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Abstract

The latest Devonian (Famennian) is characterized by an extensive Southern Hemisphere glaciation. Deposits resulting from this glaciation are present in several formations in the mid-Atlantic region, including the Hampshire, Catskill, Rockwell, and Spechty Kopf. The Hampshire (= Catskill) Formation exhibits a noticeable stratigraphic change upsection from the middle to the top. The middle part consists of thick intervals of red, channel-phase sandstones with thin overbank siltstone and mudstone. These mudstones contain poorly developed, calcareous paleosols. The top of the Hampshire Formation consists of greenish-gray sandstones containing abundant coaly plant fragments, coalified logs, and pyrite, interbedded with thick paleo-Vertisols. The upsection increase in preserved terrestrial organic matter suggests the onset of environmental conditions that became increasingly wet. The Late Devonian escalation in climate wetness culminated in the development of a stratigraphically and spatially restricted succession of diamictite-mudstone-sandstone interpreted as having formed in glacial and proglacial environments. These glacial environments are recorded in the lower Rockwell Formation of western Maryland and contemporaneously deposited intervals of the Spechty Kopf Formation of northeastern Pennsylvania. Sheared and massive diamictite facies are interpreted as lodgement and meltout deposits, respectively; whereas, bedded diamictites are interpreted as resedimented deposits. The diamictite facies is locally overlain by a mudstone facies with variable characteristics. Both the massive and deformed mudstone lithofacies are interpreted as a clast-poor, subaqueous glaciolacustrine deposit. Laminated mudstones are interpreted as forming in quiet glaciolacustrine environments. The pebbly sandstone facies is interpreted as proglacial braided outwash deposits that both preceded glacial advance and followed glacial retreat.

Introduction

The Late Devonian represents a time of change in the depositional character of the strata of the Appalachian basin. It is during this interval of time that the basin transitioned from marine to primarily terrestrial environments with the progradation of the Catskill clastic wedge. Throughout the central Appalachian basin, the Famennian Catskill Formation and equivalent Hampshire Formation of Maryland and West Virginia comprise a thick wedge of interbedded red sandstone, siltstone, and meter-scale paleosol horizons (Sevon, 1985). Near the top of the Catskill its dominantly red-brown character is replaced progressively by alternating greenish and reddish layers, and then ultimately by dominantly greenish-gray strata. This vertical change through the Catskill wedge has been interpreted as reflecting a transition from semiarid to humid conditions (Brezinski et al., 2009). Near the top of this transitional succession of strata, an interval of glacigenic facies has been recognized. These strata extend from northeastern Pennsylvania to the eastern panhandle of West Virginia (Fig. 1).

These glacial deposits have been shown to be contemporaneous with glacial deposits long identified from South America (Caputo, 1985; Isaacson et al., 1999). Thus, the progressive upsection changes in color within the upper several hundred meters of the Catskill and Hampshire Formations appear to reflect the gradual climatic change that achieved its maximum with the onset of glacial advances near the very end of the Devonian. The purpose of this field trip is to illustrate the depositional character of the upper Catskill transition facies and to illustrate the glacigenic facies that appear to have developed at the maximum of this Late Devonian climatic cooling episode.

Catskill-Hampshire Stratigraphy of the Central Appalachians

During the Middle to Late Devonian, a thick clastic wedge, known as the Catskill or “Old Red” clastic wedge, prograded westward, filling the Appalachian foreland basin (Faill, 1985; Ettensohn, 1985; Harper, 1999). This wedge of Late Devonian clastic sediment produced a gradational shift in facies from deepwater black shales to nearshore marine to deltaic and finally to alluvial plain (Harper, 1999). Consequently, by the Famennian, the red alluvial plain clastic sediments of the Catskill Formation (Hampshire Formation of Maryland and West Virginia) extended across most of Pennsylvania and Maryland (Fig. 2A). The Catskill-Hampshire Formations exhibit dramatic thinning, from more than 2500 m in central Pennsylvania to less than 300 m in northern West Virginia (Boswell et al., 1987; Cotter and Driese, 1998).

Figure 1.

Map of western Maryland and distribution of the Rockwell Formation (from Brezinski and Conkwright, 2013). Field-trip locations are identified by number. Inset map illustrates known locations of Rockwell and Spechty Kopf diamictite succession (from Brezinski et al., 2010).

Figure 1.

Map of western Maryland and distribution of the Rockwell Formation (from Brezinski and Conkwright, 2013). Field-trip locations are identified by number. Inset map illustrates known locations of Rockwell and Spechty Kopf diamictite succession (from Brezinski et al., 2010).

The lower strata of the Hampshire (Catskill) Formation are gradational with the underlying Foreknobs Formation of the Greenland Gap Group. These gradational strata consist of interbedded greenish-gray, bioturbated sandstones and reddish, mudcracked shales. In central to eastern Pennsylvania, this interval is termed the Irish Valley Member of the Catskill Formation.

Figure 2.

Interpreted depositional history of the central Appalachians during the Late Devonian (Famennian Stage). (A) Maximum progradation of the Catskill-Hampshire clastic wedge during the Famennian forms a semiarid alluvial plain experiencing high levels of evaporation. (B) Latest Devonian piedmont glaciation is concomitant with global sea-level drop. Modified from Brezinski et al. (2010). (C) Glacial retreat at the end of the Devonian results in sea-level rise and flooding of the Appalachian basin to form the Sunbury and Riddlesburg Shales.

Figure 2.

Interpreted depositional history of the central Appalachians during the Late Devonian (Famennian Stage). (A) Maximum progradation of the Catskill-Hampshire clastic wedge during the Famennian forms a semiarid alluvial plain experiencing high levels of evaporation. (B) Latest Devonian piedmont glaciation is concomitant with global sea-level drop. Modified from Brezinski et al. (2010). (C) Glacial retreat at the end of the Devonian results in sea-level rise and flooding of the Appalachian basin to form the Sunbury and Riddlesburg Shales.

The Irish Valley Member has been interpreted as representing a marginal marine coastal fringe (Walker, 1971; Cotter and Driese, 1998). Even though this unit’s name is not applied to the Hampshire Formation of south-central Pennsylvania, Maryland, or West Virginia, similar facies exist near the base of the Hampshire strata.

The Irish Valley facies is replaced upsection by a succession dominated by thick red sandstone intervals. This part of the Hampshire and Catskill Formations consists of red to reddish-brown, massive to thick-bedded sandstones that exhibit sharp, presumably erosional, bases, containing shale-pebble lag conglomerates, and fine upward into intervals of interbedded red sandstone, siltstone, and mudstone. The mudstone intervals typically have poorly defined root zones that contain layers of carbonate blebs and nodules. This part of the Catskill Formation of central Pennsylvania has been named the Sherman Creek and Duncannon Members (Dyson, 1967). Similar lithologies are present within the Hampshire Formation in West Virginia and Maryland, even though these member names are not applied there. Several of the thicker sandstone intervals within the Sherman Creek and lower Duncannon Members have been interpreted as incised channel fills produced during Famennian sea-level drops (Cotter and Driese, 1998).

Throughout the central Appalachians, the upper 100-200 m of the Catskill-Hampshire Formations displays a distinct and recognizable upsection change in color and lithology from the thick, red sandstone with thin interbeds of reddish siltstones and mudstone of the Sherman Creek facies, to interbedded reddish and greenish-gray sandstone and variegated to greenish-gray mudstone. Unlike the underlying part of the Catskill, the sandstone units in this part of the formation contain basal lag conglomerates containing coaly plant fragments, tree trunks, and pyrite nodules. Furthermore, this part of the formations also contains thick, rooted mudstone intervals that commonly contain pedogenic slickensides (Retallack, 2006; Brezinski et al., 2009).

Based upon intrinsic evidence, much of the lower and middle Catskill and Hampshire Formations were deposited under a dry subhumid climate (Brezinski et al., 2009). The thin, carbonate-rich paleosols that lack deep rooting or coaly plant fragments suggest a dry climate. Thus, it appears that conditions were not conducive to plant growth and/or preservation. Furthermore, the thin calcareous paleosols that characterize the Sherman Creek Member are suggestive of relatively dry seasonal conditions (Pettijohn, 1975; Retallack, 2001; Driese et al., 2006). These thin paleo-Entisols differ significantly from the highly rooted, thick, noncalcic, paleo-Vertisols developed within the upper transitional facies. Parallelling the upsection change in the thickness and prominence of paleosols is the change in the color of the sandstone intervals from reddish to greenish-gray (Cecil et al., 1998). Moreover, these sandstones of the upper Catskill pervasively contain abundant coaly plant fragments, and locally carbonized logs as well as pyrite.

Brezinski et al. (2009) interpreted the changes in color and character near the top of the Catskill and Hampshire Formations in the central Appalachian basin as resulting from progressive increase in the amount and/or decrease in the seasonality of precipitation near the end of Catskill deposition. The thick, wellrooted, non-calcic Vertisols in this part of the formations suggest increased wetness (Cecil et al., 2004; Retallack, 2001). Moreover, the widespread pedogenic slickensides indicate the swelling and shrinking of the paleosol clays during repeated seasonal wetting and drying (Retallack, 2001). Concomitant with the increase in wetness is the development and preservation of some of the earliest autochthonous coal beds and the appearance of the earliest known seed plants (Gillespie et al., 1981; Scheckler, 1986a, 1986b).

The late Famennian Catskill-Hampshire transitional strata are not an artifact of local climatic factors. Streel et al. (2000) demonstrated that the global climate exhibited a general increase to more humid conditions during the late Famennian. This gradual change of global climate toward what appears to be wetter conditions in terrestrial settings may have reached its maximum during the very latest Famennian with the onset of high latitude glaciation (Caputo, 1985). In the central Appalachian basin, this resulted in the deposition of the Spechty Kopf-Rockwell diamictite succession (Brezinski et al., 2010) (Fig. 2B). Concurrently, the marine realm experienced a significant sea-level drop, coincident with the expansion of black shale deposition. Earth’s marine biota also experienced a major catastrophe, the Hangenberg extinction event. This event severely affected many of the groups that had not suffered during the Frasnian/Famennian extinction earlier in the Late Devonian (Caplan and Bustin, 1999). Groups such as cephalopods and calcareous algae sustained significant diversity declines during the Hangenberg event (Fagerstrom, 1994; House, 1985), and several other groups (stromatoporoids and placoderm fish) went extinct (Hallam and Wignall, 1997). Immediately following the sea-level drop and formation of the purported glacigenic strata at the end of the Devonian, sea-level rise drowned much of the previously exposed shelf areas. In Europe, this resulted in the formation of the Hangenberg Limestone, and in the central Appalachian basin the Riddlesburg and Sunbury Shales were deposited (Brezinski et al., 2008) (Fig. 2C).

Lower Rockwell Glacigenic Facies

Throughout eastern and central Pennsylvania, western Maryland, and eastern West Virginia, the upper Catskill-Hampshire transition interval is overlain by a succession of greenish-gray sandstone, gray mudstone, and diamictite within the lower Rockwell Formation. This puzzling lithology also is known in the lower Spechty Kopf Formation of northeastern Pennsylvania (Berg, 1999). Its origin has always been suspect, but Sevon (1969, 1979), Bjerstedt (1986), and Bjerstedt and Kammer (1988) postulated a localized submarine debris flow as its environment of deposition. More recently, the Rockwell and Spechty Kopf diamictite successions have been interpreted as glacigenic in origin (Cecil et al., 2004; Brezinski et al., 2008, 2010). On this trip, we will examine the Hampshire-Rockwell transition strata that appear to reflect the climatic wetness that led to deposition of the Rockwell glacigenic facies.

The Rockwell glacigenic facies exhibit a complex vertical and lateral arrangement of lithologies (Brezinski et al., 2010, their figure 7) (Fig. 3). The diamictite facies consists of a number of bedding variations including massive, thin-bedded, and deformed. The massive form of diamictite is most widespread, with the bedded and deformed types typically being confined to the basal strata beneath the massive forms. The massive diamictite is unbedded, medium-gray to reddish-brown, with a matrix of clay, silt, and, locally, sandy mudstone. Massive diamictite deposits vary in thickness from 5 to 15 m and contain polylithic clasts from granules to boulders up to 2 m in diameter. Many of the clasts are exotic lithologies including granite, metagray-wacke, and volcaniclastics (Sevon, 1969; Suter, 1991; also see Skema and Smith’s “Character and Probable Origin of Volcanic Clasts from the Rockwell Diamictite” section). Large blocks, several meters in diameter, derived from Catskill or Hampshire strata, are locally contained within the massive lithology.

