|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 497-531|
Princeton University, Princeton, New Jersey
Upper Triassic Lockatong lacustrine deposits in central New Jersey and adjacent Pennsylvania are arranged in short asymmetrical "detrital" and "chemical" cycles that resulted from expansion and waning of the lake, presumably due to cyclic variation in climate.
Detrital short cycles, averaging 14 to 20 feet thick, comprise several feet of black shale succeeded by platy dark-gray, carbonate-rich mudstone in the lower part and gray, tough, massive calcareous silty mudstone in the upper. The massive mudstone has a small-scale contorted fabric produced largely by crumpled shrinkage cracks and burrows. Thicker, coarser grained detrital cycles contain 2 to 5-foot layers and lenses of thin-bedded, commonly cross-stratified siltstone and very fine grained sandstone, locally with small-scale convoluted bedding.
More common chemical short cycles average 8 to 13 feet thick. Lower beds are alternating dark-gray to black, platy dolomite-rich mudstone and marlstone 1 to 8 cm thick, extensively broken by crumpled shrinkage cracks. Locally initial deposits are crystalline pyrite or calcite as much as 2 cm thick. In the middle, several feet of dark-gray mudstone encloses 2 to 8 cm-thick layers of gray marlstone disrupted by syneresis. The upper part is gray, tough, massive analcime- and carbonate-rich mudstone containing as much as 7 percent soda and as little as 47 percent silica, and a maximum of about 35 to 40 percent analcime. The mudstone is brecciated on a microscopic scale, probably the product of syneresis. Much of the mudstone is also disrupted by slender crumpled shrinkage cracks irregularly filled with crystalline dolomite and analcime.
Some thinner chemical cycles are grayish red, especially in the uppermost part of the formation. These contain layers of greenish-gray weathering mudstone variously disrupted by shrinkage cracking. Thinner beds are broken into mosaic intraformational breccia; thicker ones are disrupted by long intricately crumpled cracks. Small lozenge-shaped pseudomorphs of dolomite and analcime after gypsum? or glauberite? are common in the grayish-red mudstone. Analcime and dolomite are also concentrated in shrinkage cracks and between mosaic flakes in dark dusky-red mudstone.
Varve-counts of black mudstone suggest that short cycles resulted from 21,000-year precession cycles. Bundles of detrital and of chemical short cycles occur in intermediate cycles 70 to 90 feet thick; these in turn occur in long cycles 325 to 350 feet thick. The patterns apparently resulted from alternating wetter and drier phases of intermediate and long climatic cycles, producing through-flowing drainage and a bundle of detrital cycles or a closed lake and a bundle of chemical cycles.
The Upper Triassic Lockatong Formation in central New Jersey and adjacent Pennsylvania (Fig. 1) is a thick lacustrine deposit characterized by abundant analcime, small-scale disturbed bedding, and sedimentary cycles.
Figure 1--Geologic map of central New Jersey and adjacent southeastern Pennsylvania. Interfingering Lockatong and Brunswick lithofacies based largely on mapping by McLaughlin (1946, 1959). A larger version of this figure is available.
The most obvious cycles are asymmetrical ones generally 10 to 20 feet thick (Fig. 2). These short cycles, repeated with almost monotonous uniformity of successive sedimentary features, occur in patterned sequences or bundles of two different orders of magnitude: intermediate ones 70 to 90 feet thick, which are oscillations within long sequences 325 to 350 feet thick (Fig. 2, 19A).
Figure 2--Generalized stratigraphic sections of Lockatong Formation showing detrital and chemical short cycles. A, Roadcut along New Jersey Highway 29, 6 to 8 miles south of Frenchtown, N.J. Pattern of intermediate bundles of detrital and chemical cycles indicated along right side of section. B, Quarry at Rushland, Pennsylvania, about 13 miles west-northwest of Trenton, N.J. Detrital and chemical cycles indicated along right side of section. C, Chemical syscles in quarry along Pennsylvania Highway 32, 7 miles northwest of Trenton, N.J. D, Detrital cycles in quarry 2.5 miles west of Warrington and 4 miles southwest of Doylestown, Pennsylvania. A larger version of this figure is available.
Not only was cyclic sedimentation a dominant process controlling deposition of the formation (Van Houten, 1962), but it was also a basic factor in the concentration of the abundant soda now present in one of the largest-known deposits of nonvolcanic sedimentary analcime.
Detailed variations in structures and textures which reflect changing conditions during the course of a short cycle and disruption during dewatering and compaction of the muddy sediments are described here as a part of a study of the Lockatong Formation that has been supported largely by a grant from the National Science Foundation (grant G-5032).
Several short cycles can be traced for nearly 1/2 mile in a large quarry west of Warrington, Pennsylvania, and show a lateral variation in thickness of no more than a few inches. But tracing of cycles regionally and reconstruction of the formation are hampered by limited exposures and by fragmentation of the basin into several tilted fault blocks. For this reason the present study relies heavily on analyses of vertical profiles. In the regional aspects of the interpretation, I have had to depend upon probabilities, thus making the conclusions insecure and tentative.
The Lockatong Formation of the Upper Triassic Newark Group is a huge lens a maximum of 3,500 to 3,750 feet thick. It lies conformably on the Stockton arkose and reddish-brown, sandy mudstone (a maximum of 5,000 feet thick) and under the reddish-brown Brunswick Shale (at least 6,000 feet thick; see, McLaughlin, 1959, p. 99-102). In general aspect each of these thick formations is an unusually uniform nonmarine lithofacies, pointing to prolonged persistence of established conditions of deposition in the Triassic intermontane basin in New Jersey and eastern Pennsylvania.
The Lockatong Formation grades downward into the Stockton arkose through several hundred feet of tough, well-bedded, very fine grained sandstone, siltstone, and mudstone, and interfingers laterally and upward into the Brunswick mudstone facies.
Lockatong deposits are thickest and most distinctive in the central part of the basin (in the northwestern fault block), wedging out in southeastern Pennsylvania and in northeastern New Jersey.
At its southwest end, 10 miles west of Phoenixville, Pennsylvania, the Lockatong Formation wedges out between the Stockton arkose and a conglomeratic facies of the Brunswick Shale (Hawkins, 1914, p. 148; Bascom, 1938, p. 72). Toward the northeast the southernmost belt of Lockatong rocks is covered by Cretaceous deposits east of New Brunswick, New Jersey, but has been traced in subsurface to Port Reading, 12 miles south-southwest of Newark, where recent drilling has revealed that argillite hornfels is at least 500 feet thick. Twenty miles farther north-northeast along strike, as seen in the old Granton quarry in North Bergen, the Lockatong hornfels facies apparently wedges out into the uppermost Stockton arkose (see, Bock, 1959, p. 130), in an area of conglomeratic Newark deposits. Apparently the Lockatong lake occupied a lowland between two major sites of fanglomerate deposition (Fig. 3).
Figure 3--Sketch map of Triassic sedimentary rocks in central New Jersey and adjacent Pennsylvania, showing relation of the Lockatong Formation to distribution of fanglomerate facies and conglomeratic deposits of Brunswick Formation.
On the basis of available data the Lockatong lacustrine facies is at least 90 miles long and may be as much as 110 miles long. The analcime now present in the central 45 miles of the formation has been traced to within 25 miles of its southwestern end. Toward the northeast analcime must have originally occurred beyond its known limit (just west of New Brunswick), where it was present in rocks which are now albite-rich hornfels.
Along the northwestern margin (in the northwestern fault block) fine-grained Lockatong deposits lie within several miles of the northwest border-fault and may, in fact, interfinger into coarser deposits of the Newark Group (McLaughlin, 1946). These relations indicate that no great influx of detritus spread across the basin from the northwest during Lockatong deposition. This fact, together with the feldspathic composition of Lockatong detrital deposits, points to a major source in crystalline rocks to the east and southeast that also supplied most of the Stockton arkose as well as much of the feldspathic Brunswick mudstone (Sturm, 1956, p. 159) in the central and southeastern part of the basin.
Toward the southeastern border of the basin, the Lockatong Formation thins to about 1,500 feet (Hawkins, 1914, p. 148). Here analcime-rich mudstone within a few tens of feet of the underlying Stockton arkose, as seen in the vicinity of Princeton, New Jersey, suggests that the thinning was accomplished largely by progressive lateral overlap of the upper part of the lacustrine deposits on the fluvial Stockton facies. The presence of abundant analcime along the eroded southeastern margin also suggests that the Lockatong Formation extended several miles farther to the southeast and thus had a total width of at least 25 miles and possibly considerably more. Sanders' (1963, p. 504) recent defense of a broad terrane reconstruction of the Triassic basins proposes a rift valley that was 50 to 70 miles wide.
(1) Rocks in the southwestern extent of the formation and in the lower 400 to 500 feet of the thick section in the northwestern fault block consist principally of dark grayish-red and greenish-gray tough, micaceous mudstone, siltstone, and minor very fine grained sandstone. These deposits are characterized by small-scale irregular bedding and ripple- and cross-lamination. Moreover, the bedding is extensively disturbed by mudcracks and animal burrows, but bedding planes are seldom distinctly ripple marked.
(2) The characteristic rock type of the formation is tough, massive, homogeneous, very fine grained to aphanitic mudstone (argillite as a field term), most of which is medium to dark gray. Some of it is reddish brown to grayish red. This rock is used for building blocks and crushed stone, and is the type seen most commonly in outcrops.
