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Home , Fault block, Graben, Newark Basin, Stockton Formation, Tectonics, Tectonic subsidence

VOLUME 98 NUMBER 2

THE JOURNAL OF March 1990

QUANTITATIVE FILLING MODEL FOR CONTINENTAL EXTENSIONAL BASINS WITH APPLICATIONS TO EARLY OF EASTERN '

ROY W. SCHLISCHE AND PAUL E. OLSEN Department of Geological Sciences and Lamont-Doherty Geological Observatory of Columbia University, Palisades, 10964

ABSTRACT In many half-, strata progressively onlap the hanging wall block of the basins, indicating that both the basins and their depositional surface areas were growing in size through time. Based on these con- straints, we have constructed a quantitative model for the stratigraphic evolution of extensional basins with the simplifying assumptions of constant volume input of sediments and water per unit time, as well as a uniform subsidence rate and a fixed outlet level. The model predicts (1) a transition from fluvial to lacustrine deposition, (2) systematically decreasing accumulation rates in lacustrine strata, and (3) a rapid increase in depth after the onset of lacustrine deposition, followed by a systematic decrease. When parameterized for the early Mesozoic basins of eastern North America, the model's predictions match trends observed in late -age rocks. Significant deviations from the model's predictions occur in Early -age strata, in which markedly higher accumulation rates and greater lake depths point to an increased extension rate that led to increased asymmetry in these half-graben. The model makes it possible to extract from the sedimentary record those events in the history of an extensional basin that are due solely to the filling of a basin growing in size through time and those that are due to changes in , climate, or sediment and water budgets.

INTRODUCTION trast to the study of post- deposits, in A fundamental problem in the study of ex- which the fundamental aspects of the stratig- tensional basins is the quantitative determi- raphy are explained in terms of thermally nation of the first-order controls on basin driven subsidence, sediment loading, sedi- stratigraphy. This problem is especially acute ment , and eustatic for non-marine basins, which lack a sea-level change (e.g., McKenzie 1978; Steckler and datum. Traditionally, each major change in Watts 1978; Bond and Kominz 1984; Steckler depositional environment and sedimentation et al. 1988). has been interpreted as a result of some "tec- Quantitative stratigraphic modeling of ex- tonic" event (such as increased boundary tensional basins has proven difficult because movement) or major climatic change, of the lack of appreciation of the critical ef- yet predictive, quantitative models for the fects of basin geometry on the stratigraphic stratigraphic development of extensional ba- record. In this paper, we develop a simple sins are notably lacking. This stands in con- model of extensional basin filling that appears to explain many of the elements of the strati- ' Manuscript received May 18, 1989; accepted graphic record of early Mesozoic rift basins October 17, 1989. of eastern North America. The model is based on the filling of a basin growing in size [JOURNALOF GEOLOGY,1990, vol. 98, p. 135-1551 0 1990 by The University of Chicago. All rights through time. Even under conditions of uni- reserved. form subsidence and constant volume of sedi- 0022-1376/90/9802-001$1.00 ment and water input per unit time, the model R. W. SCHLISCHE AND P. E. OLSEN

. . - - - Post-rift sediments

- - - - / - 6 ^ '/I 5 km - B - -/ . 8

FIG. 1.-Cross section and seismic lines of typical half-graben, showing the wedge-shaped geometry and the progressive onlap of younger strata onto the hanging wall basement block on the hinged side of the (A) Newark (see figs. 5 and 7 for location), (B) Atlantis (see fig. 5 for location), and (C) Railroad basins. Abbreviations for stratigraphic units in the are: B, ; F, Feltville Forma- tion; H, Hook ; L, ; 0, Orange Mountain Basalt; P,Passaic Forma- tion; Pd, Palisades ; Pr, Preakness Basalt; S, ; and T, . TWTT is two-wav travel time. Seismic ~rofileswere traced from figures presented by Hutchinson et al. (1986) and reela and and Berrong (1979): predicts transitions in major depositional en- drill hole data record the presence and thick- vironments and trends in accumulation rates ening of the Lower Barren Beds (B. Cornet and lake depth. Furthermore, the model's pers. comm. 1989). At Tennycape, Nova predictions can be "subtracted" from the Scotia, strata of the Triassic Wolfville For- actual stratigraphic record, yielding the tec- mation clearly onlap the tonic and climatic signal and allowing for the "basement" rocks in the hinge of the Fundy parameterization of sediment and water half-graben. Furthermore, the beds of the budgets. Wolfville Formation dip at a shallower angle than the between the Triassic EXTENSIONAL BASIN GEOMETRY AND and Carboniferous rocks (Olsen et al. 1989). STRATIGRAPHIC ARCHITECTURE McLaughlin stated that the basal Stockton In many extensional basins, younger strata Formation onlapped the northeastern, south- progressively onlap the "basement" rocks of ern, and southwestern margins of the Newark the hanging wall block (fig. 1). Charles Lyell basin (McLaughlin 1945; McLaughlin and (1847) first noted this onlap geometry in the Willard 1949; Willard et al. 1959; see fig. 1A). Blackheath region of the hinge area of the Seismic reflection profiles reveal this onlap Triassic Richmond basin of Virginia. Outcrop geometry in (1) the Cenozoic basins of the studies and coal mine excavations indicate Basin and Range-the Dixie Valley basin, that the lowermost stratigraphic unit-the (Anderson et al. 1983, their fig. 4), Lower Barren Beds-is absent from this re- the northern Fallon basin of the Carson De- gion and that the stratigraphically higher Pro- sert, Nevada (Anderson et al. 1983, their fig. ductive Coal Measures rest directly on base- S), the Diamond Valley basin, Nevada (An- ment. Further toward the border fault of the derson et al. 1983, their fig. 6), the Railroad half-graben, seismic reflection profiles and Valley basin, Nevada (fig. 1C; Vreeland and QUANTITATIVE FILLING MODEL Berrong 1979, their fig. 8), and the Great Salt and Schlische (1988a, 1988b) have demon- Lake basin, Utah (Smith and Bruhn 1984, strated how evolving basin geometry as a their fig. 10); (2) Lake Tanganyika of East consequence of filling under conditions of (Burgess et al. 1988, their fig. 35-12); uniform subsidence and uniform rate of input (3) the North Viking graben of the of sediments and water could produce much (Badley et al. 1988, their figs. 6 and 8); (4) the the same stratigraphic architecture and in ad- Hopedale and Sagiek basins of the Labrador dition make specific predictions about the margin (Balkwill 1987, his figs. 9 and 11); and changes in accumulation rates and times of (5) the Mesozoic basins of the U.S. Atlantic transitions from one environment to another. -the Long , Nan- tucket, and Atlantis basins (fig. 1B; Hutch- inson et al. 1986, their figs. 3, 7, 11, and 15). EXTENSIONAL BASIN FILLING MODEL Anderson et al. (1983) cited this onlap pat- The analytically simplest model consists tern as evidence that the extensional basins of a basin, trapezoidal in cross-section, had grown in size through time. Leeder and bounded by planar faults that dip at equal Gawthorpe (1987) related the onlap pattern to angles, and bounded along-strike by vertical the migration of the basin's hinge or fulcrum surfaces (fig. 2). In this full-graben model, (reflecting the line basinward of which the uniform extension results in uniform subsi- hanging wall has experienced subsidence) dence along the boundary faults (see fig. 3), away from the border fault and have incor- and hence the depth (D) of the basin in- porated this concept into their tectono- creases linearly. To maintain simplicity in the sedimentary facies models. We agree with model, the volumetric sedimentation rate these authors' interpretations and extend (V,, the volume of sediment added to the ba- them one step further: not only were the ex- sin per unit time) and the available volume of tensional basins growing in size, but the de- water (Vw) are constant. The outlet of the positional surface also was growing in area basin also is assumed to be held fixed with through time as a consequence of the filling of respect to an external reference system. the basin. This conclusion serves as the foun- If the basin subsides slowly enough or V, is dation of the extensional basin filling model. large enough, the basin initially fills com- Continental extensional basins often dis- pletely with sediments, and excess sediment play a tripartite stratigraphic architecture and water leave the basin (fig. 3A.1). We (Lambiase 1990) which, from bottom up, con- characterize this mode of sedimentation as sists of: (1) a fluvial unit indicating through- fluvial; the basin is sedimentologically and going drainage and open-basin conditions; (2) hydrologically open. During this phase, ac- a lacustrine unit with deepest-water facies cumulation rate (thickness of sediment de- near its base, gradually shoaling upward, in- posited per unit time) is constant and equal to dicating predominantly closed-basin condi- the subsidence rate: with an excess of sedi- tions; and (3) a fluvial unit reflecting once ments available, the thickness of sediment again through-going drainage. deposited is governed solely by the space These major changes in depositional envi- made available through subsidence. As the ronments (on a basin-wide scale) have been graben continues to grow, the same volume explained in terms of changes in basin subsi- of sediment added per unit time is spread out dence (Lambiase 1990). The fluvial unit is in- over a larger and larger depositional surface terpreted to have been deposited during ini- area. Ultimately, a point is reached when the tial, slow basin subsidence, perhaps in a sediments exactly fill the basin (fig. 3A.2). number of linked, small sub-basins. The Thereafter, the sediments no longer can com- deep-water lacustrine unit is interpreted to pletely fill the growing basin. The basin is reflect a deepening of the basin resulting from sedimentologically closed (all sediments en- increased subsidence and the coalescence tering the basin cannot leave the basin), and a of sub-basins. The upward-shoaling is inter- lake can occupy that portion of the basin be- preted to reflect the gradual infilling of the tween the sediment surface and the basin's basin, with fluvial sedimentation returning af- outlet (fig. 3A.3). After the onset of lacustrine ter the basin had filled to the lowest outlet of deposition, accumulation rates decrease sys- the basin. Taking a different approach, Olsen tematically, driven by the increasing size of R. W. SCHLISCHE AND P. E. OLSEN

