Triassic – Jurassic rifting and opening of the Atlantic: An overview

Chapter 3

Triassic - Jurassic rifting and opening of the Atlantic: An overview WARREN MANSPEIZER ABSTRACT Events leading to the breakup of the Pangean plate and evolution of the Atlantic passive margins are recorded in the rock record of more than 40 offshore and onshore Late Triassic - Early Jurassic synrift basins that formed on the Variscan - Alleghanian orogen. The record shows that rifting took place along low-angle detachment faults, giving rise to half-grabens along a conjugate set of lower and upper plate margins that are noteably asymmetric. The American plate was marked by a broad belt of marginal plateaus with many northeast-trending detrital basins that were linked to eachother by transfer faults and displaced by cross faults. The Moroccan plate, on the other hand, was marked by few broadly subsiding evaporite basins. Typically each half-graben on the American plate was bordered by a hinged margin and one major basin-bounding fault, which delineated the surface trace of synthetic or antithetic listric faults on a seaward-dipping detachment zone. The American plate (during the Late Triassic) was dominated by high relief with high-altitude fluvial-lacustrine basins along the western part of the orogen, and by low-relief sea-level evaporite basins proximal to the future spreading axis. During detachment faulting, in the Late Triassic - Early Jurassic, the lower plate must have been uplifted isostatically into a broad central arch that migrated seaward, as the load of the overlying upper plate continued to be reduced by erosion and listric faulting. This had the consequence of elevating Late Triassic marine strata that lay near the proto-Atlantic axis. During the Lias, these marine basins were eroded and their strata reworked and transported landward toward the onshore basins of Morocco and North America. The topographic reversal is thought to reflect the easterly migration of upwelling asthenosphere, in response to tectonic thinning along the newly forming margin. It was a time of major crustal thinning with development of the postrift unconformity (COST G-2 cores), and adiabatic decompression on the upwelling asthenosphere. Whereas the earliest melts yielded off-axis alkaline-rich volcanics (as in Morocco), subsequent melts, which were derived from later partial melt derivatives, were tholeiitic (as in the Palisades). As the upwelling asthenosphere migrated eastward in response to tectonic thinning, the 'abandoned' rift-stage crust cooled and subsided, thereby ushering in the drifting phase of the margin. The Moroccan plate, by contrast, was a broad region of low relief throughout most of the Triassic and Liassic. It was distinguished by few detrital basins, and almost all of these occurred along the South Atlas fracture zone, as Triassic strike-slip basins in the High Atlas. Except for the offshore Essaouira basin, which is a seaward extension of the High Atlas Argana basin, the Moroccan margin (unlike the American) consists of few documented Triassic - Liassic rift basins. Triassic rifting of the Middle Atlas (e.g. at Bab-Bou-Idir and Berkane) broke the orogen into the Oranian and Moroccan mesetas, and is manifested by a thick carbonate sequence. The majority of the intraplate basins of North African occur on the mesetas, and are nonrift; typically they contain nonclastic, marine and fresh water evaporites of Liassic and younger strata that formed in broad, shallow, drift-type basins on a generally subsiding terrane of low relief, near the very end of synrift time.

Introduction The tectonic regime leading to the rifting of Pangea and concomitant formation of the Atlantic passive margins was influenced primarily by the basement fabric, which was fashioned largely by the late Paleozoic Variscan - Alleghanian orogeny. The main phase of that orogeny ended along a 1000 km wide dextral shear zone that thrusted the newly formed orogen onto the adjacent cratons of Africa and North America (Arthaud and Matte, 1977), and destroyed major Carboniferous pull-apart basins (including the Magdalen and Narragansett) that had formed within the dextral shear zone in the northern Appalachians

42

W. Manspeizer

(Bradley, 1982). The lithosphere was thus broken by a system of pervasive thrust faults which, in the Triassic, under a new extensional regime, were reactivated as listric and planar normal faults (see Cook et al., 1979; Ando et al., 1984). Hutchinson and Klitgord (1988, this volume) suggest that some of these early Mesozoic faults sole into a master decollement within a zone of thin-skinned tectonics, while others penetrate the entire crust, thus lending support to various detachment models (e.g., proposed by Lister et al., 1986; Bosworth, 1987) for rifting and evolution of passive margins. From the Late Carboniferous to the Early Permian, the orogen was marked by general subsidence with small Variscan basins extending over a broad terrane that included the Moroccan, Oranian, Iberian and Meguma mesetas (Cousminer and Manspeizer, 1977). The terrane, filled with coarse fluvial elastics and acidic to intermediate lavas, was mildly deformed and intruded by acidic plutons in the Middle to Late Permian (see Van Houten, 1977). Subsequent rifting of the Variscan - Alleghanian orogen was short-lived, extending from the Middle Triassic in the Fundy basin (the Late Triassic elsewhere) through the Early Jurassic or, perhaps, earliest Middle Jurassic. At that time the rift basins were aborted and a new tectonic phase shifted to the proto-Atlantic axis, where subsidence and sea-floor spreading were beginning. Geophysical studies of the Atlantic margins (e.g. Grow, 1981; Schlee and Jansa, 1981; Klitgord et al., 1982; and Hutchinson and Klitgord, 1988, this volume) including the Bay of Biscay (Montadert et al., 1979), thus reveal that the margins consist of two early Mesozoic tectonostratigraphic facies: an older synrift sequence that is geographically confined to the aborted rift basins; and an overlying blanket-like sequence of postrift, or drift, strata that in places unconformably overlies the synrift sequence. The unconformity, marking the change from synrift to postrift deposition, is the postrift unconformity or the Breakup Unconformity of Falvey (1974). Although its age assignment is debatable, it appears to be time-transgressive, embracing most of the Late Triassic on the Scotian Shelf and much of the Lower Jurassic on Georges Bank and the western High Atlas of Morocco. The unconformity reflects intermittent episodes of uplift and crustal thinning over an interval of about 20 million years, with the major uplift and thinning (both erosional and tectonic) occurring along the future spreading axis. The unconformity thus marks a fundamental change in the thermal history of the newly forming margin (Watts, 1981). The rift stage involving thermal doming, uplift and stretching of the crust, was accompanied by faulting, igneous activity and rapid deposition in moderately deep, elongate, faultgoverned troughs. The drift or postrift stage involved the slow cooling and subsidence of the plate over a wide area, and hence is marked by stratigraphic onlap over a broad terrane of eroded rift basins lying on thinned continental and rift-stage crusts. Whereas cooling and subsidence have generally characterized the offshore basins on the American margin since the Middle Jurassic, uplift and erosion have accompanied the onshore basins. The Moroccan margin, unlike the American margin, was uplifted in the Tertiary during the Alpine orogeny, and thus exposes critically important aspects of its synrift and postrift histories that are neither recorded in the American onshore basins nor evident from offshore geophysical studies of the American plate. The primary objective of this chapter is to present an overview of the early Mesozoic onshore and offshore rift basins of North America and North Africa. We shall examine their tectonic framework, structure and stratigraphy, summarize various theories of formation, and integrate this information into an historical account of the cratonic breakup and evolution of the Atlantic margins.

43

Triassic-Jurassic rifting

Tectonic setting Late Triassic to Early Jurassic rifting of the North American and African plates extended from the Cobequid - Chedabucto - Gibraltar fracture zone on the north to the Bahamas fracture zone on the south, and across the Variscan - Alleghanian orogen onto the bordering cratons of Africa and North America (Figs. 3-1 and 3-2). Within this broad region lay about 40 northeast-trending elongate rift basins, whose trends typically follow the fabric of the Variscan - Alleghanian orogen. Rifting also occurred at this time from Texas to Florida along a zone of transforms that includes the Alabama - Arkansas fault system (see Thomas, 1988, this volume), the mafic volcanic province of Florida, and the Bahamas fracture zone, which, according to Klitgord and Schouten (1980), developed in the Jurassic as a transform connecting the spreading centers of the Atlantic and the Gulf of Mexico. About 25 of these rift basins have been identified on the American plate, where they occur in a broad band between and adjacent to the Piedmont Gravity High on the west and the East Coast Magnetic High on the east (Fig. 3-3). The exposed onshore basins broadly follow the trend of the Piedmont Gravity High of the Appalachians, and thus cluster about the Proterozoic shelf edge; the offshore basins and many of those beneath the Coastal Plain cover from Florida to Long Island, cluster about the basement hinge zone and the East Coast Magnetic Anamoly and thus group about the early Mesozoic shelf edge (Fig. 3-3). The basement hinge zone, a fundamental division in the crust according to Uchupi and Austin (1979), separates the slightly extended and thinned continental crust with its comparatively shallow Late Triassic - Early Jurassic rift basins (e.g., the New York Bight, Long Island and Nantucket basins) from the more highly extended rift-stage crust with its deeper and substantially thicker rift and postrift marginal basins (e.g., the Georges Bank basin, and the Baltimore Canyon trough; see Hutchinson et al., 1986). Seaward of the hinge zone, the basement deepens from about 2 - 4 km to 8 km (see Grow, 1981). Offshore basins landward of the hinge contain tilted subparallel reflectors that are truncated by the postrift unconformity, while those seaward of the hinge contain wedge-shaped strata that may show only minor truncation at the unconformity (Hutchinson and Klitgord, 1988, this volume). Late Triassic dinoflagellates, extracted from COST G-2 cores on Georges Bank (Cousminer, 1983; see discussion below), however, indicate that rifting was more or less contemporaneous across the orogen (and thus hinge zone) for several hundred kilometers, and that while Late Triassic continental deposition occurred primarily landward of the hinge zone, shallow water marine tongues from the Tethys Seaway to the east, and/or perhaps from Arctic Canada to the north, transgressed the basins seaward of the hinge (Manspeizer and Cousminer, 1988; see also discussion below). The gravity high in the Southern Appalachians is located in the Piedmont Province, and the exposed basins are distributed about that axis (Fig. 3-3). The Durham - Sanford Wadesboro - Crowburg basins, for example, are situated southeast of the gravity high and have west-facing border faults with east-dipping strata, and form a complementary pair of basins with the Danville and Davie County basin to the northwest with its east-facing border fault and west-dipping strata (Fig. 3-3). A similar paired basin complex occurs in the Northern Appalachians, where one branch of the gravity high extends east of the Newark - Gettysburg - Culpeper basins with their east-facing border faults and west-dipping strata, and west of the Hartford - Deerfield - New York Bight basins with their west-facing border faults and east-dipping strata. Another branch of the gravity high extends along the western Gulf of Maine through the Fundy basin, along the Cobequid - Chedabucto fault

44

W. Manspeizer

AFRICAN PLATE

NORTH AMERICAN PLATE BASINS

ONSHORE BASINS

I

OFFSHORE BASINS

|GEORGES BKSCOTIAN SHELF

ONSHORE BASINS CONT. MARGIN

GRAND BANKS

3Π CL Q

< CALLOVIAN BATHON1AN BAJOCIAN

I

SINE. HETT. RHAET1AN

■

m

IB i

NORIAN

a

LEGEND

PRE-TRI

M

mi n

UNCONFORMITY SHALE

ANDES1TE

LIMESTONE

THOLEIITIC LAVA

CLASTICS HIATUS GRAY-TO-BLACK SHALE SILTSTONE STROMATOLITES STEPHANIANAUTUN1AN

SALT

LAD1NIAN ANISIAN SCYTHIAN

rM

POTASH SALT DOLOMITE ANHYDRITE BASEMENT MARINE B1VALUES ANOPLOPHORA ZONE

PALYNOFLORAL ZONE CORES

m ΨΛ

REFERENCES

Fig. 3-2. Intercontinental time-stratigraphic correlation chart. See Fig. 3-1 or 3-3 for basin location. Stratigraphic horizons listed numerically on the chart are: / = Watchung lava flows; 2 = PassaicFm.;5 = Lockatong Fm.; 4 = Stockton Fm.; 5 = Osprey Evaporites; 6 = Iroquois Fm.; 7 = Argo Salt; 8 = Osprey Evaporites; 9 = Kettle red beds; 10 = TamaroutFm.; 11 = Bigoudine Fm.; 12 = Timesgadouine Fm.; 13 = IkarenFm.; 14 - Ouikaimeden Ss (modified from Manspeizer et al., 1978). References listed alphabetically are: A = Olsen et al., 1982; B = Cornet and Olsen, 1985; C = Manspeizer, 1981; D = Puffer et al., 1981; E = Cousminer et al., 1984; F = Manspeizer and Cousminer, 1988; G = Barss et al., 1979; H = Holser et al., 1988, this volume; I - Hinz, 1982; J = Brown, 1980; K - Tixeront, 1971; L = Cousminer and Manspeizer, 1976; M = Biron, 1982; N = El Youssi, 1986; O = Mattis, 1977. The reader is directed to Figure 3-5; in that correlation scheme the lavas of the Newark Supergroup are restricted to the Mettangian.

