Abstract

Silurian and Devonian beds that are chiefly carbonates crop out in small areas in nearly all states of the Midcontinent and in central and west Texas. The outcrops are connected through the subsurface except between the Texas outcrops. The rocks were deposited in a miogeosyncline which extended from Arkansas across Oklahoma and southward and eastward across Texas; and over the shelf area to the north and northwest of the trough. The tectonic framework, basins, and shelf areas remained essentially the same from Cambrian through Devonian time, except for secondary uplifting and warping, chiefly the Ozark Island and the Texas Arch. Local epeirogenic uplifts and oscillations affected various basins at different times initiating local disconformities in diffferent basins. A major regional emergence resulted in an unconformity between Middle Silurian and Lower Devonian throughout the Midcontinent. The regional setting was greatly altered by Pennsylvanian tectonics.

The carbonate beds of Silurian and Devonian age in the central Midcontinent are known as the Hunton Group in the subsurface and at their crop out area in the Arbuckle Mountains. The microfauna of arenaceous Foraminifera found in the subsurface of Kansas and similar microfaunas previously described from Oklahoma, Missouri, and elsewhere provide information for identification and correlation of the Hunton Group locally and regionally. The lithologic samples lack diagnostic criteria, but the acid residues and especially the contained arenaceous Foraminifera make it possible to zone the beds, identify faults, pinchouts and other stratigraphic characteristics that previously have not been determined. The study of lithologic samples, acid residues, and the Foraminifera define zones which can be correlated with outcrops.

The Hunton Group in Kansas can be subdivided into five Silurian zones and two Devonian zones. Silurian Zone 1 is the equivalent of the Keel-Ideal Quarry Member of the Chimneyhill Formation² in the Arbuckle Mountains and the Edgewood Limestone of Missouri. Zone 2 is the equivalent of the Cochrane Member of the Chimneyhill and the Brassfield Limestone of Missouri. Foraminifera are found in Zone 3 only and this microfauna suggests correlation with the Clarita Member of the Chimneyhill and of the Osgood Limestone of Missouri and areas to the east. Zones 4 and 5 are correlated as the equivalent of the Henryhouse Formation of the Arbuckle Mountains, the St. Clair Limestone of northeastern Oklahoma and the Bainbridge Limestone of Missouri. The two Devonian zones are not correlated with the Lower Devonian part of the Hunton in the Arbuckle Mountains section, but with the Middle Devonian of Missouri.

Arenaceous Foraminifera cannot be used for direct correlation of all units in the subsurface, but where present, as in Zone 3 of the Silurian, they are distinctly diagnostic and help to establish the position of that portion of the section, and they also may assist in determination of the boundaries of the zones immediately above and below Zone 3. Foraminifera are not always present in Silurian Zone 3 but their occurrence in about half of the wells scattered throughout the Midcontinent facilitates the construction of cross-sections and allows extrapolations from which distribution and thicknesses of other zones lacking Foraminifera can be interpreted.