Final1 The Sandersville Limestone: A Study Of Depositional Environment

This topic submitted by Jenny Germano ( germanjm@miamioh.edu ) on 12/7/00 .

Introduction
The Sandersville Limestone is located on the Coastal Plain of south central Georgia. This cream to white colored fossiliferous, clay-rich limestone was deposited during the Eocene in a deep-sea marine environment.
Few studies have been done on the Sandersville Limestone. By studying its biostratigraphy and focusing on the micropaleontology, we are able to further refine the age of the limestone. The microfossils found with the layers of the limestone also hint at the changing depositional environment of the area. A study of the clay mineralogy of the Sandersville site through X-ray diffractometry and insoluble residues helps support the evidence of the fossil record. Over depositional time, the sea over the site shallowed and regressed.
Most of the micropaleontological work that has been done on the Sandersville Limestone has been in the field of invertebrates. This semester long continuation has been focused on the few remains of vertebrates that were found in the summer of 2000. These remains include both shark and bony fish teeth. The analysis has been to see if they too, show a change in the strata over time.
This project is part of a three-year study of the Sandersville Limestone. It has been run through the Atlanta Consortium for Research in the Earth Sciences, an NSF funded program, since the summer of 1999 and will continue through the summer of 2001. The data and samples collected and studied during these three years will help to enhance what is currently known about the Sandersville Limestone.


Location
Geologically, Georgia is divided into four distinct parts. These are the Valley and Ridge, the Blue Ridge, the Piedmont, and the Coastal Plain (see Figure 1). The Fall Line is the contact line that divides the Piedmont Providence to the north and the Coastal Plain to the south. It is named for the common occurrence of waterfalls and rapids in the rivers along this line. The Coastal Plain consists mainly of sedimentary rocks of Mesozoic and Tertiary age. The Piedmont and Blue Ridge are composed predominately of metasedimentary and metaigneous rocks. The Valley and Ridge Providence is composed of folded Paleozoic rocks (Geology of Georgia).
The Sandersville Limestone site is found in the upper Coastal Plain of Georgia just below the Fall Line in central Washington County. The samples were taken from three locations along the Saffold Road outcrop.

Eocene Background
The Sandersville fossils are Late Eocene in age. The presence of Spiropora majuscula dated them to Middle Jacksonian or about 38 million years old (Canu and Bassler). For this reason, I will give a brief summary of events occurring in the Eocene Epoch.
The Eocene Epoch is part of the Tertiary Period in the Cenozoic Era. It lasted from 54 to 38 million years ago. It was during this epoch that many small mammals evolved and that much of the vegetation on land was changing. The modern continents were drifting independently and the Tethys Sea was slowly disappearing. Global sea levels were high and it is also thought that there was high precipitation and the highest mean annual temperatures of the entire Cenozoic Era (PALEOMAP…).
During the Eocene Epoch, many of the spreading centers and faults were changing all over the world. This had a huge impact on the climate of the Earth, which in turn, impacted the organisms that were found both in the oceans and on land. One of the most important tectonic changes was the appearance of a spreading ridge in between Antarctica and Australia. By pushing these two continents apart, a new deep-water passage was opened up and this created the circum-Antarctic Current. This new current changed the ocean circulation patterns and led to a global cooling period during the end of the Eocene (Eocene…).
On the modern continent of North America, the land was beginning to take shape. Much of what is now the Southeast part of the United States including Louisiana, parts of Texas, Florida, and the southern half of Georgia were underwater. This is shown in these two maps from Dr. Blakey. Notice how on the map below, the contact line between the land and the ocean runs from the southwest corner of what is now Georgia to the northeast, similar to the Fall Line in current Georgia geology. What is now the Coastal Plain of Georgia was almost all completely underwater during the late Eocene including our site in Sandersville, Georgia.