It comprises two varieties which occur in different kinds of short cycles. Both exhibit extensively disturbed or disrupted fabric on a small scale.
(a) One kind which breaks with a hackly or gnarly fracture is a detrital deposit composed of calcareous, feldspathic siltstone and silty mudstone with only a minor amount of quartz. It has indistinct irregular to wispy carbonaceous laminae that are extensively disturbed and generally deformed into a small-scale irregularly contorted fabric (Fig. 4C).
(b) The other kind of argillite is chiefly a colloidal-chemical deposit that is more brittle and breaks with a subconchoidal fracture. This rock type is rich in carbonate minerals of which dolomite is the more common. It contains as little as 47 percent silica and as much as 7 percent soda which is concentrated largely in analcime and albite. Analcime-rich argillite predominates in the upper part of the formation where some of it is grayish red. Because of its variation in color, colloidal-chemical argillite is the preferred building stone.
The groundmass generally has a unique brecciated aspect on a microscopic scale, and commonly is speckled with white patches of crystalline analcime and carbonates, producing a fabric similar to that of "birdseye" limestone.
(3) The rock type second in abundance in the Lockatong Formation is platy, very dark-gray to black, laminated, carbonate-rich mudstone and marlstone in which dolomite commonly predominates. Some of these rocks are varved (Van Houten, 1962, Pl. 2A), and many are extensively disrupted by shrinkage cracks (Fig. 15A, 15D, 15E).
(4) Thin-bedded, calcareous, well-sorted feldspathic siltstone with minor very fine grained sandstone occurs in 2 to 5 foot-thick layers and broad lenses in detrital argillite (type 2a). Only very rarely is it associated with colloidal-chemical argillite (type 2b). This rock is characterized by distinct black micaceous laminae and irregular small-scale ripple-bedding, lenticular cross-bedding, convoluted bedding, and rare graded bedding (Fig. 5A, 5B, 5C). Microstylolites and isolated animal burrow-casts are common, and many of the thin black laminae are broken by slender, indistinct mudcracks and marked by small, indeterminate "tracks."
(5) Laminated black silty, calcareous shale, commonly with sublenticular laminae (Fig. 4A, 4B) constitutes a minor amount of the formation. Reddish-brown shale occurs in the uppermost part of the formation where it interfingers with the Brunswick Shale. All of these rock types normally contain illite and some chlorite, albite, calcite or dolomite, and scattered pyrite. Quartz and potash feldspars are unusually scarce. Toward the southwest end of the basin, however, quartz is more common. The extent to which albite in the unmetamorphosed colloidal-chemical argillite is authigenic has not been determined.
Figure 4--Sedimentary features of detrital short cycles. A, Photomicrograph of lower black shale. Lower half composed of undisturbed, commonly discontinuous, black carbonaceous films in silty mudstone. Upper part massive, fine-grained siltstone with scattered pyrite, mottled by indistinct burrow-casts 0.1 to 0.2 mm in diameter. X 4.2. B, Photomicrograph of lower black shale with delicately disturbed bedding. Lower two-thirds composed of irregular films and laminae of black, dolomitic mudstone and sublenticular laminae of finely crystalline calcite; bedding apparently disturbed by organisms. Thicker bundles of black films recur about 0.4 to 1 mm apart. Upper third consists of several layers of black dolomitic mudstone with abundant scattered calcite crystals, disturbed by burrowers. Black lens with speckled center probably a coprolite. X 3.9. C, Photomicrograph of upper dark-gray, massive, silty mudstone with extensively disturbed fabric. Lighter, silty structure in lower left quarter may be burrow-cast. Black patch near center is pyrite. X 3.1. D, Photomicrograph of upper dark-gray, silty mudstone with siltier crumpled burrow-casts, and mottled or bioturbated fabric. Black spots are pyrite. X 3.7. E, Lower black shale with delicately disturbed films and laminae of black mudstone (white in directly reflected light) interbedded with very finely crystalline calcite (black in photograph). Thicker bundles of black films recur about 0.5 to 1.5 mm apart. Large black nodules are coprolites. X 1.7. F, Upper dark-gray, massive, silty mudstone extensively disrupted by siltier shrinkage crack casts offset by horizontal compaction shearing and by vertical zones of brecciation produced by upward-moving water and fluid mud. Upper part, medium-gray, siltier, and calcareous. White specks in lower part are pyrite, locally in shrinkage cracks and in thin zones of shearing. X 0.62.
Figure 5--Sedimentary features of detrital short cycles. A, Upper massive dark-gray silty mudstone with abundant slender crinkled shrinkage-crack casts; overlain by calcareous, cross-bedded well-sorted siltstone. X 0.54. B, Upper massive dark-gray silty mudstone mottled or bioturbated by organic burrowing. Overlain by calcareous ripple- and cross-bedded well-sorted siltstone with scouring and small load casts at base. X 0.54. C, Interbedded light-gray well-sorted siltstone and dark-gray mudstone of siltstone unit in upper part of cycle. White spots are pyrite mostly concentrated along base of beds. Contorted bedding in middle may be organic disturbance. X 0.58. D, Interbedded light-gray well-sorted siltstone and dark-gray mudstone of siltstone unit in upper part of cycle. Layers with distinct convoluted bedding and streaked-out "ripples." Load cast? in lower right corner and marked scouring of dark-gray mudstone below. X 0.58. E, Interbedded light-gray well-sorted siltstone and dark-gray mudstone of siltstone unit in upper part of cycle. Layers of cross-bedding and convoluted bedding with streaked-out "ripples." Several solitary burrow-casts parallel to bedding and a large cast at left rising vertically. X 0.58.
An orderly asymmetrical repetition of the common rock types, reflected in weathered profiles, is accompanied by a regular vertical variation in color, composition, and sedimentary structures. Approximately 80 short cycles are present in a well-exposed 1,400-foot section of the middle part of the formation in a long road cut on New Jersey Highway 29, 6 to 8 miles south of Frenchtown on the Delaware River (Fig. 2A, 19A). Short sequences of cycles are also well displayed in several large quarries in southeastern Pennsylvania (Fig. 2B, 2C, 2D; Van Houten, 1962, Pl. 1B).
In general form each short cycle consists of lower black shale and platy mudstone and marlstone, and of upper massive mudstone commonly mudcracked on top. There is no evidence of erosion between cycles but the most marked change in rock type occurs between the uppermost argillite and the basal shale or platy mudstone of the succeeding cycle. Observed in detail short cycles are of two rather distinct types, here informally referred to as detrital and chemical varieties (Fig. 6).
Figure 6--Model of detrital and chemical short cycles, showing distribution and qualitative estimates of prevalence of sedimentary structures, selected minerals, and sedimentary environments; data on thickness of cycles. Numbers along left side of chemical cycle indicate units referred to in text.
The lower several feet of most detrital cycles consists of black shale succeeded by platy black to very dark-gray mudstone (laminae 0.5 to 2 mm thick) and interbedded tan-weathering disrupted, dark-gray, calcitic or dolomitic marlstone (laminae 0.1 to 2 mm thick) in alternating layers 1 to 6 cm thick. The fissile and platy beds normally are marked by rare short burrow-casts, but some thin beds of better-sorted siltstone are mottled by very small burrows (Fig. 4A). The upper part of a cycle is tough, massive dolomitic or calcitic silty mudstone and siltstone with indistinct irregular laminae. Only rarely are there distinct laminae 0.2 to 1 mm thick and some of these are graded. Instead, most of these rocks have been churned up on a small to microscopic scale to produce a contorted and disrupted fabric (Fig. 4C, 4D, 4F, 5A, 5B). In addition they commonly contain a 2 to 5-foot lens of rather well-sorted calcitic, feldspathic, laminated siltstone and rare very fine grained sandstone, much like deposits in the lower 500 feet of the formation (Fig. 5C, 5D, 5E).
Minute specks and larger crystals of pyrite are widespread in detrital cycles. Deep impressions of cubes of pyrite as much as 2 mm wide on surfaces of black laminae both above and below suggest growth of the crystals before complete induration of the mud. Coarsely crystalline patches of pyrite as much as 2 cm long occur in siltstone lenses, presumably as diagenetic concentrations in the more porous deposits.
Detrital cycles generally range from 14 to 20 feet in thickness, comprising the thicker short cycles; but they constitute only about one-fourth of the short cycles in the thickest section of the formation in northwestern New Jersey (Fig. 2A). In contrast, throughout the western third of the formation, west of Montgomeryville, Pennsylvania, only detrital cycles are present and these contain more quartz than occurs in cycles in the central part of the basin.
A second, more common type of short cycle differs principally in containing less detrital and more colloidal-chemical sediment. These cycles generally range from 8 to 13 feet in thickness.
The lower part consists of interbedded, commonly disrupted black to very dark-gray, platy, laminated, dolomitic and calcitic mudstone and marlstone (Van Houten, 1962, Pl. 1C) and rare layers and lenses of crystalline carbonate (Fig. 7). The upper part is massive analcime- and carbonate- (predominately dolomite) rich mudstone brecciated on a microscopic scale. Presumably this nonlaminated mudstone resulted from flocculation of a colloidal sediment in the presence of a concentration of cations which produced random orientation of the clay minerals (White, 1961, p. 561). The analcime in these cycles was derived largely from saline water of a closed lake in which the mud itself was laid down. It may have been more nearly diagenetic than syngenetic, having been formed below the surface of the bottom mud, but it was introduced before the rock was lithified or deeply buried.
