Newark Basin
Newark Basin
Newark Basin
The EPS alumni newsletter, “The Redbeds,” derives its name from the red-colored sedimentary rocks prevalent in the New Brunswick / Piscataway area. The strata are part of the Passaic Formation that accumulated in Late Triassic time (about 220 million years ago) in the Newark rift basin, a depression that formed on the downthrown side of a normal fault that was active during continental stretching preceding the opening of the North Atlantic Ocean. This web essay provides a comprehensive overview of the geology of the Newark basin, information that was stitched together from geologic mapping, multiple seismic-reflection profiles that imaged the subsurface geometry, and scores of cores. We also compare the Newark basin to other rift basins in eastern North America and around the world.
This web essay is a substantially updated version of one first posted in 2002. All figures are now displayed at a higher resolution than the originals, and most figures have undergone substantial revision to reflect the most recent research. The web essay includes more than 50 illustrations that you can view by clicking on the figure call-outs in the text. This will open a new window containing one or more diagrams and an explanatory caption. Click on the X in the bottom right corner to close the illustration. Clicking on a citation will bring you to the full reference opened in a separate window or tab. Some names appear repeatedly in the bibliography: Martha Withjack, Paul Olsen, and Dennis Kent. Their collective work has enriched this essay.
Roy W. Schlische
Department of Earth & Planetary Sciences, Rutgers University, Piscataway, NJ 08854-8066,
Contents
- Introduction
- Newark Basin Geology--Maps and Cross Sections
- Stratigraphy from Cores and Drill Sites
- Lake-Level Cycles
- ADVANCED TOPIC: Large-Scale Stratigraphy and Basin-Filling Model
- ADVANCED TOPIC: Uplift and Erosion
- ADVANCED TOPIC: Comparisons with Other Rift Basins
Introduction
The Newark rift basin contains Triassic and Jurassic rocks that accumulated in a sedimentary basin that formed during the breakup of Pangea [Figure 01 >>], the giant continent that reached its maximum extent about 250 to 200 million years ago. Breakup of that supercontinent caused the formation of the Newark and related basins and led to the opening of the central North Atlantic Ocean (e.g., Withjack et al., 2012). The Triassic-Jurassic boundary occurs within the basin (e.g., Fowell et al., 1994); the end-Triassic extinction is one of the “Big 5” mass extinctions during the last 500 million years. Newark basin rocks include basalts that formed during fissure eruptions ~200 million years ago. These basalts (extrusives) and related igneous intrusions belong to the Central Atlantic Magmatic Province [Figure 02 >>] (Marzoli et al., 1999), which was emplaced in a geologically short time (< 1 million years) (e.g., Olsen et al., 2003; Whiteside et al., 2007). Given its massive extent (covering large parts of eastern North America, northeastern South America, western Africa, and western Europe) and age, eruptions associated with this large igneous province are a strong candidate for the cause of the end-Triassic mass extinction (e.g., Blackburn et al., 2013; Davies et al., 2017). Data from the Newark basin show that eruptions of CAMP-related basalts caused atmospheric carbon-dioxide concentrations (already at a high level of ~2000 ppm, ~5 times higher than present-day values) to very rapidly double and then rapidly decline as chemical weathering of the newly erupted basalts pulled carbon dioxide out of the atmosphere (Schaller et al., 2011, 2012).
Rutgers University’s New Brunswick and Newark campuses overlie rocks of the Newark basin, which is located in the Piedmont physiographic province. Because the Newark basin makes up more that 95% of the Piedmont province, most geologists simply refer to this region as the Newark basin [Figure 03 >>]. 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. As noted above, the Newark basin also contains lava flows and intrusions, the largest and best known of which is the Palisade sill. The Newark basin is bounded to its northwest by a large fault, known locally as the Ramapo fault [Figure 04 >>]; 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 New Jersey; rather, it continues into southern New York state and southwestern Pennsylvania [Figure 05 >>]. The Newark basin is one of several rift basins containing Triassic-Jurassic rocks in eastern North America. The rocks of some of these basins, like the Newark basin, 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 06 >>]. We know about the presence of these basins from drilling and seismic-reflection profiling, the technique that uses sound waves to "see" into the Earth, just like x-rays use radiation to "see" inside the body.
