Late Middle Ordovician Research Articles

Lower Ordovician sediments in the Wagga Trough: Discovery of early Bendigonian graptolites near Eskdale, north east Victoria

Abstract An assemblage of Early Ordovician dichograptids has been discovered in a spotted slate from the Omeo Metamorphic Complex just north of Eskdale in northeast Victoria. Identifiable species present are Tetragraptus fruticosus (4 br.), T. acclinans, and Didymograptus cf. D. ensjoensis. Together these indicate an age corresponding to zone Bel of the early Bendigonian. They are therefore the first recorded fossils of pre‐Darriwilian age discovered east of the Howqua Zone in Victoria. The depositional history of the Wagga Trough is thus demonstrated to have commenced at least early in the Ordovician, rather than in the late Middle Ordovician as previously thought.

Borings in solitary rugose corals of the Selkirk Member, Red River Formation (late Middle or Upper Ordovician), southern Manitoba

Borings occur in solitary rugose corals from the Selkirk Member of the late Middle or Upper Ordovician Red River Formation in southern Manitoba. They are assigned to Dictyoporus garsonensis n. ichnosp., which was produced by algae, and Trypanites weisei Mägdefrau 1932, made by spionid polychaete annelids. Most, and possibly all, boring occurred while the host corals were alive and in life position. The location and relative abundance of borings support interpretations that unattached curved solitary corals lay with the convex cardinal side in the sediment and the concave counter side facing upward during life, whereas straight conical forms were oriented upright in the sediment. These ichnospecies suggest that host corals lived in very shallow marine environments.

Middle and Upper Ordovician Paleogeography of Region Bordering Transcontinental Arch: ABSTRACT

Following a period of Whiterockian erosion, Middle Ordovician seas transgressed into the North American continental interior, and a diachronous sheet of clastic sediments (St. Peter, McLish, Winnipeg) lapped onto the margins of the Transcontinental arch. The arch served as a major clastic source region during subsequent deposition (especially Decorah). Middle Ordovician clastic sediments adjacent to the Transcontinental arch and in the Williston basin were probably deposited in a humid climatic regime, and carbonate deposition predominated in an arid belt that included Wisconsin, southeastern Iowa, Missouri, and Illinois, where carbonate oolites and evaporites are noted. Clastic source areas on the Transcontinental arch were inundated, as the marine transgression continue , and a continuous sheet of carbonate sediments (Galena, Viola, Red River, Bighorn) spread across most of the continental interior by the close of the Edenian. Migration of North America across latitudinally-related climatic belts brought the Williston basin area into an arid climatic regime by middle-Late Ordovician as evidenced by the deposition of upper Red River evaporites. Widespread carbonate deposition was replaced by Dubuque-Maquoketa and Stony Mountain carbonate-clastic deposition as source regions emerged on the Transcontinental arch and Ozark uplift. The lower Maquoketa is conformable with the underlying Dubuque over most of the eastern Mid-Continent, and the joint occurrence of organic-rich shales and phosphorites is attributed to a low-oxygen, phosphate-rich water mass in the Maquoketa sea. The Ordovician closed with red clastic deposition, intermixed with carbonates and/or ironstones, adjacent to the arch. End_of_Article - Last_Page 805------------

Ordovician chitinous hydroids from Peary Land, eastern North Greenland

Three specimens of chitinous hydroids recovered from the Børglum River Formation (Middle-Late Ordovician) of central Peary Land, eastern North Greenland, are briefly described and illustrated by scanning e1ectron microscopy. In two specimens the entrance to the theca is greatly reduced by a well developed diaphragm which is absent in the third specimen.

Paleozoic Base Maps

This paper contains 50 maps which have been designed for use by the geologic community in preparing paleogeographic, biogeographic, climatologic, and tectonic reconstructions of the Paleozoic periods. Seven maps for each of seven Paleozoic intervals are included, plus a suture map showing the outlines of the Paleozoic continents in their present positions. The intervals chosen are the Late Cambrian (Franconian), Middle Ordovician (Llandeilian-earliest Caradocian), Middle Silurian (Wenlockian), Early Devonian (Emsian), Early Carboniferous (Visean), Late Carboniferous (Westphalian CD), and Late Permian (Kazanian). The paleomagnetic information used to orient the continents is given. For each interval, three types of maps are included, one locality map with place names labelled, four paleogeographic maps with our interpretation of the distribution of mountains, lowlands, shallow seas, and deep oceans, and two outline maps for those who prefer to make their own paleogeographic interpretations. Several projections are used-Mercator, Mollweide, and stereographic polar-to suit the various requirements of paleogeographic work.