Triassic-Jurassic rifting

pp. 45-46

4 <\\-[·\

wk

gig

DETRITAL BASINS SALT BASINS POTASH SALT BASINS CARBONATE BASINS LAND DOLERITE DIKES

&

CONT. FRACTURE ZONE TRANSFER FAULTS WHITE MOUNTAIN MAGMA SERIES NEW ENGLAND SEAMOUNT CHAIN

TRIASSIC-LIASSIC BASINS THICKNESS (in meters)

.

1

MAIN SOUTH GEORGIA RIFT

20

ATLANTIS BASIN

5,000

2

RIDDLEVILLE-DUNBARTON-FLORENCE

21

GEORGES BANK

4,000 +

3

DURHAM-SANFORD-WADESBOROCROWBURG

22

SCOTIAN-SABLE IS. SUBBASIN

12,000

4

FARMVILLE-ROANOKE-SCOTTSBURG

23

SCOTIAN-ABENAKI SUBBASIN

5

DAN RIVER-DANVILLE-DAVIE CO.

3,200

24

AVALON<;ARSON SUBBASIN

2,500

6

RICHMOND-TAYLORSVILLE

1,670 +

25

LUSITANfA

500

7

CULPEPER

6,800

26

JERADA

8

GETTYSBURG

9,000

27

BERKANE

9

NEWARK

7,200

28

GUERCIF

10

HARTFORD-DEERFIELD

5.800

29

MOUSA ON SALAH-TAMDAFELT

950

11

GULF OF MAINE

30

KEROUCHEN

600

12

FUNDY

1,100

31

CENTRAL HIGH ATLAS

1,000

13

SCOTIANORPHEUS GRABEN

6,000

32

TISI N' TEST

1,500

14

BLAKE PLATEAU

5,000

33

ARGANA-ESSAOUIRA

4,500

3,000

15

CAROLINA TROUGH

7,000

34

DOUKKALA

1,400

16

BALTIMORE CANYON

8,000

35

BERRICHID

1,000

17

N.Y. BIGHT BASIN

3,000

36

BOUBEFRANE-KHEMISSET

1,000

18

LONG ISLAND BASIN

3,500

37

AAIUM

19

NANTUCKET

3,000

38

SENEGAL

Fig. 3-1. Late Triassic - Early Jurassic reconstruction of eastern North America and northwestern Africa, outlining the basins and lithofacies that developed on the Variscan - Alleghanian orogenic belt (modified from Manspeizer, 1981); line A-B marks the trend of cross section given in Fig. 3-9.

W. Manspeizer

pp. 47-48

V

m

EXPOSED RIFT BASINS, SHOWING FAULT CONTACT, SEDIMENTARY OVERLAP AND AGE(S):

BASINS

15 MAIN SOUTH GEORGIA RIFT BASIN

21 CULPEPER BASIN

10 WADESBORO BASIN

16 DAVIE CO. BASIN DAVIE CO. FAULT

1t CROWBURG BASIN

17 DAN RIVER BASIN

6 RANDOLPH BASIN

12 FLORENCE BASIN

7 SCOTTSBURG BASIN

13 DUNBARTON BASIN

fc&ffi

UNDIFFERENTIATED

2 RICHMOND BASIN HYLAS FAULT

^^M

NORIAN M CARNIAN

INFERRED TRIASSIC-LIASS1C BASINS BURIED BENEATH COASTAL PLAIN OR CONTINENTAL SHELF COST G-2 WELL Fig. 3-3.

Effijffl BRUNSWICK ANOMALY 20 BARBOURSVILLE BASIN

1 TAYORSVILLE BASIN

LATE TRIASSIC

DEEP RIVER BASINS JONESBORO FAULT

14 RIDDLEVILLE BASIN

EARLY JURASSIC

3 FARMVILLE BASIN 4

BRIERY CREEK BASIN

5 ROANOKE QREEK BASIN

8 DURHAM BASIN Θ SANFORD BASIN

MAGNETIC & GRAVITY HIGH

26 MIDDLETON BASIN BLOODY BLUFF FAULT

BULL R U N M T N . FAULT

27 FUNDY BASIN

22

GETTYSBURG BASIN

28 CHIGNECTO BASIN

23

NEWARK BASIN RAMAPO FAULT

29 CHEDABUCTO BASIN CHEDABUCTO FAULT

18 DANVILLE BASIN CHATHAM FAULT

24

POMPERAUG BASIN

30 GULF OF MAINE BASIN

19 SCOTTSVILLE BASIN

25

HARTFORD-DEERFIELD

POM!»ERAUG FAULT

MINERAL HILL FAULT

31 FRANKLIN BASIN BASIN

32 GEORGES BANK BASIN

33 ATLANTIS BASIN 34 NANTUCKET BASIN 35 LONG ISLAND BASIN 36 NEW YORK BIGHT BASIN 37 BALTIMORE CANYON TROUGH 38 NORFOLK BASIN 39 CAROLINA TROUGH

fytm;

PIEDMONT GRAVITY HIGH

= = - ^ BASEMENT HINGE ZONE ^ J

MESOZOIC STOCK/BATHOLITH

*ψ*

NEW ENGLAND SEAMOUNTS

• ·

MESOZOIC KIMBERLITE DIKES

Ξ Β

MESOZOIC THOLEIITIC DIKES

Triassic - Jurassic rifting

49

zone (Fig. 3-1; Mayhew, 1975). Seismic surveys of the Long Island Platform (Hutchinson et al., 1986) also document the presence of paired offshore rift basins, e.g., the Nantucket and Atlantic basins (Fig. 3-1). These studies further suggest that the Newark and New York Bight basins may have had a common graben history until the Lias when a regional uplift along the margin severed the graben into two basins (see Hutchinson and Klitgord, 1988, this volume; and discussion below of the postrift unconformity). Almost all recent geological and geophysical studies of these onshore and offshore basins indicate, or support the notion, that they developed primarily along low-angle detachment surfaces that, in the late Paleozoic, were Alleghanian thrust faults or dextral strike slip faults (see references below). The Fundy basin occurs along the Avalon and Meguma suture, which according to Brown (1986) is a Variscan thrust fault (the Fundy Decollement) that formed as a compressional component of the Cobequid - Chedabucto transform. Whereas dextral slip and thrusting dominated these terranes during the late Paleozoic, sinistral slip (about 75 km) along the geofracture (Keppie, 1982) and extensional slip (at minimum 9 km) down the decollement (Brown, 1986) combined to create the Fundy and Chignecto basins during the Triassic (Fig. 3-3). The studies of Belt (1968), Webb (1969) and Bradley (1982) indicate that strata in these basins unconformably overlie older (middle to late Paleozoic) rift and drift sequences, and thus are structurally related as successor basins along the transform. A similar successor basin complex is inferred by Ballard and Uchupi (1975) for the Gulf of Maine, and by Manspeizer and Cousminer (1988) for the Georges Bank basin, where late Paleozoic faulting had localized the development of subsequent Triassic rifts. Kaye (1983) has shown that the Middleton basin of eastern Massachusetts may have developed along the older Bloody Bluff fault system. COCORP deep seismic reflection profiles (Ando et al., 1984; also in deBoer and Clifford, 1988, Fig. 11-5, this volume) in New England indicate that the Hartford - Deerfield basin of Connecticut and Massachusetts lies on a transitional crust of eastward-dipping, thrust-imbricated ramps of Precambrian Grenville basement and younger metasediments. See also Zen et al. (1983), whose bedrock geologic map of Massachusetts depicts the eastern border of the Hartford - Deerfield basin as a reactivated west-dipping listric normal (antithetic) fault that, according to MacFayden et al. (1978), was a high-angle reverse fault of possible late Paleozoic age. Bain (1932), more than 50 years ago, also suggested that the eastern border fault of the Hartford basin was a pre-Triassic thrust fault. In like manner, the western borderfault to the Newark basin lies along an older Alleghanian thrust zone, separating Precambrian Blue Ridge and Paleozoic terranes (Aggerwal and Sykes, 1978; Ratcliffe and Burton, 1985; see also Fig. 3-4). The corridor or Narrow Neck (basin), connecting the Newark and Gettysburg basins, occurs along a major east - west lineament, including: (a) the N40°-Kelvin transform (Van Houten, 1977; Manspeizer et al., 1977; Manspeizer, 1981); (b) the Transylvania continental fracture zone (Root and Hoskins, 1977); (c) the Chalfont strike-slip fault (Sanders, 1963); (d) a prominent east-west deflection of the basement hinge zone (Hutchinson and Klitgord, 1988, this volume) and structural bend in the Appalachian orogen (Drake and Woodward, 1983); and (e) a wrench zone of sinistral shear that may have been synchronous with extension in the Newark and Gettysburg basins (see Manspeizer, 1981; Lucas et al., 1988, this volume). See also Rast (1988, this volume) who, suggests that the Northern and Southern Appalachians are separated by a strike-slip fault that underlies the Newark basin. Structurally paired basins

50

W. Manspeizer

(e.g., the Newark-New York Bight, and Nantucket - Atlantis) with inward-dipping listric normal border faults and outward-dipping synrift strata occur along the lineament, suggesting that these basins (half-grabens) may have formed as north-trending pull-aparts along an east - west-trending transform, that intersects a late Paleozoic detachment surface (see examples given by Bally, 1981, figs. 21-24). The offshore basins of New Jersey, Delaware and Virginia are similarly interpreted by Benson and Doyle (1988, this volume) as grabens or half-grabens, bounded by listric and planar faults that appear to intersect subhorizontal detachement surfaces; see particularly their interpretation of USGS seismic lines 10, 11, 25, 26, and 28. Reflection seismic studies of the Culpeper basin indicate that the basement consists of stacked thrust sheets of Proterozoic and lower Paleozoic age (Costain et al., in press). The Richmond - Taylorsville, Roanoke, Farmville and Scotsburg basins of North Carolina and Virginia seem to occur along a wrench-fault complex that is superimposed over a reactivated Alleghanian thrust belt, including the Hylas fracture zone (Glover et al., 1980; Bobyarchick, 1981; Resseter and Taylor, 1988, this volume; Venkatakrishnan and Lutz, 1988, this volume). The Chatham - Stony Ridge fault zone of North Carolina, the dominant pre-Mesozoic structural control for the development of the Danville basin (Thayer et al., 1970; Glover et al., 1980), may extend south into the Davie County basin and perhaps north into the Scottsville and Culpeper basins (Swanson, 1986). Seismic reflection profiles indicate that the Riddleville basin, which lies beneath the Coastal Plain cover of Georgia, is bound by a listric normal fault (the Magruder fault) that dips east and merges at depth with the Augusta thrust fault of late Paleozoic age (Cook et al., 1981; Petersen et al., 1984). It is tempting to project the Augusta fault zone north to the Dunbarton basin (Marine and Siple, 1974), and south to the main South Georgia basin (Daniels et al., 1983), and to speculate that these basins were activated along the Brunswick Anomaly, a Paleozoic suture (Fig. 3-3; see also the Suwannee - Wiggins suture of Thomas, 1988, this volume; Chowns and Williams, 1983). Elsewhere around the Atlantic margin, as in Morocco and Iberia, Triassic extension exploited older Hercynian structures. The major detrital rift basins of North Africa developed within the South Atlas fracture zone, a late Hercynian dextral fault (Mattauer et al., 1972), over 2000 km long that may have been continuous with the N40° Kelvin lineament, and acted

w ψτ^Ψ^

HI

ALLUVIAL FAN

LACUSTRINE

FLUVIAL-DELTAIC

FLUVIAL-PLAYA

Fig. 3-4. Geologic cross section of the Newark basin, drawn along the Delaware River, where the border fault is depicted as listric normal merging with low-angle detachment faults that are inclined beneath the continental margin, and the major lithofacies are largely a function of the half-graben geometry.

