Polygonal Fracturing in South Texas Petrified Wood
By Singleton, Scott
Karnes and Live Oak counties and surrounding areas in South Texas (fig. 2) are popular petrified-wood hunting grounds because the late Eocene to Oligocene sediments have undergone extensive primary and secondary mineralization, producing wonderfully colored and patterned specimens. In fact, this same mineralization is responsible for the leaching and subsequent concen tration of uranium at the unconformity between the uppermost Jackson Group (Eocene) and the Oligocene Catahoula Formation in Karnes County. Because of the importance of uranium mining, the geology of this area has received consid erable attention in the literature (e.g., McBride, Lindemann, and Freeman 1968; Galloway and Kaiser 1980). The mineralogy of Karnes County from a collector’s point of view has been summarized by Cherepon (1996a, b). Some of the petrified wood specimens from this area are truly outstanding. The superb patterning and color contrasts (figs. 1, 3) of the fossil replacement makes fine cutting material that is highly desired by lapidaries. Although not readily vis ible, the specimens in both figures 1 and 3 show clear indication of palm vascular bundles (i.e., “straw”) in their center portions, clearly identifying them as Palmox ylon. In figure 1, hexagonal patterning is only present in the outer portions of the specimen, but in figure 3, polygonal patterning is present throughout the entire piece. In both specimens, the vascular bundles should be present throughout the trunk of the palm. They cannot be seen in the outer portions of either specimen, however, because extreme silicification has wiped out all of the original plant cell structures, leaving essentially pure silica behind.
The nature of the fractures can be observed easily by viewing a cut section of this material with a hand lens (fig. 4). A close look at the polygonal patterning shows how it interacts with the fossil cell structures (in this case, palm vascular bundles). Sometimes the polygonal pattern ing intersects cell structures, splitting them into two parts, each located in adjacent polygons. This is not the result of original growth, but results from mineralization after the plant was fossilized.
Apparently, the previously fossilized specimen was subject to a stress that broke it into columnar, polygonal sections, somewhat similar to the shape of mudcracks on a dry lake bottom or the shape of basalt columns after rapid cooling of magma (compare fig. 6 to fig. 7). These fractures were then filled with golden-colored silica material, chalcedony. (Agates from this part of Texas have a characteristic den dritic pattern filled with this same golden chalcedony.)
Careful inspection of many of these polygonally fractured specimens reveals portions where the hexagonal and pen tagonal structures break down. Figure 5 shows the polygonal pattern collapsing into a mass of golden-colored silica. When the original silicified fossil material no longer exists because it is replaced by variously patterned and colored chalcedony, the fossil is said to have been agatized.
Polygonal patterning is usually seen in Palmoxylon because this is a common plant in the area. However, it is not restricted to Palmoxylon from this portion of Texas. I have also observed it in other kinds of fossil wood (figs. 8, 9). This again demonstrates that this type of patterning is a secondary mineralization feature that is unrelated to the actual fossil material.
A common perception among collectors of minerals and fossils in eastern and southern Texas is that the presence of polygonal fracturing indicates that the specimen is a fossil cycad. Although both polygo nally fractured wood and fossil cycads might superficially resemble each other, a more detailed analysis shows that they are unrelated (compare figs. 3 and 10; Singleton 2006). In fact, their only similarity is that they both have polygonal patterning. Careful observation shows the following dif ferences:
1. Cycads have rhombohedral to diamond-shaped patterns; there are predominantly hexagonal or pentag onal patterns in the South Texas fossil wood specimens.
2. Cycads have rhombohedral pat terns in only the armor section, and these can be seen only when viewed perpendicular to the exterior. (This is because the rhombs are actually traces of leaves that grow outward from the cortex area of the trunk. The anatomy of a cycad trunk consists of pith sur rounded by xylem and phloem, fol lowed by the cortex and then the outer armor [Singleton 2006; Dayvault and Hatch 2005; Stewart and Rothwell 1993]). In contrast, the South Texas fossil wood may have polygonal pat terning throughout the specimen. The polygonal sections are columnar and are oriented approximately parallel to the axis of the trunk. Thus, these polygons are best viewed in transverse section.
3. Microscopically, the interiors of the polygons in South Texas specimens consist of original fossil plant cell struc?tures that identify the tree type as hardwood, softwood, or palm. The patterns of these cell structures indicate that the original tree type can be any of the species that are normally found in that area. Conversely, the interior of fossil cycads contains only cycad cell structures.
4. The fracture-fill in the South Texas specimens consists of agate (banded chalcedony). The region between leaf traces in the armor of cycads consists of ramentum (pro?tective scales on the exterior of the trunk that become between leaf traces as the cycad grows [Single?ton 2006]).
I thank Johnny French and Robert Herring for granting permis?sion to use their specimens in this article, Richard Dayvault and Thomas Dutro for reviewing the manuscript, and William Besse for preparing the map.
Cherepon, A. 1996a. Minerals of the Karnes uranium district-The Franklin of Texas (abstract). Mineralogical Record 27 (1): 26.
_____. 1996b. Minerals of the Karnes uranium district- The Franklin of Texas, http://www.wma-minelife.com/uranium/ papers/ karnes.htm.
Dayvault, R. D., and H. S. Hatch. 2005. Cycads from the Upper Jurassic and Lower Cretaceous rocks of Southeastern Utah. Rocks & Minerals 80:412-32.
Galloway, W. E., and W. R. Kaiser. 1980. Catahoula Formation of the Texas coastal plain: Origin, geochemical evolution, and character?istics of uranium deposits, Bureau of Economic Geology report of investigations 100. University of Texas at Austin.
McBride, E. F., W. L. Lindemann, and P. S. Freeman. 1968. Lithology and petrology of the Gueydan (Catahoula) Formation in South Texas. Bureau of Economic Geology report of investigations 63. University of Texas at Austin.
Singleton, S. 2006. Cycad anatomy and fossil occurrences in Texas. Fossil News 12 (9): 4-9.
Stewart, W. N., and G. W. Rothwell. 1993. Paleobotany and the evo?lution of plants. 2nd ed. Cambridge, UK: Cambridge University Press.
3631 Grennoch Lane.
Houston, Texas 77025
Scott Singleton, a geophysicist in the oil industry, has been collecting and studying fossil woods on the Texas Gulf Coast for about ten years.
Copyright Heldref Publications Mar/Apr 2008
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