John H. Calder

The relationship between coal petrography and depositional environments from selected coal basins in Canada

Abstract This paper reviews current research in Canada relating to the sedimentology of coal-bearing sequences and the petrographic character of contained seams from selected intermontane and foreland basin settings. The intermontane Cumberland and Stellarton basins of Nova Scotia contain Westphalian coal-bearing strata of different character. Coal seams that formed in fluvial-dominated depositional environments of the Springhill Coalfield, Cumberland Basin, commonly brighten upward and laterally display distinct areal zones: a piedmont zone formed by interfingering of mire and distal fan sheetflow deposits; an inner mire zone largely devoid of partings and a riverine zone with large-scale seam splits about multistorey sandstone bodies. Maceral-based groundwater influence and vegetation indices, combined with miospore analyses, indicate that the peat precursors of the No. 3 seam of the inner mire zone accumulated from a groundwater-influenced (rheotrophic to mesotrophic) forest mire (swamp, fen). Both the gelification of lignin-derived maceral precursors and the domination of mire vegetation by lycopsid trees reflect this groundwater influence. Coal that formed from the frequently flooded paleomires of the Stellarton Basin is generally dull, microbanded and mineral-rich. The location of mires within the basin was largely determined by basin subsidence rates. When subsidence rates were high, mires developed around the margin of a central basin lake. Coal seams that formed from these mires (e.g. McLeod seam) thicken toward basin margins and grade into sapropelic shale near the basin centre. Petrographic analyses of the McLeod seam suggest development from limnic to wet forest swamp conditions and a return to a limnic environment at the top of the seam. Thick coal (e.g. Foord seam) developed near the basin centre during periods of lower subsidence rates and grades into red beds and paleosols toward the basin margin. The Foord seam of the basin centre is characterized by low mineral matter contents and a high proportion of degraded vitrinite macerals in the central parts of the seam, interpreted to reflect prolonged peat accumulation with very limited clastic input. The top and the base of the seam, in contrast, were subjected to significant clastic input, as indicated by microbanding associated with significant amounts of detrital minerals and macerals. Coal seams of the Lower Cretaceous Gates Formation in the Western Canada Basin formed in depositional settings ranging from coastal swamps (strandplains) to the upper delta plain. Strandplain coal is characterized by great lateral continuity and substantial thickness, whereas coal of the upper delta plain is thin and discontinous. The strandplain coal is characterized by variable lithotype sequences (oscillatory, brightening-up, dulling-up). Maceral-based facies parameters indicate a wide range of mire facies for these lithotype successions: open moor, forest moor, herbaceous and/or shrubby marshes and slightly raised mires. Results from megafossil and phyteral analyses suggest that the coastal plain mires were dominated by taxodiaceous conifers, whereas ferns, angiosperms and herbaceous lycopods are thought to have occupied the more marshy and/or marginal (stream/lake side) environments.

The impact of climate change, tectonism and hydrology on the formation of Carboniferous tropical intermontane mires: the Springhill coalfield, Cumberland Basin, Nova Scotia

