143-6 Solving a stratigraphic mystery: using pXRF to field locate an unconformity by detecting abrupt provenance change in sandstones from the Narragansett Basin of Massachusetts

Abstract:

The Narragansett Basin of Southeastern Massachusetts contains the Wamsutta Formation, assigned to Late Carboniferous (Bashkirian) age based on plant fossils in its uppermost strata. A previous study dated a rhyolite flow within lower strata suggesting a portion of the Wamsutta Fm. was ~60 million years older (U-Pb; 373 +/- 2Ma); therefore, an unconformity must exist between the lowermost and uppermost terrestrial redbeds. We used portable X-ray fluorescence (pXRF) to characterize visually and structurally indistinguishable strata and to delineate a potential unconformity, predicting that a shift in sediment provenance would appear in the bulk geochemistry of the sandstone units. We measured freshly broken fine to coarse sandstones from the basin’s northwestern Wamsutta outcrop area with pXRF 5-10 times each. We derived data correction factors from linear regressions on standard reference materials and applied them to raw concentration values. We assessed and retained corrected elemental data within each sample suite based on their quality using Tukey’s Interquartile Range method, then transformed it to centered-log-ration values. Our resulting dataset was restricted to six provenance-relevant elements (Sr, Nb, Rb, Ti, Y, and Zr) plus Si and Fe for testing grain size, sorting, and matrix effects. Results of a principal components analysis (PCA) at the 6-element level were significant, explaining a combined 66.9% of total variance on two axes. PC1 was driven primarily by Sr, Nb, and Rb, while PC2 was driven by Ti and Y. A k-means cluster analysis recovered two distinct geochemical groups (0.496 silhouette) separated by only a few vertical meters. This boundary lies above the Devonian rhyolite and indicates a major provenance shift from K-feldspar (high Rb) rich to plagioclase and Ti-oxide-bearing sources (carrying Sr and Nb), likely reflecting a change in sediment supply from intrabasin volcanics and Quincy Granite-like lithologies to those with more mafic components. We determined grain size and sorting not to be a factor in the analysis, as it varied minimally across all sandstones sampled, and elements associated with heavy minerals were not correlated with changes in grain size or sorting. While a shift in provenance does not in itself indicate an unconformity, one this significant (PC1 p=0.0000) and abrupt is unlikely to have been generated without one. Index fossils, if discovered above and below the geochemical boundary, could confirm our hypothesis. 

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