51

Triassic-Jurassic rifting

as a transform in the opening of the Atlantic Ocean (see Manspeizer et al., 1978; Laville, 1988, this volume; Beauchamp, 1988, this volume). In Iberia, Late Permian and Triassic basins developed along the Mauritanides - Variscan suture, a late Hercynian lineament joining the Laurasian and African plates (Sopena et al., 1988, this volume). Newark Supergroup Rocks within these basins, comprise the Newark Supergroup (Fig. 3-5) and consist primarily of basal and border fanglomerates, arkosic and lithic arenites, gray-to-black siltstones and shale, and red-brown mudstones. Interbeds of basaltic lavas occur only in the upper part of the synrift sequence and were emplaced during the Early Jurassic, i.e. about 25 million years after the onset of rifting and the accumulation of 3 - 5 km of synrift elastics. Intrusives, ranging in age from Liassic to Cretaceous, cut many of these basins and adjacent highlands. Evaporites, dune sands, coal, and kerogen-rich beds are present locally (as described elsewhere in this volume). Volumetrically, the majority of these beds were deposited in a lacustrine setting, and are observed to be laterally extensive gray-black siltstones (Olsen 1980b). See also Olsen (1988, this volume) for a discussion of the Newark Supergroup. Recurrent and differential subsidence provided space to accumulate about 7 - 9 km of synrift (Late Triassic/Early Jurassic) strata in the Newark - Gettysburg basin (Olsen et al., 1982), 4 km in the Hartford basin (Hubert et al., 1978), 4 km in the Durham basin (Bain and Harvey, 1977), 7 km in the Culpeper basin (Olsen et al., 1982), and as much as 5 - 8 km of synrift strata in the Baltimore Canyon, Georges Bank and Carolina trough basins. The onshore covered basins of Georgia, South Carolina, and Florida are thought to be notably thinner, containing about 1 km of strata in Dunbarton, 2.2 km in Riddleville and 3.5 km in the main rift in Georgia (Daniels et al., 1983). Synrift sedimentation typically began across the orogen as a diachronous Anisian to Carnian event (Fig. 3-5), in downsags or in small grabens that subsequently developed into asymmetric half-grabens along listric normal faults. Sedimentation was strongly influenced by the asymmetry of the basin. Perennial stream drained the larger hinged margin of the basin, while smaller ephemeral streams drained the uparched and faulted margins. Deposition generally occurred in a closed basin (see Smoot, 1985) with internal drainage and perched outlets that drained into topographically lower basins (e.g., Lake Magadi, East Africa), and was thus sensitive to subtle changes in climate. Deposition began with fluvial to perhaps lacustrine sandstones and conglomerates that thicken toward the axis of the basin where they interfinger with deeper-water alkaline-rich lake deposits (as with the Stockton Formation of the Newark - Gettysburg basin; see Van Houten, 1969; Turner-Peterson and Smoot, 1985) or with paludal deposits including thin coal seams (as in the Richmond, Sanford, and Taylorsville basins; see Reinemund, 1955). In some lakes, as in Lake Lockatong of the Newark basin, sedimentation formed in moderately deep water with perennial anoxic bottom conditions. Varved strata characterize these beds, and record periods of major expansion and contraction of the lake, which may have had an areal extent greater than 7500 km 2 . Besides numerous fish species, these lake beds abounded with amorphous algal kerogen, Zooplankton, pollen, spores and plant cuticles (Olsen, 1980 a,b), and thus are potential sources of hydrocarbons (Manspeizer, 1981; Katz et al., 1988, this volume; Smith and Robison, 1988, this volume). As climates became more arid in the Norian Stage, and many of these lakes began to desiccate, extensive playas and mud flats with gypsum, anhydrite, glauberite and halite formed from the Culpeper basin north through

32 NEW OXFORD-LOCKATONG 33 LOWER PASSAICHEIDLERSBURG

3 TAYLORSV1LLE GROUP 4 CHATHAM GROUP

35 COROLLINA MEYERIANA 36 COROLLINA TOROSUS 37 COROLLINA MURPHII

8 CHATHAM GROUP 9 COW BRANCH FORMATION

30 SCOTS BAY FORMATION

29 NORTH MOUNTAIN BASALT

28 BLOMIDON FORMATION

27 WOLVILLE FORMATION

26 TURNERS FALLS SANDSTONE

25 DEERFIELD BASALT

24 SUGAR LOAF ARKOSE

23 PORTLAND FORMATION

22 HARTFORD BASIN BASALTS

21 NEW HAVEN ARKOSE

20 WATCHUNG BASALTS

19 PASSAIC FORMATION

18 LOCKATONG FORMATION

17 STOCKTON FORMATION

16 ASPERS BASALT

15 GETTYSBURG SHALE

14 NEW OXFORD FORMATION

13 MOUNT ZION CHURCH BASALT

12 BALLS BLUFF SILTSTONE

11 MANASSAS SANDSTONE

10 STONEVILLE FORMATION

34 MANASSAS-UPPER PASSIAC

6 CUMNOCK FORMATION 7 SANFORD FORMATION

5 PEKIN FORMATION

2 TUCKAHOE GROUP

31 CHATHAM-RICHMONDTAYLORSVILLE

PALYNOZONES

1 CHESTERFIELD GROUP

SOME KEY STRATIGRAPHIC HORIZONS

Fig. 3-5. Time-stratigraphic correlation chart of Newark strata, based on palynofloral zones and extrusive horizons, data primarily from Cornet and Olsen, 1985. Correlation is also made with lower Mesozoic strata from the COST G-2 cores (Manspeizer and Cousminer, 1988).

BASINS

is*

Triassic-Jurassic rifting

53

the Hartford, Deerfield and Fundy basins (see Hubert et al., 1978). The upland climates generally were humid, and produced an ample supply of clay (as recorded by the mudstones) that accumulated on the basin floor, which had a more arid climate. A later and distinctively different episode of basin filling started near the beginning of the Jurassic and is marked by interbeds of tholeiitic lavas and lacustrine strata, and diabase intrusion (Fig. 3-5). This phase of tectonism is included within the time encompassed by the postrift unconformity (Fig. 3-2), and probably is related to broad thermal uplift along the orogen. From the volcanic province of Florida to the Fundy basin (and perhaps into Newfoundland) and east into the Atlas mountains of Morocco, plutons intruded the upper plate, feeding tholeiitic lavas that flowed along the valley floor where they impounded surface runoff, forming pillow lavas in lakes (Manspeizer, 1980). The occurrence of lakes was also a consequence of increased rainfall due to orographic uplift of maritime air as it crossed the newly elevated orogen (see Manspeizer, 1981, 1983). It is thus believed by this writer that many of these Liassic lakes had high-altitude rift shoulders, similar to the setting of Lakes Tanganyika and Kivu of East Africa. As both in the Dead Sea and East Africa (Manspeizer, 1985), high discharge ephemeral streams transported coarse-grained clasts to prograding alluvial fans and fan-delta complexes along the upthrown faulted margins, while clay and silt-bearing perennial streams prograded deltas along their hinged and axial margins. Differences of facies is thus related to the size and slope of the drainage basins. See the papers by Olsen, Gore, Smoot, Ressetar and Hubert (1988, this volume) for detailed analyses of sedimentary facies within these basins facies. Many of the Jurassic lake beds, e.g. those of the Culpeper, Hartford and Newark basins, accumulated in perennially stratified, eutrophic, seasonally expanding and contracting, calcite-precipitating lakes that occupied subbasins along or near, the major border fault (Olsen, 1980 a,b; Hentz, 1985). Wherever the Liassic strata are exposed, as in the Gettysburg, Newark, and Hartford basins, they are marked by a sequence of 3 - 13 tholeiitic lava flows that typically are intercalated with lacustrine strata. As many as 13 lava flows are reported from the Culpeper basin. Many flows are about 50-200 m thick, commonly composed of multiple flow units, and intercalated with about 25 - 300 m of sedimentary strata. Their absence in the exposed southern basins seems to be the result of deep erosion, since basaltic lavas of similar age are reported in more than 50 wells in South Carolina, Georgia, Alabama, and northern Florida (for details, see Daniels et al., 1983). Paleomagnetic data in conjunction with geological descriptions of cores enabled Phillips (1983) to identify 23 different lava flows in subsurface cores at the Clubhouse Crossroad in South Carolina. Although Early Jurassic volcanics are reported from the Gulf of Maine, they have not been found on the adjacent continental margin. Middle Jurassic volcaniclastics, however, have been reported from both the Scotian Shelf and Georges Bank, and may be related to a remnant volcanic cone located in the exploratory Exxon (133) Well on Georges Bank (Hurtubise et al., 1987). The volcanic synrift sequence may have been laid down in the narrow time interval of about 500,000 years during the Hettangian Epoch (P.E. Olsen, pers. commun., 1985). Volcanism began at least 20 million years after the onset of sedimentation in the Anisian to Carnian, and then only after 2 - 6 km of coarse elastics had accumulated in these basins (Fig. 3-5). The volcanic emplacements were concurrent with other changes in the geometry and stratigraphy of the basins, thus reflecting fundamental changes in the history of the margin. As multiple lava flows, consisting of individual flow units, they record periodic episodes of extension within each basin. It is unlikely that the Triassic border faults, as they break the surface, served as conduits for the egress of lava. Paleoflow studies in the Newark - Gettysburg basins

54

W. Manspeizer

(Manspeizer, 1980) show that feeder dikes lie along the axes of these basins and generally do not intersect nor crop out along border faults. Geochemical studies by Puffer et al. (1981) demonstrate that where multiple flows occur in adjacent basins (e.g., in Newark and Hartford) the magmas concurrently and rapidly underwent similar chemical changes, so that the lava flows are both rock and time-rock correlatives. As such, they are effective stratigraphic datums (Olsen, written commun., 1985; see Fig. 3-5). In addition to the diabase sills and dikes (e.g., the Palisades Sill in New Jersey, West Rock in Connecticut, and the intrusives of Gettysburg, Pennsylvania, which were emplaced during the depositional history of the basins), a younger set of dikes appear to cut across the synrift strata. This latter set is most densely concentrated in the Carolinas and its age relationship to the lavas and intrusives in the northern basins is not altogether clear (see McHone, 1988, this volume). May (1971) has shown that, on a Bullard reconstruction of the continents, the dike swarm forms a radial pattern centered over the Blake Plateau. The dikes coincide with the Appalachian Gravity High and with the East Coast Gravity Anomaly. DeBoer and Snider (1979) relate the dike swarm to a hotspot located northeast of a triple junction in northern Florida. Swanson (1982) concluded that the dikes occur along a transform system that parallels the orogen and is related to the migration of North America around a pole of rotation in the Sahara. McHone and Butler (1983) recognizing 5 alkalic igneous rock provinces of Mesozoic age, show that these provinces are located where cross-trending fracture zones intersect Appalachian orogenic structures. Bedard (1985) also has shown that older inherited structures (e.g., the Ottawa - Bonnechere graben and the Kelvin fracture zone) have controlled the emplacement of the Monteregian intrusives and Kelvin Seamounts, respectively. Age Determining the age of the Newark Supergroup has posed a problem because the 'red beds' were long thought to be barren of fossils and particularly lacked marine fossils, which are generally used for interregional biostratigraphic correlations. Isotopic age dating and paleomagnetic dating alone are too imprecise to date this stratigraphic sequence. The Newark Supergroup has been considered to be partly or solely Early Jurassic (Rogers, 1842; Lyell, 1847; Redfield, 1856), Late Permian - Triassic (Emmons, 1857), and Jurassic or Late Triassic (Fontaine, 1883). On the bases of rare vertebrates and plant fossils, it was considered (Ward, 1891; Eastman, 1913) to be solely Triassic (see also Reeside et al., 1957). Radiometrie ages determined from the lavas provide an Early to Middle Jurassic time span ranging from 200 to 175 Ma (see Sutter, 1985). With the use of palynomorphs, Cornet et al. (1973) and Cornet and Traverse (1975), and with well-preserved vertebrates (Olsen et al., 1982); Cornet and Olsen (1985), these workers have shown that many of the onshore (and presumably the offshore) basins contain Lower Jurassic strata (Figs. 3-2 and 3-5). These, and related, studies help to fix a Late Triassic age (ranging from middle to late Carnian) for the strata in the Richmond, Taylorsville, Scottsburg, Sanford, Durham and Dan River basins. More recent palynological studies by Traverse (1987) indicate that the date of earliest sedimentation in the Richmond, Taylorsville and Sanford basins is at least earliest Carnian to perhaps Ladinian. The strata in the Culpeper, Gettysburg, Newark, Deerfield and Fundy basins, however, range in age from Late Triassic (Carnian to Norian) to Early Jurassic (Hettangian to Toarcian). Synrift deposition in the Fundy basin may have begun in the Middle Triassic (Anisian), as suggested through palynological studies by Cornet and Olsen (1985) and Nadon and Middleton (1985).

