Dissertations / Theses on the topic 'Lower Silurian'

The storm-dominated paleotropical carbonate succession exposed on Anticosti Island in Eastern Canada represents one of the most complete, thickest, and well-preserved successions in the world spanning the O/S Boundary. This study develops a new high resolution integrated lithostratigraphic, cyclostratigraphic, and chemostratigraphic framework for the upper Hirnantian to lower Telychian (Upper Ordovician to lower Silurian) succession on Anticosti, by examining ~450 m of strata from a recent stratigraphic drill core (Martin La Mer), supplemented by ~120 m of outcrop, all from the south-central part of the island. Four facies assemblages and three time-specific facies were identified in this succession and can be organized into three orders of superimposed transgressive-regressive cycles. New high resolution isotopic curves were produced by sampling well-preserved bulk micrite at a resolution of 0.5-1.0 m per sample; in total 443 samples were taken from core and 168 from outcrop, corresponding to the Ellis Bay, Becscie, Merrimack, Gun River, Menier, Jupiter and Chicotte formations. Four distinct positive carbon isotope excursions are recognized in the study interval; the upper Hirnantian (+5‰), Lower Aeronian (+2‰), Upper Aeronian (+6‰), and Valgu (+3.5‰) excursions. These δ13C excursions accompany lithology, and sea level changes and are likely driven by transitions between humid and arid climate states coupled with ocean changes. Multi-ordered δ18O trends are recognized to occur in association with δ13C trends; long-term and intermediate scale δ18O fluctuations are likely controlled by glacio-eustasy and Silurian climate fluctuations, while high-frequency fluctuations may record an astronomical forcing signal.

Anticosti Island, located in Eastern Canada, displays one of the most complete, best exposed, and most fossiliferous carbonate successions spanning the Ordovician-Silurian (O/S) Boundary in the World. This study develops a new high-resolution framework for the post End-Ordovician extinction strata (~ 260 m thick) exposed in coastal outcrops and recovered from a continuous drill core (La Loutre #1), both located in the western part of the island. In total, eight facies, all associated with a storm-dominated carbonate system, were recognized and organized into a multi-order depositional cycles. A new high resolution isotopic curve with more than 300 data points from well-preserved bulk micrite samples covers the late Hirnantian to Early Aeronian time interval and corresponds to the upper Ellis Bay, Becscie, Merrimack and lower Gun River formations. Two distinct positive carbon isotope excursions are present in the late Hirnantian part of the Ellis Bay Formation (+5‰) and in the lower Aeronian part of the Gun River Formation (+2‰). These positive isotopic carbon excursions provide a distinctive chemostratigraphic signature for regional and global correlations with other O/S sections. Like the Quaternary δ18O marine signal, our δ18O record is largely coupled with multi-order cyclic facies changes. This study demonstrates the importance of glacio-eustasy following the End-Ordovician glacial maxima as one of the primary factors controlling the stratigraphic architecture of paleotropical neritic carbonates during the Early Silurian.

Well-exposed lower Wenlock sections and borehole sequences, representing various facies along a shelf-basin transect in Wales and the Welsh Borderland, have been sampled for palynomorphs. Primary attention is paid to the type area in Shropshire, including stratotypes of both lower and upper boundaries of the Sheinwoodian Stage, with sampling as close as 10cm through the boundary horizons. The study has been extended into other sequences on the shelf to nearshore facies in the Bristol area and to basinal sections in North Wales. Total organic residues were recovered using quantitative processing techniques and absolute palynomorph abundances were determined. Both transmitted light and scanning electron microscopes were used to work on strew-mounted residues allowing detailed morphological study of the palynomorphs. Techniques were developed for allowing remounting of gold coated SEM cover slips, for transmitted light study and for permanent records. Taxonomic focus is on the acritarchs and the chitinozoans; forty-one acritarch genera and one hundred and seven species and eleven chitinozoan genera and twenty-eight species are systematically described. Ten acritarch species, one chitinozoan genus and three chitinozoan species are new. One genus and species of trilete spore is also systematically described. In addition scolecodonts, graptolite fragments, melanosclerites, chitinous hydroids and amorphous kerogen were recovered and their distribution noted. The exceptionally well-preserved assemblages recovered from the deeper water shelf sections (including the Eastnor Park and Lower Hill Farm boreholes and Whitwell Coppice section) contain 80 2000 acritarchs/g, and 10-60 chitinozoans/g, while the species diversity index (Fisher et al. 1943) for these sections varies between 0.35 and 30.2. The nearshore/shallow water sections (including Tortworth and Dolyhir) yield a well preserved palynomorph assemblage of low abundance (0.024 to 1.14 palynomorphs/g) and low species diversity (0.35 to 3.8). The poorly preserved