July 1, 2000 - Cretaceous Fossils, Sumter Co, AL

This report (installed on Sept. 27) is late because of the month-long trip to South Africa that I took from mid-July to mid-August. As a result, I can only roughly describe what the BPS did this day. There were three sites in Sumter County that were visited, all well-known and previously visited by the BPS.

There are numerous Late Cretaceous chalk exposures in Sumter Co. One of these is found in a field near Livingston, and includes numerous shells, shell fragments, and other fossils (see Figures 1 and 2). The site is visited annually by various groups, and is known for the tiny echinoids (Boletechinus mcglamerii) which can be found there. Several were found on this day, one of which is shown in Figure 3. group
Fig. 1 - BPS members and guests collecting Late Cretaceous marine fossils.
shell fragments
Fig. 2 - Close-up view of a small part of the outcrop, showing numerous shells and shell fragments. The whole area shows a similar distribution of fossils.
tiny echinoid
Fig. 3 - One of the small echinoids this site is famous for having. The echinoids are called Boletechinus mcglamerii, and are named for one of Alabama’s state paleontologists, Winnie McGlamery, according to Dave Kopaska-Merkel.
One of the highlights of the visit was the apparent fossil "claw" held by Alan Collins in Figure 4. The interpretation as a "claw" is not clearcut , however. Dave Kopaska-Merkel (Geological Survey of Alabama) suspects the piece could also be the hinge of a large oyster. oyster hinge
Fig. 4 - Possible claw (or, alternatively, the hinge of a large oyster), held by Alan Collins.


Figure 5 shows a general mix of the types of fossils found at this site in abundance, including internal molds of bivalved mollusks, steinkerns, small scallops, and oyster shells. Larger oyster shells of the genus Exogyra are also found. I originally thought the internal molds were of brachiopods, but Dave Kopaska-Merkel notes that brachiopods have calcite shells while many mollusks have aragonite shells. We mostly see internal molds of these bivalve mollusks because aragonite is much more susceptible to dissolution. fossil shells
Fig. 5 - A sampling of the fossils from the site, including oyster valves, internal molds of bivalve mollusks, steinkerns, and a small scallop shell.
After spending more than an hour at this site in the intense heat, the BPS went to a site on the Tombigbee River where fossils around the K-T boundary are exposed (see Figures 6 and 7). The K-T boundary is the boundary between the end of the Cretaceous period and the beginning of the Tertiary period of Earth history. The site is well-studied and the personal favorite I have heard of Dr. Charles C. Smith of the Geological Survey of Alabama. Dr. Smith has written articles about the site and has published a stratigraphic profile showing the connection between the Late Cretaceous Prairie Bluff Chalk and the overlying Tertiary Clayton Formation. Once we arrived at the site, Dr. Andy Rindsberg (also of the GSA) gave the BPS a brief discussion of the significance of the site and what we could expect to find there. After spending a month in South Africa where fossil collecting is highly regulated, I am, in retrospect, surprised that we are allowed to collect at this site. Still, it was a great pleasure to do this and extremely educational.
k-t boundary Fig. 6 - Tombigbee River, in Sumter County. The K-T boundary is to the left of the backpack, at the point where the chalk color changes from light gray to a light brown.
Fig. 7 - View of river facing in opposite direction from Figure 6.
shells Fig. 8 - Cretaceous fossils from the light brown area, just below the K-T boundary in Figure 6. Includes oyster shells (Exogyra costata), some steinkerns, and internal molds of bivalve mollusks affected by boring sponges.
On looking at the boundary, one readily notices that the fossils below are different from the ones above. Figures 8 and 9 show some of the lower fossils while Figure 10 shows the upper fossils. Oysters from below include Exogyra costata, while those from above include Ostrea pulaskensis ("pulies" for short according to Andy Rindsberg). I think the pulies are the small gray oysters in Figure 10. Figure 9 shows some gastropods, internal molds, and a fragment of a cephalopod shell that I at first thought was a crinoid stem. Also, Figures 8 and 9 show strange fossils, probably connected with large bivalve mollusks, that have a "brain matter" look. According to Dave Kopaska-Merkel, sponges bored into the aragonitic shells of the bivalves, and their borings become filled with calcitic mud. Later, the aragonite dissolved, but the filled borings remained and these preserved the general form of the shells.
fossil shells
Fig. 9 - Cretaceous fossils from closer to the river, including gastropods, internal molds of bivalve mollusks with holes due to boring sponges, and a fragment of a cephalopod shell.
tertiary fossils
Fig. 10 - Tertiary fossils from the light gray area, just above the K-T boundary in Figure 6. The small gray shells are Ostrea pulaskensis.

The third site the BPS had planned to visit was the oyster fields in the Belmont area. I did not attend this part of today’s trip, but I have been there before, and show in Figures 11 and 12 photographs I took two years ago. The site is absolutely remarkable in the large numbers of giant Exogyra and Pycnodonte oyster shells. A variety of shells are found at this site and are summarized in a pamphlet provided by the GSA.
oyster shells
Fig. 11 - A field of Late Cretaceous oyster shells, near Belmont, Alabama.


Fig. 12 - A close-up of a typical area in one of the above fields, showing shells mainly of the genera Exogyra and Pycnodonte.

In summary, this was a very pleasant and interesting field trip for the BPS. Given how much time many of us have spent at the Union Chapel Mine collecting tracks and plant fossils recently, today’s trip was a nice break from the strip mine rock piles. Thanks to Dave Kopaska-Merkel for comments and revisions to this belated report!