Nomenclature Of Rocks Research Articles

Revised Geologic Nomenclature for Pre-Cretaceous Rocks of British Honduras: GEOLOGICAL NOTES

New field data suggest that the Maya and Macal Series are a single unit, and that the intrusive Bladen Porphyry is a volcanic-pyroclastic unit within the Maya-Macal. Because of the similarity between the Maya-Macal of British Honduras and the Santa Rosa Group of the rest of Central America, we propose that the term, Maya-Macal, be replaced by Santa Rosa Group, and that the Bladen Porphyry be replaced by Bladen Volcanic Member.

On the Petrophysical Properties of Granitic Rocks

Abstract Classification of granitic rocks on rock density determinations is applied to a granitic complex on Bornholm. It is shown that the system presented by Kopf (1966, 1967) cannot be applied generally. It is concluded that the variations in rock densities mainly depend on the contents of mafic minerals, and as the nomenclature of granitic rocks mainly are based on the salic minerals, the densities cannot be used to classify the granitic rocks. It is shown that density determinations can be used sucessfully in mapping of granitic complexes looking homogeneous. Another physical property of rocks is the magnetic susceptibility. It has been proposed by Kopf (1966, 1967) that there should exist a correlation between increasing susceptibility and increasing degree of chloritization of the mafic minerals in the rocks (hornblende and biotite). It is shown that such correlation does not exist generally.

A Modification of Sandstone Classification

The system includes a complete rock nomenclature based on parameters of sedimentary structure, grain texture, maturity, cementing material and primary detrital mineral composition. Detrital composition is based on a three-pole arrangement in which are delineated quartzose, feldspathic and lithic suites. Special names are provided for certain rock types within the suites. Each suite has particular tectonic significance.

Nomenclature for Sedimentary Rocks: ABSTRACT

Despite the many excellent papers on classification of sedimentary rocks, there is still so much loose and variable usage of names that it is not possible to be certain--or, at times, to have even a good notion--of the intended meanings of names found in the literature. If intelligible communication contributes anything to progress in sedimentary petrology, then the existing state of affairs must constitute an appreciable drag on progress. This paper is presented to re-draw End_Page 483------------------------------ attention to this lamentable state, to plead for wider employment of clearly defined names, and to offer a general system of nomenclature which could provide a means of achieving greater uniformity and precision in terminology. This system is not in the least revolutionary but merely proposes restricted meaning for many existing names and a general scheme for systematically deriving other names. Guiding principles are: (1) names should be based on petrographic features; (2) these features should be established, not inferred; (3) the system for naming rocks should be flexible enough to apply both in the field and in the laboratory; and (4) names should be just as quantitative as the means of examination permits. The classical concept of sedimentary rocks as mixtures of mechanical and chemical fractions is the principal nomenclatural basis in this system. The main name reflects the most abundant constituent of the dominant fraction. A finer division of silicate sandstone than is customarily made is believed practical and desirable. Three terms are proposed to fill a conspicuous gap in terminology. Aggregal and integral describe textures of mechanical and chemical origin, respectively, but are defined petrographically. Accretic, a correlative term with clastic, describes aggregal textures composed of grains formed by accretion. Integral textures include those which are crystalline and amorphous. The most important point concerning nomenclature is that the reader understand the terminology employed by the writer. This can be assured by reference to an explicit classification or nomenclatural system. End_of_Article - Last_Page 484------------

