Walchian Conifers from the Mid-Late Pennsylvanian Conemaugh Group in the Appalachian Basin: Stratigraphic and Depositional Context, and Paleoclimatic Significance
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International Journal of Coal Geology 171 (2017) 153–168 Contents lists available at ScienceDirect International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo Walchian conifers from the Mid-Late Pennsylvanian Conemaugh Group in the Appalachian Basin: Stratigraphic and depositional context, and paleoclimatic significance Ronald L. Martino Department of Geology, Marshall University, Huntington, WV 25755, United States article info abstract Article history: Walchian conifers are indicators of seasonally dry habitats of the Euramerican subtropics and tropics and are of Received 6 August 2016 considerable value in tracking both short term and long term Pennsylvanian-Permian paleoclimatic changes in Received in revised form 28 December 2016 central Pangea. Walchian conifer macrofossils first appeared in North America during the Middle Pennsylvanian, Accepted 11 January 2017 but are rare in central Pangean coal basins until the Permian. An abrupt climate change occurred near the Available online 12 January 2017 Desmoinesian-Missourian boundary that was characterized by global warming, stronger seasonality, and shorter Keywords: wet phases. This change coincided with the regional extinction of most tree lycopsids and the appearance of Walchia widespread, high chroma, calcic vertisols and aridisols. Only four occurrences of Walchia have been reported Paleoclimate from the Pennsylvanian of the Appalachian Basin: 1) 7-11 Mine of eastern Ohio, 2) Rennersville in southwest Cyclothems Pennsylvania, and 3) Charleston and 4) Cedar Run, both in southern West Virginia. This paper uses recently ac- Sequence stratigraphy quired outcrop data to more fully document and reevaluate the depositional and stratigraphic context of the West Virginia assemblages and their paleoclimatic and paleogeographic implications. The Cedar Run Walchia assemblage occurs in olive mudshale of an abandoned fluvial channel-fill 15.8–16.9 m above the base of the Ames Limestone, indicating an early Virgilian age. It consists of compressions and impres- sions of branches and branchlets of Walchia, Lepidophylloides, Cordaites, and rare neuropteroid pinnules. The channel-fill at this location is a component of the Grafton Sandstone incised valley-fill previously described from the study area. Correlation of paleosol-bounded, marine-cored cyclothems in the study area with their nonmarine cyclothem equivalents at Charleston, West Virginia indicates the Walchia previously reported at the Mahoning coal horizon, occurs between the Brush Creek and Bakerstown coals, and is therefore Missourian, not late Desmoinesian, and similar to the revised age for the 7-11 Mine Walchia reported from eastern Ohio. Late Pennsylvanian, upland (or dryland) conifer communities were comprised of Walchia, Cordaites,andSigillaria which produced a forest with seed ferns as an understory. The Cedar Run assemblage was probably transported into the valley from adjacent, well-drained coastal plain uplands formed during valley incision, and deposited within the early transgressive systems tract. It is also possible that Walchia expanded into the valleys when drain- age became ephemeral during more arid climatic phases. The revised correlations of Appalachian Basin Walchia horizons indicate their appearance closely followed, rather than predated the abrupt climate change and extinc- tions at the Desmoinesian-Missourian boundary. © 2017 Elsevier B.V. All rights reserved. 1. Introduction DiMichele, 2014). Plant fossils provide a snapshot of paleoclimatic con- ditions. A full understanding of their spatial and temporal distributions The late Middle and Late Pennsylvanian strata of the Appalachian and their sedimentologic context is essential to the accuracy of interpre- Basin have been the focus of much attention because they record both tations regarding paleoclimate trends and cycles and evolutionary pat- short-term climate cycles related to glacioeustasy and longer term terns. The importance of their sequence stratigraphic context in trends related to 1) the northward drift of Pangea across the equator, accurate paleoenvironmental reconstructions has also become apparent 2) orographic effects of the rising Appalachian Mountains, and 3) possi- (e.g. Demko et al., 1998; Falcon-Lang et al., 2009; Gastaldo and Demko, bly atmospheric changes in carbon dioxide (Donaldson et al., 1985; 2010). The paleoenvironmental and paleoclimatic significance of Cecil, 1990; Frakes et al., 1992; Heckel, 1994; Otto-Bleisner, 2003; Walchia has received much attention over the past three decades and the various interpretations have been recently been reviewed in detail by DiMichele (2014). Walchia was a primitive conifer represented by E-mail address: [email protected]. woody trees with small, helically arranged, needle-like leaves 1–5mm http://dx.doi.org/10.1016/j.coal.2017.01.005 0166-5162/© 2017 Elsevier B.V. All rights reserved. 