The bedded diamictite exhibits poorly defined layering with individual beds ranging from 0.3 to 2 m in thickness. These strata tend to pinch out laterally and commonly exhibit a faint normal grading. These beds are typically clast-rich and matrix- or clast-supported. Where present, this lithology is most commonly situated near the top or base of the diamictite facies.

The deformed diamictite facies consists of reddish to gray, sheared diamictite. Locally, lenticular sandstone units are present within this lithology. These sandstone units are composed of pebbly, reddish-brown or greenish-gray sandstone to laminated siltstone and exhibit a sharp (erosional?) base and a flat upper surface. Although commonly deformed, these sandstone bodies can be several meters in diameter and 1-3 m thick.

Associated with and commonly overlying the diamictite facies is a mudstone lithology that is highly variable in thickness. This mudstone facies takes on a variety of characters from thinly laminated to deformed to massive. Where massive, this mudstone resembles a clast-poor diamictite, containing sand and granular quartz. The massive mudstone intervals are commonly interbedded with deformed mudstone. This lithology consists of grayish-green to reddish-brown silty and sandy mudrock. Isolated pebbles, layers of sand or diamictite, or laminated layers are commonly present; ball-and-pillow and thin deformed sandstone strata also may be present. Where the latter lithologies are present, they are contorted and apparently were deformed penecontemporaneously.

Related to and commonly interstratified with the deformed mudstone are intervals of laminated mudstone. Laminations are 1 millimeter thick, composed of fine sand and silt or silt and clay. Sand/silt laminations typically are coarse, from 0.5 to 1.0 cm in thickness. Locally, disrupting the laminae are quartz and sandstone clasts that vary from granule- to pebble-size.

The diamictite and mudstone facies are commonly sandwiched between variably thick intervals of pebbly sandstone. Most commonly, the pebbly sandstone facies are massive, tan to brown, coarse-grained, and festoon, trough cross-bedded to planar-bedded. Rounded quartz granules and pebbles occur as isolated clasts and as pebble trains along individual cross-bed foresets. The sandstone intervals can vary from between 5 and 25 m in thickness.

Figure 3.

Cross section of Late Devonian and lower Mississippian strata of western Maryland. Modified from Brezinski (1989a, 1989b) and Brezinski et al. (2010).

Figure 3.

Cross section of Late Devonian and lower Mississippian strata of western Maryland. Modified from Brezinski (1989a, 1989b) and Brezinski et al. (2010).

Where this lithology overlies the diamictite or mudstone lithologies, it typically fines upward. However, when this facies underlies the diamictite, it exhibits an upward coarsening character. In places where the pebbly sandstone facies underlies the diamictite, it contains large inclusions (up to 1 m in diameter) of diamictite and shale. Furthermore, this part of the pebbly sandstone facies commonly exhibits a level of contorted bedding, indicating that the unit was locally deformed.

Lower Rockwell Glacigenic Environmental Interpretation

The lower Rockwell diamictite, mudstone, and sandstone facies were interpreted by Brezinski et al. (2010) to be depositionally related. These facies were proposed to have formed during a single glacial advance and retreat (Brezinski et al., 2008, 2010). The bedded clast-rich diamictite strata were interpreted as local debris flows that formed in advance of, or lateral to, glacial ice, and tend to be preserved in what are interpreted to be both glacial advance and retreat environments. Such debris flows form from meltout materials that become supersaturated and unstable on the sloping ice surface. The recognizable stratification, grading, and local clast-supported character are consistent with resedimented deposits (Benn and Evans, 1998). The bedded diamicite intervals that occur near the base of the diamictite succession are commonly associated with the sheared diamictite. Sheared diamictite intervals were interpreted by Brezinski et al. (2008, 2010) as lodgement material formed beneath an actively advancing glacier.

The massive diamictite has been interpreted as forming in a number of ways. In glacial settings, such massive diamictons can form from ice-contact meltout, or as debris flow avalanches. The lower Rockwell diamictite is interpreted to have formed by both of these mechanisms (Brezinski et al., 2010). The localized lenticular and folded sandstone bodies within the diamictite are interpreted as cut-and-fill, subglacial meltwater channels, called Nye channels (Eyles et al., 1982; Eyles, 1993; Miller, 1996). Nye channels commonly erode through sub-ice substrates to form a network of streams that are widespread in subglacial lodgement tills.

The mudstone facies, like the diamictite facies, formed in several different ways. This lithology can be interbedded with the massive and bedded diamictite. In fact, the massive mudstone was considered a clast-poor diamictite by a number of previous workers (Sevon 1969; Sevon et al., 1997; Suter, 1991). The massive and deformed clast-poor mudstone commonly displays contorted layers of bedded diamictite and sandstone, and frequently exhibits evidence of soft-sediment deformation. The contorted and deformed strata are indicative of penecontemporaneous soft sediment slumps. Solitary pebbles and cobbles that pervade this lithology are interpreted to be ice-rafted dropstones. Clast-poor and deformed mudstones associated with glacial diamictites have been interpreted as subaqueous deposits (Deynoux, 1985; Eyles and Eyles, 1983; Miller, 1989). They are commonly associated with diamictite strata, but form as subaqueous debris flow deposits. Thus, the massive mudstone lithology of the Rockwell Formation is interpreted as a glacial lake deposit formed in an area of high sediment input proximal to the active glacier.

Commonly interbedded with or overlying the massive and deformed mudstone facies is the laminated mudstone lithology. Brezinski et al. (2010) interpreted this facies as a glaciolacustrine varvite. The fine silt-clay laminations do show the regular rhythmicity consistent with quiet, cold-water stratified lakes (Miller, 1996; Benn and Evans, 1998). The layered and lenticular sand and gravel occurring in the facies are interpreted to be ice-rafted till dumps (Thomas and Connell, 1985), whereas the isolated clasts are interpreted as ice-rafted dropstones (Pettijohn, 1975; Miller, 1996; Benn and Evans, 1998; Crowell, 1999).

Because of its persistence and resistance to erosion, the pebbly sandstone facies was identified at all outcrops of the lower Rockwell glacigenic sequence. This facies also commonly occurs as the basal lithology within the glacigenic succession when the diamictite is not resting directly on the Catskill-Hampshire strata. The pervasive distribution, relatively flat base, sand and granule- to pebble-sized clasts, and planar to trough cross-bedding that characterizes this facies are consistent with a low sinuosity braided fluvial environment of deposition (Miall, 1977). The interpreted braided fluvial origin for these deposits suggests a sediment-choked river system. Such river systems are typical of glaciofluvial environments (Edwards, 1978; Miall, 1977; Miller, 1989, 1996; Hambrey, 1994; Benn and Evans, 1998). With this interpretation in mind, the pebbly sandstone facies is interpreted as representing a braided outwash plain. This facies appears to have formed during both glacial advance and retreat.

Brezinski et al. (2010) illustrated masses of diamictite within the pebbly sandstone facies, and interpreted these masses as till balls. Till balls form during high velocity glacial outburst floods wherein the elevated velocity of the flowing water rips up and carries large masses of previously deposited till well out onto the outwash plain (Shaw, 1987; Benn and Evans, 1998).

Lateral Equivalents to the Lower Rockwell Glacigenic Facies

To the west and northwest of the area occupied by the diamictite facies, the lower Rockwell Formation consists of a thick interval of medium-gray, medium- to coarse-grained, epsilon cross-bedded, fining-upward sandstone containing coaly plant fragments and large carbonaceous logs, and displaying an erosional base with basal quartz and shale-pebble lag conglomerate (Fig. 3). In western Pennsylvania, this unit is termed the Cussewago Sandstone. The Cussewago Sandstone is equivalent to the Bedford Shale-Berea Sandstone (Berg et al., 1983; Dennison et al., 1986). The lower Rockwell Formation strata above the Cussewago Sandstone consist of thin, gray to tan, lenticular sandstone, micaceous siltstone, gray rooted claystone paleosols, and thin coal beds or coaly shales. This stratigraphic interval is characterized by well-developed paleo-Vertisols, paleo-Spodosols, and paleo-Histosols (Brezinski et al., 2009) (Fig. 3).

Suprajacent Lithologies

Throughout south-central Pennsylvania and western Maryland, the lower Rockwell glacigenic sequence is overlain by dark-gray, calcareous, fossiliferous, marine shale that Girty (1928) termed the Riddlesburg Shale. This widespread marine shale has been shown to be the nearshore restricted marine facies of an early Carboniferous transgressive event. Dennison et al. (1986), Bjerstedt and Kammer (1988), and Carter and Kammer (1990) have shown that the Riddlesburg Shale can be traced westward into Ohio, where it is equivalent to the earliest Mississippian Sunbury Shale. The Sunbury Shale has been correlated with the Hangenberg Limestone of western Europe (Brezinski, et al., 2010, their figure 15).

Road Log and Stops

Leave the Baltimore Convention Center and head south on I-395.

(km)miDirections
(2.0)1.2Merge onto I-95 west.
(5.1)3.2Take Exit 49, I-95 South, on to Baltimore I-695 (Inner Loop).
(8.3)5.2Take Exit 16 off of Inner Loop I-695 to I-70 west.
(1.5)0.9Merge onto and begin I-70 west.
(2.9)1.8I-70 Exit 87, U.S. 29 south.
(8.2)5.1Merge from U.S. 40 west.
(43.0)26.7Exit 56, East Patrick Street, Frederick, Maryland.
(4.2)2.6Interchange with I-270.
(5.1)3.2Braddock Mountain, exposed are strata of 600-m.y.-old Catoctin Formation.
(16.9)10.5South Mountain, western ridge of Maryland’s Blue Ridge.
(10.5)6.5Exit 32, U.S. 40 west, Hagerstown, Maryland.
(9.5)5.9Exit 26, Interchange with I-81.
(35.7)22.2Exit 3, Hancock, Maryland.
(3.1)1.9Exit 1B, U.S. 522 south.
(0.9)0.6Exit from I-70 and merge onto I-68.
(5.5)3.4Take Exit 77, Woodmont Road. Turn left at end of ramp, then again left onto Maryland Route 144. Turn right on Woodmont Road.
(9.9)0.6Intersection with Pearre Road. C&O Canal is on the left.
(2.0)1.2Left off of Pearre Road to C&O Canal Lock 56 and WMRT (Western Maryland Rail Trail) parking.
(km)miDirections
(2.0)1.2Merge onto I-95 west.
(5.1)3.2Take Exit 49, I-95 South, on to Baltimore I-695 (Inner Loop).
(8.3)5.2Take Exit 16 off of Inner Loop I-695 to I-70 west.
(1.5)0.9Merge onto and begin I-70 west.
(2.9)1.8I-70 Exit 87, U.S. 29 south.
(8.2)5.1Merge from U.S. 40 west.
(43.0)26.7Exit 56, East Patrick Street, Frederick, Maryland.
(4.2)2.6Interchange with I-270.
(5.1)3.2Braddock Mountain, exposed are strata of 600-m.y.-old Catoctin Formation.
(16.9)10.5South Mountain, western ridge of Maryland’s Blue Ridge.
(10.5)6.5Exit 32, U.S. 40 west, Hagerstown, Maryland.
(9.5)5.9Exit 26, Interchange with I-81.
(35.7)22.2Exit 3, Hancock, Maryland.
(3.1)1.9Exit 1B, U.S. 522 south.
(0.9)0.6Exit from I-70 and merge onto I-68.
(5.5)3.4Take Exit 77, Woodmont Road. Turn left at end of ramp, then again left onto Maryland Route 144. Turn right on Woodmont Road.
(9.9)0.6Intersection with Pearre Road. C&O Canal is on the left.
(2.0)1.2Left off of Pearre Road to C&O Canal Lock 56 and WMRT (Western Maryland Rail Trail) parking.

