THE TACONIC FORELAND BASIN OF EASTERN PENNSYLVANIA: GEOCHEMICAL CONSTRAINTS ON PROVENANCE FROM MARTINSBURG FORMATION SLATES

The Upper Ordovician Martinsburg Formation (~455-450 Ma) consists of a thick sequence of claystone, siltstone, and sandstone deposited in a foreland basin that developed in response to the Taconic orogeny. Subsequent lower greenschist facies metamorphism and development of slaty cleavage accompanied Alleghenian deformation. Slate samples were collected along a transect perpendicular to the outcrop belt in eastern Pennsylvania. Sample locations extended from argillaceous limestones of the underlying Jacksonburg Formation to the Silurian unconformity at the top of the Martinsburg Formation. Geochemical analyses of the samples were used to constrain provenance and possible temporal changes in source area geochemistry.

The chemical index of alteration (CIA) varied from 64 to 78 and when corrected for K-metasomatism varied from 69 to 90. Elements which are generally immobile during weathering exhibited a positive correlation with CIA. Rare earth elements (REE) were slightly enriched relative to average upper continental crust (UCC) and North American shales (NASC) and showed the greatest enrichment in middle REE. Moderate Eu-anomalies (Eu/Eu^*=0.53-0.75) and weak Ce-anomalies (Ce/Ce^*=0.78-1.08) were present. Typical samples had Th/Sc ratios (0.8-1) similar to UCC (~1). Lanthanum/thorium versus Hf was indicative of an acidic/UCC source. Elevated REE and La/Th, negative Ce-anomalies, and lower Th/Sc were observed in several samples from the top of the section; possibly indicating a mixed felsic/basic source or the effects of a high degree of source rock weathering.