The Euramerican Coal Province: controls on Late Paleozoic peat accumulation

Palaeogeography, Palaeoclimatology, Palaeoecology, 106 (1994): 1-21 1 Elsevier Science B.V., Amsterdam

The Euramerican Coal Province: controls on Late Paleozoic peat accumulation

J .H . C a l d e r a a n d M . R . G i b l i n g b

"Coal Resources Section, Nova Scotia Department of Natural Resources, Box 698, Halifax, Nova Scotia, Canada B3J 2T9

bDepartment of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5

(Received November 9, 1993)

ABSTRACT

Calder, J.H. and Gibling, M.R., 1994. The Euramerican Coal Province: controls on Late Paleozoic peat accumulation. Palaeogeogr., Palaeoclimatol., Palaeoecol., 106: 1-21.

The ancestral, tropical peats and associated strata of the Euramerican Coal Province record the interwoven effects of climate, tectonism and eustasy during the Late Paleozoic. Many peats formed in foreland and intermontane basins associated with equatorial orogenic belts along the borderlands of Euramerica and Gondwana, while other peats formed in adjacent cratonic basins. The resolution of controls on Euramerican peat formation requires testing against a well-constrained temporal framework.

The Late Paleozoic "coal age" followed the evolution of vascular plants in the late Silurian and was associated with assemblage of the Pangean supercontinent. Devonian and Early Carboniferous peats developed locally. The major phase of peat formation commenced in the Namurian A and persisted to the end of the Stephanian, spanning 10-30 Ma in different regions of Euramerica. It is ascribed in part to the residence of northward-drifting Euramerica within the equatorial rainy belt. Within individual depocenters, basin hydrology was suitable for peatland formation during a period (106-107 years) represented by the stratigraphic distribution of coal measures and termed here the basin-fill "coal window".

Repeated successions (cyclothems) of coal, siliciclastic and carbonate strata punctuate the coal window and typically represent 104-10 s years. These stratal successions are widely ascribed to orbitally driven glacioeustasy, although cyclothems that lack marine strata may represent associated climatic shifts. Cyclothem groupings (mesothems) of 106 years duration are recognised locally, and may have eustatic or tectonic causes. Cyclothems produced by autocyclic events such as delta switching should have durations in the order of 103 years.

The coal beds themselves represent autogenic processes of peat formation over periods of 103-104 years, constrained by longer term allogenic effects. Recorded in the coal bed are tolerable allogenic events such as fires and floods caused by short- term climatic excursions. The termination of most mires probably occurred when single or combined allogenic effects surpassed the inherent ability of the ecosystem to adapt.

Introduction

Dur ing the Late Paleozoic, extensive peat-

forming ecosystems flourished across the southern,

equatorial and, to a lesser degree, nor the rn temper-

ate lowlands of Euramer ica as the landmass amal-

gamated with G o n d w a n a to form the Pangean

supercont inent . Preserved deposits of these peat-

lands, principally Pennsy lvan ian in age, formed

the impor t an t b i t uminous coal resources of Europe

and Nor th America that would fuel the Indust r ia l

Revolu t ion some 300 m.y. later, and which con-

t inue to serve as a major energy source for the

ravenous appetite of the industrial ized world.

Our reliance on these fossil peats provides reason

enough to delve into the study of their origins, bu t

the reasons, both practical and scientific, go

beyond curiosity. Careful analysis of individual

Carboni ferous peat (coal) deposits can reveal eco-

system dynamics at time scales greater than those

normal ly available to ecologists. The sensitive

pa leoenvi ronmenta l record of these ancient mires

can provide impor t an t insight into basinal pro-

cesses, eustatic and climatic change when incorpo-

SSDI 0031-0182(93)E0197-2

2 J.H. CALDER AND M.R. GIBLING

rated into broad-based studies. From an applied standpoint, knowledge of the controls on ancestral mire formation provides a powerful predictive tool in coal exploration and mine planning.

In May 1992, geoscientists from several coun- tries gathered at Acadia University, Nova Scotia during the annual meeting of the Geological Association of Canada to consider the record of the Carboniferous peat-forming ecosystems of Euramerica and controls on their formation. The interdisciplinary nature of the symposium, "The Euramerican Coal Province: Controls on Tropical Peat Accumulation in the Late Paleozoic", and frank discussion of the validity of our interpreta- tive methods was reminiscent of the seminal Crystal Cliffs Conferences on the Origin and Constitution of Coal held in Nova Scotia in the early 1950's. The present introductory paper high- lights three issues that arose repeatedly in discussion (see Gibling and Calder, 1993): (1) the setting of the coal basins, most of which developed in association with equatorial uplands, some of Andean dimensions; (2) whether modern equatorial peats can provide suitable counterparts for interpreting the origin of Euramerican coal beds; and (3) factors that controlled Late Paleozoic peat accumulation and the interaction between these factors. We emphasize in this paper studies of Late Paleozoic strata in Euramerica and possible modern counterparts, while drawing on concepts arising from many parts of the geological record. This special issue of Palaeogeography, Palaeo- climatology, Palaeoecology contains papers arising from this meeting and provides another contribu- tion to special publications on coal and coal- bearing strata, principally of Carboniferous and Cretaceous age (e.g. Dapples and Hopkins, 1969; Rahmani and Flores, 1984; Phillips and Cecil, 1985; Scott, 1987; Lyons and Alpern, 1989; Bertand, 1991; McCabe and Parrish, 1992; Cobb and Cecil, 1993).

Geographic and climatic setting of the Euramerican Coal Province

The Euramerican landmass (Fig. 1) became a distinct entity during the Silurian with the joining of Laurentia (North America) and Baltica

(Europe) (Ziegler et al., 1977; Scotese and McKerrow, 1990). By the Late Carboniferous, southern Euramerica had merged with Gondwana and, with the northward drift of Pangea, lay within the equatorial humid belt. Here were formed the precursor peatlands to the vast Pennsylvanian deposits of the Euramerican coals. Tropical mires (peat-forming wetlands: Moore, 1987) were also widespread on the microcontinents of North and South China (Liu, 1990), distant from Euramerica and not shown in Fig. 1. The Siberian landmass, which lay adjacent to Euramerica, resided in the temperate rainy belt (Witzke, 1990; Scotese and McKerrow, 1990). In the southern hemisphere, glacial conditions prevailed across high latitude regions of Gondwana during the Late Carboniferous and Early Permian (Crowell, 1978; Veevers and Powell, 1987). Siberian and Gondwanan coals are principally Permian in age.

