Dissertations / Theses on the topic 'Early Ordovician'

The Early Ordovician, Upper Knox Group consists of meter-scale shallowing-upward cycles that were deposited on a low-sloping ramp. Cycles formed in response to short term (<100 k.y.) eustatic sea-level fluctuations and typically have well developed tidal flat caps. Cycles are bundled into five transgressive-regressive sequences which correspond to third order (1-10 m.y.) sea-level fluctuations defined by Fischer plots. The Upper Knox Group is 90% dolomite of which greater than 75% predates Middle Ordovician, Knox Unconformity development. Early dolomitization occurred penecontemporaneously with tidal flat progradation during fifth-order (up to 100 k.y.) sea-level falls as indicated by: abundant dolomite in cycles with well-developed tidal flat caps and scarce dolomite in cycles with no or thin laminite caps; decrease in dolomite abundance with distance below tidal flat caps; dolomitized cycles decrease basinward; and dolomite clasts veneer cycle tops and the Knox Unconformity surface. Third-order sea-level fluctuations also strongly controlled early dolomitization as indicated by Fischer plots; limestone, subtidal-dominated cycles correspond to third-order sea level rises and completely dolomitized, peritidal-dominated cycles correspond to third-order sea level falls. "Early" dolomite was metastable and its geochemical composition was modified during initial stabilization by marine brines during progradation of each cycle, and by mixed fresh/marine waters of the Knox aquifer associated with unconformity development. Much "early" dolomite however, remained metastable into the deep burial environment where it was replaced and overgrown by burial fluids as suggested by: covariant trends between crystal size, mole % CaCO₃, Sr²⁺, Mn²⁺ and δ¹⁸O; similar regional trends defined by stable isotope values of "early" dolomites and burial dolomites; and water-rock modeling of trace element and stable isotopic trends. Trace element and stable isotope compositions of least-altered "early" dolomite however, record a memory of a precursor evaporative dolomite. Cathodoluminescent dolomite stratigraphy defines five generations of burial dolomite that can be correlated over 100,000 km². Burial dolomites postdate a regional dissolution event attributed to migration of organic acid-rich fluids through the Knox carbonates. Regional dolomitization occurred coeval with Late Paleozoic deformation and was closely associated with MVT mineralization and hydrocarbon migration. The δ¹⁸O values and trace element contents of burial dolomites in conjunction with fluid inclusions, suggest that burial fluids were warm (135 to 200°C), saline (13 to 22 wt. % NaCl equiv.), ¹⁸O-enriched (+2 to +9 % SMOW) fluids with geochemical compositions similar to present day basinal brines. Mn²⁺ and Fe²⁺ contents of the dolomites suggest a redox control over Mn and Fe fluid chemistry, and in conjunction with regional δ¹³C trends, likely record precipitation from organic acid-rich fluids. Regional trace element and δ¹⁸O trends record a basinal fluid source and regional northwestward flow. Stable isotope values of burial dolomites and fluid inclusions from dolomites and associated minerals, define a prograderetrograde sequence that formed during basinwide, gravity-driven fluid flow which developed in response to Late Paleozoic thrusting and uplift.
Ph. D.

