Radiometric ages of the Fire Clay tonstein [Pennsylvanian (Upper Carboniferous), Westphalian, Duckmantian]: A comparison of U–Pb zircon single-crystal ages and 40Ar/39Ar sanidine single-crystal plateau ages
Introduction
Tonsteins–kaolinized deposits of volcanic ash that result from explosive volcanism–are known worldwide as reliable marker beds for stratigraphic analysis and for paleogeographic reconstructions in the Pennsylvanian. In addition, they are very useful for sedimentological analysis because they are precise time markers. Tonsteins occur mainly in coal beds so they also provide a datum for regional coal-bed correlations that are essential for coal-bed-resource calculations.
Another aspect of tonsteins is that they contain relict volcanic minerals such as sanidine, zircon, and monazite that can be dated radiometrically. The dating of tonsteins is important for worldwide correlations and for the calibration of the late Paleozoic time scale. The volcanic mineral in the upper Paleozoic of Euramerica most often dated is sanidine, which has been mainly dated by 40Ar/39Ar plateau radiometric techniques (e.g. see Hess and Lippolt, 1986, Hess et al., 1988, Keiser, 1989, Rice et al., 1990, Lyons et al., 1992, Kunk and Rice, 1994). These ages have no internal check on their accuracy. However, 206Pb/238U and 207Pb/235U ages can be compared to test for concordancy. Where both U–Pb ages are precise, this provides at least a first-order test that the zircon remained a closed system for Paleozoic ages (see discussion that follows).
In this paper, a U–Pb single-crystal zircon age is reported and compared with previously reported single-crystal sanidine 40Ar/39Ar plateau ages of the Fire Clay tonstein (late Duckmantian Stage, Westphalian Series, Pennsylvanian), a widespread marker bed in the Middle Pennsylvanian (Westphalian) of the central Appalachian basin, USA. This tonstein has been the subject of a number of studies (e.g. Seiders, 1965, Huddle and Englund, 1966, Bohor and Triplehorn, 1981, Chesnut, 1985, Keiser et al., 1989, Belkin and Rice, 1989, Rice et al., 1990, Rice et al., 1994a, Outerbridge et al., 1991, Lyons et al., 1992, Lyons et al., 1993, Andrews et al., 1994, Greb et al., 1999, Hower et al., 1999, Outerbridge, 2003). The geographic extent of the Fire Clay tonstein and its correlatives has been recently published (Outerbridge, 2003). The Magoffin Member (Fig. 1, Fig. 2), a marine member of the Breathitt Formation, which occurs just above the Fire Clay tonstein, has been shown by paleontological analysis to be late Duckmantian (formerly late Westphalian B) (Greb et al., 1999, Blake et al., 2002). The Magoffin Member has been correlated with the Aegir Marine Band marking the boundary of the Duckmantian and Bolsovian stages in Europe (Lyons et al., 1997). Thus, the palynological and faunal ages of this marine member establishes its overall correlation with European sequences.
Section snippets
Sampling and analytical procedures
The zircon samples for U–Pb dating were handpicked after digestion of the kaolinite-rich and micrograined quartz matrix with hydrofluoric acid (HF). From a population of several dozen zircons, five zircon grains (designated Z-1 through Z-5; see Table 1) were selected for dating because of their clarity, euhedral shape, and lack of visible cores. These were also the freshest zircons and had the fewest cracks. The five zircon crystals were abraded using the techniques described in Krogh (1982).
Results and discussion of dating
Four of the five zircon crystals showed significant inheritance (Fig. 3). However, one of the crystals (Z-3) is concordant within error. The results of the age dating and errors are summarized in Table 1. The 2σ ages of zircon crystal Z-3 are: 206Pb/238U age of 314.6 ± 0.9 Ma, 207Pb/235U age of 314.2 ± 1.3 Ma, and a 207Pb/206Pb age of 312.9 ± 9.5 Ma. The 206Pb/238U system gives the most precise age and is the most robust with respect to analytical biases (e.g. isotopic composition of common Pb);
Implications for the Carboniferous time scale
The proposed age of 314.6 ± 0.9 Ma for the Fire Clay tonstein is an important spike for the Paleozoic time scale worldwide. The Fire Clay tonstein is stratigraphically below the Magoffin Marine Member of Kentucky, Virginia, and Tennessee, which is stratigraphically equivalent to the Winifrede Shale of West Virginia (Fig. 1). Lyons et al. (1997) correlated the Magoffin Member with the Aegir Marine Band of Great Britain, which marks the base of the Bolsovian Stage (formerly Westphalian C). Thus,
Depositional rates for the Duckmantian
If the Duckmantian Stage lasted about 3 Ma (considering errors involved), then considering lithologic and decompaction factors, depositional rates can be calculated for the Duckmantian Stage in various basins or regions. This rate of deposition is 212 to 330 m/Ma for the Duckmantian of the central Appalachian basin, which is considerably greater than the depositional rate of 142 m/Ma for the Duckmantian in the East Pennine coal fields of England based on stratigraphic data in Fulton et al.
Conclusions
The age of the Fire Clay tonstein is 314.6 ± 0.9 Ma (2σ) based on 206Pb/238U dating, which is considered to be the most accurate of the dating systems considered here. This is an important spike for the late Paleozoic time scale. The Duckmantian Stage–Bolsovian Stage boundary is here placed at 314 ± 1 Ma, which is older than previous estimates for this boundary. The Duckmantian Stage of the Westphalian Series lasted 2 ± 1 Ma, which is of less duration than previous estimates, and this has important
Acknowledgements
B. M. Blake, Jr. and the late K. J. Englund provided information on the age and correlation of coal beds in the central Appalachian basin. J. F. Sutter and J. C. Hower are thanked for critical reviews of this paper and for giving helpful suggestions for its improvement.
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