Methods
In the summer of 1999, an ACRES group began the initial study of the Sandersville Limestone by looking mainly at the macrofossil and some microfossil fauna along the Saffold Road outcrop.
During this past summer (2000), a more in-depth study was begun. The beds of the Sandersville outcrop were measured and categorized. Then, large limestone samples were taken from strata at 2, 3, 5, and 8 feet down from the top of the limestone bed. These samples were taken back to Dr. Crawford Elliot’s lab at Georgia State University. In the lab, the samples were boiled with a Quaternary-O solution for one to two days. Then, they were boiled with distilled water. Next, the sediments that had broken away from the original sample rock were removed and sieved to separate them into size gradients.
The limestone particles from each of the four study strata were observed under the microscope. All of the microfossils were picked out and glued onto slides with water-soluble glue. Scanning electron microscope photographs were taken of multiple fossils. Some of the teeth were taken to the Georgia Institute of Paper Science and Technology to perform EDS (Energy Dispersive X-ray Spectrometry) tests. Dr. John Anderson and myself then identified the microfossils.
Both Slam Method using 10% HCl acid and Slow Dissolution using a 2.5 M acetic acid buffer were used to obtain insoluble residues from the four groups of limestone samples. These insoluble residues along with whole rock samples were tested for their clay mineralogical content through X-ray Diffractometry. Dr. Crawford and Sara Tourscher analyzed the results.
During the fall of 2000, I have worked on more literary research and have been contacting numerous paleontologists to try to find more resources about the bony fish teeth that were found in the 2, 3, and 5 foot strata. I have also done a much more in-depth study of the microteeth that we found during the past summer and have classified them into eleven basic groups based on shape and dimension. I was then able to use these groups to statistically analyze and compare the three strata that contained teeth based on numbers and types of teeth found.

Results
Invertebrate Faunal List For Strata 2 Feet From
The Top Of The Sandersville Limestone

Phylum: Arthropoda
Subphylum: Crustacea
Genus: Balanus sp.

Phylum: Echinodermata
Subphylum: Echinozoa
Class: Echinoidea
Numerous echinoid spines and echinoid plate fragments.

Phylum: Mollusca
Class: Bivalvia
Various bivalve fragments

Phylum: Mollusca
Class Gastropoda
Order: Neogastropoda
Genus: Turritella sp. (steinkern)
Genus: Mitra sp. (steinkern)
Genus: Phalum sp.

Kingdom: Protoctista
Phylum: Sarcodina
Class: Rhizopoda
Order: Foraminifera
Genus: Cibicides sp.
Genus: Camerina jacksonensis

Invertebrate Faunal List For Strata 3 Feet From
The Top Of The Sandersville Limestone

Phylum: Arthropoda
Subphylum: Crustacea
Subclass: Cirripedia
Genus: Balanus sp.

Phylum: Arthropoda
Subphylum: Crustacea
Class: Ostracoda

Phylum: Bryozoa
Order: Cryptostomata
Genus: Lunulites distans Londsdale

Phylum: Echinodermata
Subphylum: Echinozoa
Class: Echinoidea
Numerous echinoid spines

Kingdom: Protoctista
Phylum: Sarcodina
Class: Rhizopoda
Order: Foraminifera
Genus: Valvulineria jacksonensis

Invertebrate Faunal List For Strata 5 Feet From
The Top Of The Sandersville Limestone

Phylum: Arthropoda
Subphylum: Crustacea
Class: Ostracoda

Phylum: Bryozoa
Kingdom: Protoctista
Class: Rhizopoda
Order: Foraminifera
Genus: Cibicides sp.


