Michael C. Rygel

Stratigraphic imprint of the Late Palaeozoic Ice Age in eastern Australia: a record of alternating glacial and nonglacial climate regime

Stratigraphic and sedimentological data from New South Wales and Queensland, eastern Australia, indicate that the Late Palaeozoic Ice Age comprised at least eight discrete glacial intervals (each 1–8 Ma in duration, here termed ‘glaciations’), separated by nonglacial intervals of comparable duration. These events spanned an interval from mid-Carboniferous (c. 327 Ma) to the early Late Permian (c. 260 Ma), and illustrate a pattern of increasing climatic austerity and increasingly widespread glacial ice from initial onset until an acme in the late Early Permian, followed by an opposite trend towards the final demise of glaciation in the Late Permian. The alternating glacial–nonglacial motif suggests that the Late Palaeozoic Ice Age was considerably more dynamic than previously thought. These patterns are remarkably consistent with recent interpretations of palaeofloral change, eustatic sea-level fluctuations and CO2–climate–glaciation relationships for this interval of time. The detailed record of alternating glacial and nonglacial climate mode disclosed herein may facilitate more closely resolved evaluations of stratigraphic records elsewhere, notably in far-field, ice-distal, northern hemisphere successions.

Role of evaporite withdrawal in the preservation of a unique coal-bearing succession: Pennsylvanian Joggins Formation, Nova Scotia

The Pennsylvanian Joggins Formation, in the Cumberland Group of Nova Scotia, is widely regarded as the worlds best exposure of coal-bearing Carboniferous strata. This 1.5-km-thick coal-bearing unit is famous for upright fossil lycopsid trees, and is preferentially preserved in the Athol syncline in the western Cumberland basin. New seismic profiles show that the Athol syncline is atop a salt weld and that the Joggins Formation thins on the flanks of adjacent evaporite-cored anticlines. These observations indicate that during deposition of the Joggins Formation, at least 1 km of syndepositional subsidence was facilitated by flow of underlying salt into the adjacent anticlines. In contrast, halo-kinesis was mainly active during the Mississippian in the eastern Cumberland basin (Tatamagouche syncline); minibasins were filled by Mabou Group sediments, whereas the Cumberland Group is thin and lacks significant coals. This basinwide comparison shows that much of the subsidence responsible for the preservation of the coal-bearing Joggins Formation was the result of salt withdrawal at depth.

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 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.

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.

Fluvial response to paleo-equatorial climate fluctuations during the late Paleozoic ice age

The Maritimes Basin of Atlantic Canada preserves a 12-km-thick stratigraphic succession deposited within a low-latitude, predominantly continental setting during the late Paleozoic. This paper examines the utility of using such successions for paleoclimate analysis, with specific reference to changes in fluvial style. Modern rivers in the subhumid and semiarid tropics preserve a suite of features that are distinct from those in humid and arid environments. These features include an abundance of sedimentary structures formed under Froude transcritical to supercritical flow conditions and the presence of in situ vegetation growing within the channel environment. This fluvial style is indicative of prolonged low-flow conditions punctuated by intense precipitation events in a strongly seasonal climatic setting. Fluvial channel bodies within the Maritimes Basin preserve a remarkably similar style compared with these modern systems, suggesting that a subhumid, strongly seasonal paleoclimate was the dominant imprint on parts of the preserved successions. Strongly seasonal deposits are concentrated within four discrete stratigraphic intervals (E1–E4), ranging in duration from 2 to 6 m.y. These intervals are correlated across the entire basin and between basins characterized by markedly different tectonic regimes, which suggests that a coherent regional climate signal is recorded. Intervals are separated by deposits characteristic of humid and arid river systems, and the alternation between these different fluvial styles implies that several pronounced, long-term changes in precipitation and runoff regimes occurred during the Carboniferous within the region. Furthermore, these intervals broadly coincide with major periods of Southern Hemisphere glaciation, which suggests that glaciation had a profound and controlling effect on paleotropical climate.

MARINE INFLUENCE IN THE UPPER ORDOVICIAN JUNIATA FORMATION (POTTERS MILLS, PENNSYLVANIA): IMPLICATIONS FOR THE HISTORY OF LIFE ON LAND

Abstract The Juniata Formation comprises Upper Ordovician sandstone and mudstone that crops out in the Appalachian region of the eastern United States from Pennsylvania to Tennessee. An outcrop at Potters Mills, central Pennsylvania, has previously been attributed to a terrestrial environment. Because this outcrop contains numerous sub-vertical burrows and evidence for pedogenesis, it has regularly been cited as the oldest evidence for several aspects of continental ecosystem development, including the first evidence for terrestrial infauna and animal-plant interactions. We present evidence from both original fieldwork and published literature that collectively sheds considerable doubt on previous interpretations. The evidence suggests that the Juniata Formation at Potters Mills was deposited in a marginal marine setting and, as such, no evidence for early life on land can be inferred from its strata. This has significant implications for the numerous studies that have cited the Juniata Formation as providing a key record of early terrestrial evolution. Removing it from the dataset of studies that deal with the history of life on land, we conclude that currently the majority of fossil evidence from localities worldwide supports the appearance of terrestrial infauna and animal-plant interactions in the Silurian–Devonian.

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.