During the Early Palaeozoic, the southern British Isles were part of a microcontinent Eastern Avalonia. In the early Ordovician Eastern Avalonia was close to the continent of Gondwana. in high southern latitudes. Northward drift of Eastern Avalonia during the Ordovician brought it towards Baltica and Laurentia, resulting in continental collision in the Silurian. Three major tectonic events have been postulated in the earlier history of the microcontinent rifting from Gondwana, onset of subduction and subduction of the mid-ocean ridge. Evidence for such events may be sought in the contemporaneous sedimentary basins: their type and geometry and the depositional environment and composition of their sediment fill. The Skiddaw Group (Tremadoc to Llanvirn) of Cumbria. comprises turbidites. debrites and a major olistostrome. Sediment was sourced from an orogenic terrain with noncoeval continental volcanic arcs, unroofed plutons, metamorphic basement and sedimentary cover. An additional source of quartzose sediment (e.g. a sandy shelf or delta) was present Two periods of submarine fan development display the following facies associations: depositional lobe, lobe fringe and interlobe, distributary channel, and proximal channel-levee. The first spans the Tremadoc-Arenig boundary and channels distributed sediment to both east and west Axial flow along a trough orientated approximately east-west is inferred. Contemporaneous depositional lobe fades association in the Isle of Man could represent a more distal portion of this system. The Manx Group displays sediment of similar facies and compositions to the Skiddaw Group. Turbidity current flow on a submarine fan of Arenig age was strongly influenced by sea floor topography. A palaeocurrent interpretation is presented which uses the theory of oblique reflection of turbidity currents (Kneller, Edwards, McCaffrey and Moore, 1991). The angular relationship between sole mark current directions and ripple current directions is used to infer the orientation of intra-basinal topographical features. With additional evidence from thickness variations of sandstone bodies and the orientation of pre-mid Ordovician faults, a tectonic model of syn-depositional extensional faults trending northnorthwest, with fault blocks tilted to the northeast is proposed. The controls of fan development are discussed. Gravitational deformation in the Llanvirn is expressed as slump folds in partially lithified sediment, debrites and an olistostrome with sandstone rafts. Slump fold orientations are used to demonstrate the development of a regional westerly palaeoslope which preceded uplift of the depositional basin prior to subaerial volcanism of the Eycott and Borrowdale volcanic groups. The undated Ingleton Group is described and contrasted with the Skiddaw and Manx Groups. A lithological correlation with Arenig sediments recovered from the Beckermonds Scar borehole is supported, but a continental margin volcanic arc provenance and deposition as a coarse grained lobe of a submarine fan suggests the dispersal system was separate from that of the Skiddaw Group. Lithological comparison is made with contemporaneous strata of southeast Ireland. With consideration of early Ordovician geology across the Southern British Isles, a tectonic model is proposed which incorporates transtensional and transpressional phases of oblique sinistral slip in the overriding plate above a subduction zone of general southerly dip. The present day active margin of Chile is used as an analogue. The long and complex history of Eastern Avalonia's northern margin is analysed to identify the three tectonic events mentioned above.

Lower-Middle Ordovician (Arenig) successions in the East Baltic have been investigated for more than one hundred and fifty years. Nevertheless detailed sampling still yields new species and better knowledge of the environment in which these organisms lived. The successions are well suited for bed by bed sampling because of the lack of tectonic disturbance and because the sequences are well documented.

This study analyses collections of Billingen-Volkhov age mainly from the St. Petersburg region, but also from Estonia. A great deal of the material was obtained from the marly to clayey, soft sediment that intercalates the compact packstones and wackestones in the succession. Twenty-nine of these clay horizons were used for diversity estimates on the fauna through the succession. The most thoroughly investigated groups for this investigation were rhynchonelliformean brachiopods, conodonts and ostracodes. The results indicate that variances in diversity and abundance levels for these groups were not correlated, either to each other or to the small-scale sea level fluctuations that have been suggested for the region. However, diversity dynamics of brachiopods and ostracodes confirm the large-scale upward shallowing of the basin into the Upper Volkhov. Comparison with fossils from the limestones did not reveal any differences in faunal composition between the two preservation modes.

The detailed sampling, coupled with sampling of the recently described mud mounds that occur in several outcrops, yielded large numbers of specimens. This enabled revision of earlier poorly known rhynchonelliformean genera such as Ujukella Andreev, as well as better known genera such as Porambonites Pander. In total the examined faunas include 31 genera assigned to 53 species of rhynchonelliformean brachiopods. Of these Leoniorthis and Eoporambonites are defined as new genera, and the following new species are described: Neumania paucicostata, Ranorthis rotunda, Orthidium gambolovensis, Orthidium lavensis, Skenidioides minutus, Tetralobula peregrina, Idiostrophia prima and Idiostrophia tenuicostata.

