Ordovician Glaciation Research Articles

Graptolite biogeography: implications for palaeogeography and palaeoceanography

Abstract Two faunal regions, a cool-water, Atlantic and a tropical-water, Pacific, may be distinguished among Tremadoc into Ashgill planktic graptolite faunas. The zenith of graptolite provincialism was during the Arenig-Llanvirn when Laurentia, Australia-New Zealand, North China, South China, Siberia, Argentine Precordillera, and parts of Kazachkstan were provinces within the Pacific Region. Coeval, Southern Hemisphere, Atlantic Region provinces were: England-Wales, Baltoscania, Bolivia-Peru-Northern Argentina, western Europe, North Africa, and possibly part of modern Tien Shan. South China Arenig-Llanvirn faunas include many incursions of Atlantic Region taxa, probably reflective of current circulation changes linked to development of seasonal monsoons. Mid-Ordovician plate motion included northward movement of Baltoscania into the tropics. That motion resulted in Baltoscania becoming a province in the Pacific Region in the Late Ordovician. Late Ordovician glaciation led to restriction of graptolites to the tropics (Pacific Region) during the latest Ordovician. Deglaciation was followed by re-establishment of a tropical, Pacific Region fauna and a cool-water, Atlantic Region fauna during the Llandovery and Wenlock. Graptolite faunas appear to have been only tropical from the Ludlow to their extinction in the Pragian (Early Devonian). The graptolite regions and provinces are essentially consistent with plate positions suggested by palaeomagnetic and lithofacies data.

There are extinctions and extinctions: examples from the Lower Palaeozoic

The extinction events at the Cambrian-Ordovician and Ordovician-Silurian boundaries are compared and contrasted. A simple theoretical model shows that times of increased cladogenesis produce elevated rates of taxonomic pseudoextinctions, according to the recognition of paraphyletic groups. Taxonomists have traditionally placed stratigraphically early and morphologically primitive members of clades into paraphyletic groups. The Cambrian-Ordovician boundary coincided with such a period of cladogenesis. Extinctions occurred among shelf taxa: deeper-water taxa were mostly unaffected. The various explanations that have been proposed to explain Cambrian-Ordovician extinctions are evaluated. The Cambrian-Ordovician boundary event was probably similar to ‘biomere-type’ events that preceded it in the Cambrian and followed in the Ordovician. However, the rapid, but apparently staggered appearance of major new taxa at this time elevated taxonomic pseudoextinctions. In contrast, the Ordovician-Silurian extinction event terminated many major clades. An important ‘oceanic’ event (or events) profoundly affected outer- to off-shelf taxa (including plankton), some having had long and stable histories. The late Ordovician glaciation produced changes in shelf taxa, but changes in brachiopod faunal composition were spread over a long time compared with that for oceanic events. The likely role of anoxia in explaining deeper water end-Ordovician extinctions at the time of deglaciation is discussed.

Trace fossil associations related to facies of an upper Ordovician low wave energy shoreface and shelf, Oslo‐Asker district, Norway

Three trace fossil associations have been identified from facies interpreted as low wave energy upper, middle and lower shoreface deposits, transitional shoreface/shelf deposits and open epicontinental shelf deposits. These were developed in pre-regressive, regressive and transgressive phases associated with eustatic sea level changes caused by the upper Ordovician glaciation. The three associations are the Skolithos-Diplocraterion association, the Diplichnites-Phycodes association and the Thalassinoides association. The Skolithos-Diplocraterion association inhabited upper, middle and lower shoreface environments, the Diplichnites-Phycodes association a lowermost shoreface environment and the Thalassinoides association inhabited offshore epicontinental shelf environmcnts. The Skolithos-Diplocraferion and Diplichnites-Phycodes associations bear strong affinities with previously described Skolithos ichnofacies assemblages (indicating shoreline environments) and the Cruziana ichnofacies forms (indicating shallow water environments down to wave base) respectively. The Thalassinoides assemblage, however, closely compares with assemblages described from Mesozoic epicontinental shelf sequences. Trace fossil diversity achieves a maximum in the lowermost shoreface environment and reduces in deeper water and shallower water environments. This accords with diversity patterns identified in analogous Jurassic epicontinental palaeonvironments. The stratigraphic distribution of some trace fossil association boundaries accords with those of contemporary body fossil associations. However, several body fossil associations are included within the volume of distribution of a single trace fossil assemblage. □Trace fossil association, shoreface and shelf, Oslo district, upper Ordovician.

The lithostratigraphy of the upper Ordovician of central Portugal

A lithostratigraphic scheme for the upper Ordovician of central Portugal is described incorporating previously-erected local lithostratigraphic terms and new units. Thirteen formations (10 new), including 11 members (eight new), with one named bed, are arranged into four groups (three new). A description of each unit is given, together with comments on its age and correlation. Deposition in a storm-dominated, shallow-shelf environment through Llandeilo and Caradoc time is indicated. Differential subsidence occurred to the NE and SW of the Dornes/Maçâo ‘rise’ particularly during the Caradoc. Lower to mid-Ashgill sequences in the north are dominated by volcanic rocks with subordinate carbonate deposition. Farther south a hiatus preceded the regressive deposits associated with the onset of the upper Ordovician glaciation. Hirnantian glaciomarine deposits occur across most of the area, except for the regions of carbonate and volcanic build-ups, and appear to represent the period of waning of the glaciation. The Ordovician/Silurian boundary is difficult to place, but probably lies close to or within a sheet sand-body which underlies a condensed Llandovery sequence. Lithostratigraphic correlation of the study area with W. Brittany, France, is generally good; the relationship of some of the upper Ordovician units in central Portugal aids the interpretation of the relationships of the equivalent deposits in Brittany.

