Sarah J. Davies

The Pennsylvanian tropical biome reconstructed from the Joggins Formation of Nova Scotia, Canada

The Pennsylvanian (Langsettian) Joggins Formation contains a diverse fossil assemblage, first made famous by Lyell and Dawson in the mid-19th century. Collector curves based on c. 150 years of observation suggest that the Joggins fossil record is relatively complete. A key feature of the site is that fossils occur in (par)autochthonous assemblages within a narrow time interval (<1 Ma). Analysis of co-occurring taxa within a precise facies context permits ecosystem reconstruction, and three main communities are recognized in this study. Brackish seas, the distal extension of European marine bands, were populated by Foraminifera, molluscs, annelids, arthropods, fishes, and aquatic tetrapods. Poorly drained coastal plains were covered by rainforests of lycopsids, calamiteans, ferns, pteridosperms, and cordaitaleans, inhabited by a terrestrial fauna of molluscs, annelids, arthropods, and tetrapods, including the earliest known reptiles. Well-drained alluvial plains were covered by fire-prone cordaitalean scrub containing a low-diversity fauna of molluscs, arthropods, and tetrapods, locally preserved in waterholes. These three environments repeatedly interchanged with one another in response to base-level fluctuations forced by tectonism and glacioeustasy. Located further inland than other well-studied Pennsylvanian tropical sites, the Joggins Formation is significant because it contains a record of intra-continental terrestrial ecosystems.

The influence of fault array evolution on synrift sedimentation patterns: Controls on deposition in the Strathspey-Brent-Statfjord half graben, northern North Sea

The dispersal and deposition of sediments in a rift basin are controlled by the sediment supply and the generation of accommodation space; hence, for the facies mosaic and depositional architecture of synrift sediments to be understood, both these variables must be constrained. Subsurface data sets, comprising three-dimensional (3-D) seismic and well data, provide the opportunity to quantify the rates of sediment supply and accommodation generation for the duration of the extensional event and over an area of regional extent. In this article, we address the controls on synrift sedimentation through detailed analysis of a high-resolution subsurface data set from the Late Jurassic northern North Sea rift basin. The sedimentation history in the study basin comprises four discrete stages intimately linked to the growth of the normal fault population. The earliest stage of rifting is characterized by a distributed fault population comprising a large number of faults with low slip rates. Sediment supply outpaced tectonic subsidence at this time, and rising eustatic sea level was the primary control on sedimentation. As rifting progressed, strain was localized onto a smaller number of active structures with higher displacement rates. The basin developed a grabenlike geometry, and the lateral propagation and linkage of fault strands controlled accommodation generation. Although the basin was flooded, the rate of sediment supply remained high and largely kept pace with the rate of tectonic subsidence. During the final two stages, the fully linked, half-graben bounding fault was the only active structure in the basin; the rate of sediment supply at this time was greatly exceeded by the rate of tectonic subsidence, and the basin became underfilled. Significantly, the final stage of the sedimentation history is characterized by large-scale fault interactions that changed the fault-controlled basin floor topography; hence, modified sediment dispersal and deposition. We conclude that sediment dynamics and facies distribution in a rift can only be understood in the context of the coevally active fault population. As the faults active at the close of a rift event are very different in location and character to those active during the initiation of rifting, this further emphasizes the need to integrate structural, sedimentary, and stratigraphic studies in rift basins.

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.

Developing diatom-based transfer functions for Central Mexican lakes

This paper is the first attempt to produce diatom-based transfer functions for the northern tropical Americas. A dataset of 53 modern diatom samples and associated hydrochemical variables from 31 sites in the volcanic highlands of central Mexico is presented. The relationship between diatom species distribution and water chemistry is explored using canonical correspondence analysis (CCA) and partial CCA. Variance partitioning indicates that ionic strength and ion type both account for significant and independent portions of this variation. Transfer functions are developed for electrical conductivity (r2 = 0.91) and alkalinity (as a percentage of total anions) (r2 = 0.90), reflecting ionic strength and ionic composition respectively. Prediction errors, estimated using jack-knifing, are low for the conductivity model, but the carbonate transfer function performs less well. This study highlights the potential for diatom-based quantitative palaeoenvironmental reconstructions in central Mexico. However, a number of key diatom species found in fossil material are not represented in the modern flora. Sampling of additional sites may resolve this, but it is thought that the lack of modern analogues may reflect the high degree of anthropogenic disturbance in many of the catchments. This highlights the problem of trying to reconstruct pre-disturbance environmental changes in highly modified ecosystems. One possible solution is to merge the central Mexican data with the African dataset, which includes sites of similar chemical composition, but which have not suffered the same degree of disturbance.