Abstract Both climate and tectonism are influential in determining basin and mire hydrology. In the rapidly subsiding intermontane basins of Euramerica which formed in response to the Variscan-Hercynian-Alleghenian orogeny, however, the record of climate change has been muted and may therefore go unrecognized. The complex interplay of climate and tectonism in a tropical continental setting is recorded in the > 4 km thick Westphalian A-B fill of the Cumberland Basin. Thick peat accumulation was favoured at the southern basin margin where rheotrophic forested mires 4–9 km wide flourished between basin-axis rivers and coalesced alluvial fans derived from the Cobequid highlands. As a result of their geomorphic setting the thicker coals (⩽ 4.3 m) exhibit distinct areal zones: piedmont, inner mire and riverine. Evidence that the forest swamps were nourished both by rainfall and by groundwater discharge from alluvial fans includes the stratigraphic affinity of thick coals and basin-margin conglomerates, interfingering of distal fan sheetflood and mire deposits, petrographic evidence of elevated pH (partial gelification of tissues), and predominance of the arboreous lycopsids Lepidodendron and Paralycopodites . Whereas such groundwater supply is crucial to maintenance of modern wetlands in drier periods, a subhumid climate is implied. Coals at Joggins, further removed from groundwater discharge at the basin margin, are less well developed. A 600-m thick “coal window” within the fining and reddening upward basin-fill sequence at Springhill is inferred to represent a coincidence of optimal groundwater supply and net subsidence. The ultimate decline of peat accumulation in the basin may reflect decrease in catchment area and fan size and loss of rainshadow as the Cobequid Massif was denuded and the basin filled. The coal window [ca. (6−9) × 10 5 yr] is punctuated by non-marine cyclothems of coal, grey sideritic mudrock and multistorey sandstone that share similar time frames with astronomic cycles of axial precession [ca. (1−4) × 10 4 yr]. The stress of related climate change is inferred to have caused peat accumulation to fall into disequilibrium with sediment flux and basin subsidence. The Late Quaternary history of certain African mires illustrates the dramatic impact of climate change (especially temperature and seasonality) and orographic effects on tropical continental peat accumulation. Aspects of the groundwater-recharge model may be applicable to the Westphalian-Stephanian intermontane mires of Spain and Mesozoic coal basins of northern China.

An early Pennsylvanian waterhole deposit and its fossil biota in a dryland alluvial plain setting, Joggins, Nova Scotia

The terrestrial ecology of Pennsylvanian tropical wetlands is understood in detail, but coeval dryland ecosystems remain highly enigmatic. To fill this gap in our knowledge, a Pennsylvanian (Langsettian) continental red-bed succession was studied at the classic Joggins locality, Nova Scotia. These units represent the deposits of seasonally dry, alluvial plains traversed by anastomosed drainage networks. One channel complex informally known as the ‘Hebert beds’ (the focus of this study) contains an unusual fossil assemblage and is interpreted as an alluvial waterhole deposit that formed following drought-induced cessation of channel flow. Adpressed and charred fossil plant remains indicate that the alluvial plain surrounding the waterhole was covered by fire-prone cordaite vegetation, with hydrophilic lycopsids and sphenopsids restricted to waterlogged riparian niches. Gigantic unionoid freshwater bivalves, locally in life position, and occurring in large numbers in the waterhole, were probably infaunal suspension feeders during periods of fluvial activity, but aestivated in channel bottom muds when flow ceased. Abundant terrestrial gastropods found clustered around fossil plant detritus may have been deposit feeders scavenging dry portions of channel floors. Common partially articulated remains of small to medium-sized tetrapods possibly represent animals drawn to the waterhole during drought when surface water was scarce elsewhere. In terms of both sedimentology and biology, the Hebert beds alluvial complex bears a very close similarity to the seasonal drainages and waterholes of present-day central and northern Australia. This unique deposit sheds significant new light on the nature of Pennsylvanian dryland tropical ecology.

The Euramerican Coal Province: controls on Late Paleozoic peat accumulation

Abstract 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 (10 6 –10 7 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 10 4 –10 5 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 10 6 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 10 3 years. The coal beds themselves represent autogenic processes of peat formation over periods of 10 3 –10 4 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.

