Roy W. Schlische
Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854-8066, firstname.lastname@example.org
The Newark rift basin contains Triassic and Jurassic rocks deposited in a large sedimentary basin that formed during the breakup of Pangea [Figure>>], the giant continent that existed about 250 to 200 million years ago. The Newark basin forms the largest physiographic province (Piedmont province) in the northern half of New Jersey. Because the Newark basin makes up more that 95% of the Piedmont province, most geologists simply refer to this area as the Newark basin [Figure>>]. The rocks within the Newark basin consist predominantly of siltstone and shale, along with sandstone and conglomerate. Most of these rocks have a reddish color, which gives the soils in the Newark basin a reddish hue. The Newark basin also contains lava flows and intrusions, the largest and best known of which is the Palisades sill. The Newark basin is bounded to its northwest by a large fault zone, known locally as the Ramapo fault [Figure>>]; as we will see, this fault was primarily responsible for forming the Newark basin. [A fault is a break or fracture in the Earth's crust along which movement takes place.]
The Newark basin is not confined to NJ; rather, it extends into southern New York state and southwestern Pennsylvania [Figure>>]. The Newark basin is one of several rift basins containing Triassic-Jurassic rocks in eastern North America. The rocks of some of these basins are exposed at the present-day land surface. For other basins the rocks are buried underneath younger Coastal Plain deposits and beneath the sediments on the continental shelf [Figure>>]. We know about the presence of these basins from drilling and seismic-reflection profiling (this technique uses sound waves to "see" into the Earth, just like X-rays use radiation to "see" inside the body).
Two tectonic events [Figure>>] are critical to understanding the origin of the Newark basin. ("Tectonic" refers to the movement of the large plates making up the Earth's outer shell.) The first event was the Appalachian orogeny ("orogeny" means mountain-building), which was related to the assembly of the supercontinent of Pangea. The final phase of the Appalachian orogeny resulted from the collision of North America and Africa. The second event was a reversal of the first, in which North America and Africa began to drift apart. [One mechanism that may have produced this rifting was convection within the interior of the Earth (Figure>>).] As these two continents moved away from one another, the crust in between them began to stetch, much like pulling apart a piece of chewing gum. As the crust stetched, it began to get thinner, through a combination of continuous flow in the lower crust and normal faulting in the upper crust [Figure>>]. Rift basins formed in association with the normal faults. Many of these faults partially or totally utilized older faults that had formed during the earlier Appalachian orogeny because it was easier to reactivate an old fault than it was to break rock and form a new fault. Eventually, North America and Africa broke apart, leading to the formation and growth of the Atlantic Ocean [Figure>>].
Stretching of the crust leads to the formation of normal faults [Figure>>]. Downward movement of one of the fault blocks creates a topographic depression, which will fill with sediments and may contain lakes [Figure>>]. Upward movement of the other fault block creates a topographic high, which will erode and supply sediment to the topographic depression--the rift basin. In cross-sectional view (a vertical slice, corresponding to the front view of the block diagram), the half graben has a triangular geometry. The steeper side of the triangle is the normal fault responsible for forming the basin. This fault also separates the basin from other rocks, and is therefore called a border fault. The gentler side of the triangle is the "floor" of the basin, and separates rift-basin sediments from rocks that pre-date the formation of the basin. The last leg of the triangle is the Earth's surface.
Newark Basin Geology--Maps and Cross Sections
The geology of a feature is best displayed on geologic maps and cross sections. The geologic map [Figure>>] of the entire Newark basin in New York, New Jersey, and Pennsylvania shows the different rock units within the basin, their arrangement, and any structural features (for example, faults) affecting these rock units. Each mappable rock unit--known as a formation--is shown with a different color and/or pattern on the geologic map. The formations in the Newark basin are, from oldest to youngest, the Stockton Formation, Lockatong Formation, Passaic Formation, Orange Mountain Basalt, Feltville Formation, Preakness Basalt, Towaco Formation, Hook Mountain Basalt, and Boonton Formation. Older formations tend to be present on the southwest side of the basin; younger formations tend to be present on the northwest side, where the border fault is present. The boundaries (contacts) between the formations and form lines within some of the formations show the attitude of the rock units. Most contacts and form lines are oriented NE-SW, subparallel to the long axis of the basin and the trend of the border-fault system. Some of the contacts and form lines have a more complicated geometry. In the northeastern part of the basin, the Early Jurassic-age formations have an arcuate geometry, indicating that the rocks have been warped or folded [Figure>>]. Diabase intrusions may intrude parallel to lithologic contacts (e.g., the Palisades sill) or cut across lithologic contacts; the latter tend to have a more complicated geometry (e.g., the diabase intrusions adjacent to the border fault in Pennsylvania).
