PROTEROZOIC MIDDLE RUN FORMATION OF EASTERN MIDWEST, USA: A TORRIDONIAN EQUIVALENT?

This study uses SHRIMP U-Pb methodology to obtain 85 ages on 57 detrital zircons of the unfossiliferous Proterozoic Middle Run Formation that underlies the Middle Cambrian Mount Simon Formation of southwestern Ohio. These zircons come from thin alluvial cycles near the top of the Middle Run Fm. in Ohio Geological Survey drill hole 2627. We first examined these zircons for overgowths of diagenetic xenotime, but none were sufficiently large to probe without including part of the detrital grain. Consequently, we examined the outer portions of the zircons to obtain their youngest possible ages (Preliminary scans by BSE and CL permitted us to avoid old cores, fractures, and damaged zones within the grains). When two or more analyses were made on a single grain, we selected the youngest. For each age we calculated a 95 % confidence interval based on 4 to 7 scans by the microprobe. Below we report only on grains with up to 1 percent discordance.

Two broad conclusions follow from our results. The youngest age of an outer rim is 1036 ± 14 Ma. Allowing 5 to 10 m.y. for uplift and erosion, this suggests a maximum deposition age of about 1,012 Ma. Secondly, judging from our sample of 57 grains, most of the sand in the top of the Middle Run Fm.in southwestern Ohio was derived from the Grenville orogen to the east and northeast (78% of the detrital grains have ages between 1022 and 1280 Ma) with a major contribution from the Composite Arc and Frontenac-Adirondack belts.

These results suggest that the Middle Run Fm. is a foreland basin deposit derived mostly from the uplift and erosion of the Grenville orogen and that it is broadly equivalent in age to the Neoproterozoic Torridonian of Scotland. The distribution of the Middle Run Fm. west and north of the Grenville Front is consistent with deposition in a closely coupled foreland basin. In contrast, a much later depositional event – such as the Vendian – would be less likely to be restricted to areas only west and north of the front, because the relief of the orogen would have been eroded long before sedimentation.