The Pennsylvanian Joggins Formation of Nova Scotia: sedimentological log and stratigraphic framework of the historic fossil cliffs

Carboniferous strata of the famous Joggins fossil cliffs hold a unique place in the history of geology. Made famous by the fossil discoveries of Lyell and Dawson in the mid 1800s, the cliffs continue to yield important information about paleobiology. The Joggins Formation (of probable Langsettian age) has been completely remeasured for the first time since Logan and Dawson’s pioneering studies, and a visual log and a map of the foreshore illustrate the 915.5 m of strata along Chignecto Bay. Formation boundaries are formally described, and two informal members are abandoned. The formation is divided into 14 cycles, most of which commence with major transgressions represented by the openwater facies assemblage, some faunal elements of which show a restricted-marine affinity. Higher in the cycles, the re-advance of coastal and alluvial systems yielded poorly and well drained facies assemblages, respectively. The main levels of standing trees, dominated by lycopsids, were entombed where distributary channels brought sand into coastal wetlands. Some trees contain tetrapods and invertebrates, which may have sought refuge or become trapped in hollow trees. Cordaitalean (gymnosperm) forests covered the alluvial plains and basin-margin uplands, and were periodically swept by wildfires. The predominance of flooding surfaces and the apparent absence of lowstand exposure surfaces reflect the rapid subsidence of the Cumberland Basin controlled by active basin-margin faults and salt withdrawal. The cycles may reflect tectonic vents, glacioeustatic sea-level fluctuations, and/or variations in sediment flux. Resume Les strates carboniferes des celebres falaises fossiliferes de Joggins occupent une place unique au sein de l’histoire de la geologie. Devenues celebres a la suite des decouvertes de fossiles de Lyell et Dawson vers le milieu du 19e siecle, les falaises continuent a fournir des donnees precieuses au sujet de la paleobiologie. La Formation de Joggins (qui remonte vraisemblablement au Langsettien) a ete entierement remesuree pour la premiere fois depuis les premieres etudes importantes du secteur realisees par Logan et Dawson; une description visuelle et une carte de l’estran illustrent les 915,5 metres de strates le long de la baie Chignectou. L’etude decrit officiellement les limites de la formation et abandonne deux membres officieux. La formation est subdivisee en 14 cycles dont la majorite commencent avec des transgressions importantes representees par l’assemblage de facies en eaux libres, dont certains elements fauniques presentent une affinite marine restreinte. A des niveaux superieurs des cycles, la recurrence des systemes cotiers et alluviaux fournit des assemblages de facies mal draines et bien draines, respectivement. Les principaux niveaux d’arbres sur pieds, a predominance de lycopsides, ont ete enfouis dans des secteurs ou des canaux tertiaires ont apporte du sable a l’interieur des terres humides cotieres. Certains arbres renferment des tetrapodes et des invertebres, lesquels pourraient avoir cherche refuge ou s’etre retrouves prisonniers dans des arbres creux. Des forets cordaitaleennes (gymnospermes) ont couvert les plaines alluviales et les terres hautes de marge de bassin, et ont periodiquement ete balayees par des incendies de foret. La predominance de surfaces d’inondation et l’absence apparente de surfaces d’affleurement de bas niveau temoignent de la subsidence rapide du bassin de Cumberland, controlee par des failles de marge de bassin actives et un retrait du sel. Les cycles pourraient correspondre a des evenements tectoniques, a des fluctuations glacio-eustatiques du niveau de la mer ou a des variations du debit de sediments.

Metalliferous coals of the Westphalian A Joggins Formation, Cumberland Basin, Nova Scotia, Canada: petrology, geochemistry, and palynology