-84 FIG.2.-Full-graben model, showing meaning of symbols used in the equations in the text. the depositional surface and constant rate of sedimentation, cumulative sediment thick- input of sediments (fig. 3A.3-6), ness (Y) and the depth of the graben (D) are Assuming an unlimited supply of water, given by: lake depth (W) would be equal to the differ- ence between the height of the basin's outlet above the floor of the graben (defined as the basement-sediment contact) and the cumula- where SR is the subsidence rate and t is time. tive sediment thickness (fig. 3A.3). However, Accumulation rate (S) is the derivative of the the supply of water is not infinite, and a point cumulative thickness curve: is reached when the graben has grown to such a size that the finite volume of water just occupies the volume between the ba- sin's outlet and the depositional surface (fig. After the onset of lacustrine sedimentation 3A.4). Thereafter, the basin is hydrologically (sedimentologic closure), cumulative sedi- closed, and lake depth decreases as a func- ment thickness and accumulation rate are tion of the growing size of the basin and non- given by: linear losses to evaporation from a lake whose surface area is growing through time tan a Y=Yf+- (fig. 3A.5-6). 2 If the subsidence rate is fast enough or V. is small enough, the basin never fills completely with sediments, and lacustrine deposition can occur from the outset (fig. 35.1). As the gra- ben grows in size through time, the rate of increase of cumulative sediment thickness and the accumulation rate both decrease. Lake depth increases quickly until the finite volume of water just fills the graben (fig. 35.2) and thereafter decreases (fig. 35.3). After the onset of lacustrine deposition, ac- cumulation rate should decrease indefinitely. However, graben do not continue to subside indefinitely. If the subsidence rate slows sufficiently or stops and V, remains con- stant, the accumulation rate and lake depth would continue to decrease until the depo- sitional surface could reach the basin's where Yf is the thickness of fluvial sediments, outlet, thereby heralding the return of flu- B is the width of the base of the graben, L is vial sedimentation at a rate equal to the sub- the length of the graben, a is the dip of the sidence rate. border faults, and ti is time after the onset of Quantitatively, under conditions of fluvial lacustrine deposition. While the basin is hy- QUANTITATIVE FILLING MODEL 139 drologically open, water depth (W) is given by:

D-Y (34

Equation (3A) also applies in the case of fluvial sedimentation and correctly predicts zero lake depth since D = Y. The equation that describes lake depth after hydrologic clo- sure is:

w=- tan a 2L

I x J{BL + ~YLcot a)' + ~LV,,,CO~a

- B + 2Ycota 2 cot a

where Vw is the volume of water available. The derivation of equations (1B) and (3B) are as follows, using the geometry depicted in figure 2. After each increment of subsidence [equation (lA)], the capacity of the basin (the volume of the trapezoidal trough) is cal- culated. If the capacity is less than the vol- ume of sediment that can be supplied in that increment, the basin is filled with sediments, and the thickness of sediments deposited in that increment is equal to the amount of sub- sidence for that increment. If the capacity of the basin exceeds the volume of sediment that can be supplied in that increment, the basin cannot completely fill. An integrated approach can then be applied to derive the expression for cumulative sediment thickness [equation (IB)]. From the geometry depicted in figure 2, the volume (V) of sediments is given by

FIG.3.-Diagrammatic representation of the sa- V = V,tl = (x cot a)(x)(L) lient features of the filling of a full-graben under conditions of uniform subsidence and constant vol- ume input of sediment per unit time; e equals per- cent extension. See text for further explanation. where V, is the volumetric sedimentation rate, tl is the time after the onset of lacustrine sedimentation, a is the dip of the border faults, L is the length of the basin, B is the initial fault spacing, Yf is the thickness of fluvial sediments, and x is the next increment 140 R. W. SCHLISCHE AND P. E. OLSEN of lacustrine sediment thickness. Rearranging Drill Hole terms yields:

Vs t (cot a)x2 + (B + 2Yfcot a)x - - = 0 L (5) Drill Hole Data The positive root of x is obtained from the quadratic formula. Equation (1B) is then given by

