Geologic and aeromagnetic data show that a major tectonic unit underlies rocks of the Musconetcong nappe in the Great Valley of eastern Pennsylvania and New Jersey. This structure, the Lyon Station-Paulins Kill nappe, can be traced from Lyon Station, Pa., at least to Branchville, N.J., a distance of about 120 km. The nappe has a core of Precambrian crystalline rocks as shown by an aeromagnetic anomaly that has the same signature as the outcropping Precambrian rocks of the Musconetcong nappe. This core extends at least 70 km east from Lyon Station to Bangor, Pa., the eastern limit of the aeromagnetic survey. Carbonate rocks in the upper limb of the nappe are exposed in the Whitehall window and in an unnamed window near Catasauqua, Pa., and in the Paulins Kill Valley of New Jersey, which is a very large window through the Musconetcong nappe. These carbonate rocks are of a more shoreward facies than the rocks in the Musconetcong nappe, showing that the Lyon Station-Paulins Kill nappe is a frontal as well as tectonically lower structure. The Lyon Station-Paulins Kill nappe has a lower limb, as is proved by three inner windows within the Paulins Kill window in New Jersey. The nappe has no crystalline core this far east. The Lyon Station-Paulins Kill nappe interfaces with the overlying Musconetcong nappe along the major Portland fault. This fault shears upsection through the Musconetcong nappe, bringing lower-limb rocks of that nappe into contact with the Lyon Station-Paulins Kill nappe in the Whitehall window and bringing upper-limb Musconetcong rocks into contact with the lower nappe in the Paulins Kill window. The Portland fault, though folded, is a late tectonic event and is thought to be a strong imbricate splay from the major ddcollement that lies just above the basement in the central Appalachians. The Portland fault, therefore, telescoped nappes formed during the Taconic orogeny and was folded with them during the Alleghenian orogeny. Far-traveled tectonic units within eastern Pennsylvania and New Jersey are recognized to belong to the very complex Musconetcong nappe system. Near Allentown, Pa., this system consists from lowest to highest, of the Lyon Station-Paulins Kill nappe, the Musconetcong nappe (sensu stricto), and the South Mountain nappe. Another structure, the Applebutter thrust sheet, belongs to this system, but its position is unknown. The Musconetcong nappe system is tectonically overlain by the Lebanon Valley nappe system near Reading, Pa., suggesting that all the far-traveled units of these two systems should be included in a Reading Prong nappe megasystem. 25 50 75 100 MILES FIGURE 1.-Map showing the divisions of the Great Valley of Pennsylvania and New Jersey and their relations to adjacent geologic terranes. Cambrian z and Ordovician rocks of the Great Valley (light shading) ; Precambrian rocks of the Reading Prong and South Mountain anticlinorium (stippled) ; e Silurian and younger Paleozoic rocks (unshaded) ; Triassic rocks of the Newark Basin (dark shading) ; and Precambrian and lower Paleo...
