Lower Ordovician Sequence Research Articles

Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco

Echinoderms are one of the major components of benthic faunas in the Lower Ordovician sequence near Zagora, central Anti-Atlas, Morocco. The Fezouata Shale (Tremadocian–late Floian) has yielded numerous, exquisitely preserved echinoderm assemblages, ranging through several stratigraphic levels and palaeoenvironmental conditions. These associations offer a unique opportunity to document both evolutionary and palaeoecological aspects of echinoderm diversification in high-latitude, siliciclastic-dominated western Gondwana sediments, where rapid in situ burials facilitated excellent faunal census conditions. Lower shoreface deposits of the Fezouata Shale provide the most complete record of successive echinoderm faunas. In late Tremadocian times, these relatively shallow shelf deposits show the progressive replacement of low-diversity, opportunistic, Cambrian-like, dwarfed communities dominated by cornute stylophorans, in unhospitable, dysoxic environmental conditions by higher diversity benthic assemblages, more typical of the Great Ordovician Biodiversification Event and dominated by blastozoans, on well-oxygenated sea-floors. The turnover of the Ordovician radiation was apparently slightly delayed in more proximal settings. Eocrinoid meadows persisted in shallower environmental conditions up to the middle Floian. In the late Floian, they were replaced by diploporite-dominated communities, typical of later Ordovician high-latitude peri-Gondwanan faunas. From a palaeobiogeographic point of view, low-diversity assemblages display relatively strong affinities with cosmopolitan late Cambrian echinoderm faunas, whereas high-diversity communities are dominated by peri-Gondwanan taxa.

Carbon-isotope stratigraphy of the Lower Ordovician succession in Northeast Greenland: Implications for correlations with St. George Group in western Newfoundland (Canada) and beyond

The Lower Ordovician sequence on the Ella Ø Island in Northeast (N-E) Greenland is a thick shallow marine platform carbonate sequence (∼ 1415 m thick) and constitutes the major part of the Kong Oscar Fjord Group. It consists, from bottom to top, of the Antiklinalbugt, Septembersø, and Cape Weber formations, which are believed to be respectively coeval with the Watts Bight, Boat Harbour, and Catoche formations of the St. George Group in western Newfoundland, Canada. Samples were collected from outcrops at high-resolution intervals and micritic materials were extracted by microdrilling after screening their petrographic and geochemical criteria to evaluate the degree of preservation. The δ 13C and δ 18O values of well preserved micrite microsamples range from −5.2‰ to 0.5‰ (VPDB) and from −10.3‰ to−6.5‰ (VPDB), respectively. The δ 13C carb profile of the sequence reveals few negative shifts, which vary between ∼ 2 and 4.7‰ and are associated with unconformities/disconformities, thus reflecting the effect of significant sea-level changes. The δ 13C shifts can be correlated with counterparts on the St. George Group and also on the global Lower Ordovician δ 13C profiles around the early Tremadoc (∼ 2.3‰) and late Tremadoc–early Arenig (∼ 4.7‰).

Lower Ordovician Sequences of the Ebeta Antiform, the Southern Urals

Lower Ordovician sequences of the Ebeta antiform, a southern extension of the Uraltau zone, were deposited at the conjugation of paleocontinental and paleoceanic sectors of the southern Urals. Four types of sections were formed on opposite sides of cordillera that existed at the margin of the East European paleocontinent on the Preordovician volcanic belt. Sections of the first three types made up the western apron on the uplift that served as a provenance. Lateral and vertical relationships of various sedimentary associations, as well as their variable (in space and time) facies patterns and sedimentation conditions are considered. An important role of redeposition, slumping, and faulting in the apron development has been revealed. The apron fringed the eastern wall of the Sakmara marginal riftogenic basin that arose at the Cambrian-Ordovician boundary. On another side of the marginal uplift in the east, a slightly modified perioceanic environment existed in the Early Ordovician. Sections of another type formed here at the periphery of Uralian paleocean. These sections are characterized by the universal occurrence of ophiolithoclastic olistostrome with fragments of an older oceanic crust.