Textural evidence, high carbonate concentration and septarian cracks in dolomite and calcite concretions and beds of the Middle Ordovician Cloridorme and Cow Head formations indicate early carbonate precipitation during very shallow burial in uncompacted or little compacted host-sediments. These carbonates started to form at a depth not exceeding 5 m from the seafloor, where porosities were as high as 90%, and continued to grow in the sediment column to subsurface depth of 150 meters. Concretions of the Silurian La Vieille Formation and Lower Ordovician Levis Formation started to form at somewhat greater depths of 50 and 350 meters, when porosities amounted to 65 and 25%, respectively, and continued to grow to depths of 350 and 600 meters, respectively.
Covariation between $ delta sp{13}$C and $ delta sp{18}$O values of these carbonates further supports early diagenetic concretion growth.
Variation of major and minor elements (i.e., Ca, Mg and Sr) observed along AAS traverses in the Cloridorme, Levis and Cow Head carbonates indicate that they grew in continually changing chemical environments.

The Cambrian – Lower Ordovician Potsdam Group is a mostly siliciclastic unit that provides important insight into the paleoenvironmental, geologic and tectonic history of Early Paleozoic Laurentia. Nevertheless, in spite of 178 years of study the Potsdam in the Ottawa Embayment and Quebec Basin remains poorly understood. Also poorly understood is how the Potsdam relates with coeval strata regionally. In this work six siliciclastic paleoenvironments are recognized: (a) braided fluvial, (b) ephemeral fluvial, (c) aeolian, (d) coastal sabkha, (e) tide-dominated marine and (f) open-coast tidal flat. Fluvial strata were examined in particular detail and interpreted to consist of two end-member kinds. Braided fluvial deposits are dominated by low-relief bars formed in wide, shallow channels; however where basement structures limited the lateral growth of channels, flows were deeper and bar deposits thicker and higher angle. In contrast, ephemeral fluvial strata are dominated by sheetflood splay sedimentation with rare preservation of scour-filling supercritical bedform strata – all later subjected to aeolian reworking. In the upper Potsdam, alternating ephemeral and braided fluvial strata provide a record of climate change, which, respectively, correlate with documented global cool (arid) and warm (humid) periods during the Late Cambrian and Early Ordovician. Three allounits are recognized in Potsdam strata, recording regional episodes of sedimentation and facilitating correlation with coeval strata throughout eastern North America. These correlations, aided with provenance data from detrital zircons, show that changes in the areal distribution of sediment supply, accommodation and deposition/erosion were principally controlled by episodic reactivation of the Neoproterozoic Ottawa graben, which then periodically modified the stratigraphic expression of the ongoing Sauk transgression. Specifically, episodes of tectonic reactivation occurred during late Early to Middle Cambrian (allounit 1), late Middle to early Late Cambrian (allounits 2 – 3 unconformity), and Earliest Ordovician (allounits 3 – 4 unconformity). The earliest episode is correlated to regional extension of southern Laurentia, whereas the latter two are linked to peri-Laurentian accretion events that triggered reactivation of the Ottawa graben via the Missisquoi oceanic fracture zone.

