Darriwilian graptolites of the Hamburg succession (Dauphin Formation), Pennsylvania, and their geologic significance

Abstract

Graptolites from the Dauphin Formation in the allochthonous Hamburg succession of the Appalachians in Pennsylvania, USA, are late Darriwilian (Da) 3 to early Da 4 age (Middle Ordovician); this age range constrains the timing of the latest depositional episode before the terrane was tectonically mobilized. These rocks were emplaced into the Martinsburg foreland basin of Laurentia during the Taconic orogeny in the early Caradoc (Late Ordovician). Nineteen taxa are described defining a narrow biostratigraphic interval. Among the characteristic fauna collected from of the Da 4 Zone are Pterograptus elegans Holm, Cryptograptus schaeferi Lapworth, Hustedograptus teretiusculus (Hisinger) ?, Haddingograptus oliveri (Bouček), Glossograptus hincksii (Hopkinson), Pseudophyllograptus angustifolius s.l. (J. Hall), and Archiclimacograptus cf. riddellensis (Harris). Tetragraptus cf. erectus Mu et al. found with the above suggests a level low in the Da 4 Zone and a limited occurrence of Bergstromograptus crawfordi (Harris) may indicate some strata within the Da 3 Zone. Proposed new forms include Pseudotrigonograptus ? ricardo sp. nov., and at least two reteograptids. Four examples of Kalpinograptus and Kalpinograptus ? may be new.