The Hunneberg Stage (Ordovician) in the area east of St. Petersburg, north-western Russia

The bio- and lithostratigraphy of the poorly known early Ordovician (Arenig) Hunneberg Stage is described from the area east of St. Petersburg (Putilovo Quarry and Lava river), where the Lakity Beds (Leetse Fm.) consist of glauconitic sands and silty clays. The described fauna includes graptolites, conodonts, brachiopods, acritarchs, foraminifers, and several problematic groups. The Lakity Beds are unique within the lower Ordovician sequence of the East Baltic in the abundance of graptolites, co-occurring with conodonts. The conodonts from the lower part of the Lakity Beds belong to the uppermost subzone (Oelandodus elongatus — Acodus deltatus deltatus) of theParoistodus proteus Zone, coinciding with the lower part of theTetragraptus phyllograptoides Zone. This suggests that there is a hiatus between the Lakity Beds and the underlying Nazya Formation. The lower Lakity Beds also contain the brachiopodsLeptembolon lingulaeformis andEosiphonotreta cf.acrotretomorpha, which suggest that they may be correlated with the Joa Member of the Leetse Formation in North Estonia. In the upper part of the Lakity Beds, conodonts of thePrioniodus elegans Zone co-occur with graptolites indicating the upper subzone of theTetragraptus phyllograptoides Zone in the Scandinavian sections. There are still problems of defining and correlating the base of the Billingen Regional Stage, which are outside the scope of this paper.

Alteration of magnetic properties of Palaeozoic platform carbonate rocks during burial diagenesis (Lower Ordovician sequence, Texas, USA)

Abstract Palaeomagnetic and sedimentological investigations of samples from two sections of correlative Iapetan platform carbonate rocks from Texas, USA, were made to test whether their magnetic properties reflect diagenetic alteration associated with regional and local tectonism. The Honeycut Formation (Llano Uplift area, central Texas), in close proximity to the late Palaeozoic Ouachita orogenic belt, exhibits a distinct correlation between magnetization intensity, magnetization age (direction) and lithofacies. Mudstones preserve their weak primary Early Ordovician magnetization, whereas dolo-grainstones carry a strong Pennsylvanian magnetization residing in authigenic magnetite. Fluid migration associated with the Ouachita Orogeny has been focused in lithofacies with high permeability and caused dolomite recrystallization and pervasive remagnetization. Magnetization intensity trends covary with fluid/rock ratios. However, aquitards were either not affected or less affected by these fluids. Unlike the Honeycut Formation, permeable rocks of the El Paso Group (Franklin Mountains, west Texas) carry only a non-pervasive Pennsylvanian magnetization. Therefore, a larger percentage of El Paso Group samples retain a primary Early Ordovician signature. This area is further removed from the Ouachita front, and, thus, the influence by Pennsylvanian orogenic fluids was less pronounced.

The influence of dolomitization on the magnetic properties of Lower Palaeozoic carbonate rocks in Estonia

Abstract The low-field magnetic susceptibility and saturation isothermal remanent magnetization of Estonian Lower Palaeozoic carbonate core and outcrop samples were studied, together with their chemical composition. Dolomites of different age and genesis were found to be of different chemical composition and different magnetic properties. The dolomitization of the Lower and Middle Ordovician sequences is diagenetic and is represented by widespread dolomitized layers as well as dolomitization associated with tectonic disturbances. All studied dolomites have an increasing total iron content. In Upper Ordovician and Silurian dolomites this increase is not enough to change magnetic properties significantly. In the rocks, sampled from a fracture zone and from a dolomite layer of the Väo Formation in the Middle Ordovician sequence and another widespread, several metres thick dolomite layer of the Volkhov stage from the Lower Ordovician sequence, the magnetic susceptibility of dolomites was found to be several times higher than that of limestones of the same age. Changes in magnetic susceptibility were found to be associated with substitution of Fe 2+ for Mg 2+ in the crystalline lattice of dolomites.

A Lower Ordovician Carbonate Contourite Drift on the Margin of the South China Paleocontinent at Jiuxi, Northern Hunan

Abstract Carbonate contourite drift at Jiuxi, Taoyuan, northern Hunan, was developed in a deep‐water area of northern Hunan on the margin of the Early Ordovician South China paleocontinent. The Lower Ordovician sequence in the area is more than 350 m thick and contains well‐developed contourites that can be grouped into the following five types: the calcilutitic, the arenitic, the siltitic, the fine ruditic and the bioclastic. The first three often constitute a complete or incomplete contourite succession. The arenitic contourite is nearly uniformly distributed as interlayers throughout the succession, creating a monotonously rhythmic texture in the contourite drift. The pattern of spatial distribution of the succession shows that the contourite drift is a huge ridge‐like sedimentary body extending along the trend of paleoslope. Numerous marks of flow direction have pointed to an eastward paleoflow direction along the slope.

