FOSSIL WOOD OF THE OLIGOCENE CATAHOULA FORMATION, JASPER COUNTY, TEXAS
by Scott Singleton
Member of the Houston Gem and Mineral Society
fossilwood@houston.rr.com

s evidenced by the fossil wood ID seminar at the recent AFMS/SCFMS show in Arlington, Texas (June, 2001), interest in fossil wood collecting in Texas runs high. Abundant collecting sites along the Gulf Coast have provided beautiful and well-preserved specimens for me and for a number of other avid fossil wood rockhounds. Of particular importance is the Oligocene Catahoula Formation in East Texas and Louisiana. The following is part of my on-going study of fossilized woods—its occurrence, formation, and identification. Ultimately this text will be updated with other genera and localities, will include microphotographs to aid in identification, and will be published by the Paleontology Section of the HGMS so that all can enjoy this hobby more fully. Correspondence is encouraged, and the reader is advised to refer to the wood anatomy texts in my references for definitions and illustrations of the cellular characteristics described.

Environment of Deposition: The Tertiary subsidence of the Gulf Coast basin preserved a thick sequence of sediments, including the landward fluvial and coastal plain facies of many Tertiary stratigraphic units. Preservation of fluvial and coastal plain sediments is significant as they are the depositional environments where we might find fossilized wood. This type of sediment is currently exposed within the early Eocene Wilcox group, upper Eocene Yegua Formation and overlying Jackson group, Oligocene Catahoula Formation, and in Miocene sediments (Goliad and Fleming Formations in particular). These formations produce fossilized wood throughout the Gulf Coast from Alabama to Mexico.

Two additional requirements are essential for the preservation of wood. First, the specimen must be rapidly buried in an oxygen-free environment. Wood left in an oxygenated environment can decompose completely within 3 years. Woods of lower durability experience a majority of this decomposition within the first year. Therefore, it is important to remove the wood from access to oxygen and other agents of decay as quickly as possible. The most common method in the Gulf Coast of removing wood from an oxygenated environment is rapid burial in a sediment layer, usually by a flood event. Sands are usually not conducive to petrifaction because sand is porous and allows the free flow of ground water and therefore oxygen, thereby leading to decay. However, an upsurge in the Rocky Mountains during the Oligocene and Miocene provided an abundant amount of sediment to the coastal plains of Texas that allowed the rapid burial and sealing of the soon-to-be-petrified wood.

The second requirement is an alkaline groundwater. Acidic water leads to the dissolution of cellulose fibers while the converse is true with alkaline water. A common source of groundwater alkalinity is volcanic ash. The ash not only reduces the acidity but supplies silica to deposit in cellular spaces and also supplies the minerals necessary to give color. On the Gulf Coast, volcanic ash is sparse in the Eocene of northeast Texas. Many of the fossilized wood specimens there are lacking in silica and are bland in color. However volcanoes were active in the Oligocene and early Miocene in west Texas and Mexico, thus giving an abundance of siliceous and colored fossil woods in these sediments.

Jasper County was an Oligocene sand depocenter, thus making it conducive for rapid burial of large amounts of woody material. Leaf imprints have been found in fine sediments whereas only trunks could be preserved in the coarse sediments. However, rot and decay are evident in many of the trunks found in these coarse-grained deposits. Good examples are the mineral-filled cavities found in Jasper County fossil oaks which appear remarkably similar to "pocket rot."

Group 1: The most common fossil wood in the Oligocene of Jasper County is Quercus (oak), specifically live oak. These woods are identical in appearance to Quercus virginiana (live oak), but I refrain from identifying the Jasper County live oaks as this species because it is possible they actually represent an ancestor to Quercus virginiana. This can be confirmed only by microscopic evaluation which I have not completed at this point. Live oak is readily identified by its thick rays and vessels that have the appearance of bubbles streaming outward from the center of the trunk.

This Oligocene Quercus sp. makes up possibly half of all fossil woods found in Jasper County. It is interesting to note that Quercus is considerably less frequent in the Oligocene of south Texas, which existed at the same time as the Jasper locality. This indicates an environment change between the two areas, similar to what exists today. Also, oaks are completely absent in the Eocene of Texas although there are several recorded Eocene localities in the western US. This is due to the tropical climate existing in Texas during the Eocene that prevented oaks from gaining a foothold.