Rocks composed of volcanic fragments and their classification

Summary Wentworth and Williams ' (1932) and Blythe 's (1940) reviews of pyroclastic nomenclature are excellent accounts of pre-1940 terminology. Changes or additions to pyroclastic names since that time have come mainly from vigorous research on processes of pyroclastic flow and their resulting depositional product, ignimbrite. It has long been realized that pyroclastic fragments become mixed with other types of fragments, but only recently has this been recognized in classifications. Russian workers ( Blokhina et al., 1959 ; Vlodavets et al., 1962 ), Panto (1959) , Fisher (1961) and Torok (1962) have attempted to unify pyroclastic and epiclastic rock terminology. Russian authors do this by classifying rock mixtures composed of pyroclastic and non-volcanic (“sedimentary”) mixtures. Fisher (1961) attempts to unify the terminology by naming genetic terms for processes of fragmentation, viz., pyroclastic, epiclastic and autoclastic, to which may be added alloclastic ( Wright and Bowes , 1963 ) and hyaloclastic ( Rittmann , 1960 ), and by using the standard clast-size limits of the Wentworth Scale for all of the genetic types. Perhaps the most fundamental disagreements in volcaniclastic names arise from disagreement or misconceptions about some common terms such as “sedimentary”, “volcanic”, “epiclastic” and “pyroclastic”. For example, the terms pyroclastic and epiclastic refer to different processes of fragmentation, not to different processes of deposition. Both types of particles may be deposited by streams or wind in any physiographic environment, but this does not alter their original mode of fragmentation. The terms sedimentary and volcanic are much broader in meaning than epiclastic and pyroclastic. International agreement on nomenclature of volcaniclastic sediments and rocks will very likely never come about until agreement is reached on the meaning and use of these four terms.

Igneous Rock Nomenclature Chart for Petrology Students

A glossary of igneous rock nomenclature is presented in chart form for courses in graduate and undergraduate petrology and petrography. This chart, which is based upon the familiar parameters of mineralogy and texture, derives its utility from the wide range of igneous rocks included. It provides a simple diagram from which the students of petrology and petrography can readily grasp the approximate mineralogy and texture of the igneous rock terms encountered in their reading of the text and literature.

Revisions in Stratigraphic Nomenclature in Tucumcari-Sabinoso Area, Northeastern New Mexico: GEOLOGICAL NOTES

It is proposed to revise the stratigraphic nomenclature of rocks in the Tucumcari-Sabinoso area in Guadelupe, Harding, San Miguel, and Quay counties in northeastern New Mexico. The following terminology for the Triassic, Jurassic, and Cretaceous units are proposed: Cretaceous (3 formations) -- Pajarita shale, Mesa Rica sandstone, Tucumcari shale; Jurassic (3 formations) -- Morrison formation, Bell Ranch formation, Entrada sandstone (latter two are part of the San Rafael group); Triassic (3 formations) -- Redonda formation, Chinle formation, Santa Rosa sandstone (latter two are part of the Dockum group).

Stratigraphy of Jackson Group (Eocene), South-Central Texas

Discovery of uranium and renewed exploratory activity for oil and gas in the outcrop area of the Jackson group of Eocene age in south-central Texas require setting up a standard nomenclature for rocks of the group in that area. In general, the names which Ellisor proposed in 1933 have been used, but modified to agree with current usage by most geologists. The Jackson group, as proposed, consists of four formations, in descending order: Whitsett, McElroy, Wellborn sandstone, and Caddell. The Whitsett formation, comprising the Fashing clay, Calliham sandstone, Dubose, and Stones Switch sandstone members, consists chiefly of bentonitic clays and tuff near the top and sandstones near the base. The McElroy formation consists of the Conquista clay (a new name substituted for the pre-empted name Falls City shale of Ellisor, 1933), the Dilworth sandstone, and the Manning clay members. The Wellborn sandstone and the clayey Caddell formation are indivisible into members in south-central Texas.