55

Triassic- Jurassic rifting

Attempts at determining the age of the offshore rift basins have proved less successful, as no wells on the U.S. margin, and only a few wells on the Canadian margin, have penetrated the entire synrift sequence [see discussions in this volume (1988) by Cousminer and Steinkraus, Benson and Doyle, Hutchinson and Klitgord, and Holser et al.]. However, because the geometry of both the offshore and onshore rift basins are similar, and because both sets of basins generally parallel the fabric of the Appalachian - Alleghanian orogen, Klitgord and Behrendt (1979) concluded that they formed from a rift system having a common pole of rotation (see also Grow and Sheridan, 1981). Notable, in this context, is the offshore New York Bight basin, which lies along the tectonic axis of the onshore Hartford basin. Determining the age(s) of the offshore basins are further complication by the presence of older onshore rift basins, e.g., the Narragansett and Boston basins, which continue into the offshore where they contain Carboniferous to Permian elastics and volcanics and occur within the zone of Triassic rift basins (Ballard and Uchupi, 1972, 1975; McMaster et al., 1980). The association of Triassic and Carboniferous rift basins is perhaps best documented in the Fundy basin of Nova Scotia, where Triassic strata lie entirely within the fault boundaries of the Carboniferous rift (see Klein, 1962; Belt, 1968; Bradley, 1982; Brown, 1986). There it rests on slightly deformed Pictou drift strata (Stephanian - Autunian or Late Carboniferous to Early Permian) or with pronounced angular unconformity on pre-Pictou synrift strata. The Triassic age assignment for the offshore basins is also compromised by the occurrence of Stephanian - Autunian drift strata in several onshore 'Triassic' basins of Morocco (Fig. 3-2; Manspeizer et al., 1978; and Beauchamp, 1988, this volume). Based on the distribution of palynoflorule assemblages, including Potonieisporites and Vittatina, Cousminer and Manspeizer (1976) inferred that Stephanian - Autunian drift-type deposition occurred over a large area extending from the mesetas of Morocco to the Pictou Group of Nova Scotia and to the type Autun basin in France. Because sedimentation at that time occurred over a broad area (see Van Houten, 1977), it would be difficult to document the age(s) of rifting by seismic data alone, and/or by the age of the oldest strata in the basins, which may predate the time of rifting. By analogy with the onshore Fundy and Magdellana basins and onshore basins of Morocco, it is reasonable to speculate that some offshore rift basins (and deeper sections of the onshore basins) contain older Carboniferous and Permian rift and drift strata respectively. The age determination of these offshore basins is one of the more pressing unresolved issues facing researchers today (see further discussion below under COST G-2 cores). Structure Most Newark-type basins are asymmetric half-grabens, bounded on one side by a zone of high-angle border faults that characteristically show en-echelon fault offset in map view, and on the other side by a gently sloping basement that is unconformably overlain by synrift strata and cut by minor faults. The exposed basins are aligned with pronounced rightstepping offset in the Northern Appalachians and general left-stepping offset in the Southern Appalachians. They thus appear to be linked to eachother by strike-slip faults that have been identified in other basins as transform segments (Bally, 1981), transfer faults (Gibbs, 1984; Tankard and Welsink, 1988, this volume), and accomodation zones (Burgess et al., 1988, this volume). In this context, cross faults, which are common in these basins, appear to link segments of the same rift basins that have undergone different rates of extension. Whereas none of the exposed basins conform to a classical graben structure, paired basins on the Long

56

NEWARK BASIN

W. Manspeizer

Triassic - Jurassic rifting

57

Island platform and those astride the Piedmont Gravity High with opposite-facing border faults may have had an earlier graben history. Hence, eastward-facing listric border faults appear to mark surface traces of eastward-dipping detachment surfaces, and westwardfacing border faults trace antithetic faults to that zone (see Bally, 1981; Gibbs, 1984). Historically, the principal basin bounding faults of the Triassic basins have been modelled as single, planar high-angle faults (Barrell, 1915), as parallel step faults (Faill, 1973; Sumner, 1977; Wenk, 1984), and most recently as low-angle planar (Ratcliffe et al., 1986), and as listric normal faults (Cook et al., 1981; Hutchinson and Klitgord, 1988, this volume; Ressetar and Taylor, 1988, this volume). Based on recent vibroseis and core data of the Newark basin (Ratcliffe et al., 1986), geophysical data from the offshore paired basins of the Long Island platform, (Hutchinson and Klitgord, 1988, this volume), seismic profiles (25 and 26) of the basins offshore New Jersey and Delaware (see Benson and Doyle, 1988, this volume), and by analogy with the Hartford - Deerfield basin (Ando et al., 1984), the Culpeper basin (Costain et al., in press), and the COCORP data from the Southern Appalachians (Cook et al., 1981), we may model the Newark basin as a half-graben with a listric border fault that soles into an east-facing detachment surface of thrust-imbricated Precambrian and Paleozoic basement (Figs. 3-4 and 3-6). In this context, the basin margin is characterized by two groups of faults: listric normal faults that trend parallel to the basin and cross or transfer faults that offset the basin margin. Whereas extension is accomodated mainly by listric fault sets, oblique-trending cross faults or transfer faults serve to connect segments of adjacent subbasins subject to different rates and amounts of extension (see Tankard and Welsink, 1988, this volume; Burgess et al., 1988, this volume). Cross faults are thus an integral part of the rifting process, and consequently have significant strike-slip components. For the Newark basin, listric fault fans with related border fanglomerates and moderately deep-water lacustrine beds formed adjacent to the active fault margin, which we infer had a prominent escarpment that was eroded by high discharge ephemeral streams (Fig. 3-4). On the other hand, the gently sloping basement block with its a substantially larger drainage basin was drained by west-flowing perennial streams feeding fluvial-deltaic deposits. Strata within each basin generally dip about 1 0 - 15° towards the principal border fault, where they are commonly bent into broad synforms and antiforms (Fig. 3-6), termed warps by Wheeler (1939) and/or into more tightly compressed en-echelon folds (Davis, 1898). Fig. 3-6.A. Geologic map of the Newark basin, showing distribution of formations and clusters of detrital cycles (parallel black lines) in the Passaic Formation (from Olsen, 1980a). Abbreviation of formations and diabase bodies as follows: B - Boonton Formation; C - Coffman Hill Diabase; Cd = Cushetunk Mountain Diabase; F = Feltville Formation; H = Hook Mountain Basalt; Hd = Haycock Mountain Diabase; Jb = Jacksonwald Basalt; L = Lockatong Formation; O = Orange Mountain Basalt; P = Passaic Formation; Pb = Preakness Basalt; Pd = Palisades Diabase; PA: = Perkasie Member of Passaic Formation; Rd = Rocky Hill Diabase; S = Stockton Formation; Sc = carbonate facies of the Stockton Formation; Sd = Sourland Mountain diabase; T = Towaco Formation. B. Structural features of the Newark basin (from Olsen, 1980a). Faults are drawn as normal with dots on downthrown side; portions of the basin margin not mapped as faults should be regarded as onlaps. While all the faults are mapped as normal, it is clear that many, if not all, have some component of strike slip. Symbols for the names of structural features are: A = Montgomery - Chester fault block; B = Bucks - Hunterdon fault block; C = Sourland Mountain fault block; D = Watchung syncline; E = New Germantown syncline; F = Flemington syncline; G = Sand Brook Syncline; H = Jacksonwald syncline; / = Ramapo fault; J = braided connection between Ramapo and Hopewell faults; K = Flemington fault; L = Chalfont fault; M = Hopewell fault (see Olsen, 1980a, for references).

58

W. Manspeizer

Whereas Wheeler (1939) related broad warps in the Newark and Hartford basins to differential dip slip along the border fault, later studies by Sanders (1963), Faill (1973), Manspeizer (1980) and Ratcliffe (1980) related folds within these basins to horizontal stress in a compressional and perhaps wrench-tectonic setting, as suggested by Manspeizer (1981) for the structures of the basin and the Narrow Neck. Fold sets also occur on the hanging wall adjacent to major cross faults within the basins; there they are related to synfault deformation. Enechelon foreland-type folds with axial plane spaced-cleavage have been mapped at Lepreau Harbor in the Fundy basin (Stringer and Lajtai, 1979) and in the Jacksonwald syncline along the Narrow Neck that connects the Newark and Gettysburg basins (Lucas et al., 1988, this volume). A rigorous structural analyses of these folds is given by Lucas et al., 1988, this volume. Triassic cleavage has also been reported from the Richmond basin of Virginia (Shaler and Woodworth, 1898), the Deerfield basin of Massachusetts (Goldstein, 1975) and in coal seams of the Sanford (Deep River) Basin of North Carolina (Reinemund, 1955). Typically both the strata and basement are cut by major oblique-trending cross faults or transfer faults, some having as much as 20 km of horizontal displacement and 3 km of vertical displacements in the Newark basin (Sanders, 1963), and about 12 km of sinistral offset in the Hartford basin (DeBoer and Clifford, 1988, this volume). Published geophysical and subsurface data from the Newark - Gettysburg basin (Cloos and Pettijohn, 1973; Sumner, 1977), Hartford basin (Wenk, 1984), the Durham basin (Bain and Harvey, 1977), and the Sanford (Deep River basin; Randazzo et al., 1970) show that the basement is cut by cross faults creating intrabasin grabens and horsts that, most probably, formed concurrently with synrift deposition (see Closs and Pettijohn, 1973; Sumner, 1977; and deBoer and Clifford, 1988, this volume). However, Faill (1973) and Lucas et al. (1988, this volume) conclude on the basis of structural data that, in the absence of identifiable growth faults and in the presence of both folded and faulted Liassic strata, deformation in the Newark basin must postdate deposition. Although the age of deformation is poorly constrained by structural data alone, there is a large body of field data that, when taken together, indicate that deformation and basin filling occurred concurrently. The issue is important because it bears on the various models concerning the origin of these basins. Syntectonic deposition with recurrent activity along the border fault has been inferred by many workers largely from the excessively thick sequence (perhaps up to 8 km) of time-transgressive fanglomerates along the fault margins of these basins. Much of the field evidence for syndepositional faulting and folding is presented by Ratcliffe (1980, pp. 288, 292), who also has concluded from his own mapping programs that the oldest synrift lavas (early Liassic) in the Newark basin rest unconformably on previously folded and faulted Triassic strata. Although unequivocal evidence of growth faults has not been observed, Olsen (1980a,b) notes that in the Hartford, Culpeper and Newark basins most formations thicken faster as the border faults are approached and thus concludes (Olsen, in press) that the geometry of these basins is consistent with subsidence along listric faults. Bally (1981) further notes that if, '. . . listric faults show updip convergence of beds, we are dealing with growth faults that formed over long periods of time'. Another clue to the age of folding comes from a rather curious, and apparently heretofore unreported, feature of these synrift basins (the Newark, Hartford, Narrow Neck, and Argana in Morocco), i.e. the presence of phacolith-like structures. In the Newark basin and Narrow Neck they typically occur along the hinge of synforms and consist of York Haventype tholeiites, dated about 200 Ma (see Lucas et al., 1988, this volume). If intrusion occurred at the time of folding, as seems likely to this writer, then the onset of folding is early Lias

59

Triassic - Jurassic rifting

(Hettangian-Sinemurian). As the younger, and apparently undeformed, Rossville dikes in the Narrow Neck and elsewhere in the Gettysburg basin, crosscut folded synrift strata (Smith et al., 1975), they help to constrain the uppermost age of folding from the early Lias to the early Middle Jurassic [which is the age assigned to these dikes by Sutter and Smith (1979)]. This writer also assigns a Liassic to early Middle Jurassic age to growth folding in Morocco, to regional uplift astride the future spreading axis, and to the postrift unconformity (see discussions below, and papers in this volume by Cousminer and Steinkraus; and Hutchinson and Klitgord). Basin origin After almost 150 years of study and debate, the origin of these basins remains controversial. Early workers, e.g., Rogers (1858), Davis (1886) and Barrell (1915), considered the Triassic basins to be simple asymmetric fault grabens, produced by extension more or less orthogonal to the basin margins. The Broad Terrane Hypothesis, formulated by Russell (1892), and later modified by Sanders (1963), speculates that the Newark and Hartford basins formed initially as a single large graben within an extensional regime; it was then arched, and subsequently eroded, producing 2 asymmetric half-grabens. The model, invoking a complex sequence or recurrent changes in the stress field, finds support in the occurrence of paired basins of the Long Island platform that are eroded and unconformably overlain by the postrift strata (Fig. 3-2). Hutchinson and Klitgord (1988, this volume) suggest, for example, that the Newark - New York Bight basin may have formed initially as a graben along the edge of the Appalachian detachment surface. Late rifting in their model, resulting from regional uplift, caused tilting and erosion of synrift strata in basins landward of the basement hinge zone and subsidence seaward of the hinge. The age of uplift may be dated as Lias to early Middle Jurassic by the postrift unconformity in the COST G-2 cores (see discussion under COST G-2 cores). A variation on the extensional theme is presented by deBoer and Clifford (1988, this volume), who show a two-phase history of rifting and shifting that consists of: Late Triassic - Middle Jurassic basin formation through N W - S E extension; and a Middle-Late Jurassic shifting event, in which the basin strata were deformed by sinistral shear along NE-trending fault zones. Ratcliffe and Burton (1985) also ascribe to an extensional model, in which extension is mainly controlled by reactivation of Paleozoic thrusts. Faill (1973), noting the lack of field data in support of syndepositional faulting, concluded that the Newark - Gettysburg basin formed as a crustal downwarp, similar to the Baikal rift. Cioos and Pettijohn (1973) and Sumner (1977), demonstrated, however, through subsurface data, that normal faulting developed grabens early in the history of the Newark - Gettysburg basin. Other studies have focused on the role of Late Triassic transforms as the possible mechanism through which wrench-induced, pull-apart basins and grabens are formed. Manspeizer (1981) interprets the Newark basin as a strike-slip basin, which evolved through an east - west-trending, left-lateral shear couple. Lucas et al. (1988, this volume), however, have shown that while the Narrow Neck may have originated in a wrench zone with sinistral shear, the Newark and Gettysburg basins formed in sinistral transtension. An east-westtrending, left-lateral shear is also postulated by Ballard and Uchupi (1975) for the origin of the Gulf of Maine basin, and is consonant with data from the Fundy basin, showing Late Triassic - Early Jurassic sinistral slip of 75 km (Keppie, 1984). Brown (1986) also interprets the Chignecto basin as a small pull-apart basin along the Minas Geofracture. Swanson