Many of the platy mudstone layers in the lower part of chemical cycles are composed of regularly alternating black and light-gray, carbonate-rich laminae. These couplets, which range from 0.05 to 0.2 mm in thickness, resemble carbonate- and organic-rich varves in the Eocene Green River lacustrine deposits (Bradley, 1930, p. 95-96). Basal beds of platy gray mudstone in some cycles consist of graded laminae of silt and clay ranging in thickness from 0.2 to 2 mm.
Fossils found in the Lockatong Formation record a fauna conspicuously devoid of large aquatic bottom dwellers. It comprises, instead, mostly remains of elasmobranch, coelocanth, and palaeoniscid fishes, estheriids and ostracodes, plant fragments (cycads, equisitales, ferns, and possibly dasycladacean algae), and indeterminate microscopic spines or setae, as well as coprolites as much as 6 cm long and insect larvae faecal pellets about 2 mm long. In addition, there are traces of several kinds of small reptiles, phytosaurs, and large squat amphibians. There are also rare indistinct large footprints, presumably of large phytosaurs and tracks of small dinosaurs, rare small tetrapod tracks and minute indeterminate "tracks and trails."
Most of the fossils were recovered from the shaly and platy beds in the lower part of cycles, but estheriids occur in abundance in the upper part of detrital cycles as well. Plant remains are much more rare than in most lacustrine deposits, and efforts to recover spores or pollen from the mudstone have yielded very little.
Isolated silty casts of animal burrows 1 mm or less in diameter are scattered through the lower shale and platy mudstone. Rarely, irregular laminae of siltstone 1 to 2 mm thick in the black shale consist almost entirely of minute crumpled casts. In detrital cycles crumpled burrow-casts 1 to 3 mm in diameter are abundant in the upper gray muddy siltstone (Fig. 4D; Van Houten, 1962, Pl. 2C) which commonly is indistinctly mottled (see, Moore and Scruton, 1957, p. 2725-2731). In associated better-sorted coarser siltstone and very fine grained sandstone burrow-casts are rare, and bioturbation by bottom crawlers is preserved locally. Beds of reddish-brown to grayish-red mudstone and siltstone in the uppermost part of the Stockton Formation, in the lower 500 feet of the Lockatong Formation, and in the Brunswick Formation contain abundant burrow-casts as much as 1 cm in diameter. Locally these strata are mottled.
The fact that the fish faunas from the three formations of the Newark Group differ only in detail (Bock, 1959, p. 130-132) implies that the Lockatong deposits, like the Stockton and Brunswick Formations, are of nonmarine origin (Schaeffer, 1952, p. 58).
Most of the fish, amphibians, and reptiles found in the Lockatong Formation were recovered from its more marginal southwestern and northeastern parts, or from its lowest part near the middle. Although this distribution may have been controlled largely by available outcrops very few vertebrate remains have been found in quarries in the upper part of the formation in western New Jersey. According to these data, fossils occur mainly in detrital cycles and there apparently was a paucity of vertebrate life in the central part of the Lockatong lake.
Of all the distinctive sedimentary features of the Lockatong Formation, crumpled shrinkage-crack casts and bedding disturbed by burowing or by hydroplastic disruption are the most ubiquitous and varied, and generally point to slow accumulation of mud. In contrast, ripple-marked strata are rare. A greater thickness of most detrital cycles compared with chemical ones reflects a more rapid rate of detrital deposition.
In both lithic features and fauna the Lockatong Formation shares many characters with the mid-Devonian Caithness Flagstone of Scotland (Crampton, 1914), Mississippian Albert Shale of New Brunswick (Greiner, 1962), Permo-Pennsylvanian Dunkard Group in Pennsylvania, West Virginia, and Ohio (Beerbower, 1961), Triassic Blomiden Formation of Nova Scotia and New Brunswick (Klein, 1962a), Triassic Keuper rocks (Bosworth, 1912, p. 51-116; Elliot, 1961), and Eocene Green River Formation (Bradley, 1931), each largely of lacustrine origin.
Detailed comparison of the Lockatong lacustrine deposits with these formations emphasizes the uniqueness of the chemical cycles and the features they share with the distinctive 5 to 10-foot cycles in the Green River Formation. Lockatong detrital cycles, on the other hand, are more like lacustrine and alluvial Dunkard cyclothems and cyclic deposits in the Papigoe beds and Thurso flagstone of the Caithness Flagstone. In addition, detrital cycles share significant features with some deposits of marine tidal lagoons (Van Straaten, 1954). Nevertheless, the lagoonal deposits differ in possessing more numerous small channel and gully lenses pointing to some current action, abundant load casts, wave ripple marks, and ripple bedding (but only minor current ripple marks), a marine fauna, and shell beds, and abundant drifted plant fragments.
Members of the detrital and chemical short cycles record an asymmetrical sequence of events resulting from expansion and waning that recurred relentlessly during the life of the Lockatong lake. Each cycle began with the slow filling of the basin to form swamps and marshy ponds, and then extensive shallow lakes. During this initial stage mud accumulated in a reducing environment noxious to large bottom dwellers and burrowers. Gradually the inflow increased and the redox potential decreased. Then inflow waned and some cycles ended with exposure of the lake bottom; yet no erosion ensued, thus suggesting but little hiatus between cycles.
Reconstruction of an extensive shallow lake in which only slight differences in elevation of the basin floor could cause widespread differences in conditions of deposition implies that members of a cycle may vary from place to place. Moreover, the paucity of bottom scour in these shallow-water sediments requires that there was no single large lake where wind-driven waves could easily have been generated, or that some deterrent, such as aquatic vegetation, restricted wave action.
In their study of two Pennsylvanian black shales Zangerl and Richardson (1963, p. 117-122) have recently explored the problem of accounting for an absence of disturbance of bottom mud below shallow water. As a solution they have elaborated the role of an algal flotant, a mat of floating vegetation as much as 3 feet thick which protected the mud from wave action. The flotant may also have prevented spores and leaves from reaching the bottom (p. 218-219) while it showered abundant microscopic plant debris on the mud. A flotant may have had a similar role in the Lockatong lake, but there is no direct evidence of its presence. In fact, Lockatong mudstone contains no such abundance of decomposition products of plants as occurs in the Pennsylvanian shales (p. 105-106). Moreover, a flotant could not have been present during oxidizing episodes that produced grayish-red chemical cycles.
The present attempt to account for the short cycles assumes that both detrital and chemical varieties had a common control, that the base of each is correctly identified, and that they accumulated in a lake. In its general setting the thick Newark Group accumulated in a continuously sinking basin supplied by a rising source area to the east or southeast as well as to the northwest. Secular fluctuations imposed on this framework produced the distinctive Lockatong cycles. But no obvious interpretation follows. Not only are there several possible causes of cyclic sedimentation but authors disagree as to which one may have been the fundamental control of a particular cyclic deposit (Weller, 1956; Goodlet, 1959; Wells, 1960; Beerbower, 1961).
Cyclic sedimentation has been attributed to continuous subsidence accompanied by periodic compaction (Van der Heide, 1950), or by shifting of the pattern of sedimentation (Goodlet, 1959; Duff and Walton, 1962), as well as to a more regional tectonic control, either periodic subsidence (Trueman, 1948; Zangerl and Richardson, 1963) or repeated uplift and submergence as envisaged by Weller (1956). In contrast, theories of worldwide control have called upon eustatic rise and fall of sea level (Ham, 1960) or cyclic variation in climate accompanied by compaction in a continuously sinking basin (Brough, 1928; Beerbower, 1961).
My prejudice in favor of climatic control, rather than periodic sinking, stems largely from an assumption that alternations in climate are apt to be more regular and persistent than intermittent subsidence (see, Gilbert, 1895, p. 123, 127). Moreover, they account more reasonably for the sequence of members in the Lockatong short cycles. Admittedly, detrital short cycles alone could be attributed to tectonic control, but the chemical ones point, instead, to a climatic control. Significantly, climatic cycles have recently been advocated by Beerbower (1961) as the basic control of alluvial and lacustrine Dunkard cyclothems of Permo-Pennsylvanian age, Wanless (1963) has favored climatic oscillations and glacial episodes as basic causes of late Paleozoic cyclothems, and Elliot (1961) has ascribed cycles in the Triassic Keuper Series to a climatic control.
A tentative estimate of the duration of a short cycle, based on varves in the lower part, gives an average of 22,000 years, which agrees well with the 21,000-year precession cycle. Support for this interpretation is afforded by similar cycles in the Eocene Green River lacustrine deposits of Wyoming and Colorado. Varve-counts of these 5- to 10-foot cycles indicate an average duration of 21,630 years, suggesting control by the precession cycle (Bradley, 1930, p. 105-106).
The characteristic features of detrital and chemical short cycles vary considerably in detail. Analysis of these variations yields basic data for a paleogeographic reconstruction (see, Duff and Walton, 1962).
The detrital cycles measured range in thickness from 10 to 25 feet (Fig. 6). The 25 to 75 percent class is 14 to 20 feet thick; the median is 17 feet.
