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Turtleback faults of Death Valley, California: A reinterpretation
Turtlebacks are smooth, curved surfaces, which form north-northwestward-plunging elongate domes on the east side of Death Valley. These surfaces are roughly parallel to bedding or foliation of anticlines in Precambrian schist, gneiss, and marble. Late Cenozoic fan and playa deposits are faulted over these surfaces along the turtleback faults. Previously the turtleback faults have been interpreted as part of a thrust fault, perhaps the Amargosa thrust fault, that was arched after thrusting. They are interpreted here as individual normal faults younger than the thrust fault and, contrary to previous interpretations, much younger than the formation of the anticlines in the Precambrian rocks. The tectonic history of this unusual area is here considered to include the following events: (1) Precambrian folding of the Precambrian rocks; (2) post-Paleozoic and pre-middle(?) Tertiary Amargosa thrusting; (3) uplift and erosion of Paleozoic strata and the Amargosa thrust fault, down to the folded Precambrian rocks in the Black Mountains block; (4) Middle (?) Tertiary rhyolite extrusions and the accumulation of later Tertiary fan and playa deposits; (5) Pliocene or Pleistocene uplift of the Black Mountains relative to Death Valley, along the Black Mountains fault system, with consequent removal of support for the Tertiary deposits on the turtleback surfaces, and the development of the turtleback faults by normal faulting, or sliding, of the Tertiary sedimentary rocks down the turtleback surfaces toward Death Valley; and, (6) Pleistocene to Recent renewal of movement on the Black Mountains fault system.
Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis
Pressures of interstitial fluids significantly greater than the normal hydrostatic pressure are known in many parts of the world. Many occurrences are in thick sections of relatively young sediments; some are in areas that have been intensely deformed. Abnormal fluid pressures in the Gulf Coast region are associated with thick bodies of shale or mudstone, and with high hydraulic gradients across bedding. The rocks there have been buried rather rapidly and are evidently not yet fully compacted. The mechanism by which clay consolidates under pressure affords a quantitative relationship among the variables—depth, strength of clay, and fluid pressure—and this relationship indicates that the Gulf Coast examples agree fairly well with observations on depth and porosity in Paleozoic shales of Oklahoma and Tertiary shales of Venezuela. Critical data are lacking, but permeability clearly decreases tremendously as clay rocks are compacted. This decrease in permeability provides a self-sealing mechanism that greatly retards the escape of pore water from deeply buried clay rocks. The relationship between rate of compaction and the development of abnormal fluid pressures probably applies not only to clay rocks but also to carbonates and possibly to micaceous and chloritic metamorphic rocks. Conditions of geosynclinal deposition are, in general, those most favorable to the development of abnormal fluid pressures. The hypothesis that large-scale overthrusting is facilitated by abnormal fluid pressures which, in turn, are associated with geosynclinal deposition is applied to the overthrust belt of western Wyoming and adjacent States. This is a long curving belt of several bedding-plane faults which have an aggregate horizontal displacement across the belt of 50 miles or more. The sedimentary rocks that make up the belt were evidently deposited in a major geosyncline bordered by uplands not far to the west. At any given locality, the rate of deposition of the sediments increased continuously until the beginning of intense deformation and overthrusting. The geosynclinal axis and the bordering uplands probably migrated slowly eastward across the belt. Several lines of indirect evidence suggest that abnormal fluid pressures developed in this region during final stages of rapid geosynclinal sinking and that thick plates of Paleozoic and Mesozoic sedimentary rocks sheared off from the underlying rocks and moved slowly eastward. Rate of movement probably was controlled by erosion of upfolds that arose at the front of each moving plate. The fundamental cause of the lateral stresses that propelled the overthrusts is not known, but it may be examined instructively in the light of the fluid-pressure hypothesis. The thrust sheets might, for example, have slid by simple gravitation down the western limb of the geosyncline on reasonable slopes and not improbable fluid pressure-overburden ratios. Such large-scale slumping of thrust sheets, however, seems to require gaps at the rear of the thrust sheets. The long intermontane valleys of Idaho and Utah may possibly have originated as such gaps or rifts, but no proof has yet been recognized that they were formed in this manner. An alternative possibility, regional compression, requires concentration of the lateral stresses within the upper few miles of the earth's crust; in this general region emplacement of the Idaho batholith seems the most likely source of such superficially concentrated stresses. However, this batholith is so far from the front edge of the overthrust belt that it would require extremely high fluid pressure-overburden ratios over a wide area. Perhaps some combination of the two forces—pushing wide thrust plates down a gentle slope—is the most likely explanation.
