Conodonts from the Devonian and associated clays of Minnesota

Conodonts occur in the shaly partings of the Cedar Valley limestone in southern Minnesota. Devonian conodonts also occur in the clays above the same limestone in the vicinity of Austin, Minnesota. These latter, however, are mingled with others of undoubted Ordovician origin, hence it seems certain that the fauna of this clay deposit has had a dual origin. A number of new species are described from these several horizons. T HE Devonian of Minnesota consists of a variable series of calcareous beds included in the Cedar Valley limestone and rests disconformably on the Upper Ordovician. The Devonian, or Cedar Valley limestone, is not extensively exposed within the state, and some of its beds are known chiefly from well samples or from outcrops a short distance farther south in Iowa. A generalized section for Howard County, Iowa, given by Calvin (1) shows the thickness to be 128 feet. It is probably about the same in Minnesota (2) but may be slightly less at places owing to pre-Cretaceous or preglacial weathering and erosion. Outcrops of the Cedar Valley limestone occur in Fillmore and Mower counties, but it underlies the drift in some of the counties farther north and west. At LeRoy, Minnesota, the middle division (the Stromatoporella beds of the Cedar Valley limestone, as designated in Iowa) crops out and is quarried to some extent. These beds consist chiefly of an exceedingly high grade white limestone with some layers of yellow to brown dolomitic limestone and thin beds or partings of shale. These shale partings have yielded a number of conodonts. In the vicinity of Austin about 10 feet of pale-blue to light bluish-gray clay, part of which may be red, occur on top of the Cedar Valley limestone. This has been exposed in cuts, such as at the underpass in Austin or the clay pit at the old brick and tile plant, but it is also penetrated by the drilled wells that extend downward into bed rock. This clay or shale lies at the base of the drift or between the drift and the limestone and is the clay that has been called Cretaceous. N. H. Winche!l found it to contain both angiosperm and gymnosperm leaves (3). During the present study no megascopic fossils were collected from it except a few corals that were found in the shales along Rose Creek, where they appear to be residual Devonian material. The clay samples from the Austin region have been thoroughly explored for microfossils, and nothing younger than Devonian has been found in them. The exposures of the clay shale at present are too unsatisfactory to discuss in detail, but the fact that they are overlain by drift and that the lower part of the clay seems to bear a residual relation to the bedrock below suggests that this deposit is preglacial in origin, although its upper part may be contaminated by drift material such as granite boulders (4). The microscopic fauna is limited to conodonts, most of which are characteristic Devonian genera, but the clay also contains such other genera as Belodus, Oistodus, Paltodus, Dichognathus, Ozarkodina, etc., not usually found in the Devonian. Moreover, some of the species appear to be identical with well-known Ordovician forms. It appears, therefore, that the clay contains a mixed Ordovician and Devonian microfauna, the Ordovician element at least being residual material brought in from the north and west. The Devonian element, if residual, need not have been carried very far, for the clay directly overlies rocks of Devonian age and may itself be, in part, of that age. The conodont fauna from the Cedar Valley limestone outcrops of southeastern Minnesota and from the same clay in the Austin region, is figured and described herewith. Many of the species found in the clay occur also in the linestone, and their De-