Arenig Series Research Articles

Revision of the Oxford Formation (Arenig) of southeastern Ontario and northern New York State

The Oxford Dolomite of southeastern Ontario is considered to be equivalent to the Ogdensburg Dolomite of northern New York State. The Oxford Formation is redefined in terms of its lithology, distribution, thickness, sedimentary environment, and age.A series of representative sections can be correlated with wells drilled in southeastern Ontario. The formation rests disconformably upon the March Formation (Tremadocian) and is overlain disconformably by Middle Ordovician (Chazyan) sediments. The lower contact of the formation is widely exposed, but the upper contact is known only from boreholes.Lithologies are not distinctive; a lower division in which quartz sand is abundant and an upper division in which it is rare or absent can be recognized. The total thickness of the formation is not known; however, a minimum thickness of 605 ft (184.5 m) is present in a well near Williamsburg, Ontario. Conodont faunas indicate that the age of the Oxford is Lower Arenig and the disconformity at the base of the formation marks the base of the Arenig series in the Ottawa – St. Lawrence Lowlands.

Faunal Evidence bearing on the Age of Late Cambrian – Early Ordovician Metamorphism in Britain and Norway

IT can no longer be stated categorically that metamorphism of the Dalradian Series in Connemara occurred before the Didymograptus extensus zone of the Arenig Series, because those sediments generally accepted as the earliest post-tectonic sediments in the area have provided a Didymograptus bifidus zone (Lower Llanvirn) graptolite fauna. This means that the only stratigraphical younger age limit which can be placed on the Dalradian metamorphism in Connemara is some time plane at or near the base of the bifidus zone. This upward extension of the younger, minimum stratigraphical limit of the Dalradian metamorphism necessarily extends considerably that interval in Late Cambrian–Early Ordovician times during which an important phase of orogeny could have taken place. Furthermore, the statement by Leggo et al.1 that “the base of the Arenig would seem to be younger than 510 ± 35 m.y.”, on the evidence of the rubidium/strontium age given by the post-metamorphic and (hitherto) pre-Arenig Oughterard granite, should be amended to refer to the base of the bifidus zone.

Le Cambrien et la base de l'Ordovicien dans la partie orientale et meridionale du Tafilalt (Maroc)

Abstract A recent structural study of the Tafilalt area (Morocco), including a correlation study between the El-Aness and Ouzina sections, showed that the Cambrian units at the two localities are correlative and can be divided into two series--the Georgian series characterized by detrital material and arkosic beds, and an upper Acadian series which is thicker than the lower series. The lower Ordovician is characterized by a red sandstone horizon representing the upper Tremadoc transgression, and overlain by the lower Arenig series of green shales with graptolites. Between the Arenig and Tremadoc is a green glauconitic bed. It can be deduced that deposition decreased from south to north during Acadian and Georgian time. The seas did not reach the El-Aness massif during the Tremadoc but only during the lower Arenig.

The Volcanic Series of Trefgarn, Roch, and Ambleston, Pembrokeshire

The area dealt with in this communication and represented by the appended map (Pl. XL) is situated some 5 miles north of Haverfordwest, 7 miles south of Fishguard, and from 8 to 14 miles east of St. David's. About 25 square miles in extent, it lies immediately north of a district recently resurveyed by the officers of H.M. Geological Survey, and south and east of a tract of country surveyed by one of us (in collaboration with Prof. O. T. Jones) in 1912. It is occupied mainly by Arenig and associated Upper Cambrian rocks, of which the Arenig Series is of especial interest, since it contains two distinct sets of volcanic rocks—one of which immediately underlies the lowest visible Arenig sediments. In addition, an important series of rhyolitic lavas and ashes of uncertain age traverses the greater part of the district from Roch to Trefgarn. As is usual in Pembrokeshire, the general strike is in an east-north-easterly to west-south-westerly direction; but strike-faulting on an intensive scale has greatly disturbed the geological succession, and complicated the structure of the district. On the south, this area of Arenig and Upper Cambrian rocks is followed by extensive outcrops of the higher Ordovician beds of the Haverfordwest district, while its northern boundary is constituted, in large measure, by the pre-Cambrian massif of Hayscastle. Topographically, the district is rather more varied than is usually the case in Western Pembrokeshire. Considerable portions of the area are occupied by the 400-foot plain; but out of this Topographically

III.—The Lower Ordovician Rocks of Scandinavia, with a Comparison of British and Scandinavian Tremadoc and Arenig Rocks

As a worker among the Lower Palæozoic rocks of Wales I have for some years devoted a good deal of time to the study of Tremadoc and Arenig rocks, and in common with others have therefore often wished to check the results attained in that district of great thicknesses and tectonic complication by reference to the thinner and more fossiliferous series of the undisturbed areas of Northern Europe. When, therefore, the Senate of the University of Cambridge granted me the Worts Fund for travelling scholars, and instructed me to study the interrelationships of the Tremadoc and Arenig Series in Scandinavia, I at once took advantage of the opportunity, and spent the months of May, June, and July in hammering over those rocks in the field.

II.—Some Graptolite Zones in the Arenig Rocks of Wales

The Arenig Series, as originally defined by Sedgwick, has undergone much subsequent modification by its founder and other authors. There have been separated off from it the Tremadoc Series at the base, and the Llandeilo Series above, and a certain amount of ambiguity has arisen as to what constitutes the Arenig thus restricted; possibly, therefore, the recognition of three well-defined graptolite sub-faunas within the series in certain districts may help in some degree towards the solution of the problem.

Manod and the Moelwyns

On the north and west of the great mass of Harlech Grit which forms the centre of the Merioneth anticlinal, the Lingula and Tremadoc strata dip under the mountains of the Moelwyns and Manods, and are there overlain hy the Arenig ashes and slates. The beds exposed on the southern slopes of these mountains have therefore the same general stratigraphical age as those cropping out on the northern slopes of Cader Idris and on the western aspect of Arenig and the Arans. The general relations of the rock-masses of this difficult district have been admirably elucidated by the officers of the Geological Survey, and the results of their work published in Sir A. Kamsay's invaluable memoir on the Geology of North Wales. In the present paper, which is the result of observations carried on interruptedly for several years, our objects are :— To offer conclusive evidence as to the intrusive nature of the great crystalline mass known as the Syenite of Tan-y-grisiau. To correct what seems to us an inaccuracy of some importance in the correlation of beds in different parts of the range, as interpreted in the map and memoir of the Geological Survey. To trace with greater completeness the position and constancy of the beds of slate in the Arenig series ; a subject of considerable local and practical importance, when we remember that those who are engaged in slate-quarrying have so far had to rely mainly on their personal and unaided observation. In order to make the