Recovery of Miocene terrestrial microvertebrates from the Fleming Formation in east Texas.
One goal of vertebrate paleontological work in the Miocene of western Louisiana has been correlation with the long-established sequence of Miocene vertebrate sites in eastern Texas, studied by Wilson (1956), Patton (1969), Quinn (1955) and Prothero & Manning (1987), among others. Schiebout (1997a) summarized this correlation effort, pointing out the fact that the lack of small vertebrates at most of the classic east Texas sites represents a major bar to comparison with the western Louisiana sites, where small vertebrates are the main fossils recovered (Schiebout 1994; 1996; Schiebout et al. 1996). The western Louisiana Miocene fauna shows some affinities to the Burkeville Local Fauna, although Schiebout concluded that (based on large vertebrates) it is closer to the overlying Cold Spring Local Fauna (Schiebout 1997a). Recovery of terrestrial microvertebrate specimens from the Cold Spring sites, which previously has lacked them, has become a priority.
Burkeville, Texas is approximately 10 km west of the Texas/Louisiana boundary. Coldspring, Texas is approximately 135 km west of the boundary, in San Jacinto County south of Lake Livingston and is clearly stratigraphically higher in the Fleming outcrop than beds near Burkeville.
Work by Anderson (1958) recognized conglomerate layers rich in calcium carbonate nodules at some of the classic Cold Spring sites. Detailed locality data on sites visited by LSU field crews is on file at the LSU Museum of Natural Science, Vertebrate Paleontology Section. Two visits by LSU field crews in 1996 to the vicinity of site 2 of Anderson (1958), approximately 2.4 km (1.5 miles) north of Coldspring, Texas, revealed that some of the old vertebrate fossil quarries are still visible, although partially filled and grassed. The level of human occupation of the region has risen, and the sites are now in a pasture. The exposures in which Anderson measured section are likewise over-grown, but the LSU field crew recovered a boulder of conglomerate in a creek adjacent to the quarries, along which Anderson had measured section (Anderson 1958).
METHODS AND RESULTS
The microvertebrates from the western Louisiana sites are recovered by acid dissolution and screening from conglomerates rich in pedogenic nodules (Schiebout et al. 1998). Wherever these conglomerates have been found in the Castor Creek Member of the Fleming Formation in western Louisiana, they have proved productive of teeth of terrestrial vertebrates, freshwater fish teeth and bone fragments.
The boulder of conglomerate recovered from the Anderson site 2 near Coldspring was a poorly sorted, subrounded granule to pebble grade conglomerate with a sand and mud matrix (Hinds, pers. comm.). It was rich in nodules (Figs. 1 & 2), usually rounded and many showing septarian features, similar to nodules interpreted as pedogenic in the western Louisiana fossiliferous conglomerates (Schiebout 1994: Figs. 3-5; Jones 1995; Schiebout 1997b: Photographs 10-13). When treated with dilute acetic acid and screened, approximately 10 kg of this Coldspring conglomerate has yielded seven teeth of the cricetid rodent Copemys, a single tooth of a heteromyid rodent, and insectivore or chiropteran tooth fragments, along with fish teeth and fragments of larger vertebrate teeth and bones. The Copemys falls within the size range seen at the Fort Polk Miocene sites. It is a genus previously unknown from the Cold Spring Local Fauna, for which heteromyids, insectivores, and chiropterans have also not been recorded previously (Stevens, pers. comm.).
Initial study of the Coldspring micromammal fauna confirms the possibility that the western Louisiana sites may be as young as the Cold Spring Local Fauna, in that the fossils recovered are similar both to them and to specimens from older Burkeville Local Fauna sites in east Texas, Town Bluff and Trinity River (Dorsey 1977), from which specimens were recovered by screening sandy material (Jacobs, pers. comm.). It seems likely that larger samples of microvertebrates from Cold Spring levels in Texas will reveal faunal differences from the Burkeville levels, similar to differences in the large mammals used in developing the sequence of Local Faunas (Wilson 1956), but it is also possible that some of the common small mammals may have been stratigraphically long ranging in the Gulf coast of the Miocene. Schiebout's summary of correlation of the western Louisiana sites with east Texas sites, prior to the recovery of Coldspring micromammals, gives a figure showing the sequence of Fleming Formation Local Faunas (Schiebout 1997a: Fig. 5).