“Such a section as never was put together before”: Logan, Dawson, Lyell, and mid-Nineteenth-Century measurements of the Pennsylvanian Joggins section of Nova Scotia

William Edmond Logan assumed his duties as the first head of the Geological Survey of Canada in June 1843. Two previously overlooked field notebooks provide new insight into his first field project that summer: measurement of the “Joggins section,” a classic Carboniferous locality in Nova Scotia. Inspired by reports of 40-foot-tall fossil trees, Logan spent five days measuring 14 570 feet 11 inches of strata exposed along the shore of the Bay of Fundy. Widely regarded as a meticulous, bed-by-bed measured section, closer examination reveals that many thickness values were calculated using paced distances. Realizing that his measured section was too detailed for scientific journals of the day, Logan published his work in a relatively obscure government publication where it went largely unnoticed for nearly a decade. Unaware of Logan’s measured section, John William Dawson and Charles Lyell visited Joggins in 1852 and measured the section for themselves. Dawson later stated that the two sections contain only minor differences, but careful comparison shows that they have radically different descriptions and measurements for even the most distinctive beds. Dawson disguised these discrepancies in post-1855 editions of his book Acadian Geology by rewriting much of the measured section and abandoning many of his own observations. Although over 200 subsequent Joggins studies build upon these measured sections, the present study represents the first detailed examination of the two historical sections and reveals previously unknown discrepancies between two of the most important early geologic studies undertaken in Nova Scotia. Resume William Edmond Logan est devenu le premier responsable de la Commission geologique du Canada en juin 1843. Deux carnets de travaux sur le terrain, precedemment negliges, fournissent un nouvel eclairage sur son premier projet sur le terrain cet ete-la : le mesurage du « stratotype de Joggins », un secteur carbonifere classique en Nouvelle-Ecosse. Inspire par des comptes rendus de la presence d’arbres fossiles de 40 pieds de hauteur, Logan a consacre cinq jours a mesurer 14 570 pieds 11 pouces de strates affleurant le long du rivage de la baie de Fundy. Un examen plus attentif de l’endroit, largement considere comme un stratotype meticuleusement mesure couche par couche, revele que de nombreuses donnees d’epaisseur ont ete calculees au nombre de pas. Se rendant compte que le stratotype qu’il avait mesure etait trop detaille pour les revues scientifiques de l’epoque, Logan avait publie ses travaux dans une publication gouvernementale relativement obscure ou ils sont demeures pratiquement inapercus pendant pres d’une decennie. John William Dawson et Charles Lyell, qui n’etaient pas au courant du stratotype mesure par Logan, se sont rendus a Joggins en 1852 et ont mesure le stratotype eux-memes. Dawson a ulterieurement laisse entendre que les deux stratotypes presentaient seulement des differences minimes, mais une comparaison attentive revele que leurs descriptions et leurs mesures sont radicalement differentes, meme dans le cas des couches les plus caracteristiques. Dawson a deguise ces divergences dans des editions ulterieures a 1855 de son livre Acadian Geology en remaniant une vaste part du stratotype mesure et en abandonnant nombre de ses propres observations. Meme si plus de 200 etudes subsequentes de Joggins se sont appuyees sur les stratotypes mesures, la presente etude represente le premier examen detaille des stratotypes et elle revele des divergences auparavant inconnues entre deux des premieres etudes geologiques les plus importantes realisees en Nouvelle-Ecosse. [Traduit par la redaction]

Discussion on 'Tectonic and environmental controls on Palaeozoic fluvial environments: reassessing the impacts of early land plants on sedimentation'. Journal of the Geological Society, https://doi.org/10.1144/jgs2016-063

The first-order importance of tectonic and environmental controls for terrigenous sediment supply has rarely been questioned, but the role of vegetation in the modification of ancient alluvial signatures has been observed since the mid-20th century (Vogt 1941). Studies of sparsely vegetated rivers (Schumm 1968) and alluvial stratigraphic variation (Cotter 1978; Davies & Gibling 2010) led to observations of (1) plant modulation of alluvial signatures and (2) Palaeozoic facies shifts (PFS): unidirectional changes to facies diversity and frequency, in stratigraphic alliance with the plant fossil record. One PFS is the Siluro-Devonian appearance of mud-rich, architecturally complex alluvium, traditionally ascribed to meandering rivers, and sedimentologically distinct from pre-vegetation strata (Davies & Gibling 2010; Long 2011). Using selected secondary data, Santos et al. (2017) dispute the correlation of these observations using three key points, as follows. (1) The mid-Palaeozoic was typified by orogenic assembly of low-gradient equatorial continents and elevated sea-level, which led to tropical weathering (abundant fine sediment) and extensive alluvial plains. This drove the PFS by promoting river meandering independently of vegetation. (2) Meandering does not require vegetation; this is shown by examples in Precambrian deposits, on other planets, and in ‘non-vegetated’ deserts. Meandering rivers were more abundant than the pre-vegetation rock record suggests, owing to selective bypass and deflation of fine material. (3) Early Siluro-Devonian (meaning Ludlow–Early Devonian) land plants were too small, their biomass and cover too limited, and their wetland habitat too narrow to have stabilized meandering channels, influencing landscape little more than earlier microbial communities. We contest the conclusions and method of the paper, and deal with each point in turn.