We use the term "Euramerican Coal Province" to designate the extensive areas of present-day Europe and North America that are underlain by Late Paleozoic, principally Pennsylvanian, coal- bearing formations. The term provides a conve- nient label for coal-bearing regions that were con- tiguous during the Late Paleozoic but have since been separated by sea-floor spreading or restruc- tured during subsequent tectonic activity. The Euramerican Floral Province covers much of the Euramerican Coal Province but is not contermi- nous with it. Much of the precursor peat formed within about 20 ° of the equator (Fig. 1), an area bounded roughly by the Tropics of Cancer and Capricorn; the peats were thus "tropical" in a latitudinal sense. The bulk of the coals fall within the Pennsylvanian, as defined in North America; frequent usage is made here of stages within the Silesian, defined in Europe.

The paleoclimate of tropical Euramerica during the Namurian and Westphalian (Lower and Middle Pennsylvanian) is inferred to have been generally humid, ranging from everwet to seasonally wet, but with short term excursions and longer, periodic alternations of climatic maxima and minima (Cecil, 1990; Perlmutter and Matthews, 1989, 1993). Periodically drier conditions and lowstands, probably linked at least in part to orbitally forced glaciation, accompanied climatic minima.

THE EURAMERICAN COAL PROVINCE: CONTROLS ON LATE PALEOZOIC PEAT ACCUMULATION 3

MAJOR EURAMERICAN

COAL BASINS

NORTH AMERICA

AN Anthracite Fields AP Appalachians BW Black Warrior I I l l inois M Maritimes

MD Mid¢ontinental MI Michigan N Narragansett

EUROPE C Cantabrian Mountains D Donetz IB SW Iberia IS tntrasudetic MC Massif Central MS Moscow MV Midland Valley NP Nord-Pas de Calais P Pennine

PY Pyrenees R Ruhr

SL Saar-Lorraine S Silesian W South Wales

~ Deep Water

Marine conditions predominant

Subaerial conditions predominant

Regions with uplands

Fig. 1. Generalized Late Carboniferous (306 Ma) paleogeographic reconstruction to show distribution of major coal-bearing basins of Euramerica. Paleomagnetic reconstruction from Scotese and McKerrow (1990) with addition of geomorphic elements from coloured maps, courtesy of University of Texas at Austin. Upland areas include high-elevation mountain belts, as in parts of the U.S. Appalachians, and areas of modest relief, as in parts of Europe and Atlantic Canada. Basin designations indicate the approximate location of major coal-bearing regions. Many of these regions are structural remnants of formerly more extensive depositional basins and were probably interconnected during the Late Carboniferous; other regions comprise numerous, formerly isolated coal-bearing depocenters. The relative positions of some basins reflects Late Carboniferous and subsequent tectonic translocation. Marine transgressions periodically covered substantial areas shown as subaerial.

Strong monsoonal circulation prevailed (Parrish, 1993), with rainshadows created by equatorial mountain chains resulting in significant variation in the distribution of precipitation across Euramerica. Interpretations of Euramerican climatic change share consensus on increasingly drier conditions in the Stephanian relative to the Westphalian (through Middle to Upper Pennsylvanian), but differ in the general trend of relative humidity experienced through the Westphalian (Fig. 3).

The same zonal cells of atmospheric circulation are inferred to have existed in the Late Paleozoic as today (Scotese and Barrett, 1990), although their latitudinal disposition would have varied between climatic maxima and minima (Perlmutter and Matthews, 1989). Elements of Late Paleozoic global geography (Fig. 1), however, had potential to disrupt the zonal climate pattern. These

included: (1) orographic barriers: the rising equatorial Appalachian and Ouachita Mountains, potentially of Andean height (Slingerland and Furlong, 1990) in the west, Hercynian and Ural uplands in the east and Mauritanides of Gondwana to the south (Rowley et al., 1985); (2) the amalgamated Pangean landmass across which zonal patterns would have decayed (Scotese and Barrett, ibid.); and (3) the tropical paleolatitude of Pangea, which would have fostered strong monsoonal circulation (Fairbridge, 1986; Rowley et al., 1985; Perlmutter and Matthews, 1989, Parrish, 1993). A number of less elevated uplands may also have influenced climatic patterns, including landmasses in northern Europe and Greenland (Stemmerik and Worsley, 1989; Cliff et al., 1991) and ancestral orogenic belts of western North America (Sloss, 1988).

The changing disposition of oceans through time is of fundamental importance in assessing the role

4 J . H . C A L D E R A N D M . R . G I B L I N G

of eustasy and climate in Late Paleozoic peat formation and preservation. An epicontinental sea waxed and waned across wide areas of the North American craton throughout much of the Carboniferous and early Permian, with deeper water to the south (Donaldson and Shumaker, 1981; Heckel, 1991). In present-day Europe, marine conditions were common during the Early Carboniferous and widespread incursions are recorded through the Westphalian A-C (Fig. 2). Open sea existed to the north in Scandinavia and across the Russian Platform, and a Rheno- Hercynian Ocean may have lain to the south of present-day Europe (Leeder, 1988a). The continu- ity of North American and European oceans through a Carboniferous seaway has yet to be established. The Maritimes Basin of eastern Canada has recently yielded evidence for the periodic incursion of brackish waters during the Westphalian A to Stephanian, based on the micro-

faunal record (Wightman et al., 1993, 1994) (Fig. 3).

Major coal basins of the Euramerican Coal Province

The Euramerican Coal Province embraces a plethora of local basin and coalfield names, and we make no attempt to catalogue them all here. The coal beds are predominantly Pennsylvanian in age, with relatively few Mississippian and Permian coal deposits. The location and subsidence history of many basins can be related to tectonic activity associated with the orogenic belts shown in Fig. 1, and foreland-basin, strike-slip and rift settings are all represented. Some Euramerican coal basins formed in the foredeeps of the equatorial orogenic belts that arose from the amalgamation of Euramerica with Gondwana, whereas others were located on distant cratons. Numerous intermon-

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coal-bearing ~ - ~ red beds ~ cong, . . . . to ~ ] moj . . . . fine hori . . . . . . . . . . fortuity ~ carbonot . . . . . poHtes

m interbedded marine strata

Fig. 2. Stratigraphic distribution and age of coal-bearing strata and major marine horizons in some major coal basins of Euramerica from Phillips et al. (1985), Dolby (1991), Ramsbottom et al. (1978), C.F. Eble and D. Chesnut (pers. comm., 1992), Wagner (1971), Strhlau, (1990).