The structure map [Figure>>] highlights the geometry of faults. The border-fault system, shown in red, consists of a number of discrete segments. Many of these segments are parallel to thrust faults (black) that formed during the Appalachian orogeny. Both the border-fault segments and the thrust faults dip (are inclined) to the southeast (the symbols on the faults--ball and bar for normal fault, triangle for thrust fault--are on the southeast side of the fault line). As discussed further below, it is likely that border-fault system reactivated preexisting faults. Intrabasinal faults (blue) are mainly normal faults and generally strike more northerly than the border-fault system fault segments. Furthermore, the average trend of the intrabasinal faults is subparallel to the average trend of diabase dikes (generally thin igneous intrusions that cut across preexisting layering). It is likely that both the intrabasinal faults and the dikes formed perpendicular to the Mesozoic extension direction (the direction in which the lithosphere was stretched apart), whereas the border-fault system was somewhat oblique to the extension direction (see idealized rift basin in Figure>>).
The structure map [Figure>>] also shows the attitude of bedding within the Newark basin. Most strata within the basin strike subparallel to the long axis of the basin and dip toward the border-fault system, as would be expected in an idealized half graben [Figure>>]. However, complications to this simple geometry occur in areas adjacent to large intrabasinal faults and areas of folding.
The geology of the Newark basin below the Earth's surface may be inferred using three different techniques: seismic-reflection profiling, projecting surface information to depth, and drilling. NB-1 [Figure>>] is a regional seismic reflection profile that crosses much of the Newark basin and its border fault. The interpretation of the subsurface geology of the Newark basin is based on the geometry of the reflections as well as the known surface geology (locations of contacts, faults, etc.). The border fault dips to the southeast at an angle of <30°, which is atypical for a normal fault but is typical for a thrust fault. This suggests that the border fault is an old thrust fault (originally formed during the Appalachian orogeny) that was reactivated during Mesozoic rifting. Strata within the basin generally dip to the northwest, although strata are subhorizontal adjacent to the border fault, where the seismic line crosses a fold. The basin itself shows a fairly classic "triangular" half-graben geometry [Figure>>]. The Flemington-Furlong fault system is associated with its own "triangle" [Figure>>]. The Stockton Formation notably thickens toward the border fault, as does a unit that does not outcrop at the surface ("buried Stockton"); older strata dip more steeply than younger strata; and conglomeratic facies are inferred to be present adjacent to the border fault. All three features are prime evidence that sedimentation (deposition) was occurring during faulting [Figure>>]. Paleozoic rocks below the rift basin show evidence of reverse displacement [Figure>>], which likely occurred prior to rifting during the Appalachian orogeny.
Cross sections based on projecting outcrop data to the subsurface also show the classic half-graben geometry [Figure>>]. Cross section A-A' is relatively simple. It shows a SE-dipping border fault and generally NW-dipping strata (again, with folding adjacent to the border-fault system). All formations within the Newark basin are present on this cross section. The geometry of the Palisades sill is not well known, but the sill is known to "step-up" into younger rock units in map view. Cross section B-B' shows a SE-dipping border fault and SE-dipping intrabasinal faults. The intrabasinal faults have considerable offset along them and repeat the Late Triassic stratigraphy (Stockton-Lockatong-Passaic). Strata dip to the NW. None of the Jurassic formations is present in the area of this section. In fact, only the lower third of the Passaic Formation outcrops adjacent to the border fault. Organic matter, which changes appearance based on temperature (which increases with depth), indicates that rocks now at the surface were once buried by an additional 5 km of strata (M.L. Malinconico, personal communication, 2002). Thus, considerable erosion has occurred in this part of the Newark basin after sedimentation.