W. Manspeizer

60

(1982), however, proposes that these basins formed under a dextral shear couple and were later deformed through sinistral shear. Although the wrench-induced pull-apart model maybe more applicable to Triassic basins lying astride transform segments, as in the Narrow Neck and Fundy basins on the American plate and the High Atlas basins of the African plate (Laville, 1988, this volume; Beauchamp, 1988, this volume), it may not be applicable to other east coast basins of the passive margin. It is enlightening to apply a modification of Gibbs' model of extensional basins in the North Sea or Bosworth's model of the Gregory rift in East Africa to the east coast rift basins. In that context, the Triassic - Jurassic basins may be viewed as a series of half-grabens or subbasins that are linked together by a complex zone of wrench and oblique-slip transfer faults. Thus the Newark and Hartford basins, offset in map view, are bordered by opposing basin-bounding faults that mark the surface traces of synthetic (Newark basin) and antithetic (Hartford basin) listric faults on a northeast-dipping detachment zone. Wrench-dominated accomodation zones (e.g., the Narrow Neck) are viewed as connecting basins of different extensional histories. A consequence of the half-graben model, as suggested by Bally (1981), is that it gives rise to a conjugate set of asymmetric margins along detachment faults. (See discussion below.) COST wells and postrift unconformity The Triassic - Jurassic data base for the U.S. margin relies heavily on multichannel seismic reflection profiles that have been supplemented by magnetic and gravity studies, and by the COST wells in Georges Bank. Even the offshore drill hole data from the Triassic strata (Fig. 3-2) of the Canadian margin are quite limited. Of 23 deep wells on the Scotian Shelf (Barss et al., 1979), only 2 may reach uppermost Triassic strata. The Mohican 1-100, which commonly is used as a standard for correlation to the U.S. margin (see Poag, 1982, fig. 24) does not even penetrate the Jurassic-Triassic systemic boundary (Barss et al., 1979). Only the Sandpiper 2J-77, the Osprey H-84 and the Spoonbill C-30 wells of Grand Banks penetrate virtually the entire Upper Triassic section (Barss et al., 1979; Fig. 3-2; Holser et al., 1988, this volume; Tankard and Welsink, 1988, this volume). The COST G-2 well off Georges Bank is the deepest and most important stratigraphic well on the U.S. margin (Fig. 3-7). It serves as the offshore stratigraphic standard for the U.S. margin; yet considerable controversy surrounds the age determinations of strata in the well. It was drilled to a depth of 6667 m (i.e. 1769 m deeper than the COST G-2 well). Based on suites of age-diagnostic palynomorphs from Cores Nos. 4 and 5, Cousminer (Cousminer, 1983; Cousminer and Steinkraus, 1988, this volume) determined that the well had penetrated a thick Upper Triassic section of dolomite with limestone and anhydrite, bottoming in Upper Triassic salt. Palynomorphs, extracted from both cores (Cousminer and Steinkraus, 1988, this volume) and cuttings (B. Cornet, written commun., 1983, 1984) indicate that the postrift unconformity occurs within an attenuated Liassic section (less than 300 m thick) of carbonates and evaporites that lies near the base (4153 m) of the Middle Jurassic Mohican Formation, which according to Given (1977) represents the first drift deposits to transgress the newly formed margin after the breakup of the North American and Afro-European plate. These same researchers further report the presence of Carnian - Norian phytoplankton in core No. 5 at 4441 m, indicating that intermittent marine conditions were present on Georges Bank as early as the Late Triassic. The basal salt in the well, tentatively dated through the rare occurrence of palynomorphs in Core No. 7, can be correlated with the Triassic Osprey

61

Triassic-Jurassic rifting

Salt on Grand Banks. Using seismic data, however, Poag (1982) correlated the basal salt with the Early Jurassic Argo Salt of the Scotian Shelf (see also Holser et al., 1988, this volume). But the basal salt in the COST G-2 well occurs more than 2000 m below Core No. 5 with its Triassic flora, suggesting to some workers that the fauna in Core No. 5 maybe reworked. The assemblage, however, shows no evidence of reworking. Moreover, industry researchers

AGE CORE (COUSMINER, STEINKRAUS AND HALL, 1984)

PALYNOMORPHS CORE NO. 4

MIDDLE JURASSIC

BAJOCIAN

LIASSIC

HETTANGIAN-TOARCIAN*

TRIASSIC

CARNIAN-NORIAN

ψψιιΜι»[\ •

GLISCOPOLLIS MEYERIANA KYRTOMISPORITES LAEVIGATUS CAMEROSPORITES VERRUCATUS PORCELLISPORA LONGDONENSIS VERRUCOSISPORITES SP. AFF. V. CHENEYI DICTYOTRILETES SP. GRANULOPERCULATISPORITES RUDIS TODISPORITES ROTUNDIFORMIS CONVOLUTISPORA KLUKIFORMA CYCADOPITES SPP. VESICASPORA SP. AFF. V. FUSCA CARNISPORITES LEVIORNATUS ARATRISPORITES SP. AFF. A. FIMBRIATUS CORE NO. 5 CAMEROSPORITES SP. (TETRAD) PATINASPORITES DENSUS PSEUDOENZONALAPOLLENITES SUMMUS TASMANITES SP. LUNATISPORITES SP. OVALIPOLLIS SP. CAMEROSPORITES SECATUS CHORDASPORITES SP. ALISPORITES SPP. PATINASPORITES DENSUS NORICYSTA SP. AFF. FIMBRIATA ICAMEROSPORITES SP. (TETRAD) loVALIPOLLIS SP.

DEPTH IFT. M. h

11,000

h

12,000

h

13,000

: 3500-

4000

14,000

fr

15,000 4500 "1

h16,000 5000 h17,000

Γ 18,000 EXPLANATION

LATE

LIMESTONE

TRIASSIC ?

5500 •19,000

DOLOMITE ANHYDRITE

EJ[

Lr20,000 6000

SALT f-21,000 SANDSTONE/SILTSTONE SHALE

•CORNET,1981, PERSONAL CONN.

T.D.

21,874 FTl

•

6500 •22,000

CORE, NO. 3-9

Fig. 3-7. Geologic column of the COST G-2 Well, Georges Bank, showing major lithologies, age designations, core locations and palynomorphs extracted from cores 4 and 5; data primarily from Poag, 1982; Cousminer et al., 1984; Cousminer and Steinkraus, 1988, this volume; Manspeizer and Cousminer, 1988).

62

W. Manspeizer

now reveal that similar dinoflagellate assemblages have been identified in 5 other proprietary wells. Most significantly, the fact remains that the dinoflagellate assemblage also documents the occurrence of a Late Triassic marine transgression in the outboard regions, whether or not the flora in Core No. 5 have been reworked. This paper does not postulate, nor does the assemblage require, that a Late Triassic ocean floor existed at this time in the central Atlantic. Further, Core No. 4 (4035-4051 m) contains abundant Middle Jurassic (Bajocian) flora along with reworked Triassic flora; the assemblage constrains the upper age of the postrift unconformity on Georges Bank, and supports the notion that a broad regional uplift of Early Jurassic age occurred astride the spreading axis. Seismic data also reveal that the basal salt in the COST G-2 well is underlain by perhaps as much as 5 km of strata, indicating that the section may also include older Triassic and/or Late Paleozoic synrift (?) strata. Other marginal basins Although none of the other COST wells drilled off the eastern U.S. penetrated Upper Triassic - Lower Jurassic strata, geophysical data show that, like the Georges Bank basin, as much as 5 km of Triassic (?) synrift deposits lie within the rifted basement of the Baltimore Canyon and Carolina troughs and the Blake Plateau basin (see Grow, 1981). These marginal basins are major depocenters that have developed seaward of the basement hinge zone, on a rifted crust. The postrift wedge of strata, on the order of 8 - 13 km thick, overlies the postrift unconformity, and except for the Blake Plateau basin, is cut by salt diapirs. The unconformity has been reported in the Baltimore Canyon trough (seismic line 25), the Carolina trough (seismic line 32) and the Blake Plateau basin (profile FC-3; see Schlee and Jansa, 1981; and Grow, 1981). In the Scotian basin, which has been extensively drilled and is the subject of many papers (including Jansa and Wade, 1975; Ascoli, 1976; Given, 1977; and Holser et al., 1988, this volume) the synrift sequence consists of Upper Triassic red beds of the Eurydice Formation that give way seaward and upward in section, to salt and shale, with minor anhydrite of the Argo Formation of Rhaetian to Hettangian - Sinemurian age (Fig. 3-2). Toward the shallower parts of the basin, the red-bed and evaporite sequence is cut by a prominent unconformity, which has been correlated by Jansa and Wade (1975) with the early Cimmerian tectonic event of Stille (1924). The unconformity is not recognized in deeper parts of the basin, where deposition may have proceeded uninterrupted. Lying above the Cimmerian unconformity on the Scotian Shelf is the Iroquois Formation, a Sinemurian to Toarcian shallow-marine carbonate sequence (Fig. 3-2). The carbonates are dolomites with anhydrite and stromatolites in the lower part of the sequence, and skeletal limestone in the upper part. Marine conditions were short-lived, however, as terrestrial sedimentation resumed in the early Middle Jurassic with the deposition of the overlying red beds of the Mohican Formation. The stratigraphic and structural record at Grand Banks also records two major episodes of rifting and drifting, which in this case spanned at least 110 Ma, from the Late Triassic to middle Early Cretaceous (Tankard and Welsink, 1988, this volume). The earlier rifting episode, lasting from the Late Triassic to the Early Jurassic, is marked by the accumulation of continental red beds and evaporites in aborted rifts, similar to the Newark basins in the United States. Late Callovian to Aptian rifting, although more intense than Triassic rifting, occurred along listric normal faults that were inherited from the Late Triassic rifts, and merge with low-angle detachment zones at depth. The Osprey H-84 well, which is fairly representative of the early Mesozoic rift-drift sequence, drilled through more than 2500 m

63

Triassic-Jurassic rifting

of Upper Triassic - Lower Jurassic evaporites, carbonates, and red beds that rest unconformably on the Avalon basement (Fig. 3-2; Jansa et al., 1980). The oldest strata encountered in the well are Late Triassic Kettle red beds, a series of continental reddish brown sandstones and shale carrying Patinaspontes densus and other diagnostic Carnian - Norian palynomorphs. The red beds are conformably overlain by slightly more than 2000 m of almost pure halite with minor interbeds of reddish shale, dolomite, fine elastics, and red anhydrite of the Osprey evaporites. They yield a Carnian - Norian to Hettangian - Sinemurian palynological assemblage, indicating that the Argo Formation corresponds only to the upper part of the Osprey evaporites (Jansa et al., 1980). The Murre carbonate, a sequence of dolomite and anhydrite, conformably overlies the Osprey Formation but is unconformably overlain by younger carbonates (see Tankard and Welsink, 1988, Fig. 6-3, this volume). Multiple rift/drift cycles Evolution of the Atlantic margins has produced a two-fold depositional couplet of rifting and drifting, yielding in cross section a 'steer's head' depositional configuration (see McKenzie, 1978; Watts, 1981; Bradley, 1982). Onshore field studies also demonstrate that some Late Triassic - Early Jurassic synrift strata are underlain by an older couplet of rift and drift strata, thereby recording (as in the Fundy basin) two major episodes of thermal expansion and subsidence. A two-stage rifting process also is recorded in Grand Banks (Tankard and Welsink, 1988, this volume). On the Avalon terrane of the Scotian Shelf, for example, early Mesozoic synrift strata are underlain by an older passive margin sequence consisting of Upper Devonian Carboniferous synrift strata, and Late Carboniferous to Lower Permian drift strata (see Belt, 1968; Webb, 1969). Where these older terranes occur on the passive margin, as in Georges Bank and the Scotian Shelf, we speculate that the margin also consists of two synrift/drift sequences. In some deep basins, e.g., the Maine basin of Georges Bank, where more than 5 km of Triassic synrift strata have accumulated, the Triassic may rest unconformably on Permo-Carboniferous drift strata, as first suggested by Ballard and Uchupi (1975) and Given (1977). The succession of multiple rift/drift cycles for the Atlantic margin is shown diagrammatically in Figure 3 - 8 . Each tectonostratigraphic cycle is marked by first order characteristics, e.g., geometry, unconformities, stratigraphy and structure. The primary data that are used to establish time-stratigraphic correlations between the onshore and offshore strata are based on biostratigraphic data from the COST G-2 cores and from outcrop. Note particularly that: (a) the onshore and offshore tectonostratigraphic units seem to be out-of-phase; i.e. the onshore volcanic-lacustrine sequence is a time correlative of the offshore postrift unconformity; (b) the postrift unconformity is diachronous, embracing varying Early Jurassic to perhaps Late Triassic intervals; (c) two (or more) salt formations occur on the shelf, one of Carnian age (Osprey Salt) and the other (Argo Salt) of Hettangian - Sinemurian age; and (d) the oldest Mesozoic marine transgression, documented by dinoflagellates in the COST G-2 core, is Carnian to Norian age; it is a time-stratigraphic equivalent of the typical continental red-bed facies.