Two tectonic events [Figure 07 >>] 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 08 >>).] As these two continents moved away from one another, the crust in between them began to stretch, much like pulling apart a piece of chewing gum. As the crust stretched, it began to get thinner, through a combination of continuous flow in the lower crust and normal faulting in the upper crust [Figure 07 >>]. Rift basins formed in association with the normal faults. Many of these faults partially or totally utilized older faults that had formed during the older 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 01 >>].
Stretching of the crust leads to the formation of normal faults [Figure 09 >>]. Downward movement of one of the fault blocks creates a topographic depression, which will fill with sediments and may contain lakes [Figure 10 >>]. Upward movement of the opposite 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 rift basin (also known as a 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
We use geologic maps to display the geologic units that are present at Earth's surface in a given region. The geologic map [Figure 11 >>] 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 and folds) affecting these rock units. Each mappable rock unit--known as a formation--has a different color and/or pattern on the geologic map. The formations in the Newark basin are, from oldest to youngest, the Stockton Formation (Fig. 12A>>), Lockatong Formation (Fig. 12B>>), Passaic Formation [example 1 (Fig. 12C>>), example 2 (Fig. 12D>>), example 3 (Fig. 12E>>)], Orange Mountain Basalt (Fig. 12F>>), Feltville Formation, Preakness Basalt (Fig. 12G>>), Towaco Formation (Fig. 12H>>), Hook Mountain Basalt, and Boonton Formation (Fig. 12H>>). [The three basalt-flow formations have an age (200 Ma) and composition that make them part of the Central Atlantic Magmatic Province, the world's largest large-igneous province present in parts of eastern North America, northern South America, Western Africa, and western Europe [Figure 02 >>].
The geologic map [Figure 11 >>] shows that older formations within the basin tend to be present on the southeast side of the basin; younger formations tend to be present on the northwest side, where the border-fault system is present. Paleozoic and older rocks lie to the northwest of the border-fault system; coastal plain strata are present to the southeast of the basin. The boundaries (contacts) between the formations in the basin show the attitude (orientation) of the rock units. Most contacts are oriented NE-SW, subparallel to the long axis of the basin and the trend of the border-fault system. Some contacts have a more complicated geometry. In the northern part of the basin, the lava flows and interbedded strata have an arcuate geometry, indicating that the rocks have been warped or folded [Figure 13 >>]. Diabase intrusions may be parallel to lithologic contacts (e.g., the Palisade sill) or cut across lithologic contacts; the latter tend to have a more complicated geometry (e.g., the diabase bodies adjacent to the border fault in Pennsylvania).
The structure map [Figure 14 >>] highlights the geometry of faults. The border-fault system, shown in the heavy black line, consists of multiple discrete segments. Many of these segments are parallel to thrust faults (green) 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--tick mark 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 (thinner black lines) 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 15 >>).
The structure map [Figure 14 >>] 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 10 >>]. 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 techniques: seismic-reflection profiling, projecting surface information to depth, and drilling. NB-1 [Figure 16 >>] 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 10 >>]. The Flemington-Furlong fault system is associated with its own "triangle" [Figure 16 >>]. The Stockton Formation notably thickens toward the border fault, as does a unit that does not outcrop at the surface ("unexposed synrift"); 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 evidence that sedimentation (deposition) was occurring during faulting [Figure 17 >>]. Paleozoic rocks below the rift basin show evidence of reverse displacement [Figure 16 >>], which likely occurred prior to rifting during the Appalachian orogenies.
Cross sections based on projecting outcrop data to the subsurface also show the classic half-graben geometry [Figure 18 >>]. Cross section B-B' 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 Palisade sill is inferred, but the sill is known to "step-up" into younger rock units in map view. Seismic line Sandia 101 [Figure 19 >>] also shows that the sill steps upward on the NW end of the profile. Cross section A-A' [Figure 18 >>] 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 formations). Strata dip to the NW. None of the basalt-flow 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 up to an additional 6 km of strata (see section on uplift and erosion). Thus, considerable erosion has occurred after sedimentation.