assemblages of the basin (including the Pistyll Quarry section and the Llanrwst and Conway composite sections) contain 0 to 4.6 palynomorphs/g and species diversity varies between 0 and 4.8. Palynomorph absolute abundances and species diversity are compared and contrasted, both are considerably higher in the inner shelf and shelf sections than in the nearshore/shallow water and outer shelf and basinal sections. Distribution of the organic residues through the different sections is illustrated and discussed and the acritarchs and chitinozoans are used for biostratigraphical refinement. Taxa ranges and relative frequencies are illustrated by computer drafted figures for each section; graphical techniques are also used for correlation purposes as are summary logs of range data. In addition to vertical palynomorph distributional patterns through a studied section, palynomorph assemblage distributional patterns are also discussed and illustrated by graphical representations for the different palaeoenvironments represented by the shelf-basin transect. It is noted that the chitinozoans generally prefer deeper water; on the outer shelf the genera Ancyrochitina Eisenack 1955a and Cingulochitina Paris 1981 and in the basin the genera Sphaerochitina Eisenack 1955a and Conochitina Eisenack 1931 are dominant. With the acritarchs thin-walled leiospheres and shortspined MlchrystrId1um Deflandre 1937 emend. Staplin 1961 appear to have a preference for nearshore/shallow water environments. The acritarchs are most abundant and diverse on the shelf with the acanthomorphs being the dominant group. Basinal sections are dominated by small thick-walled leiospheres and relatively abundant short-spined fat-bodied Veryhachium Deunff ex Downie 1959. Marine and inshore indices adapted from Richardson & Rasul (1990) are also used to highlight assemblage contrasts over the shelf and basin. From the biostratigraphical results a new biozonational scheme for the early Wenlock is proposed, based on the recorded stratigraphical ranges of diagnostic taxa. Three existing acritarch biozones (the Deunffia brevispinosa, Deunffia furcata and Eisenackidium wenlockensis biozones) have their boundaries changed on new stratigraphical range information and one new zone, the Helosphaerldium malvernensis Biozone is proposed. Two new chitinozoan biozones, the Calplchitlna (Densichitina) densa and Clngulochitlna cingulata biozones are also proposed. The palynomorph biozones are related to the established graptolite biozones (see Bassett at al. 1975) in the Whitwell Coppice section and the Lower Hill Farm borehole in the Wenlock type area. The thermal maturity of the different sections is calculated by the use of the Acritarch Alteration Index (AAI) of Legall et al. 1981, which is a method of calibrating palaeotemperatures. For consistency in results only the acritarch genus Leiosphaeridia Eisenack 1958 emend Downie & Sarjeant 1963 was used. For the shelf sections the AAI is low and varies between 2 and 4 (indicating palaeotemperatures of 60-70⁰C ), in contrast the thermal maturity of the basinal sections is much higher, with an AAI of 5 showing palaeotemperatures in the range 90-460⁰C and probably towards the higher end of that range.

This thesis focuses on the development of the black shales belonging to the lower Silurian Formigoso Formation (Fm.) situated within the Cantabrian Fold Belt, Northern Spain. The geochemical data obtained by the use of an energy-dispersive Niton XL3t XRF analyser under controlled laboratory conditions is comparable to that of conventional wavelength dispersive XRF analysers. The in-situ field analysis resulted in similar geochemical signals and elemental concentrations to that of the laboratory analysed samples. The high-resolution geochemical database, consisting of 4148 readings of the Formigoso Fm. was used to characterise the shales of the Formigoso Fm. relative to a number of international shale standards. Following this geochemical characterisation, a number of palaeoenvironmental proxies for [1] anoxia reconstructions, [2] palaeosalinity, [3] palaeohumidity, [4] weathering indices, and finally [5] bioproductivity were utilised. The combined outcome of these proxies together with field-based sedimentological observations led to a detailed reconstruction of the environment that prevailed during the deposition of the Bernesga Mb. black shales. A generic model is developed for the formation of these organically enriched sediments. This model is subsequently compared to previously published Silurian, black ‘hot’ shale models. The major parameters controlling the development of the Cantabrian black shale deposits were mainly the presence of a pronounced palaeorelief, which was generated by a combination of tectonic pulses related to the widening of the Rheic Ocean to the north and glaciogenic processes of the Hirnantian ice masses to the south. Orbital parameters or ‘deep time cycles’, mainly Obliquity and Eccentricity in combination with tectonic pulses led to high resolution sea-level oscillations and consequent cyclic behaviour of the redox elements recorded within the ‘hot’ shale deposits of the southern Cantabrian Basin. Additionally, 209 measurements of 76 international and inter-laboratory standards were performed to formulate new equations for the precise calibration of major and trace elements.