Abstract Five coals of Westphalian A (early Middle Pennsylvanian) age were sampled from the Joggins Formation section exposed along Chignecto Bay at Joggins, Nova Scotia. Coal beds along the bay were mined beginning in the early 17th century, yet there have been few detailed published investigation of the coal beds of this classic section. The lowermost coal, the Upper Coal 28 (Upper Fundy), is a high-vitrinite coal with a spore assemblage dominated by arboreous lycopsid spores with tree ferns subdominant. The upper portions of the coal bed have the highest ratio of well-preserved to poorly-preserved telinite of any of the coals investigated. Coal 19 (“clam coal”) has 88% total vitrinite but, unlike the Fundy coal bed, the telinite has a poor preservation ratio and half the total vitrinite population comprises gelocollinite and vitrodetrinite. The latter coal bed is directly overlain by a basin-wide limestone bed. The Lower Kimberly coal shows good preservation of vitrinite with relatively abundant telinite among the total vitrinite. The Middle Kimberly coal, which underlies the tetrapod-bearing lycopsid trees found by Lyell and Dawson in 1852, exhibits an upward decrease in arboreous lycopod spores and an upward increase in the tree fern spore Punctatisporites minutus . Telinite preservation increases upwards in the Middle Kimberly but overall is well below the preservation ratio of the Upper Fundy coal bed. The coals all have high sulfur contents, yielding up to 13.7% total sulfur for the lower lithotype of the Upper Fundy coal bed. The Kimberly coals are not only high in total and pyritic sulfur, but also have high concentrations of chalcophile elements. Zinc, ranging up to 15,000 ppm (ash basis), is present as sphalerite in fusain lumens. Arsenic and lead each exceed 6000 ppm (ash basis) in separate lithotypes of the Kimberly coals. Together these data are consistent with elevated pH in planar mires. The source of the elemental enrichment in this presumed continental section is enigmatic.

Stratigraphy and sedimentology of early Pennsylvanian red beds at Lower Cove, Nova Scotia, Canada: the Little River Formation with redefinition of the Joggins Formation

The coastal cliffs along the eastern shore of Chignecto Bay, Nova Scotia contain one of the finest Carboniferous sections in the world. In 1843, Sir William Logan measured the entire section as the first project of the Geological Survey of Canada, and defined eight stratigraphic divisions. We have re-measured a section corresponding almost exactly with Logan’s Division 5 in bed-by-bed detail. The strata are exposed in the wave-cut platform and low-relief bluffs of a 2 km-long section at Lower Cove, near Joggins, north and south of Little River. This 635.8 metre-thick succession until now has been included within the basal part of the Joggins Formation, and overlies the Boss Point Formation. However, the studied strata are lithologically distinct, and are formally recognized as the new Little River Formation. This formation is bounded by regionally important surfaces and is traceable inland for 30 kilometres from its Lower Cove type section. Facies analysis indicates that it represents the deposits of a well-drained alluvial plain dissected by shallow rivers characterized by flashy flow. It can be clearly distinguished from the underlying Boss Point Formation (Logan’s Division 6) by its much smaller channels, and from the overlying Joggins Formation (Logan’s Division 4) by lack of coal seams and bivalve-bearing limestone beds. Palynological assemblages indicate that the Little River Formation is of probable late Namurian to basal Westphalian (basal Langsettian) age, and is a likely time-equivalent of the informal Grand-Anse formation of southeast New Brunswick. Resume Les falaises cotieres longeant le rivage oriental de la baie Chignectou, en Nouvelle-Ecosse, abritent l’un des stratotypes carboniferes les plus interessants dans le monde. Sir William Logan avait mesure en 1843 l’ensemble du stratotype dans le cadre du premier projet de la Commission geologique du Canada et il avait defini huit divisions stratigraphiques. Nous avons mesure a nouveau un stratotype correspondant presque exactement dans ses details couche par couche a la division 5 de Logan. Les strates affleurent dans une plate-forme d’erosion et des falaises de relief emousse d’un secteur de deux kilometres de longueur a l’anse Lower, pres de Joggins, au nord et au sud de la riviere Little. Cette succession de 635,8 metres d’epaisseur avait jusqu’a maintenant ete incluse a l’interieur de la partie basale de la Formation de Joggins et elle recouvre la Formation de Boss Point. Les strates etudiees sont cependant lithologiquement distinctes et on les reconnait officiellement en tant que nouvelle Formation de Little River. Cette formation est limitee par des surfaces importantes a l’echelle regionale; on peut la retracer a l’interieur des terres sur 30 kilometres a partir de son stratotype de l’anse Lower. Une analyse du facies revele qu’il represente les depots d’une plaine alluviale bien drainee, sectionnee par des rivieres peu profondes caracterisees par des crues eclair. On peut nettement la distinguer de la Formation sous-jacente de Boss Point (division 6 de Logan), grâce a ses canaux beaucoup plus petits, ainsi que de la Formation sus-jacente de Joggins (division 4 de Logan), par l’absence de couches houilleres et de couches de calcaire abritant des lamellibranches. Les assemblages palynologiques revelent que la Formation de Little River remonte probablement a la periode du Namurien tardif au Westphalien basal (Langsettien basal) et qu’elle constitue vraisemblablement un equivalent chronologique de la Formation officieuse de Grande-Anse dans le sud-est du Nouveau-Brunswick.