Stop 1. Western Maryland Rail Trail (WMRT), Mouth of Sideling Hill Creek: Upper Hampshire Formation Transitional Strata and Lower Rockwell Glacigenic Section (Coordinates for top of section: 39°38’23”N, 78°20O1”W)

This is one of the most complete sections of the Hampshire Formation in the region. More than 1400 m of the formation is exposed (Fig. 4). The section begins at the eastern portal of Indigo tunnel within the upper strata of the Foreknobs Formation. The Pound Sandstone Member forms the ridge immediately to the west of the portal. The Pound Sandstone marks the boundary between the Frasnian and Famennian (McGhee and Dennison, 1980; McGhee, 1996), which marks a major biotic crisis for marine biota. This interval is known as the Kellwasser event in Europe. This crisis greatly reduced alpha and beta diversity in trilobites, brachiopods, corals, bryozoans, and crinoids (Hallam and Wignall, 1997). In this part of the Appalachians, the crisis horizon is contained within the Pound Sandstone. The Pound Sandstone has been interpreted as an incised, valley-filling sandstone that can be up to 35 m thick (Brezinski, 2011).

Approximately 100 m of the Foreknobs Formation is exposed at the base of this exposure. This interval of thinly bedded, greenish-gray shale and sandstone is assignable to the Red Lick Member. The lowest 400 m of the Hampshire Formation is gradational with the underlying Foreknobs Formation. These strata consist of cyclically interbedded greenish-gray, bioturbated sandstones and reddish, mud-cracked shales. This lithology is termed the Irish Valley Member of the Catskill in Pennsylvania. This part of the Hampshire represents a lower alluvial plain (Walker, 1971; Cotter and Driese, 1998) that was periodically submerged by marine inundations.

The alternating greenish-gray to reddish lithologic units of the Irish Valley facies of the Hampshire are replaced upsection by ~700 m of red to reddish-brown, massive- to thick-bedded sandstones that exhibit erosional bases; contain shale-pebble lag conglomerates; and fine upward into thin intervals of interbedded red sandstone, siltstone, and rooted mudstone (Fig. 5A). This part of the Hampshire Formation is equivalent to the Sherman Creek Member of the Catskill Formation. The sandstone intervals vary from 3 to 15 m in thickness and locally may have a thin (< 2 m), greenish-gray zone at the base. Thin, reddish-brown siltstone and mudstone intervals several meters thick separate the sandstone units. Some of these siltstone strata are indicative of incipient soils.

Figure 4.

Stop 1. Measured section of the Hampshire and lower Rockwell Formations along the Western Maryland Rail Trail at Sideling Hill Creek.

Figure 4.

Stop 1. Measured section of the Hampshire and lower Rockwell Formations along the Western Maryland Rail Trail at Sideling Hill Creek.

This part of the Hampshire Formation has been interpreted as an alluvial plain consisting primarily of fluvial-channel phase sandstones (Sevon, 1985). The thin siltstone and mudstone intervals separating the thick sandstone units are interpreted as fine-grained overbank deposits containing thin paleosols. Locally, carbonate-rich layers or carbonate nodules are present within these paleosols. These carbonate-rich paleosols are interpreted to be incipient fossil caliche layers developed within thin, poorly developed paleosols.

The upper 200 m of the Hampshire Formation takes on a very different appearance from that of the Sherman Creek facies. This part of the formation consists of thick intervals of reddish-brown to greenish-gray, slickensided claystone and thin intervals of greenish-gray sandstone (Figs. 5B-5D). Brezinski et al. (2009) proposed that this part of the Hampshire Formation represented a transition between the red-dominated Hampshire and the grayish strata of the overlying Rockwell Formation. The transition, they hypothesized, reflected a progressive increase in the amount of precipitation experienced prior to the onset of the Late Devonian glacial episode.

Figure 5.

Stop 1, Hampshire (Catskill) Rockwell Transition. (A) Thin reddish paleosol sandwiched between two thick, channel-phase sandstones within the middle Hampshire Formation. (B) Thick paleosol within the transitional facies of the upper Hampshire Formation. (C) Greenish-gray sandstone of the upper Hampshire Formation. (D) Archaeopteris? stump near top of Hampshire Formation at Stop 1.

Figure 5.

Stop 1, Hampshire (Catskill) Rockwell Transition. (A) Thin reddish paleosol sandwiched between two thick, channel-phase sandstones within the middle Hampshire Formation. (B) Thick paleosol within the transitional facies of the upper Hampshire Formation. (C) Greenish-gray sandstone of the upper Hampshire Formation. (D) Archaeopteris? stump near top of Hampshire Formation at Stop 1.

The top of the Hampshire is concealed by the Sideling Hill Creek valley. The basal Rockwell Formation is exposed, however, along the rail trail on the eastern side of the creek. At this part of Stop 1, the basal Rockwell strata consist of a succession of diamictite, laminated mudstone, and cross-bedded, pebbly sandstone. These lithologies are interpreted to be part of the Rockwell glacigenic sequence. The character of this succession will be revealed more fully as we examine it at the remainder of the stops on this trip (Fig. 6).

(km)miDirections
(12.3)(3.8)Retrace route back to I-68 westbound.
7.62.4West on I-68 to Sideling Hill visitor center.
(km)miDirections
(12.3)(3.8)Retrace route back to I-68 westbound.
7.62.4West on I-68 to Sideling Hill visitor center.

Stop 2. Lunch, Sideling Hill Visitor Center: Lower Rockwell Glacigenic Succession, Diamictite and Sandstone Facies, and Overlying Lower Mississippian Riddlesburg Shale (Coordinates: 39°43’12”N, 78°17 11”W)

Highway construction in 1985 exposed a nearly complete section of the lower Rockwell glacigenic sequence. This unusual lithology had been identified elsewhere in the Rockwell Formation, locally (Sevon, 1979). This perplexing Rockwell lithology initially was proposed as representing a localized submarine debris flow (Sevon, 1979; Bjerstedt, 1986, Bjerstedt and Kammer, 1988), but when striated and faceted clasts were identified from nearby outcrops (Cecil et al., 2002), a closer examination of the diamictite and associated lithologies was initiated (Brezinski et al., 2006, 2008).

The succession of lower Rockwell strata is graphically displayed in Figure 7. At this location, the diamictite is underlain by 16 m of coarsening upward, pebbly sandstone. The bedding in the upper several meters of this sandstone appears to be deformed, and large bodies of mudstone and diamictite can be observed in it (Fig. 8A). This sandstone is overlain by 1 m of reddish-gray mudstone, above which is 12 m of gray, massive diamictite. Clasts within the massive diamictite consist of boulders of red mudstone, sandstone, and cobbles and pebbles of various exotic composition. These exotic clasts include granite, basalt, metagraywacke, and metaquartzite (Figs. 8B, 8C). Their origin can be traced to potential sources within the Maryland Piedmont Province 180 km to the east (see Smith and Skema’s “Character and Probable Origin of Volcanic Clasts from the Rockwell Diamictite” section). The uppermost lithology in the Rockwell glacigenic succession at this location is an upward fining, cross-bedded, pebbly sandstone ~10 m thick. The entire glacigenic package is succeeded upsection by medium- to dark-gray siltstone containing thin coal beds and then by dark-gray, silty, fossiliferous shale. This shale contains bivalves and brachiopods and has been correlated with the Riddlesburg Shale of the Broad-top synclinorium of Pennsylvania (Girty, 1928). The Riddlesburg Shale has been traced westward into the coeval Mississippian Sunbury Shale of Ohio.

The upsection change in lithology exhibited within the lower Rockwell diamictite succession led Brezinski et al. (2010) to interpret these stacked facies as a preserved record of a single glacial advance and retreat. The upward-fining, deformed sandstone beneath the diamictite was interpreted as a proglacial outwash braidplain. The deformed bedding within the upper part of this sandstone was interpreted as shearing and loading of unconsolidated sediment beneath advancing glacial ice. The diamictite at this stop was interpreted as a massive meltout till produced during glacial retreat. Likewise, the overlying pebbly, crossbedded sandstone was presumed to represent a braided outwash plain formed by a retreating glacier. The overlying Riddlesburg Shale was interpreted to be a transgressive marine facies that submerged the Appalachian basin concomitantly with the melting of the glacial ice (Brezinski et al., 2008; 2010) (Fig. 2C).

Figure 6.

Idealized Piedmont glacial facies illustrating interpreted locations of field-trip stops 2-5.

Figure 6.

Idealized Piedmont glacial facies illustrating interpreted locations of field-trip stops 2-5.

Figure 7.

Measured section through the lower Rockwell Formation along Interstate 68 on the western flank of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 7.

Measured section through the lower Rockwell Formation along Interstate 68 on the western flank of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

(km)miDirections
(4.6)2.9Proceed westward on I-68 to Exit 72 for High Germany Road. Turn left on ramp to re-enter I-68 eastbound.
(3.2)2.0Take Exit 74, exit onto Scenic 40, Mountain Road eastbound.
(4.2)2.6Pull off MD 144 at State Highway barn. Stop 3.
(km)miDirections
(4.6)2.9Proceed westward on I-68 to Exit 72 for High Germany Road. Turn left on ramp to re-enter I-68 eastbound.
(3.2)2.0Take Exit 74, exit onto Scenic 40, Mountain Road eastbound.
(4.2)2.6Pull off MD 144 at State Highway barn. Stop 3.

Stop 3. Maryland Route 144, Sideling Hill, Maryland: Lower Rockwell Glacigenic Succession, Diamictite and Sandstone Facies (Coordinates: 39°48’12”N, 78°1734”W)

Along Maryland Route 144 (“old U.S. 40”), a similar, although not identical, section through the lower Rockwell glacigenics is exposed (Figs. 9, 10A). Just as along I-68, the diamictite at this stop is directly underlain and overlain by pebbly sandstone. The lower sandstone consists of more than 20 m of trough cross-bedded, medium- to coarse-grained sandstone overlain by 7 m of sandstone with contorted and deformed bedding, which also contains bodies of reddish-gray diamictite and shale (Fig. 10B). Above this deformed sandstone unit is 2 m of polymictic diamictite that is clast-supported. This is overlain by 8 m of massive, trough cross-bedded sandstone. The stratification at the top of this sandstone interval appears to be contorted and deformed and grades into ~2 m of tan, massive diamictite. Within this diamictite is a lenticular coarse-grained sandstone stratum (Fig. 10C). This sandstone lens is ~1 m thick along the top of the outcrop, but thickens to a maximum of nearly 3 m. Above the lenticular sandstone bed the diamictite is massive, and contains large bodies of red mudstone identical to the underlying Hampshire Formation (Fig. 10D). The top of the diamictite grades into nearly 2 m of massive mudstone. This mudstone looks identical to the diamictite except that it lacks the abundant clasts. The massive mudstone is sharply overlain by an upward-fining, crossbedded, pebbly sandstone that is 15 m thick. Unlike the succession along I-68, at Stop 3 the shaly, marine strata overlying the glacigenic package are concealed.

