Lower Silurian Rocks Research Articles

Paleomagnetic testing of the Lachlan Orocline hypothesis: Overprinting of remanence in Paleozoic rocks of the Lachlan Orogen in southeast Australia

Paleomagnetic studies in the Lachlan Orogen have indicated rotation of elements within it, but the evidence for rotation is limited and has significant statistical uncertainty. This study presents a detailed paleomagnetic investigation, assisted by X-ray analysis of magnetic mineralogy of middle Cambrian to lower Silurian sedimentary and igneous rocks in Victoria and New South Wales. Four demagnetisation behaviours (types A, B, C and D) have been recognised. Type A is characterised by a single component, resulting in an approximately linear path over the magnetite unblocking temperature range. Type B specimens exhibit slow progressive demagnetisation over a wide temperature range, with clustering of demagnetised directions above the magnetite Curie temperature, indicating hematite. Behaviour C is also characterised by clustering of directions, but this extends over a wider temperature range. Samples with behaviour D show noisy and non-informative demagnetisation, indicating very low magnetic stability. Hysteresis data confirm that samples with behaviour B and C have high coercivities and remain unsaturated above 300 mT. Mineral liberation analysis of selected samples indicates presence of iron oxides, magnetite, titanomagnetite, botryoidal hematite and iron hydroxides. Only three sites yielded specimens with type A primary magnetisation, with a poorly defined pole from the Dookie Volcanics on the central limb of the orocline, which does match a previous result from the same district. Two paleopoles, of type B and C specimens from the rocks in Tabberabbera Zone and the Rockley Volcanics, plot within the confidence limit of poles defining the late Silurian to Early Devonian Australian apparent polar wander path. These poles match other poles representing Devonian overprints around the margins of the Lachlan Orogen that have been interpreted to be the result of alteration mineralisation produced by fluids driven by tectonic compression during events in the Bindian (420–410 Ma) or early Tabberabberan (405–380 Ma) orogenies. KEY POINTS Paleomagnetic techniques have been used to test the rotation within the elements of the Lachlan Orocline in southeast Australia. Results suggest that the original remanence in the rocks is obscured owing to post-deformation overprint. This overprint is widespread in the Lachlan Orogen and is associated with the deformation during Bindian/Tabberabberan orogenic events.

Tectono-thermal Evolution of the Lower Paleozoic Petroleum Source Rocks in the Southern Lublin Trough: Implications for Shale Gas Exploration from Maturity Modelling

The Lower Paleozoic basins of eastern Poland have recently been the focus of intensive exploration for shale gas. In the Lublin Basin potential unconventional play is related to Lower Silurian source rocks. In order to assess petroleum charge history of these shale gas reservoirs, 1-D maturity modeling has been performed. In the Łopiennik IG-1 well, which is the only well that penetrated Lower Paleozoic strata in the study area, the uniform vitrinite reflectance values within the Paleozoic section are interpreted as being mainly the result of higher heat flow in the Late Carboniferous to Early Permian times and ~3500 m thick overburden eroded due to the Variscan inversion. Moreover, our model has been supported by zircon helium and apatite fission track dating. The Lower Paleozoic strata in the study area reached maximum temperature in the Late Carboniferous time. Accomplished tectono-thermal model allowed establishing that petroleum generation in the Lower Silurian source rocks developed mainly in the Devonian – Carboniferous period. Whereas, during Mesozoic burial, hydrocarbon generation processes did not develop again. This has negative influence on potential durability of shale gas reservoirs.

Structural analysis of the Baolun gold deposit, Hainan Island, South China: Implications for metallogeny

The Baolun gold deposit is a mesothermal orogenic gold deposit located in the southwestern part of Hainan Island, South China. The deposit comprises a series of NNW-trending quartz-sulfide lodes situated within a parallel array of fault zones traversing a sequence of variably foliated flysch siliciclastic rocks of the Lower Silurian. Detailed field mapping documented at least five phases of deformation in the deposit including NNW-trending folding of the Lower Silurian rocks (D1), development of NNW-trending, steeply dipping ductile shear zones with an oblique dextral sense corresponding to NNE-SSW shortening (D2), WNW-ESE shortening and extension associated with an early oblique sinistral ductile shearing along the NNW-trending fault zones (D3), ENE-WSW shortening (D4), and near N-S extension (D5). The gold-bearing quartz lodes cut the strata folded in the D1, show some laminar layering related to ductile shear in the D2 and are overprinted by brittle structures formed in the D3 to D5. 40Ar–39Ar dating on muscovite from an auriferous quartz lode yielded an age of 242±2.5Ma, which, together with the age of 232±2.5Ma for an aplite vein in the deposit, suggests that the mineralization may be related to a tectono-thermal event in the Triassic. In the context of the southern South China plate tectonics, the formation of the Baolun gold deposit is interpreted to be related to the oblique dextral ductile shearing (D2) along the NNW-trending fault zones during the Indosinian orogeny, in relation to the convergence between the Indochina and South China plates.