Invertebrate Faunal List For Strata 8 Feet From
The Top Of The Sandersville Limestone

Phylum: Bryozoa
Subphylum: Entoprocta
Class: Gymnolaemata
Order: Cheilostomata
Suborder: Anasca
Family: Lunulitidae
Genus: Lunulites distans Lonsdale
Genus: Lunulites jacksonensis (Canu and Bassler)

Order: Cyclostomata
Genus: Diaperocia varians (Ulrich)
Genus: Spiropora majuscula Canu and Bassler
Genus: Erkosonea parkeri

Phylum: Echinodermata
Subphylum: Echinozoa
Class: Echinoidea
Numerous echinoid spines

Phylum: Mollusca
Class: Gastropoda
Order: Neogastropoda
Genus: Phalum sp. (steinkerns)

Phylum: Arthropoda
Subphylum: Crustacea
Class: Ostracoda
Order: Podocopida
Suborder: Podocopina
Superfamily: Cytheracea
Family: Cytherideidae
Subfamily: Cuneocytherinae
Genus: Cuneocythere sp.

Superfamily: Cypridacea
Family: Cyclocyprididae
Genus: Cypria sp.

Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
Infraorder: Anomura
Superfamily: Galatheidae
Family: Galatheidae
Subfamily: Galatheinae
Genus: Galathea sp.

Mystery fossil

Eleven Micro-Teeth Types Found In Sandersville

Identification Letter/Physical Descriptions/Possible Identification/Strata Found In (# Individuals)

A/Shark tooth//2’(2), 5’(1)
B/Conical and rounded w/point/Sparus sp./2’(11), 3’(10), 5’(9)
C/Flatter topped ball on stem/2’(3)
D/Flatter, curved/Sphyraena bognorensis/2’(6), 3’(4), 5’(1)
E/Flat, almost shark like/Sphyraena striata or sp./2’(1), 3’(1), 5’(2)
F/Flat, round disk-like/Parabula marylandica/2’(3), 3’(5)
G/Pointed, skinny/Amia sp./2’(1)
H/Striped, ridged edges/Gar fish/2’(1)
I/Triangular pointed top on stem/Trichiurides sagittidens or Trichiurus sp./2’(1)
J/Pointed stem type/2’(2), 3’(3)
K/Bulbulous/3’(1)



Discussion
Invertebrates
By looking at the change in the fauna from the oldest layer (8 feet from the top of the limestone) to the youngest layer (2 feet from the top), we can draw some conclusions about how the depositional environment changed over time.
Modern barnacles live in tidal zones where they can feed during high tides and fill their shells with water during low tide periods. They live fixed to objects in upper zones where the water reaches only during the high tide (van Egmond). One could take this to mean that finding their remains infers a shallow, tidal marine environment. In our study of the microfossils of the Sandersville limestone at the Saffold Road location, the remains of this tidal area crustacean were found in the form of barnacle plates. These plates were only found in the strata 2 and 3 feet below the top of the Sandersville limestone. There were no barnacle plates found in the 5 and 8 feet strata. This suggests that over time, the depositional marine environment changed from a slightly deeper marine environment to a shallow, tidal area marine environment.
Another item in the paleontological record of the Sandersville limestone at the Saffold Road location that is worth mentioning is the bryozoa. According to Stauch (1936, 1937), “definite relationships exist between various zoarial types and their habitat.” Our study showed a shift in the bryozoa from a mix of Vinculariform and Lunulitiform in the 8’ level to only the Lunulitiform in the 5’ and 3’. The Vinculariform of bryozoa live in low energy zones, which are characteristic of deep or quiet waters. The Lunulitiform on the other hand lives in sandy bottom areas where there is a strong current action. They live in shallower water and at the very most can live in water 15 fathoms deep (27.45 m or 90 ft). This change, like the appearance of the barnacles within only the two shallowest strata studied, shows a transformation from deep water to shallower.

