Sunday, July 31, 2011

Accretionary Wedge #37 - Sexy Geology

No, Geology is not a fetish for me. Sometimes an obsession, but not a fetish - there is a difference.  Without going into any unnecessary detail, I will just list my favorite rock units, as this part of AW #37.

For photographic purposes, I related my "current love" for the Jurassic Navajo Sandstone, in my "What a Geologist Sees - Part 36".  I haven't been in the outcrop area of the Navajo Sandstone for many years, but through some of my old 35 mm slides and digital photos given to me by friends - I have developed a great appreciation for the eolian cross-beds and the diverse settings in which the Navajo is exposed on the Colorado Plateau.

For fossil-collecting purposes, I would have to say that the Late Eocene limestones of the Georgia Coastal Plain and the Florida peninsula.  Of greatest interest - to me - are the diverse array of irregular echinoids with a small, but nice assemblage of different scallops (Chlamys and Pectin sp.).  On a smaller scale, there are also small comatulid crinoids and very small brachiopods at some of the localities.  In the Savannah River area of Burke County, there are occurrences of the large oysters Ostrea gigantissima in the Griffins Landing Member of the Late Eocene Dry Branch Fm.

The widespread distribution of limestones (close to the Fall Line) and other evidence suggests that the Eocene, especially the Late Eocene was a time of warm temperatures, friendly to biodivesity.  The Ocala Limestone in Florida is the dominant Late Eocene rock unit in Florida.  Updip in Georgia, there are a number of different limestone units of interest, as well as the sandy Clinchfield Formation.

The first Late Eocene locality that I visited was the Tivola Limestone, in the Perry, Ga. area, in Houston County.  This sand dollar was from that undergrad field trip, from the old Medusa Quarry, SE of Perry.  This sand dollar is also found in the Tivola-equivalent at the Rich Hill quarry, just 2 miles from the Fall Line, NW of Macon.

Unless there have been some interpretational changes in the strat sections, the Sandersville Limestone is one of the youngest of the Late Eocene units in the Georgia Coastal Plain and the Periarchus quinquefarius was the last of the 3 (or 4) Periarchus species in Georgia.  The Ocmulgee Fm. is a downdip equivalent of the Sandersville.  Periarchus lyelli, from the uppermost part of the underlying Middle Eocene Lisbon Fm. and the Clinchfield is the first of these, followed by P. pileussinensis, then P. quinquefarius.

This smaller irregular echinoid is from near Leesburg, GA, in the old Starkville quarry.  If memory serves me correctly, this is the equivalent to the limestone exposures at the northern end of Albany, near the "power dam" on the Flint River.

I traded for this specimen from the Florida peninsula.  There are numerous other Late Eocene echinoids in my collection, and there are other Georgia collecting localities, some now inaccessible.
Presumably, it was the end-of-Eocene Chesapeake bolide impact that triggered a regional extinction event, which wiped out a number of the Late Eocene echinoid taxa or at least those in certain updip facies. 

Yeah, I guess I am sort of in love with the Late Eocene of Ga. and Fla.  Just don't tell my wife.

[Update:  I seem to be having trouble with my Blogger account, it is giving me grief when I try to comment on my own post.  So thanks to all that leave positive and useful comments.  I am not ignoring you, the system just has a glitch.]

1 comment:

  1. Nice! As fossils go, I'm pretty enamored with echinoderms myself.