Microvertebrate concentrations in pedogenic nodule conglomerates: Recognizing the rock and recovering and interpreting the fossils

J. A. Schiebout, Suyin Ting, and J. T. Sankey

Plain Language Abstract

When calcium carbonate nodules formed in ancient soils are naturally concentrated by erosion into coarse sedimentary deposits (conglomerates, Figure 20), remains of small animals (Figure 1, 16, 34) are often included. These remains may eventually become informative fossils. In very wet areas soluble mineral matter may be washed out of the soils entirely, but in other areas (e.g. parts of the modern and ancient Gulf Coast) mineral matter from the top soil layer may be concentrated in lower layers to form nodules or crusts (sometimes called caliche). The soil-formed (pedogenic) nodules discussed herein are composed mainly of calcium carbonate.

Three Gulf Coast sites (Figure 2), from the Late Cretaceous and Paleocene of west Texas and the Miocene of east Texas and western Louisiana, illustrate how fossils can be recovered from pedogenic-nodule-rich conglomerates and interpreted. The rocks can be partially dissolved in dilute acetic acid, so that the residues can be screened and picked to retrieve the fossils. Amounts of rock are large in contrast to the amounts used in the investigation of small invertebrate fossils, often involving processing of tons of rock material (Movie1, 2, 3, 4, and 5). The effort required to process so much rock is worth it, because remains of tiny animals can be recovered, many of which would often be overlooked in surface search. These erosion-resistant, conglomeratic rocks form ledges (Figure 29) that can be recognized even in small gullies in heavily vegetated areas, where the possibility of finding fossil vertebrates is otherwise low. Surface finds such as the lower jaw of an approximately fourteen-million-year-old camel relative (Figure 32) are very rare in areas like western Louisiana. Nevertheless, such remains have been recovered by processing pedogenic nodules.

Taphonomy, the study of how fossils form, including all the processes undergone between death and study of the fossil, sheds light on the specimens from the nodule-rich conglomerates. They form in areas where soil-formed nodules and vertebrate remains are a substantial part of the coarse fraction of sediments available; i.e., they are likely to be prominent where most of the deposition is clay (Figure 21), where calcareous soils tend to form, and where rates of sediment deposition are comparatively slow, allowing good development of soils.

Concentration of nodules and bones can take place in rivers and between them during floods and in periods of widespread erosion, which can leave extensive blankets of material winnowed from soils to be buried and preserved later. Geochemical studies of the nodules can give information on the chemistry of vegetation and rainfall at the time the nodules were forming, and thus give information on ancient ecology.

Glossary

Acetic acid - The chief acid in vinegar
Cretaceous - The end of the time of the dinosaurs
Miocene - The middle part of the Age of Mammals Paleocene - The beginning of the Age of Mammals

Free lay booklet on research on the Miocene fossils of Fort Polk. 1996.
Schiebout, J. A. and B. S. Dooley, Fort Polk Fossils, June, 1996 edition, Popular Report Submitted to Fort Polk Environmental Learning Center, 23 p. This short booklet is aimed at elementary school children, and teachers and others are welcome to reproduce it. For a free copy, send a stamped, self-addressed envelope to:
Department of the Army
HQ, JRTC and Fort Polk
AFZX-PW-EC
(Attn. J. Grafton, re Miocene Fossil Booklet) Fort Polk, Louisiana, 71459-7100

Other popular articles on sites discussed in the article include:
1986. Schiebout, J. A. Big Bend National Park, A crossroads at the beginning of the Age of mammals. In Geology of the Big Bend Area and Solitario Dome, Texas. West Texas Geological Society Guidebook, Publ. 86-82:129-134.
1998. Schiebout, J. A. When Elephants and Rhinos Roamed Louisiana: Hunting Miocene Fossils on Fort Polk. Phi Kappa Phi Kappa Phi National Forum Winter Edition: 26-29.

Groups within Louisiana may request a speech by Dr. Schiebout on the Fort Polk Miocene research entitled "When Rhinos and Elephants Roamed Louisiana" from the Speaking of Science Program sponsored by NSF/EPSCoR-SI and the Board of Regents by calling 504-342-4253 or E-mailing This email address is being protected from spambots. You need JavaScript enabled to view it.

Web pages may be contacted for more information on the LSU Museum of Natural Science (http://www.museum.lsu.edu/), the LSU Department of Geology and Geophysics (http://www.geol.lsu.edu/), and the Louisiana Geological Survey (http://www.lgs.lsu.edu/).