Upper Ordovician graptolites from the Cape Phillips Formation, Canadian Arctic Islands

Ashgill age graptolites have been collected from seven sections of the Cape Phillips Formation across most of its outcrop belt. The earliest graptolite zone recognisable is that of Orthograptus fastigatus. It is correla­ted with the Dicellograptus ornatus · Zone of the northern Canadian Cordilllera and the Dicellograptus complexus Subzone of the Dicel/ograptus anceps Zone of Great Britain although no dicellograptids have been found at any of the present sections. The overlying zone is that of Paraorthograptus pacificus, an ea­sily recognisable zone around much of the world. Graptolites of the C/imacograptus extraordinarius and Glyptograptus persculptus zones appear to be en­tirely absent from this formation. This is attributed to the Late Ordovician glaciation which has induced regression and submarine erosion in many areas worldwide. The earliest recognisable Silurian zone varies from section to section due to buried or barren intervals and/or hiatuses of varying length. The Parakidograptus acuminatus Zone has been recognised at only one section. At the others, the Atavograptus atavus, the Lagarograptus acinaces-Coronograptus gregarius, the Monograptus convolutus or the Monograptus spiralis Zone (s.1.) are the earliest recognisable Silurian fau­nas. Relatively low fauna! diversities in the Ashgill and lowest Llandovery portion of the section and the to­tal lack of dicellograptids are interpreted to be due to relatively shallow water, outer shelf or carbonate ramp depositional environment.

The uppermost Ordovician (Hirnantian) trilobites of Girvan, SW Scotland with a review of coeval trilobite faunas

ABSTRACTThe uppermost Ordovician (Hirnantian) trilobite fauna of the High Mains Formation in the Craighead Inlier near Girvan comprises species of Flexicalymene, Achatella, Hemiarges, Isotelus and an indeterminate proetid. All are illustrated and the first three including Hemiarges extremus sp. nov. are described. This is one of the most diverse Hirnantian trilobite assemblages known. The beginning of the Hirnantian marked a profound global decrease in trilobite diversity and an overall increase in endemicity with many genera persisting only as local relicts. Many faunas include a dalmanitid (Mucronaspis or Dalmanitina) and the absence of this family suggests a shallow shelf environment for the High Mains fauna which comprises relicts of earlier lineages in the area. Some genera have yet to be discovered in the Hirnantian, but are recorded from both older and younger strata. Like many of the known Hirnantian taxa, they must have survived in localised faunas before becoming widespread again in the Silurian. Limited evidence suggests that the spread of cold/deep water trilobites, notably Mucronaspis / Dalmanitina, on to low latitude shelves preceded the major trilobite extinction at the Rawtheyan-Hirnantian boundary, but this would still be within the timescale suggested by other workers for a rapid onset of the late Ordovician glaciation and the consequent ecological disruption.

Glaciations, preservation of their sedimentary record and sea level changes — A discussion based on the Late Precambrian and Lower Palaeozoic sequence in Norway

Terrestrial glacial sediments have a low preservation potential in the sedimentary record. Tillites and other glacial sediments of pre-Quaternary age are often found to represent marginal facies in subsiding marine basins or continental rift environments. Continental rifting provides an ideal tectonic setting for both accumulation of ice and preservation of glacial sediments in the rapidly subsiding rift basins. Uplifted horsts along the rifts may be sites for mountain glaciations at relatively low paleolatitude. The late Precambrian sparagmite basin of South Norway may be an example of this. Major pre-Quaternary glaciations should be expected to be accompanied by eustatic sea level changes. Records of sea level changes in tectonically stable areas may provide some constrains on the timing and magnitude of ancient continental glaciations. The Upper Ordovician sequence in the Oslo Region presents a continuous shallow marine carbonate environment which is sensitive to sea level changes. The records here suggest that the Ordovician glaciations in West Africa were either very minor glaciations compared to the Quaternary glaciation causing less than 20 m of eustatic sea level changes, or that the glaciations occurred in periods marked by tectonic unconformities.

Influence of the Late Ordovician glaciation on basin configuration of the Yangtze Platform in China

The Late Ordovician paleogeographic changes in the Yangtze Platform indicate a widespread regression and consequent subaerial exposure probably caused by the ‘locking up’ of substantial amounts of marine waters during the Late Ordovician glaciation. The subsequent rapid paleogeographic changes on the Yangtze Platform during the earliest Silurian suggest a rapid melting of the ice. The relatively cool waters in the partially closed Yangtze basin during the latest Ordovician were replaced by open and warm waters during the earliest Silurian. Sedimentation rates and composition of faunal assemblages underwent marked changes during latest Ordovician and Early Silurian. Glaciation, Ordovician, Silurian, Paleogeography, sedimentary rate, faunal change.

