Quartzite Layers Research Articles

Structural sequence in the metamorphosed Precambrian Rocks of the Frankland and Wilmot Ranges, south-western Tasmania

Four groups of deformational events are recognised in the low grade Precambrian quartzite and phyllite of the Frankland and Wilmot Ranges. The rocks generally show the presence of a penetrative D foliation with associated folding.D3 folds are the most pronounced structures occurring on all scales in all lithologies and control the present attitude of the earliest foliation. Steeply inclined foliations are the rule. D2 structures are variable; some inter-foliated quartzite/phyllite zones display strong F2 and S2 (a crenulations foliation) whereas these may be totally lacking in the same material, particularly to the south. The few mesoscopic F2 folds in zones of massive quartzite layers involve localised rotation of S1. D4 structures only occur on a minor scale with little cleavage development. All D1-4 structures have been rotated by a major wrench system thought to have operated during the Cambrian which accounts for the swing of the main quartzite ridges from N/S to E/W.

Selective buckling of composite layers with contrasted rheological properties, a theory for simultaneous formation of several orders of folds

Affected by lateral compression, a “competent” sheet of rock enclosed in a less “competent” host, shortens by two unlike mechanisms; viz., 1. (1) by buckling (buckle shortening) and 2. (2) by uniform strain along the sheet (layer shortening). The relative rate of these two types of shortening depends chiefly upon the relative competency of the, host and the enclosed sheet. The greater the contrast in competency (viscosity, rigidity), the higher the rate of buckle shortening relative to the rate of layer shortening. Therefore, if rock sheets with unlike competency are enclosed in a given host e.g., layers of limestone, marl, quartzite and dolomite in shale, a given amount of compression is likely to result in quite unlike finite strain patterns of the several sheets. The least competent sheet (probably the marl) becomes shortened and thickened with only negligible folding, whereas the more competent sheet (e.g., the quartzite) responds to the compressive stress essentially by buckling. Layers of intermediate competency, such as limestone and dolomite in this example, are affected by buckling and layer shortening in relative degrees according to their competency (effective viscosity, rigidity). These relationships are treated quantitatively by the tools of fluid dynamics and the results are applied to the development of dragfolds, anticlinoria and synclinoria, and other structures involving unlike orders of folds. For relatively competent layers resting on the earth's surface, the relation between buckle shortening and layer shortening is not only controlled by relative competency of layer and substratum, but also to a great extent by the magnitude of the deviatoric compressive stress affecting the layer. The larger the stress, the greater the rate of buckling relative to the rate of layer shortening. But there are limits to the deviatoric stress which may be applied in real systems. For gravity-affected surficial layers, the upper limit is determined by the ultimate strength of the materials in question and the lower limit by the fact that weak stresses create no buckling, but only slow layer shortening.

Geology of Northern Soledad Basin, Los Angeles County, California

The Soledad basin is situated north of the San Gabriel Mountains, in Los Angeles County, California, and its center is about 35 miles north of the Los Angeles civic center. Roughly an elongate parallelogram in shape, this basin has dimensions of about 10 by 30 miles, with the longer dimension oriented east-west. The Sierra Pelona and the San Gabriel Mountains form the northern and southern boundaries, respectively. The San Andreas fault and the San Gabriel fault, both of which trend northwest in this region, bound the basin on its northeast and southwest borders, respectively. Only the northern part of the basin is discussed in this paper. The pre-Cretaceous Pelona schist, the oldest unit in the area, is a thick sequence of muscovite schist, muscovite-chlorite schist, and chlorite-actinolite schist with rare layers of quartzite and marble. This unit makes up the Sierra Pelona, an elongate mountain mass that trends east. Granitic intrusive rocks, probably late Mesozoic in age, underlie a complex section of Tertiary sedimentary and volcanic rocks in most of the eastern Soledad basin. Gneisses, some of which may represent highly injected Pelona schist, also are present in a belt that trends essentially parallel with and south of the Sierra Pelona. In general, the sedimentary beds at the surface are younger from east to west in the Soledad basin. The marine Martinez formation of Paleocene age is the oldest sedimentary unit in the region. It is overlain by the Vasquez formation of doubtful Oligocene age, which crops out over a wide area and has a maximum known thickness of about 16,700 feet. This younger unit is composed of interlayered fanglomerate and volcanic rocks. Where the Martinez formation was removed by pre-Vasquez erosion the Vasquez beds rest directly on the pre-Tertiary crystalline rocks. The Vasquez beds were deposited in three basins separated by ridges. Burial of the intervening ridges by thick alluvial fans building northward and westward from the ancestral San Gabriel Mountains coalesced these basins into a singl broad area of sedimentation late in Vasquez time. The upper lower Miocene Tick Canyon formation and the upper Miocene Mint Canyon formation which lie unconformably above the Vasquez formation, also represent non-marine deposition of dominantly coarse-grained sedimentary material. These units are widespread in the western part of the Soledad basin. Marine sandstone and siltstone of the Castaic formation rest unconformably on the Mint Canyon beds, and reflect an eastward encroachment of a late Miocene sea over the western part of the basin. West of the area mapped, the marine Pliocene Towsley formation and the non-marine Plio-Pleistocene Saugus formation, which grades westward into a marine facies, overlie the older rocks. Terrace deposits of late Pleistocene age are common over much of the area. Recent alluvium is present in all of th major valley bottoms and locally in some of the minor valleys. The date of the folding and metamorphism of the Pelona schist is not known, but it assuredly is pre-Tertiary. Tertiary rocks have been either tilted or deformed into broad open folds, although nearly isoclinal folds are found locally near the major faults. Almost without exception, these folds plunge gently west or southwest. End_Page 1812------------------------------ The most prominent structural features of the Soledad basin are numerous faults. High-angle dip-slip faults that trend generally east were formed in post-Martinez, pre-Vasquez time. Displacements occurred throughout Vasquez time and ceased prior to Tick Canyon time. Net slip on these faults may be as much as 10,000-15,000 feet. The normal faults indicate that the minimum compressive stress in this region was oriented nearly north-south prior to Mint Canyon time. On the other hand, the post-Mint Canyon, pre-Saugus faults indicate a maximum compressive stress oriented north-south, which resulted in a number of northeast-trending left-lateral strike-slip faults with displacements up to 10,000 feet. None of the faults of the Soledad basin has been reactivated in spite of Pleistocene and R cent offsets along the nearby San Andreas and San Gabriel faults.