Provenance and depositional setting of Lower Silurian siliciclastic rocks on Hainan Island, South China: Implications for a passive margin environment of South China in Gondwana

Diverse models have been proposed for interpreting the early Paleozoic tectonic setting of the Cathaysia Block in South China. However, few of them undertook a comprehensive study of clastic sedimentary rocks in the Cathaysia Block. This study reports the petrography and sedimentary facies, the whole-rock geochemistry and Sm–Nd isotopic data, and the LA-ICP-MS U–Pb ages of detrital zircon from the Lower Silurian meta-siliciclastics on Hainan Island in order to constrain the early Paleozoic tectonic setting. The source rocks for the Lower Silurian meta-siliciclastics have suffered moderate to intense weathering, indicated by the weathering indices, like Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Weathering Index (W index). The Lower Silurian rocks have high Th/Sc (generally between 1.1 and 2.0) and Th/U (generally between 4 and 6) ratios, LREE-enriched REE patterns with negative Eu anomalies (avg. 0.55), and low εNd(t) values (−16.2 to −8.5), suggesting a continental margin basin receiving detritus mostly derived from a recycled-orogen or cratonic provenance. This Early Silurian turbiditic succession with rhythmic flysch and incomplete Bouma sequences also reveals a neritic to abyssal facies, continental shelf to continental slope depositional setting. The geochemical features like the pattern of REEs, the ratios of trace elements (e.g., Th/Sc, Th/Co, La/Sc, Cr/Th, La/Th), and the binary plots and ternary diagrams (e.g., A–CN–K and FMW diagram), demonstrate that the source-areas are dominated by felsic rocks. Together with the two-stage Nd depleted mantle model age (TDM-2) (ca. 2.3Ga), the detrital zircon U–Pb age populations with peaks mainly at ca. 1635Ma confirm that the source rocks are dominated by the Paleo- to Mesoproterozoic Baoban Group and ca. 1430Ma granitoids on Hainan Island. Linked to the early Paleozoic tectonic development of South China, the Lower Silurian siliciclastic rocks on Hainan Island can be interpreted as depositing in a passive continent margin.

Study of an Early Silurian–Early Permian paleo-weathering profile in Zunyi, north Guizhou Province, China

To understand and help settle the controversy around the living time of Pinnatiramosus qianensis Geng, a paleo-weathering profile situated in the town of Yongle near the city of Zunyi, north Guizhou Province, China, was analyzed. The profile formed during a break in sedimentation between the Early Silurian and the Early Permian. Paleosol developed with a depth of several meters. The fossil plant P. qianensis Geng is present, but only in the lower portions (Layer 2) of the paleosol. Another plant with an irregularly branching system is found in Layers 2 and 3. The distinct geochemical characteristics of the lower and upper portions of the Gaojiayan paleosol indicate a compound genesis. Its lower portions (Layers 1 and 2) resulted from in situ weathering of silty mudstone of the lower Silurian Hanjiadian Formation. The upper portions (Layer 3) are allochthonous. Transgression brought substantial concentrations K and Na, and led to K- and Na-enrichment in the profile. Pumping of vascular plants and downward leaching enhanced the K enrichment in the middle portions. A superior preservation of P. qianensis Geng was observed in an exposure of Layer 2. Mass balance calculation indicates a great K enrichment related to bioaccumulation in the top of Layer 2 and a K loss in Layer 3. Fossil plants (e.g., P. qianensis Geng) preserved in the paleosol are Permian rooting systems growing down into the lower Silurian rocks.

