TEMPORAL CHANGES IN ND ISOTOPIC COMPOSITION OF SEDIMENTARY-ROCKS IN THE SEVIER AND TACONIC FORELAND BASINS - INCREASING INFLUENCE OF JUVENILE SOURCES

Initial Nd-143/Nd-144 ratios of elastic sedimentary rocks in the Ordov ician Taconic and Sevier foreland basins increase with decreasing stra tigraphic age. This shift in isotopic composition reflects an increase d influx of juvenile sediment to both basins. For the Taconic basin, g eologic and isotopic data are consistent with the juvenile sediment be ing derived from uplifted Grenville crust, upthrust as part of the Tac onic allochthons, sediment derived from the Bronson Hill are, or a com bination of both. For the Sevier foreland basin, there are few geologi c constraints on source regions, but the similar shift in isotopic rat ios with time suggests a similar increase in the influence of juvenile sources such as exotic terranes. Sevier basin sedimentary rocks have higher epsilon(Nd) values than those within the Taconic basin, thus ju venile source areas were of greater importance to Sevier basin sedimen ts.