3.3. Tectonics of Rifting and Drifting: Pangea Breakup

3.3.2. Extracting Tectonic Information from Cores in Rift Basins

Roy W. Schlische
Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854-8066 U.S.A.

Rift basins are tectonic features. Thus, a goal of any large-scale study involving rift basins should be a better understanding of the tectonic processes controlling basin formation and infilling. The NSF-funded Newark Basin Coring Project (NBCP) demonstrated that it is possible to extract tectonic information from cores in rift basins. This section reviews the successes of NBCP in terms of tectonics and basin evolution and briefly highlights some of the unanswered questions.
Figure Schematic cross section of the Newark basin showing offset coring technique; marker unit at the base of one core correlates with same marker unit at top of adjacent core. From Schlische (2000).

NBCP used an offset drilling technique (e.g., Olsen et al., 1996a) to take advantage of the eroded half-graben geology of the Newark basin. Core sites were positioned so that the bottom of one hole overlapped with the top of an adjacent hole in a distinctive stratigraphic interval (Figure Correlations are based on cyclostratigraphy and magnetostratigraphy (Olsen et al., 1996a; Kent et al., 1995). The overlap sections allowed construction of a composite stratigraphic section (Olsen et al., 1996a; section 3.2.2). In addition, the overlap sections by themselves provide useful tectonic information. For example, the overlap section between the Rutgers and Somerset cores shows that stratigraphic units thicken by ~12% across the 10 km between these holes (Figure  In addition, lake facies are deeper in Somerset than Rutgers.  All overlap sections thicken from the lateral edge of the basin toward the basin center (e.g., Rutgers-Titusville) and from the hinged margin toward the intrabasinal faults and/or border fault system (e.g., Nursery-Titusville) (Figure The simplest interpretation of these variations in thickness and facies is variations in basin subsidence caused by syndepositional faulting (see Figure
Figure  Overlap section of the Rutgers and Somerset cores, showing pronounced increase in thickness and proportion of deeper-water mudstones (gray and black units) from Rutgers to Somerset. Based on data in Olsen et al. (1996a).

Click on the image at left to view a larger version.

Figure  Geologic map of the north-central part of the Newark basin showing the locations of the seven NBCP drill sites. Arrows indicate the amount of thickening between overlap sections of stratigraphically adjacent cores based on correlations in Olsen et al. (1996a). Abbreviations for drill holes are: M, Martinsville; N, Nursery; P, Princeton; R, Rutgers; S, Somerset; T, Titusville; W, Weston. From Schlische (2000).

Correlations of the NBCP cored sections to outcrop sections is also extremely useful (e.g., Silvestri, 1994, 1997; Olsen et al., 1996a; Schlische, 1999). The Perkasie Member of the Passaic Formation extends across 125 km of the Newark basin (Figure Variations in thickness of the Perkasie Member indicate that the Newark basin is a large longitudinal syncline, consistent with border-fault displacement being highest near its center and declining towards its lateral ends (e.g., Schlische, 1992) (also see Figure Additional core-to-outcrop correlations, coupled with seismic-reflection profiles, indicate that a hierarchy of fault-related folds along the border fault system and intrabasinal faults formed, at least in part, syndepositionally (Jones, 1994; Schlische, 1992, 1995; Reynolds, 1994; Olsen et al., 1996).
Figure Basinwide correlation of the Perkasie Member of the Passaic Formation showing variations in thickness and facies. Inset sketch map of Newark basin shows locations of sections. Modified from Olsen et al. (1996a).

Click on the image at left for a larger version.

The large-scale basin geometry outlined above, the large-scale stratigraphic architecture present in the NBCP composite section (see Figures 2.4 and, and onlap relationships revealed by seismic data can be reproduced in quantitative basin-filling models (see Figure  Although these basin-filling models successfully explain many aspects of the stratigraphy of the Newark basin and many other non-marine rift basins (e.g., Lambiase, 1990; Schlische and Olsen, 1990; Olsen, 1997), this simply indicates that the models are viable. The models are bolstered because they also make quantitative predictions about accumulation rates. In the Newark basin, accumulation rates are derived from Milankovitch lacustrine cycles in the NBCP composite section (Figure; Contreras et al., 1997; Olsen and Kent, 1999). The most sophisticated numerical basin filling models (which incorporate self-similar faulting, flexure, isostasy, and sediment diffusion; Contreras et al., 1997) broadly account for observed trends in accumulation rates in the NBCP data (Figure In addition, the basin-filling models place constraints on the boundary conditions of the rifting process (constant strain-rate conditions are favored over constant fault-lengthening rate), the rheology of the crust and lithosphere, and the nature of fault growth (Schlische, 1991; Schlische and Anders, 1996; Contreras et al., 1997).
Figure  Accumulation rate data derived from NBCP cyclostratigraphy (a) and constant strain-rate basin-filling model (b). The two accumulation rate data sets were normalized by the maximum accumulation rate in each data set to facilitate comparisons. The numbered curves in (b) were derived from vertical drill holes through the model rift basin shown in map view on right. Curve 2 fits the Newark basin data reasonably well, but fails to reproduce the marked increase in accumulation rates at ~27.5 M.yr. since the onset of extension (Early Jurassic extrusive interval). Modified from Olsen (1997), Contreras et al. (1997), and Schlische (2000).

Deviations from the predictions of the models are also important. The most notable deviation in the Newark basin concerns the markedly higher accumulation rates (Figure and deeper lake facies present in the Early Jurassic strata (tectonostratigraphic (TS) package IV; see Figure compared with those in TS-III (Olsen et al., 1996a, b). Strata belonging to TS-IV are interbedded with a series of lava flows (CAMP flows) that were emplaced in as little as 650 kyr (Olsen et al., 1996b). Schlische and Olsen (1990) postulated that accelerated faulting and tilting would markedly increase basin asymmetry. This would cause sediments and water to shift toward the basin depocenter, increasing accumulation rates and average lake depths. This anomaly is not just limited to the Newark basin: a plot of cumulative stratigraphic thickness versus age (Figure shows marked increases in accumulation rates for all of the eastern North American rifts containing Early Jurassic strata (Schlische and Anders, 1996). Thus, tectonics is likely responsible for this "anomaly", although the relationship of CAMP volcanism to this tectonic anomaly is not clear.
Figure  Cumulative stratigraphic thickness versus geologic age for various exposed rift basins in eastern North America. The Culpeper (C), Deerfield (D), Fundy (F), Hartford (H), and Newark (N) basins all show pronounced increases in stratal thickness in earliest Jurassic time (extrusive interval). Other abbreviations are DR, Deep River; DV, Danville; and R, Richmond basins. Modified from Schlische and Anders (1996).


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