CECIL e_t a1.,(1985) PHILLIPS, PEPPERS DONALDSON rainf--all

and and evapotranspiration DiMICHELE (1985) RENTON ( 1985 )

wettest driest wet drier

\ \

? (' \

I t

Euromer /co ppo oc loin

WINSTON CECIL(1990] and

STANTON {1989)

tropical least high low rainy arid wettest wet

Humid subtropical to semiarid

Wet-dry tropical to humid

pical

Wet -dry to tropical rainy

-1

I I I I I k \

B Eastern U 5 ,~

Central ,~ppolochlon B. Central lJppoloch/on E'

Fig. 3. Interpreted paleoclimate trends for Euramerican coal basins during the Late Paleozoic.

tane basins, typically smaller but host to thick coals and basin fills, formed within the orogenic belts. Marginal marine, coastal plain settings included deltaic and back-barrier ("paralic") coals, whereas non-marine, continental ("limnic") sites included lacustrine and alluvial plain coals, with some coals adjacent to basin-margin alluvial fans.

The majority of the coal beds are associated with terrigenous strata, reflecting in part the wide distribution of upland source areas across the Euramerican equatorial belt. Elsewhere in Euramerica, coeval strata lack coal deposits. Shallow-marine carbonates and evaporites were deposited across much of western and northern North America and on the Russian Platform. Deeper water deposits are present in the Ouachita foreland basin of the southern USA and the Culm facies of southwest England.

Coal basins cover large areas west of the Appalachian Mountain belt in western Euramerica (Fig. 1). The Appalachian and Black Warrior Basins lie adjacent to the Appalachian orogenic

belt, and are foreland basins where subsidence was closely related to development of the fold-and- thrust belt (Beaumont et al., 1987; Hatcher et al., 1989). The anthracite and associated coalfields of Pennsylvania represent erosional remnants of the Appalachian foreland basin within the deformation zone (Levine and Davis, 1989). The Midcontinental, Illinois and Michigan Basins developed further west on the North American Craton, possibly with influence from coeval Appalachian tectonism (Quinlan and Beaumont, 1984; Leighton et al., 1991; Archer et al., 1994). The Narragansett Basin and several associated small basins in New England formed as intermontane basins within the Appalachian mountain belt (Skehan et al., 1986). The Maritimes Basin of Atlantic Canada is a complex of formerly interconnected and locally strike-slip bounded intermontane depocenters that lies within the northern Appalachians and sits astride terranes of both North American and Gondwanan affinity (Gibling et al., 1992; Grant, 1994; White et al., 1994). Peat


accumulation was important locally in these basins from the Namurian to the Stephanian (Fig. 2).

Many coal-bearing depocenters in eastern Euramerica (Fig. 1) developed in foreland-basin, extensional and strike-slip settings associated with or post-dating major phases of Hercynian deformation. The tectonic history of parts of the Hercynian belt has been outlined by Matte (1986, 1991) and Leeder (1988b). The coals in the basins of eastern Euramerica are mainly Namurian to Westphalian C in age, with some coals as young as Stephanian (Fig. 2). A paralic coal belt stretched from Ireland to Poland during the Namurian and Westphalian, much of it in a foreland basin setting. Depocenters were often interconnected and influenced by similar paleogeographic, tectonic and eustatic controls (R.H. Wagner, pers. comm., 1993). Their margins are not everywhere clearly defined, with a general absence of alluvial-fan deposits, and a subdued Westphalian paleotopog- raphy may have been characteristic of the northern European basins (Calver, 1968; Wagner, ibid.). Basins of Britain and Ireland include the Midland Valley of Scotland, the South Wales Basin, and the Pennine Basin of north-central England which extends under the southern North Sea (Besly and Kelling, 1988; Fraser and Gawthorpe, 1990; Leeder and Hardman, 1990). North of the central European Variscides, the Kent-Nord Pas de Calais and Ruhr foreland basins cover parts of England, France, Belgium, the Netherlands and Germany (Littke, 1987; Strehlau, 1990; Drozdzewski, 1993). At the eastern Variscan margin, the Upper Silesian Basin of Poland and the Czech Republic was an early, eastern component of the paralic coal belt (Dopita and Kumpera, 1993). The Silesian and fault-bounded Intrasudetic basins, (Mastalerz and Wilks, 1994) which lie adjacent to the Bohemian massif, were influenced by Variscan tectonism and contain both paralic and limnic coals, of mainly Namurian age. The Donetz Basin of the Ukraine and Russia and the Moscow Basin of Russia developed on the easternmost plate of Baltica (Wagner et al., 1979; Matveev, 1990).

Basins with coal deposits of Namurian to Autunian age are widely distributed within the Hercynian belt. Fault-bounded, largely limnic basins are present in southern and eastern France and westernmost Germany, including the

Saar-Lorraine Basin and numerous depocenters in the Massif Central (Vetter, 1986; Courel, 1988). Strike-slip faults were influential in the development of some basins, including the Pefiarroya coal basin of southwestern Iberia which comprises several strike-slip fault controlled depocenters (Andreis and Wagner, 1983; Gabaldon and Quesada, 1986). Late Westphalian-Stephanian coal basins of the Cantabrian Mountains region (Asturias, northern Leon, northern Palencia, southwest Cantabria) of northern Spain (Heward, 1978; Wagner and Fernandez-Garcia, 1984; Colmenero and Prado, 1993), and the Pyrenees (Nijman and Savage, 1989; Besly and Collinson, 1991) were at times connected to the sea and exhibit a complex history of syn- and post- depositional faulting. Smaller, post-Leonian and post-Asturian successor basins in northern Spain are characterized by high sedimentation rates and tectonic mobility, with paleovalleys and alluvial fan deposits (Iwaniw, 1984). Many of these basins were intermontane in the sense that they lay within upland areas, although topographic relief was not necessarily great.

Coal deposits of northern Euramerica, typically comprising thin beds mainly of Early Carboniferous age, are present in northern Canada, northern and eastern Greenland and in the Svalbard-Barents Sea area (Stemmerik and Worsley, 1989; Stemmerik et al., 1991; Cameron et al., 1994). Many of the basins are fault-bounded and related to rifting.