Stratification and grain size
Lamination of the lower fissile, black, pyritic mudstone varies considerably. Some of the mudstone with indistinct black films occurs in beds 0.5 to 3 mm thick (Fig. 4A). Very rarely there are minor scours 5 mm deep with tiny streaked-out "ripples" at the bottom. Distinct lamination 0.02 to 1 mm thick varies from a succession of delicately wrinkled black laminae with scattered silt and minute calcite crystals to wrinkled and delicately disturbed films and bundles of films of black mud with sublenticular laminae of silt or very finely crystalline calcite (Fig. 4B, 4E). Thicker bundles of black films commonly recur about 0.4 to 1.5 mm apart. In some beds the calcite and silt occur as sharply defined layers, stringers, and lenses 0.05 to 0.5 mm thick. Only very locally are laminae of black mud distinctly graded.
The pyritic, black shale in the lower part of detrital cycles apparently accumulated in marshy ponds and lakes as did the thin coaly shale (humulite) overlying the coal of some Pennsylvanian cyclothems of Illinois (Zangerl and Richardson, 1963, p. 69-74, 225-226). In contrast, the humulite contains abundant shells and microscopic flaky plant debris, and it is overlain by a transgressive marine facies of gray to black shale that was deposited in marginal lagoons along a deltaic coastal plain (p. 24, 226-227).
Thicker detrital cycles are consistently siltier in the upper part which contains a distinct 2 to 5-foot unit of gray, laminated (laminae commonly 0.2 to 1 mm thick, rarely 3 mm thick), well-sorted siltstone and fine-grained sandstone. Within each unit some of the siltstone and sandstone has irregularly paralled lamination, some has small-scale, ripple- and cross-lamination (Fig. 5C, 5D, SF) and rarely some of the coarsest sandstone has crudely graded beds 1 to 2 mm thick. In some cycles the siltstone unit is scoured broadly a few feet into the darker mudstone below and contains very dark-gray intraformational mud clasts 1 to 10 mm long.
These coarse units mark the episode of maximum inflow, producing repeated scour (Fig. 5A, 5B) and spread of relatively well-sorted silt and very fine grained sand. Normally the coarse unit in the upper silty mudstone does not extend to the top of a cycle, suggesting a decrease in transportation energy toward the end of a cycle.
Thinner detrital cycles commonly begin with platy mudcracked mudstone and dolomitic marlstone, and have no distinct upper siltstone unit; the thinnest ones consist only of mudstone in the upper part, approaching chemical cycles in their sedimentary features. In fact, some cycles about 10 feet thick are intermediate in character between detrital and chemical varieties and contain traces of analcime.
Inorganic--Platy marlstone and mudstone in the lower part of detrital cycles commonly are conspicuously disrupted by shrinkage cracks of several different patterns like those in the platy lower part of chemical cycles.
Silty mudstone in the upper part of detrital cycles is marked by abundant long, slender, crumpled shrinkage-crack casts (Fig. 5A) and very irregular vertical brecciated zones 2 to 5 mm wide that apparently were paths of upward moving water and fluid mud (Fig. 4F). Disrupted bedding surfaces appear "shattered" by a complex of randomly oriented, slender, straight crack-casts rather than by a simple polygonal pattern. Locally, specks of pyrite are more abundant in crackfillings and vaguely outline a mosaic type of intraformational breccia with all the fragments parallel to bedding. Much of this small-scale shrinkage disruption may have occurred under shallow water (see, Van Straaten, 1954, p. 38).
In profile, crumpled casts of silt vary from rare to abundant, and from obvious crackfillings to obscure structures (Fig. 4C, 4F, SA). Some may be crumpled siltstone dikes (Shelton, 1962). Some may fill pockets and passageways formed by escaping gas (Cloud, 1960) or water released during compaction. Without adequate evidence, however, they cannot be distinguished from obscure shrinkage-crack casts or from animal burrow-casts.
Commonly, crumpled structures have been offset laterally (in profile) a few millimeters by shearing parallel to bedding on planes 2 to 15 mm apart (Fig. 4F), similar to slip-layers in fine-grained detrital deposits in the Keuper Series (Elliot, 1961, p. 199-202). Presumably the shearing resulted from local lateral flowage during dewatering and compaction. Vertical displacement of offset crumpled casts points to further compaction after shearing.
In a few detrital cycles bedding surfaces in the upper silty mudstone are marked by shrinkage cracks as much as 5 cm wide that outline polygons as much as 35 cm wide, like those "suncracks" in mudstone of the Caithness Flagstone (Crampton, 1914, Pl. VI, 2).
Many of the siltstone and fine-grained sandstone units are marked by layers of conspicuous irregular bedding (Fig. 5C, 5D, 5E), and by deformed load casts of silt protruding into underlying silty mudstone (Fig. 5C). Minor scouring and slumping or flowage over a distance measured in millimeters are common, and delicately convoluted beds with folds about 2 to 5 mm high are associated with streak-out "ripples" of the underlying mudstone (Fig. 5E). These features probably resulted from frictional drag of silt- and sand-laden currents on soft but cohesive mud over which the sediment was spread (Sanders, 1960), and in part, perhaps, from vertical loading (McKee and others, 1962). As pointed out by Sanders (p. 414), convoluted laminae in other deposits, such as the Keuper Series (Elliot, 1961, p. 202-204), are characteristically limited to sediments of silt and very fine grained sand sizes.
Rarely, anastomosing sharp-crested, slightly asymmetrical current ripplemarks with secondary crests are preserved on the surface of 1 to 2 cm-thick beds of well-sorted siltstone that are marked by conspicuous ripple-bedding, intraformational mud-chips, and scouring at the base. The main ripples have wave lengths of about 3 to 3.5 cm and amplitudes of 3 to 4 mm (ripple index = 8 to 12). Secondary crests are displaced somewhat toward the downcurrent side of each ripple.
In a few of the siltstone and sandstone units in the upper part of detrital cycles, as well as in similar rock types in the lowest part of the formation, lamination is disrupted by irregular vertical zones of brecciation as much as 2 cm wide and 8 cm high. At their bases these structures join shrinkage-crack casts, and like smaller ones in the silty mudstone (Fig. 4F), probably resulted from dilatation of sand and silt by gas, water, and fluid mud moving upward along shrinkage cracks during compaction.
The assemblage of structures in the siltstone and sandstone units is remarkably like that in cyclothemic Carboniferous deposits in England (Greensmith, 1956) and the regressive Dakota deposits in North Dakota (Shelton, 1962).
Organic--Although much of the crumpled fabric of the silty mudstone is the result of inorganic disturbance some irregularly contorted structures are animal burrow-casts 1 to 3 mm in diameter that seldom have septal laminae and can be traced only a few centimeters (Fig. 4C). In siltier mudstone, burrow-casts crumpled by compaction are more abundant (Fig. 4D, 5B), as they are, for example, in the siltier Baggy Beds of the Old Red Sandstone (Goldring, 1962, p. 242, 248). Locally, the siltstone is indistinctly mottled. In many cycles, muddy siltstone with abundant burrowing (bioturbation or mottling) is interbedded with units of parallel- and cross-laminated well-sorted siltstone and fine-grained sandstone that slightly truncate the extensively burrowed beds (Fig. 5B). Apparently the spreading sand sheet killed off the abundant mud-dwelling burrowers, for the well-sorted deposits, in contrast, are penetrated only by solitary burrow-casts as much as 5 mm in diameter and commonly with septal laminae (Fig. 5E). These features record minor variations in scour and sedimentation during accumulation of the coarser units.
Locally, several inches of the laminated siltstone and very fine-grained sandstone unit has an irregularly and intricately contorted fabric of very lobate and crumpled folds as much as 1.5 mm high. The complex pattern suggests disturbance by bottom crawlers rather than by an inorganic agent.
As observed in other studies (Van Straaten, 1954, p. 29; Greensmith, 1956, p. 352; Goldring, 1962, p. 248; Middlemiss, 1962) muddy silt to very fine grained sand is the favorable range of sediment size for abundant burrowers in lakes, lagoons, and tidal flats, and extensive bioturbation or mottling points to a relative slow rate of deposition.
Many of the prominent features of the upper part of detrital cycles, and especially those in the coarser siltstone and very fine grained sandstone units, resemble features in the Devonian Psammites du Condroz which Van Straaten (1954, p. 43-44) believes accumulated in a marine lagoon that was covered by water most of the time.
Thickness and grain size
The measured chemical cycles range in thickness from 4.5 to 17 feet. The 25 to 75 percent class is 8 to 13 feet thick, and the median is 10 feet. The basal black shale that aids in demarcating detrital cycles in outcrop is present only in the thicker chemical cycles. Thinner ones begin with platy mudstone and marlstone and are less evident. The thinnest chemical cycles are composed mostly of grayish-red mudstone.
Several analcime-rich sequences between distinct cycle bases range from 18 to 25 feet thick. A vague 6 to 12-inch zone of shrinkage-cracking in the middle of some of these anomalously thick successions suggests that each may comprise two poorly demarcated cycles. If correctly interpreted, these long chemical sequences apparently resulted from relatively little change in conditions of deposition during two successive climatic cycles.
Almost every chemical cycle consists only of silty mudstone and colloidal-chemical sediments. In the upper part of a few, however, there is a 1 to 2-foot unit of delicately laminated (laminae generally 0.3 to 1.5 mm thick) calcareous siltstone with isolated burrow-casts. Locally the thinnest laminae are graded; more commonly all are sublenticular and disturbed. Rarely, part of the siltstone unit is mottled.