FAULT IN PALEOZOIC ROCKS NEAR FREDERICK, MARYLAND

Diamond drill cores from an area 5 miles southwest of Frederick, Maryland, reveal significant stratigraphic and structural information. The Antietam and Tomstown (Lower Cambrian) and Grove formation (Ordovician) are involved. The Antietam has some quartzite layers but is predominantly quartzose phyllite. The lower 40 feet of Tomstown is dolomite with sericite partings. The remaining 140 feet is impure, thin-bedded dolomitic limestone. The Grove is a light-gray slightly dolomitic limestone with red staining in the matrix of breccias and along fractures. The Antietam and Tomstown dip easterly 25°–30°. Overlying the Tomstown, above a fault which also dips 25°–30° easterly, is Grove limestone which is overturned, dipping 75°–85° easterly. Along the fault are up to 5 feet of black clay gouge, 30 feet of breccia, and slickensides. The absence of the Frederick limestone and the angular relationship of the Grove bedding to the fault surface provide additional evidence of faulting. The fault, having younger rocks resting on older, could be normal, but the low angle plus overturning of the Grove limestone suggest a subsequent shear thrust. The authors postulate that this fault or a series of similar faults may explain anomalous stratigraphic relations in the Lower Paleozoic rocks along the eastern edge of Catoctin Mountain. Locally, where the Antietam and/or Tomstown formations are missing, faulting could be the cause instead of facies changes or one or more unconformities.

STRUCTURE AND PETROLOGY OF THE SUNAPEE QUADRANGLE, NEW HAMPSHIRE

The rocks of the Sunapee quadrangle, in southwestern New Hampshire, are chiefly metamorphic in origin and range in age from upper Ordovician (?) to lower Devonian. The pre-Devonian rocks, constituted by the Ammonoosuc volcanics, the Clough formation, and the Fitch formation, are confined to the northwest part of the quadrangle. They are nearly completely separated from the rest of the area by the Grantham fault, which is probably Triassic in age. The structure northwest of this normal fault consists of an elongate dome of granodiorite gneiss flanked and overlain by concordant layers of amphibolite, biotite gneiss, quartzite, and schist. Immediately southeast of the fault is the Bethlehem gneiss. This mass occupies the central half of the quadrangle and is composed of well-foliated, “porphyritic” granitic gneiss. East of the gneiss is the Littleton formation. The western (older) portion of this belt of rocks consists of well-bedded mica-sillimanite schist and quartzite. To the east the rocks of this belt become somewhat coarser-grained and more gneissic. The most easterly portions contain little schist but consist predominately of mica and microcline gneiss. East of the Littleton formation is the Kinsman quartz monzonite. This rock resembles the Bethlehem gneiss but is generally coarser-textured and more poorly foliated. Metamorphism, during the late Devonian time, was not uniform throughout the quadrangle. In general zoning is well developed and ranges from the staurolite-kyanite subfacies, in the western part, to the lower portion of the granulite fades in the eastern part of the area. The structural relations and origin of foliation, lineation, major folds, tabular inclusions, large feldspar crystals, and pegmatites are considered in detail. It is concluded that the area southeast of the Grantham fault originally consisted of a fairly uniform sequence of pelitic rocks of the Littleton formation. As a result of regional metamorphism, these rocks were in large part converted to coarse-grained, “porphyritic” feldspathic gneiss (Bethlehem gneiss and Kinsman quartz monzonite). Recrystallization and reconstitution of the original sediments were the predominant processes involved. Certain constituents are considered to have migrated from distances up to many miles, but the source of this nomadic material may have been largely from the more deeply buried sedimentary rocks. In the Bethlehem gneiss, bedding planes controlled the development of foliation, and recrystallization converted much muscovite into large porphyroblasts of potash feldspar. Considerable sodium and some calcium were introduced to form oligoclase-andesine. In the Kinsman quartz monzonite, recrystallization was so severe that foliation was more poorly developed, grain size was greatly increased, and nearly all muscovite was converted to huge microcline porphyroblasts. Also contributing to the formation of micro-dine was the potassium released in the conversion of biotite in the granulite (eastern) facies. Sodium and calcium were introduced quite thoroughly to form oligoclase-andesine. In the eastern part of the area, considerable potassium was introduced to develop the phases very rich in microcline. The history of these metamorphic-metasomatic rocks is extremely complicated but certain processes and sequences are clear. There has been, apparently, a whole sale introduction of sodium, but potassium has been introduced only locally. Recrystallization and reconstitution were long-continued processes which began prior to the introduction of material. Minor bodies of magma were intruded, largely in the form of dikes and sills, in the later stages. These intrusives replaced much of the adjacent country rock and were themselves altered by the introduction of material which was highly mobile up to a very late stage.