012345 Time (millions of years) After the onset of lacustrine deposition, the capacity of the basin is again calculated knowing Y. If it is less than the available vol- ume of water (Vw), water depth (W) is cal- culated as the difference between the total depth of the basin and Y. If the basin's capac- ity exceeds V,,,, lake depth is derived as fol- 0 Basin Averaae lows, again based on the geometry in figure 2. Time (millions of years) Vw = (B + 2 Y cot a)(W)(L) FIG. 4.-Half-graben filling model. (A) Cross + (W cot a)(W)(L) (7) section of half-graben loosely based on the geome- try of the Atlantis basin (fig. I), in which stratal (L cot a)W2 + (BL + 2Y cot a)W - V,,,= 0 surfaces, corresponding to time slices of 550,000 (8) yrs, delineate areas of 1.5 x lo6 m2 which were iteratively fit to the basin. Cumulative sediment thickness (B) and accumulation rate (C) show simi- Solving for the positive root of W yields equa- lar trends as in a full-graben (cf. fig. 9). tion (3B). In principle, the above analysis should hold for any concave-upward basin in which the of the volumetric sedimentation rate) is itera- depositional surface area grows through time tively fit to the observed geometry of a half- under conditions of uniform subsidence, con- graben (fig. 4). Throughout the "filling" pro- stant rate of input of sediments and water, cess, both the half-graben's geometry and the and constant position of the outlet fixed to an stratal geometry are maintained. As seen in external reference system, although the form figures 45 and 4C, both the cumulative thick- of the equations will differ depending on the ness and accumulation rate curves for both a evolving geometry of the basin. In a half- drill hole through the deepest part of the ba- graben growing in volume through time (in- sin and the average thickness of each stratal ferred from onlap geometry) under conditions package are quite similar in form to the trends of uniform extension, it therefore seems rea- produced in a full-graben (cf. figs. 4 and 9), sonable to assume that the changes in aver- implying that full- and half-graben fill simi- age accumulation rate and the time of transi- larly. tion from fluvial to lacustrine sedimentation The evolution of half-graben, however, can should follow a similar path as for a full- be more complicated than that proposed in graben as long as the area of the depositional the model. The depositional surface area of a surface continues to grow. Although three- half-graben ceases to grow when the onlap- dimensional modeling of the filling of a half- ping edge of the basin fill passes inside the graben has not yet been undertaken, prelimi- margin of previously deposited sediments, nary two-dimensional modeling indicates that i.e., when strata cease to onlap the "base- the above assumption may be valid. In the ment" rocks and begin to onlap previously two-dimensional approach, a constant cross- deposited strata. At that point, under condi- sectional area (the two-dimensional analogue tions of uniform subsidence, the accumula- QUANTITATIVE FILLING MODEL 141 tion rate would cease to change or actually the basin filling model warrants additional decrease. Additional complications arise in discussion. The model assumes a constant half-graben bounded by domino-style rota- volumetric sedimentation rate, yet as the ba- tional faults. With continued extension, the sin fills, younger strata progressively onlap normal faults progressively rotate to shal- potential source areas of new sediment. As a lower dips (Wernicke and Burchfiel 1982). result, volumetric sedimentation rate might Even under conditions of uniform extension, decrease. However, in the East African rifts, therefore, subsidence will not be uniform. most sediments are derived from the hanging Furthermore, subsidence can .be expected to wall block and from rivers flowing along the decrease from a maximum along the bound- axis of the rift system (Lambiase 1990). If this ary fault to a minimum at the hinged margin axial component is a general feature of most of the half-graben. We also expect significant rift systems, then the effect of onlap onto along-strike changes in subsidence, ranging source regions may not critically affect sedi- from a maximum near the center of the ment supply, and the assumption of constant boundary fault to zero at its tips. This cumu- V. is probably a good starting point. lative variation in subsidence would then The model also assumes that the outlet of mimic the nature of subsidence observed fol- the depositional basin is fixed with respect to lowing slip on normal faults, such as the 1959 an external reference system. Based again on Hebgen Lake in Montana (Fraser the East African rifts, the lowest elevation et al. 1964). outlet is commonly found at the along-strike The qualitative model presented above pre- end of an individual half-graben (Lambiase dicts major changes in depositional environ- 1990). As this is the region that experiences ments as a consequence of the filling of the minimal hanging wall subsidence and foot- basin: fluvial sedimentation (basin is sedi- wall uplift, then the outlet may indeed be as- mentologically and hydrologically open) giv- sumed to be stationary. ing way to hydrologically open lacustrine As for subsidence and volume of water sedimentation followed by hydrologically available, there are no reliable, long-term, closed lacustrine sedimentation. Further- modern data. Parsimony therefore dictates more, the model can be quantified to predict that we assume these parameters to be con- the timing of the major transitions in deposi- stant. The model is specifically designed to tional environments and trends in accumula- assess the intrinsic effects of the filling of the tion rates and lake depth. Clearly, however, basin. Only when these are understood can the model cannot address all aspects of ex- we begin to examine and decipher the causes tensional basin stratigraphy. For example, of the extrinsic effects (changes in subsidence during "lacustrine deposition," fluvial and rate, climate, and rates of sediment and water fluvio-deltaic deposits will ring the lacustrine input). This approach is illustrated below deposits (see facies models of Leeder and with reference to the extensional basins con- Gawthorpe 1987). Furthermore, near the taining the rocks of the early Mesozoic-age main border faults of the basin, lacustrine , particularly the Newark and alluvial fan deposits will interfinger. Re- basin. peated motions along the border fault will lead to the formation of tectonic cyclothems APPLICATION OF THE BASIN FILLING MODEL (Blair and Bilodeau 1988). In addition, the Geologic Overview of the Newark Super- facies distribution will be influenced by group.-Eastern North America forms part climatic changes (Olsen 1984a, 1984b, 1986): of the classic Atlantic-type passive continen- the may shrink and swell, and the po- tal margin (Bally 1981) created by rifting of sition of the marginal lacustrine facies will the of . All along the migrate accordingly. The model therefore axis of the future from Green- is only intended as a guide to understanding land to Mexico, the Triassic initiation of the the gross stratigraphic development of ex- breakup was marked by the formation of ex- tensional basins, reflecting perhaps those tended . Early Mesozoic rift basins of processes operating on a scale of lo6 or 10' eastern North America are exposed from years. South Carolina to Nova Scotia; numerous The rationale of the assumptions used in others have been discovered or inferred by R. W. SCHLISCHE AND P. E. OLSEN

FIG.5.-Early Mesozoic rift basins of eastern North America. Names of basins mentioned in the text are bold-faced. ~odifiedfrom Olsen et al. (1989). drilling, seismic reflection profiling, and grav- stand, and (3) regression plus low-stand ity studies to be buried both by post-rift sedi- facies (Olsen 1986). These cycles were appar- ments on the continental shelf and below ently produced by the basin-wide rise and coastal plain sediments (fig. 5). Thousands of fall of water level of very large lakes (Van meters of exclusively continental strata, Houten 1964; Olsen 1984a, 1984b, 1986; tholeiitic lava flows, and diabase plutons Olsen et al. 1989). Fourier analysis of strati- filled the exposed rift basins. The faulted, graphic sections has shown that the Van tilted, and eroded strata are termed the Houten cycles have a periodicity of approxi- : Newark Supergroup (Van Houten 1977; mately 21,000 yrs (Olsen 1986). Furthermore, Olsen 1978; Froelich and Olsen 1984). the 21,000-yr-long cycles are hierarchically Milankovitch-Period Lacustrine Cycles.- arranged in compound cycles of 100,000 and The lacustrine rocks of Newark Supergroup 400,000 yrs. It is this hierarchy that provides basins show a common theme of repetitive the best evidence that lake levels were cli- cycles named in honor of their discover, matically driven by orbital variation accord- F. B. Van Houten, by Olsen (1986). The fun- ing to the Milankovitch theory (Olsen 1986). damental Van Houten cycle consists of three These cycles permit well-constrained esti- lithologically-distinct divisions which are in- mates of accumulation rate (thickness of cy- terpreted as (1) lake transgression, (2) high cle divided by its duration) in the lacustrine QUANTITATIVE FILLING MODEL "Maximum"