Sedimentary features of gray cycles
Inasmuch as chemical cycles are the distinctive ones and reflect more clearly the unique conditions of deposition of the Lockatong Formation, variations within them will be described in detail in a sequence of rather arbitrary units (Fig. 6) of each cycle.
Unit 1--The lower several feet of a cycle consists of black platy carbonate-bearing mudstone with thin layers, lenses, and scattered patches of coarsely crystalline carbonates and rare short burrow-casts less than 1 mm in diameter. Deep impressions of carbonate and pyrite crystals as much as 3 mm wide on the surface of black mudstone above and below indicate that the crystals grew before complete induration of the mud. Commonly, bedding surfaces are also marked with small indeterminate "tracks and trails."
Bedding is 1 to 10 mm thick, with laminae normally 0.2 to 1 mm thick, but where the lowest beds are undisturbed by shrinkage-cracking they are commonly composed of varves 0.05 to 0.2 mm thick.
In several cycles with undisturbed lower beds, the initial deposit is a 2 to 7 mm-thick layer or lens of coarsely crystalline pyrite overlain by a 2 to 7 mm-thick layer of crystalline calcite and subordinate dolomite with interbedded black dolomitic mudstone containing abundant scattered crystals of dolomite and pyrite. In other cycles pyrite is less common and the initial deposit consists of 0.5 to 2 cm-thick, pinching and swelling layers of coarsely crystalline calcite and subordinate dolomite with a distinct pseudopellet texture which are interbedded with irregular layers of black dolomitic mudstone 2 to 4 mm thick (Fig. 7).
Figure 7--Lower platy black mudstone and gray marlstone of gray chemical short cycle, showing various types of disruption by shrinkage. (1) Very dark-gray analcime-rich mudstone at top of preceding cycle. (2) Very light-gray crystalline calcite with layers of dark-gray dolomitic mudstone. (3) Light-gray, extensively disrupted dolomitic marlstone with scattered small patches of calcite. (4) Medium-gray brecciated dolomitic marlstone with shrinkage-crack casts of gray calcitic and dolomitic or of black, dolomitic mudstone, offset laterally by compactional shearing. (5) Very dark gray to black dolomitic mudstone with medium-gray, dolomitic crumpled shrinkage-crack casts. (6) Black dolomitic mudstone with densely scattered crystals of calcite and medium-gray, dolomitic shrinkage-crack casts. Vertical zone of brecciation by upward-moving fluid in lower block. Specimen is 81 cm high.
As in detrital cycles, dolomite generally predominates in fine-grained mudstone and aphanitic marlstone. In contrast, the layers and lenses of coarsely crystalline carbonate commonly consist mainly of calcite. Where coarsely crystalline dolomite predominates a minor amount of analcime is present. Moreover, scattered crystalline patches suggestive of minute salt casts contain only dolomite and analcime, and these patches are more common in the upper part of a cycle.
The lower part of many chemical cycles shows a distinct pattern of 1 to 6 cm-thick layers of black to very dark-gray mudstone alternating with 1 to 6 cm-thick layers of tan-weathering gray marlstone and aphanitic dolomite, locally disrupted by "pull-aparts" (Fig. 8). More commonly the alternating layers have been extensively disrupted by shrinkage cracks with distinctly different surface pattern, depth of penetration, and kinds of crumpled casts (Fig. 7). The surfaces of some layers of black mudstone are marked by a delicate tracery of incomplete polygons no more than 1.5 cm wide. Where layers and lenses of crystalline carbonates are common 0.5 to 1 cm-thick layers of black, dolomitic mudstone speckled with calcite patches show distinct shrinkage polygons a maximum of 6 cm wide and outlined by stout dolomitic crack-casts 1 to 4 mm wide (Fig. 7 (6), 9A). Layers of black mudstone only a few millimeters thick between crystalline calcite layers have a more randomly "shattered" pattern of slender cracks (Fig. 7 (2), 9B).
Figure 8--Small-scale pull-aparts in beds of medium-gray, dolomite-rich marlstone and aphanitic dolomite set in matrix of dark-gray, dolomitic mudstone, in upper part of unit 1 of gray chemical cycle. Shrinkage produced by dewatering after burial.
Figure 9--Bedding plane pattern of shrinkage cracks in black dolomitic mudstone in lower few feet of gray chemical cycle. Casts are tan-weathering aphanitic dolomite and marlstone. A, Layers 0.5 to 1 cm thick with scattered small patches of calcite. Specimen is 45 cm wide. B, Layers 1 to 2 mm thick between layers of crystalline calcite. Specimen is 35 cm wide.
Layers of black mudstone more than 1 cm thick normally have complexly cracked surfaces and shrinkage-crack casts of aphanitic dolomite and marlstone that are thick and crumpled and anastomose downward, as in gray beds in grayish-red cycles (Fig. 17). In contrast, associated layers of brecciated marlstone have slender, slightly crumpled crack-casts of black mudstone or gray dolomite. Some of the abundant cracking in unit 1 probably resulted from subaerial shrinkage, but the fact that the cracks in many layers were filled from above and below suggests that much of the shrinkage resulted from syneresis under shallow water and after burial.
As in detrital cycles, crumpled crack-casts in chemical cycles commonly have been offset a few millimeters by shearing parallel to bedding. Moreover, thicker beds of both mudstone and marlstone in this lower sequence are disrupted by irregular vertical zones of brecciation as much as several centimeters wide which join shrinkage-crack casts at their base (Fig. 7).
In several chemical cycles, the basal few inches contain a 2 to 4 cm-thick layer of black, laminated dolomitic mudstone broken by conspicuous shrinkage cracks 1 to 2.5 cm wide. These outline simple polygons as much as 15 cm wide (Fig. 10). The cracks are filled with thick, broken casts of aphanitic dolomite. During compaction the more rigid dolomitic casts were flattened, but they were compressed less than the cracked layer of black mud. Consequently, compaction "folds" developed in the beds above and below the casts. The pattern of the "folds" now reflects the polygonal pattern of the shrinkage cracks in the black mudstone.
Figure 10--Shrinkage-crack pattern and casts deformed during compaction, in basal part of short cycles. A, Surface (above) of platy black mudstone 3 cm above base of chemical cycle, and profile (below) of basal 6 cm of cycle showing thick broken and compressed crack-casts of aphanitic medium-gray dolomite and compaction "folds" developed in beds above and below. Specimen is 35 cm wide. Bed 1. Mediumgray dolomitic silty mudstone. Bed 2. Black dolomitic mudstone with some analcime, containing broken casts of aphanitic dolomite with trace of analcime. Bed 3. Black dolomitic mudstone with stringers of finely crystalline dolomite and analcime. B, Profile of basal 4 cm of detrital cycle, showing more slender crumpled crack-casts of aphanitic medium-gray dolomite which was locally injected into surrounding mudstone during compaction. Bed 1. Medium-gray, calcitic silty mudstone with trace of dolomite. Bed 2. Black mudstone containing crumpled casts of aphanitic dolomite. Bed 3. Black, dolomitic mudstone with trace of calcite.
A similar feature occurs in the carbonate-rich basal part of a thin, extensively cracked and brecciated cycle in a sequence of grayish-red cycles. Here a layer of dark-gray dolomitic mudstone is broken by wide cracks that outline irregular polygons as much as 15 cm wide (Fig. 11). These cracks are filled with unique flattened casts of aphanitic dolomite about 2.5 cm thick. Each cast is irregularly oval to lobate and shows internal plastic flowage and lateral injection into the enclosing mudstone. Layers of crystalline calcite 1 to 2 cm thick above and below the mudstone and dolomitic lobate casts apparently served as buttresses between which the plastic dolomitic crack-filling was squeezed during compaction of the dark-gray mud.
Figure 11--Irregular shrinkage-crack polygons and casts deformed during compaction, in basal part of chemical cycle. Surface (above) of dark-gray mudstone about 4 cm above basal bed of crystalline calcite, and profile (below) showing thick, plastically deformed casts of aphanitic dolomite compressed between layers of coarsely crystalline calcite. Specimen is 35 cm wide. Bed 1. Medium-gray, dolomitic marlstone with densely scattered calcite crystals in lower part. Bed 2. Coarsely crystalline calcite. Bed 3. Dark-gray, dolomitic mudstone with lobate casts of aphanitic dolomite. Bed 4. Coarsely crystalline calcite underlain by thin, discontinuous layer of black mudstone.
The preservation of delicate seasonal laminae and of undisturbed dead fish in pyritic black mudstone of unit 1 in some chemical cycles points to accumulation on a stagnant bottom, probably in a warm-climate, thermally stratified lake with cooler bottom water low in oxygen, and with no active benthic fauna, waves, or currents. Although these very features have been cited as evidence of deep water, such an interpretation is not required. Instead, shallow water with deposition below wave-base, as elaborated for other deposits of black shale (Richter, 1931; Engels, 1957; Beerbower, 1960; Conant and Swanson, 1961; Zangerl and Richardson, 1963) accounts satisfactorily for the Lockatong features, thus suggesting a lake with depths measured in a few tens of feet at most, and perhaps similar to shallow alkaline lakes with "bottomless" mud in Kenya (Jenkins, 1932, p. 546-547). As pointed out previously (p. 12-13) this interpretation does introduce the problem of accounting for no bottom stir-up in an extensive shallow lake.
