GRAPTOLITES, CONODONTS, STRATIGRAPHY, AND STRUCTURE: MAPPING THE TACONIC FORELAND IN THE GREAT VALLEY OF PENNSYLVANIA

The Martinsburg Formation of the Great Valley represents the Late Ordovician Taconic foreland. In Pennsylvania, from just west of the Susquehanna River eastward nearly to the Lehigh River, part of the rock within this strike belt has long been recognized as older than, and quite different from, the traditional Martinsburg. Two competing models explain this anomalous geology: 1) a Taconic thrust placed an allochthonous “Hamburg klippe” over the Martinsburg; 2) allochthonous rocks were emplaced below, or mixed within, the Martinsburg sediment (see Ganis et al., 2001, GSA Bulletin, v. 113, no. 4, p.109-128 for summary). In this area of limited and discontinuous outcrop, it is difficult to differentiate one gray shale from another, or to discern the geometry of the “exotic” lithologies – limestone, green and red shale, and chert. Sorting out the Martinsburg from older strata and their strato-structural relationships seemed an intractable problem.

In the past several decades, newly acquired data from graptolites and conodonts has enabled extensive dating of the rocks in this belt, leading to identification of field characteristics that can be used to distinguish the older allochthonous units from the Martinsburg. Paleontology has provided the basis for definition of an internal stratigraphy in the allochthonous sequence and, for the first time, allowed for sensible separation and mapping of the allochthonous strata and the Martinsburg.

Biostratigraphy shows that the oldest (L. Cambrian to E. Ordovician) rocks of the allochthonous Dauphin Formation are olistoliths, from cobble-size to map-scale, contained within a M. Ordovician trench matrix. This package was structurally moved from the trench across the Laurentian carbonate platform, emplaced within the Taconic foreland, and covered by the Martinsburg Formation in the mid-Late Ordovician. Graptolite ages allowed identification of a piggyback basin unit separating map-scale allochthonous sheets, constraining allochthon emplacement to the early Late Ordovician. Regional mapping has revealed that the structure of this part of the Great Valley is dominated by a large-scale Alleghanian anticlinorium with the allochthons in the core and the overlying Martinsburg on the flanks.