The Teeth
The in-depth study of the teeth found in the strata of the Sandersville limestone also indicate a change over time, though their implications are somewhat less obvious due to the lack of previous research done in the area of bony fish and microteeth in paleontology. The numbers that the teeth do provide do show some interesting statistical differences between the layers that had gone previously unnoticed.
This past semester, I have focused mainly on the strata 2’, 3’, and 5’ below the top of the Sandersville limestone, because these were the three layers that contained teeth. There were no bony fish or shark teeth found among the microfossils collected in the layer 8’ below the top of the limestone.
Through the three layers studied this semester, it was found that the layers became more species rich the younger they were in age. The Margalef’s index of species richness was used instead of the Shannon diversity index, since it minimizes the effect of sample size bias (Odum 1971). The sample 2’ from the top had an index of 2.6. The 3’ sample had an index of 1.6 and the 5’ had an index of 1.2. Going along with this higher diversity, the top layer studied also had the lowest index of dominance. In this area, the 2’ sample had .19459, the 3’ had .2639, and the 5’ had .5148. This shows that the youngest layers had a higher level of diversity and that this diversity was somewhat spread out since it was not dominated by a few species as the older layers were.
An index of similarity was performed to see how the strata differed. This showed that strata 2’ and 3’ below the top of the limestone were the most similar, having an index of .75. Strata 3’ and 5’ below the top had an index of .60 and 2’ and 5’ had an index of .57. This shows that the strata closest together are the most similar. The two layers that were most similar were the two that were within one foot of each other, then the ones with two feet between them, and finally the strata that were three feet apart were the most dissimilar out of the group. This is reasonable, because the strata represent different periods of time and a longer period of time appears as a larger distance between strata. Given more time, the species content of an area has a greater chance to evolve more to have greater differences as was shown in this study.
Of the teeth found within our Sandersville microfossil samples, 45.6% of them were found in the uppermost layer studied, 2’ from the top of the limestone. The 3’ strata contained 35.3% of the teeth and the 5’ strata held 19.1% of the teeth found. This may be caused by two factors. The youngest strata may hold the most teeth samples because it occurred latest in time, giving the area time to fill it niches with more aquatic organisms. It may also be related to the shallowing upward of the sea and may have been affected by a higher energy zone.
The dominance of certain teeth types within each stratum did differ but only slightly through the layers. Tooth Type B held the highest percentage of abundance in all three of the layers. Tooth Type D was also a very prevalent type in all three strata.

Clay Mineralogy – Courtesy of Sara Tourscher, Bryn Mawr College
From looking at the clay mineralogy, it was found that kaolinite is only appears in the 2 feet sample. This is consistent with a near shore environment. Kaolinite is formed from weathered feldspars and micas found in soils. These weathered particles wash into streams and rivers and may eventually end up in estuaries. There were not large amounts of kaolinite in our sample; therefore, it is possible that some traces of kaolinite would be found in shallow marine environments.
Smectites are found in deeper marine environments and are most commonly formed by weathered volcanic glass. The majority of smectite found is Na- smectite; Ca-smectite was found in the 8 feet sample. The d-spacings one of the smectites was measured to be 15 Å. This is indicative of a divalent 6-fold coordination; because we are looking at a limestone, we drew the conclusion that it is a Ca-smectite. In the 5 feet sample, there was a small X-ray diffraction peak next to a smectite peak- this was vermiculite. Vermiculite is a clay formed by the alteration of pre-existing clay. There was also observed weathered surfaces in our samples collected at five feet.
The amount of quartz decreased deeper into the limestone. In the eight feet sample, there was no quartz measured. There was also significantly fewer quartz peaks in the sample five feet from the top of the limestone. The aluminum that was measured in the samples is due to the slide used in the X- ray Diffractometer (XRD).
One interpretation for this data is that there was a facies change during the depositional period of the Sandersville limestone. It looks to be a local regression from shallow shelf marine to near shore marine environments. We expect to see more quartz grains in a near shore environment. This is consistent with our hypothesis.

Conclusion
The depositional environment of the Sandersville limestone was not static one. Instead it was changing over time as is shown in the paleontological record by the change in both vertebrate and invertebrate microfauna throughout the strata. The invertebrate microfossils, in particular the barnacle plates and the bryozoa, and the clay mineralogy indicate a shallowing marine environment over time.

References
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Anderson, John R., and Elzea, Jessica, 1999, Tertiary/Cretaceous stratigraphy and paleontology of the north central coastal plain of Georgia, Field Trip 8, Southeastern Section, Geological Society of America, 39 p.

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