Additional problems concerning unit 1 remain. It is not clear why the Lockatong lake contained such an abundance of calcium and magnesium carbonate at the beginning of a chemical cycle, or why a lake that produced beds of crystalline limestone did not support abundant organic activity that would have yielded deposits rich in organic carbon and calcareous algae. It is reasonable to expect that algae did participate in concentrating the carbonates, but there is no direct evidence that they did.
In the upper part of the platy unit 1, thicker layers of tan-weathering marlstone predominate, and these are extensively broken by slender, irregular shrinkage cracks now filled with dark-gray mudstone that was only slightly crumpled during compaction (Fig. 12A). The small-scale fragmented fabric of some of these thicker layers and their relation to overlying deposits indicates that they were disrupted after burial, during dewatering and compaction.
Figure 12--Sedimentary features of middle part of gray chemical short cycle. A, Tan-weathering mediumgray marlstone in upper part of unit 1 brecciated by shrinkage cracks. X 0.66. 1. Profile shoWing shrinkage cracks only slightly crumpled. Overlying dark-gray, dolomitic mudstone speckled with small patches of dolomite and analcime. 2. Bedding surface showing shattered pattern of shrinkage cracking. B, Very dark-gray, analcime-bearing dolomitic mudstone with beds of tan-weathering medium-gray dolomitic marlstone in unit 2. Beds of marlstone in lower part of outcrop are fragmented by shrinkage cracks filled with dark-gray mudstone, but fragments are not widely scattered. Lower beds are 4 to 6 cm thick. Tape is 12 inches long.
Only in the most analcime-rich cycles are there traces of analcime in the lower part of unit 1. Locally, minor amounts of crystalline analcime mixed with crystalline dolomite occur in lenses as much as 5 mm thick, and in densely scattered small crystalline patches in black mudstone layers (Fig. 12A).
Unit 2--Above the distinctly platy, cracked sequence there are several feet of extensively disrupted 2 to 8 cm-thick layers of tan-weathering marlstone (Fig. 12; Van Houten, 1962, Pl. 1C) embedded in very dark-gray analcime-bearing mudstone. The pattern of fragmentation is similar to that of a flow-type, penecontemporaneous breccia developed in mudstone of the Keuper Series (Elliot, 1961, p. 205-206). These dolomite-rich layers apparently hardened and shrank more rapidly after burial than the surrounding dark-gray colloidal mud (see, Boswell, 1961, p. 67). Some of the disruption may have begun as syneresis cracks, then fragmentation by post-burial shrinkage upset the physical system so that the surrounding black colloidal gel reverted thixotropically to a sol. With loss of support the firmer dolomitic fragments drifted apart and colloidal mud in the sol state was forced around the firmer pieces.
This interpretation is consistent with Boswell's (1961, p. 50) report that lime added to clay-rich sediments reduces the thixotropy, that the plasticity of lime-rich mud is low, and that they compact more rapidly and fracture with ease if disturbed in the course of diagensis (p. 86, 98, 102).
In the middle of some chemical cycles fragmented marlstone layers have been widely dispersed, yet a vague continuity of layering may be preserved by an alignment of angular pieces, or the fragments may be scattered upward through the overlying analcime-bearing mudstone in a large-scale "explosive" pattern (Fig. 13A; Van Houten, 1962, Pl. 1D) that suggests forceful disruption of layers of quick-setting dolomite-rich mud by localized upward moving muddy sol and water released during compaction.
Figure 13--Sedimentary features of middle part of chemical short cycles. A, Disrupted tan-weathering medium-gray, dolomitic marlstone layers scattered widely through very dark-gray analcime-bearing mudstone of gray cycle. Pattern of dispersal suggests forceful disruption after burial by upward-moving water and fluid mud. Small fragments of marlstone thinly dispersed through dark-gray mudstone above and below central feature. Tape is 6 inches long. B, Greenish-gray weathering, medium-gray mudstone in grayish-red cycle. Upper part extensively brecciated and immersed in grayish-red mudstone. Lower part disrupted by long, slender crinkled and injected shrinkage-crack casts of grayish-red mudstone. Many casts are offset laterally by compaction shearing. Crinkled vein of hematite and analcime in upper left. X 0.7.
The analcime-bearing mudstone in unit 2 commonly is brecciated and disturbed on a small scale as a result of physical disruption of colloidal-chemical mud. A minor part of the fabric may be bioturbation, but no certain evidence of burrowing has been found. In a few cycles a fabric of minute shredded and wispy fragments suggests that an intricate churning up of fluid mud with scattered firmer muddy films has occurred. The pattern resembles that of some of the complexly disturbed, indistinctly laminated mudstone in detrital cycles, but has no structures like burrow-casts.
The markedly different fabrics and patterns produced by disrupted layers in the middle of most chemical cycles apparently resulted mainly from differing thicknesses and proportions of firmer, quick-setting dolomite-rich beds and colloidal mud. It is of interest to note that they are not dependent upon the presence of montmorillonite which possesses remarkable thixotropic properties.
In the middle and upper part of some chemical cycles there are conspicuous patterns of irregular, white crystalline, crudely lozenge-shaped patches of analcime and dolomite as much as 7 mm long. Smaller patches occur inside indistinctly outlined lozenge-shaped areas of dark-gray mudstone. Some are densely scattered in zones 2 to 4 cm thick parallel to bedding; some occur in crudely radiating patterns as much as 10 cm in diameter. Others are distributed in irregular downward and outward diverging stringers that form sprays in a zone as much as 20 cm thick. The shape and distribution of these large crystalline patches of analcime and dolomite suggest that they may be pseudomorphs after gypsum or glauberite (Hawkins, 1914, p. 163-164; Wherry, 1916; Schaller, 1932, Pl. 1) that grew in soft and drying mud during low-water stages of chemical cycles. Once trapped, the soluble salts presumably were isolated from resolution and the ions migrated downward to places of lower concentration.
In the brecciated, dolomite-rich unit 2 of analcime-rich gray and grayish-red cycles in the upper part of the formation long crinkled shrinkage-crack casts are conspicuously offset or deflected along shear planes or thin laminae of sheared matrix parallel to bedding and about 2 to 5 cm apart (Fig. 13B). In some of these cycles, where seen on a large joint face, the shear planes have been deformed by plastic flow into a secondary structural pattern of broad upward-concaved arches (Fig. 14) ranging from 15 to 30 cm high and with wave lengths of 0.5 to 1 meter. In anyone cycle there is only one zone of uparched shear planes and all arches are about the same height.
Figure 14--Secondary structural pattern of upward-concaved planes and thin zones of shearing in dolomitic and analcime-bearing mudstone in middle part of chemical cycle. A, Complete pattern of arched shear planes showing abundant crumpled shrinkage cracks offset along shear planes, and distinctly brecciated, somewhat more dolomitic basal layer involved in structure. Right limb steeper and overriden. Dashed rectangle encloses area of B. B, Apex of another arched structure, equivalent to enclosed area of A. Right limb steeper and overriden.
The arcuate shearing ends abruptly at the base of the structure with a distinctly fragmented horizontal layer bounded by horizontal shear planes along which most of the plastic flow took place. In the lower central part of each structure the crests are lower and the rock in intensely brecciated, suggesting flow of material inward and upward. At the apex of the intersecting arcs the crests are sharp; one limb commonly overrides the other a few centimeters; the overriding is in opposite directions on different crests in a cycle, and the slope of the shear planes is a little steeper on the overridden limb. Above the crest the sheared and brecciated layers are vaguely outlined as the arcuate pattern fades out upward. Because of the limited nature of Lockatong outcrops there is little information about the third dimension of these structures. From available observations they appear to be crudely conical structures. Apparently the cones formed in firm but plastic mud during dewatering and compaction of each cycle by the uparching of horizontal shear planes by inward and upward localized flowage concentrated on the basal planes of shearing.
The following sequence of nearly contemporaneous post-depositional events in analcime-rich cycles is suggested by the observed relationships:
Unit 3--The upper part of most chemical cycles is characterized by medium-gray to dark brownish-gray, analcime- and carbonate-rich mudstone with abundant tiny rhombs of dolomite and small patches of analcime. On a microscopic scale the rock is extensively disrupted and locally contains very small, thinly dispersed fragments of marlstone (Fig. 13A). Irregularly disrupted layers of tan-weathering marlstone, like that in unit 2, recur about every 3 to 4 feet in the gray, massive mudstone. In a few chemical cycles there is a 1 to 2-foot bed of siltstone with delicately disturbed laminae, some of which are graded, and with only rare small burrow-casts.
The distinctive microscopic fabric of only slightly displaced fragments suggests small-scale brecciation of a flocculated colloidal mud by dewatering after burial. The shape and size of the microbreccia fragments differ in detail from one cycle to another. In a few cycles the fragments are barely discernible; in others they are wispy and shredded or irregularly blocky (Fig. 15A, 15B, 15C). As yet no trend in these differences is apparent, but they probably reflect variations in the colloidal and chemical environment of deposition, such as varying concentrations and kinds of cations present (White, 1961, p. 569).
In addition to its distinctive microbrecciation, the upper massive mudstone has a fabric of indistinct, delicate and discontinuous crinkled cracks partially and irregularly filled with crystalline dolomite (rarely calcite) and analcime forming patches as much as 8 mm long. The pattern of delicate fissure-filling is like that of syneresis cracks in mud where the process of shrinkage continued until little curds or islands of mud formed with cracks on all sides (White, 1961, Pl. 2).