Lithofacies Codes and Proposed Depositional Environments for the Lower Rockwell Diamicite Succession as used During this Field Trip

Table 1.
Lithofacies Codes and Proposed Depositional Environments for the Lower Rockwell Diamicite Succession as used During this Field Trip
Lithofacies codeLithologyInterpreted environment of deposition
GtGravel, trough cross-beddedBraidplain
DmmDiamictite, matrix supported, massiveMeltout tillite, rainout deposit
Dmm (s)Diamictite, matrix-supported, massiveStratified-debris/gravity flow
DmsDiamictite, matrix supported, stratifiedDebris/gravity flow
DmdDiamictite, matrix supported, deformedLodgement/deformation tillite
Dms (r)Diamictite, matrix supported, resedimentedDebris/gravity flow
DcsDiamictite, clast supported, stratifiedDebris/gravity flow
Dcs (r)Diamictite, clast supported, stratified, resedimentedDebris/gravity flow
Ds (r)Diamictite, stratified, resedimentedDebris/gravity flow
FmFine-grained, massiveGlaciolacustrine, proximal
Fm (d)Fine-grained, massive, dropstonesGlaciolacustrine
Fm (l)Fine-grained, massive, load structureGlaciolacustrine
FlFine-grained, laminatedGlaciolacustrine, deep, distal
Fl (d)Fine-grained, laminated, dropstoneGlaciolacustrine, ice rafted
SmSandstone, massiveOutwash braidplain
SmtSandstone, massive, trough cross-beddedBraidplain channel dune
Sm (d)Sandstone, massive, diamictite inclusionsProximal outwash braidplain
StSandstone, trough cross-beddedBraidplain
SdSandstone, deformedPreglacial outwash plain
SpSandstone, planar cross-beddedBraidplain channel dune
ShSandstone, horizontal beddingGlaciolacustrine, deltaic
Lithofacies codeLithologyInterpreted environment of deposition
GtGravel, trough cross-beddedBraidplain
DmmDiamictite, matrix supported, massiveMeltout tillite, rainout deposit
Dmm (s)Diamictite, matrix-supported, massiveStratified-debris/gravity flow
DmsDiamictite, matrix supported, stratifiedDebris/gravity flow
DmdDiamictite, matrix supported, deformedLodgement/deformation tillite
Dms (r)Diamictite, matrix supported, resedimentedDebris/gravity flow
DcsDiamictite, clast supported, stratifiedDebris/gravity flow
Dcs (r)Diamictite, clast supported, stratified, resedimentedDebris/gravity flow
Ds (r)Diamictite, stratified, resedimentedDebris/gravity flow
FmFine-grained, massiveGlaciolacustrine, proximal
Fm (d)Fine-grained, massive, dropstonesGlaciolacustrine
Fm (l)Fine-grained, massive, load structureGlaciolacustrine
FlFine-grained, laminatedGlaciolacustrine, deep, distal
Fl (d)Fine-grained, laminated, dropstoneGlaciolacustrine, ice rafted
SmSandstone, massiveOutwash braidplain
SmtSandstone, massive, trough cross-beddedBraidplain channel dune
Sm (d)Sandstone, massive, diamictite inclusionsProximal outwash braidplain
StSandstone, trough cross-beddedBraidplain
SdSandstone, deformedPreglacial outwash plain
SpSandstone, planar cross-beddedBraidplain channel dune
ShSandstone, horizontal beddingGlaciolacustrine, deltaic

Notes: Lithofacies codes based upon Eyles et al. (1983).

Figure 8.

Stop 2. Sideling Hill glacigenic facies along Interstate 68. (A) Upper part of the sandstone underlying the diamictite illustrating deformed bedding and diamictite/mudstone inclusion. Basal diamictite in the upper right of image. (B, C) Massive diamictite containing exotic pebbles and cobbles.

Figure 8.

Stop 2. Sideling Hill glacigenic facies along Interstate 68. (A) Upper part of the sandstone underlying the diamictite illustrating deformed bedding and diamictite/mudstone inclusion. Basal diamictite in the upper right of image. (B, C) Massive diamictite containing exotic pebbles and cobbles.

The section at Stop 3 is interpreted to have had a similar origin to that exposed along I-68. The basal sandstone here also shows some evidence of deformation and contains diamictite bodies. These diamictite bodies have been interpreted, as elsewhere in the Rockwell Formation, as till balls (Brezinski et al., 2010). Till balls typically form from catastrophic outwash floods that rip up previously deposited till and reincorporate it in the outwash plain. The bedded, grain-rich diamictite at the base of the diamictite facies can be interpreted as debris flows shed off of the front and sides of the advancing glacier (Brezinski et al., 2010). The depositional origin of the overlying lenticular sandstone bed is in question, but it may represent a subice channel similar to meltwater Nye channels. Overlying this channel-form sandstone, the massive diamictite can be interpreted as a meltout till similar to that seen along Interstate-68. Similarly, the overlying fining-upward sandstone is interpreted as an outwash braidplain.

Figure 9.

Measured section for Stop 3. Section in the lower Rockwell Formation glacigenic facies along Maryland Route 144 on the east side of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 9.

Measured section for Stop 3. Section in the lower Rockwell Formation glacigenic facies along Maryland Route 144 on the east side of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 10.

Stop 3. Lower Rockwell glacigenic succession exposed along Maryland Route 144, Sideling Hill, Maryland. (A) Overview of glacigenic section. (B) Diamictite clast within the sandstone underlying main diamictite interval. (C) Lenticular sandstone within the lower diamictite facies. (D) Meter-scale red shale clast within the upper part of the diamictite facies.

Figure 10.

Stop 3. Lower Rockwell glacigenic succession exposed along Maryland Route 144, Sideling Hill, Maryland. (A) Overview of glacigenic section. (B) Diamictite clast within the sandstone underlying main diamictite interval. (C) Lenticular sandstone within the lower diamictite facies. (D) Meter-scale red shale clast within the upper part of the diamictite facies.

(km)miDirections
(5.6)3.5Continue east on Maryland Route 144 to
Sandy Mile Road into Pennsylvania.
(9.1)5.7Intersection with Pennsylvania Route 484.
Turn left and proceed over Sideling Hill.
(2.7)1.7Pull off on right. Stop 4.
(km)miDirections
(5.6)3.5Continue east on Maryland Route 144 to
Sandy Mile Road into Pennsylvania.
(9.1)5.7Intersection with Pennsylvania Route 484.
Turn left and proceed over Sideling Hill.
(2.7)1.7Pull off on right. Stop 4.

Stop 4. Pennsylvania Route 484, Sideling Hill, Pennsylvania: Lower Rockwell Glacigenic Succession, Diamictite and Laminated Mudstone Facies (Coordinates: 39°45’14”N, 78°1633”W)

At this stop along Pennsylvania Route 484, the glacigenic succession exhibits a different lithologic arrangement than that seen at Stops 2 and 3 (Fig. 11). This section begins with a poorly exposed sandstone that sits beneath the massive diamictite. Approximately 3 m of the coarse-grained, cross-bedded sandstone is exposed. The base of the diamictite is also poorly exposed, but consists of massive, reddish-brown diamictite that is 9 m thick. The massive diamictite grades upsection into 6 m of massive, red-brown mudstone containing rare rounded pebbles and cobbles. Above the massive mudstone is 11 m of red to tan, laminated mudstone containing scattered outsized clasts of rounded sandstone and white quartz (Figs. 12A, 12B). Overlying the laminated mudstone is an upward-fining, cross-bedded, pebbly sandstone that is 9 m thick.

Figure 11.

Stop 4. Measured section through the lower Rockwell Formation exposed along Pennsylvania Route 484 on the west side of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 11.

Stop 4. Measured section through the lower Rockwell Formation exposed along Pennsylvania Route 484 on the west side of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

The section at Stop 4 is interpreted as having a glacigenic origin partially similar to that discussed at earlier stops. As at the other exposures examined on this trip, the stratigraphic succession of Stop 4 is interpreted as representing a single glacial advance and retreat (Brezinski et al., 2010). The basal sandstone and massive diamictite at Stop 4 are interpreted as being formed during glacial advance in a braidplain and by meltout processes, respectively. What is different at this stop is the presence of a thick interval of massive and laminated mudstone facies containing outsized clasts. The massive mudstone is interpreted as a subaqueously deposited till. Workers elsewhere have interpreted such clast-poor tillites as glaciolacustrine deposits (Deynoux, 1985). The laminated mudstone above the massive mudstone consists of fine alternations of light and dark laminae that may represent glacial varves (Brezinski et al., 2010). Numerous outsized rounded clasts and graded conglomeratic sandstone layers are interpreted as dropstones and till dumps (Brezinski et al., 2008). As with other stops on this trip, the overlying, massive, fining-upward sandstone is interpreted as an outwash braidplain deposit.

Figure 12.

Stop 4. Lower Rockwell glacigenic succession, mudstone facies. (A) Laminite mudstone. (B) Laminated mudstone facies with oversized pebble.

Figure 12.

Stop 4. Lower Rockwell glacigenic succession, mudstone facies. (A) Laminite mudstone. (B) Laminated mudstone facies with oversized pebble.

(km)miDirections
(0.6)0.4Continue west on PA 484, then left onto Harmonia Road, PA Route 3002.
(0.72)0.4Left on County Road 308 south.
(4.1)2.5Exit Mountain Road onto I-68 west.
(3.9)2.4Exit I 72.
(4.4)2.7Take Exit I 68, Orleans Road. Right on ramp
to North Orleans Road.
(0.8)0.5Left on Maryland 144 (old Route 40).
(2.1)1.3Pull off on left. Stop 5.
(km)miDirections
(0.6)0.4Continue west on PA 484, then left onto Harmonia Road, PA Route 3002.
(0.72)0.4Left on County Road 308 south.
(4.1)2.5Exit Mountain Road onto I-68 west.
(3.9)2.4Exit I 72.
(4.4)2.7Take Exit I 68, Orleans Road. Right on ramp
to North Orleans Road.
(0.8)0.5Left on Maryland 144 (old Route 40).
(2.1)1.3Pull off on left. Stop 5.

Stop 5. Maryland Route 144, Town Hill: Lower Rockwell Glacigenic Succession, Lower and Upper Sandstone Facies (Coordinates: 39°42’11”N, 78°23’41”W)

At Stop 5, the massive diamictite facies is poorly exposed. However, the proglacial sandstone units below and above the diamictite are well exposed (Fig. 13) and are the emphasis of this stop. The basal part of the glacigenic sequence here consists of greenish-gray, cross-laminated, fine-grained sandstone (Fig. 14A). This sandstone coarsens upsection into a massive, trough cross-bedded, coarse-grained to pebbly sandstone (Fig. 14B). Above a thin concealed interval this sandstone contains layers of rounded gravels and disrupted stratification. These polymictic conglomerate beds are exceptionally well developed near the top of this unit. These conglomerate strata are followed upsection by a 5-m-thick interval that is largely concealed at road level. This concealed interval represents the massive diamictite facies (Fig. 13). Above the concealed diamictite interval, the succession consists of interbedded massive reddish mudstone and contorted beds of argillaceous, fine-grained sandstone. This mudstone interval is succeeded upsection by ~25 m of massive to thick-bedded, trough cross-bedded sandstone (Figs. 14C, 14D). At the top of the exposure, the Mississippian Riddlesburg Shale is exposed in the roadside borrow pit.

Figure 13.

Stop 5. Measured section through the lower Rockwell Formation exposed along Maryland Route 144 on the east flank of Town Hill. See Figure 3 for lithologic symbols and Table 1 for lithologie codes and interpreted environments of deposition.

Figure 13.

Stop 5. Measured section through the lower Rockwell Formation exposed along Maryland Route 144 on the east flank of Town Hill. See Figure 3 for lithologic symbols and Table 1 for lithologie codes and interpreted environments of deposition.

The sandstone interval that underlies the thin diamictite lithofacies exhibits a distinct coarsening upward character and even consists of polymictic gravels at its top. This upward coarsening sequence has been postulated to reflect the proximity of the sandstone to the ice front (Brezinski et al., 2010). That is, the well-sorted, fine-grained sandstone at the base of the succession was deposited at a greater distance from the ice contact environments than the gravels lying directly beneath the diamictite; these gravels likely were deposited immediately in front of the advancing/retreating glacier. The thick sandstone overlying the mudstone facies at this exposure represents just the opposite situation—the lower part of the sandstone was deposited nearer the glacier than the upper part.

Following Stop 5, the trip will return to Exit 68, Orleans Road and Interstate 68. From there we shall continue eastward to Interstate 70 and then return to the Baltimore Convention Center.

Additional Contributions

Character and Probable Origin of Volcanic Clasts from the Rockwell Diamictite

Viktoras W. Skema, Pennsylvania Topographic and Geologic Survey (retired), 3240 Schoolhouse Road, Middletown, Pennsylvania 17057, USA

Robert C. Smith II, Pennsylvania Topographic and Geologic Survey (retired), 3240 Schoolhouse Road, Middletown, Pennsylvania 17057, USA

Exotic igneous clasts have been recovered from both the Rockwell diamictite and coeval Late Devonian strata that lie to the west of the main diamictite outcrop belt. The most unusual of these outlier occurrences is a three ton granitic boulder known from the upper Cleveland Shale of east-central Kentucky (Lierman et al., 2009). Igneous pebbles also are known from this stratigraphic interval as channel lag conglomerates within the Murrysville Sandstone in the Conemaugh and Youghiogheny River gorges through Chestnut and Laurel Hill ridges of western Pennsylvania (Stevenson, 1878; Brezinski et al., 2010).