Identifying cryptic tidal influences within deltaic successions: an example from the Marsdenian (Namurian) interval of the Pennine Basin, UK

Research literature abounds on the depositional processes and products associated with macro-tidal regimes, whereas there is little available literature on sediments deposited in micro-tidal regimes. This paper presents new field-based sedimentological interpretations of the Marsdenian (Namurian, Carboniferous) interval of the Pennine Basin, a basin-fill that is classically regarded as the archetypal fluvial-dominated delta system. This paper reinterprets discrete lithostratigraphic units, and suggests they were deposited under the influence of weak tidal currents. We highlight three lithofacies that contain tidally influenced deposits within the Marsdenian interval of the Pennine Basin; a cross-bedded sandstone with mud drapes and reactivation surfaces, a heterolithic ripple-laminated sandstone with muddy drapes and silty mudstone interlaminations, and a rhythmic–parallel-bedded sandstone with mudstone–siltstone interlaminations. Evidence for cryptic tidal signatures in tractionally transported and reworked sediments is qualitative, and largely dependent on the sedimentologists view of what constitutes a diagnostic number of tidal indicators (i.e. mud-drape couplets, reactivation surfaces). In areas away from either tractional deposition or reworking, sediments deposited from suspension are more likely to preserve indicators of tidal processes. This paper focuses on a lithofacies interpreted as a tidally influenced sand-rich delta-front mouthbar deposited from a buoyant effluent plume. Time-series analysis of laminae thickness variations in this facies implies that these variations are rhythmic. We review how the interaction of tidal currents and buoyant plume processes modifies depositional products. This model implies that the rhythmic variation observed in the Marsdenian interval is attributed to the modulation of plume deposition by tidal currents with a semi-diurnal and diurnal tidal periodicity.

Exhumation of a Cretaceous rift complex within a Late Cenozoic restraining bend, southern Mongolia : implications for the crustal evolution of the Gobi Altai region

Abstract: The Gobi Altai region of southern Mongolia contains intramontane basins dominated by Jurassic–Cretaceous clastic and volcanic deposits. The origin of the basins is poorly documented because Late Cenozoic transpressional mountain building and associated alluvial sedimentation have overprinted and obscured the older Mesozoic history. In this study, we report the discovery of Cretaceous high-angle normal faults and a major low-angle extensional detachment fault bordering the Altan Uul range in southern Mongolia, which indicates that NW–SE crustal stretching was responsible for creating the accommodation space for thick Jurassic–Cretaceous sedimentary accumulations in southern Mongolia. The detachment fault separates chlorite-grade metasedimentary rocks in the footwall from intensely sheared and flattened unmetamorphosed conglomerates and breccias in the hanging wall. The significant metamorphic break across the detachment fault, associated extensional structures in the hanging wall, and domal form of the footwall block indicate that NW Altan Uul has some structural and topographic characteristics typically associated with metamorphic core complexes. However, NW Altan Uul lacks ductile extensional fabrics along the detachment fault and in the adjacent footwall, and limited stretching in the upper plate suggests less total extensional slip than is reported from typical metamorphic core complexes. Unconformable relations in hanging-wall strata and palynological data indicate that extensional detachment faulting at NW Altan Uul occurred in the Aptian (120–112 Ma) similar in time to metamorphic core complex development previously reported in Mongolia–China border areas and the Daqing Shan of northern China. Post-detachment fault Cretaceous sedimentation buried NW Altan Uul, which is now exposed only as a result of erosional denudation associated with uplift of the recent Nemegt–Altan Uul restraining bend. The discovery of significant Aptian crustal extension in the southern Gobi Altai suggests that Early Cretaceous diffuse rifting encompassed an even wider region than was previously recognized, including areas of the eastern Altai, central, southern and eastern Mongolia, and adjacent areas of northern and northeastern China and Transbaikalia, constituting one of Earths largest continental interior extensional provinces. Workers investigating the neotectonic development of the Gobi Altai should consider the extent to which the pre-existing rift basin architecture may have influenced the recent range development and network of seismically active faults in the region.

Use of spectral gamma-ray data to refine subsurface fluvial stratigraphy: late Cretaceous strata in the Book Cliffs, Utah, USA