The geological history of the Early Palaeozoic in eastern Australia is not known precisely. The eastern margin of the outcropping Precambrian Craton 'Tasman Line' is poorly understood. The Thomson Orogen, which underlies much of eastern Queensland, lies to the east of the Tasman Line. Basement to the Tasman Orogenic Zone is poorly understood, but knowledge of this basement is critical to our understanding to the processes that formed the eastern margin of the Precambrian craton. The Lolworth-Ravenswood Province lies to the east of the Tasman Line in northeast Queensland. A study of basement terranes in the Lolworth-Ravenswood Province will therefore provide some insights as to the nature of crust beneath this area, and therefore to the basement to the Thomson Orogen. The Fat Hen Creek Complex comprises para-authchthonous bodies of granitoid within middle to upper amphibolite facies metamorphic rocks. Data contained herein demonstrate that the composition and geochemistry of the granitoid are compatible with the generation of the granitoid by partial anatexis of the metamorphic rocks that are part of the Cape River Metamorphics. Temperature and pressure of anatexis is determined to be between 800-850OC and 5-9kb. Under these conditions, experimental data indicate that meta-pelite and meta-greywacke will produce between 5-10% melt coexisting with biotite, cordierite, garnet and plagioclase. The mineralogy of the granitoid bodies in the Fat Hen Creek Complex is consistent with partial anatexis of meta-greywacke at these temperatures and pressures. 5-10% melt is generally insufficient to allow efficient separation of melt and restite. The granitoids of the Fat Hen Creek Complex are interpreted as being a closed system with melt generated during high-grade metamorphism not separating from the residium. U/Pb dating of zircon from the Fat Hen Creek Complex indicate two distinct periods of zircon growth. The older episode occurred during the Late Cambrian to Early Ordovician. A second episode is dated as Middle Ordovician. This younger age coincides with the onset of regional compression, and may be related to exhumation of a mid-crustal layer during thrusting. The Lolworth Batholith is one of three granite batholiths in the Lolworth-Ravenswood Province. It comprises mainly muscovite-biotite granite, with smaller areas of hornblende-biotite granite to granodiorite. Sills and dykes of muscovite and garnet-muscovite leucogranite extensively intrude both of these types. The hornblende-biotite granite to granodiorite is metaluminous, with petrographic and geochemical characteristics similar to the adjacent Ravenswood Batholith. U-Pb SHRIMP ages also overlap with those from the Ravenswood Batholith. ENd(tc) values of ~-3 suggest a significant crustal contribution in the magma. Zircon populations determined using the SHRIMP suggest some inheritance from a Neoproterozoic source. The two-mica granites make up over 80% of the batholith and show little variation throughout. Aluminium Saturation indices range dominantly from 1-1.1, in keeping with the muscovite-bearing nature of the granites. U-Pb ages are significantly younger than the hornblende-biotite granitoids. ENd(tc) is ~-10, suggesting a greater role for crustal material in these granites than in the hornblende-bearing varieties. Previously, these granites were interpreted as S-types, mainly on the basis of the presence of muscovite. Low Na/Ca and Na greater than K are both considered as indicators of source compositions and both are characteristic of a mafic igneous rather than a meta-sedimentary source. Anatexis of mafic igneous rocks at temperatures less than~1000OC are found experimentally to produce peraluminous melts similar to those which produced the two-mica granites. The third major rock-type in the Lolworth Batholith is muscovite leucogranite, which occurs as sills and dykes intruding older granites and basement. The age of the leucogranite was not determined, but it has sharp contacts with the two-mica granite suggesting that the latter had cooled prior to intrusion of the former. The leucogranite is strongly peraluminous and is deemed to have been derived from anatexis of a supra-crustal (meta-sedimentary) source. The batholith is therefore deemed to comprise three different elements. The hornblende-biotite granitoids are the western extension of the adjacent Ravenswood Batholith. The two-mica granite and muscovite leucogranite are derived from different sources, but may be part of the same crustal anatexis event. During the Early Palaeozoic, the Lolworth-Ravenswood Province saw the intrusion of three granite batholiths into a basement of Late Neoproterozoic to Cambrian meta-sedimentary rocks. Also, Late Cambrian to Early Ordovician and Middle Ordovician high-grade metamorphism accompanied by partial anatexis is recorded at several sites across northeast Queensland. Although this metamorphism is restricted to these sites, they are widespread across the area suggestive of a widespread metamorphic event at these times. Similar metamorphism is recorded in the Arunta Inlier in Central Australia increasing the possible extent of this event. The geochemistry, isotopic characteristics and zircon populations of granites in the Lolworth-Ravenswood Province are used to characterise their source rocks; and thus the basement to the Province. Precambrian basement is indicated to underlie the entire province. However, the source rocks for the eastern part of the Province (Ravenswood and into the Lolworth Batholiths) are different to source rocks for the western part of the Province. Georgetown-type crust extends eastwards from the outcropping area, extending under the western Lolworth-Ravenswood Province. Late Mesoproterozoic rocks are recorded from the Cape River area adjacent to the Lolworth Batholith. They are also indicated as source-rocks for granites in the Ravenswood Batholith. Rocks of this age are characteristic of Grenvillian-age mobile belts in the United States. Their presence in north Qeensland has implications for the breakup of Rodinia, the Mesoproterozoic-age super continent that broke up during the Neoproterozoic.

