Patterns of fossil diagenesis caused by groundwater leaching provide insight into how shells are altered. This study analyzes fossils in unconsolidated terrigenous sediments from the Virginia Coastal Plain, unlike previous studies conducted mostly in carbonate terranes. The vertical and lateral distribution of diagenetic states was mapped in an outcrop (63 m by 2.1 m) of the Yorktown Formation. A paleostream channel located at one end was incised during the Pleistocene and filled with sediments of the Shirley Formation. The Tabb Formation unconformably overlies the outcrop. Acidic groundwater caused the observed patterns of fossil and sediment diagenesis. These patterns include zones of fossil alteration, diagenetic stratification of the sediment, and fossil diagenesis on a microstructural level. Groundwater movement, controlled by the presence of the paleochannel, caused diagenetic alteration or complete dissolution of the fossils, and possibly caused precipitation of fine-grained iron oxyhydroxides. All carbonate material in the vicinity of the paleochannel is completely dissolved away, although iron oxyhydroxide coatings of fossils remain. Away from the paleochannel Crepidula fornicata (gastropod; aragonite), Ostrea sp. (bivalve; calcite), Balanus sp. (barnacle; calcite), and bryozoans (calcite) are found in parallel zones of alteration that dip toward the paleochannel and cut across horizontal sedimentologic and fossiliferous layers. Groundwater also leached the Yorktown sediments. This alteration caused a diagenetic stratification of the sediment, with unaltered greenish-gray silty fine sand along the base of the outcrop, overlain by leached yellowish-brown silty fine sand and areas of concentrated iron oxyhydroxides. The preservation of both aragonitic and calcitic shells was affected by groundwater movement. Original aragonitic shell material is found as chalky, uncrystallized specimens or neomorphosed shells, or is completely dissolved with only molds or ghosts remaining. Neomorphosed specimens typically consist of calcite-replaced shell material with pockets of original aragonite, and sparry calcite filling empty shell cavities. Original calcitic shell material is either chalky or unaltered. Chalky shells range from relatively hard to soft and pasty. Crepidula shells of intermediate chalkiness tend to separate into thin flakes, caused by dissolution along growth surfaces. Chalkiness of pasty shells is caused by dissolution of shell material (without recrystallization) and not simply loss of organic matrix. SEM photos of Crepidula reveal the more porous and leached appearance of chalky shells in contrast with hard; unaltered shells. The presence of chalky aragonitic and calcitic shells indicates that chalky textures are, to some degree, independent of mineralogy and microstructure. / M.S.

Biostratigraphic study of the Late Silurian-Early Devonian strata of western Virginia reveals the presence of at least three potentially useful multielement conodont apparatuses. Two multielement species of the genus Ozarkodina may prove to be useful in determining the boundary between the Silurian and Devonian Systems in the Appalachian Basin. Currently, a single form species, Icriodus woschmidti woschmidti, which was found in the lowermost New Creek Limestone at Low Moor, Virginia, is used by previous agreement to denote the base of the Devonian system. In the absence of this standard paleontologic indicator, multielement species of the genus Ozarkodina aay be useful. Ozarkodina steinhornensis eosteinhornensis is identified from the Silurian Tonoloway Formation in the area of New Castle, Virginia. Ozarkodina steinhornensis remscheidensis, which is transitional to Oz. s. eosteinhornensis, has been identified in the overlying Late Silurian-Early Devonian Keyser Formation and Early Devonian New Creek Limestone near New Castle and Low Moor. Oz. s. remscheidensis, which is reported to occur first in the Gedinnian, has been suggested as a possible alternative indicator of lowest Devonian strata (Walliser, 1971). However, because of the transitional relationship of these two multielement species, abundant yields of conodonts are necessary to determine the first appearance of Oz. s. remscheidensis. Elements belonging to a third conodont apparatus, Delotaxis elegans, have been recovered from the Keyser and New Creek intervals at the same locations. Delotaxis elegans may ultimately prove to be a significant biostratigraphic marker with further study of these units. An overview of the Late Silurian-Early Devonian strata in Alleghany, Botetourt, Craig, and Roanoke Counties is developed to provide a regional perspective of this stratigraphic package and to illustrate significant biostratigraphic markers. Unconformities, wedge-outs, facies changes, and thickness variations are examined within the study area based on nine measured sections and a review of pertinent literature. Clastic units and quartzose carbonates within the Helderberg Group are discussed with regard to source area. / Master of Science

Studies in modern ecology indicate that most species are distributed independently along environmental gradients according to their individual requirements. Steep gradients often produce species associations separated by discontinuities; gradual gradients produce broadly-overlapping distributions. Approaching the distribution of species populations as a continuum, using gradient analysis, avoids artificial sub-division of totally intergrading distributions, yet permits discontinuities to emerge where present. Faunas of the Martinsburg Formation (Ordovician) in southwestern Virginia offer an excellent opportunity to test the applicability of gradient analysis in a paleoecological setting. A broad spectrum of environments, from nearshore to open-marine, elastic to carbonate-dominated facies, provide both temporal and geographic variation against which to evaluate changes in species distributions. Five classical, Petersen-type communities were recognized in the Martinsburg using cluster analysis: 1) Lingula, 2) bivalve, 3) Rafinesquina, 4) Onniella, and 5) Sowerbyella-dominated associations. Two gradient analysis techniques, ordination and Markov analysis, revealed the same basic associations. However, ordination and Markov analysis permit arrangement of these associations along one or more interpreted environmental gradients. Factors related to water depth and distance from elastic source areas, particularly bottom stability and disturbance frequency, appear to have been the most important of a complex of interrelated physical parameters. The high-stress, nearshore end of the Martinsburg gradient complex was occupied by a Lingula association, followed seaward by an association of bivalves adapted to less stressed environments. Low-stress, open-shelf environments were occupied by Rafinesquina, Onniella, or Sowerbyella-dorninated associations. Broad overlap among these articulate brachiopod communities reflects variations within the open-shelf habitat. / Ph. D.