Deciphering eustatic and tectonic influences during parasequence development in the Mesoprotozoic Helena/Wallace Formation, Belt Supergroup, Montana and Idaho

The stratigraphic architecture of sedimentary basins results from a combination of changes in relative sea-level and tectonism, and resulting changes in sediment supply. The Helena/Wallace formations, Mesoproterozoic Belt Supergroup, Montana and Idaho, consists of >500 meters of stacked meter-scale cycles (parasequences) that record in situ carbonate deposition as well as siliciclastic deposition from both the Laurentian craton and an unknown (tectonically active?) western source. In this study, statistical methods and 2-D forward modeling are combined with geochemical provenance analysis to examine parasequence stacking patterns and decipher the relative eustatic and tectonic controls on sequence development.

Helena/Wallace parasequences are typically composed of a lower siliciclastic half cycle, which thickens and coarsens to the west, and an upper carbonate half cycle that contains sandy-to-silty siliciclastic interbeds. Parasequences are bounded by scoured surfaces and are frequently marked by a basal intraclast packstone lag that represents reworking of subjacent strata. Parasequences are interpreted as 4^th or 5^th order cycles likely driven by Milankovitch frequency eustasy.

Statistical analysis of cycle thicknesses suggests non-random packaging of thicker- and thinner-than-average parasequence sets. Modified Fischer plots, which examine changes in both cycle thickness and siliciclastic:carbonate ratio within the cycles, indicate that compositional variability and parasequence packaging (3^rd order cyclicity) was likely controlled by changing tectonic regime (subsidence rates or influx of siliciclastic sediment) rather than longer-term eustatic changes in sea level. 2-D forward modeling of how subsidence, sediment influx, sea-level fluctuations may have contributed to both changes in cycle thickness and compositional variability also supports a tectonic origin for parasequence sets and suggests that episodic increases in basin subsidence rates are responsible for observed parasequence stacking patterns.

X-ray fluorescence analysis of siliciclastic residues from Helena/Wallace formation samples was performed to determine if tectonism along the western basin margin resulted in changes in the sediment source that can be identified based on the geochemical composition of Helena/Wallace formation rocks. Results of the analysis suggest that, despite tectonically driven changes in subsidence, the sediment source area did not change throughout deposition of the Helena/Wallace formations. The results of the analysis are consistent with sediment sourced via drainage from a cratonal source area that is geochemically distinct from the Laurentian craton.

Siliciclastic strata of the >15 km thick lower Belt Supergroup has long been recognized to record episodic rifting of the Belt basin. This study, however, is the first to suggest that shallow marine, mixed carbonate-siliciclastic strata of the Helena/Wallace formations also contain a subtle record of continued basin extension.