Zircon from a bentonite bed in Martinsburg shale (Ordovician) at Fisher's Hill, Virginia
No abstract available.
Chemical composition of argillites of the cobalt series (precambrian) and the problem of soda-rich sediments
Two new chemical analyses show that the varved argillites of the Precambrian Cobalt series of Ontario, Canada, contain more Na 2 O than K 2 O and have a Na 2 O/K 2 O ratio higher than most pelitic rocks but similar to that found in many graywackes. Published chemical data substantiate these observations. The Cobalt materials are also low in CaO but otherwise have a composition compatible with their presumed glacial origin. Albitization of the detrital plagioclase feldspar seems to best account for the low lime and high soda content of these rocks. X-ray data confirm the presence of albite. Spectrographic analyses of the argillites are also given.
Froth veins, formed by immiscible hydrothermal fluids, in Mercury deposits, California
No abstract available.
Ion exchange in clays and other minerals
Ion exchange in clays and other minerals is dependent on the crystalline structure of the mineral and on the chemical composition of any solution in contact with the mineral. The structures of clay minerals and zeolites are briefly described to provide a background for the discussion of their ion-exchange reactions. Ion exchange in these minerals is a reversible chemical reaction that takes place between ions held near a mineral surface by unbalanced electrical charges within the mineral framework and ions in a solution in contact with the mineral. Generally the excess charge on the mineral is negative, and it attracts cations from the solution to neutralize this charge. The chemical reactions in ion exchange follow the law of mass action, but the reactions are restricted by the number of exchange sites on the mineral and by the strength of the bonding of the exchangeable cations to the mineral surface. Titration of H-clays with bases shows that montmorillonites and "illites" behave like a mixture of two or three different acids, whereas kaolinite, with an indefinite number of exchange sites, behaves like an indefinite number of acids. Ion-exchange capacity is measured in chemical equivalents of base adsorbed at pH 7. Each clay mineral has a range of exchange capacities because of differences in structure and in chemical composition. The ranges (in milliequivalents per 100 grams) are kaolinite, 3-15; halloysite (2H 2 O), 5-10; halloysite (4H 2 O), 40-50; montmorillonite, 70-100; "illite," 10-40; vermiculite, 100-150; glauconite, 11-20; attapulgite, 20-30; and allophane, 70. The common metallic cations found in exchange positions in clay minerals are Ca +2 , Mg +2 , Na + , and K + . At low pH values H + replaces other cations. The order of replaceability of the common cations has been found to be: Li + < Na + < K + < Rb + < Cs + and Mg +2 < Ca+2 < Sr +2 < Ba +2 Bivalent cations enter the exchange sites preferentially to univalent cations. The common exchangeable cation in most clay minerals in soils is Ca +2 . Other exchange phenomena discussed are anion exchange, fixation of cations and anions by clay minerals, effect of environment on cation exchange, and the exchange capacity of zeolites, of rocks, of other minerals, of organic matter and organic complexes, and of amorphous mineral material.
Granitization, migmatization, and fusion in the northern Entiat Mountains, Washington
A tabular quartz diorite complex extends along the Entiat Mountains , which form a southeast-trending spur of the Northern Cascade Mountains . The country rocks of the complex are biotite gneiss, probably derived from arkosic sedimentary rocks, and hornblende schist, probably derived from basic volcanic rocks. Their present mineral composition is typical of the upper epidote amphibolite and amphibolite facies of regional metamorphism. During regional metamorphism the biotite gneiss was granitized to biotite-quartz diorite gneiss and the hornblende schist to hornblende-quartz diorite gneiss. The metamorphic origin of these quartz diorite gneisses involving relatively little replacement is shown by their chemical and mineralogical similarity to the original biotite gneiss and hornblende schist, by their enclosing long, thin, undisturbed layers of country rocks, and by their association with migmatites formed by metamorphic processes. Here and there in the quartz diorite gneisses massive rocks with hypidiomorphic granular "igneous" textures have formed principally by recrystallization, and these features are associated with small-scale swirling of the foliation and some intrusive features, which show that the gneiss was rendered plastic and mobile during granitization . Migmatites containing replacement bodies and secretions of leucocratic quartz diorite occur in the biotite gneiss and in most of the rocks of the complex - particularly in the biotite-quartz diorite gneiss. During the formation of the complex, the felsic material contained in the leucocratic rocks, principally sodium, potassium, and silicon, was generated by metamorphic differentiation. During the granitization , anatectic magmas were formed by fusion of hornblende-quartz diorite gneiss, and some similar magma intruded from below. Melting without metamorphic differentiation is indicated by the chemical similarity between the anatectic quartz diorite and the parent hornblende-quartz diorite gneiss. Smearing out of inclusions and crystals and the tabular, sill-like nature of the bodies indicate that fusion may have been triggered by differential movement and shearing. The anatectic rocks contain inclusions of hornblende schist that survived both the granitization to hornblende-quartz diorite and the fusion . Crystallization differentiation of the anatectic magmas yielded potassium-rich pegmatites and local granodioritic masses. Regional metamorphism continued after the anatectic magmas in the sills had solidified; felsic metamorphic differentiates occur in leucocratic quartz diorite that partly replaces the borders of the sills. The leucocratic material was plastic and facilitated differential movement between the sill and its walls.