Enigmatic occurrence of Permian plant roots in Lower Silurian rocks, Guizhou Province, China

Pinnatiramosus qianensis Geng, 1986, is a plant with a complex, extensive pinnate branching system and pitted tracheids, collected from marine Lower Silurian (Llandovery; c. 430 Ma) rocks in Guizhou Province, China. It challenges long-held theories on the origin and early evolution of vascular plants in the Silurian and Devonian. However, there is a hypothesis that the fossils were not syngenetic with the entombing rock, but were the rooting systems of much younger plants, probably of Permian age. New sections and collections of P. qianensis have been subjected to detailed analyses, which indicate that P. qianensis belongs to an early Permian (c. 285 Ma) rooting system growing down into lower Silurian rocks.

Hydrocarbon-Generation in Cambrian-Silurian High- to Over- Mature Source Rocks, Middle and Upper Yangtze Region, China

Two main Cambrian-Silurian high- to over-mature hydrocarbon source rocks occur in the Middle and Upper Yangtze region: the Lower Cambrian Niutitang Formation (mostly shale), and the Lower Silurian Longmaxi and Upper Ordovician Wufeng formations (mostly graptolite shale). Due to several complicated tectonic events occurred in the study area, the source rocks experienced inhomogeneous uplift, deformation, and burial, resulting in discontinuous and episodic hydrocarbon generation evolution. Based on a detailed study of the tectonic burial history and an analysis of the current degree of thermal evolution of the source rocks, this paper studied the burial history, the thermal history, and the hydrocarbon-generation history of the Cambrian-Silurian source rocks. It used basin modeling technology and thermal simulation tests on source rocks to reveal the process and the regional regularity of the hydrocarbon-generation evolution, and to expound the tectonic phases and the time-spatial distribution of hydrocarbon generation. It shows that the region from western Hubei to eastern Chongqing is the major source area of Lower Cambrian rocks, and the region from southeastern Sichuan to central Guizhou is the major source area of Lower Silurian rocks. Reconstruction of hydrocarbon-generation history allows simulated estimation of the remaining hydrocarbon-generation potential of the high-over mature marine source rocks.

The Salinic Orogeny in northern New Brunswick: geochronological constraints and implications for Silurian stratigraphic nomenclature1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Salinic Orogeny is defined to encompass tectonic interactions that affect all elements of Ganderia involved in the closure of the Tetagouche–Exploits back-arc basin between the Late Ordovician and Early Devonian. Hence, the D1 and D2 deformations in the Miramichi Highlands and Elmtree Inlier of northern New Brunswick are Salinic events, and onlap of Lower Silurian rocks onto exhumed parts of the Brunswick Subduction Complex represents the earliest (Salinic A) of three Silurian unconformities in the region. Upper Ordovician to Lower Silurian rocks of the Matapédia successor basin contain widespread evidence of Middle Silurian tectonism (e.g., disconformities, angular unconformities, and fold interference patterns) created by Devonian overprinting of Silurian folds lacking axial planar cleavage (Salinic B). Recent U–Pb radioisotopic dating of chemically abraded zircon from rhyolite just above the Salinic B unconformity has yielded an age of 422.3 ± 0.3 Ma; combined with late Early Silurian fossil ages just below the unconformity, this indicates a ca. 5 million year Middle Silurian hiatus. Finally, Upper Silurian (Ludfordian) rocks are locally disconformably overlain by polymictic conglomerates that form the base of the Devonian section (Salinic C). All Silurian rocks in northeastern New Brunswick have historically been included in the Chaleurs Group; however, unconformities and local stratigraphic variations (especially compared with the type locality) support the introduction of new higher rank names in New Brunswick. Hence, the Quinn Point Group is introduced to incorporate Lower Silurian rocks, the Petit Rocher Group to include Upper Silurian sedimentary rocks in the Nigadoo River Syncline, and the Dickie Cove Group for Upper Silurian volcanic rocks in the Charlo – Jacquet River area. Upper Silurian rocks west of Campbellton that are contiguous with the Chaleurs Group in Quebec, will remain part of the Chaleurs Group.

