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

Abstract

Abstract The Nd isotopic composition of core samples from the mid-Carboniferous to Upper Permian Finnmark carbonate platform is interpreted as representing their siliciclastic content, the Nd content of the carbonate fraction being relatively insignificant. Trends of upward increase in 143Nd/144Nd in these strata correlate with the two largest-scale (second-order) depositional sequences defined by previous sedimentologic study. These trends are interpreted as the result of mixing between two hypothetical siliciclastic provenance terranes. The relatively “old” (low 143Nd/144Nd) material dominating the base of each cycle is suggested to be derived from local Caledonian basement. The relatively “young” (high 143Nd/144Nd) material comprising the upper part of each cycle is suggested to be an open-marine siliciclastic composition representing mixing of more widespread sources. Dominance of the basement provenance signature at the base of each second-order sequence could 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 uplifts. 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 of major siliciclastic bypass, suggesting the possible existence in the Nordkapp Basin of extensive sandstones of this age. The observed pattern of cyclic, asymmetric 143Nd/144Nd variation should be a universal characteristic of Finnmark platform stratigraphy and may therefore be useful for stratigraphic correlation between widely separated well locations. The broader interest of these results is the demonstration that: (1) the Nd isotopic composition of even relatively pure carbonate rocks can reflect mainly the provenance signature of the siliciclastic component; and (2) Nd isotopic profiling of carbonate strata can indicate patterns of tectonic influence on depositional cyclicity.