Group 2: The second group of woods is from genera which, like Quercus, possibly have existing relatives but which are less frequent in Jasper County sediments. These genera include Legume (pea family) Ulmus (elm), Acer (maple), and Gleditsia (honey locust). In many cases legumes appear to be very similar to Laurels (described below) and are often confused with them.  The one possible distinguishing factor is thick or continuous banded parenchyma that also surrounds vessels.  Under low-power lens, elms can be distinguished by the presence of wavy bands of latewood vessels connected by light-colored parenchyma cells. This arrangement is called ulmiform, from the family name, Ulmus. Maples are identified by abundant, diffuse-porous vessels (evenly spaced with no variation in size), and two widths of rays. One width of ray is a single cell wide and the other width is perhaps 5 cells wide, wide enough to be visible with a low-power lens. Honey locust contains large pores around the growth rings, but the size decreases dramatically away from the growth ring (called "ring-porous"). In addition, the latewood vessels are connected with short strands of light-colored parenchyma cells.

Group 3: The final group contains extinct genera. Extinct genera are possible, even expected, because the earth's climate was warm from the Cretaceous through the end of the Eocene. Temperatures cooled throughout the Oligocene, and the current temperature regime was established by the Miocene. Scientific journals have described three Texas fossil wood families in this group: Lauraceae (laurels), Juglandaceae (walnuts), and a conifer family, Cupressaceae (which contains cedars and junipers).

The family Lauraceae is represented by the extinct genus Ulminium. Laurels are readily identified by the presence of light-colored parenchyma cells surrounding each vessel (called vascentric parenchyma). In addition, vessels may clump in radial groups of 2 or 3 with flattened intersections between adjoining vessels. Rays are thin but visible with a hand lens and bend somewhat around vessels. Note that laurels are easily confused with legumes, and positive ID is often very difficult in the field. Ulminium (and/or legume) is fairly common in Jasper County fossil woods but is even more common in Eocene sediments. A Yegua locality north of Lake Livingston yielded either Ulminium or legume almost exclusively. The popular George West-Three Rivers-Karnes City area also contains a high percentage of these two species. The Ulminium  genus has been described from the Eocene in Oregon and Yellowstone National Park and the upper Cretaceous in California. Today, the laurel family is represented on the West Coast by the California laurel or pacific myrtle and in the Northeast by sassafras, but laurel otherwise inhabits tropical or subtropical environments.

Engelhardioxylon sp., from the family Juglandaceae, was also prevalent in the early Tertiary. Wood from this family possesses a characteristic light-colored parenchyma pattern consisting of wavy, tangential lines that intersect at right angles with the rays. The rays and vessels look similar to Ulminium. Today the Engelhardieae are represented by one genus in eastern Asia and two genera in Central America. Engelhardioxylon wood has also been identified in the upper Eocene through Miocene of the George West-Three Rivers-Karnes City area and in the upper Eocene of the Lake Somerville area.

A published description from a Yegua locality north of Lake Livingston assigns this wood to a new species, Engelhardioxylon texana. Similar woods are common in the Oligocene of Jasper County. However, juglandaceous (walnut-like) leaves and fruit from an Oligocene locality near Huntsville are assigned to the extant Mexican Engelhardieae genera, Oreomunnea mexicana, whose wood structure closely resembles Engelhardioxylon texana.

Conifer species have few representatives in Jasper County or in all of the Texan Eocene, Oligocene, and Miocene. This is in direct contrast with the current floral assemblage of East Texas which has a high percentage of conifers (Southern Pine). The sole representative (to date) of the gymnosperm Order Coniferales is an extinct genera of the family Cupressaceae, whose members include cedars and junipers. This genera, Cupressinoxylon, is macroscopically similar to other members of the Cupressaceae family in that it contains abundant zonate parenchyma (forming light-colored tangential bands a single cell wide), but it can be separated from other members by its very narrow and abrupt latewood zone. This genus has been described from the Eocene Yegua of south Texas, the Oligocene of Mississippi, and from Yellowstone National Park.

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Manchester, S. R., 1983. Fossil wood of the Engelhardieae (Juglandaceae) from the Eocene of North America: Engelhardioxylon gen. Nov. Bot. Gaz. 144:157-163.

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