Chronostratigraphic correlation across Euramerica is best achieved through absolute dating of volcanic ash-falls (tonsteins), especially using 4°Ar/39Ar dating of sanidine (Hess and Lippolt, 1986; Hess et al., 1988; Leeder and McMahon, 1988). Unfortunately, relatively few tonsteins have been identified and dated (Lyons et al., 1994). The main correlative tool for the Namurian and Westphalian of the Euramerican Coal Province continues to be miospore-based palynostratigraphy (Smith and Butterworth, 1967; Clayton et al., 1977; Phillips et al., 1985; Dolby, 1991). The later Westphalian D and Stephanian, however, is better defined using the macroflora (Pfefferkorn and Gillespie, 1980; Wagner, 1984). An important consideration is whether or not floral appearances and disappearances which


define stage boundaries are synchronous across Euramerica, given its varied paleogeography and paleoclimate. Magnetostratigraphy is not possible for much of the Late Carboniferous when magnetic reversals were few (Permo-Carboniferous Reversed Superchron) but can assist in correlating Early Carboniferous strata (Roy and Morris, 1983). The stratigraphic distribution of coal beds in selected Euramerican basins is shown in Fig. 2.

Actualism and the Euramerican ecosystem

The interpretation of Late Paleozoic peat- forming systems has drawn heavily on comparison with present-day peatlands, but in some cases has exceeded the validity of the analogue. The Late Paleozoic mire vegetation was fundamentally different from modern wetland floras in its physiology, ecology and growth strategy (see DiMichele and Phillips, 1993, for comprehensive discussion; also Collinson and Scott, 1987a; Cross and Phillips, 1992). Nonetheless, certain actualistic principles should have governed the development of peat-forming ecosystems in the Late Paleozoic. The most fundamental of these principles is the hydrology of mire formation.

Mires, both ancient and modern, can be classified on the basis of ionic input through their source of water (Moore, 1987, 1989). Those peatlands which receive ionic input both from precipitation and from groundwater (both surface and subsur- face) are classified as rheotrophic; there is compel- ling evidence that such mires require groundwater recharge to supplement seasonally drier periods, and their recognition can provide important paleoclimatic information (Cecil et al., 1985; Calder, 1994). Under conditions of abundant rainfall, low seasonality and a low ratio of evaporation to precipitation, the surface elevation of peatlands can rise above the influence of local groundwater; such mires are ombrogenous (raised) and ombrotrophic (solely rainfed). The terms "planar" and "domed" are commonly used in place of rheotrophic and ombrotrophic but are less meaningful from a genetic standpoint. In the case of ancient deposits, planar and domed involve secondary inference of topography, which is difficult to determine. Mires generally originate under partial influence of groundwater and may evolve under

suitable climate toward solely rainfed systems. Because the ancestral mires of thick, economic coal beds may have persisted for 103 10 4 years (Teichmfiller, 1982), this hydrologic evolution has the potential to result from longer term climate change.

Many low-ash and low-sulphur coal beds originated as raised, ombrotrophic mires (McCabe, 1984; Cecil et al., 1985; Moore, 1987), whereas others originated as planar, rheotrophic mires (Staub and Richards, 1993; Calder, 1994). Criteria have been suggested for the resolution in coal beds of these fundamental mire types (table 1 of Cecil et al. 1985; Calder et al., 1991; Staub and Esterle, 1992; Fig. 2 of Calder, 1994). Coal petrography, perhaps more than any other method, has been used to attempt reconstruction of ancient mire types (Hacquebard and Donaldson, 1969; Diessel, 1982; Mukhopadhyay, 1989; Calder et al., 1991). Single disciplinary methods of mire analysis such as the maceral diagrams of Diessel (1982) and Calder et al. (1991), however, require complemen- tary paleobotanical and geochemical data (Kalkreuth et al., 1994; for discussion, see Calder, 1994 and DiMichele and Phillips, 1994) and interpretation in the context of their geological setting. In certain coal beds, one need look no further than abundant siliciclastic partings for direct evidence of groundwater influence (Teichmfiller, 1989).

The physiology, growth strategies and life histor- ies of Late Paleozoic peat-forming plants (Phillips, 1979; DiMichele and DeMaris, 1987; Collinson and Scott, 1987a; DiMichele and Phillips, 1994) and geochemical evidence, governed by essentially timeless principles (Cecil et al., 1985), offer direct insight into the trophic status of Carboniferous peatlands. As insight into the paleoecology of the mire vegetation grows with multidisciplinary study of coal beds, the flora provide important information on edaphic conditions, hence mire type (Winston, 1990; Eble et al., 1994). Lepidodendron and Paralycopodites (Anabathra), for example, appear to have favoured relatively high water and nutrient levels (Phillips, 1979; DiMichele et al., 1985), conditions found in rheotrophic mires. They may occur as early colonizing phases in raised mire successions (Bartram, 1987), or as the dominant flora in groundwater-nourished ecosystems (Calder, 1994). Definitive ombrotrophic flora


remain elusive. Smith (1962) interpreted dens- ospore-producing plants as the climax vegetation of raised mires in the British Westphalian, but caution has been suggested until more is known of the parent vegetation (DiMichele and Phillips, 1993).

Even though the Euramerican mire flora have no precise modern analogue, studies of modern peatland formation, especially in the context of regional geologic and climatic setting and past environmental change (Cecil et al., 1993), are proving fruitful. The imperiled, raised bog forests of Malaysia and Indonesia (Polak, 1975; Anderson, 1964, 1983; Cameron et al., 1989) provide an analogue for certain low ash, coastal plain coals of Euramerica (Teichmiiller, 1962; McCabe, 1984; Grady et al., 1993; Supardi et al., 1993; Moore, 1987; Staub et al., 1991; Staub and Esterle, 1992). Neotropical peatlands of coastal Central America (Cohen et al., 1989) offer insight into raised and planar mire complexes in back barrier environments. The Snuggedy and Okefenokee swamps of southeastern USA are examples of rheotrophic to mesotrophic peatlands that may serve as analogues for thinner, moderate ash coal beds (Staub and Cohen, 1985; Cohen, 1984). Peats of the Mississippi Delta provide models for formation of discontinuous, locally high-ash coal beds during sea-level fluctuation (Kosters and Suter, 1993).

