LIBRAR~ A Review and Interpretation of the Geologic Setting of the Watchung Basalt Flows, New Jersey GEOLOGICAL SURVEY PROFESSIONAL PAPER 864-A .. I ~ .•.~ i A Review and Interpretation of the Geologic Setting of the Watchung Basalt Flows, New Jersey By GEORGE T. FAUST STUDIES ON THE WATC·HUNG BASALT FLOWS OF NEW JERSEY GEOLOGICAL SURVEY PROFESSIONAL PAPER 864-A A general introduction to the restudy of the volcanism in theNewark basin of New] ersey UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1975 UNITED STATES DEPARTMENT OF THE INTERIOR STANLEY K. HATHAWAY, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Faust, George Tobias, 1908- A review and interpretation of the geologic setting of the Watchung Basalt flows, New Jersey. (Studies on the Watchung Basalt flows of New Jersey) (Geological Survey professional paper ; 864-A) Bibliography: p. Supt. of Docs. no.: I 19.16 :864-A 1. Geology-New Jersey. 2. Volcanism-New Jersey. Geology, Stratigraphic-Triassic. I. Title. II. Series. III. Series: United States. Geological Survey. Professional paper ; 864-A. QE141.F38 551.7'6 74-20937 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-02652-0 CONTENTS Page Page The nature and products of the basaltic eruptions­ ~\bstract ----------------------------------------- A1 Introduction ------------------------------------- 1 Continued Acknowledgments _____________ ------------------- 2 Microscopic character of the basalts -----------­ A25 Location and physiography of the Watchungs -------- 3 The probable character of the pristine gases in Geologic setting ____________ ----_______________ --- 4 the Watchung Basalt ----------------------­ 26 Summary of previous views on the structural History of tectonic subsidence and volcanism in the characteristics __ ----- _..:. ______________________ --- 7 ~ewark basin as it relates to the Watchung The continental glaciers over the Watchungs -------­ s· Mountains ------------------------------------- 26 Stratigraphy and geologic age ---·-----------------­ 9 Structure of the ~ewark basin at the close of Local stratigraphic relations ------------------­ 9 the Triassic Period ------------------------­ 26 Timing of fissure eruptions of the Upper Triassic Structure of the ~ewark basin after the post- in ~ew Jersey ----------------------------­ 14 Triassic deformation _______ ----------------- 27 Geologic age of the igneous activity ------------- 16 The formation of a regional arch --------------­ 29 Regional stratigraphic relations ---------------- 18 Triassic tensional history, its effect on the de­ The nature and products of the basaltic eruptions ----­ 20 velopment of the ~ewark basin and its global Sites of the fissures -------·-------------------­ 20 cause---a summary -------------------------­ 31 Preeruptive character of the graben floor -------­ 20 Supplementary topics ----------------------------- 32 Surface character of the vesicular tops ---------­ 21 Some characteristics of fissure eruptions ________ _ 32 Areal extent and volume relations of the Vesiculation in basaltic magma ---------------­ 35 Watchung Basalt flows ---------------------­ 21 The process of bubble formation and move- Geologic processes responsible for the jointing ment ---------------------------------- 35 systems _ ----------__________ ---__ -----____ - 22 Bubble formation in a natural lava flow ---- 35 Petrochemistry ______________________ ----____ _ 22 Literature cited ---------------------------------- 36 Megascopic characteristics of the basalt --------- 24 Index -------------------------------------------- 41 ILLUSTRATIONS Page FIGURE 1. Outline maps of area -------------------------------------------------------------------------- A4 2. Map showing southern limit of the Wisconsin ice sheet over the Watchungs ---------------------- 8 3. Stratigraphic columns for the Triassic rocks in the ~ewark basin in ~ew Jersey ---------------------- 10 4. Generalized geologic cross section from west of Pine Brook on Third Watchung Mountain to an exposure just north of West Orange on First Watchung Mountain -------------------------------------- 12 5. Diagram showing estimated sequence in time of the extrusion of the Watchung Basalt flows of· ~ew Jersey ------------------------------------------------------------------------------------ 15 6. Diagram showing estimated sequence in time of the extrusion of the Watchung Basalt flows of ~ew Jersey ----------------------------------------------------~-------------------------------- 17 7. Chart showing generalized stratigraphic section of the Triassic of southern Connecticut --------------- 18 8. Generalized cross section of the Triassic of northeastern ~ orth America showing the postulated arching of the