6 March 2014
The Wallbridge Unconformity is a surface of stratigraphic hiatus or erosion between the depositional influence of the Tippecanoe and Kaskaskia epeiric seas. After Alan Pitts and I located ourselves in the Oriskany Sandstone (terminal Tippecanoe stratum), we looked stratigraphically above the quartz sandstone for the overlying unit, which should be the Needmore Formation shale (beginning of the Kaskaskia sequence). Indeed, the quartz sandstone was overlain by a black shale at the site:
This road cut is on the south side of the highway, facing north. Hence, it was in deep shade when we visited in late afternoon. Uncharacteristically, I see I failed to include a sense of scale in that photo, but the peaklike sandstone outcrop at left is probably about 7 meters tall.
A better exposure can be seen on the north side of the road, where it’s not only sunny, but the contact is more discernible.
I remembered to include a sense of scale for this shot. Thanks, Alan!
Here’s what strikes me about this exposure: the contact between quartz arenite below and black shale above does not appear to be abrupt or erosional, like I would expect a disconformity to appear. Instead, there are gradually thinner and thinner sandstone beds, and thicker and thicker shale beds. Let’s zoom in:
Fundamentally, this looks transitional (conformable). It doesn’t look like an unconformity. But it’s supposed to be one. So what gives?