The geological history of the Early Palaeozoic in eastern Australia is not known precisely. The eastern margin of the outcropping Precambrian Craton 'Tasman Line' is poorly understood. The Thomson Orogen, which underlies much of eastern Queensland, lies to the east of the Tasman Line. Basement to the Tasman Orogenic Zone is poorly understood, but knowledge of this basement is critical to our understanding to the processes that formed the eastern margin of the Precambrian craton. The Lolworth-Ravenswood Province lies to the east of the Tasman Line in northeast Queensland. A study of basement terranes in the Lolworth-Ravenswood Province will therefore provide some insights as to the nature of crust beneath this area, and therefore to the basement to the Thomson Orogen. The Fat Hen Creek Complex comprises para-authchthonous bodies of granitoid within middle to upper amphibolite facies metamorphic rocks. Data contained herein demonstrate that the composition and geochemistry of the granitoid are compatible with the generation of the granitoid by partial anatexis of the metamorphic rocks that are part of the Cape River Metamorphics. Temperature and pressure of anatexis is determined to be between 800-850OC and 5-9kb. Under these conditions, experimental data indicate that meta-pelite and meta-greywacke will produce between 5-10% melt coexisting with biotite, cordierite, garnet and plagioclase. The mineralogy of the granitoid bodies in the Fat Hen Creek Complex is consistent with partial anatexis of meta-greywacke at these temperatures and pressures. 5-10% melt is generally insufficient to allow efficient separation of melt and restite. The granitoids of the Fat Hen Creek Complex are interpreted as being a closed system with melt generated during high-grade metamorphism not separating from the residium. U/Pb dating of zircon from the Fat Hen Creek Complex indicate two distinct periods of zircon growth. The older episode occurred during the Late Cambrian to Early Ordovician. A second episode is dated as Middle Ordovician. This younger age coincides with the onset of regional compression, and may be related to exhumation of a mid-crustal layer during thrusting. The Lolworth Batholith is one of three granite batholiths in the Lolworth-Ravenswood Province. It comprises mainly muscovite-biotite granite, with smaller areas of hornblende-biotite granite to granodiorite. Sills and dykes of muscovite and garnet-muscovite leucogranite extensively intrude both of these types. The hornblende-biotite granite to granodiorite is metaluminous, with petrographic and geochemical characteristics similar to the adjacent Ravenswood Batholith. U-Pb SHRIMP ages also overlap with those from the Ravenswood Batholith. ENd(tc) values of ~-3 suggest a significant crustal contribution in the magma. Zircon populations determined using the SHRIMP suggest some inheritance from a Neoproterozoic source. The two-mica granites make up over 80% of the batholith and show little variation throughout. Aluminium Saturation indices range dominantly from 1-1.1, in keeping with the muscovite-bearing nature of the granites. U-Pb ages are significantly younger than the hornblende-biotite granitoids. ENd(tc) is ~-10, suggesting a greater role for crustal material in these granites than in the hornblende-bearing varieties. Previously, these granites were interpreted as S-types, mainly on the basis of the presence of muscovite. Low Na/Ca and Na greater than K are both considered as indicators of source compositions and both are characteristic of a mafic igneous rather than a meta-sedimentary source. Anatexis of mafic igneous rocks at temperatures less than~1000OC are found experimentally to produce peraluminous melts similar to those which produced the two-mica granites. The third major rock-type in the Lolworth Batholith is muscovite leucogranite, which occurs as sills and dykes intruding older granites and basement. The age of the leucogranite was not determined, but it has sharp contacts with the two-mica granite suggesting that the latter had cooled prior to intrusion of the former. The leucogranite is strongly peraluminous and is deemed to have been derived from anatexis of a supra-crustal (meta-sedimentary) source. The batholith is therefore deemed to comprise three different elements. The hornblende-biotite granitoids are the western extension of the adjacent Ravenswood Batholith. The two-mica granite and muscovite leucogranite are derived from different sources, but may be part of the same crustal anatexis event. During the Early Palaeozoic, the Lolworth-Ravenswood Province saw the intrusion of three granite batholiths into a basement of Late Neoproterozoic to Cambrian meta-sedimentary rocks. Also, Late Cambrian to Early Ordovician and Middle Ordovician high-grade metamorphism accompanied by partial anatexis is recorded at several sites across northeast Queensland. Although this metamorphism is restricted to these sites, they are widespread across the area suggestive of a widespread metamorphic event at these times. Similar metamorphism is recorded in the Arunta Inlier in Central Australia increasing the possible extent of this event. The geochemistry, isotopic characteristics and zircon populations of granites in the Lolworth-Ravenswood Province are used to characterise their source rocks; and thus the basement to the Province. Precambrian basement is indicated to underlie the entire province. However, the source rocks for the eastern part of the Province (Ravenswood and into the Lolworth Batholiths) are different to source rocks for the western part of the Province. Georgetown-type crust extends eastwards from the outcropping area, extending under the western Lolworth-Ravenswood Province. Late Mesoproterozoic rocks are recorded from the Cape River area adjacent to the Lolworth Batholith. They are also indicated as source-rocks for granites in the Ravenswood Batholith. Rocks of this age are characteristic of Grenvillian-age mobile belts in the United States. Their presence in north Qeensland has implications for the breakup of Rodinia, the Mesoproterozoic-age super continent that broke up during the Neoproterozoic.

