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Most of the conodont data presented in this report (table 1) were acquired to support 1:100,000-scale geologic mapping of the Winchester 30' x 60' quadrangle by the U.S. Geological Survey (USGS). Conodonts were chosen to provide a biostratigraphic framework for the Upper Cambrian to Mississippian marine carbonate rocks that make up about 25 percent of the Paleozoic strata exposed in the quadrangle ( ~ 2,130 m of the approximately 9, 450 m) of Paleozoic strata exposed in the quadrangle). Thicknesses of stratigraphic units are from McDowell (1991), our own measurements, and from many of the stratigraphic reports and geologic maps listed in the references cited. Conodont biostratigraphic and color alteration index (CAl) analyses help identify stratigraphic units and structural discontinuities, particularly in the Upper Cambrian to Middle Ordovician chiefly carbonate rocks of the Shenandoah Valley and North Mountain fault zone. Conodont biofacies analyses provide additional information about the provincial affinities of the conodonts and the depositional environment of the rocks that contain them. Lithostratigraphic, biostratigraphic, paleoenvironmental, and CAl data for all conodont samples are given in table 1. Conodonts are toothlike, relatively transparent, lamellar, apatitic microfossils (commonly 0.1-1 mm in size) that are biostratigraphic and organic metamorphic indices. Conodonts are the only known hard parts of an extinct group of primitive, probably jawless fishes that ar~ found in marine rocks of Late Cambrian through Triassic age. They are among the foremost biostratigraphic indices throughout their geologic range. More than 100 conodont zones have been used to subdivide latest Cambrian through Mississippian time. At least 27 of these zones were recognized in the carbonate rocks of the Winchester quadrangle (see accompanying stratigraphic column). Conodonts are most easily recovered from carbonate rocks and poorly indurated clastic rocks, although their abundance generally varies inversely with the rate of sedimentation. About 75 percent of the Paleozoic rocks in the Winchester quadrangle · are of inappropriate lithology for conodont extraction or are too old to contain true conodonts. These include indurated fineto coarse-grained, relatively rapidly deposited siliciclastic rocks, rocks that formed in restricted marine and nonmarine environments, and rocks of Middle Cambrian age or older. Despite the great biostratigraphic utility of conodonts, their lifestyle(s) remains uncertain. The widespread occurrence of conodonts in marine rocks deposited in a variety of depositional environments has influenced many researchers to propose a pelagic mode of life for the conodont animal. Many taxa, however, are not obviously cosmopolitan or even provincial, and exhibit a spatial distribution influenced by poorly constrained, paleoenvironmental parameters. Undoubtedly, water temperature and chemistry, trophic resources, and turbidity were among the chief factors that influenced conodont distribution. The paleogeographic distribution of conodonts suggests they undoubtedly had more than one mode of life ranging from nektobenthic to fully pelagic. Conodont elements grew throughout the life of the animal by periodic addition of a centrifugally deposited apatite layer followed by a layer of organic matter (e.g., Burnett and Hall, 1992). During heating, the organic matter sealed between transparent apatite lamellae undergoes carbonization that produces gradual visible color changes from pale yellow, to amber, light brown, dark brown, and black in the range of 50° to 300°C (Epstein and others, 1977). Above 300°C, conodonts change from black, to gray, opaque white, and, finally, to crystal clear as a result of carbon loss, release of water of crystallization, and recrystallization. All these color changes have been observed in natural collections and reproduced and calibrated by pyrolysis experiments in the laboratory (Epstein and others, 1977; Rejebian and others, 1987). Because conodonts are index fossils as well as organic metamorphic indices, they have been used to produce time-based isograd maps that can, in turn, be used to assess depth and duration of burial, hydrocarbon and mineralization potential, and timing of tectonic events (e.g., Harris and others, 1978, 1980; Burnett, 1987; Nowlan and Barnes, 1987; Read and others, 1992).

miscellaneous field studies map

Conodont color alteration index (CAI) map and conodont-based age determinations for the Winchester 30' x 60' Quadrangle and adjacent area, Virginia, West Virginia, and Maryland