Counterparts for the widespread intermontane coal deposits of Euramerica, however, have received much less attention. The Meervlakte, as yet neither explored nor defiled, is a 250 x 50 km 2 intermontane basin in equatorial Irian Jaya that drains to the Pacific Ocean and is floored by eutric histosol (FAO-Unesco, 1979). Inland, alluvial fan- bordered reaches of the Mahakam Basin in southern Kalimantan (Flores, 1986) may offer a partial analogue. The strongly seasonal climate and active tectonic setting of East African rift valley mires (Beadle, 1974; Lind, 1955; Thompson and Hamilton, 1983; Chateauneuf et al., 1988) invites comparison with similar settings in the Euramerican Coal Province. The strike-slip Hula Valley within the Dead Sea rift accumulated thick deposits of high-ash peat during the Late Quaternary (Brenner et al., 1978). The Okavango swamps of Botswana bear witness to the impor-

tance of supplementary groundwater in supporting peat growth where precipitation is seasonal (McCarthy et al., 1991; Lottes and Ziegler, 1994).

In summary, the value of modern tropical analogues may lie less in direct floral and petrographic comparison than in the broader environmental and geological context of their formation.

Controls on Euramerican peat and coal formation

Key factors

Controls on the formation of Euramerican ancestral peatlands have been discussed since the earliest years of coal-basin research. Cyclic recurrences of coal (cyclothems: see below), for example, have been attributed by various authors to glacioeustasy, tectonism, climate, intrabasinal processes such as delta switching and compaction, and com- binations of these factors (see reviews by Westoll, 1968; Heckel, 1984; Riegel, 1991; Buchanan and Maples, 1992; Langenheim and Nelson, 1992). Vegetational factors and plant/sediment inter- actions during peat formation have traditionally received less attention (see Scott, 1979).

Above all else, the accumulation of peat is influenced by climate. Of particular importance is the seasonality of precipitation and temperature and especially whether the greater evaporation of the summer months coincides with, and is partially offset by, a wet season (Lottes and Ziegler, 1994). Furthermore, as noted earlier, seasonality is believed to play a key role in determining the trophic status and topography of mires (Cecil et al., 1985; Moore, 1987). Indicators of climate and especially of seasonality include paleosol types (Cecil, 1990) and styles of channel sedimentation (Gibling and Rust, 1990). The paleoclimate that prevailed while intervening siliciclastic or carbonate rocks formed may differ, however, from climatic conditions that prevailed while the coal bed formed (Cecil, 1990). Although sedimentology increasingly provides evidence of greater seasonality during intervals between coal bed formation, seasonal growth rings in Euramerican fossil trees are rarely observed (Chaloner and Creber, 1990). It is probable, therefore, that seasonality in the Euramerican Coal Province involved variation in precipitation but little change in temperature or day length.


An important but particularly elusive component of paleoclimatic impact on peat formation is temperature, which is an important factor in evapotranspiration. Isotopic data can offer insight when extricated from diagenetic effects. For example, 8180 and 613C isotopes of primary aragonite in bivalves have been used to infer temperature and water source (Brand, 1994). 613C isotope analysis may hold promise in searching for C3 versus C 4 plants, which are affected differently by CO2 levels and temperature (Moore,1989). Temperature change over spans of 104 year may have been especially influential in the formation and termination of continental, intermontane mires (Thompson and Hamilton, 1983). Climate and sediment supply are closely related (Cecil, 1990; Perlmutter and Matthews, 1989): climate changes on the scale of a few thousand years or less can profoundly affect the stratigraphic architecture of basinal fills, as noted in Quaternary deposits of the Gulf Coast by Blum and Valastro (1989) and Blum (in press).

Tectonic forces shape the landscape and drainage patterns of basins supporting peat-forming ecosystems: these drainage patterns are of particular significance in the sustenance of rheotrophic peatlands, perhaps more so under seasonally drier conditions (Calder, 1994). Moreover, tectonism

can engender orographic climate change (Perlmutter and Matthews, 1989). Historically, however, tectonism has commonly been considered to have had a negative impact, peat formation being possible only during periods of slow subsidence (Courel, 1986; McCabe, 1991). The importance of foreland basins as hosts to economic coal deposits has been linked to their favourable subsidence rates (McCabe, ibid.). The formation of very thick coal beds almost certainly requires conditions whereby peat accumulation keeps abreast with basin subsidence for unusually long periods. Tectonism (including halokinesis) also provides accommodation space for preservation of the peat deposits.

A long recognized control on coastal-plain peat formation is eustasy, although its influence on inland mires is less clear. Sea-level lowstands can potentially expose large shelf areas suitable for peatland development, but may also diminish the beneficial effects of a maritime climate. Coastal peat development may be favoured landward of maximum transgressive shorelines during periods of rapid sea-level rise when groundwater and nutrient conditions are especially suitable and accommodation space is available (Kosters and Suter, 1993). Thin peats and carbonaceous muds can form at periods of stillstand during which coastal

T A B L E 1

The d u r a t i o n a n d p e r i o d i c i t y o f c o n t r o l s o n p e a t f o r m a t i o n t h r o u g h g e o l o g i c t ime

TIMEFRAME I FACTOR EFFECT

Duration Periodicity

since 400 Ma

107 - lO s yr

107 yr

10 6 - 10 7 yr

10 4 - lOs yr

10 3 - 10 4 yr

10 3 yr

I - 10: yr

evolution of vascular plants

continental evolution & amalgamation

continental drift through latitudinal climate belts

continental tectonics: basin evolution, regional orographic climate variation, optimal local groundwater level & subsidence

recurring allogenic processes: astronomic climate cycles, eustasy

mire genesis: autogonic processes of paludification, terrestrialization

autocyclicity in neighbouring systems (fluvial avulsion, delta switching)

tolerable allogenic events/impositions (fire, flood)

Late Paleozoic wetland ecosystems

greenhouse/icehouse cycles; Coal Ages of Late Paleozoic, Mesozoic, Tertiary

optimal climatic/geographic position for peat growth

optimal regional conditions for peat, and basin-fill "coal windows"

on/off peat cyclicity (cyclotbems)

vegetative & edaphic/chemical succession

major interruptions in peat formation (splits; termination of poorly established mires; some cyclothems)

successional interruptions

10 J . H . C A L D E R A N D M . R . G I B L I N G

progradation can create a stable platform suitable for vegetative growth (Woodroffe et al., 1985). Coal beds associated with fluvial and estuarine strata are components of some Carboniferous paleovalley fills (Archer et al., 1994).

