NEOPROTEROZOIC TRANSCONTINENTAL RIVERS AND A POSSIBLE JACOBSVILLE/BAYFIELD GROUP -- UINTA MOUNTAIN GROUP CONNECTION

The Neoproterozoic (770 – 740 Ma) Uinta Mountain Group (UMG), UT records fluvial, deltaic, and marine deposition within a north-tilted intracratonic half-graben basin that formed mainly in the hanging wall of a south-dipping normal fault. Detrital zircon data include significant Grenville-age grain-populations. Transcontinental river systems draining the Grenville orogenic belt have been invoked to explain the presence of this zircon population in the UMG and other western Laurentian basins. Here we present new detrital zircon data from the Proterozoic Bayfield Group, the youngest of the post-rift sediments that infilled the ~1 Ga Midcontinent Rift (MCR) in northern WI, and correlative (? or younger) Jacobsville Sandstone (MI), which accumulated outside of the active rift. This new data provides a test for a potential fluvial system connecting the UMG and MCR after 770 Ma.

Both the UMG and MCR sandstones contain scattered Archean grains 2.5-2.8 Ga, no grains between 2.5 and 1.9 Ga, and a wide distribution of grains from 1.5 to 1.0 Ga. Both contain Grenville-age peaks near 1.1 and 1.2 Ma, and syn-MCR grains between 1.1 and 1.0 Ga. MCR samples have an age-peak at about 1.9 Ga that is not present in the UMG. MCR samples show a significant peak between 1.3 – 1.4 Ga; this peak is present but not strong in UMG samples. Paleocurrents from the MCR and UMG are complex, but do show trends: the dominant Bayfield Group trend is to the rift center from either flank. Jacobsville sandstone mostly trends to the north and west. UMG paleocurrents mostly trend to the west.

Grain-ages from Bayfield and Jacobsville sandstones likely reflect a southern or southeastern Laurentian Grenville provenance, mixed with some grains of local MCR derivation. This suggests transportation north and west from the complex Grenville orogen. The similarity in detrital zircon populations suggests the possibility that large rivers carried grains from the eastern Grenville orogen through the Midcontinent rift to the UMG trough after 770 Ma. The Bayfield Group/Jacobsville sediments may have been deposited in environments influenced by this post-770 Ma river system, or deposited much earlier and simply reworked.