Reservoir Potential of Silurian Carbonate Mud Mounds in the Southern Sichuan Basin, Central China

: Lower Silurian mud mounds of the Shinuilan Formation, located in the southern Sichuan Basin, China, have developed in open shelf settings in deeper water than shallow-water reef-bearing limestones that occur in the region. An integration of the outcrop, drill data and seismic profiles show that contemporaneous faults have controlled the boundary and distribution of the sedimentary facies of Lower Silurian rocks in the southern Sichuan Basin. Mounds appear to have developed in the topographic lows formed by synsedimentary faulting, on the shelf of the Yangtze Platform. Average mound thickness is 20 m, maximum 35 m. Mounds are composed mainly of micrite, possibly microbially bound, and are overlain by shales. Mound tops are preferentially dolomitized, with the Mg2+ source probably from the clay content of the mound-top carbonate. Microfacies analysis and reconstruction of the diagenetic history reveal that the mound tops have higher porosity, and are gas targets; in contrast, mound cores and limbs show pores filled by three generations of calcite cement, and therefore have a low gas potential.

A structural study of western Anticosti Island, St. Lawrence platform, Quebec: a fracture analysis that integrates surface and subsurface structural data

Abstract Field data from fracture sets, folds and faults affecting generally flat lying Lower Ordovician to Lower Silurian strata on western Anticosti Island, Quebec, are integrated with remote sensing and seismic reflection data. Spatial and temporal relationships among different structural features observed on the Island and in the Gulf of St. Lawrence are assessed with the aim of understanding the role of these structural features for petroleum migration through the Anticosti Platform. The tectonic impact of Appalachian orogenic events on Anticosti Platform is considered, together with Jurassic and Quaternary tectonics. Four fracture sets are identified in Lower Silurian rocks: 1) a NS-EW orthogonal fracture system (Sets I and II); 2) a N40–70 set (Set III); 3) and a N135–165 set (Set IV). Some irregular, randomly oriented fractures are also observed. The orthogonal system may be related to Early Acadian events, which may have induced extensional stresses within the Platform during Late Silurian-Early Devonian time. Normal faults observed in the Lower Silurian succession may be related to this extensional event. The N135–165 and N40–70 sets may have developed either during the Middle Devonian Acadian orogeny or later in the Jurassic. The irregular fractures could have formed during the final stages of fracture development in response to erosion of the Lower Silurian and younger succession, or in response to isostatic rebound following glaciation. Shuttle Radar Digital Elevation Model lineaments are consistent either with observed fracture set orientations, or subsurface Taconian fault orientations. We infer some Taconian faults and Silurian fracture zones were reactivated, probably during the opening of the Atlantic Ocean, as indicated by the intrusion of two Jurassic dikes. In the subsurface of the Island, major SW-dipping normal faults and extensional fractures affect the Lower to Upper Ordovician succession most. These probably formed in response to an extensional stress field in front of the Taconian deformation front during Late Ordovician time. WSW-trending typically open folds occur on southern Anticosti Island in Chicotte Formation and in the Gulf of St. Lawrence. They involve Lower Silurian to Devonian rocks, and may have formed in a compressive regional setting during the Middle Devonian Acadian orogeny. Thermal maturation and petroleum generation on Anticosti Platform are controlled mainly by the burial history. However, the structures described in this study, in particular Taconian and Early Acadian fractures and faults systems, may have played an important role in petroleum migration and in potential reservoir development.

Geochemistry of Sainte-Marguerite volcanic rocks: implications for the evolution of Silurian–Devonian volcanism in the Gaspé Peninsula

The Lower Devonian Sainte-Marguerite volcanic rocks are part of a Silurian–Devonian volcanic sequence deposited between the Taconian and Acadian orogenies in the Gaspé Peninsula, Quebec, Canada. The Sainte-Marguerite unit includes basaltic and dacitic lava flows with calc-alkaline and volcanic-arc affinities. Such affinities are also recorded by the trace-element signature in Lower Silurian and most Lower Devonian volcanic units of the Gaspé Peninsula. However, most of the other Silurian–Devonian volcanic rocks occurring in the Gaspé Peninsula have been previously interpreted to have erupted in an intracontinental setting. A back-arc setting for the Gaspé Peninsula between the Taconian and Acadian orogenies could account for these subduction volcanic-arc signatures, though a metasomatized lithospheric mantle magma source, unrelated to subduction, cannot be excluded. Lower Silurian and Lower Devonian volcanic rocks in the central part of the Gaspé Peninsula show an arc affinity, whereas Upper Silurian and Lower to Middle Devonian volcanic rocks, located in the south and north of the Gaspé Peninsula, respectively, show a within-plate affinity. The Lower Devonian Archibald Settlement and Boutet volcanic rocks of the southern and northern Gaspé Peninsula, respectively, show a trend toward a within-plate affinity. This suggests that within-plate volcanism migrated from south to north through time in an evolving back-arc environment and that the subduction signature of Lower Silurian and Lower Devonian rocks results from a source that melted only under the central part of the Gaspé Peninsula.