[FIGURE 1 OMITTED]
Efforts to extend the biostratigraphic and geographic range of research on the microvertebrates from nodule-rich conglomerates in the Miocene of the Gulf Coast also include work in Louisiana on members underlying the productive Castor Creek Member of the Fleming Formation (Hinds 1998). Detailed mapping in the Fort Polk area of western Louisiana is currently underway by LSU researchers, who are alert for the conglomerates, especially in the clay-rich units. The Fleming in Louisiana is considered to consist of six members, with clay-rich and sand-rich members alternating (Rogers & Callandro 1965). The clay-rich members, from stratigraphically lowest to highest, are the Lena, Dough Hills, and the Castor Creek. Turcan et al. (1996: Table 1) working with data from electric logs included beds outcropping near Burkeville, Texas and the Castor Creek Member in western Louisiana in their contiguous Burkeville aquiclude. The current geological mapping should help clarify whether the clay-rich and sand-rich zones are localized and interfingering deposits, or are actually relatively continuous from western Louisiana through east Texas.
[FIGURE 2 OMITTED]
The fossiliferous conglomerates in western Louisiana occur in fluvial mudstones and have been attributed to reworking of soils and concentration of debris including animal remains, during drops in base level in which rivers entrenched and the floodplains were subject to increased erosion (Schiebout 1994: 679; Jones et al. 1995). Smith & Kitching (1997) reported a bed of pedogenic nodule conglomerate rich in terrestrial vertebrate remains, including skulls and partial skeletons of cynodonts, in floodplain deposits of Lower Jurassic age in South Africa, which they likewise attributed to a regional base level drop.
One purpose of this study is to record occurrences of these conglomerates in both east Texas and Louisiana in the hope that scientists whose work takes them to areas of Fleming Formation outcrop will notice conglomerates, alert us, and confer about sending a sample for trial processing for microvertebrates in our bulk acid lab. Fossiliferous conglomerates range from 10-25 cm thick in western Louisiana (Jones et al. 1995). The nodules produce a very distinctive speckled appearance (Fig. 1). Very worn bone fragments are the fossils most likely to be observed on conglomerate outcrop surfaces, with the small teeth usually represented only by very rare bits of shiny enamel. The teeth of terrestrial vertebrates from these conglomerates are usually between one and two mm in maximum crown length. The rocks vary in degree of cementation by calcium carbonate. Some can be crumbled by hand and others required a jackhammer to break for transport to the laboratory. They tend to be the most resistant rocks in their intervals, so they can be recognized in very small outcrops.
We wish to thank Frances and Earl Hodges, landowners near Cold-spring, for their cooperation, and individuals at Fort Polk, especially Director of Public Works, Rory A. Salimbene, LTC, EN and his environmental staff, including Dr. Charles Stagg, James Grafton, and Bob Hays, for their help, which made the western Louisiana research possible. Pam Borne and Megan Jones helped with Coldspring field work. I appreciate being allowed to study specimens in the collections of the Shuler Museum of Paleontology, Southern Methodist University, and in the Frick Collection of AMNH. Louis Jacobs, Ruth Hubert and Paul Heinrich provided helpful comments on the manuscript. Helpful discussions are acknowledged with Margaret S. Stevens (Lamar University Department of Geology), Louis L. Jacobs (SMU Department of Geological Sciences), and David J. Hinds (LSU Department of Geology & Geophysics).
Paleontological research and geologic mapping on Fort Polk is currently supported by U. S. Army FORSCOM, on contracts to the LSU Museum of Natural Science and the Louisiana Geological Survey, administered by Dr. Frank Servello, Fort Worth District, U. S. Army Corps of Engineers, issued to Prewitt and Associates, Inc. Mapping in the vicinity has also been supported by the United States Geological Survey via the EDMAP program. Support has also been provided by the LSU Museum of Natural Science.
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Judith A. Schiebout and Suyin Ting
Louisiana State University, Museum of Natural Science
Baton Rouge, Louisiana 70803
JAS at: email@example.com
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|Author:||Schiebout, Judith A.; Ting, Suyin|
|Publication:||The Texas Journal of Science|
|Date:||Aug 1, 1998|
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