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W . Manspeizer

OFFSHORE EVENTS/STRATIGRAPHY REGIONAL SUBSIDENCE MARINE TRANSGRESSION CARBONATE DEPOSITION BAJOCIAN MICROFLORA WITH REWORKED NORIAN PHYTOPLANKTON PARALIC MARINE POSTRIFT UNCONFORMITY REGIONAL UPLIFT ARGO SALT & IROQUOIS FORMATION CARNIAN-NORIAN PALYNOMORPHS SUPRATIDAL-SUBTIDAL SABKHAS EVAPORITE DEPOSITION RESTRICTED MARINE SEDIMENTATION TETHYAN MARINE TRANSGRESSION GEORGES BANK : SALT FLUVIAL-LACUSTRINE SEDIMENTATION

REGIONAL SUBSIDENCE FLUVIAL - MARGINAL MARINE

CONTINENTAL SEDIMENTATION WITH OCCASIONAL VOLCANICS AND EVAPORITES

55

LIMESTONE BLACK SHALE & SILTSTONE

DOLOMITE W/ *LA φτ ¥/ // ANHYDRITE vVV

EXTRUSIVES

■ " · .■ * ' · ' · ' . ' ' . ' . *

COARSE C

SALT

!■ + + + *■ + + + ►++t

BASEMENT

UNCONFORMITY

CORES 4,5,9

Fig. 3-8. Diagrammatic representation of multiple rift/drift cycles off Georges Bank of the U.S. margin. Correlation of offshore and onshore events based on paleontologic correlation, see Figs. 3-2, 3-5 and 3-7.

African plate: Morocco The onshore Triassic - Liassic rocks of Morocco and North America are substantially different (Fig. 3-2). Unlike the inboard Newark Supergroup, which is primarily a continental synrift deposit, the Moroccan sequence is largely marine and consists of three Hthologically and chronologically different facies, namely: (1) an older Middle to Late Triassic marine carbonate and evaporite facies with a basal andesitic lava of questionable Triassic age of the Oranian meseta; (2) a thick clastic and tholeiitic synrift continental and marginal marine facies of Late Triassic to Early Jurassic age of the High Atlas; and (3) a younger, reddishbrown mudstone and evaporite paralic marine facies of latest Triassic to earliest Jurassic age of the Moroccan meseta (Fig. 3-2; Manspeizer et al., 1978).

Triassic-Jurassic rifting

65

The section on the Oranian meseta unconformably overlies either a late Paleozoic basement that was deformed during the Namurian to Westphalian (Late Carboniferous), or cross-bedded red sandstones of Autunian (Early Permian) age (Cousminer and Manspeizer, 1977). These Autunian sandstones are correlatives of the Pictou Group in the Maritime Provinces, and may have been contiguous with that drift sequence. Significantly both drift sequences are overlain by very different stratigraphic sections, thus reflecting different Mesozoic thermal histories. The Oranian meseta sequence consists of basal high-potassic and vesicular lavas that are interbedded with and overlain by several meters of pink to buff dolomite containing the Ladinian (Middle Triassic; upper Muschelkalk stage) pelecypod, Anoplophora lettica. Several hundred meters of carbonate strata, presumed to be Late Triassic, but without reported age-diagnostic fossils, overlie the dolomites. Locally throughout Algeria and Morocco, salt gypsum and anhydrite are abundant in this horizon (Augier, 1967; Salvan, 1968). Similar carbonate and volcanic sections occur in the Middle Atlas province, where the lavas have been dated at 211 Ma, thereby supporting a Ladinian age (see Manspeizer et al., 1978). Trachytic lavas also occur at the base of well-documented Carnian red beds and above Hercynian schists in the High Atlas mountains (see Mattis, 1977). These lavas, however, are now considered by some workers to be Permian (see Beauchamp, 1988, this volume), thus raising questions about the Triassic age assigned to the basal volcanics on the Oranian meseta. The age issue is further complicated because Middle Triassic andesitic lavas and evaporites occur extensively across the Oranian meseta throughout Algeria and Tunisia (see Van Houten, 1977). Only in the High Atlas province, where Late Triassic rifting reactivated the late Paleozoic South Atlas fracture zone, did Newark-type sedimentation occur. While the origin of the basins on the High and Middle Atlas provinces has been interpreted traditionally through extensional tectonics (see Van Houten, 1977; Brown, 1980; Stets and Wurster, 1982), an alternate view is that they may have been controlled by wrench tectonics along a complex set of anastomosing faults, forming transtensional rift basins (see Laville and Petit, 1984; Laville, 1988, this volume; Beauchamp, 1988, this volume). Like the onshore basins of North America, those of the High Atlas contain a synrift continental clastic sequence, measuring 1000-5000 m thick, that typically overlies eroded and deformed Variscan basement rock. The thickest and best known of these basins, the Argana basin of the western High Atlas, contains an important Late Triassic - Early Jurassic synrift sequence that becomes progressively finer-grained upward in section, where it gives way to Liassic marine carbonates and evaporites. The Triassic component of the section (Brown, 1980), includes: (1) a basal alluvial fan and alluvial plain facies, (up to 2500 m thick), and stream-laid conglomerates and sandstones of the Ikakern Formation; (2) a medial 2000-mthick formation, the Timesgadiouine, consisting of a lower alluvial fan complex and an upper sequence of lacustrine red mudstones; and (3) the Bigoudine Formation, which is about 1300 m thick and consists of delta-front and delta-plain sediments overlain by red-brown playa-type mudstones alternating with massive beds of gypsum (Brown, 1980). Paleocurrent data (Brown, 1980) and petrographic analyses (Jones, 1975) indicate that the basin was filled with detritus derived from an easterly source of lower Paleozoic low-grade metamorphics, Hercynian volcanics, and Stephanian - Autunian late orogenic intramontane deposits. The upper part of the Triassic section may be traced westward to the thick sequence of evaporites, red mudstones and basaltic lavas exposed along the salt diapiric province at Essaouiria and Tidsi (Robb, 1971; Beck and Lehner, 1974; Uchupi et al., 1976; Hinz et al., 1982). According to Hinz et al. (1982) and Holser et al. (1988, this volume), the salt province off Morocco,

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including the salts drilled at DSDP site 546 (at the foot of the Mezagan escarpment), may have been contiguous with the salt province of the Scotian Shelf. Olivine and quartz-tholeiitic lava flows, yielding isotopic ages of about 195 Ma, overlie the Triassic sequence and, like those of North America, are correlated with the basal Lias (Hettangian to Sinemurian) (Manspeizer et al., 1978). Both the lavas and overlying channel deposits are thickest (about 400 m) in structural lows and thin to a feather edge along adjacent basement highs, indicating that both growth faulting and folding were active during the Lias. The channel deposit is noteworthy because it rest uncorformably on eroded lake beds, and consists of lacustrine pebbles that make up large, eastward-dipping planar cross-bed sets, indicating that these clasts were transported inboard from an uplifted and eroded basin to the west. We speculate that regional uplift occurred along the future plate margins, and that it corresponds in time and place to the Liassic uplift that is manifested by the postrift unconformity in the COST G-2 cores. Note that the COST G-2 well and the Argana basin may have been only 300 to 500 km apart during the Lias (see Fig. 3-1). The channel sands are overlain by a sequence of shallow-water, pelletic, skeletal and oolitic carbonates with interbeds of sabkha-type evaporites (including nodular anhydrite) that is noteably similar to the Iroquois Formation on the Scotian Shelf (see also Holser et al., 1988, this volume). The carbonates of the Tamarout Formation contain an abundant mollusk, echinoderm, and brachiopod assemblage, including Zeilleria lycetti, a Toarcian brachiopod (Harding, 1975). About 50 km to the west, at Jebel Amsittene, the carbonates rest on Triassic salt and give way to limestone reefs, thus marking the first open-marine transgression of the High Atlas. The limestones carry a fossil assemblage of echinoderms, bivalves, belemnites, and brachiopods, including Spiririna, suggesting an early Liassic - Sinemurian age for the marine transgression (Societe Cherifienne des Petroles, 1966; Ager, 1974). In Western Europe a similar assemblage is found only along the western coast of Portugal, marking the transgression of marine waters from an opening Atlantic Ocean to the west (Ager, 1974). To the east, in the central High Atlas, the synrift section thins and begins with interbeds of conglomerate and trachytic lava (Mattis, 1977) that yield radiogenic ages of 203 and 262 Ma (Van Houten, 1977). Whereas Manspeizer et al. (1978) correlated these lavas with the andesitic volcanics of the Oranian meseta and the Middle Atlas province, they are now considered by some workers to be Permian (Fig. 3-2; Beauchamp, 1988, this volume). The lavas are overlain by a thick sequence of redbeds, the most important of which is the Oukaimeden Sandstone, for it contains age diagnostic Carnian palynomorphs (Cousminer and Manspeizer, 1976) and intercalations of marine strata with brachiopods, echinoids, and lamellibranchs (Biron, 1982). The Liassic part of the section, above the tholeiitic lavas, consists primarily of supratidal deposits (chicken-wire gypsum, caliche crust, vadose pisoliths); intertidal deposits (algal laminated boundstones, bioturbated pelletiferous mudstones, wackestones with disruptive channels and storm sequences); and subtidal (skeletal lime packstones, oolitic tidal deltas, offshore bars, oncolites, and occasional coral and Opisoma reefs (Lee and Burgess, 1978). Facies maps show that extensive sabkhas existed along the southwest end of the basin, while carbonate platforms developed to the northeast where, according to Crevello et al. (1987), their distribution was controlled by basin-bounding faults and by pre-existing or actively forming structural highs within transtensional rift basins. We may infer that during the Lias, as in the preceding Triassic, a shallow tongue of the Tethys Seaway transgressed the eastern half of the central High (and Middle) Atlas, but it did not connect with the embryo Atlantic waters in the Argana - Essaouira basins, which also shoaled against the rising Tichka Massif to its east (Fig. 1). Crevello et al. (1987) also note that the

67

Triassic - Jurassic rifting

Liassic platforms of the central High Atlas were destroyed by a Toarcian drowning event, and that a second episode of carbonate platforms of Aalenian - Bajocian age were destroyed by the onset of continental sedimentation, thus marking the end of the High Atlas Seaway. Two clearly different isochronous lithofacies of Liassic, and perhaps latest Triassic, age occur on the Moroccan meseta: a thin (1000 m) cratonic deltaic facies of sandstone and shale with gypsum and dolomitic limestones; and a slightly thicker (about 1400 m) evaporite facies of halite, gypsum, and shale. Both facies are intercalated with low-alkaline quartz tholeiites, having isotopic ages of about 186 Ma (Manspeizer et al., 1978). Massive Triassic rock salt, uniquely enriched with potassium minerals and interbedded with anhydrite and mudstone, of the Sei Inferieur occur below the lavas in the Berrichid, Khemisset, Doukkala, and Boufekrane basins of northwestern Morocco, indicating a time of extreme aridity (Salvan, 1972). A similar mineral assemblage was cored at DSDP site 546 (Holser et al., 1988, this volume). Massive Liassic salt deposits, 100-300 m thick, of the Sei Superieur occur above the lavas in these same basins. On the bases of geochemical and stratigraphic data (Holser and others, this volume) recognize two evaporite provinces: Atlantic and Atlas. According to these workers, the Atlantic type, e.g., the evaporites from the Berrechid, Essaouira, and Khemisset basins were deposited in large interconnected marginal-marine basins that were fed by both marine and continental waters, and concentrated over 75 times. Evaporites of the Atlas province were precipitated in local continental playas or sabkhas that were fed by nonmarine waters. Correlation Events leading to the breakup of the craton and the formation of the Atlantic passive margins may be inferred from the early Mesozoic stratigraphic record of eastern North America and northwest Africa (see Correlation Chart, Fig. 3-2). The bases of that correlation, presented by Manspeizer et al. (1978), is that the volcanics on both sides of the spreading center form a paragenetic time-stratigraphic sequence that is essentially isochronous. The data base for the correlation includes basalt geochemistry and isotopic age dates, and pollen from both continental red beds and marine strata. On the basis of cross-cutting field relationships in the Gettysburg and Newark basins, Smith et al. (1975) demonstrated that these synrift igneous rocks comprise a timestratigraphic sequence of igneous activity throughout the Central Appalachians. These researchers showed that: (1) olivine-normative types formed early in the tectogenetic history; (2) quartz tholeiites, like the Watchung - York Haven - Holyoke lavas and the Palisades Sill, formed later in the history; and (3) subalkaline quartz tholeiites, such as the Rossville - Hamden lavas formed last in the sequence. Later studies (e.g., Puffer et al., 1981) showed that the same paragenetic sequence occurs from the Newark basin to Fundy basin. In Morocco, the volcanics occur in three distinct, partially synchronous, volcanicsedimentologic provinces: the Oran meseta, the High and Middle Atlas provinces, and the Moroccan meseta. The oldest volcanics, dated at 211 Ma, are alkaline-rich basalts that are interbedded with dolomites, containing the Middle Triassic Anoplophora fauna. Although these alkaline-rich rocks of Ladinian age have not been reported in North America, in Morocco they form an important datum that was used to correlate Mesozoic strata of the Oran meseta with those of the High Atlas. This correlation may have been in error, as the basal alkaline-rich lavas in the High Atlas are now considered to be Permian (see Beauchamp, 1988, this volume). These lavas are overlain by a thick red-bed sequence carry-

