RIFT TECTONICS INFERRED FROM VOLCANIC AND CLASTIC STRUCTURES WARREN MANSPEIZER Department of Geological Sciences Rutgers University, Newark, N,j, INTRODUCTION Tholeiitic basaltic lava flows and intrusives of Late On the bases of these studies we infer (Fig, 4) that: (1) Triassic and Early Jurassic age clearly m~rk and crustal thinning (through erosion, necking and exten­ distinguish the upper part of the rift-valley stratigraphic sion) was very long·lived, perhaps 50-75 m.y., and sequence in the onshore and offshore basins of eastern followed an episode of Late Paleozoic Hercynian North America and northwest Africa. These igneous crustal thickening and mountain building through con­ rocks were emplaced over a vast area, perhaps 2500 km tinental collision; (2) rifting and clastic deposition was long and 1000 km wide, during an episode of crustal ex­ widespread by the beginning of the .Late Triassic in tension and sea-floor spreading and hence exhibit a many small rift basins marginal to the proto-Atlantic variety of structures reflecting the complex history of Ocean; and (3) a second phase of crustal extension the basin at that time. Where these volcanics are in­ resulting primarily from horizontal shear developed at terbedded with sediments· having paleocurrent and/or the beginning of the Jurassic Period and was accom­ paleo-environmental attributes, important conclusions panied by vulcanism, sea-floor spreading and collapse may be deduced about the tectonic behavior of the basin of the continental margins (Manspeizer and other, from each rock type. 1978), The objective of this field trip is to study a typical rift The largest of these onshore basins in North America facies of interbedded volcanics and sediments, thereby is the combined Newark-Gettysburg Basin, which ex­ providing a frame-work for interpreting the tectonics of tends from Rockland County, New York through cen­ the basin during crustal extension and sea-floor tral New Jersey to Lancaster, Pennsylvania, The basin, spreading. Figure 1, a diagrammatic sketch through the a half-graben, is step· faulted in the subsurface (Sumner, section, shows the kinds of structures and facies that we 1978). The rift fill consists of 5 to 6 km of continental shall observe on the field trip. Many of these features, sediments interbedded with tholeiitic lavas of the such as tumuli, subaqueous flow lobes and palagonite Watchungs and intruded by diabases of the Palisade forset bedding have not been reported from rift Sill-Rocky Hill Complex, The sequence, resting uncon­ volcanics of eastern North America. Figure 2 shows the formably on Paleozoic metamorphics and dipping route of our field trip. westward towards the east-facing high angle border fault, is folded and faulted with considerable vertical A Stratigraphic Perspective: Geologic Setting and strike-slip displacement (Sanders, 1962) . Field studies (Manspeizer and others, 1978) in North STRATIGRAPHY America and Morocco show that the major elements of Triassic~Jurassic stratigraphy in basins marginal to the Intrusive sheets, such as the Palisade sm and the Atlantic Ocean are: (1) Mesozoic rocks resting with pro­ Rocky Hill-Lambertville sheets, extend collectively found unconformity on Autunian sandstones (Upper throughout the basin while the volcanics are restricted Carboniferous and Lower Permian) and Paleozoic to its northern end. Paleoflow data (Manspeizer, 1969) metamorphics; (2) Rifting and syntectonic clastic and geochemical data (Puffer and Lechler, 1980) show deposition began in the Carnian Stage (Lower Upper that these lavas, now preserved as erosional remnants in Triassic), along a zone marginal to the axis of the proto­ the north, originally extended 80-150 km south to their Atlantic Ocean; and (3) Vulcanism began about 195 probable source in the eastern part of Pennsylvania. m.y. ago, i.e. about 10-15 m.y. after the onset of rifting Field studies also indicate that the lava flows extended and the accumulation of 3-5 km of clastics in the rift west of the Ramapo border fault. The lavas, inter­ valleys. (Fig. 3) calated within the sediments of the "Brunswick" For- 314 315 FIELD STUDIES OF NEW JERSEY GEOLOGY AND GUIDE TO FIELD TRIPS PALEOFLOW DIRECTION AMYGDALOIDAL & VESICULAR FLOW TOPS PALAGONITE PORSET BEDOIN z: PAHOEHOE LOBES o TUMUL! io IJ... DELTAIC FORSET BEDDING SUBAQUEOUS FANS SHELF CARBONATES INTRUSIVES BEDDED PILLOW BUDS SUBAQUEOUS FLOW LOBES SPIRACLES UPPER COLONNADE FLUV tAL SANDS Fig. 1 Schematic diagram of the volcanic and sedimentary structures to be studied on the field trip. mation, form three marked stratigraphic units: the Formation, Preakness Basalt, Towaco Formation, First, Second, and Third Watchung basalts, which Hook Mountain Basalt t and Boonton Formation (Fig. average about 185, 