Thesis (M.S.)--State University of New York at Buffalo, 2008.
Title from PDF title page (viewed on Nov. 13, 2008) Available through UMI ProQuest Digital Dissertations. Thesis adviser: Maletz, Jorg Includes bibliographical references.

Here, I evaluate biospheric evolution during the Ordovician using high-resolution inorganic carbon and sulfur (carbonate-associated sulfate and pyrite) isotope profiles for Early Ordovician to early Late Ordovician strata from geographically distant sections in Western Newfoundland and the Argentine Precordillera. Additionally, I present new, high-resolution U-Pb ages for volcanic ash beds within strata of the Argentine Precordillera. Carbon isotope data record subdued variation that is typical of Early- to Middle Ordovician strata worldwide. By contrast, sulfur-isotopic compositions of carbonate-associated sulfate reveal a complex signal of short-term, rhythmic variation superimposed over a longer-term signal. This short-term, rhythmic variation occurs in all sections and appears to be unrelated to lithology or depositional environment, suggesting preservation of an oceanographic signal. I interpret this signal to reflect a combination of a marine sulfate reservoir that was likely much smaller than the modern, the persistence of a substantial deep-ocean hydrogen sulfide reservoir, and the episodic oxidation of a portion of the deep-ocean euxinic reservoir. Persistent euxinia likely resulted from decreased solubility of oxygen in warmer water and/or sluggish oceanic circulation during greenhouse conditions that reduced vertical ventilation. A dramatic change in the behavior of carbonate-associated sulfate and pyrite in the Middle Ordovician is interpreted to reflect a major oceanographic event that records the initial transition from Ordovician greenhouse to icehouse states. I suggest that the initiation of downwelling of increasingly cool, oxygen-rich surface water resulted in widespread oxidation of much of the deep ocean hydrogen sulfide reservoir and concomitant limitation of marine pyrite formation. It is unknown, however, why sea surface temperatures declined through the Early to Middle Ordovician. Explosive volcanism does not appear to be a primary climate driver, based on the timing of Argentinian K-bentonite formations relative to marine records of sea surface temperature, carbon and strontium isotopic composition. Rather, long-term positive feedback between organic carbon burial rates and productivity may have increased carbon dioxide drawdown, ultimately driving a gradual decrease in sea surface temperatures in the Early to Middle Ordovician.

Flat-lying Early and Middle Ordovician limestones exposed on the North margin of Estonia provide key insights into the early Paleozoic biosphere and climatic history of the Baltic Platform, and potentially offer a site for calibrating the duration of the proposed Moyero River Reversed Superchron. Past paleomagnetic analyses on these rocks have been focused primarily on determining paleomagnetic pole positions and have been hampered by relatively weak remanent magnetizations. We therefore applied techniques of the Rock and Paleomagnetic Instrument Development (RAPID) consortium using thin-walled, low-noise quartz glass sample holders on an automatic system to enhance magnetostratigraphic resolution. Our results, based on over 300 oriented core samples spanning the stratigraphic interval from the Volkhov stage, up through the Lasnamägi stage, confirm previous work isolating a stable characteristic magnetization of reversed polarity, and furthermore confirm the presence of an interval of magnetically Reversed polarity spanning an interval of at least 15 million year duration. In addition, we recognize a magnetic overprint of presumed Normal polarity held in antiferromagnetic phases, of presumed Permian age, based on the apparent polar wander path given by (Plado et al., 2010).