PARASEQUENCE STACKING PATTERNS IN THE MESOPROTEROZOIC HELENA/WALLACE FORMATIONS, BELT SUPERGROUP: DECIPHERING EUSTATIC AND TECTONIC SIGNALS

The stratigraphic architecure of sedimentary basins results from a combination of changes in relative sea-level, tectonism, and sediment supply. The Helena/Wallace formations, Mesoproterozoic Belt Supergroup, 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. Here we combine statistical methods and 2-D forward modelling to examine parasequence stacking patterns and decipher the relative roles eustatic and tectonic controls play in 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 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 modelling 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 the genesis of observed parasequence stacking patterns.

Siliciclastic strata of the >15 km thick Lower Belt 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 contains a subtle record of continued basin extension.