Fig. 4.10. The Late Cretaceous to Eocene reversal sequence recorded at Gubbio, Italy. The observed polarity zones are compared with the GPTS. Normal polarity zones are in black. After Lowrie and Heller (1982).

Silva, 1964). The sections contains a long, continuous sequence of magnetic polarity zones intercalibrated with planktonic foraminiferal zones (Alvarez et al., 1977; Arthur and Fischer, 1977; Lowrie and Alvarez, 1977; Premoli Silva, 1977; Roggenthen and Napoleone, 1977). The sequence has become a classic (see Fig. 4.10) and is particularly valuable because of its unambiguous correlation to magnetic anomaly profiles and to oceanic cores (Tauxe et al., 1983b).

4.3.5 Late Triassic GPTS

Extension of the GPTS back beyond about 175 Ma is currently difficult because there is no extant modern ocean floor that allows the construction of a complete reference sequence of polarity reversals. In general the development of a GPTS for earlier times has relied on the piecing together of separate magnetostratigraphic records of variable length, variable reliability, and usually with insufficient absolute or relative chronological control. However, studies in the Newark rift basin of eastern North America show the potential for GPTS extension in thick, complete continental sedimentary sections.

The initial magnetostratigraphic results (Mcintosh et al., 1985) from the Newark basin were from discontinuous outcrop and industry boreholes. An improved documentation of the polarity sequence was provided in later studies by Witte and Kent (1989) and Witte et al. (1991). These later studies were based on more dense sampling and better demagnetization procedures, and they incorporated field tests to constrain the age of magnetization. These studies showed the potential for detailed correlation of cyclostratigraphy and magnetostratigraphy in the Newark basin and stimulated a comprehensive drilling program. Under this program continuous coring with near-complete recovery at seven drill sites produced a total of 6770 m of core. This represented almost the whole of the Upper Triassic continental lacustrine sediments together with some of the lowermost Jurassic interbedded continental sediments and lavas of the Newark igneous extrusive zone. Stratigraphic overlapping of the cored sections provided about 30% redundancy and it was possible to assemble a (normalized) 4660-m-thick composite section (Kent et al., 1995). The remaining part of the Jurassic section was studied using test borings by the Army Corps of Engineers (Fedosh and Smoot, 1988; Witte and Kent, 1990; Witte et al., 1991; Olsen et al., 1996b).

The lacustrine deposits display a pronounced cyclic variation in lithofacies linked to the McLoughlin cycle (a Milankovitch cycle). Furthermore, the Watchung basalts at the top of the sedimentary sequence are known to be contemporaneous with the Palisades Sill, which is well dated at 201±2.7 Ma (40Ar/3 Ar; Sutter, 1988) and 202±1 Ma (U-Pb zircon; Dunning and Hodych, 1990). Hence, it is possible to date events within the drill cores by using the cyclostratigraphy to count down from the Watchung basalts.

Combining the magnetostratigraphic analysis of the 4660-m-thick composite section with the cyclostratigraphic analysis, it was possible to provide a GPTS for almost the whole of the Late Triassic (Kent et al., 1995; Olsen and Kent, 1996; Olsen et al., 1996a). Recently, by undertaking additional sampling to confirm short polarity intervals and to determine the thickness and duration of polarity transition zones, and by extending the cyclostratigraphy to older strata, Kent and Olsen (1999) presented a refinement of the initial magnetostratigraphic results and were able to calculate an astronomically tuned GPTS for almost 25 million years of the Late Triassic. They further extended the record by 6 Myr by downward extrapolation of the sedimentation rates determined by cyclostratigraphy. The lithostratigraphy, magnetostratigraphy, and cyclostratigraphy of the composite section are shown in Fig. 4.11.

The details of the resulting GPTS are provided in Table 4.5. It is impressive, and encouraging for the future potential for extending the GPTS through magnetostratigraphy, that this GPTS is good enough to perform a statistical analysis of the reversal sequence. Kent and Olsen (1999) note that the interval lengths have a mean duration of about 0.54 Myr (corresponding to an average reversal rate of about 1.8 Myr"1) and that there is no discernible polarity bias.

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