Stratigraphy from Cores and Drill Sites
The Newark basin boasts an extensive series of cores and drill holes [Figure>>] that fall into three series: (1) two deep holes drilled in Pennsylvania as part of hydrocarbon exploration; (2) the seven ~1-km-deep Newark Basin Coring Project (NBCP) cores from the central Newark basin in New Jersey; and (3) the numerous Army Corps of Engineers (ACE) cores obtained along a transect in the northeastern Newark basin in New Jersey. Continuous core was not recovered from the hydrocarbon-exploration drill sites, but cuttings from the holes allow identification of the formations. Subsurface stratigraphic contacts determined from the drill site closest to the Delaware River constrain the interpretation of the NB-1 seismic line [Figure>>].
Dr. Paul E. Olsen (Lamont-Doherty Earth Observatory of Columbia University) and Dr. Dennis V. Kent (then of Lamont-Doherty, now at Rutgers University) initiated the NBCP [Figure>>] expressly to recover continuous core to provide detailed information about the Late Triassic and earliest Jurassic-age stratigraphy of the Newark basin. To accomplish this goal, Olsen and Kent could have drilled one very deep hole in a part of the basin where all of the targeted stratigraphic units are present in the subsurface (e.g., a hole located in the lower Preakness Formation along cross section A-A'; Figure>>). However, the cost of such a deep hole would have been prohibitive, and the drill hole may have drilled through unrecognized normal faults, which could have omitted part of the stratigraphy. Instead, Olsen and Kent chose to use the "offset coring method" which involved drilling seven holes in carefully selected areas [Figure>>]: the hole should not intersect any major faults or intrusive igneous bodies, the top of the hole had to recover a well-known stratigraphic marker unit, the bottom of the hole had to recover another well-known stratigraphic marker unit, and the stratigraphy in the top of one hole had to overlap with the stratigraphy at the bottom of an adjacent drill hole. For example, the top of the Nursery hole intersected the contact between the Lockatong and Passaic formations, and the bottom intersected the contact between the Lockatong and Stockton formations. The top of the Somerset core and the bottom of the Weston core overlap, and there is an excellent match in the stratigraphy [Figure>>], although the units in Weston are about 11% thicker than the same units in Somerset (the significance of this is discussed below). The NBCP project recovered over 25,000 ft of core, which is currently archived in the New Jersey Core Repository on the Livingston Campus of Rutgers University.
The overlap sections of the seven NBCP allowed Olsen and Kent to construct a single composite stratigraphic section [Figure>>]. Because of the variations in thickness of correlative units from one core to the adjacent core, all cores were scaled to the Rutgers core, which covers the central part of the composite section. The composite section covers the interval from the Stockton Formation through the base of the Preakness Basalt and is >15,000 ft. thick. The Stockton Formation is the coarsest-grained formation in the section, consisting of purple, white, and red sandstone, siltstone, and conglomerate. The Lockatong Formation consists of mostly gray and black shale and siltstone, with subordinate purple and red mudstone (shale + siltstone). The Passaic Formation consists of mostly red mudstone and subordinate purple, gray, and black mudstone, with the percentage of non-red units generally decreasing upward. The Feltville Formation is lithologically similar to the lower Passaic Formation. The Orange Mountain and Preakness basalts consist of multiple lava flows.
The Army Corps of Engineers (ACE) cores are short to moderate-length cores recovered along the proposed path of the Passaic River diversionary tunnel [Figure>>]. The wide spacing of the mostly shallow cores in the Passaic Formation and the lack of well-defined marker beds makes construction of a composite section for this stratigraphic interval problematic. However, the close spacing of the cores in the Jurassic formations allows construction of a composite stratigraphic section [Figure>>], which complements the NBCP composite section. The Orange Mountain, Preakness, and Hook Mountain basalts each consist of three lava-flow units. The Feltville, Towaco, and Boonton formations consist of mostly red mudstone and subordinate black, gray, and purple mudstone as well as variously colored sandstone and conglomerate. These three formations are similar to the Passaic Formation, except that packages of non-red units are considerably thicker in the Jurassic formations than in the Passaic Formation. ACE cores covering the uppermost Passaic Formation and the Jurassic units are archived in the NJ Core Respository at Rutgers University.