Stratigraphy from Cores and Drill Sites
The Newark basin boasts an extensive series of cores and drill holes [Figure 20 >>] that include: (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; (3) the numerous Army Corps of Engineers (ACE) cores obtained along a transect in the northeastern Newark basin in New Jersey; and (4) two wells in the northernmost Newark basin drilled as part of the Tricarb Project (to study potential for carbon sequestration); the project also obtained a seismic-reflection profile [Figure 19 >>] that is subparallel to the border between New York and New Jersey. Continuous core was not recovered from the hydrocarbon-exploration drill sites in Pennsylvania, but cuttings from the holes allow identification of the formations. Subsurface stratigraphic contacts determined from the Cabot well (the drill site closest to the Delaware River) constrain the interpretation of the NB-1 seismic line [Figure 16 >>]. The USGS has also drilled a number of holes adjacent to the border-fault system to determine its subsurface geometry and relationship to modern earthquake activity (e.g., Ratcliffe et al., 1986).
Dr. Paul E. Olsen (Lamont-Doherty Earth Observatory of Columbia University) and Dr. Dennis V. Kent (then at Lamont-Doherty, later at Rutgers University) initiated NBCP [Figure 21 >>] to recover continuous core to provide detailed information about the Late Triassic and earliest Jurassic-age stratigraphy of the Newark basin (Olsen and Kent, 2000). 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 B-B'; Figure 18 >>); however, the cost of such a very 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 21 >>]: 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 22 >>], 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 that is currently archived in the Rutgers IODP Core Repository on the Livingston Campus.
The overlap sections of the seven NBCP allowed Olsen and Kent to construct a single composite stratigraphic section [Figure 23 >>]. 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 (Fig. 12A>>) is the coarsest-grained formation in the section, consisting of purple, white, and red sandstone, siltstone, and conglomerate. The Lockatong Formation (Fig. 12B>>) consists of mostly gray and black shale and siltstone, with subordinate purple and red mudstone (shale + siltstone). The Passaic Formation (example 1 (Fig. 12C>>), example 2 (Fig. 12D>>), example 3 (Fig. 12E>>) 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 Basalt (Fig. 12F>>) and Preakness Basalt (Fig. 12G>>) consist of multiple lava flows.
The Army Corps of Engineers (ACE) cores (Fedosh & Smoot, 1988) are short to moderate-length cores recovered along the proposed path of the Passaic River diversionary tunnel [Figure 24 >>]. 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 part of the stratigraphic interval problematic. However, the close spacing of the cores in the post-Passaic formations allows construction of a composite stratigraphic section [Figure 25 >>], which expands on 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 younger units are archived in the Rutgers IODP Core Repository.
Lake-Level Cycles
With the exception of the lower Stockton Formation, the sedimentary formations of the Newark basin are highly cyclical [Figure 26 >>] (Olsen, 1986). 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 27 >>]. 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. [These units have a depth rank of 5; depth rank assigns a number to lake deposits based on color and sedimentary fabrics.] 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); increase in "redness" (indicating more oxygen-rich conditions); and increasing presence of mudcracks, root traces, and reptile footprints (indicating drying out of the lake and depth rank of 0). 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 28 >>]. P.E. Olsen names 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 1960s.
The nature of the lake-level cycles changes through the sedimentary section. For example, in the Rutgers core [Figure 27 >>], 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 29 >>], 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 30 >>]. The formal named members--for example, the Perkasie Member of the Passaic Formation--can be mapped throughout the Newark basin [Figure 31 >>], 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 32 >>] 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 relies 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 33 >>] and that the end-Triassic extinction level is ~40,000 years older than the base of the Orange Mountain Basalt.
As discussed above, the Perkasie Member, a McLaughlin cycle, is mappable throughout much of the Newark basin [Figure 31 >>]. 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 21 >>] 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 34 >>]. 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 35 >>]. All overlap sections show some change in thickness [Figure 36 >>]. 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 deeper—and could allow a thicker package of sediment to accumulate—at 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 15 >>], 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 37 >>].
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 31 >>] thickens and its lake-highstand deposits deepen toward the center of the basin and toward the border-fault system. Strata also get coarser grained with proximity to the border-fault system and the lateral edges of the basin. The Ukrainian Member [Figure 38 >>] is another unit that is mappable throughout the Newark basin, and generally shows the same trends in thickness and facies documented for 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 13 >>], 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 39 >>]. 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 during 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 distinct from one another and also quite distinct from the Stockton Formationa [Figure 26 >>]. The lower Stockton Formation, which is considerably coarser than other units in the NBCP composite, is a mostly fluvial (river) deposit. The upper Stockton and lower Lockatong formations represent lacustrine strata in which the lake-highstand deposits got progressively deeper up-section. 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 up-section. Thus, the large-scale stratigraphy in the Newark basin Triassic-age section consists of three 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"; it is also present in many rift basins from around the world (Lambiase, 1990).