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W. Manspeizer

ing palynomorphs assigned to the Minutosaccus-Patinosporites concurrent range zone of middle Carnian age (Cousminer and Manspeizer, 1976). The red beds are a time-stratigraphic correlative of the Swiss and English middle Keuper, the type Carnian of Austria, and the lower portion of the Triassic section in the Taylorsville and Richmond basins of Virginia and the Deep River basin of North Carolina, the lower and middle New Oxford Formation of the Gettysburg basin in Pennsylvania, and of the middle Passaic Formation of the Newark Group. The clastic section in the High Atlas is overlain by olivine and quartz tholeiites that yield isotopic dates averaging 196 Ma; they are rock- and time-stratigraphic equivalents of the York Haven - Palisades suite, and probably formed at the beginning of the Liassic Stage. The youngest volcanics in the sequence, the low-alkaline quartz tholeiites of the Moroccan meseta, are dated at 186 Ma and are intercalated with evaporites; they are time and rockstratigraphic equivalents of the Rossville tholeiites of Pennsylvania. The overlying Tamarout Formation of the Argana basin in the High Atlas includes carbonate reefs, and carries a marine fauna ranging from Sinemurian to Toarcian; it has been correlated with the Iroquois Formation of the Scotian Shelf (Jansa and Wiedmann, 1982). Its stratigraphic position above the Late Triassic Rhaetic salt leads one to conclude that it is also a time-stratigraphic correlative of the carbonate reefs in the G-2 well, as described by Poag (1982). Early history of the Atlantic margins The Atlantic passive margins evolved through a protracted history of recurrent plate boundary activity that accompanied several cycles of Proterozoic and Paleozoic accretionary and rifting tectonics. Different tectonic regimes were superimposed onto a previously deformed basement, so that near the end of the Paleozoic Era, the regional basement was a collage of inherited fabrics that lay along the core of the Variscan - Alleghanian orogen (see for example, Arthaud and Matte, 1977; Lefort and Van der Voo, 1981; Williams and Hatcher, 1983; Ziegler, 1982). The main accretionary phase of the orogeny culminated along a 1000 km wide and 10,000 km long dextral shear zone that led to crustal shortening of at least 200 km at both ends of the megashear with concomitant thrusting of the Urals and Mauritanides to the east and the Appalachians to the west (Arthaud and Matte, 1977). Subsequent rifting of the orogen and opening of the central Atlantic occurred primarily along many of these same late Paleozoic thrust faults and sutures (see for example, Cook et al., 1979; Ando et al., 1983; Hutchinson and Klitgord, 1988, this volume). Dextral shear of the Appalachians continued into the Late Carboniferous (see Gates et al., 1986), when according to Bradley (1982) it led to the deformation of many late Paleozoic (Late Devonian to Carboniferous) strike-slip basins in the Northern Appalachians (including the Magdalen, Minas, Stellarton, Cumberland, and Narragansett). Late Paleozoic strike slip with major dextral displacement is also recorded along the South Atlas fracture zone (Mattauer et al., 1972), but there it led to wrench faulting along the Tizi n' Test branch of the South Atlas fracture zone and to Appalachian-type foreland thrusting of the Variscan fold belt over the African craton (Michard et al., 1982). A comparable set of late Paleozoic, Northern Appalachian-type strike-slip basins has not been reported from North Africa. From the Late Carboniferous to the Early Permian, following an interval of uplift and erosion, Pictou-type nonmarine drift deposits and acidic to intermediate lavas blanketed a vast area, as general subsidence extended primarily from the Autun province of France to the mesetas of Morocco - AlgeriaTunisia, Iberia and the Maritime Provinces of Canada (Van Houten, 1977; Cousminer and Manspeizer, 1977).

Triassic-Jurassic rifting

69

At the end of the orogeny, continental accretion had formed a high-standing, broadly convex landmass that, we speculate, extended from about 80°S to 70°S. It had an area of about 184 x 106 km 2 , a broad central arch standing about 1.7 km high and 720 km across, and an average elevation of more than 1300 m above the early Mesozoic sea level (Hay et al., 1981). By the onset of rifting in the Middle to Late Triassic, the future proto-Atlantic margins, from Florida to Grand Banks, lay essentially between 5°S and 20°N latitudes, thereby encompassing climatic regimes that ranged from equatorial rainforest to tropical savannah (see Robinson, 1973; Barron et al., 1981). This distribution has important paleoclimatic implications (Manspeizer, 1981). As the landmass drifted further north transgressing about 10 to 15° of latitude, from the Late Triassic to Middle Jurassic, the northern rift basins (e.g., Grand Banks, and the Moroccan - Oranian meseta) encountered increasing aridity as they moved under the subtropical high-pressure cell. The southern basins (e.g., the Danville and Richmond), however, stayed humid as they remained under the influence of the equatorial low-pressure system. Superimposed on this long-term trend were the seasonal monsoons that swept the proto-Atlantic region with cold dry winter winds from the northwest, and warm moist summer winds from the transgressing Tethys Sea to the east. The rift topography also profoundly affected the climate of the source regions and depositional basins by establishing more humid climatic conditions in the mountains and more arid climates in the valleys than otherwise would have occurred at about 30° latitude; this is a consequence of adiabatic cooling and warming of air masses as they are swept up and down and mountain slope respectively. In as much as rifting had created an asymmetric topography with high-standing rift basins on the American plate and along the narrow zone of the protoAtlantic axis, and low-lying landscapes over the extensive belt of mesetas to the east, the prevailing climates (beneath the subtropical high-pressure cell) were arid over the mesetas and humid over the mountains. The studies of Morgan (1981) and Crough (1981) indicate that, as the newly consolidated Pangaean plate began to move north about 200 m.y. ago, its Laurasian component moved northwestward over the Canary hot spot, and its Gondwanaland component moved westward over the St. Helena hot spot. We speculate that heating and doming led to lithospheric attenuation through both surface erosion (as documented by the ubiquitous and profound Permo-Triassic unconformity) and technically induced listric faulting (as recorded by seismic profiles of these basins). The breakup of the Pangaean plate (between North America and northwest Africa) involved faulting and basin filling along sutures and fracture zones, with a change in the locus of rifting from the thrust and fold belt of the Variscan - Alleghanian orogen to a more medial position along the orogen (see Van Houten, 1977). Rifting initially was centered about the Minas geofracture, and perhaps along the South Atlas fracture zone (see correlation chart, Fig. 3-2). It is noteworthy that the Fundy basin (Brown, 1986) and the High Atlas basins (Beauchamp, 1988, this volume; Laville, 1988, this volume) are both strike slip basins, lying along well-documented continental fracture zones that have long histories of late Paleozoic dextral faulting, and Middle-Late Triassic sinistral faulting. Sinistral displacement along these east - west-trending fracture zones and extension along older Paleozoic thrust faults appears to have produced a family of northeast-trending half-grabens. A consequence of the half-graben model, as suggested by Bally (1981), is that it gives rise to a conjugate set of asymmetric margins along detachment faults, which break the crust into upper and lower plate margins (Lister et al., 1986). Figure 3-9, a modification of detachment models, presented by Klitgord et al. (1988) and Bosworth (1987), Bally (1981), and Lister et

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W. Manspeizer

al. (1986), attempts to synthesize some of the data and concepts presented in this paper about the evolution of the Atlantic margins. The asymmetry of the margins are reflected by substantially different stratigraphic records at the time of breakup (Fig. 3-2). We may infer from that record that the American plate was marked by a broad belt of marginal plateaus with many northeast-trending detrital basins that were linked to eachother by transfer faults and displaced by cross faults. The Moroccan plate, on the other hand, was marked by few broadly subsiding evaporite basins. Typically each half-graben on the American plate was bordered by a hinged margin and one major basin-bounding fault, which marked the surface trace of synthetic and antithetic listric faults on a detachment zone that was inclined towards the newly forming ocean. The American plate (Late Triassic) was dominated by high relief, high altitude fluvial-lacustrine basins along the western part of the orogen, and by low-relief A

LATE TRIASSIC CONTINENTAL BASINS

MOROCCAN MESETA COST G-2 TETHYAN TRANSGRESSION

EROSIONAL SURFACE

"OSPREY SALT-

UPPER PLATE

PARTIAL MELT ASTHENOSPHERE

B

EARLY JURASSIC HIGH ALTITUDE LAKES

BASIN REWORKING POST RIFT UNCONFORMITY

EVAPORITE PLATFORM

MANTLE UPWELLING THERMAL UPLIFT

C M IDDLE JURASSIC

SPREADING CENTER BAJOCIAN TRANSGRESSION

Fig. 3-9. Crustal evolution model for the Atlantic-type margins based on low-angle detachment faulting and the formation of lower and upper plate margins (concept and caption modified from Klitgord et al., 1988). Frame A, Late Triassic detachment faulting with uplift and arching of the lower American plate, as the load of the upper plate is technically removed and displaced laterally, thereby wedging the Moroccan plate upward so that it becomes a broad erosional surface. Late Triassic marine seas transgress the toe of the wedge, depositing evaporites and carbonates on the upper plate. Frame B, Early Jurassic uplift and partial melting. As tectonic thinning of the upper plate migrates eastward, the locus of partial melting and thermal uplift migrates to the proto-Atlantic axial basins, which are uplifted and eroded during the formation of the post-rift unconformity. As the cooler Moroccan plate subsides, it becoming a broad evaporite platform. Frame C, Bajocian cooling, subsidence and sea-floor spreading.

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sea-level evaporite basins near the future spreading axis. During detachment faulting, in the Late Triassic - Early Jurassic, the lower plate must have been uplifted isostatically into a broad central arch, as the load of the overlying upper plate continued to be reduced by erosion and listric faulting. This had the consequence of elevating Late Triassic marine basins that lay near the proto-Atlantic axis. By the Lias, many of these marine basins were eroded and their strata reworked and transported landward towards the onshore basins of Morocco and North America. The topographic reversal is thought to reflect the eastward migration and upwelling of the asthenosphere. It was a time of major crustal thinning with development of the postrift unconformity (COST G-2 cores), and adiabatic decompression on the upwelling asthenosphere. Whereas the earliest melts are expected to have yielded off-axis alkalinerich volcanics (as in Morocco), subsequent melts are though to have been derived from later partial melt derivatives and are expected to have been more tholeiitic (see discussion by Bedard, 1985). As the upwelling asthenosphere migrated eastward in response to tectonic thinning, the abandoned' rift-stage crust cooled, ushering in the drift phase of the margin. The Moroccan plate, by contrast, was a broad region of low relief throughout most of the Middle to Late Triassic and Early Jurassic. It was distinguished by few detrital basins, and almost all of these occur along the South Atlas fracture zone, as Triassic strike slip basins in the High Atlas. Except for the offshore Essaouira basin, which is a seaward extension of the High Atlas Argana basin, the Moroccan margin consists of a very narrow band of early Mesozoic rift bands. Triassic rift basins of the Middle Atlas (e.g., Bab-Bou-Idir and Berkane), which record the breakup of the meseta into an Oranian and Moroccan component consist primarily of a thick carbonate sequence (see Manspeizer et al., 1978). The majority of the intraplate basins of North African occur on the mesetas, and are nonrift; typically they contain nonclastic, marine and fresh-water evaporites of Liassic and younger strata that formed in broad, shallow, drift-type basins on a generally subsiding terrane of low relief, near the very end of synrift time. The first stratigraphic record of a Tethyan transgression is found in late Middle Triassic rocks (Ladinian Stage) of northeastern Morocco, along the Gibralter shear zone (Fig. 3-1). There, andesites are interbedded with carbonates bearing the Lettenkohle Anoplophora faunas that are overlain by several hundred meters of massively bedded micrites. A similar marine carbonate and andesite sequence also occurs to the southwest, in the Middle Atlas. Farther to the south, in the central High Atlas, Ladinian andesites at the base of the section are overlain by Late Triassic continental red beds that, according to Biron (1982), are interbedded with sandstones containing Carnian brachiopods and echinoids (see also Beauchamp, 1988, this volume). As the Carnian and Norian seas trangressed southwestward along the South Atlas fracture zone, the waters shoaled against the rising Tichka massif and thus were blocked from entering the embryo Atlantic basin through the Argana basin of the western High Atlas (Fig. 3-1). The first marine invasion of the central Atlantic basin occurred in the Late Triassic (probably Carnian), as shallow hypersaline Tethyan waters transgressed westward through the Gibralter fracture zone (Fig. 3-1; Jansa and Wade, 1975; Manspeizer et al., 1978; Jansa et al., 1980; Lancelot, 1980; Manspeizer, 1982), or south from Canada through eastern Greenland (see below). As fluvial and lacustrine sedimentation occurred in the inboard and outboard detrital basins along the central Atlantic, carbonates and sulphates formed in marine basins from the Alps to Southern Spain, and halite with minor amounts of anhydrite and dolomite formed in evaporite basins in Algeria, Tunisia, and the Aquitane of southern France (Busson, 1972; Jansa et al., 1980; and Holser et al., 1988, this volume). Further west

