Re–Os age for the Lower–Middle Pennsylvanian Boundary and comparison with associated palynoflora

Abstract

The Betsie Shale Member is a relatively thick and continuous unit that serves as a marker bed across the central Appalachian basin, in part because it includes an organic-rich shale unit at its base that is observable in drill logs. Deposited during a marine transgression, the Betsie Shale Member has been correlated to units in both Wales and Germany and has been proposed to mark the boundary between the Lower and Middle Pennsylvanian Series within North America. This investigation assigns a new Re-Os date to the base of the Betsie and examines the palynoflora and maceral composition of the underlying Matewan coal bed in the context of that date. The Matewan coal bed contains abundant lycopsid tree spores along its base with assemblage diversity and inertinite content increasing upsection, as sulfur content and ash yield decrease. Taken together, these palynologic and organic petrographic results suggest a submerged paleomire that transitioned to an exposed peat surface. Notably, separating the lower and upper benches of the Matewan is a parting with very high sulfur content (28 wt.%), perhaps representing an early marine pulse prior to the full on transgression responsible for depositing the Betsie. Results from Re-Os geochronology date the base of the Betsie at 323 ± 7.8 Ma, consistent with previously determined age constraints as well as the palynoflora assemblage presented herein. The Betsie Shale Member is also highly enriched in Re (ranging from 319.7 to 1213 ng/g), with high 187 Re/ 188 Os values ranging from 3644 to 5737 likely resultant from varying redox conditions between the pore water and overlying water column during deposition and early condensing of the section.Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Radiometric dating of marine-influenced coal using Re–Os geochronology

“…The core is characterized by sandstones and shales with interbedded coal horizons. Geboy et al (2015) identified five coal facies within the Matewan coal bed (total bed thickness of 0.75 m). These five coal facies can be sub-divided into two groups: (i) the lowermost three facies are similar with moderate to high sulfur concentrations and high vitrinite content, whereas (ii) the uppermost two facies contain lower sulfur and proportionally higher inertinite.…”

“…Seven black shale samples from the Betsie Shale Member were also analyzed in an effort to improve the precision of the Re-Os age reported in our earlier study (Geboy et al, 2015). These shale samples were collected within a stratigraphic interval of ~3 inches (7.6 cm) and are ~2 ft (~61 cm) above the shale layer studied by Geboy et al (2015). For each of the coal and shale samples, several grams were finely ground using an agate ball mill; aliquots of the same sample powders were used for Rock-Eval and Re-Os isotopic analyses.…”

“…Specifically, a U-Pb zircon age for the underlying Upper Banner tonstein (316 ± 1 Ma; Lyons et al, 1997), and concordant single zircon age for the overlying Fire Clay tonstein (314.6 ± 0.9 Ma; Lyons et al, 2006) provide age constraint for the Betsie Shale Member. Further, reported 40 Ar-39 Ar ages for sanidine range from ~311 to ~315 Ma (summarized in Geboy et al, 2015), with recent inter-laboratory comparisons yielding ages of 314.6 ± 1.1 or 315.3 ± 0.7 Ma, depending on the calibration used (Hemming et al, 2012). More recently, we reported a Model 1 Re-Os age of 323 ± 8 Ma for the Betsie Shale…”

“…1). The Matewan coal bed, with vitrinite bands (~5-10 mm thick) throughout, is highly volatile bituminous A in rank (ASTM rank (D-388) standard; Ruppert et al, 2010), with high sulfur content (~2.7 %, dry basis; Geboy et al, 2015).…”

“…The base of the Betsie Shale Member is the proposed stage boundary for Lower-Middle Pennsylvanian in North America (Geboy et al, 2015 and references therein) and the proposed age for this boundary is 315.2 ± 0.2 Ma (Gradstein et al, 2012). Earlier reported radiometric ages for the sedimentary sequences bracketing the Matewan coal bed and Betsie Shale Member are consistent with each other (Geboy et al, 2015;Hemming et al, 2012;Lyons et al, 1997Lyons et al, , 2006 and confirm an age of ~315 Ma for the Betsie shale ( Fig. 2).…”