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 40 >>]. 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. No matter how wet the climate, the lake will never be deeper than the difference in elevation between the outlet and the depositional surface. 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 scenarios 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 simple model for the evolution and filling of a rift basin [Figure 41 >>], 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 and the water-supply are constant and initially larger than incremental capacity, then we predict a tripartite stratigraphy while extension is active. Initially, basin capacity is small, and sediment supply exceeds capacity, thus resulting in fluvial deposition (Stage 1A). As the basin capacity progressively increases, a point is reached where the incremental capacity and sediment supply are balanced (Stage 1B). Thereafter, lacustrine deposition can take place as incremental capacity is greater than sediment supply; lake depth is initially shallow as the excess incremental capacity (i.e., the part of the basin not filled with sediment) is small (Stage 2A). As basin growth continues, water supply eventually matches the excess incremental capacity, producing the deepest lake (Stage 2B). After this point, basin growth results in sediment and water being spread out over a large basin, causing gradually diminishing lake depths. This basin-filling model is only one of several explanations for the larger-scale stratigraphy of the Newark basin, but it is one of simplest, and scientists generally prefer simpler explanations to more complex explanations. Note that the lake-level cycles discussed previously are a result of much shorter-period changes in climate. The basin filling model explains the large-scale character of the Stockton, Lockatong and Passaic formations (at the scale of 100s to 1000s of meters, whereas Milankovitch cycles explains the cycles at the scale of meters to 10s of meters).
ADVANCED TOPIC: Uplift and Erosion
Let’s begin this section by revisiting seismic-line NB-1 [Figure 16 >>]. The geologic map in the area of this seismic line shows that the Passaic Formation is the youngest unit present. In fact, the Perkasie Member of the Passaic Formation crops out at the NW end of the seismic profile, i.e., in proximity to the border fault; this means that only the lower third of the Passaic Formation is present in this part of the basin [Figure 30 >>]. Were the upper Passaic and overlying formations never deposited? Alternatively, were they once present and subsequently removed by erosion. Work by Withjack et al. (2013, 2020) indicates that up to 6 km of synrift strata have been eroded from the Newark basin during uplift. Two key pieces of evidence in support of this are vitrinite reflectance and variations in sonic-transit time.
Vitrinite reflectance uses the appearance of organic matter in rocks (from cores, outcrops, etc.) to estimate the maximum temperature that the organic matter experienced during burial. Once the temperature is known, we can use knowledge of the geothermal gradient (the rate at which temperature increases with depth in the Earth) to estimate how much rock was once above the sample location. Suppose that a sample currently exposed at Earth’s surface contains organic matter with a reflectance indicating it experienced a temperature of 100°C. If the geothermal gradient was 25°C, then its burial depth was 4 km. Thus, the amount of erosion and uplift to bring that sample to Earth’s surface was 4 km. This description is greatly simplified, and the interested reader should look at Malinconico (2010) and Withjack et al. (2013, 2020).
Sonic transit time is a geophysical logging technique that the uses the travel time of sound waves to estimate the porosity of shales in boreholes. Because of compaction, the porosity of shales exponentially decreases with depth in modern basins. Porosities estimated from shales in Newark basin boreholes all indicate a lower porosity than is typical for their depth with respect to modern basins. Essentially, this means that the shales are overcompacted. The amount of overcompaction allows calculation of the maximum burial depth. The difference between the maximum burial depth and the present-day depth in a borehole is the amount of erosion. The sonic-transit time and vitrinite-reflectance analyses yield comparable values for erosion (Durcanin et al., 2017; Withjack et al. 2020). In the rest of this discussion, we summarize the results obtained by Withjack et al. (2013).