Igneous clasts are most prominent in exposures of the Rockwell Formation diamictite in south-central Pennsylvania, western Maryland, and adjacent northeastern West Virginia. Typically, sedimentary clasts outnumber the igneous ones by a large percentage, but the rare exotic igneous clasts are important because they can give insight as to the source of the glacigenic succession. Efforts to determine the provenance of these igneous pebbles led to sampling of the Rockwell diamictite from exposures along Interstate 70 at Crystal Spring, Pennsylvania; Interstate 68 at Sideling Hill (Stops 2 and 3) and La Vale, Maryland; along the Potomac River at Pearre, Maryland (Stop 1); and at Bismarck and Paw Paw, West Virginia. Most igneous clasts recovered are distinctly porphyritic, and all appear to be of volcanic or hypabyssal (subvolcanic) origin. Forty-four of these samples have been chemically analyzed for this study. Using the alkali versus silica diagram of LeBas et al. (1986), these samples yielded a range of compositions from picro-basalt through basalt, basaltic andesite, andesite, and dacite to rhyolite (Fig. 15A). A few pyroclastic clasts of andesitic lithic welded tuffs also were found.

Figure 14.

Stop 5. Lower Rockwell glacigenic succession, trough cross-bedded sandstone facies. (A) Trough cross-bedded, coarse-grained sandstone interval subjacent to main diamictite interval. (B) Graded gravel and sandstone strata beneath the diamictite. (C) Massive crossbedded sandstone overlying diamictite interval. (D) Close-up of trough cross-bedded massive unit illustrated in C.

Figure 14.

Stop 5. Lower Rockwell glacigenic succession, trough cross-bedded sandstone facies. (A) Trough cross-bedded, coarse-grained sandstone interval subjacent to main diamictite interval. (B) Graded gravel and sandstone strata beneath the diamictite. (C) Massive crossbedded sandstone overlying diamictite interval. (D) Close-up of trough cross-bedded massive unit illustrated in C.

These pebbles are relatively unmetamorphosed and lack any macro-scale structural fabric.

Trace element analyses on the collected samples define rare earth element groups that suggest related multiple and/or evolving volcanic source complexes for the pebbles. Basalts through rhyolites were found at most of the diamictite localities. On a Th-Hf/3-Ta diagram, most of the samples plot in the calc-alkaline portion of the destructive plate margin field (Fig. 15B). In looking at Ti-Zr content, samples having <62% SiO2 plot as calc-alkaline. Cr-Y and other diagrams suggest predominantly volcanic arc basalts.

U-Pb analyses by the ID-TIMS (isotope dilution-thermal ionization mass spectrometry) method were performed by Jahan Ramezani at the Massachusetts Institute of Technology on single zircons from two dacitic igneous cobbles in diamictite collected at the Crystal Spring location (Fig. 15C). Results based on 3-4 zircon analyses per sample yielded coherent clusters of data with weighted mean 206Pb/238U dates of 452.70 ± 0.55 Ma and 453.9 ± 2.1 Ma that are indistinguishable within uncertainties (Smith and Skema, 2012). The new, high-precision geochronology along with chemistry of the diamictite pebbles suggest that the most likely source is in the belt of Appalachian Late Ordovician volcanic arc units found in the Piedmont of Maryland and Virginia. Specifically, these include the Rowlandsville Granite of Harford County, Maryland, and the Carysbrook Granodiorite and portions of the Ellisville Granodiorite of central Virginia (Smith and Skema, 2014). The Rowlandsville pluton and the central Virginia plutons are close to, and on opposite sides of, major shear zones along which there has been interpreted dextral movement. These shear zones are the Rock Run shear zone (Fleming and Drake, 1998; Orndorff, 1999) and the sheared Chopawamsic fault zone of Virginia (Hughes et al., 2013). Present-day, straight-line distance between these chemically similar plutons is ~330 km.

The Rockwell diamictite at Sideling Hill has been known to contain igneous clasts since before the road cut was opened. Suter (1991) identified clasts of welded tuff, dacite breccia, and mafic igneous rocks. She estimated that these volcanics comprised less than 1% of the total “grains” in the diamictite deposit. She speculated that the source area for the exotic volcanic clasts was the Catoctin Formation and the Chopawamsic volcanic-arc terrane of northern Virginia and Maryland (specifically the James Run Formation of Maryland and the Occoquan Granite of northern Virginia). Suter (1991) did not test her hypothesis with any comparative chemical analyses. However, the analyses of volcanic and hypabyssal pebbles collected at Sideling Hill, and other nearby sites in Maryland, Pennsylvania, and West Virginia for this study, showed that they lacked any reasonable similarity with Catoctin metarhyolite or metabasalts (Smith and Barnes, 2010). The Catoctin volcanics were the product of rifting, whereas the chemistry of the Sideling Hill pebbles indicates a convergent plate origin. This is more consistent with the Chopawamsic volcanic-arc terrane as a possible source area.

Figure 15.

Possible provenance of the Rockwell diamictite igneous clasts. (A) Composition of all igneous pebble samples plotted against alkali versus silica diagram of LeBas et al. (1986). Two siliceous rhyolites are slightly off the diagram to the right, and are not shown. (B) Most analyzed clasts plotted against destructive plate margin field of a Th-Hf/3-Ta diagram. Stars are clasts from Crystal Spring. (C) Th-Hf/3-Ta diagram comparing dated Crystal Spring cobbles (stars) against the Rowlandsville Granite (dots) and Carysbrook Granodiorite (open circles).

Figure 15.

Possible provenance of the Rockwell diamictite igneous clasts. (A) Composition of all igneous pebble samples plotted against alkali versus silica diagram of LeBas et al. (1986). Two siliceous rhyolites are slightly off the diagram to the right, and are not shown. (B) Most analyzed clasts plotted against destructive plate margin field of a Th-Hf/3-Ta diagram. Stars are clasts from Crystal Spring. (C) Th-Hf/3-Ta diagram comparing dated Crystal Spring cobbles (stars) against the Rowlandsville Granite (dots) and Carysbrook Granodiorite (open circles).

Collections from Sideling Hill (Stops 2 and 3) produced 11 igneous pebbles and cobbles. These include one basaltic andesite, three andesites, five dacites, and two siliceous rhyolites. Many were porphyritic, and a few were welded tuff breccias.

Igneous clasts from the Pearre section (Stop 1) tended to have thick weathering rinds. However, four suitable igneous clasts were found: picrobasalt, andesite, dacite, and rhyolite. The latter was a welded tuff breccia confirming that some of the clasts are extrusive volcanics. Volcaniclastic cobbles also were found.

Paleobotany at the Devonian-Mississippian Transition: A Brief Review

William A. DiMichele, National Museum of Natural History, Washington, D.C., 20560, USA

William E. Stein, Department of Biological Sciences, Binghamton University, Binghamton, New York 13902, USA

C. Blaine Cecil, U.S. Geological Survey, Reston, Virginia 21092, USA

The time interval from Middle Devonian (Givetian) through the early Mississippian (Tournaisian) contains some of the most important evolutionary and ecological changes in the history of land plants (Scheckler, 1986a). With the exception of angiosperms (flowering plants), all of the major evolutionary lineages of plants appeared during that time, as well as some that are no longer extant. Representatives today include familiar groups—ferns, sphenopsids (horsetails), gymnosperms (seed plants), and lycopsids (club mosses). Extinct groups include the cladoxylopsids, iridopterids, stenokolealeans, rhacophytaleans (traditionally considered “ferns”), zosterophylls, leclerqioid and cormose lycopsids, barinophytes, aneurophytalean and archaeopteridalean “progymnosperms” (the lignophyte clade from which seed plants evolved), as well as the first pteridosperms (the earliest seed plants). Interestingly, several of the major lineages also partitioned the ecological space along taxonomic lines (DiMichele and Bateman, 1996; DiMichele et al., 2001). As a result, features of topography, local environment, and microclimate were associated with floral composition in a way quite unlike the modern world (today, with some notable exceptions, flowering seed plants or coniferous seed plants dominate nearly all terrestrial ecosystems). Nonetheless, many of the aspects of modern ecosystem structure and function, including growth habits and life history strategies, were progressively assembled in ways that continue to characterize terrestrial ecosystems today.

Numerous major ecological innovations appeared among plants as they radiated morphologically (Chaloner and Sheerin, 1979; Bateman et al., 1998). These include the independent evolution of tree habit in several different lineages (Meyer-Berthaud et al., 2010; Retallack and Huang, 2011). Trees, assembled into forests, altered the fundamental structure of ecosystems in several ways (Stein et al., 2012). Vertical tiering provided a physical framework for the evolution of vines and epiphytes. Segregation of the plant growth cycle between permanent architectural versus semiyearly deciduous organs fundamentally modified ground litter microenvironments, as well as altered the patterns and intensities of light reaching plant photosynthetic surfaces. The evolution of the seed (Rothwell et al., 1989) was equally profound, allowing seed plants to escape the constraints of waterdependent fertilization, thus opening the vast world of interior continental surfaces (Fig. 16A). Also of exceptional importance was the evolution of root systems, which took place independently in a number of lineages, forever altering both physical and biotic components of Earth’s highly essential, but often overlooked, rhizosphere. Additional major innovations include the development of a diversity of plant organ and tissue types that permitted increased water transport and photosynthate movement within larger plants (e.g., Decombeix and Meyer-Berthaud, 2013), which in turn fed back to increased plant size. Perhaps the most important of these were secondary xylem (“wood”), found in several groups of plants (cladoxylopsids, progymnosperms, gymnosperms, lycopsids, sphenopsids), and specialized “bark” tissues, typical of tree lycopsids, which would later become a major component of peat/coal deposits that are of Pennsylvanian age. Arthropod herbivory also appeared in the Devonian and may have been a major selective factor in the development of plant physical and chemical defenses (Labandeira, 2007).

There were, of course, numerous significant physical changes in Earth systems that resulted from these evolutionary innovations. In many cases, it appears that important ecosystem changes emerged from strong interactions between morphological features in plants and their physical environment involving positive ecological feedback loops. For example, it has been suggested that increased rooting depth contributed to the drawdown of high atmospheric CO2 levels through its effects on soil formation and rock weathering (Retallack, 1997; Elick et al., 1998; Beerling and Berner, 2005; Algeo and Scheckler, 2010), with consequent effects on nutrient runoff to coastal regions, and further effects on marine ecosystems. Also, whereas the evolution of the seed allowed colonization of areas characterized by low or fluctuating soil moisture, life in those places would not have been possible without the independent evolution of deeply penetrating root systems (Fig. 16B). This, in turn, promoted further occupation of land surfaces by other plants, profoundly affecting changes in plant physiology, productivity, and architecture, and allowing significant expansion of organic carbon fixation. This ultimately led to changes in the fluvial environment (Davies and Gibling, 2010), the beginnings of significant peat/coal accumulations (Scheckler, 1986a, 1986b; Dai et al., 2006), and the onset of fire as a major ecosystem process (Cressler, 2001); the latter may be a plant-related consequence of rising levels of atmospheric oxygen (Glasspool and Scott, 2011).

Reconstructions of Middle and Late Devonian landscapes have been slow in emerging, compared to studies of some other time intervals of the Phanerozoic. This reflects, in part, the extremely simple morphology of primitive plants, which lack easily recognizable plant organs that would permit identification to specific major groups. Another difficulty, perhaps, is lack of current surface exposures as are found, for example, in desert or semi-arid regions of the western United States. Some of the most prominent examples of reconstructed later Devonian terrestrial ecosystems include the Middle Devonian Gilboa forest of New York state (Fig. 16C) (Stein et al., 2012), the Late Devonian (Famennian) Red Hill site in Pennsylvania (Cressler, 2006), and broader analyses of Late Devonian central Appalachian landscapes from fossil assemblages and facies analysis (Retallack and Huang, 2011; Mintz et al., 2010; Scheckler, 1986a).

The diversification of plant form, reflected in proxy by phylogenetic and/or taxonomic diversification, appears to have been accompanied by increasing ecological specialization at ever more refined levels (e.g., Cressler et al., 2010). Evidence from the Middle Devonian suggests considerable habitat overlap between cladoxylopsids, aneurophytalean progymnosperms, and perhaps cormose arborescent lycopsids (e.g., Berry and Fairon-Demaret, 2001) within, in one instance, a periodically disturbed environment (Stein et al., 2012). Archaeopteridalean progymnosperms make their first appearance in the fossil record at about this time, but they may have been restricted to less disturbed lowland habitats (Stein, 2015, personal observations). It has been suggested that climate fluctuations may have affected the relative abundances of progymnosperm woodlands, composed of progymnosperms and coastal swamps, dominated by cladoxylopsids (Retallack and Huang, 2011). However, recent evidence shows that cladoxylopsids were not ecologically restricted to specific paleosols or to wet environments, and were likely much more widespread on the ancient landscape than traditionally thought (Stein, 2015, personal observation; Morris et al., 2015). By the latest Devonian (Famennian), partitioning of the landscape may have been more fully developed (Scheckler, 1986a; Algeo and Scheckler, 1998; Cressler et al., 2010), evidenced by the presence of specialists in swamp habitats (Rhacophyton and small lycopsid trees) and on well-drained, but moist alluvial plains (most Archaeopteris species and early seed plants) (Fig. 16D).

