Neodymium isotopic profiling of carbonate platform strata, correlation between siliciclastic provenance signature and sequence stratigraphy

The Nd isotopic composition of core samples from the mid-Carboniferous to U pper Permian Finnmark carbonate platform is interpreted as representing the ir siliciclastic content, the Nd content of the carbonate fraction being re latively insignificant. Trends of upward increase in Nd-143/Nd-144 in these strata correlate with the two largest-scale (second-order) depositional se quences defined by previous sedimentologic study. These trends are interpre ted as the result of mixing between two hypothetical siliciclastic provenan ce terranes. The relatively "old" (low Nd-143/Nd-144) material dominating t he base of each cycle is suggested to be derived from local Caledonian base ment. The relatively "young" (high Nd-143/Nd-144) material comprising the u pper part of each cycle is suggested to be an open-marine siliciclastic com position representing mixing of more widespread sources. Dominance of the b asement provenance signature at the base of each second-order sequence coul d reflect tectonic movements that exposed local basement areas to erosion. The subsequent gradual shift to "younger" open-marine provenance signature could result from progressive peneplanation or burial of the basement uplif ts. Higher-frequency depositional sequences have little to no effect on Nd composition, suggesting that these fluctuations in facies distribution were largely unrelated to patterns of siliciclastic supply. This model implies that the mid-Asselian second-order lowstand was a time o f major siliciclastic bypass, suggesting the possible existence in the Nord kapp Basin of extensive sandstones of this age. The observed pattern of cyc lic, asymmetric Nd-143/Nd-144 variation should be a universal characteristi c of Finnmark platform stratigraphy and may therefore be useful for stratig raphic correlation between widely separated well locations. The broader int erest of these results is the demonstration that: (1) the Nd isotopic compo sition of even relatively pure carbonate rocks can reflect mainly the prove nance signature of the siliciclastic component; and (2) Nd isotopic profili ng of carbonate strata can indicate patterns of tectonic influence on depos itional cyclicity. (C) 2000 Elsevier Science B.V. All rights reserved.