Vitrinite-reflectance data indicate that the basin has undergone 1 to 6 km of erosion, with higher values near the SE edge of the basin and lower values adjacent to the border fault [Figure 42 >>]. The estimates of the amount of eroded section provide constraints on a how the basin appeared at the end of rifting by adding back the eroded section and removing the effects of post-depositional tilting, faulting, and folding (Figure 43 >>). Using this reconstructed basin geometry, we can sequentially remove the formations to show how the geometry of the basin evolved. (Figure 44 >>). The basin was initially narrow (<25 km) and visibly asymmetric, with the two oldest synrift units showing significant thickening toward the border fault. As rifting continued, the basin became much wider (possibly >100 km), deeper (up to 10 km), and less asymmetric; syn-rift strata exhibit subtle thickening toward the border-fault zone. Subsequent late rift and post-rift deformation and erosion (up to 6 km) significantly reduced the size of the Newark basin. Withjack et al. (2020) discuss various mechanisms to produce the uplift; their preferred explanation is that this region still retained some of the elevation inherited from Appalachian mountain-building. Reduction of the elevation is responsible for the erosion and exhumation.
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 45 >>]. All rift basins shown in the figure contain Late Triassic-age strata. Southern-segment rift basins contain only Triassic-age strata. Central-segment rift basins also contain Triassic through Early Jurassic-age strata. Northern-segment rift basins contain Triassic through Late Jurassic / Early Cretaceous-age strata. We interpret this to mean that the duration of rifting was longest in the north and shortest in the south. Because of post-rifting erosion, rifting may have lasted longer than the ranges shown in the figure, but studies on organic maturity by MaryAnn Malinconico indicate that the fundamental conclusion does not change appreciably. Some central-segment rift basins also contain Permian strata. Other basins may have strata older than Late Triassic age, given that onlap geometries (as observed on seismic line NB-1, Figure 16 >>; see dark orange unit (unexposed synrift) at base of synrift section) mean that the oldest synrift strata are not exposed at the Earth's surface.
All exposed rift basins have exclusively non-marine deposits. Marine deposits are present in some subsurface rift basins studied with seismic profiling and drilling, including the Jeanne d’Arc, Orpheus, and Scotian-shelf basins (all of which contain evaporites) [Figure 46 >>]. The Orpheus basin is bounded by the same border-fault system as the Fundy basin. The presence of salt in Orpheus but none in Fundy suggests that the more easterly Orpheus basin was located at a lower elevation and thus subject to incursions of ocean waters.
The oldest known strata in all rift basins shown in the figure are fluvial. In most basins the basal fluvial deposits are succeeded by lacustrine strata and fit into the tripartite stratigraphy discussed previously. Therefore, it is possible that the same processes that produced the tripartite stratigraphy in the Newark basin also produced the stratigraphy in the other basins. The fluvial-lacustrine transition is not present in Triassic-age strata from the Pomperaug, Hartford and Deerfield basins. It is possible that basin capacity was sufficiently small and/or sediment supply sufficiently large that the fluvial-lacustrine transition never occurred.
Synrift CAMP-related lava-flow unit are present only in central- and northern-segment rift basins [Figure 45 >>], suggesting that the southern-segment basins had become inactive prior to eruption of CAMP basalts. In the Newark basin, CAMP-related flows are the Orange Mountain Basalt, Preakness Basalt, and Hook Mountain Basalt. The strata interbedded and succeeding the lava flow units in the Newark basin are highly cyclical [Figure 26 >>] and quite similar to sequences of the same age in the Hartford subbasin of the Connecticut Valley basin. For example, the East Berlin Formation of the Hartford subbasin is virtually identical to the Towaco Formation of the Newark basin [Figure 47 >>]. Although these basins may never have been physically connected, their lakes responded to the same regional climatic events. The CAMP-related lava flows have a similar stratigraphy and geochemistry [Figure 48 >>]. 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 49 >>] indicate that the ~600,000 year duration of extrusive activity in the Newark basin [Figure 33 >>] also applies to the other rift basins.