Lake Depth (Milankovitch Cycles) High *

FIG.6.-Stratigraphic pattern of the Newark basin. Black lines in the stratigraphic column represent the most prominent gray and black units for which their proper stratigraphic position is known. Accumulation rates were calculated using the thicknesses of the 21,000-yr-long Van Houten cycles. Letters refer to specific measured sections: a, Lower Lockatong Formation, Lumberville, PA (Olsen unpubl. data); b, Middle Lockatong Formation along NJ 29, near Byram, NJ (Olsen 1986); c, Perkasie Member of the , US 202, Dilts Comer, NJ (Olsen et al. 1989); d; Ukrainian Member of the Passaic Formation, Ukrainian Village, NJ (Olsen unpubl. data); e, f, and g, Feltville, Towaco, and Boonton formations, respectively, from Army Corps of Engineers drill cores, Pompton Plains to Clifton, NJ (Fedosh and Smoot 1988; Olsen and Fedosh 1988; Olsen et al. 1989). "Maximum" lake depth is the enveloping surface of the Milankovitch-period fluctuations in lake level. The extrusive interval consists of the following formations (in stratierauhic order): Orange Mountain Basalt, , Preakness Basalt, Towaco Forma- tion, and~bokMountain ~asalt. intervals of these extensional basins, thereby the onset of lacustrine deposition, accumula- providing a high-resolution test of the predic- tion rate (as calibrated by the thickness of tions of the basin filling model. 21,000-yr-long Van Houten cycles) slowly Stratigraphic Pattern.-Two markedly dif- and systematically decreased through the re- ferent stratigraphic patterns are present mainder of the Triassic section (Lockatong within the basins of the Newark Supergroup and Passaic formations). Lake depths were and are exemplified by the stratigraphy of the controlled by orbitally induced climatic Newark basin (Olsen 1980). The first strati- changes, and the lakes dried out every 21,000 graphic pattern is of mostly age yrs (Olsen 1986). However, certain longer- and consists of (1) the basal red and brown term trends are also apparent. The first mi- fluvial strata of the Stockton Formation (de- crolaminated black shales formed about posited during the first 3 m.y. of basin subsi- 800,000 yrs after the onset of lacustrine depo- dence), overlain by (2) the mostly gray and sition and must have been deposited in at black lacustrine rocks of the Lockatong For- least 70-100 m of water, below wave base, at mation, which is in turn overlain by (3) the the bottom of an anoxic, turbulently stratified mostly red, lacustrine and marginal lacustrine lake (Manspeizer and Olsen 1981; Olsen strata of the Passaic Formation (fig. 6). After 1984~).The best developed microlaminated 144 R. W. SCHLISCHE AND P. E. OLSEN units, suggesting the deepest lakes, are found Where the early Mesozoic extension direc- in strata deposited 1.4 m. y. after lacustrine de- tion was normal to the strike of pre-existing position began. Higher in the Lockatong For- structures, Paleozoic thrust faults of the Ap- mation, red massive mudstones become more palachian orogen were reactivated principally and more abundant. With the exception of as dip-slip normal faults, as in the Newark the Ukrainian Member of the Passaic Forma- basin (Ratcliffe and Burton 1985), forming tion and the Early Jurassic-age sedimentary half-graben in their collapsed hanging walls. rocks, the last microlaminated unit is present Based on surface geology and seismic reflec- near the base of the Passaic Formation. Gray tion profiles, the basin fill of these half-graben ' and black shales are still present within the is wedge-shaped (see fig. I), indicating that rest of the Passaic Formation, but both their normal faulting responsible for basin subsi- frequency and extent of development de- dence was active during sedimentation. Fur- crease up-section (fig. 6). For ease in model- thermore, younger strata dip less steeply than ing, we have defined "maximum" lake depth older strata, indicating syndepositional rota- as the depth of the deepest lake produced by tion of the hanging wall block. As mentioned each 400,000 yr climate cycle; it is the bound- above, published seismic reflection profiles of ing curve for the entire fluctuating lake level offshore basins clearly show onlap onto the curve (see fig. 6). "Maximum" lake depth be- hanging wall, and studies of outcrops, drill gan shallow at the onset of lacustrine deposi- holes, and proprietary seismic reflection pro- tion, rapidly deepened during the deposition files of the Newark, Fundy, and Richmond of the lower Lockatong Formation, and then basins lead to a similar conclusion. slowly and systematically decreased toward The Newark basin of New York, New the Triassic-Jurassic boundary (fig. 6). Jersey, and is a half-graben The second stratigraphic sequence of bounded on its northwestern side by a south- mostly Early Jurassic age consists of tho- east-dipping border fault system (figs. 1, 7). leiitic lava flows interbedded with and suc- The dip of the border fault ranges from 60' to ceeded by mostly lacustrine strata, which less than 25' (Ratcliffe and Burton 1985). The show marked changes in accumulation rate basin is approximately 190 km long (exclud- and "maximum" lake depth. Approximately ing the narrow neck between the Newark 30 m below the palynologically determined and Gettysburg basins), presently 30-50 km Triassic-Jurassic boundary (Cornet 1977), wide, and contains cumulatively >7 km of both the accumulation rate and "maximum" sedimentary rocks, lava flows, and diabase lake depth greatly increased (fig. 6). The high- sheets and dikes. Strata within the basin gen- est accumulation rates and the deepest lake erally dip at less than 20' toward the border facies (well-developed microlaminated units) fault system, although they are locally more are associated with the thick extrusive basalt steeply dipping within transverse folds adja- flows. After the extrusion of the last basalt cent to the border and intrabasinal faults (see flow, accumulation rate and "maximum" fig. 7). McLaughlin's studies of the Stockton lake depth again decreased systematically. Formation (McLaughlin 1945; McLaughlin Basin Geometry.-Eastern North Ameri- and Willard 1949; Willard et al. 1959) and our can rift basins were profoundly influenced by mapping of the thickness of Van Houten cy- pre-existing structural control (Ratcliffe and cles (Olsen et al. 1989; Olsen 1989) clearly Burton 1985; Swanson 1986; Schlische and show that units of all ages thicken noticeably Olsen 1987, 1988). Where the basins trended toward the border fault side of the basin, indi- oblique to the regional extension direction, cating that the faulting responsible for basin extensional strike-slip duplexes, flower struc- subsidence was occurring throughout the tures, and small syndepositional horsts and filling of the basin (contra Faill 1973). The basins were developed in strike-slip zones basin is cut by a number of intrabasinal (Schlische and Olsen 1987; Olsen et al. 1989). faults, but these appear to have developed In both volume of sedimentary fill and sur- late in the history of the basin, after deposi- face area, however, these strike-slip regions tion of most of the preserved sedimentary are subordinate to the predominantly dip-slip rocks (Olsen 1985; Faill 1988; Schlische and half-graben. Olsen 1988). FIG. 7.-Geologic map of the Newark basin of Pennsylvania, , and New York. Dark shading represents diabase intrusions, light shading represents lava flows. Dotted lines are from lines of bedding, and thin black lines are gray and black units in the Passaic Formation. Balls are on downthrown side of normal faults. A is position of cross section shown in figure 1; cross section B-B' shown in figure 8A. Abbreviations are: B, Boonton Formation; F, Feltville Formation; H, Hook Mountain Basalt; I, basement inliers; J, Jacksonwald Basalt; L, Lockatong Formation; 0, Orange Mountain Basalt; P, Passaic Formation; Pd, Palisades diabase; Pk, Perkasie Member of Passaic Formation; Pr, Preakness Basalt; 5, Stockton Formation; and T, Towaco Formation. Numbers refer to specific sections discussed in text and figure 9:1, Stockton, NJ; 2, Lumberville, PA; 3, Byram, NJ; 4, Dilts Comer, NJ; 5, Ukrainian Village, NJ; 6, 7, 8, Army Corps of Engineers cores, Pompton Lakes to Clifton, NJ. Modified from Schlische and Olsen (1988). R. W. SCHLISCHE AND P. E. OLSEN

