Distribution of REE in the Eastern Belt Supergroup (Montana, U.S.A.): Implications for stratigraphic correlations and basin evolution

Abstract

Shales of the Mid-Proterozoic Newland Formation, Belt basin, contain a geochemical marker horizon that is characterized by the appearance of negative Eu anomalies in shale REE patterns. REE pattern changes appear to be related to changes in weathering intensity and tectonic activity in the hinterland. Stratigraphic and sedimentologic considerations suggest that this REE marker horizon defines an approximate time-line within the sequence. Comparison of REE patterns of the Newland Formation and the coarse clastic LaHood Formation that was deposited to the south, shows that the lower portions of the Newland Formation were deposited prior to LaHood sedimentation. Such a correlation implies that early Belt sediments may have covered a much larger area than delineated by the outline of the present-day Belt basin and that, contrary to earlier views of basin evolution, the half-graben configuration of the eastern Belt basin was established at some later point of basin history.