Although the latest Triassic – earliest Jurassic-age CAMP-related flows and interbedded strata are quite similar in some of the central-segment rift basins, the Late Triassic-age strata exhibit some important regional differences [Figure 46 >>]. 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 (this does not apply to the Feltville, Towaco, and Boonton formations). 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 50 >>]. 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 (also indicated by the evaporites in the adjacent Orpheus rift basin). 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 ~12° [Figure 32 >>], only about 1300 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 01 >>]. The Newark basin (and the other basins in eastern North America) gradually drifted northward as indicated by the increase in the paleolatitudes [Figure 32 >>]. 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 American rift system [Figure 51 >>] include: 1) the Newark basin is part of a large rift zone that includes the Narrow Neck, Gettysburg, and Culpeper rift basins; the dip direction of the border faults is always to the S to SE to ESE for this rift zone. This rift zone is roughly the same length as Lake Tanganyika in the East African rift system; the dip direction of the border faults alternate along the length of that rift zone. 2) As noted above, southern-segment basins (Richmond, Dan River, and Deep River) do not contain any latest Triassic to earliest Jurassic-age strata or CAMP-related lava flows; northern-segment rift basins contain synrift strata as young as Early Cretaceous.3) Diabase dikes are mostly NE-striking in and near the Newark and Connecticut Valley basins and NW- and N-striking for the Danville/Dan River and Deep River basins; these dikes in the southern segment of the rift system apparently cross-cut the border faults indicating emplacement after the border faults became inactive; this is consistent with the youngest synrift strata being Late Triassic in age. 4) Most basins in cross sections perpendicular to the border-fault system exhibit the classic asymmetric half-graben geometry. 5) The majority of 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 as well as some basins on the Scotian shelf have border faults dipping to the west or northwest, presumably reflecting the local orientation of reactivated Paleozoic faults. Most border faults or segments of border faults are inclined at angles more typical of thrust faults (30°) than normal faults (60°), also suggesting reactivation of preexisting weaknesses. 6) Basins with highly ductile units (e.g., Mohican, Orpheus, Jeanne d’Arc, and Flemish Pass) tend to be more structurally complex than basins lacking these units. 7) Some basins, especially in the north, are quite wide; others like the Danville / Dan River basin are very narrow, perhaps reflecting more post-rift erosion. 8) Several basins (Fundy, Mohican) have faults with components of reverse offset and associated folds (i.e., inversion structures which form when a rift basin characterized by subsidence and normal faulting is inverted by reverse faulting and uplift).
The maps and cross sections of the eastern North American rift basins form the basis of idealized rift-basin geometries [Figure 15 >>]. The Newark, Gettysburg, Culpeper, and Pomperaug 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 between the Newark and Gettysburg basins is like the northern margin of the idealized basin in Case c.
In comparing the Newark basin with other rift basins from around the world [Figure 52 >>], we note the following for the basins in cross section: 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, Voring, and Viking basins have a relatively low dip angle, especially along their deeper parts, suggesting reactivation of preexisting thrust faults as normal faults; the border faults of the Upper Rhine and Suez rift basins dip moderately to steeply and probably are not reactivated structures. 3) The Fundy rift basin has been modified by a period of postrift shortening that produced inversion structures. 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 strata in the Voring rift basin and the Dampier subbasin are related to the presence of prerift salt and thick shales. Because of these considerations, the geometry of rift basins in cross section can be quite variable.
Regarding the worldwide rift basins in map view, we note the following: 1) For the Connecticut Valley, Newark, and Dampier basins, the extension direction is oblique to the overall trend of the rift basin, which has produced secondary faults oblique to the regional trend of the basins. For the Suez and Rhine rift basins, the estimated extension direction is (sub)perpendicular to the regional trend of the basins. 2) For the Jeanne d’Arc basin, the extension direction changed during its protracted rift history. 3) In the Newark basin, the border fault is (sub)parallel to preexisting faults in pre-rift rocks. In the Dampier basin, thick shales act as a highly ductile layer that decouples the deep and shallow deformation. In the Suez rift basin, the dip direction of border faults and the dip direction of syn-rift strata vary along the length of the rift.
Withjack et al. (2002) noted that a standard rift basin [Figure 53 >>] has a moderately dipping border fault perpendicular to the extension direction and parallel to secondary normal faults; the border fault bounds an asymmetric basin (half graben) with a triangular geometry in a cross section perpendicular to the border fault. They proposed that the following factors produce deviations from this standard geometry: 1) If highly ductile units like evaporites or thick shale are present in synrift or prerift rocks, there is a decoupling of faulting-dominated deformation at depth from folding-dominated deformation at shallower levels, producing a more symmetrical syncline-shaped basin. 2) The presence of preexisting zones of weakness formed during pre-rift shortening can produce border faults with anomalously shallow inclinations (dip angles). 3) If the extension direction is oblique to preexisting zones of weakness, the border fault and secondary faults will have somewhat different trends; the border fault will also not be a pure normal fault (i.e., there will be a component of strike slip). 4) If shortening takes place after rifting, normal faults formed during rifting may undergo reverse reactivation with associated folding, producing uplift. Rift basins may have two or more of these factors influencing their architecture.