Tectonic history of the Lunksoos Composite Terrane in the Maine Appalachians

Several Ordovician volcanic terranes are preserved between the Paleozoic North American margin and the Avalon Terrane in the northern Appalachians of Maine. They have been interpreted as remnants of an island arc on the basis of their current position in the Appalachians or as ocean islands near the margin of Avalon on the basis of paleontologic data. A study of one of these terranes, the Lunksoos terrane in northern Maine, was undertaken through a combination of paleomagnetic and structural analyses in order to understand its tectonic evolution. Paleomagnetic study of the pre‐Caradoeian (pre‐Late Ordovician) Stacyville volcanics revealed three components of magnetization. The final magnetization removed (D=90°, I=36°, α95=11.8°) is interpreted to be primary and yields a paleolatitude of approximately 20°S; however, a Late Ordovician remagnetization cannot be excluded. The Stacyville volcanics now overlie a Cambrian and Lower Ordovician sequence (the Grand Pitch and Shin Brook formations) that is believed to have been deposited at the southern side of Iapetus, possibly near the margin of Avalon. During the Middle and Late Ordovieian, Avalon was at paleolatitude s higher than 40°S, while the volcanics were either formed or remagnetized at a paleolatitude of 20°S. The pre‐Caradocian Stacyville volcanics therefore could not have been deposited directly on the Grand Pitch‐Shin Brook sequence but must have been tectonically emplaced. The Rockabema Diorite which intrudes the entire Cambro‐Ordovician sequence constrains the timing of this emplacement to be pre‐late Early Silurian. These relationships indicate that the Lunksoos area is a composite terrane within the Maine Appalachians. The tectonic emplacement scenario is unnecessary if the Grand Pitch and Shin Brook formations were not located on the south side of Iapetus during the Ordovician. This alternative, however, poses conflicts with the interpretation of faunal affinities.

The lower and upper boundary of the Ordovician system of some selected regions (Celtiberia, Eastern Sierra Morena) in Spain Part I: The Lower Ordovician sequence of Celtiberia

The lower and upper boundary of the Ordovician system of some selected regions (Celtiberia, Eastern Sierra Morena) in Spain Part I: The Lower Ordovician sequence of Celtiberia

The Grampian Orogeny

Synopsis Review of existing plate tectonic theories of the Lower Palaeozoic development of the north and northwest British Isles leads to the conclusion that ridge-trench impingement theory can be used to integrate all known geological observations within one unified plate tectonic theory for the area. Consideration of the consequences of the progressive impingement of a ridge-transform system with a northwesterly-dipping subduction zone in Arenig time gives explanations for: (a) termination of Dalradian sedimentation in the Arenig; (b) initiation of Barrovian metamorphism at the same time; (c) development and history of the Girvan-Ballantrae complex; (d) initiation of the Highland Boundary Fault as a dextral transform fault in the Arenig; (e) its absence in western Scotland; (f) the postulated presence of a boundary fault southeast of Connemara and perhaps at Pomeroy; (g) the peak of metamorphism (Barrovian and Buchan types) in the Dalradian and associated basic intrusions in the Arenig in both Scotland and Connemara; (h) the development of the Moine Thrust (and related foreland folds and thrusts) also in the Arenig and (i) the origin of the South Mayo trough on oceanic crust. Related events explained by this model include the location of the Donegal batholith, the generally low-grade metamorphism of Donegal and southwest Scotland, the location of the Great Glen Fault and all known faunal province and Ordovician sedimentological data from the Midland Valley and Southern Uplands. It is proposed that this Lower Ordovician sequence of related events be termed the Grampian Orogeny, distinct from the later Caledonian Orogeny, which is tectonically and geographically separate.

Brecciation, alteration and mineralization in the Central Tennessee zinc district

