DEVELOPMENT OF THE MARTINSBURG/ HAMBURG FORELAND SEGMENT

The Martinsburg/Hamburg foreland segment in Pennsylvania (a.k.a. the Hamburg klippe) formed earlier than the foreland basin elsewhere as a result of a Taconic allochthon’s forward arrival. A formative foreland basin segment developed in the early Late Ordovician Nemagraptus gracilis graptolite Zone as the first allochthons advanced. This Middle Ordovician and older allochthon carried a N. gracilis age (late Chazyan to early Blackriveran) synorogenic piggyback flysch basin (two graptolite zones younger than the youngist allochthonous rock) that was eventually thrust over the Hershey/ Myerstown Formation, a platform to foreland transitional carbonate. The Myerstown Formation, dated with conodonts, is also N. gracilis age, which is older than the Jacksonburg Formation below the Martinsburg Formation occurring outside the area where allochthons are present in the foreland. Tectonic loading of the Laurentian margin caused the Blackriver unconformity (peripheral bulge), the erosion of which provided limestone clasts to the Hershey Formation facies prior to Jacksonburg deposition. Above the formative allochthon(s) with piggyback strata, is additional allochthon emplacement(s). This is covered by turbiditic foreland basin sediment that is slightly older (mid to late Climacograptus bicornis Zone) than the Martinsburg Formation elsewhere at its base, but is correlative to the Martinsburg Formation (Dicranograptus clingani Zone) in its upper part as the foreland enlarged.

The western part of the Martinsburg/Hamburg foreland segment is divided by the Yellow Breeches fault, which thrusts rocks of Cocalico terrane (Hamburg allochthon rocks ± Martinsburg Formation?) affinity northward within the overturned limb of the Lebanon Valley nappe. Within this thrusted sequence the Hamburg succession can be seen on the Myerstown Formation. A lithologic comparison can be made between greenschist grade Hamburg succession rocks carried on the Yellow Breeches fault and allochthonous units of lower grade north of that fault emplaced within the main outcrop belt of the Martinsburg foreland basin.