Evaporite tectonics and the late Paleozoic stratigraphic development of the Cumberland basin, Appalachians of Atlantic Canada

The Cumberland basin is part of the large and deep Maritimes Basin of Atlantic Canada, interpreted to have developed at tropical latitudes in a tectonic environment of dextral strike slip. The predominantly Mississippian–Pennsylvanian basin fill includes a thick succession of Visean evaporites of the Windsor Group. An overlying clastic succession includes, in the coal-bearing Cumberland Group, fossil forests with upright trees, preserved at the Joggins Fossil Cliffs United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Heritage Site. Analysis of two-dimensional seismic profiles demonstrates that accommodation for the successions overlying the evaporites was provided by salt expulsion, which led to the development of broad open synclines separated by narrow isoclinal anticlines cored by salt. In the western part of the basin (Athol syncline), evaporites remained largely undisturbed until the Pennsylvanian, when their rapid expulsion accommodated accumulation of the thick Joggins succession. In the eastern part of the basin (Tatamagouche syncline), evaporite withdrawal began in the Visean and continued during Serpukhovian time, providing accommodation for symmetric and wedge-shaped minibasins filled by Windsor and overlying Mabou Group strata. Only a small volume of evaporites remained to be expelled during Pennsylvanian thrusting along the southern basin margin; as a result, the Cumberland Group is relatively thin. To the north, the Black River, Wallace, and Pugwash synclines developed as minibasins having a character intermediate between the Athol and Tatamagouche synclines. Many of the halokinetic structures in the Cumberland basin are similar to those on salt-bearing passive continental margins. However, the tectonic environment in narrow fault-bounded basins encouraged vertical, rather than horizontal movement of salt and overlying sediments, and has produced characteristic inequant, oval minibasin geometries. These features may be characteristic of salt tectonics in strike-slip basins. Salt expulsion has strongly influenced the distribution of hydrocarbons and other resources in the basin.

MICROBIAL MATS AND ICHNOFAUNA OF A FLUVIAL-TIDAL CHANNEL IN THE LOWER PENNSYLVANIAN JOGGINS FORMATION, CANADA

ABSTRACT A meandering fluvial channel body at Coal Mine Point in the Joggins Fossil Cliffs, Nova Scotia contains an unusual fossil assemblage. During an early stage of channel abandonment, a wrinkled surface attributed to microbial mats was traversed by large arthropleurids (Diplichnites cuithensis trackways). Closely associated are smaller Diplichnites gouldi trackways, probably made by myriapods, as well as tetrapod tracks (Pseudobradypus?, Dromillopus, Hylopus) and invertebrate traces (Cochlichnus, Gordia), collectively representing the Scoyenia ichnofacies. The mats stabilized the sediment surface, allowing excellent trackway preservation, and may have formed a food source, although no feeding traces were identified. Overlying strata yield Protichnites followed by a succession containing paired mud drapes and an impoverished Skolithos ichnofacies encompassing Skolithos, Arenicolites, Cochlichnus, and possible Rhizocorallium, collectively suggesting brackish influence. The channel deposits contain some logs and large plant axes, and were colonized in late stages by lycopsid and calamitalean trees. The assemblage indicates that Early Pennsylvanian channels on a vegetated coastal plain near the tidal limit had a diverse and interconnected aquatic and riparian ecosystem, with tetrapods and terrestrial arthropods entering the channel. Microbial mats may have been common components of Pennsylvanian channels, much as they are in modern fluvial and tidal channels.

