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Bulletin of the Geological Society of America Vol. 69 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 69. PP. 76-86. 3 FIGS. JANUARY 1948 CRITIQUE OP THE TIME-STRATIGRAPHIC CONCEPT BY HARRY E. WHEELER AND E. MAURICE BEESLEY CONTENTS Page Abstract 75 Introduction 75 Acknowledgments 76 Bright Angel group in space and time 77 General Statement 77 Bass Canyon section 78 Pioche section 78 Nopah Range section 79 A fourth dimension in stratigraphy 80 Principle of temporal transgression 82 Conclusions 84 References cited 85 ILLUSTRATIONS Page 1.—Index map of southern Great Basin region 77 2.—Trigonal block diagram of Bright Angel group in space and time 79 3.—Diagram showing apparent temporal isolation (and actual continuity) of occurrences of Prospect Mountain quartzite in part of southern Great Basin region 84 ABSTRACT The Bright Angel group of the southern Great Basin region is denned as consisting of the predominantly argillaceous strata which lie between the underlying Prospect Mountain quartzite and overlying Middle Cambrian limestones. This lithogenetic unit is shown to range in age from partly pre-Cambrian in the Nopah Range of south- eastern California to entirely Middle Cambrian in the Grand Canyon of Arizona. The Birght Angel group illustrates the fact that the problems of stratigraphic classi- fication are four-dimensional and, as such, are not amenable to treatment by the conventional dual system of stratigraphic nomenclature. The fact that rock units and unconformities may vary in age from place to place is determined as the sole factor demanding (1) a three-fold nomenclatural system, and (2) abandonment of the concept that erosional breaks may serve as time-stra- tigraphic boundaries. This variation in age of lithogenetic units is recognized as a fundamental truth in stratigraphy, nearly equal in significance to the laws of super- position and faunal succession, and is appropriately designated therefore as the principle of temporal transgression. INTRODUCTION For many years the classical approach to the problem of the classification of sedi- mentary rocks with respect to time has been based upon two fundamental truths: the law of superposition and the law of faunal succession. In consequence strati- 75 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/59/1/75/3431328/i0016-7606-59-1-75.pdf by guest on 28 September 2021 76 WHEELER AND BEESLEY—TIME-STRATIGKAPHIC CONCEPT graphic classification has been of dual nature. "Rock" units such as system, series, formation, etc., have had as their counterparts in the time scale, period, epoch, and smaller divisions. Schenck and Muller (1941) have conclusively shown that a three-fold terminology is mandatory if the known facts of sedimentary deposition are to be treated in their true relationships. These authors have accepted the generally recognized time terms. Some of those previously regarded as rock terms—namely, system, series, stage, and zone— are shown to be denned by tune, and are therefore classified as time-rock or time-stratigraphic. By virtue of the fact that lithogenetic units such as formations, members, etc., may vary hi age from place to place, they bear no definite or constant relationship to the standard. Consequently these lithogenetic units are classed by themselves as rock units. The significant fact that necessitates the separation of rock and tune-rock units in classification is, of course, their variable relationship, a fact which is perhaps known but little appreciated by most geologists, while a few, evidently, are totally unaware of its existence. Many of those who are aware do not concede the essential unity of lithogenetic units as they transgress tune from place to place. The principal purpose of this paper, therefore, is to emphasize the variable relation- ships between rock and time-stratigraphic units as a fundamental truth in stratig- raphy which is regarded as second in importance only to the laws of superposition and faunal succession. Doubltess the chief reason for the failure of conventional stratigraphic concept has been the continued custom to emphasize the concept of diastrophism as the principal basis for subdivision of the geological record, and otherwise to over-simplify sedi- mentary relationships. Critical review of the known facts indicates that most of our past attempts to classify sedimentary strata have been at best an only partially suc- cessful compromise between the results of the paleontologist on the one hand and the physical geologist on the other. Tune units are repeatedly compromised in order to make them fit rock units, and vice versa. In other words, to justify the application of the over-simplified dual classification, we have closed our eyes to the fact that there may exist no constant parallelism between rock unit surfaces and tune horizons. Also contributing to the over-simplification is the manner of our thinking on prob- lems involving spatial relationships in the lithosphere. The traditional picture is that of the one-dimensional columnar section or the two-dimensional cross section. At