High-resolution (sub-seismic) stratigraphic correlation in fluvial strata is extremely difficult using conventional subsurface core and wireline-log data (e.g. gamma-ray, neutron, density, sonic logs). Spectral gamma-ray logs provide additional data on lithological composition that can aid identification of chronostratigraphic surfaces in a high-resolution sequence stratigraphic correlation framework. The application of spectral gamma-ray data in constructing such a framework is demonstrated using data from exposures and the subsurface from the Cretaceous non-marine Blackhawk Formation and Castlegate Sandstone of the Book Cliffs area, Utah, USA. These strata provide an ideal test case for the use of spectral gamma-ray data in fluvial stratigraphy, because interpretations can be compared directly with a high-resolution sequence stratigraphic framework from coeval shallow-marine strata exposed nearby. The stratigraphic abundance of coals and relatively uniform Th/K ratio, which indicates uniform clay mineralogy, in mudstone sections of the non-marine Blackhawk Formation implies a humid, subtropical climate throughout its deposition. Variations in facies architecture and stratigraphy are therefore interpreted to have been driven principally by fluctuations in relative sea level and hinterland tectonics. Fluvial and fluvio-tidal sandstones within incised valleys, which lie above erosional sequence boundaries, have a highly variable spectral gamma-ray character, but locally are marked by distinctive heavy mineral lags and concentrations (Th consistently >3 ppm). On the interfluves of incised valleys, sequence boundaries are marked by palaeosols that locally have a distinctive leached, potassium-depleted character (Th/K ratio >17). Lateral variability in spectral gamma-ray character of sequence boundaries is consistent with facies and sequence stratigraphic models of the non-marine Blackhawk Formation and Castlegate Sandstone, but sequence boundaries do not exhibit a clear, diagnostic character in every location. In addition, high Th/K ratios of the type noted at interfluve palaeosols may be produced by potassium depletion beneath coal seams as a result of acid groundwater percolation during early diagenesis, and by thorium enrichment in heavy mineral lags within foreshore sandstones. These alternative interpretations can be eliminated if high Th/K ratios occur in intervals lacking coal seams and shallow-marine sandstones, respectively. Thus, within the context of a sound understanding of detrital mineralogy, diagenetic history, facies and appropriate sequence stratigraphic models, spectral gamma-ray data allow sequence boundaries to be identified and correlated in non-marine fluvial strata at least 80 km from the interpreted coeval shoreline.

Phylogenetic and environmental context of a Tournaisian tetrapod fauna

The end-Devonian to mid-Mississippian time interval has long been known for its depauperate palaeontological record, especially for tetrapods. This interval encapsulates the time of increasing terrestriality among tetrapods, but only two Tournaisian localities previously produced tetrapod fossils. Here we describe five new Tournaisian tetrapods (Perittodus apsconditus, Koilops herma, Ossirarus kierani, Diploradus austiumensis and Aytonerpeton microps) from two localities in their environmental context. A phylogenetic analysis retrieved three taxa as stem tetrapods, interspersed among Devonian and Carboniferous forms, and two as stem amphibians, suggesting a deep split among crown tetrapods. We also illustrate new tetrapod specimens from these and additional localities in the Scottish Borders region. The new taxa and specimens suggest that tetrapod diversification was well established by the Tournaisian. Sedimentary evidence indicates that the tetrapod fossils are usually associated with sandy siltstones overlying wetland palaeosols. Tetrapods were probably living on vegetated surfaces that were subsequently flooded. We show that atmospheric oxygen levels were stable across the Devonian/Carboniferous boundary, and did not inhibit the evolution of terrestriality. This wealth of tetrapods from Tournaisian localities highlights the potential for discoveries elsewhere.

Ostracods from freshwater and brackish environments of the Carboniferous of the Midland Valley of Scotland: the early colonization of terrestrial water bodies

The Mississippian Strathclyde Group of the Midland Valley of Scotland yields some of the earliest non-marine ostracods. The succession records shallow marine, deltaic, estuarine, lagoonal, lacustrine, fluvial and swamp environments representing a series of staging-posts between fully marine and limnetic settings. Macrofossils and ostracods are assigned to marine, marginal marine, brackish and freshwater environments based on their faunal assemblage patterns. Key brackish to freshwater ostracods are Geisina arcuata , Paraparchites circularis n. sp., Shemonaella ornata n. sp. and Silenites sp. A, associated with the bivalves Anthraconaia , Carbonicola , Cardiopteridium , Curvirimula , Naiadites , the microconchid ‘ Spirorbis ’, Spinicaudata and fish. Many Platycopina and Paraparchiticopina ostracods are interpreted as euryhaline, which corresponds with their occurrence in marine to coastal plain water bodies, and supports the ‘estuary effect’ hypothesis of non-marine colonization. The success of non-marine colonization by ostracods was dependent on the intrinsic adaptations of ostracod species to lower salinities, such as new reproductive strategies and the timing of extrinsic mechanisms to drive non-marine colonization, such as sea-level change. The genus Carbonita is the oldest and most common freshwater ostracod, and went on to dominate freshwater environments in the Late Palaeozoic.

Lake Sediments Record Sensitivity of Two Hydrologically Closed Upland Lakes in Mexico to Human Impact

Abstract We provide evidence of rapid, recent environmental change in two lakes in the highlands of central Mexico. Multiple sediment cores were obtained from Lago de Zirahuén (Michoacán) and Laguna de Juanacatlán (Jalisco). Analysis of diatom assemblages, magnetic susceptibility, and metal concentrations was carried out, with the chronology provided by 210Pb dating, 14C dating, and tephrochronology. There is evidence of catchment disturbance during the colonial period in both basins, but the most striking feature at both sites is the rapid change in diatom assemblages during the last 20 y, indicating the onset of eutrophication. Limnological data from Lago de Zirahuén support this interpretation, although none are available from Laguna de Juanacatlán. Paleolimnology is a powerful tool in tracking recent change, particularly in the absence of regular limnological monitoring programs. These lakes appear to be highly sensitive to changes in catchment exploitation, which must be considered in future drainage basin management.