With the exception of the lower Stockton Formation, the sedimentary formations of the Newark basin are highly cyclical [Figure>>]. For example, in the Passaic Formation, the predominant red mudstone is punctuated by non-red units consisting of purple and gray mudstones and black shales [Figure>>]. The black shales tend to be laminated or microlaminated (requiring very quiet conditions) and have a relatively high organic-carbon content (requiring low-oxygen conditions), both of which indicate a deep-lake environment. The units surrounding the black shales show evidence of decreasing lake levels: lower organic carbon content (hence a trend away from the black color); more bioturbation (disruption of sedimentary layering by organisms) and increase in "redness" (indicating more oxygen-rich conditions); increasing presence of mudcracks, root traces, and reptile footprints (indicating drying out of the lake). One can define a lake-level cycle as the portion of rock between two successive deepest-water deposits, and thus the diagram at right shows two complete lake-level cycles and two partial cycles. Another approach has the lake-level cycle consisting of three divisions: strata corresponding to rising lake level (transgression), lake highstand, and falling lake level (regression) [Figure>>]. P.E. Olsen calls this cycle a Van Houten cycle, after Princeton University stratigraphy Prof. Franklin Van Houten, who conducted groundbreaking research on lake-level cycles in the Newark basin in the 1960's.
The nature of the lake-level cycles changes through the sedimentary section. For example, in the Rutgers core [Figure>>], the non-red part of the core is thickest and blackest for the lowest non-red unit. Longer sections of core (and outcrops) show even more profound changes [Figure>>], and indicate that the basic lake-level cycles are arranged in a series of compound cycles. The short modulating compound cycle contains ~5 Van Houten cycles, and the McLaughlin cycle contains ~20 Van Houten cycles. The ratio of 1:5:20 (for the number of cycles within Van Houten, short-modulating, and McLaughlin cycles) is similar to the ratio of the periods of the Milankovitch cycles of precession and eccentricity (~20,000 yr : ~100,000 yr : ~400,000 yr = 1:5:20), suggesting that the Van Houten and compound cycles are Milankovitch cycles. These are changes in the Earth's orbit, which affect the amount of sunlight reaching the Earth's surface, which affects climate, which ultimately affects lake levels.
The cycles provide an opportunity to subdivide the formations in the stratigraphic section into a series of members and to date the rocks of the Newark basin at a very fine level. The members in the Lockatong and Passaic Formations are the ~400,000 year McLaughlin cycles [Figure>>]. The formal named members--for example, the Perkasie Member of the Passaic Formation--can be mapped throughout the Newark basin [Figure>>], as was accomplished by astronomer D.B. McLaughlin in his spare time during the middle part of the 20th Century. For his pioneering work, Olsen and Kent named the 400,000-year compound cycle after him. The geologic age of the Newark basin section [Figure>>] is based on an absolute date of ~202 Ma for the oldest lava flow in the Newark basin; the durations of the various lake-level cycles are then used to count back or forward in time from the 202 Ma date. [Absolute dating relays on the decay of radioactive isotopes. A "parent" isotope decays to a "daughter" isotope at a known decay rate; measuring the proportions of parent and daughter isotopes in a rock and knowing the decay rate determines the time when the rock formed.] The Van Houten and compound cycles have also indicated that the duration of extrusive igneous activity in the Newark basin is ~600,000 years [Figure>>] and that the Triassic-Jurassic boundary is ~40,000 years older than the base of the Orange Mountain Basalt.