Paleozoic and mesozoic fossils in a thick stratigraphic section in the eastern Sierra Nevada, California
A thick section of metamorphosed Paleozoic and Mesozoic rocks is exposed in two roof pend- ants, one each in the Mount Morrison and Devils Postpile quadrangles in the eastern Sierra Nevada near Mammoth Lakes, Cali- fornia (Fig. 1). In the course of geologic mapping in these quadrangles by the U. S. Geo- logical Survey in co-operation with the California Division of Mines, fossils of Ordovician, Pennsylvanian, Permian(P), and Early Jurassic ages were collected from weakly metamorphosed parts of the pendants. Detailed map- ping in the Mount Morrison quadrangle has been completed by Rinehart and Ross, and a report on this area is in preparation. Similar mapping in the Devils Postpile quadrangle by Rinehart and Huber is in progress. In the part of the Devils Postpile quadrangle not yet covered in the present study, the contacts shown on the map (Fig. 1) are after Erwin (1934). Because diagnostic fossils are extremely rare in the metamorphic rocks of the Sierra Nevada, these fossils are considered important enough to merit this short note prior to completion of the detailed mapping in the Devils Postpile quadrangle and the publication of more comprehensive reports on the two quadrangles
Subdivision of the quaternary alluvium east of the front range near Denver, Colorado
No abstract available.
Role of fluid pressure in mechanics of overthrust faulting: A reply
No abstract available.
Review of paleomagnetism
This review is an attempt to bring together and discuss relevant information concerning the magnetization of rocks, especially that having paleomagnetic significance. All paleomagnetic measurements available to the authors are here compiled and evaluated, with a key to the summary table and illustrations in English and Russian. The principles upon which the evaluation of paleomagnetic measurements is based are summarized, with special emphasis on statistical methods and on the evidence and tests for magnetic stability and paleomagnetic applicability. Evaluation of the data summarized leads to the following general conclusions: (1) The earth's average magnetic field, throughout Oligocene to Recent time, has very closely approximated that due to a dipole at the center of the earth oriented parallel to the present axis of rotation. (2) Paleomagnetic results for the Mesozoic and early Tertiary might be explained more plausibly by a relatively rapidly changing magnetic field, with or without wandering of the rotational pole, than by large-scale continental drift. (3) The Carboniferous and especially the Permian magnetic fields were relatively very “steady” and were vastly different from the present configuration of the field. (4) The Precambrian magnetic field was different from the present field configuration and, considering the time spanned, was remarkably consistent for all continents.
Use of water-well data in interpreting occurrence of aquifers in northeastern Lyon County, Minnesota
In northeastern Lyon County the areal distribution of aquifers of Cretaceous age determined from well-bottom altitudes suggests a series of interbedded sandstones striking northwestward and overlapping one another to the northeast. Probably the numerous thin sandstone aquifers in the area were deposited near the flanks of a Precambrian granite “high” area by a transgressing Late Cretaceous sea.
Late Wisconsin age of terrace alluvium along the North Loup River, central Nebraska: A revision
A radiocarbon age of 10,500±250 years B.P. (Before Present) from shell material near the base of a terrace along the North Loup River, Howard County, Nebraska, dates the alluvial sequence of silts and soils as late Wisconsin. The alluvium containing the shell material previously was reported as Kansan and Yarmouth in age.