Devonian System in Williston Basin and Central Montana

Knowledge of Devonian stratigraphy in the southern part of the Williston basin and adjacent areas has increased greatly since extensive deep drilling began in 1951. In recent years many formation names have been borrowed from the Alberta basin, the Canadian part of the Williston basin, and the northern Rocky Mountains of western Montana and applied with varying degrees of success but little uniformity to the buried Devonian rocks of central Montana and the United States part of the Williston basin. This paper summarizes the stratigraphy and geologic history of the Devonian system and, based on the facts and interpretations presented, proposes a uniform subsurface nomenclature for Devonian rocks in North Dakota, South Dakota, eastern and central Montana east of the 111 ; meridian, and the adjacent part of Wyoming. The Middle Devonian subsurface nomenclature introduced by Baillie in 1953 is followed with the exception of the Manitoba group and the Ashern formation, which is not a continuous stratigraphic unit in the report area and is included at the base of the Winnipegosis formation. The Middle Devonian series consists in ascending order of the Winnipegosis and Prairie formations of the Elk Point group and the Dawson Bay formation. The Upper Devonian series consists in ascending order of the Souris River formation, the Jefferson group made up of the Duperow and Birdbear formations, and the Three Forks formation, overlain by the Bakken formation of Late Devonian(?) and Early Mississippian age. (Lower Devonian rocks either have not been recognized or ha e not been penetrated in the subsurface, although local channel-fill deposits, assigned by Dorf in 1934 to the Beartooth Butte formation of Early Devonian age, have been mapped in the mountains of northern Wyoming and south-central Montana.) The Mobil Producing Company's Birdbear well No. 1 in Dunn County, North Dakota, near the geographical center of the Williston basin is proposed as the standard subsurface section for the Souris River, Duperow, and Three Forks formations. A standard subsurface section, as used here, replaces an inadequately defined or unpublished type section or supplements a distant type section that is not representative of the formation in the area under study. The same well serves as the type subsurface section of the Birdbear formation, which is named herein. The Birdbear replaces the Nisku formation of Baillie, which can not be correlated with the type Nisku formation of the Alberta basin. The subsurface Upper Devonian formations are correlated with the Devonian outcrop at the type locality of the Jefferson and Three Forks formations near Logan, Montana. The original contact between the Jefferson and Three Forks formations as defined by Peale in 1893 is accepted in preference End_Page 2293------------------------------ to the higher boundary proposed by Sloss and Laird at a much later date. The subsurface Jefferson group, consisting of the Duperow and Birdbear formations, is the stratigraphic equivalent of the Jefferson formation, and the uppermost part of the subsurface Souris River formation is correlative with part of the Maywood formation, which underlies the Jefferson near Logan. The potentialities of Devonian rocks as petroleum reservoirs in the southern part of the Williston basin and in central Montana are still relatively underdeveloped. Minor oil production has been developed in the Winnipegosis, Dawson Bay, Duperow, and Three Forks formations and many scattered shows of oil and gas have been found in the porous Birdbear formation.

The Distinction between Grain Size and Mineral Composition in Sedimentary-Rock Nomenclature

A system of grain-size nomenclature of terrigenous sediments and sedimentary rocks is introduced wherein fifteen major textural groups are defined on the ratios of gravel, sand, silt, and clay. Further subdivision of each class is based on the median diameter of each size fraction present. Next, the mineral composition of terrigenous sedimentary rocks is considered. A triangular diagram is used to define eight rock types (orthoquartzite, arkose, graywacke, and five transitional types) based on the mineralogy of the silt-sand-gravel fraction and ignoring clay content. The writer contends that the current practice of calling all clayey sandstones graywackes is not valid, inasmuch as it represents a confusion of texture with composition. It is suggested that sedimentary rocks may be best defined by the use of a tripartite name, based on the following pattern-(grain size): (textural maturity) (mineral composition).

On the old and the new systems of nomenclature of volcanic rocks

The nomenlature of volcanic rocks on the basis of identifiable minerals is shown in the above comparisons to be in many cases in contradiction to the actual character of the rocks under consideration. Most of the so called basalts of the Indonesian Volcanoes are for example in reality andesitic or even dacitic rocks. Thus the basicity of the plagioclase felspar phenocrysts should not be considered as an important character for a basaltic rock, but the relative proportions of the femic and salic constituents are to be considered. Applying these facts in the nomenclature based on the C.I.P.W. norm it is found that a good agreement exists between the names arrived at by this method and the new chemical method proposed byA. Rittmann.

The nomenclature and classification of sedimentary rocks

The nomenclature and classification of sedimentary rocks

Nomenclature of Triassic rocks in northeastern Utah

Nomenclature of Triassic rocks in northeastern Utah

The Classification of Igneous Rocks: An Historical Approach

There can be little doubt that most petrologists will welcome J- the classification of igneous rocks according to their grainsize as proposed by the British Association Committee on Rock Nomenclature and Classification (1). This classification has been incorporated in the new edition of the well-known textbook of Hatch and Wells (2) and has recently been very ably defended by Wells (3). To avoid misunderstanding it must be emphasized that grain-size is taken to be only one of the factors in the classification of igneous rocks and not the sole factor, and that the suggested limits between different grades of granularity must be taken at the present time to be only provisional.