The upper Llandovery succession across the Jupiter-Chicotte formational contact on Anticosti Island, Québec, allows us to study the sediment dynamics and stratigraphic architecture of a storm-dominated, carbonate ramp. The Anticosti paleotropical ramp was slowly subsiding and recording significant changes in sea level in a far field glacial setting during the early Telychian. Three facies associations, grouping nine facies, are recognized along the E-W outcrop belt, and from top to bottom as the: (FA-1) encrinitic carbonate facies, (FA-2) mixed siliciclastic and carbonate facies, and (FA-3) non-encrinitic carbonate facies. These mid to outer ramp sediments represent deposition mostly from episodic, high-energy storm events as evidenced by hummocky cross-stratification, large wave ripples, gutter casts, and wave-enhanced sediment-gravity flow deposits. Spatial and temporal changes in siliciclastic content imply basin margin depositional environments in the eastern sections and change in climate regime from arid to humid conditions. The Chicotte deposition marks a major faunal change with the domination of crinoids triggered by increasing siliciclastic supply, rapid sea level fluctuations and change in substrates. The recognition of one major transgressive-regressive (TR) sequence subdivided in distinct meter-scale cycles allows a high resolution E-W correlation. The development of the TR sequence and meter-scale cycles is driven by glacio-eustacy where the main sequence is 4th order (~400 Ky) with superimposed meter-scale cycles that are 5th and/or 6th order (~100 Ky). Nevertheless, erosional capping surfaces within the more proximal tempestites represent ancient rocky shorelines that developed during forced sea level falls. In order to explain this stratigraphic architecture, a carbonate open-ramp model is proposed with a concave-up profile and a narrow and steep inner ramp in equilibrium with a high-energy coastline.

This thesis documents and analyses the distribution of acritarchs, prasinophyte algae and palynofacies at the basal stratotype of the Ludlow Series (Silurian) at Pitch Coppice Quarry, Ludlow, Shropshire, and the nearby Goggin Road section. Ten new species are erected; Cymatiosphaera lawsonii, Cymatiosphaera multicrista, Cymatiosphaera paucimembrana, Cymatiosphaera triangula, Baltisphaeridium. spinatum, Buedingiisphaeridium reticulum, Glyptosphaera heltaskelta, Gorgonisphaeridium. listen, Leptobrachion digitatum and Pulvinosphaeridium dorningii; 31 new species are described in open nomenclature. The diagnoses of Psenotopus, Psenotopus chondrocheus, Cymatiosphaera octoplana, Dorsennidium europaeum, Dorsennidium rhomboidium, Eupoikilofusa filifera, Evittia robustispinosa, Pulvinosphaeridium oligoprojectum, Schismatosphaeridium rugulosum and Veryhachium trisphaeridium are emended. Six new combinations are proposed; Comasphaeridium brevispinosum, Dorsennidium polygonale, Dorsennidium wenlockianum, Evittia aculeata, Evittia almarada, Gorgonisphaeridium citrinum. The ranges of Buedingiisphaeridium pyramidale and Neoveryhachium mayhillense are extended, producing an overlap between biozones W3 and LI of Doming (1981a). Biozone L2 may, possibly, be recognised by G. listeri 78.4m above the series boundary (Middle Elton Formation). Zone 6 (Gorstian-Ludfordian stages) of Martin (1989) is identified by Percultisphaera stiphrospinata (+83.5m, Middle Elton Formation). Potentially important taxa for future biostratigraphic research are highlighted and these may be used to refine the existing biostratigraphic schemes. The palynofacies analysis undertaken identifies a Wenlock-Ludlow series boundary event correlating with the delta13C event of Corfield et al. (1992). Transgression induced reworking, concentrating 12C rich organic matter, is proposed as the cause. Between Pitch Coppice and Goggin Road this event is diachronous. Individual taxa, which occur through the section in ~80% of samples, are grouped by cluster analysis. These groups, and the distribution of rarer taxa, are related to the palynofacies analysis, and a series of events are proposed (fluctuations in sea-level, marine/terrestrial productivity, sedimentation rates, palaeoenvironment, palaeoceanography) These events are compared to eustatic and climatic/oceanic models.

