Darrel G.F. Long

Oxygen and carbon isotopes and event stratigraphy near the Ordovician—Silurian boundary, Anticosti Island Quebec

The growth and destruction of continental ice-sheets in North Africa and adjacent parts of Gondwana during the Hirnantian Stage had profound influence on the sedimentary style of mixed carbonate-siliciclastic sequences on Anticosti Island, Quebec, Canada. These strata, deposited in a tropical, mid-shelf setting, show direct evidence of local sea-level changes of up to 40 m. After correction for tectonic and load-induced subsidence this indicates maximum global glacio-eustatic changes of 32 to 36 m. This implies maximum ice volume of about 12 to 14 × 106 km3 and related changes in the oxygen isotopic composition of global sea water amounting to as little as 0.5 to 0.6%. Given that the oxygen isotope ratios in syn-glacial samples from the Anticosti sequence range from −8.9 to −2.8 δ18O, it is not possible to clearly distinguish the effects of ice-cap development from temperature and diagenetic controlled variation. The strong negative shift in δ13C values at the top of the Ellis Bay Formation may indicate a pronounced increase in atmospheric CO2 coincident with destruction of the Late Ordovician ice-caps.

Early Silurian Virgiana pentamerid brachiopod communities of Anticosti Island, Quebec

Morphological, taphonomical, and sedimentological data from lower Llandovery fossils and strata of Anticosti Island demonstrate that the Virgiana community occupied a wide range of substrate settings from near or above fair-weather wave base to well below normal storm wave base. Globally the large-shelled pentamerid brachiopod genus Virgiana had a relatively short temporal duration during the early Llandovery (Early Silurian) and a wide geographical distribution in shallow carbonate basins of the paleocontinents of Laurentia (North America) and Siberia. In most areas, shells of Virgiana are found in post-mortem assemblages associated with ripple-marked and cross-stratified carbonate sands, indicating that they occupied carbonate banks and shoals above fair-weather wave base, and were subject to reworking during storms. Most species of Virgiana developed a costate shell with thickened ventral umbo and flattened apex, which probably helped the organisms stay in an incurved beak-down feeding position in current-swept environments, by weighting and stabilizing the shellposterior. Virgiana shells of the Becscie Formation of Anticosti Island are most commonly preserved as post-mortem assemblages in sharp-based, poorly graded packstones, as disarticulated, broken, nested, stacked, or imbricated valves. They are intimately associated with hummocky cross-stratified carbonate sandstones and massive to laminated mudstones, indicative of deposition by storm-induced current in mid-shelf settings, below fairweather wave base. While some of the shells may have been washed in from shallower, inner-shelf environments during storms, many articulated specimens show well-preserved growth lines, and little evidence of abrasion, indicating local origin. The rare occurrences of in situ shells suggest that Virgiana communities extended into depths of 30 m. A further departure from the norm occurs in the overlying late Rhuddanian Merrimack Formation of Anticosti Island, where strongly costate, articulated shells of Virgiana mayvillensis are relatively well preserved in calcareous and micritic mudstone and calsisiltite beds. This suggests that V. mayvillensis was able to survive in deeper-water, relatively low-energy environments, in midto outer-shelf settings well below normal storm wave base. On Anticosti, there are no reefs in either of the Virgiana-bearing formations.

Birth of the northern Cordilleran orogen, as recorded by detrital zircons in Jurassic synorogenic strata and regional exhumation in Yukon