FIG.8.-(A) Cross section A-A' of the Delaware River valley (see fig. 7 for location) before and after palinspastic restoration of slip on the intrabasinal faults. (B) Cross-sectional geometry of palinspastically- restored basin just after the deposition of the Perkasie Member. (C) Full-graben equivalent of basin shown in (B).The areas of the shaded regions in (B) and (C) are equal.

Comparison of Model Predictions with Schlische and Olsen 19881, key horizons can Newark Basin Stratigraphy.-Before the be palinspastically restored as shown in full-graben model can be applied, the asym- figure 8A. In this restored basin, the base and metric Newark half-graben needs to be top of the Stockton Formation are con- "transformed" to a symmetric basin. The ap- strained by known thicknesses and by the av- proach is to determine a representative cross- erage dip of the base of the Stockton in the sectional area of the Newark basin for a given northwestern ; where these two time-slice and then by conserving cross- horizons converge is taken to be the south- sectional area convert the basin into a full- eastern edge of the Newark basin after depo- graben with border faults dipping at equal sition of the Stockton Formation. A similar angles. Thus the width of the basin after de- exercise can be performed on the Perkasie position of a certain stratigraphic horizon and Member, where the constraints are (1) the po- the depth of the basin for that time need to be sition of Perkasie outcrops in the northwest- known. Unfortunately, the Newark basin has ern fault block and (2) the height of the been substantially eroded, and its present Perkasie above the top of the Stockton For- boundaries do not reflect its limits during de- mation in the middle fault block. A projection position. Variations in thickness of units with of the line connecting these two points clearly distance from the border fault, however, do extends further to the southeast than the lim- provide important constraints. In the Dela- its of the Stockton Formation, again reflect- ware River valley, the Newark basin is cut ing the progressive onlap of younger strata up into three fault blocks with repetition of onto basement on the hanging wall margin of the same formations (fig. 8A). Outcrops in the basin. the northwestern fault block indicate that the In figure 8B, the horizon representing Stockton Formation is approximately 1800 m the Perkasie Member has been rotated thick (Olsen 19801, whereas the Stockton is to paleohorizontal. The basin was approxi- only about 900 m thick where it outcrops in mately 6 km deep in its depocenter (cor- the southeastern fault block (Willard et a]. roborated by proprietary seismic lines) and 1959). Because the intrabasinal faulting com- >60 km wide after deposition of the Perkasie pletely post-dates the preserved strata in Member. We have assumed that the border these fault blocks (Olsen 1985; Fail1 1988; fault did not rotate along with the hanging QUANTITATIVE FILLING MODEL 147 wall and hence the 30' dip shown in figure 8B tion from fluvial to lacustrine sedimentation is the same as the present day dip (Ratcliffe at the appropriate time (fig. 9: b, b', and b"). et al. 1986). The "full-graben" equivalent of It remains to be seen whether the V, needed this basin is shown in figure 8C. By preserv- to generate this transition is realistic. A po- ing cross-sectional area, choosing an initial tential test would be to calculate the volume border fault spacing of 5 km (so that the "full- of a key marker bed, such as the Perkasie graben" more closely approximates the trian- Member, over the entire basin; unfortu- gular cross-sectional geometry of the palin- nately, the trends in thickness of units along- spastically-restored Newark half-graben), strike are not yet well enough defined. The and by preserving the same 6 km thickness model also predicts a systematic decrease of strata, the "border faults" of the "full- in accumulation rate in the Lockatong and graben" must dip at approximately 13'. Be- Passaic formations (fig. 9: f). Actual strati- cause of this transformation, the output of the graphic data show a similar trend, suggesting model (always given for the deepest part of that the filling of a basin growing in size the full-graben) will most closely match the through time might indeed be responsible. stratigraphic data from the deepest portion of Notice, however, that the model's accumula- the Newark half-graben. tion rates are consistently higher than those Additional parameters entered intofie ba- measured in the basin. If we assume that the sin filling model were determined as follows. sediment added to the model basin was fully The subsidence rate can be constrained be- compacted, then the discrepancy is most cause, under fluvial sedimentation during de- likely the result of the fact that the model's position of the Stockton Formation, subsi- output applies to the deepest part of the basin dence rate equaled accumulation rate. The and that the actual stratigraphic data were not maximum thickness of the Stockton Forma- collected from its depocenter. Additional tion in the deepest part of the basin is esti- comparisons and extrapolations are not war- mated to be 2500 m (see fig. 8A), and its ranted at this time because the stratigraphic paleontologically estimated duration is 3 m.y. data come from outcrops scattered through- (Cornet and Olsen 1985), thus yielding an es- out a basin in which appreciable transverse timated accumulation rate of 0.833 mmlyr. A and along-strike changes occur. volumetric sedimentation rate (Vc = 4.15 x The decreasing accumulation rates in the 106 m31yr) was chosen to yield a transition Lockatong and Passaic formations should not from fluvial to lacustrine deposition 3 m.y. af- be necessarily construed to reflect decreasing ter the start of subsidence. The maximum subsidence rates. It can be argued that the volume of water available (V,,, = 1.10 x lo1* lake cycles present in these formations repre- m2) was chosen to yield the deepest lakes in sent filling sequences: the basin subsided, the lower Lockatong Formation. formed a lake, which subsequently filled in, It must be emphasized again that the the basin subsided, etc. In this scenario, the Newark basin did not evolve as a full-graben. decompacted cycle thickness would approxi- This exercise is only a heuristic procedure mate the deepest lake and the amount of sub- meant to demonstrate an application of the sidence. This argument is flawed on two basin filling model, not to precisely model the accounts: (1) The cycle thickness in the evolution of the Newark basin. It is clear that Lockatong is only 5.5 m, decompacted to the approach has severe limitations: namely about 10 m, implying a lake depth of 10 m, yet that it cannot predict transverse and along- the facies analysis of Olsen (1984~)requires strike changes in accumulation rate and lake the lakes that deposited microlaminated sedi- depth in an asymmetrically-subsiding basin. ments were minimally 70-100 m deep. (2) Nonetheless, the trends in accumulation rate Most of the Passaic Formation is not mi- and "maximum" lake depth generated by the crolaminated, and therefore there are no model in many instances match the same good constraints on lake depth; it is then trends observed in the rock record. plausible that subsidence approximates de- The curves generated using equations (1-3) compacted cycle thickness and was decreas- are shown in figure 9 with reference to actual ing through the deposition of the Passaic For- stratigraphic data. Using the parameters out- mation. However, the Ukrainian Member in lined above, the model can generate a transi- the middle Passaic is microlaminated and R. W. SCHLISCHE AND P. E. OLSEN would require an instantaneous 70-100 m of subsidence, which had no effect on accumu- lation rate (see below). If subsidence was rel- atively uniform as predicted by the basin filling model, the basin would have been sedi- ment-starved during Passaic deposition, and the basin need not have undergone a radical deepening to accommodate the Ukrainian lake. All we can say for certain is that the subsidence rate was greater than the accumu- lation rate. The basin filling model also predicts the I I f very rapid rise in "maximum" lake depth in- 0 5 10 15 20 25 30 Time (millions of years) ferred from the lower Lockatong Formation and its subsequent decline (fig. 9:m). It is also plausible that longer-term changes in climate were responsible for the trends in "max- imum" lake depth. Clearly, climatic changes played a large role in the basin's microstratig- raphy (Olsen 1986). However, there is no evi- dence that the trends in accumulation rate wewverned by changes in climate. For ex- ample, in the upper Passaic Formation, the ramge of facies is identical to those in the driest portions of the Lockatong Formation (Olsen 1984~)~yet the accumulation rates in the Passaic Formation are lower. Further- more, in the rare instances where deep lakes were produced in the Passaic Formation (i.e., the Ukrainian Member), there is no apprecia- ble change in accumulation rate from the sur- rounding dry facies (see fig. 9). Since trends in accumulation rate appear to be detached from a climatic origin, it seems reasonable (and parsimonious) to assume that changes in "maximum" lake depth might be similarly detached, although to a lesser extent. Note also that the model predicts that the last mi-

