How much of the USGS is actually surveyed?

[Jdeutsch]

I've been studying the USGS map http://mrdata.usgs.gov/geology/state/map.html# and was looking at some Silurian/Ordovician interface areas- Islands that are out in the waters of Green Bay- one reef in particular in which I have found a lot of Ord type fossils- bryozoan and crinoid - and never any coral. The Silurian areas around this region usually have favosites.

The map describes the west shore of Green Bay as Ordovician and the east shore as Silurian.but it identifies the reef in question as Silurian. My findings suggest Ordovician. I suppose glacial movement could account for the discrepancy, but if so, it would be unclear to me as to how a geologic age could be ascribed without boring into bedrock.

This made me curious as to the amount of inferential vs surveyed data on the USGS survey, and the density of real data. Can someone tell me how the map was constructed? I assume the quality of the information varies greatly in different regions.

[Raggedy Man]

Mining used to be big in Wisconsin. I would say that survey is fairly accurate as I've used it on multiple occasions without issues. However, the accuracy of the survey is determined by the amount of data points used to construct it. With Wisconsins known geological past, it's somewhat common to find silurian fossil beds in close proximity to ordovician fossil beds.

Best regards,

Paul

[Ridgehiker]

This is a great question.

Lots of discussion areas. However, firstly one needs to define terms. What is Ordovician, Silurian, etc.? Until the last 30 years, most of this was stil in flux. In our Paleozoic studies the most important variable is the relationship of one formation to another. Excuse the pun but this is not 'set in stone'. When you get into early Paleozoic dating...no agreement. Perhaps 25 million years or so flexibility.

Anyways, a lot depends when field work was done, the data for the maps was published, etc. Some of these maps can be based on data 75 years old...others within the decade.

Excellent point about mining. In general geology maps for regions with economic activity are quite detailed....especially mining, oil and gas.

[PetrolPete]

I'm sure in some areas the maps may have been built by applying formation dip and thickness to a topographic map based on several surveyed locations (at least that is how we did it in Structural Geology) which allows for some discrepancy, due to variations in formation thickness and further erosion of the surface rock. Erosional unconformities can also cause similar issues.