As we saw above, the Perkasie Member, a McLaughlin cycle, can be traced throughout much of the Newark basin [Figure>>]. Other compound and Van Houten cycles can also be correlated short and long distances within the Newark basin, and variations in the thickness and facies (sediment characteristics, such as grain size and color) of correlative (time-equivalent) units tell us something about how lake depth and basin geometry varied throughout the Newark basin. The overlap sections of the NBCP cores [Figure>>] provide some illustrative examples. The Kilmer, Livingston, and Metlars members of the Passaic Formation are present in the overlap sections of the Somerset and Rutgers cores [Figure>>]. The overlap section is 13.5% thicker in Somerset than in Rutgers. In addition, the lake-highstand deposits are deeper in Somerset than Rutgers (three purple units in the Livingston Member in Rutgers consist of three gray and black units in Somerset). The units in the Titusville core are 20% thicker than their correlative units in the Rutgers core [Figure>>]. All overlap sections show some change in thickness [Figure>>]. The 20% change in thickness from Rutgers to Titusville occurs mainly in the "strike direction" (parallel to the border-fault system), indicating that the basin was deeperand could allow a thicker package of sediment to accumulateat its center (near Titusville) compared to its lateral edges (at Rutgers and beyond). The 5.8% change in thickness from Nursery to Titusville occurs mainly in the "dip direction" (perpendicular to the border-fault system), indicating that the basin got deeper toward the border-fault system. In a simplified, idealized basin [Figure>>], we see that the basin deepens from its edges to the center and from the hinged margin to the border-fault system. (The same situation existed in the Newark basin, although the simple trends are complicated by the large amounts of movement on the intrabasinal faults.) These trends reflect displacement variations on the border-fault system: greater displacement toward its center compared to its ends, greater displacement at the fault compared with distance away from the fault. In addition to affecting stratal thickness, these displacement variations also affected sedimentary facies: lake-highstand deposits for correlative units are deeper toward the center of the basin and toward the border-fault system [Figure>>].
Outcrop data confirm the trends observed in the NBCP overlap sections and reveal additional information about how sedimentary units vary in thickness and facies with position in the Newark basin. The Perkasie Member of the Passaic Formation [Figure>>] thickens and its lake-highstand deposits deepen toward the center of the basin and toward the border-fault system. Strata also get coarser grained as we approach the border-fault system and the lateral edges of the basin. The Ukrainian Member [Figure>>] is another unit that can be traced throughout the Newark basin, and generally shows the same trends in thickness and facies documented by the Perkasie Member. In addition, the Ukrainian Member may also show the influence of small-scale folding on sedimentation. For example, sections 4, 5, and 6 are all located in the central fault block, but the Copper Hill section is thinner than the Flemington and Muirhead sections. The Copper Hill section is located near the crest of an upfold (anticline), whereas the other two sections located near the troughs of downfolds (synclines)). A greater thickness of section could accumulate in the troughs of synclines versus the crests of anticlines, indicating that some folding was occurring during deposition.
Numerous folds are present throughout the Newark basin [Figure>>], but are mostly present adjacent to the border-fault system and the large intrabasinal faults, suggesting that the folds are related to the faults. Folds are exceptionally well developed in the southwestern Newark basin (Jacksonwald-Sassamansville area) [Figure>>]. The synclines tend to occur near the centers of border-fault segments, whereas anticlines tend to occur near in the areas where fault segments overlap. Because synclines are downfolds and the basin is deeper at downfolds, fault displacement was higher where synclines are located (the centers of fault segments) and lower where anticlines are located. Stratigraphic units in outcrop thicken toward the trough of the Jacksonwald syncline; stratigraphic units interpreted on a seismic-reflection profile thin away from the crest of the Sassamansville anticline. The outcrop and seismic data indicate that some folding was occurring during sedimentation.
As the above examples indicate, the lake-level cycles not only tell us that the climate periodically changed in Triassic-Jurassic time but also allow us to precisely date the sedimentary rocks in the Newark basin and to understand how faults and folds controlled the large- and small-scale geometry of the Newark basin.
ADVANCED TOPIC: Large-Scale Stratigraphy and Basin-Filling Model
Previously, we have seen that climatic changes produced by Milankovitch cycles were responsible for producing the cyclicity present in the lacustrine (lake) strata of the Lockatong and Passaic formations. These two formations are quite distinct from one another and also distinct from the Stockton Formation [Figure>>]. The lower Stockton Formation is a mostly fluvial deposit (that is, it was deposited by rivers). The upper Stockton and lower Lockatong formations represent lacustrine strata in which the lake-highstand deposits got progressively deeper upsection. The deepest-water highstand deposits occur in the middle Lockatong Formation. The highstand deposits in the upper Lockatong and throughout the Passaic Formation tend to become shallower upsection. Thus, the large-scale stratigraphy in the Newark basin Triassic-age section consists of three main units: (1) fluvial deposits; (2) shallow to deep lacustrine deposits, with a fairly abrupt transition from shallow to deep; and (3) deep to shallow lacustrine deposits, with a gradual transition from deep to shallow. We refer to this stratigraphy as a "tripartite stratigraphy."