FIG. 9.-Predictions of the full-graben model strata from scattered outcrops in the basin (see fig. (solid lines), based on the geometry depicted in 7); shaded cross represents estimate of accumula- figs. 7 and 8, compared to data from the Newark tion rate in fluvial Stockton [estimated thickness basin (dotted lines and crosses). The Y + W curve (calculated from outcrops near location 1, fig. 7) represents the height of the lake above the floor of divided by estimated duration]; g, anomalously the graben. Abbreviations are: a, filling equals sub- high accumulation rate in extrusive zone, a devia- sidence resulting in fluvial sedimentation, with ac- tion from the model's predictions; h, decreasing cumulation rate (a') equal to subsidence rate; b, b', accumulation rate in the Boonton Formation; i, 100 and b", onset of lacustrine deposition and sedimen- m water depth line, the minimum depth for the tologic closure; c and c', rapid increase in lake formation of microlaminated sediments (Olsen depth as depositional surface subsides below hy- 1984~);j, actual first occurrence of microlaminated drologic outlet; d and d', deepest lake predicted by sediments; k, actual last occurrence of micro- model and onset of hydrologic closure; e, major laminated sediments in lower Passaic Formation; 1, deviation from cumulative thickness curve in Early predicted last occurrence of microlaminated sedi- Jurassic; A hyperbolic decrease in accumulation ments; m, slow decrease in lake depth; n, anoma- rate predicted by model; solid cross represents lous "superwet" climatic anomaly of the Ukrai- data on accumulation rates from cyclical lacustrine nian Member; 0,anomalously deep Jurassic lakes. QUANTITATIVE FILLING MODEL 149 crolaminated black shale should form consid- erably later than is indicated by the data (fig. 9:k).This most likely can be attributed to the fact that the model did not incorporate the effects of evaporation of water from a lake whose surface area was continually increas- ing. Thus, actual lake depth decreased more rapidly than predicted by the model. Operating under its basic assumptions (uni- form subsidence, constant rates of input of sediment and water), the model cannot ex- plain the observed Early Jurassic increases in both accumulation rate and "maximum" lake depth (fig. 9:g, o), which imply that the Newark basin had actually decreased in size. In full-graben, changes in subsidence have either no effect or the wrong effect, for ac- FIG. 10.-Effects of asymmetric basin subsi- cumulation rate and "maximum" lake depth dence. (A) Normal filling sequence under condi- are governed by the areas of the depositional tions of uniform subsidence. (B) Accelerated tilting and lake surfaces respectively. Increasing the causes sediment and water to shift toward the bor- der fault side of the basin and results in the onlap of subsidence rate only deepens the basin, but it youngest strata onto previously-deposited strata. cannot change the surface areas. Decreasing (C)Antithetic faulting creates a smaller sub-basin. the subsidence rate initially has no effect until Fault need not be located within present extent of the basin fills to its lowest outlet, whereupon the basin. Darkest shading in (B) and (C) corre- sponds to Jurassic deposits of the Newark basin. fluvial sedimentation returns. In half-graben, however, maximum subsi- dence occurs adjacent to the border fault and A dramatic increase in the volume of sedi- decreases systematically toward the basin's ment addedtothe basin per unit time also hinge. An increase in the subsidence rate re- would haVe increased the accumulation rate sults in an increase in basin asymmetry. As a in the ~urasbsection.However, this added result, sediments and water shift to the deep- volume of sediment would have raised the est portion of the basin. A rapid increase in depositional surface and thereby displaced basin asymmetry in the earliest Jurassic of the volume of water upward, perhaps even the Newark basin therefore could have re- causing lake level to reach the hydrologic sulted in (temporary) decreases in the areas outlet. In a concave-upward basin, this would of the depositional and lake surfaces, causing have had the effect of increasing the lake's increases in accumulation rate and "max- surface area, thereby decreasing "max- imum" lake depth. Systematic decreases imum" lake depth, not increasing it dramat- in accumulation rate and "maximum" lake ically. An increase in the volume of water depth recorded in the Boonton Formation (V,,,) added to the basin during the wettest (figs. 6 and 9:h) might then reflect the return portions of the 400,000 yr climate cycles to uniform subsidence, as the newly depos- would have increased "maximum" lake ited sediments progressively onlapped the depth and presumably also would have older sediments and "basement" rocks of the brought in more sediments, increasing the ac- hanging wall. cumulation rate. However, this would imply Asymmetry may have been introduced by that the accumulation rate ought to be lower two fundamentally different processes (fig. in the drier portions of the 400,000 yr cycles, 10): (1) accelerated rotation of the entire which is not the case. It therefore appears hanging wall block and (2) the formation of likely that the observed Early Jurassic in- antithetic faults between the basin's hinge creases in accumulation rate and "max- and the border fault, thus generating a nar- imum" lake depth resulted from an increase rower sub-basin. Presently available geologic in basin asymmetry. data are not good enough to distinguish be- In terms of the approach taken in this pa- tween these two processes. per, only one parameter-subsidence rate- 150 R. W. SCHLISCHE AND P. E. OLSEN