“Coal Age Galapagos”: Joggins and the Lions of Nineteenth Century Geology

The celebrated coastal section at Joggins, Nova Scotia, has played a seminal role in the development of the Earth Sciences, figuring in the careers of such lions of Nineteenth Century science as Lyell, Dawson, Darwin, Logan, Marsh, Gesner, Agassiz, Wyman and Owen, among others. The story that unfolds is not only one of scientific discovery, but one of enlightening interactions between the players that brings to life these personalities, their debates and, for some, their personal agendas. The “marvellous chapter of the big volume” of Earth’s history recorded in the sea cliffs at Joggins served as a “Coal Age Galapagos” for Lyell, Darwin, Dawson and others, furthering their case for geological and evolutionary principles that continue to inform scientific and popular views today. Coincidental with Lyell’s appearance on the scene, Logan undertook at Joggins one of the first field projects of the Geological Survey of Canada. Against the backdrop of advancing scientific thought and positions, a penny opera of professional one-upmanship was played out. Gesner sought reprimand of Lyell from Murchison, President of the Geological Society for misleading Nova Scotia’s geologists; Owen, who earlier coined the word “dinosaur”, beat Lyell and Dawson in naming their own discovery; while a young O.C. Marsh, presaging his intensely competitive dinosaur battles with Edward Cope, arrived at Joggins from Yale hot on Lyell and Dawson’s trail, only to be duped by a worldly traveller ready to oblige his desire for fame. Above all others, the work of Dawson in describing the fossil record and its ecological context established a lasting legacy of relevance for the Joggins cliffs. RESUME Le celebre secteur cotier de Joggins, en Nouvelle-Ecosse, a joue un role majeur dans l’essor des sciences de la terre : il figure parmi les carrieres de plusieurs personnages scientifiques du 19e siecle, tels que Lyell, Dawson, Darwin, Logan, Marsh, Gesner, Agassiz, Wyman et Owen, entre autres. L’histoire des lieux ne se limite pas a une decouverte scientifique; elle relate des interactions instructives entre les protagonistes mettant au jour ces personnalites, leurs debats et, dans certains cas, leurs priorites personnelles. Le « merveilleux chapitre du grand volume » de l’histoire de la terre, enregistre dans les falaises de Joggins, a constitue un « genre de Galapagos de l’âge du charbon » pour Lyell, Darwin, Dawson et d’autres : il a soutenu les principes geologiques et les principes de l’evolution qu’ils avancaient et sur lesquels continuent de s’appuyer aujourd’hui les opinions scientifiques et populaires. En meme temps que Lyell apparaissait sur la scene, Logan entreprenait a Joggins l’un des premiers projets de la Commission geologique du Canada sur le terrain. Avec le desir de faire progresser la pensee et les positions scientifiques en toile de fond, un opera aux nombreux rebondissements s’est alors joue entre chercheurs professionnels. Gesner a demande a Murchison, president de la Societe geologique, que Lyell soit reprimande pour avoir induit en erreur les geologues de la Nouvelle-Ecosse. Owen, qui avait anterieurement avance le terme de « dinosaure », a battu Lyell et Dawson en baptisant leur propre decouverte. Cependant, un jeune O. C. Marsh, pressentant ses luttes profondes intensement competitives avec Edward Cope, arrivait a Joggins en provenance de Yale, tout enthousiaste de s’engager dans le sillage de Lyell et de Dawson, mais seulement pour etre dupe par un voyageur d’experience pret a se plier a son desir de celebrite. Emergeant au-dessus de tous les autres, les travaux realises par Dawson pour decrire les fossiles presents et leur contexte ecologique ont implante un heritage durable et pertinent par rapport aux falaises de Joggins. Traduit par la redaction]