The Silurian Salinic Disturbance — newly discovered orogenic event in the northern Appalachians

In northern New Brunswick the Silurian and Ordovician rocks have been affected by two regional deformation events. The first deformation is attributed to the Salinic orogeny. It took place during the late Wenlockian — early Ludlow (426—421 Ma) and produced tectonic folds with northwest striking axial surfaces. Evidence for cleavage and faulting coeval to this event is rare. The second regional deformation took place during the lower to middle Devonian (407—390 Ma) and is know as Acadian orogeny. The Acadian deformation overprinted the Salinic structures in the lower Silurian rocks but produced single fold generation in the upper Silurian rocks. Intensive cleavage and widespread faulting are related to the Acadian revolution. The refolds in the lower Silurian rocks are steeply plunging. The refolds are overprinted by the Acadian cleavage, while the Acadian folds are dextrally transected by the same cleavage. The kinematics of the Salinic deformation is not understood but it is known that the Acadian deformation was achieved during intensive dextral shear, which resulted in transecting cleavage and an echelon arrangement of Acadian folds with respect to the high-strain shear zones. Most of the faults in the northern Appalachians appear to be synchronous or to postdate the Acadian folding. The dextral shear was penetrative on a regional scale and resulted in numerous bedding parallel faults with hidden separation.

Early Silurian brachiopods (Rhynchonellata) from the Sælabonn Formation of the Ringerike district, Norway

A revision of Kiær’s index fossils of "Etage" 6 in the Ringerike district of Norway reveals the presence of four species: Rostricellula wadti sp. nov., Platytrochalos ringerikensis sp. nov., Platytrochalos rabbei sp. nov., and Zygospiraella duboisi. Rostricellula is a common Ordovician rhynchonellide brachiopod and is known to occur in the Lower Silurian as a holdover taxon in only a few localities worldwide. Previously, Platytrochalos was known only from the Lower Silurian (Llandoverian) rocks of Anticosti Island, eastern Canada. The occurrence of this rare genus in the Ringerike district provides additional information for the early evolution and palaeogeography of the taxonomically enigmatic family Leptocoeliidae. Zygospiraella is regarded as an index genus for the Rhuddanian, and the presence of Z. duboisi provides a useful biostratigraphic control on the age of the Sælabonn Formation.

Sequence stratigraphical analysis of late ordovician and early Silurian depositional systems in the Welsh Basin: a critical assessment

Abstract Sequence stratigraphical concepts are applied to a 55 km long transect through the uppermost Ordovician and lower Silurian rocks of the Welsh Basin and the adjoining Midland Platform. The study focuses on sedimentary rocks deposited during the Llandovery epoch (about 439–430 Ma). An early Llandovery slope apron of hemipelagite and laterally supplied mudstone turbidites shows the influence of eustatic sea-level changes. Two complete depositional sequences are recognized. Transgressions and highstands on the platform were accompanied by laminated hemipelagite deposition in the basin, recording anoxic bottom waters in a stratified watermass. The intervening regressions generated unconformities on the platform and produced bioturbated basinal sediments in a more oxic environment. By contrast, the extent and timing of sandstone-turbidite and debrite systems in the basin are strongly affected by tectonic activity. Easterly derived late Llandovery facies relate directly to faulting and submarine mass wasting along the basin margin. Southerly derived late Llandovery to mid-Wenlock sandstone-lobe systems were the products of tectonic uplift in extrabasinal source areas and were partially confined within intrabasinal tilted fault blocks. Subsidence analysis confirms a basinal stretching event in late Llandovery (Telychian) time and shows that it also affected the adjacent Midland Platform. The depositional systems of this period constitute elements of a dual-sourced depositional sequence, within which eustatic effects are masked by the strong influence of relative base-level changes attendant to tectonism. The architecture of the early Silurian Welsh Basin illustrates the complexities of applying sequence stratigraphical models where there is an interleaving of several depositional systems, each modulated by a different mix of eustatic, tectonic and input controls.