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in the Lusitania basin on the Iberian meseta and in the Carson subbasin in eastern Grand Banks, over 2000 m of halite of the Osprey evaporites were precipitated above the continental Kettle red beds, without carbonates or elastics, from hypersaline waters in restricted basins (Jansa and Wade, 1975). Continued marine transgression along the proto-Atlantic axis, throughout the late Carnian and into the Norian, brought hypersaline seas and evaporite precipitation to Essaouiara and the western High Atlas basins of Morocco, as well as to Georges Bank and, perhaps south to the Carolina trough off North America and the Aaiun basin off north Africa (Fig. 3-2). Data from the COST G-2 well show that quiet, nearshore, paralic marine conditions prevailed over much of the platform lying adjacent to the coast, while offshore, dolomite and calcite with subordinate amounts of anhydrite, algal stromatolites and oolitic sands formed on extensive sabkhas that were occasionally flooded by high-energy marine waters (see Arthur, 1982; Poag, 1982). Where hypersaline waters were restricted, as in technically active rift basins, thick Carnian salt deposits formed (see Rona, 1982). That marine conditions prevailed for some time on the newly formed margin at Georges Bank, is documented by the occurrence of Carnian phytoplankton below the postrift unconformity and by reworked late Norian marine palynomorphs with in situ Bajocian microflora above the unconformity. Similar dinoflagellate assemblages have been described from the subsurface of the Canadian Arctic (Bujak and Fisher, 1976) where they occur with ammonites, indicative of a Norian age and marine habitat. The marine transgression of Georges Bank was part of a much more extensive Late Triassic flooding of the rift basins that were breaking apart the Pangean plate. At that time marine seas extended south from Arctic Canada along the North Atlantic rift zone through eastern Greenland (see Clemmensen, 1982), and west from the Tethys Sea through a complex rift system of Western Europe (see Ziegler, 1982). By the Early Jurassic, the newly evolving margins experienced further tectonic unrest. Basins were uplifted and eroded (particularly along the future Atlantic plate margins) and the seas withdrew (Fig. 3-9). The regional paleocurrents were away from the spreading axis, as earlier formed synrift strata were uplifted and reworked toward the landward basins. Onshore, it was a time of volcanism, rifting and renewed uplift of the source terranes with the concomitant deposition of fan deltas that prograded into moderately deep-water lakes. Elsewhere, as in western Europe, the uplift on Georges Bank, which we correlated with the postrift unconformity, is related to the Mid-Cimmerian tectonic event. According to Ziegler (1982), it is a major rifting event of Early to Middle Jurassic age that was synchronous with volcanism in the North Sea, eustatic lowering of sea level, and the rifting episode that preceded the onset of sea-floor spreading in the Central Atlantic. It also signaled an end to synrift deposition of the U.S. Atlantic margin, and the beginning of its drifting history. References Ager, D.V., 1974. The western High Atlas of Morocco and their significance in the history of the North Atlantic. Geol. Assoc. (London) P r o c , 85: 2 3 - 4 1 . Aggerwal, Y.P. and Sykes, L.R., 1978. Earthquakes, faults and nuclear power plants in southern New York, and northern New Jersey. Science, 200: 425-429. Ando, C.J., Czuchra, B.L., Klemperer, S.L., Brown, L.D., Cheadle, M.J., Cook, F.A., Oliver, J.E., Kaufman, S., Walsh, T., Thompson Jr., J.B., Lyons, J.B. and Rosenfeld, J.L., 1984. Crustal profile of mountain belt: COCORP deep seismic reflection in New England Appalachians and implications for architecture of convergent mountain chains. Bull. Am. Assoc. Pet. Geol., 68 (7): 819-837.

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Cloos, E. and Pettijohn, F.J., 1973. Southern border of the Triassic basin, west of York, Pennsylvania; fault or overlap. Geol. Soc. Am. Bull., 84: 523-536. Cook, F.A., Albaugh, S.D., Brown, L., Kaufman, S., Oliver, J. and Hatcher, R., 1979. Thin-skinned tectonics in the crystalline southern Appalachians: COCORP seismic reflection profiling of the Blue Ridge and Piedmont. Geology, 7: 563-567. Cook, F.A., Brown, L.D., Kaufman, S., Oliver, J.E. and Petersen, T.A., 1981. COCORP seismic profiling of the Appalachians orogen beneath the costal plain of Georgia. Geol. Soc. Am. Bull., 92: 738-748. Cornet, B. and Olsen, P.E., 1985. A summary of the biostratigraphy of the Newark Supergroup of eastern North America, with comments on early Mesozoic provinciality. In: R. Weber (Editor), Symposio Sobre Flores del Triasico Tardio su Fitografia y Paleoecologia, Memoria. Proc. 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Gates, A.E., Simpson, C. and Glover, III, L., 1986. Appalachian Carboniferous dextral strike-slip faults: an example from Brookneal, Virginia. Tectonics, 5: 113-119. Gibbs, A.D., 1984. Structural evolution of extensional basin margins. J. Geol. S o c , London, 141: 609-620. Given, M.M., 1977. Mesozoic and early Cenozoic geology of offshore Nova Scotia. Bull. Can. Pet. Geol., 25: 63-91. Glover, L., Ill, Poland, F.B., Tucker, R.D. and Bourland, W.C., 1980. Diachronous Paleozoic mylonites and structural heredity of Triassic-Jurassic Basins in Virginia. Geol. Soc. Am., Abstr. with Programs; 12 (4): 178. Goldstein, A.G., 1975. Brittle fracture history of the Montagne basin, north-central Massachusetts. Contrib. no. 25 (M.S. Thesis) Geology Department, Univ. Mass., Amherst, MA, 105 pp. Grow, J.A., 1981. The Atlantic Margin of the United States. In: W.A. Bally (Editor), Geology of Passive Continental Margins; History, Structure, and Sedimentologic Record. Am. Assoc. Pet. Geol., Educat. Course Note Ser., 19 (3): 1 - 4 1 . Grow, J.A. and Sheridan, R.E., 1981. Deep structures and evolution of the continental margin off the eastern United States. Oceanol. Acta, Proc. 26th Int. Geol. Congress, Geology of Continental Margin Symposium, Paris, 1980, pp. 1 1 - 1 9 . Harding, A.G., 1975. The stratigraphic analysis and significance of the Late Triassic to upper Lower Jurassic rocks of the western High Atlas Mountains in southwest Morocco. Master's thesis, Univ. South Carolina, Columbia, SC, 66 pp. Hatcher, R.D., Jr., Howell, D.E. and Talwani, P., 1977. Eastern Piedmont fault system: Speculations on its extent. Geology, 5: 636-640. Hay, W.W., Barron, E.J., Sloan, J.L. and Southam, J.R., 1981. Continental drift and the global pattern of sedimentation. Geol. Rundsch., 70: 302-315. Hentz, T., 1985. Early Jurassic sedimentation in a rift-valley lake: Culpeper basin, northern Virginia. Geol. Soc. Am. Bull., 96: 9 2 - 1 0 7 . Hinz, K., Dostmann, H. and Fritsch, J., 1982. The continental margin of Morocco: Seismic sequences, structural elements and geological development. In: U. von Rad, K. Hinz, M. Sarnthein and E. Seibold (Editors), Geology of Northwest African Continental Margin. Springer-Verlag, Berlin, pp. 3 4 - 6 0 . Holser, W.T., Clement, G.P., Jansa, L.F. and Wade, J.A., 1988. Evaporite deposits of the North Atlantic rift. In: W. Manspeizer (Editor), Triassic - Jurassic Rifting. Continental Breakup arid the Origin of the Atlantic Ocean and Passive Margins. Elsevier, Amsterdam, pp. 525-556. Hubert, J.F., Reed, A.A., Dowdall, W.L. and Gilchrist, J.M., 1978. Guide to the red beds of central Connecticut. 1978 Field Trip, Eastern Section, Soc. Econ. Paleontol. Mineral., 129 pp. Hurtubise, D.O., Puffer, J.H. and Cousminer, H.L., 1987. An offshore Mesozoic igneous sequence, Georges Bank basin. Geol. Soc. Am. Bull., 98 (4): 430-438. Hutchinson,, D.R. and Klitgord, K.D., 1988. Evolution of rift basins on the continental margin off southern New England. In: W. Manspeizer (Editor), Triassic - Jurassic Rifting. Continental Breakup and the Origin of the Atlantic Ocean and Passive Margins. Elsevier, Amsterdam, pp. 81 - 9 8 . Hutchinson, D.R., Klitgord, K.D. and Detrick, D.S., 1986. Rift basins of the Long Island platform. Geol. Soc. Am. Bull., 97: 688-702. Jansa, L.F. and V/ade, J.A., 1975. Geology of the continental margin off Nova Scotia and Newfoundland. In: W.J.M. Van der Linden and J.A. Wade (Editors), Offshore Geology of Eastern Canada. Can. Geol. Surv., Pap., 74-30: 5 1 - 1 0 5 . Jansa, L.F. and Wiedmann, J., 1982. Mesozoic - Cenozoic development of the eastern North American and northwest African continental margins: A comparison. In: U. von Rad, K. Hinz, M. Sarnthein and E. Siebold (Editors), Geology of the Northwest African Continental Margin. Springer, Berlin, pp. 215-269. Jansa, L.F., Bujak, J.P. and Williams, G.L., 1980. Upper Triassic salt deposits of the Western North Atlantic. Can. J. Earth Sei., 17: 547-559. Jones, D.F., 1975. Stratigraphy, environments of deposition, petrology, age, and provenance of the basal red beds of the Argana Valley, western High Atlas Mountains, Morocco. Master's thesis, New Mexico Inst. Mining and Technology, 148 pp. Katz, B.J., Robison, C.R., Jorjorian, T. and Foley, F.D., 1988. The level of organic maturity within the Newark basin and its associated implications. In: W. Manspeizer (Editor), Triassic-Jurassic Rifting. Continental Breakup and the Origin of the Atlantic Ocean and Passive Margins. Elsevier, Amsterdam, pp. 683-696. Kaye, C.A., 1983. Discovery of a Late Triassic basin north of Boston and some implications as to post-Paleozoic tectonics in northeastern Massachusetts. Am. J. Sei., 283: 1060-1079. Keppie, J.D., 1982. The Minas Geofracture. In: T. St. Julian and J. Beland (Editors), Major Structural Zones and Faults of the Northern Appalachians. Geol. Soc. Can., Spec. Pap., 24: 263-280.

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Kinsman, D.J.J., 1975. Rift Valley basins and sedimentary history of trailing continental margins. In: A.G. Fischer and S. Judson (Editors), Petroleum and Global Tectonics, pp. 8 3 - 126. Klein, G. de V., 1962. Triassic sedimentation, Maritime Provinces, Canada. Geol. Soc. Am. Bull., 73: 1127- 1146. Klitgord, K.D. and Behrendt, J.C., 1979. Basin structure of the U.S. Atlantic margin. In: J.S. Watkins, L. Montadert and P.W. Dickerson (Editors), Geological and Geophysical Investigations of Continental Margins. Am. Assoc. Pet. Geol., Mem., 29: 8 5 - 1 1 2 . Klitgord, K.D. and Schouten, H. 1977. The onset of sea-floor spreading from magnetic anomalies. In: Symposium on the Geological Development of the New York Bight. Palisades, N.Y. Lamont-Doherty Geological Observatory, pp. 1 2 - 13. Klitgord, K.D. and Schouten, H., 1980. Mesozoic evolution of the Atlantic, Caribbean, and Gulf of Mexico. In: R.H. 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