154 R.L. Martino / International Journal of Coal Geology 171 (2017) 153–168 in length (Taylor, 1981). The xeromorphic traits of Walchia, like most 2. Geologic setting modern conifers (thick cuticle, needle like leaves with low ratio of sur- face area to volume), enabled it to retain moisture and inhabit well- Strata in the study area crop out in the Central Appalachian Basin at drained soils, and drier climates (Lyons and Darrah, 1989a). the southwest end of the Dunkard Basin Synclinorium (Spencer, 1964; Walchia is a key taxon and is widespread in Permian strata. In con- Merrill, 1988). Downwarp of the Dunkard Basin may have occurred dur- trast, relatively few occurrences are know from the Mid-Late Pennsylva- ing, as well as after, the deposition of the Conemaugh Group (Merrill, nian of North America. The term ‘Methuselah taxa’ was recently 1988). The Appalachian foreland basin extends from Quebec to Ala- suggested for sporadic, anomalous occurrences of genera, like Walchia, bama, covering an area of about 536,000 km2 (Ettensohn, 2008). During many millions of years older than their more temporally continuous, the Middle-Late Pennsylvanian, thrust-loading in the Appalachian established range (Looy et al., 2014). They are named for Methuselah, Orogen caused basin subsidence which helped provide sediment ac- the oldest man (929 years) in the book of Genesis in the Hebrew commodation space (Quinlan and Beaumont, 1984; Greb et al., 2008). Bible. Methuselah floras are typically found in seasonally dry facies in The rate of subsidence was greatest in eastern West Virginia and de- between vertically adjacent wetland floras. It has been postulated that creased northwestward toward the cratonic platform in Ohio and Ken- Methuselah floras may have first appeared in upland areas with low tucky. Transgressive-regressive cycles each lasting several million years, preservation potential, and periodically migrated into basinal lowlands referred to as tectophases, resulted from tectonic loading and relaxa- during seasonally drier portions of glacioeustatic climate cycles (e.g. tion. Higher frequency, glacioeustatic transgressive–regressive cycles Dolby et al., 2011; DiMichele, 2014; Looy et al., 2014). are superimposed on the tectophases (Busch and Rollins, 1984; Busch The earliest unequivocal walchian conifer macrofossils occur in Mid- and West, 1987; Heckel, 1994, 1995; Ettensohn, 2008; Greb et al., dle Pennsylvanian (middle Desmoinesian) strata of the North American 2008). During the Late Pennsylvanian the Central Appalachian Basin Illinois Basin (Plotnick et al., 2009; Falcon-Lang et al., 2009). Walchian was positioned within a few degrees of the equator (Blakey, 2007; pollen (Potonieisporites) has been reported from the Late Mississippian Rosenau et al., 2013). During the Middle and Late Pennsylvanian, rivers (early Namurian) of Britain and Nova Scotia (Bharwadjwa, 1964; flowed west and north across West Virginia, draining the Allegheny Neves and Belt, 1971; Mapes and Gastaldo, 1986). Walchia has been re- Orogen. Channel belts in the tropical coastal plain were flanked by ported from the Pennsylvanian of Colorado, Illinois, Oklahoma, and Kan- flood basin lakes and swamps during the wetter parts of glacial-inter- sas (Elias, 1942; Rothwell, 1982; Winston, 1983; Lyons and Darrah, glacial cycles (Arkle, 1974; Donaldson, 1979; Martino, 2004, 2015). 1989a, 1989b; Falcon-Lang et al., 2009). Only four occurrences have been reported from the Late Pennsylvanian of the Appalachian Basin: 3. Stratigraphic and depositional framework one in Ohio (7-11 Mine, McComas, 1988), one in Rennersville, Pennsyl- vania (Darrah, 1969, 1975) and two in West Virginia (Charleston: Lyons The late Middle to Late Pennsylvanian Conemaugh Group extends and Darrah, 1989a, 1989b; Cedar Run, Martino and Blake, 2001; Fig. 1). from the top of the Upper Freeport coal to the base of the Pittsburgh The stratigraphic context of the Ohio occurrence has recently been re- coal (Fig. 2). It was historically referred to as ‘the lower barren mea- vised (Easterday, 2004; Falcon-Lang et al., 2011; Belt et al., 2011), but sures’ because there are relatively few minable coals in this stratigraphic the West Virginia occurrences have yet to be clearly documented. The interval (Wanless, 1939). The Conemaugh Group does contain several objectives of this paper are to fully document the paleobotanical and high volatile bituminous coals with total sulfur of 1–3%. These are usu- stratigraphic context of the Cedar Run Walchia assemblage (Martino ally minable only in the northern Appalachian Basin (Repine et al., and Blake, 2001) and re-evaluate the