crust ------------------------------------------------------------------------------- 27 9. Diagram showing geometric changes produced by folding horizontal beds to form an arch ----------- 30 10. Schematic representation of a tectonic graben and folds ------------------------------------------ 30 11. Block diagram of the Mount Eccles fissure eruption in Victoria, Australia ------------------------ 33 12. Schematic representation of a fissure eruption showing the linear formation of the late stage vents 33 TABLES Page TABLE 1. Stratigraphic sections of the upper part of the Brunswick Formation of the Triassic System in the ~ew- ark basin of ~ew Jersey -------------------------------------------------------------------- All 2. Average chemical analyses and C.I.P.W. normative data of Watchung Basalt from Third Mountain with other averages of tholeiitic rocks _______ ------ __________________________________ ~- _ ___ _ ___ _ __ 23 3. Mechanical strength of selected rocks ------------------------------------------------------------ 29 III STUDIES ON THE WATCHUNG BASALT FLOWS OF NEW JERSEY A REVIEW AND INTERPRETATION OF THE GEOLOGIC SETTING OF THE WATCHUNG BASALT FLOWS, NEW JERSEY By GEORGE T. FAUST 2 ABSTRACT of 2,000 square miles ( 6,180 km ). The lavas are tholeiites. The Watchung Basalt flows and the thick sedimentary se­ Supplementary topics include a discussion of the character­ quences as now exposed in Bergen, Essex, Union, Passaic, istics of fissure eruptions and of vesiculation in basaltic Morris, and Somerset Counties, New Jersey, are part of the magma. remnant of a former. graben. This graben has been largely destroyed by erosion. The graben is generally believed on the "Watchung Mountains-The trap basis of paleontologic evidence to have been formed in the Late 'ridges extending from Bedminster Triassic. About 21h to 1 million years before the end of Triassic and Somerville to Oakland and sedimentation in the graben, lava flowed out onto the graben Darlington are known as the floor to form the large flood basalts of First Watchung Moun­ Watchungs or Orange mountains. tain. Approximately 600,000 to 250,000 years after the first They are sometimes called First, volcanic episode a second period of basaltic eruptions produced Second and Third Mountain, be­ the lavas of Second Watchung Mountain. Following a lapse of ginning with the outermost or 111 11h million years to 600,000 years another flood of basalts eastern one. formed the lavas of Third Watchung Mountain. As the lavas solidified they accumulated body stresses and most of this stress INTRODUCTION was relieved by jointing. Subsequently, the region was folded to form an arch. During the buckling of the arch subsidiary In 1936-37, the author was teaching ·a course in border synclines were formed along the contact with the gra­ the theory of annealing, as developed by Williamson nitic walls of the horsts. This tectonic event produced joints ·and Adams (1919) and Adams and Williamson whose abundance and distribution are related to the geometry of the structure. Late in the history of the Newark basin in (1920), to graduate students at Rutgers University. New Jersey, during the Pleistocene, a continental glacier cov­ During this course, I became interested in the prob­ ered a part of the area. After the disappearance of the glacier, lems of the cooling of lava flows, such as the nearby decompression of the crust produced sheeting structures in the Watchung Basalt, and I made some preliminary cal­ basalt. Thus the jointing systems observed today are the re­ sults of the combined geologic processes of volcanism, tectonism, culations of the heat-flow relations. Subsequent and glaciation. field observations of the three Watchung Basalt The history of the events involved in the tectonic subsidence flows disclosed that the problem was more complex and volcanism in the Newark basin bears a parallel relation­ than the ·simple model. Any theory of cooling of the ship to the order of events taking place in the North American lava flow would have to account for the several plate in its location in the ancient continent of Pangaea. It is suggested here that ( 1) the rifting to form and develop the types of joints present in these flows and the Newark graben, (2) the deep rifting associated with the vol­ obviously unequal frequency of the several joint canism, and (3) the downfaulting of the graben which pro­ systems. It thus became evident that a preliminary duced the central arch and the border synclines are the re­ stage in the evaluation of the cooling history of these sult of the following events in the continent of Pangaea: (1) flows would be a study of the character of the joint­ the development of a linear