U-PB AGES FROM ZIRCONS IN THE MIDDLE ORDOVICIAN BENTONITE AND BENTONITE-LIKE HORIZONS IN CENTRAL PENNSYLVANIA

Twenty Middle Ordovician ash beds, mapped in the Rte 453 road-cut near Union Furnace, have been tagged following the alpha-numeric system of Berkheiser and Cullen-Lollis (1986), of which the Deicke B-12 and Millbrig B-14 are the most widespread. The stratigraphic range for many of these bentonites has been extended; near Ashcom Quarry the group includes 2 beds near the top of the Bellefonte Fm. and in Oak Hall Quarry ash beds are correlated to 6 feet above the base of the Antes Mbr. of the Reedsville Fm.

Zircons were recovered from the B-5, B-12 and B-13 ash beds sampled in Oak Hall quarry. Isotopic analyses on 70 grains from each sample yielded high quality data: low 204 (high ²⁰⁶Pb/²⁰⁴Pb), good precision, and no high or reverse discordance. The zircon ages are distributed among three distinct groups. The most frequent and tightly defined group is the youngest at ~451-455 Ma, in which both B-5 and B-13 zircons occur. The second group is a broader distribution of ages from 1049.1 ± 57.5 Ma to 1760.8 ± 17.7 Ma and consists of only B-13 zircons. The third group is represented by only two B-5 zircons with ages of 2476.8 ± 17.5 Ma and 2698.5 ± 20.3 Ma. Zircon morphologies are similar among the crystals analyzed regardless of age differences and bentonite source. Apparently Taconic subduction involved components of both Grenville as well as Superior provinces crust for the B-5 and B-12 events but not for B-13.

The calculated zircon age of bentonite B-5 is 455.1 ± 5 Ma and for bentonite B-13 it is 451.5 ± 5 Ma, corresponding closely to an Upper Ordovician age of 456-457 Ma estimated from Berg et al. (1993). Samson et al. (1989) used U-Pb data from zircons to determine an age of 457.1 ± 1.0 Ma for the Deicke bentonite from localities in the Midwestern United States. Presumably those bentonites correlate with the B-12 (Deicke) bentonite of Oak Hall Quarry. Despite the obvious overlap in calculated age error bars, the mean ages are consistent with stratigraphic relationships as B-5 is expected to be older than B-13. If the mean ages represent the time of ash crystallization, a 3.6 Ma difference exists between deposition of B-5 and B-13. The 41.2 meter stratigraphic separation of these bentonites indicates an average limestone depositional rate through the upper Linden Hall to the lower Salona formations of 11.4 meters per Ma.