Late Ordovician-Early Silurian Strata in Mid-Continent Area: ABSTRACT

Late Ordovician-Early Silurian strata extend from the Texas Panhandle across Oklahoma and Arkansas into eastern Missouri and Illinois. The sequence consists of upper and lower sections of organic-detrital limestone separated by calcareous shales (Sylvan, Cason, Maquoketa Shales); the limestones directly above the shales are commonly oolitic (Keel, Noix Formations). The carbonate rocks have a rich megafauna and microfauna, and the shales yield graptolites and chitinozoans. The Ordovician-Silurian boundary, as here defined, usually falls just above the oolite, but locally it occurs in the shale. The faunal and stratigraphic relations suggest continuous deposition through the oolitic beds, although there is substantial lateral gradation between shale and limestone. At most p aces, Late Ordovician (late Ashgillian) strata are overlain by Late Silurian (late Llandoverian) strata; earliest Silurian beds are poorly represented or absent. Physical evidence for an unconformity at this horizon is common, pointing to interruption in deposition accompanied, at least locally, by erosion. Late Ordovician glaciation reported from Africa and South America was possibly of sufficient magnitude to have caused an appreciable lowering of sea level; this could explain the absence of earliest Silurian strata. The Late Ordovician shales are almost devoid of benthic fossils, a condition that might have been caused by various factors including colder water; however, these shales are closely associated with organic-detrital limestones and oolites which bear a large, sessile and vag ant benthos, suggesting temperate to tropical waters. End_of_Article - Last_Page 410------------

The relation of Late Ordovician glaciation to the Ordovician‐Silurian changeover in North American brachiopod faunas

Sheehan, P. M.: The relation of Late Ordovician glaciation to the Ordovician-Silurian changeover in North American brachiopod faunas. The Ordovician-Silurian changeover of brachiopod faunas in North American epicontinental seas involved the abrupt extinction of endemic Late Ordovician stocks and subsequent repopulation of North American seas by Old World taxa. The Late Ordovician Gondwanaland glaciation may have lowered sea levels sufficiently to place severe stress on the widespread shallow marine faunas in North America, resulting in their eventual extinction. The Late Ordovician depositional history in North America is not well enough known to establish the presence of a latest Ordovician regression, but the earliest Silurian was an interval of off-lap in North America. Therefore, the glacial lowering of sea level is considered to be the most likely cause of the faunal changeover.

Possible Late Ordovician Glaciation of Nova Scotia

A thin, poorly stratified, polymictic diamictite at the the base of the White Rock Formation (perhaps Caradocian or younger) contains unsorted, clustered, faceted, and grooved clasts apparently dropped into a shaly or sandy lithotope. Ice probably transported these stones from a distant source. Other lithosomes of the formation are of almost every sedimentary/volcanic lithology, shape, and extent. Supermature orthoquartzite may be eolian or beach dune, or intertidal; immature quartzites are channel-fills, probably fluvioglacial; bouldery diamictite may be morainic or glaciomarine. Underlying and overlying conformable formations are thick, graptolitic black slate. White Rock contacts may mark glacio-eustatic lowering and rising of sea-level. Locally at the top of the Halifax Formation, small-scaled folding resulting in detailed and superficial angular discordance with the overlying White Rock. Formation may be ice-thrust phenomena.Both the gross stratigraphic succession and detail of lithosomes of the White Rock Formation are remarkably similar to those of the latest Ordovician (Ashgillian) of northwest Africa. There, this stage records extensive continental glaciation well-established by sedimentology, stratigraphy, paleomagnetism, and paleontology. Ordovician–Silurian glaciation of North America is unknown. If the White Rock Formation is glacio-genetic, then it is Ashgillian and correlative with the Dunn Point Volcanics of northeastern Nova Scotia. Moreover the underlying Halifax Formation could encompass almost all of the Ordovician, and the overlying Kentville perhaps most of the Silurian. Furthermore the hypothesis would be supported that Nova Scotia and the rest of southeastern Canada is a remnant of northwestern Africa.

Upper Ordovician Glaciation in Northwest Africa? Reply

Upper Ordovician Glaciation in Northwest Africa? Reply

Upper Ordovician Glaciation in Northwest Africa? Discussion

Faunal, paleomagnetic, and sedimentological arguments that claim to show the existence of a continental ice sheet in northwest Africa during the Ordovician and Silurian are questionable. Neither the fauna nor the relevant pole positions necessitate the presence of land ice. The existing supposedly glacial level is of uppermost Ordovician age, and its lithological and other features do not necessarily involve glacial deposition. It seems to be related to tectonic movements reflecting the Taconic orogeny. An origin by mass-flow, in harmony with the available evidence, is suggested.

La tillite de la Gara Assaba (bordure sedimentaire sud-orientale de l'Eglab, Sahara occidental)

Abstract Lithologic description, upper Ordovician glaciation, Algeria