.