Local, differential subsidence can also affect basin topography, hydrology and peat formation. Causes of locally variable subsidence may include differential compaction of underlying sediments including peats (Fielding, 1986; Ferm and Staub, 1984; Kvale and Archer, 1990; Demko and Gastaldo, 1992; Mastalerz and Wilks, 1994), variable basement topography (Weisenfluh and Ferm, 1985; Gibling and Bird, in press) or syndeposi- tional faulting (Fielding, 1984; Courel et al., 1986).

Timeframe and interaction of factors

Table 1 and Fig. 4 summarize the effects and durations or periodicities of these and other important factors that have been invoked to explain the distribution of coal within Late Paleozoic (and younger) coal-bearing strata. Figure 5 illustrates the application of these factors to the Maritimes Basin of Atlantic Canada and shows, where they can be determined, the approximate periods of influence for the various factors. The evolution of vascular land plants in the Late Silurian (Collinson and Scott, 1987b) and their subsequent expansion, probably accelerated by the continental amalgamation of Pangea, restricts significant peat accumulation to the Late Paleozoic and later (Teichmfiller, 1962; Scott, 1980; Cross and Phillips, 1990).

The Coal Ages of the Late Paleozoic and

B A S I N F I L L S

! COAL ABSENT~'

i BASlN~ "CO~. WINDOWS': OPTIMA L HYDROLOGIC

FLOODS/FIRI~ ~ CONDITIONS

ORBITAL DURATION ) }~r[ ~ IFORCING ? Rr~/DETA eOOES 10 10" I0~

' PERIODICIT Y OF SWITCHING);) i ~ - Y~r) l ' MIRE ~ PERIODICI / (103-10'yr DURATION)

RESIDENCE OF GEOGRAPHIC REGION "#(THIN OPI3MAL EQUATORIAL BELT (107yr DURATION /

Fig. 4. Schematic representation of the duration, periodicity and overlap of controls on peat formation through time.

• - ~ - " " Mo

i - u~ z

~-- -306 z

< == z

o - -3og

= -311

=~

®. Z=

_~- -- -31S

>-

~ EX o

_ _ z 31~

°

. - ~ - .

M A R I T I M E S R E G I O N A L B A S I N A T L A N T I C C A N A D A

CUMBERLAND SYDNEY BASIN BASIN

CYCLOTHEM: \ \ \ Orb~al

Forcin ? (2 x l~gsyr

Mean Ouration)

BOe,Es . ",t I ~ 1 / /

f yr Du#atlon) Residence of o ° ° Maritimes Basin

~ in OPTIMAL EQUATORIAL BELT

during continental Proximal Distal assernMy/ddff

Fans Channels (~8 Ma) "COAL WINDOW"

(2-3 Ma)

l El Red strata, ~ Conglomerates coal absent

G r e y , c o a l - 1 ] ]5C] ]~ C a r b o n a t e s , bear ing strata [ 5 ~ 5 ~ evapor i tes

Fig. 5. An example, from a Euramerican coal basin, of the geological record of controls on Late Paleozoic peat and coal formation.

Mesozoic have been linked to periods of continental amalgamation and break-up (Fischer, 1984; Veevers, 1990). CO2 release by volcanism associated with plate activity during supercontinent break-up is inferred to have generated a greenhouse state, with icehouse conditions prevalent during continental amalgamation, of Pangea, for example. Such periods have lasted for 107-108 years, and may have a periodicity of 300 Ma (Fischer, ibid.) to 400 Ma (Veevers, ibid.). During the Late Paleozic, Pangea hosted widespread tropical peatlands while coeval high-latitude parts of Gondwana were glaciated (Crowell, 1978; Veevers and Powell, 1987), an intriguing coincidence. Citing evidence of lower oceanic, and presumably global, temperature deduced from Ca:Mg ratios in marine invertebrate fossils (Yasamanov, 1981),

THE EURAMERICAN COAL PROVINCE: CONTROLS ON LATE PALEOZOIC PEAT ACCUMULATION I 1

Ross and Ross (1988) invoked the suppression of bacterial decomposition and the exposure of low- lying shelf areas as causes of the Late Paleozoic Coal Age. Undoubtedly the story is more complex. Elevated atmospheric 02 levels inferred for the Late Paleozoic (Berner and Canfield, 1989; Robinson, 1991) would have favoured plants with C4 photosynthetic systems, which in turn prefer warmer temperatures, hence low latitudes (Moore, 1989). The dominance of the lepidodendrid-based peat forming ecosystems of the Late Paleozoic may reflect evolution of a highly transmissive stem and root system needed to accommodate increased transpiration in response to elevated atmospheric 02 (Robinson, 1991).

Late Paleozoic Euramerican peat formation was most widespread during the Pennsylvanian and has been ascribed to the residence of Euramerica within the equatorial rainy belt, during a period that generally ranged from 10 to 30 Ma (Fig. 2). By the Permian, regionally drier conditions prevailed as the coalescing Pangean landmass drifted northward into the tropical subhumid belt (Schutter and Heckel, 1985; Scotese and Barrett, 1990; Witzke, 1990; Ziegler, 1990). Gondwana and Angara then became significant sites of peat formation as suitable conditions shifted to higher latitudes.

With the tectonic evolution of interacting continental plates came basin formation and the generation of local coastal and intermontane sites conducive to hosting extensive peat-forming ecosystems. The basins typically accumulated sediment for periods of 106-107 years and experienced strong local variation in climate and sedimentation. The stratigraphic distribution of coal measures in the basin-fill of each depocenter records both the period (generally about 106 years) and the location where basin hydrology was suitable for peat formation. Basin hydrology is affected by climate, tectonism and eustasy (Galloway and Hobday, 1983). In the intermontane Springhill Coalfield of Nova Scotia, this "coal window" during which peat formed has been ascribed to the interaction of basin subsidence, sediment supply and local orographic climate (Calder, 1991, 1994).