Conodonts of late Llandovery age have been collected from the Anse a Pierre-Loiselle Formation, the deposition of which represents a deepening episode in the generally shallow deposition of the Chaleur Group in southern Gaspe. The fauna is dominated numerically by elements assigned to Panderodus, Oulodus?, and Ozarkodina with elements of Aulacognathus, Carniodus, Distomodus, Johnognathus, Kockelella, Pterospathodus, and Walliserodus as well as rate or indeterminate taxa. One genus is new, represented by one species: Digitodontus bellistriatus. Other new species include: Apsidognathus sulcatus, Pseudooneotodus pyramis and Pterospathodus? ceragnathoides. Based on these faunas the unit is considered to represent deposition of Llandovery C$\sb6$--early Wenlock age. Due to the low abundance of zonal indicator species Pterospathodus celloni and P. amorphognathoides, a new taxon range zone is established to encompass the total range of the new species P.? ceragnathoides which is locally abundant. The position of the Llandovery-Wenlock boundary remains unclear. (Abstract shortened by UMI.)

This study establishes the stratigraphic framework and conodont biostratigraphy of Lower Paleozoic strata of the Northern Canadian Cordilleran Miogeocline, which document a non-passive tectonic evolution of the rifted margin of Laurentia. Only a few reconnaissance stratigraphic studies have been conducted previously in the study area. Nine key sections span an east-west transect from the Macdonald Platform to the Kechika Trough (platform-miogeocline-basin) and 3 key sections comprise a transect across the parautochthonous Cassiar Terrane. Over 12 000 m of strata from the Kechika and Skoki formations and Road River Group in northeastern British Columbia were measured and described, from which a total of 405 conodont samples (4-5 kg each) were taken. A total of 39 526 conodonts have been used to refine the Upper Cambrian to Lower Silurian conodont biostratigraphy across the transect. The stratigraphy is revised to divide the Kechika Formation (late Cambrian to early Arenig in age) into 5 formal members: Lloyd George, Quentin, Grey Peak. Haworth and Mount Sheffield members. The Skoki Formation (early to late Arenig in age) comprises 3 new formal members defined as: Sikanni Chief. Keily and Redfern members. The Road River Group is divided into 3 new formations: Ospika (early Arenig to Llanvim in age), Pesika (Lower Silurian in age) and Kwadacha (formerly the Silurian Siltstone). The Ospika Formation is further subdivided into 5 formal members: Cloudmaker, Finlay Limestone, Chesterfield, Finbow Shale and Ware. Conodonts of Late Cambrian to Early Silurian age are described taxonomically from the Kechika, Skoki, Ospika and Pesika formations across the transect. A total of 39 526 identifiable conodonts recovered from 142 productive samples indicate high species diversity and abundance in shallow water facies and less diversity and abundance with in deeper water facies. Elements are moderately to well preserved, typically with a colour alteration index (CAl) of 3-5. A total of 197 species, representing 73 genera are identified and illustrated among which 6 new genera and 39 new species are described. Fifteen of the 39 new species had too little material and were described in open nomenclature. The new genera are Graciloconus, Kallidontus, Planusodus and 3 new genera (A, B, C) treated in open nomenclature. The new species are Acodus kechikaensis n. sp., A. quentinensis n. sp., A. warenesis n. sp., Cordylodus delicatus n. sp., Colaptoconus greypeakensis n. sp., ?Diaphorodus n. sp., Drepanoistodus minutus n. sp., Graciloconus concinnus n. gen. n. sp., Kallidontus serratus n. gen. n. sp., K. nodosus n. gen. n. sp., K. princeps n. gen. n. sp., Laurentoscandodus sinuosus n. sp., Macerodus cristatus n. sp., M lunatus n. sp., Microzarkodina n. sp., Oepikodus n. sp., Oistodus n. sp., Paroistodus n. sp., Planusodus gradus n. gen. n. sp., ?Prioniodus n. sp., Protoprioniodus n. sp., Rossodus kwadachaensis n. sp., R. muskwaensis n. sp., R. sheffieldensis n. sp.. R. subtilis n. sp., Scolopodus amplus n. sp., Striatodontus strigatus n. sp., Triangulodus akiensis n. sp., Tricostatus infundibulum n. sp., T. terilinguis n. sp., 3 unnamed new genera and 3 new species and 5 new species of Drepanoistodus (A, B. C, D, E). The conodont zonation for Upper Cambrian to Lower Silurian strata is refined, using Sections 4, 5, 13 and Grey Peak as reference sections. It allows close dating of stratigraphic