Figure 16.

Late Devonian plants and environments. (A) Reconstruction of the Late Devonian seed plant, Elkinsia polymorpha, from the upper Hampshire Formation, Elkins, West Virginia. From Serbet and Rothwell (1992); used with permission of the International Journal of Plant Sciences, University of Chicago Press. (B) Rooting structures of Devonian plants. Early Devonian (left) to Late Devonian (right). rhy (rhyniohytes), tri (trimerophytes), lyc-he (herbaceous lycopsids), lyc-tr (tree lycopsids), prog-an (aneurophyte progymnosperms), progarc (archaeopterialean progymnosperms), gym (seed plants), zyg (zygopterid ferns). From Algeo and Scheckler (1998); used with permission of the Royal Society of London. (C) Gilboa Middle Devonian forest. (a) Stump of a “Gilboa tree,” Eospermatopteris. (b) Stump being removed in earlier excavations at the site. (c) The “footprint” of an Eospermatopteris tree, in situ, showing the rooted area around the tree base. (d) Map of a quarry floor showing tree distributions. Dark shading indicates Eospermatopteris trunk bases identified with high confidence; light shading, intermediate confidence; no shading, low confidence. Aneurophytalean remains, linear stems and fragments, shown as black lines. From Stein et al. (2012); reprinted with permission of Nature Publishing Group. (e) Fossil trunk and reconstruction of an Eospermatopteris tree. From Stein et al. (2007); reprinted with permission of Nature Publishing Group. (D) Reconstructions of Late Devonian plant environments. Deltaic marsh (top). Backswamp on an upland floodplain (bottom). R (Rhacophyton), A (Archaeopteris), S (seed plant), L (tree lycopsid). From Scheckler (1986a); used with permission of Stephen E. Scheckler. (E) Vegetational changes during the Devonian-Mississippian transition. Evolution proceeded in environments with low preservation potential (“uplands”) and, during environmental changes across the Devonian-Mississippian boundary, plants from these environments moved into lowland refugia where preservation was more likely. From Decombeix et al. (2011a); used with permission of the Geological Society of London.

Figure 16.

Late Devonian plants and environments. (A) Reconstruction of the Late Devonian seed plant, Elkinsia polymorpha, from the upper Hampshire Formation, Elkins, West Virginia. From Serbet and Rothwell (1992); used with permission of the International Journal of Plant Sciences, University of Chicago Press. (B) Rooting structures of Devonian plants. Early Devonian (left) to Late Devonian (right). rhy (rhyniohytes), tri (trimerophytes), lyc-he (herbaceous lycopsids), lyc-tr (tree lycopsids), prog-an (aneurophyte progymnosperms), progarc (archaeopterialean progymnosperms), gym (seed plants), zyg (zygopterid ferns). From Algeo and Scheckler (1998); used with permission of the Royal Society of London. (C) Gilboa Middle Devonian forest. (a) Stump of a “Gilboa tree,” Eospermatopteris. (b) Stump being removed in earlier excavations at the site. (c) The “footprint” of an Eospermatopteris tree, in situ, showing the rooted area around the tree base. (d) Map of a quarry floor showing tree distributions. Dark shading indicates Eospermatopteris trunk bases identified with high confidence; light shading, intermediate confidence; no shading, low confidence. Aneurophytalean remains, linear stems and fragments, shown as black lines. From Stein et al. (2012); reprinted with permission of Nature Publishing Group. (e) Fossil trunk and reconstruction of an Eospermatopteris tree. From Stein et al. (2007); reprinted with permission of Nature Publishing Group. (D) Reconstructions of Late Devonian plant environments. Deltaic marsh (top). Backswamp on an upland floodplain (bottom). R (Rhacophyton), A (Archaeopteris), S (seed plant), L (tree lycopsid). From Scheckler (1986a); used with permission of Stephen E. Scheckler. (E) Vegetational changes during the Devonian-Mississippian transition. Evolution proceeded in environments with low preservation potential (“uplands”) and, during environmental changes across the Devonian-Mississippian boundary, plants from these environments moved into lowland refugia where preservation was more likely. From Decombeix et al. (2011a); used with permission of the Geological Society of London.

There is uncertainty over the degree to which the Late Devonian can be subdivided into large floral provinces or realms. During much of the Devonian, including the later portions, plant distributions are often characterized as biogeographically homogenous (e.g., Xu and Wang, 2008). However, palynological studies through this time interval reveal that although terrestrial phytoprovinces were large, there were, nonetheless, clear patterns of climatic differentiation among the plants at large and small spatial scales (Streel et al., 2000).

Devonian-Mississippian Boundary Events

By the end of the Devonian, large forested tracts appear to have covered parts of the land surface, at least in those areas that have been studied by paleobotanists. Large, woody Archaeopteridalean trees, consisting of many species of Archaeopteris, were the most widely dispersed and abundant of forest-forming taxa. In addition, other groups, including lycopsids (e.g., Wang et al., 2005), cladoxylopsids (Soria and Meyer-Berthaud, 2004; Stein et al., 2007), zygopterid-like ferns (Scheckler, 1986a), and even giant fungi (Hueber, 2001; Boyce et al., 2007) somehow contributed to forest structures. Plants capable of forming small, often ephemeral, woodlands and colonizing disturbed areas created the possibility for highly dynamic ecosystems, although with much lower species richness than is typical of younger landscapes, and especially of that in the world of flowering plants.

During the earliest Carboniferous (earliest Mississippian), however, this picture appears to have changed dramatically. Archaeopteris disappeared from the landscape (Scheckler, 1986a), potentially leaving the world devoid of large, woody trees (Fig. 16E). Perhaps then, by default, well-drained landscapes were dominated by small, shrubby pteridosperms (DiMichele and Hook, 1992). Recent finds and reevaluation of older reports of woody plants, however, may change this picture considerably. Decombeix et al. (2005) summarize the current understanding, noting that in the earliest Mississippian, just prior to the extinction of Archaeopteris, there is unequivocal evidence of other kinds of woody plants also occurring on the landscape. Although none of the known specimens would qualify as of tree size, all belong to, or are similar to, lineages that are composed of woody trees, particularly Protopitys and Eristophyton, which have long stratigraphic ranges into the late Mississippian.

Perhaps in some ways, this makes the extinction of Archaeopteris even more mysterious. Whatever the extinction agent, it may have been specific to homosporous-to-heterosporous reproduction versus seeds among woody lignophytes. However, one needs to realize current limits to our understanding due to differences in preservational state (compressions versus permineralizations) among sites straddling the Late Devonian-Early Mississippian boundary and the very real possibility that Archaeopteridalean or related progymnosperms (for instance, Protopitys and Noeggerathiaceae) may have persisted well into the Carboniferous. Flabeliform Archaeopteris leaves are not strikingly different from Mississippian “pinnules” presumed to belong to seed plants but, in fact, of uncertain affinity. Some other plants typical of the Late Devonian also persisted into the earliest Mississippian prior to disappearing (e.g., Barinophytales; Scheckler, 1984). Furthermore, there is a nearly complete turnover in the lycopsid trees of swampy, wetland settings. Consequently, the extinction event seems to have been widespread and to require an agent with the capacity to affect whole landscapes, across different kinds of plant communities, such as climatic change (Streel et al., 2000).

This biotic event certainly correlates with the Late Devonian ice age, but a mechanistic link to the conditions in lower latitudes created by ice at 30° S would be of considerable difficulty to assess, based on present evidence. If Archaeopteris were a strictly lowland, warm temperature plant (as estimated by Retallack and Huang, 2011), the expansion of freezing conditions into the low latitudes may have been strongly influential. Such change is a reasonable explanation. Climate is one of the few natural phenomena with such broad coverage geographically, and is insensitive to local habitat variation. The most likely underlying factor during this time is the onset of the latest Devonian episode of intense glaciation (Caputo, 1985; Cecil et al., 2004; Brezinski et al., 2008, 2010; Isaacson et al., 2008), which may have been a multiphasic event (Wicander et al., 2011).

Early Mississippian

Early Mississippian/early Dinantian (Kinderhookian/Osagean; Tournaisian) plant biogeography demonstrates greater global provincialization than found in the Late Devonian. Rowley et al. (1985; see also Raymond, 1985) recognized five distinct floristic regions by the Kinderhookian-Osagean boundary; the Acadian unit (Canadian Maritimes, Wales, Scotland, and Great Britain) is perhaps the most similar to floras found more recently in eastern North America. This basic architecture, that of multiple tropical and/or equatorial realms with similarities in taxonomic makeup versus floristically distinct, north and south high-latitude, temperate realms, persisted throughout the rest of the Paleozoic, despite high turnover in the species composition of each of the regions. The floras are best known from the middle to later part of the interval (late Tournaisian/late Kinderhookian to Visean/ Osagean). With global provincialization, many new genera and species appeared, in all kinds of habitats (Scott et al., 1984; Bateman and Rothwell, 1990).

Early Mississippian floras fall principally into two time intervals: those known from the Devonian-Mississippian boundary (Kinderhookian), and those known from the middle of the early Mississippian (Kinderhookian-Osagean boundary). Some of the most informative boundary floras are preserved as petrifactions from black shale that formed during, or shortly after, the ice retreated at the end of the Devonian/earliest Mississippian. Among the best known of these is the flora of the New Albany Shale, a unit that includes several distinct black shale beds of Devonian through early Mississippian age (Rimmer et al., 2004). Originally described by Read and Campbell (1939), the New Albany Shale flora has revealed a wealth of plant remains, many with affinities clearly with the Carboniferous rather than the Devonian. Such plants recovered are presumed pteridosperms belonging to the Calamopityaceae (e.g., Matten and Trimble, 1978; Stein and Beck, 1978; Beck and Stein, 1987; Beck et al., 1992); presumed ferns (Beck, 1960; Galtier and Scott, 1985), possibly related to the Zygopteridaceae abundant throughout the Pennsylvanian and Permian (Phillips and Galtier, 2005); and lycopsids of several kinds with affinities either to long-ranging groups or intermediate in form between Devonian and later Carboniferous forms (Roy and Matten, 1989; Chitaley and Pigg, 1996; Pigg, 2001). Yet, the earliest Mississippian assemblages retain elements from lineages perhaps more typical of the Devonian. Most notable among these are the late cladoxylopsids, but with many features not previously seen in “pseudosporochnalean” cladoxylopsids of Middle Devonian age (Soria and Meyer-Berthaud, 2003; Berry and Wang, 2006).

Among the new habitats to appear in the middle of the early Mississippian were the first significant coal swamps, dominated by the tree-sized lycopsids, which left a record of coal thick enough and laterally extensive enough to have been mined economically both on the surface and underground (Brezinski and Kertis, 1987; Hower et al., 2008). The dominant swamp lycopsid trees, generally classified as Lepidodendropsis, may include more generic diversity than is presently recognized (Gensel and Pigg, 2010); that is, the Lepidodendropsis assemblage reported worldwide in wetland habitats at this stratigraphic horizon (e.g., Pal and Chaloner, 1979; Iurina and Lemoigne, 1975; Falcon-Lang, 2004; Rygel et al., 2006).

There are significant climatic implications associated with the presence of these coals in the central Appalachian basin. They occur near the Kinderhookian-Osagean transition, mid-Tournasian, at ~30° S latitude, an extra-tropical position that might be expected to have a dry or strongly wet-dry seasonality, a climate not conducive to peat formation and preservation (Cecil, 1990; Lottes and Ziegler, 1994). However, there is now considerable evidence, both isotopic and physical, of global-scale glaciation at this time (e.g., Mii et al., 1999; Saltzman, 2002; Matchen and Kammer, 2006; Buggisch et al., 2008; Isaacson et al., 2008; Wallace and Elrick, 2014). Whether this glaciation represents the development of large ice sheets, or is limited to mountain glaciation is unclear. The presence of substantial ice in the southern hemisphere would be sufficient to have created extensive moisture off the ice front, which could account for these coal beds (Cecil et al., 2004). In addition, this glacial event has profound implications for the temperature tolerances of the plants associated with these peat-forming landscapes. Rather than being the harbingers of the Pennsylvanian tropical peat-forming wetlands, these deposits might have formed under quite cool to even cold climates, populated by plants that were not directly ancestral to the lycopsids and pteridosperms of the Pennsylvanian tropics.