1 Lava Flows and Intercalated Sediments 1

HARTFORD BASIN FUNDY Bt Fluvial and Alluvial Sediments 0.25 NEWARK BASIN

Accumulation rate 0.20 (mnvyr) determined by Balls Bluff thickness of fixed-period Sinstone / lacustrine cycles

DEEP RIVER BASH . . :. ::. Sanford Sub-basil

0.10 -0.24 :..:Sandsty. ..:.. -0.40 :: : :: :.Â¥

Lower Barren Beds- I imn~vrtestimated bv by its paleontologplly estimated dur ion

FIG.11 .-Chronostratigraphic sections of several Newark Supergroup basins, showing trends in accumu- lation rate. In all of the basins, the transition from fluvial to lacustrine deposition occurred at different times. The marked increases in accumulation rate and "maximum" lake depth associated with the extrusive episode occurred simultaneouslv in all basins that contain a Jurassic section. Modified from Olsen et al.

needs to be altered to explain the marked and Forlenza 1988; Ressetar and Taylor 1988; changes in the Jurassic sequence. In terms of Olsen et al. 1989; Smoot 1989). In the Fundy, the traditional approach, the changes would Culpeper, Richmtfnind Deep River basins, be explained by increasing both the volume the fluvial units are overlain by predomi- of sediment and water added to the basin. For nantly lacustrine deposits (Reinemund 1955; example, Manspeizer (1988) cites the deep Ressetar and Taylor 1988; Smoot 1989) with Jurassic lakes as evidence of a wetter Ju- the deepest water intervals definitely occur- rassic climate. Our new approach indicates ring near the base of the lacustrine sequence that climate need not necessarily have been in the Deep River, Richmond, and Fundy responsible. basins (Olsen et al. 1989). In the Blomidon Other Basins.-The basin filling model Formation of the , studies of predicts a basic tripartite stratigraphic pat- scattered outcrops support a decrease in ac- tern consisting of (1) a fluvial base, which cumulation rate up-section (Olsen et al. would not be present if the subsidence rate is 1989). In the Hartford basin, the basal New high and/or the rate of sediment input is low), Haven Arkose is fluvial until just prior to the (2) a middle lacustrine interval showing de- eruption of the first basalt flow (Olsen et al. creasing accumulation rates and evidence of 1989). It is possible that the basin was subsid- deep lakes near its base followed by gradual ing too slowly or the volumetric sedimenta- shoaling, and (3) possibly an upper fluvial in- tion rate was too high to have permitted the terval, deposited during waning subsidence. onset of lacustrine deposition. It is also possi- These predictions provide a convenient ble that the preserved fluvial strata are mar- frame of reference against which the ob- ginal to lacustrine strata. Since paleocurrents served stratigraphy of basins can be com- flowed from east to west at the time of New pared and their histories deciphered. Haven deposition (Hubert et al. 1978) and the All of the Newark Supergroup basins strata now dip to the east, any lacustrine shown in figure 11 contain a basal fluvial unit strata undoubtedly now would be eroded. In (Reinemund 1955; Hubert et al. 1978; Hubert the upper Triassic portions of the Culpeper QUANTITATIVE FILLING MODEL and Deep River basins, a higher content of and the subsequent lower accumulation rates coarse clastic material suggests that fluvial to reflect sedimentation keeping up with sub- sedimentation may have returned (Olsen et sidence, implying that excess sediments were al. 1989), signaling a decrease in the subsi- leaving the basin. However, the stratigraphy dence rate. It also is possible that the coarse of the upper two-thirds of the Ngorora For- elastics reflect the proximity to the border mation is predominantly lacustrine (Bishop fault or that they are the marginal facies to and Pickford 1972), implying sedimentologic lacustrine strata deposited in a deeper portion closure. Thus, the decrease in accumulation of the basin. More study is needed to resolve rate up-section may be an indication of a ba- this problem. The Otterdale of the sin growing in size through time, fitting our Richmond basin is largely fluvial (Ressetar model's predictions. and Taylor 1988) and contains evidence of Cumulative sediment thickness and ac- Late Triassic paleo-canyon systems appar- cumulation rate curves for the Cenozoic Val- ently cut by rivers after subsidence had lecito-Fish Creek basin in the Imperial Valley stopped or during uplift of the basin (B. Cor- in (Johnson et al. 1983; their figs. 4 net pers. comm. 1989). and 5) bear a strong resemblance to those in In the Fundy, Hartford, and Culpeper ba- figure 9. As the sediments were deposited sins, the syn- and post-extrusive formations largely in shallow marine waters, implying record higher accumulation rates and greater that sedimentation kept up with subsidence, "maximum" lake depths over the preceding Johnson et al. (1983) interpreted the curves Triassic portions (fig. 11; Olsen et al. 1989), to reflect decreasing rates of tectonic subsi- possibly reflecting increased asymmetry in dence. Alternatively, the basin may have each of these basins in the earliest Jurassic. been subsiding uniformly and growing in size The uppermost Portland Formation of the through time. Hartford basin appears to be fluvial in origin (Olsen et al. 1989). If no lacustrine strata are SUMMARY AND DISCUSSION located downdip of the exposed fluvial strata, Stratal onlap geometry in extensional ba- then this interval recorded a slowing of basin sins indicates that the tiasins were growing subsidence, which allowed the Hartford ba- in size through time, and therefore the depo- sin to once again fill its lowest outlet. sitional surface area was also increasing Lake Bogoria in the Gregory Rift and the through time. Under conditions of uniform hydrologically-open Lake Tanganyika repre- subsidence and constant rate of sediment and sent two modern examples (Lambiase and water input, a simple basin filling model pre- Rodgers 1988; Lambiase 1990) of graben in dicts an initial period of fluvial sedimentation the first two stages, respectively, of the mod- as the volume of sediments available exceeds el's tripartite evolution. The Keweenawan the capacity of the basin, such that the ac- trough of Michigan and Wisconsin (Elmore cumulation rate equals the subsidence rate. and Engel 1988) also shows a tripartite As the given volume of sediment is spread stratigraphic architecture. over a larger and larger basin volume, a point Jones (1988) presented a graph of cumula- is reached when the sediments exactly fill the tive sediment and lava flow thickness plotted basin; subsequently, lacustrine deposition against time for the Kenya Rift east of the occurs. Accumulation rate decreases as the Elgeyo . Although there is consid- sediments are spread over a larger deposi- erable scatter, the rate of increase of cumula- tional surface area. After the onset of lacus- tive thickness decreases up-section, which trine deposition, lake depth should rapidly may indicate that the basin was increasing in deepen to a maximum (as the finite volume of size through time. A paleomagnetically and water available just fills the basin between the radiometrically calibrated plot of cumulative hydrologic outlet and the depositional sur- sediment thickness against time for the Ngor- face) and then systematically decrease as the ora Formation of the Kenya Rift shows the volume of water is spread over a larger area. same trend (Tauxe et al. 1985; their fig. 4). If the subsidence rate is fast enough or the The higher accumulation rates in the lower sediment input rate slow enough, lacustrine part of the section were interpreted to reflect deposition will occur from the outset. the rapid infilling of a topographic The basin filling model provides a frame- 152 R. W. SCHLISCHE AND P. E. OLSEN work for understanding the