Resumen en Español

CONCENTRACIONES DE MICROINVERTEBRADOS EN CONGLOMERADOS DE NÓDULOS PEDOGÉNICOS: RECONOCIMIENTO DE LAS ROCAS, RECUPERACIÓN E INTERPRETACIÓN DE LOS FÓSILES

Las concentraciones de nódulos de carbonato pedogénico son un recurso infra-utilizado en la recuperación de faunas de microinvertebrados terrestres. Estos nódulos han demostrado ser productivos en rocas del Cretácico tardío, Paleoceno tardío y Mioceno de Texas y Louisiana. Estas concentraciones de nódulos y fósiles se forman en la base de canales y por erosión de suelos en áreas interfluviales. El tratamiento de muestras grandes de conglomerados con ácido acético diluido permite la recuperación de especímenes frágiles, siendo los dientes de pequeños invertebrados el principal componente identificable. Geólogos y paleontólogos deben prestar atención al potencial de estas rocas, pues lentejones relativamente pequeños, que se pueden encontrar en los márgenes de arroyos y quebradas incluso en regiones con vegetación densa como el occidente de Louisiana, pueden contener importante fauna fósil.

Palabras clave: microinvertebrados, conglomerados, tratamiento ácido, tafonomía, nódulos pedogénicos

Traducción: This email address is being protected from spambots. You need JavaScript enabled to view it., Ocean Drilling Program, Texas A&M University, College Station, TX USA 77845-9547.

Résumé en Français

LES CONCENTRATIONS DE MICROVERTÉBRÉS DANS LES CONGLOMÉRATS À NODULES PÉDOGÉNÉTIQUES: LA RECONNAISSANCE DES GISEMENTS, ET LA RECOLTE ET L’INTERPRÉTATION DES FOSSILES

Les concentrations des nodules de carbonates pédogénétiques sont une ressource sous-utilisée pour la recolte de faunes microvertébrées terrestres. De tels gisements productifs ont étés exploités dans les strates du crétacé supérieur, du paléocène supérieure, et du miocène du Texas et de la Louisiane. Ces concentrations de nodules et des fossiles forment dans les lits de cours d’eau et par l’érosion des sols des interfluves. Le traitement des grands échantillons des conglomérats à l’acide acétique dilué permet la recolte de spécimens délicats, les dents des petits vertébrés étant les composantes identifiables principales. Les géologues et paléontologues devraient être au courant de leur potentiel, parce que des lentilles relativement petites, qu’on peut trouver dans les ravins et dans les coupes des cours d’eau même au régions fortement végétés tels la Louisiane occidentale, peuvent céder des faunes fossiles importantes.

Mots clés: microvertébrés, conglomérats, traitement d’acide, taphonomie, nodules pédogénétiques

Deutsche Zusammenfassung

KONZENTRATIONEN VON MIKROVERTEBRATEN IN KONGLOMERATEN VON PEDOGENEN KONKRETIONEN: DIE GESTEINE ERKENNEN, DIE FOSSILIEN GEWINNEN UND INTERPRETIEREN

Konzentrationen von pedogenen Karbonatkonkretionen sind eine unterschätzte Resource für terrestrische Mikrovertebraten-Faunen. Sie haben sich in Gesteinen aus der Ober-Kreide, dem späten Paläozän und Miozän von Texas und Louisiana als produktiv erwiesen. Diese Konzentrationen von Konkretionen und Fossilien haben sich an der Basis von Rinnen und durch interfluviale Erosion von Böden gebildet. Die Behandlung grosser Proben dieser Konglomerate mit verdünnter Essigsäure liefert selbst zerbrechliche Fossilien, von denen Zähne kleiner Vertebraten die Masse der identifizierbaren Komponenten ausmachen. Geologen und Paläontologen sollten sich des Potentials dieser Gesteine bewusst sein. Selbst relativ kleine Linsen, die auch in stark überwachsenen Gebieten, wie im westlichen Louisiana, im Bereich von Rinnen und Uferbereichen von Flüssen gefunden werden, können bedeutende fossile Faunen liefern.

Stichworte: Mikrovertebraten, Konglomerate, Säurebehandlung, Taphonomie, pedogene Konkretionen

Translation: Heinz Hilbrecht, Geological Institute, ETH Zentrum

Italiano

LE CONCENTRAZIONI DI MICROVERTEBRATI NEI CONGLOMERATI PEDOGENICI NODULARI: COME RICONOSCERE LE ROCCE E RECUPERARE ED INTERPRETARE I FOSSILI

Le concentrazioni nei noduli carbonatici pedogenici sono una risorsa sotto-utilizzata nel tentativo di recupero di faune di microvertebrati terrestri. Questi noduli hanno fornito molto materiale in rocce del Cretaceo superiore, del Paleocene superiore e del Miocene provenienti dal Texas e dalla Louisiana. Tali concentrazioni di noduli e di fossili si formano alla base di canali e per erosione dei suoli nelle aree interfluviali. Il trattamento di grandi campioni di conglomerati con acido acetico diluito consente il recupero di campioni delicati, dove le principali componenti identificabili sono i denti di piccoli vertebrati. I geologi ed i paleontologi dovrebbero prestare molta attenzione al potenziale di queste rocce perchè anche lenti relativamente piccole, come quelle che si possono trovare in banchi di ruscelli e torrenti, anche in aree ricoperte da abbondante vegetazione come la Louisiana occidentale, possono fornire significative faune fossili.

Parole chiave: microvertebrati, conglomerati, trattamento con acido, tafonomia, noduli pedogenici