Another important and relatively recent development regarding early Mississippian floras is the recognition that woody plants of tree habit appeared early and were important parts of terrestrial ecosystems across the Devonian-Mississippian transition (Decombeix et al., 2011a). Generally considered to be “seed ferns,” these plants have systematic affinities that are regarded as problematic (Scheckler and Galtier, 2003; Galtier and Meyer-Berthaud, 2006). Probably the best known genera are Pitus and Eristophyton, which are widespread and long ranging stratigraphically. However, there are significant new finds (e.g., Decombeix et al., 2006), including some from areas in which woody plant diversity was not previously recognized, such as Australia (Decombeix et al., 2011b).

The sudden and widespread appearance of woody plants of tree habit, either progymnospermous or gymnospermous in their broadest affinities, other than Archaeopteris, raises major questions about where and when these plants evolved. Decombeix et al. (2011a) suggest that such evolutionary innovation may have taken place in extrabasinal (sensu Pfefferkorn, 1980) settings, or “uplands,” areas in which reduced resource competition permitted the survival of derivative forms (Fig. 16E). These species then may have migrated into the basinal lowlands during the climatic changes that affected these areas during the early Mississippian. The terrestrial fossil record has significant preservational biases that are increasingly well understood, resulting in patterns such as early “precocious” or sudden appearances of taxa in the record (e.g., Looy et al., 2014); and these early Mississippian woody plant occurrences are fully in keeping with such a pattern.

As a final comment, there has been suggestion of a temporal “gap” in the terrestrial vertebrate and arthropod fossil records, encompassing much of the stratigraphic interval considered here, including the early Mississippian. Given the moniker “Romer’s Gap” (Coates and Clack, 1995), this apparent absence of terrestrial animal fossils has been thought to reflect a major environmental crisis, perhaps a period of low O2 (Ward et al., 2006). Recent finds of early Mississippian vertebrates within the gap interval (Clack, 2002; Mansky and Lucas, 2013), and of body (Lerner et al., 2013) and trace fossils (Mansky and Lucas, 2013) of terrestrial arthropods suggest that these animals were certainly in existence during this time and that their seeming absence may be a taphonomic happenstance combined with a need for more fieldwork (Mansky and Lucas, 2013). Plants are certainly present throughout the interval, though there are portions of this time period when preservation of plant remains clearly was more likely than at others (Decombeix et al., 2011a; Fig. 16E). Variable preservation of plants may reflect climatic fluctuations during this period of time, including periods of extreme aridity when the likelihood of organic preservation was severely reduced. Direct reading of the terrestrial fossil record is hazardous, as indicated by the frequent appearance of taxa millions of years earlier than well-established ranges (Looy et al., 2014) or their reappearance millions of years after apparent extinction (Mamay, 1992; Mamay and Bateman, 1991). Thus, “Romer’s Gap” may become less of a gap, or none at all, as field-based research in the latest Devonian and Early Mississippian continues.

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Acknowledgments

WD acknowledges the National Museum of Natural History Small Grants program for support of study of early Mississippian geology. We would especially like to thank S. Lucas, C.A. Kertis, and J.E. Repetski for their valuable reviews of this manuscript.

Figures & Tables

Figure 1.

Map of western Maryland and distribution of the Rockwell Formation (from Brezinski and Conkwright, 2013). Field-trip locations are identified by number. Inset map illustrates known locations of Rockwell and Spechty Kopf diamictite succession (from Brezinski et al., 2010).

Figure 1.

Map of western Maryland and distribution of the Rockwell Formation (from Brezinski and Conkwright, 2013). Field-trip locations are identified by number. Inset map illustrates known locations of Rockwell and Spechty Kopf diamictite succession (from Brezinski et al., 2010).

Figure 2.

Interpreted depositional history of the central Appalachians during the Late Devonian (Famennian Stage). (A) Maximum progradation of the Catskill-Hampshire clastic wedge during the Famennian forms a semiarid alluvial plain experiencing high levels of evaporation. (B) Latest Devonian piedmont glaciation is concomitant with global sea-level drop. Modified from Brezinski et al. (2010). (C) Glacial retreat at the end of the Devonian results in sea-level rise and flooding of the Appalachian basin to form the Sunbury and Riddlesburg Shales.

Figure 2.

Interpreted depositional history of the central Appalachians during the Late Devonian (Famennian Stage). (A) Maximum progradation of the Catskill-Hampshire clastic wedge during the Famennian forms a semiarid alluvial plain experiencing high levels of evaporation. (B) Latest Devonian piedmont glaciation is concomitant with global sea-level drop. Modified from Brezinski et al. (2010). (C) Glacial retreat at the end of the Devonian results in sea-level rise and flooding of the Appalachian basin to form the Sunbury and Riddlesburg Shales.

Figure 3.

Cross section of Late Devonian and lower Mississippian strata of western Maryland. Modified from Brezinski (1989a, 1989b) and Brezinski et al. (2010).

Figure 3.

Cross section of Late Devonian and lower Mississippian strata of western Maryland. Modified from Brezinski (1989a, 1989b) and Brezinski et al. (2010).

Figure 4.

Stop 1. Measured section of the Hampshire and lower Rockwell Formations along the Western Maryland Rail Trail at Sideling Hill Creek.

Figure 4.

Stop 1. Measured section of the Hampshire and lower Rockwell Formations along the Western Maryland Rail Trail at Sideling Hill Creek.

Figure 5.

Stop 1, Hampshire (Catskill) Rockwell Transition. (A) Thin reddish paleosol sandwiched between two thick, channel-phase sandstones within the middle Hampshire Formation. (B) Thick paleosol within the transitional facies of the upper Hampshire Formation. (C) Greenish-gray sandstone of the upper Hampshire Formation. (D) Archaeopteris? stump near top of Hampshire Formation at Stop 1.

Figure 5.

Stop 1, Hampshire (Catskill) Rockwell Transition. (A) Thin reddish paleosol sandwiched between two thick, channel-phase sandstones within the middle Hampshire Formation. (B) Thick paleosol within the transitional facies of the upper Hampshire Formation. (C) Greenish-gray sandstone of the upper Hampshire Formation. (D) Archaeopteris? stump near top of Hampshire Formation at Stop 1.

Figure 6.

Idealized Piedmont glacial facies illustrating interpreted locations of field-trip stops 2-5.

Figure 6.

Idealized Piedmont glacial facies illustrating interpreted locations of field-trip stops 2-5.

Figure 7.

Measured section through the lower Rockwell Formation along Interstate 68 on the western flank of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 7.

Measured section through the lower Rockwell Formation along Interstate 68 on the western flank of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 8.

Stop 2. Sideling Hill glacigenic facies along Interstate 68. (A) Upper part of the sandstone underlying the diamictite illustrating deformed bedding and diamictite/mudstone inclusion. Basal diamictite in the upper right of image. (B, C) Massive diamictite containing exotic pebbles and cobbles.

Figure 8.

Stop 2. Sideling Hill glacigenic facies along Interstate 68. (A) Upper part of the sandstone underlying the diamictite illustrating deformed bedding and diamictite/mudstone inclusion. Basal diamictite in the upper right of image. (B, C) Massive diamictite containing exotic pebbles and cobbles.

Figure 9.

Measured section for Stop 3. Section in the lower Rockwell Formation glacigenic facies along Maryland Route 144 on the east side of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 9.

Measured section for Stop 3. Section in the lower Rockwell Formation glacigenic facies along Maryland Route 144 on the east side of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 10.

Stop 3. Lower Rockwell glacigenic succession exposed along Maryland Route 144, Sideling Hill, Maryland. (A) Overview of glacigenic section. (B) Diamictite clast within the sandstone underlying main diamictite interval. (C) Lenticular sandstone within the lower diamictite facies. (D) Meter-scale red shale clast within the upper part of the diamictite facies.

Figure 10.

Stop 3. Lower Rockwell glacigenic succession exposed along Maryland Route 144, Sideling Hill, Maryland. (A) Overview of glacigenic section. (B) Diamictite clast within the sandstone underlying main diamictite interval. (C) Lenticular sandstone within the lower diamictite facies. (D) Meter-scale red shale clast within the upper part of the diamictite facies.

Figure 11.

Stop 4. Measured section through the lower Rockwell Formation exposed along Pennsylvania Route 484 on the west side of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 11.

Stop 4. Measured section through the lower Rockwell Formation exposed along Pennsylvania Route 484 on the west side of Sideling Hill. See Figure 3 for lithologic symbols and Table 1 for lithologic codes and interpreted environments of deposition.

Figure 12.

Stop 4. Lower Rockwell glacigenic succession, mudstone facies. (A) Laminite mudstone. (B) Laminated mudstone facies with oversized pebble.

Figure 12.

Stop 4. Lower Rockwell glacigenic succession, mudstone facies. (A) Laminite mudstone. (B) Laminated mudstone facies with oversized pebble.

Figure 13.

Stop 5. Measured section through the lower Rockwell Formation exposed along Maryland Route 144 on the east flank of Town Hill. See Figure 3 for lithologic symbols and Table 1 for lithologie codes and interpreted environments of deposition.

Figure 13.

Stop 5. Measured section through the lower Rockwell Formation exposed along Maryland Route 144 on the east flank of Town Hill. See Figure 3 for lithologic symbols and Table 1 for lithologie codes and interpreted environments of deposition.

Figure 14.

Stop 5. Lower Rockwell glacigenic succession, trough cross-bedded sandstone facies. (A) Trough cross-bedded, coarse-grained sandstone interval subjacent to main diamictite interval. (B) Graded gravel and sandstone strata beneath the diamictite. (C) Massive crossbedded sandstone overlying diamictite interval. (D) Close-up of trough cross-bedded massive unit illustrated in C.

Figure 14.

Stop 5. Lower Rockwell glacigenic succession, trough cross-bedded sandstone facies. (A) Trough cross-bedded, coarse-grained sandstone interval subjacent to main diamictite interval. (B) Graded gravel and sandstone strata beneath the diamictite. (C) Massive crossbedded sandstone overlying diamictite interval. (D) Close-up of trough cross-bedded massive unit illustrated in C.

Figure 15.

Possible provenance of the Rockwell diamictite igneous clasts. (A) Composition of all igneous pebble samples plotted against alkali versus silica diagram of LeBas et al. (1986). Two siliceous rhyolites are slightly off the diagram to the right, and are not shown. (B) Most analyzed clasts plotted against destructive plate margin field of a Th-Hf/3-Ta diagram. Stars are clasts from Crystal Spring. (C) Th-Hf/3-Ta diagram comparing dated Crystal Spring cobbles (stars) against the Rowlandsville Granite (dots) and Carysbrook Granodiorite (open circles).

Figure 15.

Possible provenance of the Rockwell diamictite igneous clasts. (A) Composition of all igneous pebble samples plotted against alkali versus silica diagram of LeBas et al. (1986). Two siliceous rhyolites are slightly off the diagram to the right, and are not shown. (B) Most analyzed clasts plotted against destructive plate margin field of a Th-Hf/3-Ta diagram. Stars are clasts from Crystal Spring. (C) Th-Hf/3-Ta diagram comparing dated Crystal Spring cobbles (stars) against the Rowlandsville Granite (dots) and Carysbrook Granodiorite (open circles).

Figure 16.

