3.2.2. Cyclicity, Climate, and Time scales

Paul E. Olsen, Lamont Doherty Earth Observatory, Palisades, New York, 10964 USA
 
 

	The NSF funded Newark Basin Coring Project (NBCP) (Fig. 3.2.2.1) resulted in the recovery of about 6.8 km of continuous core from 7 coring sites making up a combined 4.7 km stratigraphic section spanning nearly all of the Late Triassic age strata of the Newark rift basin (Olsen et al., 1996a). Additional core from the Army Corps of Engineers, completed the Jurassic age part of the Newark basin section (Olsen et al., 1996b). This core spans roughly 32 million years and provides the longest available continuous record of orbital forcing of tropical climate (25 my), as well as allowing the construction of an orbitally tuned geomagnetic polarity time scale for the Late Triassic and earliest Jurassic, including the entire zone of basaltic extrusives (Fig. 2.4). This core record and subsequent studies (see below) serves as a test of concept for the Pangean Coring Transect.
	Figure 3.2.2.1: The Princeton coring site of the Newark Basin Coring Project. This is the SHADS set up of AMOCO Production Company. Total depth at this site was 3697 ft (1087 m).

Orbital forcing in the Newark basin is revealed as a hierarchy of lake level cycles that controlled by precipitation and evaporation changes controlled by precession-related celestial mechanical cycles (Fig. 3.2.2.2). The shortest period cycles, originally described some 35 years ago (Van Houten, 1964), were controlled by climatic precession (~20 ky) and these are modulated by ~100 ky, 404 ky, and 1.75 my cycles of eccentricity (Olsen and Kent, 1996, 1999a). There is no obvious obliquity (41 ky) signal, consistent with relatively local insolation forcing in the tropics. A perhaps surprising aspect of the cyclicity is that the longest period cycles of 1.75 my is significantly shorter than the corresponding period is at present (2.35 my), a result that is plausibly due to the chaotic behavior of the planets, specifically Mars (g4) and Earth (g3). The very long Newark record in fact allows calculation of all of the major fundamental frequency relating to eccentricity (Olsen and Kent, 1999a), assuming the relative constancy of the 404 ky cycle over this time period (see Laskar, this report). A geomagnetic polarity reversal stratigraphy comprised of 60 major zones has been recovered from the Newark core record (Kent and Olsen, 1995) (Fig. 2.4). Stratigraphy of the polarity transitions has been tested against the cyclostratigraphy in the overlap intervals between stratigraphically successive cores and with outcrop and the data are completely consistent regardless of lateral changes in rock color and accumulation rate. The core paleomagnetic data also document a slow northward drift of the Newark basin from at the equator to about 10° N latitude.
Figure 2.4 (above): Newark basin time scale: A, Rock Color; B, Formations (Bt., Boonton Fm.; Hm, Hook Mt. Basalt; To., Towaco Fm.;  Pr., Preakness Basalt; Ft., Feltville Fm.; Om., Orange Mountain Basalt; Pc., Passaic Fm.; Lt., Lockatong Fm.; St., Stockton, Fm.); C, Filtered lake depth proxy (red) and rock color (green) showing the 404 ky lake-level cycles; D, Astronomically calibrated geomagnetic polarity sequence; E, Paleolatitude of the coring sites though time; F, Relative Ages; G, Numerical age based on astronomical calibration and radiometric ages from the basalts. Based on Kent and Olsen (1999) and Olsen and Kent (1999). Click on image for a more detaile veiw.

Kent and Olsen (1999) and Olsen and Kent (1999a) used the orbitally forced cyclicity as a basis for a high-resolution time-scale for the polarity reversal stratigraphy. The principle basis for this time scale is the 404 ky cycle caused by Venus and Jupiter (g2-g5), which on first principles should be stable, at the appropriate scale, over hundreds of millions of years (see Laskar, this report). The resultant astronomically tuned geomagnetic polarity time scale (GPTS) covers the entire Late Triassic and part of the Early Jurassic (Hettangian) and for the first time provides a time scale of Neogene-levels-of-resolution for much of the Early Mesozoic.

The Newark basin astronomically tuned GPTS has subsequently been used to provide a time scale and Milankovitch-level correlation for portions of the lacustrine sections of several other basins spanning 40° in paleolatitude (from north to south). These are the Dan River rift basin of North Carolina and Virginia (Kent and Olsen, 1997), the Taylorsville rift basin of Virginia (LeTourneau, 1999), the Fundy rift basin of Nova Scotia, Canada (Kent and Olsen, 1999b), and the Jameson Land basin of Greenland (Kent and Clemmensen, 1996). These basins span the coal-bearing humid equatorial region through the evaporite-rich arid tropics into the again coal-bearing humid temperate belt. Profound changes in the mode of cyclicity and sedimentary facies accompany this latitudinal transect. Although outcrop location and happenstance have limited the amount of section that can be sampled in these basins, it is possible to examine several of these basins along a latitudinal transect from about 7° to 39° N for from about 209 to 211 Ma (Fig. 3.2.2.2). There is a very close correspondence between the wettest intervals in these sections, but there is nonetheless a change from 20 ky nearly pure precession forcing in the south to precession plus obliquity forcing in the north, while the overall facies goes from fairly wet, to arid, and back to fairly wet again. This Milankovitch-level correlation necessary to test global climate models at the appropriate levels precession (Sloan and Morrill, 1998) that allow discrimination of completing hypotheses and reconciliation of seemingly disparate geological and paleontological data.
	Figure 3.2.2.2 (above):  Comparisons of correlative sections of the   Culpeper, Newark, Fundy, and Jameson Land basins. Culpeper   section is from the Lenn Bros.  no. 1 and Andrus no. 1 cores of the   Bull Run Formation (Balls Bluff Siltstone), Newark section is from   the Somerset no. 1 core of the middle Passaic Formation (E16r-E18n   are magnetic polarity zones,  and Z-FF are member names from Kent   et al. 1995 and Olsen et al. 1996b), Fundy basin section is based on   outcrop (Blomidon area, Nova Scotia) correlated by lithostratigraphy   to the GAV-3 core which is the source of the magnetostratigraphy   (Kent and Olsen, 2000b);  Jameson Land basin section is based on   Kent and Clemmensen (1996) (3M-7C are Clemmensen's designations   of ~100 ky cycles). See figure 2 for key to rock color. Figure is   adapted from Olsen and Kent, 2000).

Also extracted from the Newark basin core record, by use of the astronomically tuned GPTS, is the accumulation rate scale for the basin (Contreras, et al., 1997; Olsen and Kent, 1999a) as well as the duration of the CAMP episode in the Newark basin. Accumulation rate, aspects of the lacustrine facies, and the igneous events are strongly correlated, suggesting a strong tectonic signal (see Schlische, this report). There is a surprising degree of coherence between these same parameters over at least 1000 km, as seen in the divergent basins listed above suggesting regional-large scale tectonic controls. In as much, as this tectonic signal reflects deep-seated underlying processes, the core record provides a quantitative as well as qualitative basis for understanding fundamental physical Earth processes.

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