Gafsa Trough of Central Tunisia: Basin Evolution and Maturation of Hydrocarbons

The Gafsa trough of onshore central Tunisia is one of the more interesting and underexplored features of North Africa. It is a 5-12 km deep, east-west-trending depression along the inner edge of the Tunisian-Libyan shelf margin. The basin has had a long and virtually uninterrupted history of subsidence from the late Paleozoic into the early Cenozoic. Subsidence began in the late Carboniferous, soon after the close of the Hercynian orogeny, and resulted in deposition of a 3000+ m succession of Permo-Carboniferous carbonates and shale that pinches out southward onto the Saharan Flexure. The tectonic setting for this earliest phase of subsidence is not clear. The main episode of subsidence, which began in the Middle Triassic, continued through the Jurassic as left-lateral, transtensional rifting along the South Saharan and Maghrebian Shear zones. A set of organic maturation maps for onshore central Tunisia depicts the minimum time of entry into the oil and gas generative windows of the two potential source rocks in the region, the Lower Silurian Tannezufft Formation and Middle-Upper Jurassic basinal shales. Maturation modeling suggests that the Lower Silurian source rocks beneath the deeper portions of the Gafsa trough are overmature, even for generation of dry gas. Everywhere northmore » of the Saharan Flexure potential Paleozoic source rocks are highly mature to overmature. The Middle-Upper Jurassic basinal shales in the deeper, central portions of the Gafsa trough entered the oil generative window as early as mid-cretaceous time and into the gas generative window in the Late Cretaceous - early Tertiary. These possible source rocks are mature to highly mature beneath nearly all of the basin. The Gafsa trough is a probable gas province, with occurrences of condensate possible.« less

Silurian Proporidae and Plasmoporidae (Anthozoa, Tabulata) from the Chaleurs Bay region, Canada

Six species belonging to the families Proporidae and Plasmoporidae occur in the Lower and Upper Silurian rocks of the Limestone Point and La Vieille Formations of northern New Brunswick and of the Anse à Pierre-Loiselle, La Vieille, and Gascons Formations of the Gaspé Peninsula of Québec. The three species ofProporaare widely distributed but show varying faunal affinities, while both species ofPlasmopora, Plasmopora loganiandPlasmopora corrugata, are new and are almost endemic. Revised concepts ofProporaandPlasmoporaare proposed. The holotype specimen for the type species ofCamptolithuswas examined and confirms the genus as distinct fromPropora, rather than a synonym as has been previously suggested.The facies distribution of these corals is variable. In general, the proporids, which mostly occur in facies indicating shallow carbonate banks and patch reefs, are more restricted in distribution than the plasmoporids, which occur in these facies and also in others representing a variety of open-shelf environments.

Evolution of the Early Silurian rhynchonellid brachiopodFenestrirostrain the Anticosti Basin of Quebec

Among the abundant, well-preserved rhynchonellid brachiopods in the Upper Ordovician–Lower Silurian rocks of Anticosti Island,FenestrirostraCooper, 1955, one of the earliest camarotoechiid genera to appear in the fossil record, is probably of oligorhynchiid ancestry and seemingly endemic to the Anticosti Basin. The genus differs most obviously from other camarotoechiids in its peculiar costae, which are angular and strong posteriorly and flat and weak anteriorly.Three successive species ofFenestrirostra, found in micritic mudstones and calcareous shales of the middle and upper Merrimack and Lower Gun River Formations, provide an example of gradualistic evolution. The most primitive species,F. primaeva, occurs only in the lower part of the middle Merrimack Formation; the descendant intermediate species,F. glacialis, occurs mainly in the upper part of the middle and in the upper Merrimack and in the lowermost Gun River Formations; and the descendant terminal species,F. pyrrha, occurs only in the lower Gun River Formation. Four major morphoseries distinguish the lineage: 1) change in the proportions and size of the shells; 2) increase in the number and concomitant loss of the costae from the anterior halves of the shells; 3) change in the orientation and reduction in the size of the hinge plates and hinge fossettes; and 4) change in the height of the median septum. The evolutionary modifications, estimated to have been accomplished in no more than 1 m.y., may have been adaptive responses to persistent low-energy marine conditions.Biostratigraphically, theFenestrirostralineage provides the basis for three interval or range zones that straddle the Rhuddanian–Aeronian interstage boundary in the Anticosti Basin.

