berzelia rock, _columbia_county, _georgia_4, I_AMC7764
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© Copyright Alan Cressler 2011

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The rock that forms this flatrock outcrops is most often referred to as indurated flint kaolin. In reality it looks like there is actually very little flint kaolin in the outcrop. The rock is very sandy in places but does contain some flint kaolin. Until last weekend I had never heard of such. I did some investigating and didn't come up with much. I finally called my geologist friend David Prowell (retired USGS) and got a great explanation. It is assigned to the middle Eocene age Huber/Congaree Formation:

"Since you asked for clarification, let me give you a detailed picture. During the early middle to middle middle Eocene (nanno Zones NP12-NP14), large deltas (sediment laden rivers) spread across Georgia from Augusta to just west of Macon. If you look at sediment from these rivers, you will see crossbedded, poorly sorted, angular, kaolinitic sand. At some point these deltas entered a shallow sea where the sediment became well sorted, sub-angular, sand with beds of silty clay. The inner edge of this sea was down towards Americus and curves around to Aiken, SC. Check out a map of the Mississippi delta.

The people mapping within the fluvial environment named the stuff (upper) Huber Formation whereas the people mapping in the marine environment named the stuff Congaree Formation. The two formations intertongue with each other and can't really be mapped separately. Therefore, in a regional sense they are just one (allostratigraphic) formation.

If that weren't confusing enough, the sea cycled two times within the Huber/Congaree event making separate NP12, NP13, and NP14 depositional packages separated by unconformities. Each time, the sea transgressed a little further inland. Therefore, your deposit is probably not NP12, possibly NP13, but most likely NP14, because it is way updip and at the top of the Huber/Congaree section.

You can see your same layer all around eastern Georgia but it's only "flint kaolin" in the correct weathering environment. If you look closely at your layer, it probably has a lot of poorly sorted sand in with the kaolin. The "true" flint kaolins are predominately clay/silt particles and the stuff does resemble flint from a distance."


title berzelia rock, columbia county, georgia 4
date yyyymmdd hr:mn 2009:10:04 11:18:04
source Flickr
flickr_agent alan_cressler

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