Staurolite Zone Caradoc (Middle-Late Ordovician) Age, Old World Province Brachiopods from Penobscot Bay, Maine: Discussion

Staurolite Zone Caradoc (Middle-Late Ordovician) Age, Old World Province Brachiopods from Penobscot Bay, Maine: Discussion

Staurolite Zone Caradoc (Middle-Late Ordovician) Age, Old World Province Brachiopods from Penobscot Bay, Maine: Reply

Discovery of staurolite zone Caradoc age brachiopods in the summer of 1970 on the west side of Penobscot Bay, Maine, is the third occurrence in the world of staurolite or higher grade, identifiable fossils. These fossils are the first discovered from the lengthy belt of highly metamorphosed rocks belonging to the Penobscot Formation and will help to thoroughly revise the geology of a large region where age relations have previously been very uncertain. Age determination of rocks associated with the fossiliferous strata by the Rb-Sr whole rock method yields a 460 ± 10 m.y. isochron which is in good agreement with the paleontologic evidence. The brachiopods belong to the Old World Province and are of European rather than North American aspect, thus having great biogeographic as well as paleogeographic significance.

Staurolite Zone Caradoc (Middle-Late Ordovician) Age, Old World Province Brachiopods from Penobscot Bay, Maine

Discovery of staurolite zone Caradoc age brachiopods in the summer of 1970 on the west side of Penobscot Bay, Maine, is the third occurrence in the world of staurolite or higher grade, identifiable fossils. These fossils are the first discovered from the lengthy belt of highly metamorphosed rocks belonging to the Penobscot Formation and will help to thoroughly revise the geology of a large region where age relations have previously been very uncertain. Age determination of rocks associated with the fossiliferous strata by the Rb-Sr whole rock method yields a 460 ± 10 m.y. isochron which is in good agreement with the paleontologic evidence. The brachiopods belong to the Old World Province and are of European rather than North American aspect, thus having great biogeographic as well as paleogeographic significance.

Algae of genus Dimorphosiphon

The genus Dimorphosiphon, previously known only from the late Middle and early Upper Ordovician of Norway, is reported from the Upper Ordovician of East Kazakhstan and the North Balkhash area. Large collections were available for preparation and study in thin sections as well as etching with hydrochloric acid. A new species D. diadromum is established and differentiated from D. rectangulare HOeg on the basis of its thinner and longer thallus, occasional branching of the thallus, number and arrangement of central tubes, shape of the first order radial tubes and shape and orientation of the second order radial tubes. The age of the new species is Upper Ordovician. The genus Dimorphosiphon on the basis of this report is shown to have a very much wider geographic range and is evidently of stratigraphic importance, since it characterizes the boundary layers of the Middle and Upper Ordovician. --F. M. Hueber.

MINIMUM AGE OF MIDDLE ORDOVICIAN ROCKS IN SOUTHERN QUEBEC

K-Ar age determinations on muscovite from two granitic bodies provide a minimum date of 480 million years (m.y.) for the end of the late Middle Ordovician. The granitic bodies intruded ultramafic rocks that had intruded the Beauceville Group in the Thetford Mines-Black Lake area of southern Quebec. The Cambrian-Ordovician boundary may be dated at about 535 m.y., 35 m.y. earlier than Kulp9s (1961) estimate. It follows that either the estimated duration of the Ordovician (Kulp, 1961) should be lengthened from 75 m.y. to 110 m.y., or the Ordovician-Silurian boundary is also 35 m.y. earlier, that is, about 460 m.y. ago.

On the Magog, Quebec, graptolites

Restudy of graptolite assemblages from the classic Castle Brook locality near Magog reveals that 2 graptolite zones, those of Climacograptus bicornis and Orthograptus truncatus var. intermedius, are present there. These zones indicate a late Middle Ordovician (Wilderness and Trenton) age for the rocks comprising them. If the graptolite-bearing slates are included in the Beauceville Group, the lower part of that unit is, therefore, Wilderness and Trenton in age. The graptolite-bearing rocks near Magog and the Normanskill Formation may be correlated because the same 2 graptolite zones are present in both units. The Magog graptolite-bearing strata are thus not younger than the Normanskill Formation, as has long been thought, and the graptolite fauna of the Magog rocks does not belong in a zone distinct from those in the Normanskill Formation.

Depositional Environment of the Ordovician Succession in the Marathon Region, Texas

ABSTRACT Graptolites occur primarily in limestones in the Ordovician succession in the Marathon region. Study of thin sections of these limestones reveals that most of them were lime muds; however, oolitic and bioclastic limestones also bear graptolites. Integration of the thin section study of the several rock types occurring in the Ordovician sequence with analysis of the associated sedimentary features enables recognition of four major environments. During the Early Ordovician mud was accumulating in the region. Local sectors of it were frequently exposed for brief periods; also, for a short time, small patch reefs flourished. In the early part of the Middle Ordovician strong currents swept the area and calcium carbonate oolites and aggregates formed. Aligned biserial graptolites and the direction of dip of the cross-laminae indicate that the last current to pass over the region came from the southwest. Then a tidal mud flat developed during the latter part of the Middle Ordovician. It was occasionally swept by currents bearing pel gic organisms. Aligned biserial graptolites again indicate that the direction from which the current came was southwest. Finally during latest Middle Ordovician and all of the Late Ordovician the region was predominantly accumulating lime mud under reducing conditions.