Ordovician graptolite faunas and stratigraphic construction of the Martinsburg/Hamburg foreland segment, central Appalachians, North America

Graptolite fossils provide a means for separating Late Ordovician autochthonous Martinsburg foreland basin strata (Appalachians, Pennsylvania, USA) from tectonically emplaced allochthonous Hamburg succession rocks. The youngest allochthonous rocks (Dauphin Formation) are Middle Ordovician, late Darriwilian 3 to 4a age, approximately two graptolite zones older than the foreland strata. Nineteen taxa described from the Dauphin Formation include Archiclimacograptus cf. A. riddellenis (Harris), Cryptographs schaeferi Lapworth, Normalograptus antiquus (Ge), Pterograptus elegans Holm, Hustedograptus teretiusculus (Hisinger)?, Pseudophyllograptus augustifolius s.l. (J. Hall), Haddingograptus oliveri (Boucek), Bergstroemograptus crawfordi (Harris), Tetragraptus cf. T. erectus Mu, Geh & Yin, and Kalpinograptus spp. (nov?). Newly described taxa are Pseudotrigonograptus ? ricardo sp. nov., and two (possibly more) reteograptids.;The initial allochthonous incursion (Unit H-1; = Cocalico Formation ?) into the foreland carried a synorogenic piggyback basin of mid-to-late Nemagraptus gracilis Zone turbidites (Unit M-1). Emplacement was upon the Hershey/Myerstown Formation deposited during earliest regional foreland subsidence. Emplacement of allochthonous Unit H-2 followed which was covered by foreland Unit M-2 (late Climacograptus bicornis Zone), containing allochthonous fragments. The foreland basin then spread laterally over the Jacksonburg Formation and equivalent "basal limestones" as Unit M-3 (Martinsburg Formation. s.s.) during Dicranograptus clingani time.;The Hamburg succession (= Dauphin Formation) contains basin sediments deposited within the Octoraro Sea adjacent to the northern fringe of the microcontinent "Baltimoria". Late Cambrian through Early Ordovician age rocks are composed of quartzfeldspathic, micaceous strata overlain by phosphorites, and covered by a black shale and quartzose-ribbon limestone package; lower to middle Arenig rocks are mostly starved clastic hemipelagites. After a biostratigraphic gap of 3-4 graptolite zones those rocks were incorporated into a Middle Ordovician, trench-origin olistostrome, formed as the Octoraro Sea closed, and co-occur with turbidites, distal pelagites and extrusive/intrusive volcanics.