Major zinc deposits have recently been discovered in stratabound breccias in the Mascot Dolomite of the upper Knox Group in Central Tennessee and southern Kentucky. The Mascot Dolomite is a Lower Ordovician sequence of very finely crystalline dolostone units with interbedded limestones and coarsely crystalline dolomitized equivalents of these limestones. The breccias are similar to those of the well-known zinc deposits within the Knox Group of East Tennessee but with some distinct differences. The dominant minerals in Central Tennessee are reddish brown sphalerite, barite, fluorite, galena, and calcite. In East Tennessee pale yellow sphalerite and dolomite are the only major minerals cementing the breccias. Sulfide mineral deposition in Central Tennessee was significantly later than breccia formation and may have been as late as late Paleozoic. In East Tennessee sulfide mineralization took place in temporal proximity to limestone dissolution and dolomitization and occurred during the early Middle Ordovician.In Central Tennessee sulfide-depositing solutions were in equilibrium with calcite and could not have been responsible for dissolution and subsequent collapse of overlying dolostone beds to form ore-hosting breccias. Stratigraphic relationships suggest that secondary dolomitization and dissolution were related processes and that the dominance of limestone dissolution in preference to volume-for-volume dolomitization controlled the development of the breccia systems. Precipitation of coarse crystals of ore and gangue minerals may have resulted from slow mixing of separate fluids containing reduced sulfur and metals or by gradual reduction of sulfate present in a metal-bearing brine.

Lower Ordovician sequence and conodonts, Mount Patriarch, North-West Nelson, New Zealand

Abstract Limestone beds forming the upper part of Mount Patriarch, 1687 m, are named the Summit Limestone and are described from the section exposed on the northern ridge of the mountain. Trilobites from the basal beds and conodont faunas from the lower and upper parts of the limestone indicate that the age ranges from latest Tremadocian to early Arenigian. The development of carbonates in the Lower Ordovician of New Zealand is thus established for the first time; this contra;ts markedly with the only other known New Zealand rocks of this age—the graptolite-bearing slates and quartzites at Aorangi Mine and Preservation Inlet and turbidites of the Greenland Group at Reefton—and suggests a pattern of Lower Ordovician sedimentation in belts of contrasting facies. Twenty form-species of conodonts, from 6 samples, are described. They are grouped into two faunal assemblages, one correlated with the Drepanodus? gracilis-Scolopodus sexplicatus Zone of Australia, of basal Arenigian age, and the other with an undes...

SECTIONS THROUGH THE SKIDDAW–BORROWDALE UNCONFORMITY IN EASTERN LAKELAND

Summary Four sections are described through the controversial junction between the Skiddaw Group and the Borrowdale Volcanic Group within the Lower Ordovician sequence of eastern Lakeland. An angular unconformity between the groups is exposed in Matterdale Beck and at Causeway Foot. At Matter-dale, the underlying Skiddaw beds, yielding Lower Llanvirn graptolites, are interbedded with volcanics older than the Borrowdale volcanic sequence. Coarse Borrowdale conglomerates overlie the unconformity and are succeeded by andesites. At Low Rigg, the field relations suggest that the Threlkeld Micro-granite is pre-Borrowdale in age. The distribution of graptolite faunas within the Skiddaw outcrop supports the view of a regional unconformity, and suggests that a pre-Borrowdale anticline was developed similar in position and trend to the later Skiddaw Anticline.

Facies Variations in Ordovician Sediments of Southeast Ireland and their Paleogeographic Significance: ABSTRACT

A continuous section through the Ordovician belt in southeast Ireland is exposed in southwestern County Wexford. There an unconformity separates Lower Ordovician (mainly Arenigian) sediments from an Upper Ordovician (Caradocian) sequence. The thick Lower Ordovician sequence contains four facies, of which the most important is alternating siltstone and shale. Interfingered as minor facies are thick shale beds, paraconglomerate, and interbedded sandstone and shale. The siltstone and sandstone are interpreted as deposits from axial turbidity currents, with associated bottom-current activity, in an otherwise low-energy environment. Paleoslope data indicate that deposition was near the southeast flank (the Irish Sea landmass) of a northeast-trending basin. Only the paraconglomerates were derived from this border area; the dispersal pattern and petrology indicate a northeastern source for the other detrital sediments. Volcanic rocks (laterally equivalent to the more westerly Tramore limestones) are at the base of the Caradocian. An initial nearshore environment, progressively deepening, is probable for this sequence. The overlying sediments contain three facies; quantitatively, the most important is interbedded siltstone (turbidite) and shale. Paleocurrent directions indicate a southwestern source and deposition in a northeast-aligned basin. A close mineralogical similarity with the Arenigian sediments suggests that the Arenigian composed much of the source area. This facies shows an upward increase in shale, and is overlain by two facies of less importance--a pyritic siltstone and silty mudstone facies, and a black shale facies. This upward change implies diminishing supply of terrigenous material and a change in basin geometry to quiet, restricted, shallow depositional areas. These probably were near shore, because overlying rhyolite flows were extruded subaerially. End_of_Article - Last_Page 718------------