The Whitehorse trough is an Early to Middle Jurassic marine sedimentary basin that overlaps the Intermontane terranes in the northern Cordillera. Detrital zircon dates from eight Laberge Group sandstones from various parts of the trough all display a major Late Triassic–Early Jurassic peak (220–180 Ma) and a minor peak in the mid-Paleozoic (340–330 Ma), corresponding exactly with known igneous ages from areas surrounding the trough. Source regions generally have Early Jurassic (ca. 200–180 Ma) mica cooling dates, and the petrology of metamorphic rocks and Early Jurassic granitoid plutons flanking the trough suggests rapid exhumation during emplacement. These data suggest that subsidence and coarse clastic sedimentation in the trough occurred concurrently with rapid exhumation of the shoulders. Isolated occurrences of sandstone and conglomerate units with similar detrital zircon signatures occur west and east of the trough, as well as overlapping the Cache Creek terrane, indicating that either the trough was once more extensive, or isolated basins tapped similar sources. Development of these sedimentary basins and accompanying rapid exhumation in the northern Cordillera were coeval with the onset of orogenic activity in the hinterland of the southern Canadian Cordillera, and subsidence in the western Canada foreland sedimentary basin. The Whitehorse trough is interpreted as a forearc basin that progressively evolved into a collisional, synorogenic piggyback basin developed atop the nascent Cordilleran orogen. Upper Jurassic–Lower Cretaceous fluvial deposits overlapping the Whitehorse trough have detrital zircons that were mainly derived from recycling of the Laberge Group, but they also contain zircons exotic to the northern Intermontane terranes that are interpreted to reflect windblown detritus from the Late Jurassic–Early Cretaceous magmatic arc that developed either atop the approaching Insular terranes to the west or southern Stikinia.

Palygorskite in palaeosols from the Miocene Xiacaowan Formation of Jiangsu and Anhui Provinces, P .R. China

Abstract Strata associated with Tertiary basalts of the Xiacaowan Formation, in the North Jiangsu Basin of eastern China, contain mudstones which are rich in palygorskite and montmorillonite, with lesser concentrations of dolomite and CT-opal. Massive montmorillonite clays may have developed as vertisols by deep meteoric weathering of underlying basalt flows in a sub-humid to semi-arid climatic setting with pronounced seasonality. Laminated montmorillonite clays, higher in the sequence, accumulated in low-lying areas in inland lakes with poor external drainage. Climatic fluctuations resulted in periods of increased aridity, during which falling lake levels exposed montmorillonite-rich vertisols, lake and playa sediments, to further pedogenesis. Lowering of the water table and increased rates of evaporation allowed leaching of sodium and calcium from the soils; this, in conjunction with the introduction of silica and magnesium by rising ground water, led to the neomorphic development of extensive stratiform palygorskite deposits.

Are Outliers of the Huronian Supergroup Preserved in Structures Associated With the Collapse of the Sudbury Impact Crater

Several outliers of the Paleoproterozoic Huronian Supergroup are preserved in a 20 km wide arcuate belt northwest of the Sudbury Igneous Complex. Forty years ago it was suggested that these outliers represented the remnants of a discontinuous rim syncline developed around a domal structure. Thirteen years later it was proposed that they were preserved in grabens formed as a result of collapse of the central uplift of a multi‐ring impact crater and the simultaneous emplacement of the Sudbury Igneous Complex. This interpretation has been accepted by most investigators without question, to the extent that the position of the outliers has been used to estimate the size of the supposed crater. Our observations indicate that Huronian strata in the outliers were deposited in a series of paleo‐valleys on a terrane with considerable topographic relief. The asymmetric nature of the fill in some of the outliers suggests deposition may have taken place in half‐grabens. The outliers are preserved in synclines developed by orogenic activity prior to the 1850 Ma impact event. They are cut by later transverse and longitudinal faults. The geometry of the outliers, combined with the character and directional attributes of the sedimentary fill, indicates that the outliers are not related to the Sudbury Event.