Late Devonian plants and environments. (A) Reconstruction of the Late Devonian seed plant, Elkinsia polymorpha, from the upper Hampshire Formation, Elkins, West Virginia. From Serbet and Rothwell (1992); used with permission of the International Journal of Plant Sciences, University of Chicago Press. (B) Rooting structures of Devonian plants. Early Devonian (left) to Late Devonian (right). rhy (rhyniohytes), tri (trimerophytes), lyc-he (herbaceous lycopsids), lyc-tr (tree lycopsids), prog-an (aneurophyte progymnosperms), progarc (archaeopterialean progymnosperms), gym (seed plants), zyg (zygopterid ferns). From Algeo and Scheckler (1998); used with permission of the Royal Society of London. (C) Gilboa Middle Devonian forest. (a) Stump of a “Gilboa tree,” Eospermatopteris. (b) Stump being removed in earlier excavations at the site. (c) The “footprint” of an Eospermatopteris tree, in situ, showing the rooted area around the tree base. (d) Map of a quarry floor showing tree distributions. Dark shading indicates Eospermatopteris trunk bases identified with high confidence; light shading, intermediate confidence; no shading, low confidence. Aneurophytalean remains, linear stems and fragments, shown as black lines. From Stein et al. (2012); reprinted with permission of Nature Publishing Group. (e) Fossil trunk and reconstruction of an Eospermatopteris tree. From Stein et al. (2007); reprinted with permission of Nature Publishing Group. (D) Reconstructions of Late Devonian plant environments. Deltaic marsh (top). Backswamp on an upland floodplain (bottom). R (Rhacophyton), A (Archaeopteris), S (seed plant), L (tree lycopsid). From Scheckler (1986a); used with permission of Stephen E. Scheckler. (E) Vegetational changes during the Devonian-Mississippian transition. Evolution proceeded in environments with low preservation potential (“uplands”) and, during environmental changes across the Devonian-Mississippian boundary, plants from these environments moved into lowland refugia where preservation was more likely. From Decombeix et al. (2011a); used with permission of the Geological Society of London.

Figure 16.

Late Devonian plants and environments. (A) Reconstruction of the Late Devonian seed plant, Elkinsia polymorpha, from the upper Hampshire Formation, Elkins, West Virginia. From Serbet and Rothwell (1992); used with permission of the International Journal of Plant Sciences, University of Chicago Press. (B) Rooting structures of Devonian plants. Early Devonian (left) to Late Devonian (right). rhy (rhyniohytes), tri (trimerophytes), lyc-he (herbaceous lycopsids), lyc-tr (tree lycopsids), prog-an (aneurophyte progymnosperms), progarc (archaeopterialean progymnosperms), gym (seed plants), zyg (zygopterid ferns). From Algeo and Scheckler (1998); used with permission of the Royal Society of London. (C) Gilboa Middle Devonian forest. (a) Stump of a “Gilboa tree,” Eospermatopteris. (b) Stump being removed in earlier excavations at the site. (c) The “footprint” of an Eospermatopteris tree, in situ, showing the rooted area around the tree base. (d) Map of a quarry floor showing tree distributions. Dark shading indicates Eospermatopteris trunk bases identified with high confidence; light shading, intermediate confidence; no shading, low confidence. Aneurophytalean remains, linear stems and fragments, shown as black lines. From Stein et al. (2012); reprinted with permission of Nature Publishing Group. (e) Fossil trunk and reconstruction of an Eospermatopteris tree. From Stein et al. (2007); reprinted with permission of Nature Publishing Group. (D) Reconstructions of Late Devonian plant environments. Deltaic marsh (top). Backswamp on an upland floodplain (bottom). R (Rhacophyton), A (Archaeopteris), S (seed plant), L (tree lycopsid). From Scheckler (1986a); used with permission of Stephen E. Scheckler. (E) Vegetational changes during the Devonian-Mississippian transition. Evolution proceeded in environments with low preservation potential (“uplands”) and, during environmental changes across the Devonian-Mississippian boundary, plants from these environments moved into lowland refugia where preservation was more likely. From Decombeix et al. (2011a); used with permission of the Geological Society of London.

(km)miDirections
(2.0)1.2Merge onto I-95 west.
(5.1)3.2Take Exit 49, I-95 South, on to Baltimore I-695 (Inner Loop).
(8.3)5.2Take Exit 16 off of Inner Loop I-695 to I-70 west.
(1.5)0.9Merge onto and begin I-70 west.
(2.9)1.8I-70 Exit 87, U.S. 29 south.
(8.2)5.1Merge from U.S. 40 west.
(43.0)26.7Exit 56, East Patrick Street, Frederick, Maryland.
(4.2)2.6Interchange with I-270.
(5.1)3.2Braddock Mountain, exposed are strata of 600-m.y.-old Catoctin Formation.
(16.9)10.5South Mountain, western ridge of Maryland’s Blue Ridge.
(10.5)6.5Exit 32, U.S. 40 west, Hagerstown, Maryland.
(9.5)5.9Exit 26, Interchange with I-81.
(35.7)22.2Exit 3, Hancock, Maryland.
(3.1)1.9Exit 1B, U.S. 522 south.
(0.9)0.6Exit from I-70 and merge onto I-68.
(5.5)3.4Take Exit 77, Woodmont Road. Turn left at end of ramp, then again left onto Maryland Route 144. Turn right on Woodmont Road.
(9.9)0.6Intersection with Pearre Road. C&O Canal is on the left.
(2.0)1.2Left off of Pearre Road to C&O Canal Lock 56 and WMRT (Western Maryland Rail Trail) parking.
(km)miDirections
(2.0)1.2Merge onto I-95 west.
(5.1)3.2Take Exit 49, I-95 South, on to Baltimore I-695 (Inner Loop).
(8.3)5.2Take Exit 16 off of Inner Loop I-695 to I-70 west.
(1.5)0.9Merge onto and begin I-70 west.
(2.9)1.8I-70 Exit 87, U.S. 29 south.
(8.2)5.1Merge from U.S. 40 west.
(43.0)26.7Exit 56, East Patrick Street, Frederick, Maryland.
(4.2)2.6Interchange with I-270.
(5.1)3.2Braddock Mountain, exposed are strata of 600-m.y.-old Catoctin Formation.
(16.9)10.5South Mountain, western ridge of Maryland’s Blue Ridge.
(10.5)6.5Exit 32, U.S. 40 west, Hagerstown, Maryland.
(9.5)5.9Exit 26, Interchange with I-81.
(35.7)22.2Exit 3, Hancock, Maryland.
(3.1)1.9Exit 1B, U.S. 522 south.
(0.9)0.6Exit from I-70 and merge onto I-68.
(5.5)3.4Take Exit 77, Woodmont Road. Turn left at end of ramp, then again left onto Maryland Route 144. Turn right on Woodmont Road.
(9.9)0.6Intersection with Pearre Road. C&O Canal is on the left.
(2.0)1.2Left off of Pearre Road to C&O Canal Lock 56 and WMRT (Western Maryland Rail Trail) parking.
(km)miDirections
(12.3)(3.8)Retrace route back to I-68 westbound.
7.62.4West on I-68 to Sideling Hill visitor center.
(km)miDirections
(12.3)(3.8)Retrace route back to I-68 westbound.
7.62.4West on I-68 to Sideling Hill visitor center.
(km)miDirections
(4.6)2.9Proceed westward on I-68 to Exit 72 for High Germany Road. Turn left on ramp to re-enter I-68 eastbound.
(3.2)2.0Take Exit 74, exit onto Scenic 40, Mountain Road eastbound.
(4.2)2.6Pull off MD 144 at State Highway barn. Stop 3.
(km)miDirections
(4.6)2.9Proceed westward on I-68 to Exit 72 for High Germany Road. Turn left on ramp to re-enter I-68 eastbound.
(3.2)2.0Take Exit 74, exit onto Scenic 40, Mountain Road eastbound.
(4.2)2.6Pull off MD 144 at State Highway barn. Stop 3.

Lithofacies Codes and Proposed Depositional Environments for the Lower Rockwell Diamicite Succession as used During this Field Trip

Table 1.
Lithofacies Codes and Proposed Depositional Environments for the Lower Rockwell Diamicite Succession as used During this Field Trip
Lithofacies codeLithologyInterpreted environment of deposition
GtGravel, trough cross-beddedBraidplain
DmmDiamictite, matrix supported, massiveMeltout tillite, rainout deposit
Dmm (s)Diamictite, matrix-supported, massiveStratified-debris/gravity flow
DmsDiamictite, matrix supported, stratifiedDebris/gravity flow
DmdDiamictite, matrix supported, deformedLodgement/deformation tillite
Dms (r)Diamictite, matrix supported, resedimentedDebris/gravity flow
DcsDiamictite, clast supported, stratifiedDebris/gravity flow
Dcs (r)Diamictite, clast supported, stratified, resedimentedDebris/gravity flow
Ds (r)Diamictite, stratified, resedimentedDebris/gravity flow
FmFine-grained, massiveGlaciolacustrine, proximal
Fm (d)Fine-grained, massive, dropstonesGlaciolacustrine
Fm (l)Fine-grained, massive, load structureGlaciolacustrine
FlFine-grained, laminatedGlaciolacustrine, deep, distal
Fl (d)Fine-grained, laminated, dropstoneGlaciolacustrine, ice rafted
SmSandstone, massiveOutwash braidplain
SmtSandstone, massive, trough cross-beddedBraidplain channel dune
Sm (d)Sandstone, massive, diamictite inclusionsProximal outwash braidplain
StSandstone, trough cross-beddedBraidplain
SdSandstone, deformedPreglacial outwash plain
SpSandstone, planar cross-beddedBraidplain channel dune
ShSandstone, horizontal beddingGlaciolacustrine, deltaic
Lithofacies codeLithologyInterpreted environment of deposition
GtGravel, trough cross-beddedBraidplain
DmmDiamictite, matrix supported, massiveMeltout tillite, rainout deposit
Dmm (s)Diamictite, matrix-supported, massiveStratified-debris/gravity flow
DmsDiamictite, matrix supported, stratifiedDebris/gravity flow
DmdDiamictite, matrix supported, deformedLodgement/deformation tillite
Dms (r)Diamictite, matrix supported, resedimentedDebris/gravity flow
DcsDiamictite, clast supported, stratifiedDebris/gravity flow
Dcs (r)Diamictite, clast supported, stratified, resedimentedDebris/gravity flow
Ds (r)Diamictite, stratified, resedimentedDebris/gravity flow
FmFine-grained, massiveGlaciolacustrine, proximal
Fm (d)Fine-grained, massive, dropstonesGlaciolacustrine
Fm (l)Fine-grained, massive, load structureGlaciolacustrine
FlFine-grained, laminatedGlaciolacustrine, deep, distal
Fl (d)Fine-grained, laminated, dropstoneGlaciolacustrine, ice rafted
SmSandstone, massiveOutwash braidplain
SmtSandstone, massive, trough cross-beddedBraidplain channel dune
Sm (d)Sandstone, massive, diamictite inclusionsProximal outwash braidplain
StSandstone, trough cross-beddedBraidplain
SdSandstone, deformedPreglacial outwash plain
SpSandstone, planar cross-beddedBraidplain channel dune
ShSandstone, horizontal beddingGlaciolacustrine, deltaic

Notes: Lithofacies codes based upon Eyles et al. (1983).

(km)miDirections
(5.6)3.5Continue east on Maryland Route 144 to
Sandy Mile Road into Pennsylvania.
(9.1)5.7Intersection with Pennsylvania Route 484.
Turn left and proceed over Sideling Hill.
(2.7)1.7Pull off on right. Stop 4.
(km)miDirections
(5.6)3.5Continue east on Maryland Route 144 to
Sandy Mile Road into Pennsylvania.
(9.1)5.7Intersection with Pennsylvania Route 484.
Turn left and proceed over Sideling Hill.
(2.7)1.7Pull off on right. Stop 4.
(km)miDirections
(0.6)0.4Continue west on PA 484, then left onto Harmonia Road, PA Route 3002.
(0.72)0.4Left on County Road 308 south.
(4.1)2.5Exit Mountain Road onto I-68 west.
(3.9)2.4Exit I 72.
(4.4)2.7Take Exit I 68, Orleans Road. Right on ramp
to North Orleans Road.
(0.8)0.5Left on Maryland 144 (old Route 40).
(2.1)1.3Pull off on left. Stop 5.
(km)miDirections
(0.6)0.4Continue west on PA 484, then left onto Harmonia Road, PA Route 3002.
(0.72)0.4Left on County Road 308 south.
(4.1)2.5Exit Mountain Road onto I-68 west.
(3.9)2.4Exit I 72.
(4.4)2.7Take Exit I 68, Orleans Road. Right on ramp
to North Orleans Road.
(0.8)0.5Left on Maryland 144 (old Route 40).
(2.1)1.3Pull off on left. Stop 5.

Contents

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