boundaries. The oldest zones in the Kechika are cosmopolitan and include the Eoconodontus Zone (upper Cambrian), Cordylodus proavus and Cordylodus lindstromi zones (uppermost Cambrian) and lapetognathus Zone (base of Tremadoc). Ten higher zones are recognized and redefined for shallow water platform facies containing faunas of the Midcontinent Realm. Four of these are new (Polycostatus falsioneotensis, Rossodus tenuis, Scolopodus subrex and Acodus emanualensis zones) and 10 new subzones are established. Those for the Kechika Formation include, in ascending order, the Polycostatus falsioneotensis Zone (lower Tremadoc). Rossodus tenuis Zone (lower Tremadoc); Rossodus manitouensis Zone with R. muskwaenesis and R. sheffieldensis subzones (middle Tremadoc), Low diversity interval (upper Tremadoc), Scolopodus subrex Zone with Graciloconus concinnus and Colaptoconus bolites subzones (lower Arenig) and Acodus kechikaensis Zone with Kallidontus serratus. Diaphorodus russoi and Kallidontus nodosus subzones (lower Arenig). Those for the Skoki Formation include the Oepikodus communis Zone with Tropodus sweeti, Bergstroemognathus extensus and Juanognathus variabilis subzones (middle Arenig). The O. communis Zone spans the Kechika-Skoki boundary and the uppermost Kechika lies within the lowermost part of the O. communis zone underlying the T. sweeti Subzone. The Skoki Formation also contains the Jumudontus gananda Zone (middle Arenig) and Tripodus laevis Zone (upper Arenig). The Phragmodus undatus Zone (Upper Ordovician) lies within the Road River Group in the Cassiar Terrane. Thirteen deep water zones are recognized for basinal facies containing faunas of predominantly the North Atlantic Realm. Five new zones are established (Drepanoistodus nowlani, Acodus deltatus, Paracordylodus gracilis, Paroistodus horridus and Dzikodus tableheadensis zones) and one new subzone within the P. gracilis Zone is proposed. Those within the Kechika Formation include Cordylodus angulatus Zone (lower Tremadoc), Paltodus deltifer Zone (middle Tremadoc), Drepanoistodus nowlani Zone (middle Tremadoc), Acodus deltatus Zone, (middle Tremadoc), Paroistodus proteus Zone (upper Tremadoc), Paracordylodus gracilis Zone with Oelandodus elongatus Subzone (upper Tremadoc) and Prioniodus elegans Zone (base of Arenig). Those within the Skoki and Ospika formations include Oepikodus evae Zone (Skoki Formation, middle Arenig), Paroistodus originalis Zone (Skoki and Ospika formations, upper Arenig), Paroistodus horridus and Dzikodus tableheadensis zones (both within the Ospika Formation, lower Llanvim). The Amorphognathus tvaerensis Zone lies within the Road River of the Cassiar Terrane (Upper Ordovician). The Distomodus staurognathoides Zone lies within the Pesika Formation (middle Llandovery). The conodont faunas therefore provide detailed temporal constraints for the stratigraphic framework. Some evolutionary remarks are made for selected species involved in radiations, especially in the Tremadoc and Arenig, that are useful in further refining the standard Midcontinent Realm zonation. The Midcontinent Realm conodont faunas are used for regional correlations within North America and those of the Atlantic Realm provide calibration on an interregional scale, for example, with Baltica.
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Lower Silurian conodont biostratigraphy is poorly known and not well correlated with the parallel graptolite biozonation. The Cape Phillips Formation contains a continuous record of sedimentation, and exceptionally well preserved faunas in the unstable shelf region of the Franklinian miogeoclinal succession in the Canadian Arctic Islands and north-west Greenland and affords a rare opportunity to integrate the conodont and graptolite biozonation. Detailed biostratigraphic sampling was completed from the base of the Cape Phillips Formation (Upper Ordovician), over the Ordovician-Silurian boundary, and through the entire Lower Silurian (Llandovery and Wenlock) succession on Cornwallis Island in the Canadian Arctic Islands. A total of 180 4.5 kg, samples were collected, yielding 7,575 conodont elements, from which 59 species representing 29 genera were recognised. One new genus and species are established arom the lower Wenlock. Eleven conodont zones were recognized, 1 of which is new, and 3 of which are modified. Due to the multidisciplinary approach of the fieldwork, the conodont zones are integrated with the detailed graptolite biozonation over the entire interval.