best, we occasionally think in terms of the three spatial dimensions. The problem of rock formations and their ages, however, when viewed in regional perspective, is clearly four-dimensional. Perhaps, if we recognize it as such, we can better appre- ciate the necessity for distinction between rock units as entities in space and stratal units defined by time (time-rock units). ACKNOWLEDGMENTS In addition to those whose aid on the general problem of the Cambrian stratigraphy of Nevada and adjacent areas has been acknowledged previously, the writers are especially indebted to Byron S. Hardie of Bristol Silver, Nevada, for his able field Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/59/1/75/3431328/i0016-7606-59-1-75.pdf by guest on 28 September 2021 ACKNOWLEDGMENTS 77 assistance during the 1946 field season. Further expression of gratitude is due Prof. Allan Cree and Dr. V. P. Gianella of the University of Nevada, Drs. A. I. Levorsen and S. W. Muller of Stanford University, T. D. Overton of the Nevada State Bureau of Mines, Dr. L. L. Sloss of Northwestern University, and Dr. D. E. White of the U. S. Geological Survey, for their helpful suggestions and criticism of the manuscript. FIGURE 1.—Index map of southern Great Basin region Showing triangular area of Bright Angel group diagrammed in Figure 2. BRIGHT ANGEL GROUP IN SPACE AND TIME GENERAL STATEMENT To illustrate this four-dimensional relationship the writers have selected the most extreme case in their experience, with regard to variation in age of a lithogenetic unit. The unit selected is the Bright Angel shale as exposed in its type area in the Grand Canyon, together with its lithogenetic equivalents (Bright Angel group) over a tri- angular 12,000 square-mile area in northern Arizona, southwestern Utah, southern Nevada, and southeastern California. With Bass Canyon in Arizona at one point of the triangle, the other two points are located at Pioche, Nevada, and the Nopah Range of California (Fig. 1). The Bright Angel group, rather than one of its con- stituent formations such as the Pioche shale, is selected solely because of its greater variation in tune range. Nevertheless the same principle could be illustrated, though somewhat less strikingly, with smaller units. The employment of a group to illustrate time-stratigraphic relationships is entirely consistent with the rules for classification and nomenclature of rock units (Ashley et al., 1933), as the difference between a formation and a group is solely one of rank. The Bright Angel formation and group constitute a lithogenetic unit in which shale predominates, and which lies between the underlying Prospect Mountain quartzite (also designated in this region as "Tapeats" and "Sterling") and the variously designated overlying Middle Cambrian limestones. To illustrate the general spatial and time relationships of the Bright Angel group in the simplest possible manner, a trigonal block diagram is employed in which Bass Canyon, Pioche, and the Nopah Range are situated at the corners (Fig. 2). The regional continuity of the Bright Angel group within and beyond the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/59/1/75/3431328/i0016-7606-59-1-75.pdf by guest on 28 September 2021 78 WHEELER AND BEESLEY—TIME-STRATIGBAPHIC CONCEPT diagrammed area, together with the continuous relationship between this unit and the underlying quartzite and overlying limestone is demonstrated by the following from among the published and unpublished sections. Published: Grand Canyon, Arizona, Noble (1922); McKee (1945) Western Grand Canyon, Arizona, Schenk and Wheeler (1942) Virgin Range, Nevada, Longwell (1928) Mormon Range, Nevada, Wheeler (1943 and 1944) Pioche district, Nevada, Wheeler (1940, 1943, and 1944); Wheeler and Lemmon (1939) Delamar, Nevada, Callaghan (1936); Wheeler (1943 and 1944) Groom district, Nevada, Humphrey (1945) Spring Mountain, Nevada, Nolan (1929) Sheep Mountain, Nevada, Hewett (1931) Nopah Range, California, Hazzard (1938) Unpublished:1 Wah Wah Range, Utah Desert Range, Nevada Johnnie district, Nevada Carrara (Bare Mountain), Nevada Frenchman Mountain (Las Vegas), Nevada Eagle Mountain (Death Valley Junction), California BASS CANYON SECTION The Bright Angel shale sequence first described in great detail by Noble (1922) is the section in the Grand Canyon at Bass Canyon, Arizona, about 17 miles west of Bright Angel Canyon, its type locality. Here Noble measured 391 feet of Bright Angel, lying above the Prospect Mountain quartzite ("Tapeats sandstone") and be- neath the Muav limestone. In this portion of the Grand Canyon the Bright Angel formation lies entirely within the Middle Cambrian Series. According to Resser (1945) the higher beds of the formation are characterized by the genera Clavispidella, Ehmaniella, Elrathia, Kootenia, Niswia, Parehmania, Ptarmigania, and Soknopleu- rella; and the lower strata contain Alokistocare, Anoria, and Glossoplewa. On the basis of these faunas the entire Bright Angel formation at Bass Trail (Fig. 2, B-B') lies well above the Lower-Middle Cambrian boundary (T3).
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