Petrology of Upper Ordovician – Lower Silurian rocks of the Antigonish Highlands, Nova Scotia

Upper Ordovician to Lower Silurian rocks in the Antigonish Highlands consist of interlayered basalts, rhyodacites, arkoses, and conglomerates overlain by a thick sequence of marine clastic rocks and minor rhyolites. The stratigraphy documents a marine transgression. The volcanic rocks were deposited in a within-plate, continental, extensional environment. The basalts display alkalic and tholeiitic affinities, and the rhyodacites were formed by anatexis of the crust. The origin of the younger rhyolites is not clear: they are compositionally distinct from the rhyodacites but may be related to them as late-stage differentiates. At present, it is not possible to evaluate whether the tectonic setting and magmatic affinities are regionally or locally controlled.The geological history is very similar to that of Lower Silurian rocks immediately north of the Antigonish Highlands at Arisaig. In the simplest sense, this indicates these areas may have been juxtaposed prior to the Late Ordovician and limits cumulative post-Silurian movement on the boundary (Hollow) fault to about 40 km.

Extent and bathymetry of North American Platform Seas in the Early Silurian

The North American craton is 25% covered by Lower Silurian (Llandovery) rocks based strictly upon the geographic area of surviving marine strata preserved in outcrop and the subsurface. This figure does not take into account extensive post‐Silurian tectonics and erosion. Isolated structural basins in which these strata are found today were not necessarily defined as corresponding topographic basins in the Silurian. Analysis of interbasin structural arches and outliers provides important clues as to the actual extent of Early Silurian seas. Information on the age of tectonic origin for 15 prominent arches is reviewed. The placement and significance of 11 key outliers is also discussed. Evidence of this kind indicates Early Silurian platform seas were among the most extensive of the Paleozoic. Probably more than 65% of the North American craton was then flooded. Nothing on so vast a scale serves as a recent guide to the past. The Pliocene‐Holocene Bahamian Banks do provide, however, worthy models for the role of bathymetry and water circulation as related to the changeover from laterally monotonous blanket deposits to a more complex mosaic of facies. The original bathymetry of Early Silurian environments may be determined, correlated, and mapped on the basis of widespread carbonate cycles linked to sea level fluctuations. The range of fully integrated facies patterns includes (1) coral‐stromatoporoid/stromatolite/paleosol cycles (Williston Basin), (2) pentamerid brachiopod/coral‐stromatoporoid/stromatolite cycles (Michigan Basin), and (3) stricklandiid brachiopod/pentamerid brachiopod/coral‐stromatoporoid cycles (East Iowa Basin). With shallow bathymetries of 0–30 m the Williston class tends to represent mosaic facies, while with bathymetries of 0–60 m and 10–90 m the deeper‐water Michigan and Iowa classes typically involve blanket deposits. Bathymetry of other basin deposits generally fits one of these three classes. Maximum and minimum stands in sea level during Late Llandovery time are represented by bathymetric maps.

The heliolitid coralAcidolitesin Ordovician–Silurian rocks of eastern Canada

AcidolitesLang, Smith and Thomas occurs in upper Middle and Upper Ordovician, Lower and lower Middle Silurian rocks of Ontario and Quebec. On Anticosti Island, Quebec, the genus is represented byA. tenuis(Billings) in the Upper Ordovician (Gamachian) Ellis Bay Formation; the new speciesA. arctatus, A. compactusandA. helianthusin the Ordovician–Silurian boundary beds at the top of the Ellis Bay Formation; the new speciesA. arctatus, A. compactusandA. lindströmiin the lower Llandoverian Becscie Formation;A. arctatusin the mid-Llandoverian Gun River Formation; and an unnamed species in the upper Llandoverian Jupiter Formation. The lower Llandoverian Clemville Formation of the Gaspé Peninsula, Quebec, containsProtaraea clemvillensisParks, now considered to beAcidolites. The upper Middle to lower Upper Ordovician Cobourg Formation near Ottawa, Ontario, containsA. cf.arctatus, formerly included inProtaraea vetusta(Hall). The lower Wenlockian Amabel Formation in southern Ontario contains a species ofAcidolitesas yet unnamed.