To understand this tripartite stratigraphy, we need to consider (1) the fundamental distinction between fluvial and lacustrine sedimentation and (2) the relationships among the capacity of the basin to hold sediment, the supply of sediment entering the basin, and the supply of water available to the basin [Figure>>]. Fluvial deposition requires a slope, whereas lacustrine (ponded water) depositional systems require that the outlet of the basin be located above the depositional surface. In cases where the sediment supply exceeds the basin capacity, fluvial deposition predominates (Case 1). In cases where the basin capacity exceeds the sediment supply, lacustrine deposition predominates. The relationship between the supply of water and the excess capacity (the difference between the basin capacity and the sediment supply) of the basin determines the conditions in the lake. If the water supply exceeds the excess capacity, the lake is hydrologically open (Case 2) such that excess water leaves the basin. If the excess capacity exceeds the water supply, the lake is hydrologically closed (Case 3), and all water remains in the basin.
Given the relationships among basin capacity, sediment supply, and water supply, several mechanisms can produce the major transitions in depositional environment observed in the Newark basin. The fluvial-lacustrine transition may result from an increase in basin capacity and/or a decrease in sediment supply. The shallow-water to deep-water lacustrine transition may result from an increase in basin capacity, a decrease in the sediment supply, or an increase in the water supply. The deep-water to shallow-water lacustrine transition may result from a decrease or increase in basin capacity (depending on the geometry of the excess basin capacity), an increase in the sediment supply, and/or a decrease in the water supply. Note that all three of the transitions could result from an increase in basin capacity, which is controlled by movement on the border fault.
Let's consider a very simple model for the evolution and filling of a rift basin [Figure>>], which is controlled by the evolution of the border fault. Faults generally increase in length as fault displacement increases. Thus, as displacement on the border fault increases, the rift basin generally increases in depth, length, and width. Thus, basin capacity increases through time. If the sediment-supply rate and the water-supply rate are constant, then a tripartite stratigraphy results. This basin-filling model is one of several models that can explain the stratigraphy of the Newark basin, but it is one of simplest, and scientists generally prefer simpler explanations to more complex explanations.
ADVANCED TOPIC: Comparisons with Other Rift Basins
Outcrop, core, and seismic data from the Newark basin provide us with a fairly detailed picture about the geometry of the basin, the type of structures (faults and folds) bounding and within the basin, and the nature of the rift-basin strata. But how typical is the Newark basin? How does it compare with other basins in the eastern North American rift system, or, for that matter, other rift basins from around the world?
Let's first consider stratigraphy [Figure>>]. Rift-basin strata in eastern North America can be subdivided into a series of tectonostratigraphic (TS) packages that are separated by unconformities (gaps in the rock record). The Late Triassic-age TS3 is the most widespread TS package in the eastern North American rift system. Basal fluvial deposits, like those of the Stockton Formation, are present in TS3 in all of the other eastern North American rift basins. [Basal fluvial deposits are also present in most of the basins that have TS2 packages.] For TS3, the basal fluvial deposits are succeeded by lacustrine strata in most rift basins, and these lacustrine strata show large-scale stratigraphic trends similar to the Newark basin. Therefore, it is likely that the same processes that produced the tripartite stratigraphy in the Newark basin also produced the tripartite stratigraphy in the other basins. A tripartite stratigraphy is absent from the Pomperaug, Hartford and Deerfield basins, in which TS3 consists exclusively of fluvial deposits. It is possible that basin capacity was sufficiently small and/or sediment supply sufficiently large that the fluvial-lacustrine transition never occurred.
TS4 consists of Early Jurassic strata and lava flows. TS4 is present in all of the northern basins (including the Newark basin) but is absent in all of the southern basins. The stratigraphy of TS4 is very similar from basin to basin. For example, the East Berlin Formation of the Hartford basin is virtually identical to the Towaco Formation of the Newark basin [Figure>>]. Although these basins were probably never physically connected, their lakes responded to the same regional climatic events. The lava flows in TS4 in all of the northern basins have a similar stratigraphy and geochemistry [Figure>>]. This indicates that these lava flows were erupted at the same time and that the source magma covered a very large area. The similarity of the basalt geochemistry and lake-level cycles in Early Jurassic strata [Figure>>] indicate that the ~600,000 year duration of extrusive activity in the Newark basin [Figure>>] also applies to the other rift basins.