stratigraphic de- mensions of the basins, their geometries, and velopment of extensional basins, particularly the rates of sediment input. the Newark basin of eastern North America. On the other hand, the marked increases Many of the observed changes through the in accumulation rate and "maximum" lake stratigraphic sequence can be explained sim- depth occurred simultaneously in the earliest ply as the consequence of the filling of a Jurassic in all of the basins containing this growing basin: the transition from fluvial to sequence (regional effect; fig. 11). This anom- lacustrine deposition in the Triassic-age se- aly is attributed to a marked increase in basin quence, the decrease in accumulation rate asymmetry, causing sediment and water to (calibrated by the thickness of Milankovitch- pile up adjacent to the border fault (fig. 11). period lacustrine cycles) after the onset of This asymmetry may have resulted from a lacustrine deposition, and the inferred rapid sudden change in border fault geometry. rise and subsequent slower decline in "max- However, this seems unlikely, as it would imum" lake depth. Other observations re- mean that all border faults experienced a quire additional explanations. For example, similar change in geometry at the same time. figure 9:p shows a marked departure in More likely, the basins' asymmetry is at- "maximum" lake depth in the Ukrainian tributable to a marked increase in the exten- Member of the Passaic Formation. Prior to sion rate, which was probably governed by this type of analysis, the "excursion" would deep-seated processes. simply have been interpeted as one of the The inferred regional increase in the exten- many climatic changes thought to have in- sion rate occurred just 150,000 yrs before the fluenced the basin's stratigraphic develop- volumetrically massive early Mesozoic extru- ment. With the basin filling model and Milan- sive event. The sudden increased extension kovitch-period modulation accounting for may have thinned the crust sufficiently to al- most of the changes in the stratigraphic se- low melting in the upper mantle. That the ac- quence, this anomaly stands out as something celerated extension was a brief excursion is unique-a superwet climatic event. Accumu- supported by the brief duration (<550,000 lation rate was not affected, indicating that yrs) of the eastern North American extrusive sediment accumulation rates and climate are episode (Olsen and Fedosh 1988). Further- not causally linked (fig. 9). more, the loading of up to 700 m of basalt and Analogous to thermal modeling of passive coeval intrusives within the basins may have margins, the usefulness of the basin filling contributed to their asymmetry. model rests in subtracting the stratigraphic Both the filling of a growing extensional ba- effects due simply to the filling of the basin sin as a result of mechanical (fault-controlled) (intrinsic effects) from the observed sedimen- subsidence and subsidence induced by decay tary record. The resulting anomalies reflect of thermal anomalies on passive margins pro- departures from the basin filling model's as- duce much the same result on accumulation sumptions (e.g., constant volume of sediment rates: a decrease in rate up-section. We do input per unit time, constant inflow rate of not imply that our basin filling model is appli- water, etc.) and point to important changes in cable to deposits resulting from thermal sub- tectonics, climate, and sediment and water sidence on known passive margins. How- budgets (extrinsic effects). ever, care must be taken in interpreting Furthermore, two or more basins may be accumulation rates in older (particularly fos- compared in order to separate local from re- sil-poor) rocks for which precise tectonic and gional effects. For example, in each of the depositional settings (marine vs. continental) basins illustrated in figure 11, the transition may not be known and the cause of the subsi- from fluvial to lacustrine deposition in the dence (mechanical vs. thermal) may be con- Triassic-age sequences occurred at different fused. The resolution of this problem may be times (local effect). This is to be expected: aided somewhat by the observation that ther- even if all the basins had equal subsidence mal subsidence tends to be regionally persis- rates and began subsiding simultaneously, tent, whereas mechanical subsidence gener- their filling rates and the timing of the fluvial- ally is restricted to smaller, discrete basins, lacustrine transition would depend on the di- where diagnostic coarse-grained facies may QUANTITATIVE FILLING MODEL 153 point to normal faulting as the cause of sub- and the Ruzizi-Kigoma-Southern deposi- sidence. tional basins of Lake Tanganyika (Burgess et In basins in which fault-controlled subsi- al. 1988; Lambiase 1990). In addition, the ex- dence is thought to be the dominant mecha- isting geometry of the extensional basins nism of basin evolution, as in the basins of needs to be more tightly constrained, ideally the Newark Supergroup, the minimum subsi- through a network of intersecting transverse dence rate can be constrained, for the sub- and along-strike seismic lines. sidence rate is greater than the filling rate Despite the shortcomings of the exten- during lacustrine deposition and equal to sional basin filling model presented in this re- the filling rate during fluvial deposition. Ulti- port, the preliminary results are encouraging: mately, by combining the results of the ba- even under the simplest conditions, continen- sin filling model's subsidence analysis with tal facies in extensional basins are expected knowledge on how extensional basins grow to exhibit predictable changes through time and deform through time (e.g., Wernicke and as a consequence of the changing geometry of Burchfiel 1982; Gibbs 1984), it may be possi- the basin and not necessarily as a result of ble to estimate how the extension rate varied changing tectonic or climatic parameters. In- throughout the history of Early Mesozoic ex- deed, in order to recognize changes in the , provided the basins' geometries are extrinsic parameters, the changes intrinsic to also defined through time. the basin must first be subtracted from the Before the extensional basin filling model stratigraphic record. can be used to its full potential, several important improvements need to be made. ACKNOWLEDGMENTS.-Wethank Gerard Foremost, three-dimensional quantitative Bond, William hosworth, Roger Buck, A1 models for the filling of half-graben need to be Froelich, Joseph Lambiase, Fred Mandel- developed, models that take into account not baum, and Joseph Smoot for valuable dis- only variations in subsidence with position in cussions. This paper was greatly improved the basin but also build in the along-strike by reviews from Mark Anders, Nicholas growth of the basin. The effects of differential Christie-Blick, Bernie Coakley, Bruce Cor- relief and the erosion of uplifted blocks on the net, Michelle Kominz, Teresa Jordan, Mar- filling of the basins has yet to be addressed. jorie Levy, and Warren Manspeizer. We The non-linear losses of water volume to gratefully acknowledge support from the Do- evaporation still need to be quantified as well nors of the American Chemical Society ad- as the effects of sediment compaction. Future ministered by the Petroleum Research Fund models also need to incorporate different and the National Science Foundation (Grants filling sequences for structurally distinct yet BSR-87-17707 to PEO and EAR-87-21 103 to sedimentologically-linked half-graben, such G. Karner, M. Steckler, and PEO). This pa- as the Newark-Gettysburg-Culpeper system per is Lamont-Doherty Geological Observa- in eastern North America (Olsen et al. 1989) tory Contribution Number 4575.

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