Shorter contributions to paleontology and stratigraphy, 1989

Abstract

INTRODUCTIONThe Metaline district of northeastern Washington State ( fig. 1) has yielded some 17 million tons of zinc-lead ore since mining began there in the early 1900's. Most of this ore has come from an irregularly stratiform zone of breccia (the Josephine unit of McConnel and Anderson (1968)) just below the contact between the Metaline Limestone and the overlying Ledbetter Slate. The nature of this contact has been the subject of some dispute. Where the contact is unfaulted, it has been interpreted as being conformable and gradational (Snook and others, 1981, p. 5), possibly disconformable (Park and Cannon, 1943, p. 20; Dings and Whitebread, 1965, p. 21), and even the result of prolonged subaerial exposure (Mills, 1977). In an effort to resolve the question, J .A. Morton of the GRC Exploration Co. has carefully examined the contact in the underground workings of the Pend Oreille Mine ( fig. 1) and, in the process, has collected large numbers of well-preserved graptolites, which are the subject of this paper.Manuscript approved for publication May 16, 1988. AcknowledgmentsThe author is grateful to Jack A. Morton for providing the graptolite specimens and for a guided tour of his collecting sites in the Pend Oreille Mine. Many thanks to J. Thomas Dutro, Jr., of the U.S. Geological Survey and Roger A. Cooper of the New Zealand Geological Survey for their comments and advice on the manuscript. Thanks also to J. Eric Schuster and Nancy L. Joseph of the Washington Division of Geology and Earth Resources. GENERAl GEOlOGYThe Metaline Limestone consists of about 5,000 ft of thin-bedded limestone and shale, light-gray bedded dolomite, and gray massive limestone . The overlying Ledbetter Slate, predominantly black argillite and slate interbedded with minor amounts of limy argillite and limestone , is estimated to be between 2,000 and 4,500 ft thick. Both formations extend over much of Pend Oreille and Stevens Counties, Washington, and adjacent southeastern British Columbia. An undisturbed (unfaulted) contact between these formations is exposed in only a few places, one of which is in the Pend Oreille Mine near Metaline Falls, Wash.During his study of the Ledbetter-Metaline contact, Morton collected graptolites from a shale unit in the Metaline Limestone as well as from a short interval of the Ledbetter Slate just above the contact. The fact that all of these graptolites are characteristic of the Middle Ordovician Zone of Paraglossograptus tentaculatus gives credence to Morton's interpretation of the contact as a depositional transition (J .A. Morton, written communication, 1986). Clearly, the age of this contact is not everywhere the same, because graptolites older than those of the P. tentaculatus Zone have been found in the Ledbetter Slate at other localities in the area. THE GRAPTOLITE FAUNAGraptolites have been known from the Ledbetter Slate in the Metaline Falls area since the early 1940's, when Park and Cannon (1943, p. 20-22) published Josiah Bridge's report on faunas ranging in age from "lower Deepkill" thro...