Stratigraphic control of rare-earth pattern types in Mid-Proterozoic sediments of the Belt Supergroup, Montana, U.S.A.: Implications for basin analysis

Abstract

Within sediments of the Helena embayment, an eastern extension of the Mid-Proterozoic Belt basin, Montana, U.S.A., three different types of REE patterns were identified. The lower part of the investigated sequence (Chamberlain Shale and Lower Newland Formation) is a uniform shale interval which accumulated during a period of tectonic quiescence. The REE patterns of these shales (normalized to NASC) are either flat or LREE enriched, lacking Eu deficiencies. The superjacent unit (Newland Transition Zone) contains variable amounts of feldspathic sandstone. It indicates rejuvenation of the hinterland and regression. The uppermost unit (Upper Newland Formation) consists of alternating packages of carbonates and shales. REE patterns in shales of the Newland Transition Zone and Upper Newland Formation have negative Eu anomalies. This drastic change of REE patterns was observed in all stratigraphic sections. The source rocks of the Beltian sequence were probably dominated by granitoid gneisses and migmatites, and rocks of this composition show negative Eu anomalies (against NASC) in many other places. The observed negative Eu anomalies in the shales were probably inherited from the source rocks. However, the patterns of the Chamberlain Shale and the Lower Newland Formation are not as easy to explain, because the source rocks seem not to have changed during deposition of the Beltian sequence. Perhaps more intense chemical weathering during Chamberlain Shale—Lower Newland time obscured the negative Eu anomalies in the residual clays. Adsorption of LREE's on clays during transport may have caused the LREE-enriched patterns. A change in tectonic regime and weathering intensity coincides with the stratigraphic distribution of the different REE pattern types. Thus, stratigraphically controlled REE pattern distribution in a sedimentary basin may help to establish quasi time lines, and may allow monitoring of tectonic pulses and of weathering conditions in the hinterland.