In detail there are several distinctive associations of minerals in the patches. In some cycles minute dolomite rhombs are scattered abundantly throughout the analcime-rich mudstone, and analcime is the principal mineral in large irregular patches with lobate outlines. In other cycles analcime fills numerous thin discontinuous cracks and coarsely crystalline dolomite occurs in large patches. Less commonly small patches of analcime and crystals of dolomite are arranged in vague trends with no discernible fissures. Analcime and dolomite also occur as scattered lozenge-shaped pseudomorphs.
In addition to analcime and dolomite, the following minerals fill patches in unmetamorphosed mudstone: calcite, albite, chlorite, epidote, quartz, and amphibole.
The uppermost 2 to 8 cm of a cycle commonly is very dark-gray above gray mudstone and the top surface is mudcracked. The darker color apparently resulted from downward penetration of strong reducing effects prevailing at the beginning of the succeeding cycle. A similar situation prevailed at the top of Dunkard cycles (Beerbower, 1961, p. 1035).
Sedimentary features of grayish-red cycles
Some of the chemical cycles in the upper part of the Lockatong Formation are grayish-red in the upper part or grayish-red to grayish-red purple throughout. In the succession of cycles these occur in the most analcimerich intervals (Fig. 2) and apparently grade laterally into tongues of reddish-brown Brunswick mudstone. In the grayish-red cycles, greenish-gray weathering, light- to medium-gray, slightly dolomitic mudstone, normally with considerable analcime, proxies for the variously disrupted tan-weathering dolomitic marlstone in gray cycles. The massive grayish-red mudstone also contains more analcime and less dolomite than massive mudstone in gray cycles.
In cycles that are grayish-red in the upper part only, the color change occurs in the middle extensively brecciated unit where small greenish-gray weathering fragments of mudstone are scattered thinly through dark grayish-red massive mudstone.
As in some analcime-rich gray cycles, an arcuate pattern of shear planes has been imposed on the middle part of grayish-red cycles. It is conspicuously recorded by brecciated greenish-gray weathering beds with crinkled crack-casts cut by arcuate grayish-red zones of shearing 0.5 to 1 mm thick.
In a few cycles veins filled with specular hematite and analcime are as much as 4 feet long and 1 to 4 mm wide, and cut diagonally across the bedding (Fig. 13B). In detail, the veins are crinkled in both vertical and horizontal planes, and in profile are deflected or offset along planes or zones of horizontal shearing.
The upper massive analcime-rich mudstone commonly is grayish-red to dark reddish-brown, shows no disruption by shrinkage cracks, has only a vague fabric of microbrecciation, and contains abundant minute rhombs, clusters, and stringers of dolomite and analcime (Fig. 15F). Some are scattered lozenge-shaped pseudomorphs after gypsum, or glauberite? as much as 4 mm long. Locally, concentrations of analcime and dolomite crystals occur in crudely radiating patterns through an area about 10 cm in diameter, suggesting replacement of a salt that crystallized in a radiating pattern in soft mud. Similar rosettes of slender lozenge-shaped calcite pseudomorphs as much as 3 cm long occur in the Brunswick mudstone.
Figure 15--Photomicrographs of massive analcime-rich mudstone of chemical cycles. A, Dark-gray mudstone with shredded fabric of microbrecciation and distinctive pattern of short trains of rhombs, lozenges, and ovals of dolomite (mostly 0.2 to 0.4 mm in diameter). Trains and darkest shreds outline vague stratification from upper right to lower left of photograph. Many small irregular patches are analcime. X 3.5. B, Dark-gray mudstone with colloform fabric. Large irregular vugs (0.5 to 1 mm long) filled with analcime, dolomite and muscovite? Slender crumpled shrinkage cracks and very small patches filled mainly with analcime. X 3.5. C, Dark-gray mudstone with shredded to blocky fabric of microbrecciation. Irregular vugs (0.5 to 1.4 mm long) filled with analcime, dolomite, and muscovite? Crumpled fabric near right edge may be gas passageway. X 3.5. D, Medium-gray, homogeneous mudstone fragmented by crumpled shrinkage cracks filled with grayish-red mudstone and specular hematite, and analcime and rare dolomite. X 3.5. E, Brecciated, medium-gray mudstone immersed in grayish-red mudstone. Crumpled shrinkage cracks are filled with long patches of analcime and dolomite. X 3.5. F, Reddish-brown mudstone speckled with minute irregular patches of analcime in discontinuous shrinkage cracks around curds of mudstone. Large lozenge-shaped pseudomorphs of dolomite after gypsum or glauberite. X 3.9.
The uppermost several centimeters of many of the grayish-red cycles, as in the gray ones, show the effect of reducing conditions imposed by the succeeding cycle, and the top bedding surface is mudcracked (Fig. 16).
Figure 16--Top surface of grayish-red chemical cycle showing pattern of shrinkage cracks; and profile of uppermost 3.5 cm showing crumpled and horizontally sheared crack-casts. Vaguely brecciated light- to medium-gray, analcime-rich mudstone (1) grading down into massive grayish-red analcime-rich mudstone (2). Gray color produced by downward penetration of reducing conditions imposed by initial stage of succeeding cycle. Casts are dark-gray, dolomitic mudstone. Specimen is 80 cm wide.
Cycles that are entirely grayish-red or grayish-red purple comprise some of the thinnest of the chemical cycles and are only vaguely demarcated in outcrop. Greenish-gray weathering mudstone at the base of some of these cycles occurs in several layers 5 to 10 mm thick. Most of the layers are broken into angular fragments by shrinkage cracks filled with grayish-red mudstone (Fig. 15D), and commonly show a mosaic type of intraformational brecciation with flat, flakelike fragments oriented parallel to bedding.
In the middle and upper part of some thin grayish-red cycles 15 to 25 cm-thick layers of greenish-gray weathering mudstone are extensively brecciated into large fragments immersed in grayish-red mudstone. In other cycles 5 to 15 cm-thick layers of gray mudstone are brecciated on a small scale (Fig. 15E) and disrupted by many long, slender, intricately crumpled and delicately ramifying shrinkage cracks apparently produced by syneresis (Fig. 13B). The crack-filling was locally injected into the matrix and was laterally displaced by horizontal shearing during compaction.
In contrast to the delicately brecciated features, some layers of gray mudstone as much as 8 cm thick are broken by stout cracks as much as 3 cm wide. These outline large irregular polygons (Fig. 17). The unusually thick crack-casts of grayish-red mudstone anastomose downward through brecciated gray mudstone and are joined by short, slender, randomly oriented and crinkled casts that are laterally offset or deflected by compaction shearing. Their bedding-plane traces form a complex pattern of thin cracks within larger polygons.
Figure 17--Surface pattern of stout shrinkage-crack casts and profile of 5 to 8 cm-thick layer of gray mudstone at top of thin grayish-red chemical cycle, showing unusually thick and anastomosing casts of grayish-red mudstone that were crumpled, sheared, and injected laterally during compaction. Specimen is 60 cm wide.
The fact that cracks in gray beds are filled with grayish-red mudstone but cracks in grayish-red beds are never filled with gray mudstone suggests that the gray beds accumulated subaqueously in a weakly oxidizing or reducing environment and were readily disrupted by syneresis or by shrinkage during exposure on mudflats on which overlying grayish-red mud accumulated. Continued cracking of gray beds after burial permitted squeezing of fluid grayish-red mud into intricate syneresis cracks during compaction.
In thin grayish-red cycles the analcime-rich mudstone commonly is blackish red to very dusky red. Beds conspicuously speckled with large white patches of analcime and dolomite, are interbedded with completely aphanitic layers as much as several millimeters thick that are disrupted by intricate patterns of shrinkage cracks or by mosaic type of intraformational brecciation (Fig. 18). Many of the white patches, which may be as much as 8 mm long, are concentrated in shrinkage cracks or between mosaic flakes.
Figure 18--Pattern of shrinkage cracks A, and mosaic type intraformational breccia; B, in blackish-red to very dusky-red analcime-rich aphanitic mudstone in grayish-red chemical cycle in uppermost part of Lockatong Formation. Patches of analcime and dolomite as much as 8 mm long are concentrated in blackish-red mudstone filling cracks and spaces between mosaic flakes.
In contrast to the abundant evidence of post-burial shrinkage and disruption in gray chemical cycles, most of the structures in grayish-red cycles suggest repeated cracking when the mud stood a little above water-level during low stages of the lake. In fact, many of the patterns are suggestive of large playa cracks crossed by later less regular ones (Longwell, 1928, Fig. 6). After burial, crackfilling mud in a very fluid state apparently was squeezed along lateral passageways opened as the casts were crumpled during compaction.
Despite the abundance and variety of subaerial cracking recorded in the grayish-red cycles, they need not have been out of water most of the time. Accumulation of lacustrine mud may have persisted for long intervals, probably measured in hundreds of years, to be interrupted by brief episodes of exposure and deep cracking that may have been but one season long. Accordingly, these subaerial episodes were brief intense reversals in the long sequence of aggradation under water.
(1) In common with other lacustrine deposits the Lockatong Formation is characterized by laterally persistent units, thin-bedding, varving, local graded bedding, disturbed bedding, small-scale cross- and ripple-bedding, subaerial shrinkage cracking, abundant carbonates, and salt casts (Klein, 1962b). Animal burrow-casts are common in the Lockatong detrital cycles; syneresis disruption predominates in chemical cycles where analcime and dolomite are concentrated.