Punctuating the coal measures are the coal beds themselves. Their discrete, cyclic occurrence bears witness to the fact that conditions favouring peat

accumulation were not persistent but recurred with inherent regularity. Coal beds occur within stratal successions which are commonly referred to as cyclothems. The term cyclothem has been widely and variously used; we use the term in a descriptive, non-genetic sense, in reference to simple successions of metre-scale coal beds and intervening siliciclastic and carbonate beds (Weller, 1930). Cyclothems that include beds of marine affinity have long been ascribed to transgression and regression (Udden, 1912). The timeframe of most cyclothems, whether containing marine beds or entirely of continental affinity, accords generally with that of orbitally-forced climatic change (104-105 years: Heckel, 1986; Leeder, 1988b; Collier et al., 1990; Gibling and Bird, in press, among others). For cyclothems with marine components, this has lent support to a glacioeustatic origin, linked to Gondwanan glaciations (Wanless and Shepard, 1936; Crowell, 1978; Heckek 1986; Veevers and Powell, 1987). Eustasy less readily explains cyclic recurrences of coal beds in continental settings, for which recurrent tectonic events traditionally have been invoked (Bott and Johnson, 1967). However, in such settings, astronomically forced cycles of climate change could have played an important role in determining whether peat accumulation could maintain equilibrium with sediment flux and basin subsidence (Calder, 1994). The cyclothem record varies with paleogeographic position relative to the coast (Collier et al., 1990) and with the degree of subsidence (Klein and Willard, 1989; Klein and Kupperman, 1992). Cyclothems have also been ascribed to autocyclic channel and delta switching (D. Moore, 1959; Ferm, 1979), although the periodicity of such events should be considerably shorter, in the order of 103 years (Frazier and Osanik, 1969) (Fig. 4).

Ramsbottom (1978, 1979) recognized in the European Carboniferous unconformity-bounded stratal groupings which consist of progressively more widespread cyclothemic units terminated by a major regression. He termed these groupings "mesothems" and estimated Namurian and Westphalian mesothems to be 1-2 Ma in mean duration. Equivalent groups with an average duration of 0.9 1.5 Ma were recorded in the Appalachian Basin by Busch and Rollins (1984), and unconformity-bounded units with a mean


duration of about 2 Ma were correlated across Euramerica by Ross and Ross (1985, 1988); both workers ascribed these groupings to glacioeustasy. Transgressive-regressive units in the Appalachian Basin were inferred by Tankard (1986) to record a tectonic periodicity of 3-5 Ma. Heckel (1986) noted that, although mesothemic patterns may exist in parts of midcontinental USA, the vertical spacing of major regressions does not define uni- form stratal groupings and that mesothem-scale groupings, if present, are obscured by strongly marked cyclothemic patterns. Mesothems are not shown in Table 1 and Fig. 4 but may be an important, or dominant, aspect of stratal organization in some areas.

Recognition of transgressive-regressive sequence boundaries in the seismic stratigraphic record (Van Wagoner et al., 1988) has resulted in the application of sequence stratigraphic principles to Permo-Carboniferous cyclothems (e.g. Busch and Rollins, 1984; Ross and Ross, 1988; Read and Forsyth, 1989; Maynard, 1992; Gastaldo et al., 1993; Gibling and Bird, in press).

Individual mires typically formed during periods of 103-104 years. Rising quickly from groundwater influenced origins or, in the case of less humid settings, persisting in a rheotrophic state, peat formation began and would ultimately cease as environmental change exceeded the vegetation's capability to adapt. It has long been recognized that both allogenic (external) and autogenic (intrin- sic) change influence the development of a mire and that extricating the two in the peat record is exceedingly difficult (Tansley, 1936; Frenzl, 1983; Tallis, 1983). Plant growth and accumulation mod- ifies the topography and hydrology of the mire. As the peat mass forms, laterally flowing ground- waters are diverted around or become channelled through the mire. This fundamental autogenic change results in reduced inorganic input and nutrient supply. Mire formation can back up water flow, flooding areas into which the mire can expand. This phenomenon, termed paludification, has been described from the Pripet Marches of Poles'ye (Kulczynski, 1949) and Lake Agassiz, Minnesota (Heinselman, 1970). The process of terrestrialization occurs locally as peat accumulates to infill lakes or ponds (Cameron et al., 1989). Given sufficient rainfall, the mire evolves to local-

ized and later to widespread ombrotrophic, raised peatland.

Short-term allogenic change can be a regularly occurring phenomenon to which the ecosystem has evolved tolerance. An example is the bald cypress to slash pine and palmetto fire succession in the Everglades (Fig. 6), caused by periodic drought that accompanies the seasonal climate of the area (Toops, 1989). Wildfires can also have significant sedimentological consequences (Staub and Cohen, 1979; Nichols and Jones, 1992; Scott and Jones, 1993). Short-term climatic excursions caused by phenomena such as global oscillations in oceanic circulation (Peixoto and Oort, 1992), the 18.6 year lunar nodal cycle, 11 year sunspot cycle and 22 year Hale cycle of solar magnetic field reversal (Currie, 1984; Currie and Fairbridge, 1985) poten-

Fig. 6. Fire succession in the Everglades: an example of adapta- tion within the ecosystem to recurring allogenic change. Stumps of razed bald cypress (foreground) amidst dried algal mats, succeeded by slash pine and palmetto.


tially affect wetlands. Resulting severe fires (Johnson, 1984; Swetnam and Betancourt, 1990) and floods (Depetris and Kempe, 1990; Currie and Fairbridge, 1985) are unusual at the scale of human perception but potentially common in the context of geological time.

The ultimate termination of a mire may be the least understood aspect of peat genesis. Initiation and cessation of peat accumulation has been ascribed to basinal processes, including autocyclic channel and delta switching (Ferm, 1979) and tectonic subsidence (Copeland, 1959; Courel, 1986; Klein and Kupperman, 1992) in concert with sediment supply (Naylor et al., 1989). Orbitally forced climate change may have combined with basinal processes (Perlmutter and Matthews, 1989; Calder, 1994) to exceed the inherent ability of the peat- forming ecosystem to adapt to environmental change. Clymo (1987), however, proposed that the depth of peat in a mire has an inherent limit dependent upon the dry density of the peat. For Sphagnum peat, he calculated this limit to be 7-15 m. It is probable that Euramerican peats of differing botanical composition would have yielded coal beds of different thickness because of their compaction ratios, which may have been as little as 3:i for cordaitean wood, 7:1 for lycopsid periderm and doubtless much higher for medullosan tissues (Winston, 1986; Scott and Collinson, 1987).

In summarizing work on this immensely complex subject, we note that:

(l) Some controlling factors imposed limits to the duration of conditions suitable for Late Paleozoic peat formation and preservation in a given area. These factors mainly operated over long periods, and arose from the regional or local climatic and hydrologic consequences of global tectonics and basin development. Such conditions may have a quasi-periodic recurrence through geological time.