The Middle Cambrian Mount Roosevelt Formation (new) of northeastern British Columbia: evidence for rifting and development of the Kechika Graben System

The eastern margin of the Kechika Graben in the vicinity of Muncho and Moose lakes, northeastern British Columbia, contains Middle Cambrian rift-related strata herein named the Mount Roosevelt Formation. The formation conformably overlies a quartzite package equivalent to the uppermost clastic unit of the informal late Early Cambrian Gataga group (Bonnia–Olenellus zone) and is conformably overlain by a thick, unnamed carbonate sequence of Middle Cambrian age (Plagiura–Poliella zone). The Mount Roosevelt Formation is subdivided into three members. The lowermost member is characterized by ooid-bearing siltstone and sandstone, interbedded with dolostone, limestone, and hematitic conglomerate. Conformably above this, the middle member is a thick sequence of polymict cobbly pebble conglomerate. The upper member includes karstified dolostone, calcareous-cemented conglomerate and sandstone, and limestone. Collectively the Mount Roosevelt Formation reflects alluvial fan delta progradation into a transgressive mar...

Dispersion of Cross-Stratification as a Potential Tool in the Interpretation of Proterozoic Arenites

ABSTRACT The distribution of values of dispersion of cross-stratification (measured as variance, at outcrop level) in Proterozoic rocks can be compared directly with patterns obtained from published descriptions of Phanerozoic rocks of more certain origin in order to augment environmental interpretations of the older sequences. Fluvial patterns show characteristic peaks below a variance of 4000; marine sequences show a broad distribution of values with characteristic peaks above 4000. Patterns obtained from aeolian deposits and (?) marine deposits in which unidirectional currents were important are difficult to distinguish from fluvial patterns. Hence, in Proterozoic arenite sequences, the observed distribution of variance values should be used only in conjunction with other environmental indi ators.

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.

Fourier Transform Infrared Spectroscopy of Geological and Archaeological Chert from Southern Ontario

Fourier transform infrared spectroscopy (FT-IR) may provide a low-cost, minimally destructive method for sourcing archaeological cherts in Ontario. In this article we outline a study to characterize chert samples from geological sources in Ontario and to identify the raw material of a small number of archaeological specimens from the late sixteenth century A.D. Neutral Iroquoian Emmerson Springs site. We compare our results with those from an earlier study that used FT-IR to determine if cherts selected for excellent knapping qualities contain sufficient trace minerals to allow for accurate source characterization.

The Early Silurian Gun River Formation of Anticosti Island, eastern Canada: A key section for the mid-Llandovery of North America

The 85–100 m thick carbonate succession of the Gun River Formation is revised on the basis of nearly continuous cliff sections measured on the northeast coast, and a re-analysis and re-measurement of the stratotype sections on the southwest coast of Anticosti island, Quebec. The Gun River Formation (Llandovery: lower Aeronian) is herein formally divided into four members, from the base up, the Lachute, Innommee, Sandtop and Macgilvray members. The formation characterizes a generally deepening phase on the northwestern shelf of Laurentia, bordering the Iapetus ocean in this region. Since the base of the formation is located ca. 150 m above the O/S boundary, it provides fresh data as to the nature of the Early Silurian transgression, radiation, and re-invasion of shelf communities following the Late Ordovician mass extinctions. The Gun River strata were deposited in a midto distal, tropical, very gently sloping ( 1–3 m per km), carbonate ramp or shelf, some 40–80 km offshore from the stable Laurentian craton, and at water depths of 30 to 60 m. Much of the formation consists of rhythmically deposited, light grey micrites (with fissile muddy partings), and minor calcareous mudstones, punctuated by abundant hardgrounds, intraformational conglomerates, and shell-beds. The fauna of brachiopods and corals is generally of low diversity, marking a turnover from a latest Rhuddanian community (Merrimack–lower Gun River Fm) to a more diverse midAeronian fauna (top Gun River–lower Menier Fms) when large pentamerids become abundant. This indicates gradual changes rather than abrupt invasions, sustained deposition, and lack of major sedimentary gaps between stratigraphic units. The absence of shallow water reefs, skeletal stromatolites, and ooid or oncoid shoals, also suggests deposition near or periodically below the storm wave base on a carbonate platform in southern tropical latitudes at 20°–25° S.