The Grain-size Factor in Classification of Igneous Rocks

Although two years have passed since the British Association Committee on Rock Nomenclature and Classification reported in favour of using grain-size as an important factor inclassification, very few petrologists have accepted the Committee's invitation to discussand criticize the suggested scheme. This silence may be taken to mean consent; but in some quarters the older scheme, based on mode of occurrence of the rocks, still finds favour. It is instructive to inquire how far it is correct to speak of the scheme as new. Actually one does not have to dig deeply into petrological literature to realize that the principle of grain-size classification has been advocated for many years; indeed, it was the guidmg principle in some of the earliest scientific attempts at rock classification, notably in the well-known scheme elaborated in his Lehrbuch der Petrographie by Ferdinand Zirkel in 1866 and 1893–4. The reactionagainst the complete subordination of mode of occurrence found expression in the second edition of Rosenbusch's Mikro-skopische Physiographie (1887), in which the igneous rocks were divided into (1) deep-seated rocks, (2) dyke rocks, and (3) effusive rocks, each of these main categories being subdivided in terms of mineral content. In quite recent times A. Johannsen (1931)

Nomenclature of Eruptive Rocks

THE reviewer of my book “Eruptive Rocks ” (NATURE, Dec. 17, 1927) says that I have not accomplished my expressed purpose of cleaning up “the jungle of rock names.” The wisdom of my procedure is open to question, but the facts can be ascertained by simple enumeration. Prof. Arthur Holmes collected about 470 names of eruptive rocks in his “Nomenclature of Petrology,” and the total number in use at the present time is certainly not less than 500. Of these I have used only 56 in my system, a reduction of about 88 per cent. This seems to me to be a fairly thorough ‘cleaning-up,’ and I wonder what more the reviewer wants? Of course, I use some qualifying terms in conjunction with these names, but so does every petrologist, no matter what system he follows.

Nomenclature of Eruptive Rocks

Nomenclature of Eruptive Rocks

A Further Study of the Nomenclature of Rocks

It is hardly necessary to point out how very important it is that those on whom falls the onus of naming a rock should decide which is the most fitting name only after very careful consideration. Some there are who profess to be above the petty worries of nomenclature, and from these only a loose application of rock names can be expected. This is liable to mislead those dealing with problems of petrogenesis, who are justified in assuming that those competent to carry out geological research are also competent to apply correct names to their discoveries. That this is often not the case can be easily demonstrated from quite recent papers. The difficulty arises from two causes: the lack of a system of rock classification which in any sense approaches the ideal, and a desire for brevity. Concerning the first difficulty it is sufficient to say that little advance in placing the nomenclature of rocks upon a firm and common basis is possible until one universally recognized system of rock classification is in use. It is still open for investigators to choose, without any set standard, which of several variable characters of a rock should be the criterion upon which the systematic position of the rock depends. Thus, one writer places mode of occurrence before micro-structure, while his neighbour deems it wise completely to ignore mode of occurrence and makes micro-structure the deciding factor.

Silica Percentages of Igneous Rocks

Figs. I–VII give a graphic representation of the average silica percentage of a number of igneous rock types, so arranged as to show the relation between silica-percentage and rock nomenclature. All the analyses used were obtained from the 1917 edition of H. S. Washington's Chemical Analyses of Igneous Rocks”. This book contains some 8,000 rock analyses from many parts of the world. They are divided into two classes, superior and inferior. Only those of the former category (some 6,000 in all) have been utilized.

A classification of metamorphic rocks

Introduction The purpose of this paper is to present a comprehensive classification of metamorphic rocks, the attempt being made to work out a satisfactory scheme without the introduction of new terms. In this proposed classification the main principles are based on (1) origin of the rocks, (2) rock structure, and (3) composition. Since it is necessary to have certain important terms pertaining to metamorphic rocks clearly, defined and differentiated, this matter is considered at some length. After the terms are defined, they are, it is believed, consistently employed in the classification presented. In geological literature there is much confusion regarding the nomenclature of metamorphic rocks, and, in many eases, after a writer has clearly defined a term this same term is used in a different sense in the same book or paper. Apparently there are few published attempts to make comprehensive classifications of metamorphic rocks. Many text-books, as, for example, Pirsson’s . . .