(2) In contrast to other lacustrine deposits, the Lockatong Formation is marked by an unusual abundance of small-scale disturbed bedding as well as a paucity of fossils, algal structures, ripple-marked bedding planes, oolites, and irregular channel lenses. Among minerals there is a notable scarcity of quartz, potash feldspars, and montmorillonite, and no concentration of salts.
(3) Accumulation of the Newark Group took place in a continuously sinking basin surrounded by uplands that supplied a thick wedge of nonmarine sediments. During the course of aggradation of the Triassic basin in New Jersey and eastern Pennsylvania fluvial deposition of the Stockton arkose and mudstone ended with ponding of the longitudinal drainage of the basin. At present there is no direct evidence as to the direction of flow of the main stream or the specific cause of damming. Initial deposits of the developing new environment were ripple- and cross-laminated silt and very fine sand and mud spread across broad flats where they were mudcracked and reworked by abundant burrowers.
With the establishment of a long lake, uniform lacustrine conditions prevailed and these varied only within narrow limits for several million years (Van Houten, 1962, Table 1). Most of the sediment accumulated in shallow, thermally stratified water, below wave base, with no bottom turbulence. Interpretation of these as shallow-water deposits implies that their total thickness provides an approximate measure of the amount of subsidence of the basin.
In this setting cyclic variations in climate exerted a major control on sedimentation. A warm climate, as recorded by floras from Triassic deposits of eastern North America, probably induced deep weathering in the source areas, thus providing abundant free ferric oxide and silica as well as rapid evaporation which aided the concentration of cations in the lake. Assuming that the short cycles were produced by the 21,OOO-year precession cycle, the following reconstruction is proposed.
Chemical cycles accumulated when the lake had no outlet and chemicals from a deeply weathered source area were concentrated and precipitated in the lake. A cycle began with increasing rainfall and inflow of fresh water. As the lake level rose, laminated carbonate-rich black mud and layers of pure carbonate and pyrite were deposited on the stagnant bottom, but there is no direct evidence of precipitation of carbonates by algae. Through the course of a short cycle, rainfall began to wane; the water perhaps became somewhat shallower and less stagnant. Deposition of uniform colloidal clay predominated as the continued precipitation of carbonates left the water enriched in sodium. In this way analcime or its precursor accumulated in the colloidal mud.
In the late dry stage of many of the chemical cycles, and throughout the course of some, grayish-red mud was preserved in a dry oxidizing environment that produced a maximum concentration of sodium. At such time the size of the lake probably was reduced, leaving broad mudflats with pools of noxious brine from which scattered crystals of salt (probably gypsum or glauberite) were precipitated in the mud. In the final stage of minimum rainfall of many chemical cycles, the lake basin was covered by vast swamps, but during drier episodes the lake bottom and mudflat deposits were exposed to suncracking in the final stage and salts crystallized locally in the mudcracks. Once formed, the recently deposited cycle underwent relatively rapid compaction which accentuated lowering of the basin floor by continuous sinking. During early compaction upward-moving water probably transferred some sodium, silica, and carbonates to the upper part of the cycle where they were deposited in shrinkage cracks and salt molds. Such an upward migration apparently occurred before deposition of the analcime-lean basal beds of the overlying cycles that was initiated by renewed inflow of fresh water.
Detrital cycles resulted from a similar cyclic increase and decrease in rainfall, but they accumulated during moister episodes that maintained a through-flowing drainage which carried most of the soluble material to the sea. Only during the stage of maximum inflow were silt and very fine sand spread as a prograding facies across the lake basin, accumulating above wavebase in a setting of repeated gentle scour and fill.
Throughout the history of the Lockatong lake (Fig. 19A) deposition repeatedly reverted to a Stockton-like "deltaic" facies whenever moister phases of long climatic cycles generated through drainage and produced detrital cycles. During drier phases of the long cycles, especially in the later stage of the lake, grayish-red chemical cycles were induced by conditions that anticipated the broad expanse of oxidizing mudflats on which reddish-brown Brunswick mud accumulated.
Figure 19--A, Generalized stratigraphic section of uppermost Stockton, Lockatong, and Brunswick Formations. Black-gray units; white-grayish-red to reddish-brown units. Left of column--distribution of short chemical (C) and detrital (D) cycles in pattern of intermediate and long cycles; based on data in Fig. 2A. Long climatic cycles of wetter and drier phases and sequence of alternating geographic environments sketched in columns at right. B, Model of two long climatic cycles and associated intermediate and short climatic cycles. Arrow indicates direction of increase in detrital sediment, in moisture, and in rate of deposition. Graph shows relative amount of analcime produced during intermediate and long cycles.
In this setting chemical cycles developed in two different ways essentially related to the influx of detrital sediments. In an earlier stage of an elongate lake flooding the length of the basin, chemical cycles accumulated as a central basin facies remote from detrital contamination while mud forming detrital cycles accumulated at the periphery and graded marginward into the Stockton arkose. When accumulation of the Stockton facies gave way to the Brunswick mudflat facies at the margin the lake became more equant in shape, less long and spreading marginward to the southeast beyond earlier Lockatong deposits. Now chemical cycles achieved their maximum development as the mudflat facies with relatively little coarse detrital sediment encroached upon the lake.
(4) In their general pattern detrital cycles that accumulated in open lakes possess many of the characteristics of some cyclothems (Beerbower, 1961, p. 1040-1042). Each was controlled by symmetrical climatic cycles that produced an asymmetrical succession of depositional environments; each shows a crudely symmetrical cycle of grain size; each contains siltstone and very fine grained sandstone channel deposits developed as delta distributaries cutting into lacustrine mud accumulated in shallow water; and each records cyclic deposition preserved in a continuously sinking basin.
In contrast, chemical cycles accumulated in closed saline lakes. From comparison with modern closed lakes (Langbein, 1961) the environment was arid to semiarid, and gross evaporation exceeded precipitation of less than 25 inches a year. As in playas, salt crystals apparently grew in the mother liquor in the mud below the drying surface.
(5) With the waning of the Lockatong lake, mudflats or broad floodplains with wandering watercourses and weak external drainage prevailed during deposition of Brunswick mudstone (Fig. 19A). Now expression of the short climatic cycles was diminished again, but long cycles produced episodes of a dry oxidizing environment and thick sequences of reddish-brown mud that alternated with moister periods producing brief returns to reducing conditions and accumulation of thin sequences of dark gray mud.
(6) Similarities between Lockatong detrital cycles and Stockton fluvial deposits and between grayish-red chemical cycles and Brunswick mudflat deposits (Table 1), (a) suggest that in many ways the Lockatong conditions were transitional between those of the Stockton and the Brunswick lithofacies, and (b) support the reconstruction of a shallow lacustrine origin for the Lockatong Formation.
Table 1--Distribution of selected features in Lockatong Formation and adjacent strata of Newark Group (Fig. 19A).
|Fine sand||X||X xd||xd||xd||x|
|d--detrital short cycle||M--middle Lockatong|
|c--chemical short cycle||U--upper Lockatong|
|g--gray units||B--lower Brunswick|
(7) The fact that the grayish-red chemical cycles with abundant pseudomorphs of salt accumulated during the driest episodes in Lockatong history implies that Brunswick redbeds were also products of dry phases of the long climatic cycles. In this dry environment halite and glauberite crystallized in soft Brunswick mud (Wherry, 1916; Hawkins, 1928), and concentrated cations in ephemeral lakes on Brunswick mudflats crystallized out as anhydrite and glauberite when local lakes were overrun by lava (Schaller, 1932, p. 8).
(8) On the basis of varve-counts of beds in the lower part of chemical cycles the duration of three different cyclic patterns (Fig. 19B) has been estimated:
A similar sequential pattern of cycles of three different orders of magnitude has been identified in the Keuper Series of England (Elliot, 1961, p. 225) and has been portrayed as "composite rhythms" by Barrell (1917, p. 795-797).
(9) The sequence of facies in the Newark Group points to a general progression from fluvial to mudflat environments. Assuming that climate was a basic control, the succession of Stockton, Lockatong, and Brunswick deposits reflects a change from abundant to limited inflow in response to a general trend from moister to drier climate. Granted, this sequence of formations could have been produced by controls other than climate. The possibilities await rigorous testing.
(10) Considered in their regional setting, Lockatong cycles were part of a continuing pattern of cyclic sedimentation recorded in late Paleozoic and Triassic rocks in eastern United States. Mississippian and Pennsylvanian cyclothems of western Pennsylvania were cratonic marine to nonmarine cyclic deposits reasonably accounted for by climatic oscillations (Wanless, 1963). Latest Pennsylvanian and Permian cyclothems of the Dunkard Group are essentially like their predecessors but are wholly nonmarine and partly lacustrine. These, too, have been attributed to climatic control (Beerbower, 1961).
According to this reconstruction, cyclic climatic control of sedimentation during late Paleozoic and Triassic time was accompanied by a progressive withdrawal of the sea that produced a succession of environments from marine to nonmarine. Then a trend toward drier climate produced changes in environment from fluvial to open lakes, then to closed lakes and finally to mudflats.
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Kansas Geological Survey
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Web version March 2003, Corrected July 2005. Original publication date Dec. 1964.