230 and 90 meters thick respectively 5), The writer, adopting Olsen's as formal stratigraphic (Kummel, 1898; Darton and others 1908; Faust, 1975). names, uses the Watchung nomenclature as informal Although these names are well established in the designations in the same way that researchers use the geologic literature, cogent arguments are presented by name Columbia River Basalt. The introduction of new Olsen (see the guidebook) for dropping the names names, although bothersome to some, actually helps to Watchung and Brunswick in favor of the following elucidate complex stratigraphic relationships and is in which are listed in stratigraphically ascending order: the spirit of the Code of Stratigraphic Nomenclature Passaic Formation, Orange Mountain Basalt, Feltville (1961). This will be evident throughout the field trip. RIFT TECTONICS INFERRED FROM VOLCANIC AND CLASTIC STRUCTURES o 4 I'll I I -+-+-+--+--7~lADENTOWN TERMINAL MORAINE CRETACEOUS TRIASSIC-JURASSIC PRE-TRIASSIC & SATURDAY FIELDSTOPS @ SUNDAY FIELDSTOPS Fig. 2 Location map. showing generalized geology and field stops. Each lava flow consists of multiple flow units. At good, each flow unit may exhibit: (1) a flow top of least two, and perhaps three, flow units are recognized vesicular, amygdaloidal and/or ropy pahoehoe lava; (2) in each of the First and Second Watchungs, while at a flow base of massive columnar basalt with pipe least three flow units are recognized in the Third Watch­ vesicles and pipe amygdaloids; and (3) an advancing ung. The lateral extent of individual flow units may not flow front of pillow lavas and palagonite tuff in forset be as great as anticipated by early investigators beds. Some of the lava, however, originated as fissure (Johnson, 1957), although Van Houten (1969) states eruptions within deep-water, fault-bounded lakes, and that the Watchung lavas flowed at least 60 km as a did not flow any appreciable distance. Individual flows. single unit 450 to 500 m thick. Where the expo~ures are, are separated by thin lenses of red tuff at Millington, 317 FIELD STUDIES OF NEW JERSEY GEOLOGY AND GUIDE TO FIELD TRIPS Fig. 3 Intercontinental correlation chart. Conventional symbols depict time - and rock - stratigraphic units (Manspeizer and others, 1978). Summit and Oldwick (Johnson, 1957), New Street most useful paleoflow structures for determining flow Quarry in Paterson (Van Houten, 1969), siltstone at direction are pipe vesicles, pipe amygdaloids, and Prospect Park Quarry near Paterson (Bucher and Kerr, pillow-palagonite forset bedding. Other structures, such 1948), and sandstones at Caldwell (Faust, 1975). as tumuli and pillow lavas provide important informa­ tion about the paleoenvironment. Paleo flow Structures in Rift Volcanics (Field trip stops 2 and A) Pipe Vesicles and Pipe Amygdaloids (Du Toit, 1907) commonly occur in the lower colonnade, near the sole The application of paleocurrent structures in of the flow, in sets of parallel-to-sub-parallel, straight­ sedimentary rocks is weB-known and has been used to to-bifurcating cylindrical tubes surrounded by chilled resolve important questions on the dispersal systems, lava. The vesicles measure about 0.5 cm in diameter and basin geometry and provenance. Primary directional 10 cm long. Vesicles probably form when the heat of the structures also occur in volcanic rocks and have been lava vaporized water from the underlying sediment. As used effectively by Brinkman (1957), Waters (1960), the vapors rise into the overlying flow, their leading Schmincke (1967), and others to resolve important ques­ ends are bent in the direction of the lava flow (Figs. 6 tions about the tectonic history of various basins. Ex­ and 7), Pipe vesicles have been used effectively for basin cept for preliminary observations on Triassic-Jurassic analyses by Brinkman (1957), Waters (1960), and volcanic structures by Walker and Parsons (1922) in Schmincke (1967). Nova Scotia, Bain (1957) in Massachusetts, Chapman (1965) in Connecticut, and Iddings (1906), Fenner Vesicle Cylinders (Du Toit, 1907) are cylindrical (1908), Bucher and Kerr (1948) and Manspeizer (1969) zones of vesicular basalt that often occur in the upper in New Jersey, none of these features has been part of the lower colonnade (Fig, 6), They are. con­ systematically mapped or studied in detail. Among the siderably larger than pipe vesicles, measuring about 2 250 "", I TIME AXIS IM,Y,) NORTH AMERICA~ EPISODE I EROSION OF PERMIAN -MIDDLE TRIASSIC MOUNTAIN BUILDING INCIPIENT RIFTING CRUSTAL THINNING 75 ISOSTATIC REBOUND EPISODE II AXIAL WIDENING a RIFTING MIDDLE TRIASSIC RIFTING a CRUSTAL EXTENSION CRUSTAL THINNING -- TETHYS SHEAR a ANDESITES 5-15 TETHYs MARINE TRANSGRESSION ........."""'-i,j.l.U.lllllll CONTINUES EPISODE m CARNIAN - RHAETIAN RIFTING