Although TS4 is remarkably similar in all of the northern rift basins, TS3 shows some important regional differences [Figure>>]. For example, coal deposits (which indicate humid, swampy conditions) are restricted to the southern basins, and there are no "deeper-water" cyclical lacustrine strata north of the Newark basin. Even these "deeper-water" cyclical lacustrine strata in the Newark basin are quite different from those in the southern basins. Although all basins contain lake-level cycles, the transgressive and regressive parts of the Van Houten cycles in the Newark basin always contain mudcracks, but that is not the case in the Richmond basin [Figure>>]. Therefore, although lake levels fluctuated in both basins, the lakes dried out every 20,000 years in the Newark basin; the lakes in the Richmond basin did not dry out completely. In the Fundy basin, there are virtually no gray or black shales, but evaporites and eolian (wind-blown) deposits are present, indicating a generally arid climate. Thus, the lacustrine strata in the Richmond basin represent a humid end member, the lacustrine strata in the Fundy basin represent an arid end member, and the lacustrine strata in the Newark basin are intermediate between the two end members. The reason for these regional differences is paleolatitude. Paleomagnetism indicates that strata that are 215 million years old in the Newark basin accumulated at a paleolatitude of ~8° [Figure>>], only about 900 km north of the Late Triassic equator. Rift basins south of the Newark basin were located closer to the equator and its more humid climate, whereas rift basins north of the Newark basin were located farther from the equator and experienced a more arid climate [Figure>>]. The Newark basin (and the other basins in eastern North America) gradually drifted northward as indicated by the increase in the paleolatitudes [Figure>>]. Therefore, younger strata in the Newark basin accumulated in a more arid regional climatic setting, and this may have contributed to the shoaling of lake-highstand deposits in the Passaic Formation (in addition to the effects of basin growth and filling).
We conclude this essay by comparing the structure and basin geometry of the Newark basin with other rift basins in eastern North America and from around the world. Items to note with respect to the eastern North Americ rift system [Figure>>] include: 1) the Newark basin is part of a large rift system that includes the Narrow Neck, Gettysburg, and Culpeper rift basins. 2) Southern basins (Richmond, Dan River, and Deep River) do not contain any Jurassic strata or lava flows. 3) The basins in longitudinal section (parallel to the border-fault system) consist of (faulted) synclines or multiple synclines and anticlines. 4) Most basins in transverse section (perpendicular to the border-fault system) exhibit the classic asymmetric half-graben geometry (the Minas subbasin of the Fundy basin is an exception). 5) The majority of the basins have border faults dipping to the east or southeast, reflecting the predominant attitude of reactivated Paleozoic faults; the Connecticut Valley, Pomperaug, and Deep River basins have border faults dipping to the west or northwest, reflecting the local orientation of reactivated Paleozoic faults. The maps and cross sections of the eastern North American rift basins form the basis of idealized rift-basin geometries [Figure>>]. The Newark, Gettysburg, Culpeper, Pomperaug and Richmond basins are like the idealized basin in Case a. The Connecticut Valley and Deep River basins are like the idealized basin in Case b. The Fundy basin is like the idealized basin in Case c; the Narrow Neck is similar to the northern margin of the idealized basin in Case c.
In comparing the Newark basin with other rift basins from around the world [Figure>>], we note the following: 1) The Newark, Fundy, Jeanne d'Arc, Suez, and Voring rift basins are asymmetric features; the half-graben geometry is well developed in the Newark, Jeanne d'Arc and Suez basins; the Upper Rhine basin is a symmetric (full graben) rift basin. 2) Border faults from the Newark, Fundy, Jeanne d'Arc, and Voring basins have a relatively low dip angle, especially along their deeper parts; the border faults of the Upper Rhine and Suez rift basins dip moderately to steeply. 3) The Fundy rift has been modified by a period of postrift shortening. 4) Folding in the synrift sections of the Jeanne d'Arc and Suez rifts is related to the presence of synrift salt. 5) The relatively unfaulted, bowl-shaped synrift basins in the Voring rift basin is related to the presence of prerift salt. Because of these considerations, the geometry of rift basins in cross section can be quite variable.
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