Devonian Cavity Fillings in Subsurface Silurian Carbonates of West Texas: GEOLOGICAL NOTES

Abstract

In the subsurface of much of W. Texas, the Siluro-Devonian Hunton carbonate section lies between Woodford Shale of Upper Devonian or of Upper Devonian and Lower Mississippian age and the Ordovician Montoya Dolomite. Locally the Lower Devonian carbonate rocks are lacking, and thick limestone or dolomite containing Middle Silurian fossils occurs beneath the Woodford Shale. The cavity fillings were noted generally in the upper part of the Silurian limestone near the contact with the Woodford Shale, but locally they are as much as 100 ft. below the contact. The fillings are commonly sinuous in form, but some contacts are sharp and vertical; the diameters of the cavities range from less than 1/16 in. to several inches. The gray fine-grained dolomite in these Middle Devonian cavity fillings is distinguished readily from the Silurian white fossiliferous dolomitic limestone. Other constituents filling the cavities are conodonts, phosphatic organic remains, pyrite, and 5-10% rounded detrital minerals including quartz, feldspar, muscovite, hornblende, and zircon. The conodonts include characteristic Middle Devonian species of Icriodus, Polygnathus, Neprioniodus, and Angulodus?.