(2) Other factors, especially orbital forcing and autocyclicity of clastic systems, showed a periodicity that resulted in recurrent short-lived phases of peat accumulation within the longer term periods noted above. Very short-term, periodic events such as fires and floods profoundly influenced peat accumulation.

(3) The overlap of timeframes associated with these factors precludes recognition of a clearly

defined hierarchy of controls for Late Paleozoic peat accumulation. Controlling factors were commonly interactive, although certain factors may have been preeminent locally.

(4) Peat accumulation can affect associated terrigenous deposystems (see Warwick and Flores, 1987; McCabe and Shanley, 1992, for inferred Tertiary and Cretaceous examples, respectively), although most studies of the Late Paleozoic record have stressed environmental control of peat growth.

Discussion and suggestions for future work

Many of the early advances in our understanding of the origins of Euramerican coal beds were made by geologists who were true multidisciplinarians. Further insight into the ancestral peatlands of the Late Paleozoic will come about mainly through study of coal beds in the context of their geological setting. There is a need for an interdisciplinary approach involving paleoclimatology, basinal history, stratigraphy, sedimentology, coal petrography and paleobotany. Ascertaining controls on coal formation requires an understanding of principles of peat-forming ecosystems and of the Late Paleozoic plants and their environment, both local and global.

Although the most extensive Euramerican coalfields developed in paralic parts of foreland basins and on cratons, other Euramerican coal basins lay within upland areas. There is a need for study of tropical, intermontane settings where peat is accu- mulating in order to assess the interaction of basin tectonics, climate and hydrology in peat formation. Such studies would be most fruitful when linked to work on adjacent coastal peatlands to assess the impact of allogenic change, including eustasy, on both coastal and inland tropical settings.

Regional event correlation within the Euramerican Coal Province is needed in order to determine which events are continent-wide in scope and which reflect local, basinal factors. Such research will be enhanced by advances in chronostratigraphic resolution. The limits of resolution for absolute and biostratigraphic methods constrain the accuracy of our interpretations of peat formation in the Carboniferous (Klein, 1990). The margin of error for 4°Ar/39Ar dates of

14 J.H. CALDER AND M.R, GIBLING

Carboniferous tonsteins (105-106 years) is greater than the duration of most cycles to which peat- forming sequences are commonly ascribed, including those of the Milankovitch band (104-105 years). The use of different geologic time scales by various authors can lead to further misleading but avoidable comparisons (Klein, ibid.). For example, the time scale of Harland et al. (1982) gives the Westphalian a duration of 19 Ma, whereas the favoured 4°Ar/39Ar dates of Hess and Lippolt (1986) indicate a duration of only 9 Ma. Time represented by individual coal beds is equally problematic. Peat to coal compaction ratios have been determined (Ryer and Langer, 1980; Winston, 1989; Moore and Hilbert, 1992, among others; see discussion in Scott and Collinson, 1987a), but it is doubtful if we will ever know with certainty the range of peat accumulation rates for Carboniferous mires.

It remains uncertain whether, during periods of allogenic change, peat-forming systems migrated laterally or died out and were later re-established. Does the presence of coal beds in a vertical succession imply facies migration in accord with Walther's Law or basinwide facies replacement? Even in the face of environmental crises induced by allogenic change, basinal refuges for peat- forming ecosystems probably persisted, and peat formation may not have been simply "on or off". Peat formation is diachronous, especially at the base of peat deposits as mires spread laterally, as revealed through radiocarbon dating (Cameron et al., 1989). It is probable that the upper surfaces of coal beds more closely approximate a time line, recording allogenic change, although many coal beds exhibit "rider" splits.

Carefully integrated petrographic and botanical analysis of Late Paleozoic coal beds is needed to identify botanical components in coal (phyterals: Cady, 1942; Winston, 1986; Lapo and Drozdova, 1989) in order to better constrain interpretations of mire type, especially where coal balls are absent. Such research will require cooperation between coal petrographers and paleobotanists. New methods of enhancing tissue structure in coal, such as laser or acoustic scanning microscopy, will be helpful in this pursuit.

Vexing problems persist in the interpretation of

coal macerals, especially of inertinites inferred to have formed by processes of degradation. In searching for upward trends in peat deposits, it should be noted that the present surface of an active mire will not necessarily be the ultimate top of the bed. Furthermore, cessation of peat accumulation prior to burial of a peat bed may engender physical and chemical changes that would not occur at the surface of still active mires. There is a need for investigation of tropical mires that have stagnated in the face of climate change, such as Kamiranzovu Swamp, Rwanda (Thompson and Hamilton, 1983).

Knowledge of the controls that influenced coal formation in a given coal basin can be a powerful predictive tool in mine-scale and exploration geology programs. For example, ombrotrophic peats are precursors of low ash, low sulphur coals. Exploration models for intermontane basins could usefully utilize the fact that rheotrophic peats are intimately associated with basinal groundwater systems. Coal beds do not occur randomly.

In summary, controls on Late Paleozoic peat accumulation were interwoven, with differing degree of impact regionally across Euramerica. Accordingly, both comparative and interdisciplinary studies are required. Controls must be rigor- ously tested using actualistic principles where applicable as in mire hydrology, and knowledge of uniquely Late Paleozoic conditions where they are not applicable, as in mire ecology. Testing for possible controls against a temporal spectrum may help to identify muted influences that were nonetheless important at various stages of basin and mire evolution. This may be especially important in basins dominated by one controlling factor.

Acknowledgements

The authors wish to acknowledge the thought- provoking input and exchange of ideas that took place at the Euramerican Coal Province Symposium and through the careful input of reviewers of this volume. This manuscript in particular benefitted greatly from the insightful reviews of Drs. C. Blaine Cecil, Andrew C. Scott and Robert H. Wagner. We gratefully acknowledge conference grants from the Global Sedimentary


Geology Program (Canadian Committee) and the Natural Sciences and Engineering Research Council of Canada, financial and logistic support from the Nova Scotia Department of Natural Resources. We also thank typist Jill Cumby and draft persons Wayne Burt, Janet Webster and Cynthia Phillips of NSDNR for help in prepara- tion of this manuscript.

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The Euramerican Coal Province: controls on Late Paleozoic peat accumulation

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