Petrified wood from the Inglefield Sandstone (Upper Pennsylvanian) in Vanderburgh County, Indiana

William Elliott and Scott Beard – University of Southern Indiana

Introduction Background & Results

Upper Pennsylvanian rocks in southwest Indiana dip 1 to 2 degrees to the west on the eastern margin of the Illinois Basin (Fig 1). The Patoka Formation overlies the West Franklin Limestone of the Shelburn Formation and is capped by the Carthage Limestone of the Bond Formation. The lower part of the Patoka is subdivided into the Ditney Coal overlain by the Inglefield Sandstone. The Inglefield Sandstone consists of tan to gray, thin to thick bedded, trough cross-stratified, fine- to medium-grained quartz arenite. The Inglefield Sandstone is typically 5 to 10 m thick, with localized 20 to 25 m thick deposits in Vanderburgh and Posey Counties, Indiana (Fig. 2). These localized thick sandstone deposits are interpreted as incised paleovalleys that formed during lowstand systems tracts and subsequently back-filled during early transgression.

Eight samples of petrified wood were collected from the Inglefield Sandstone west of Evansville in Vanderburgh County, Indiana. Four specimens are identified as Dadoxylon, two specimens are Cordaites, and two specimen are coniferous. The conifer specimens have a hematite/limonite preservation and upon preparing thin sections were too degraded to identify wood type. Transverse and tangential thin sections were prepared from silicified petrified wood samples. Transverse sections are one to twelve cells high and exhibit uniseriate rays. Many of the cells are circular in shape, while others are angular, most likely caused by compaction. Tangential sections show broad tracheids in contrast to narrow rays. Growth rings are not present in any of the samples.

Regression of continental seas due to periodic Gondwana glaciation resulted in repeated exposure of the Illinois Basin and the onset of drier climates. The association of Cordaitalean and coniferous petrified wood from the Inglefield Sandstone is consistent with the remains of dry lowland vegetation that accumulated during a lowstand systems tract. Because of the decreased preservation potential of lowstand deposits, this study provides additional insights into the xerophytic floras of the Late Pennsylvanian (Kasimovian) of North America.

Petrified Wood Specimens

Figure 1. Geology and location of where petrified wood sample were found.

Figure 2. Stratigraphic column of the university of Southern Indiana, Vanderburgh County, Southwest Indiana

Figure 6. Tangential photomicrograph of specimen PF-6 in plain polarized light displaying narrow rays and broad tracheids. This sample is identified as Dadoxylon, a cordaitalean wood.

Figure 10. Transverse section of specimen PF-4 in plain polarized light showing the lack of growth rings. Note the deformed cells most likely caused by compaction.

Figure 7. Transverse section of D-1 in plain polarized light showing vascular structure consistent with conifer-like wood. This specimen is poorly preserved and mineralized by hematite and limonite.

Figure 11. Transverse section of specimen PF-6 in plain polarized light showing significant deformation likely caused by compaction. Scale bar is 1 mm in length.

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Discussion

Figure 3. Specimen PF-6 showing the irregular surface of petrified wood with tangential orientation. This specimen is silicified and contains numerous quartz-filled fractures. Scale bar is one centimeter in length.

Figure 4. Transverse cut of specimen PF-2 showing rows of cells that lack growth rings. Note the presence of iron bands in the specimen. Scale bar is one centimeter in length.

Figure 5. Tangential cut of petrified wood sample from the Inglefield Sandstone showing trachieds & narrow rays. Scale bar is one centimeter in length.

Hand Specimens of Petrified Wood from the Inglefield Sandstone

Photomicrographs of Petrified Wood from the Inglefield Sandstone

Previous studies by Jensen (1982) and Menci et al. (2009) described Pennsylvanian petrified wood from Oklahoma and Czech Republic respectively. Specifically, Jensen (1982) described samples exhibiting growth rings and identified the petrified wood as Cordaites and Dadoxylon. Menci et al. (2009) described Pennsylvanian petrified wood from the Intra Sudetic Basin of Czech Republic. These samples lacked growth rings and were identified as Dadoxylon. Likewise, Arnold (1931) identified specimens of Cordaites and Dadoxylon lacking growth rings from Pennsylvanian rocks of Michigan and Ohio.

In this study, eight specimens of petrified wood were collected from the Inglefield Sandstone of Vanderburgh County, Indiana. Tangential, transverse, and longitudinal thin sections were prepared from silicified and hematite replaced specimens. Transverse sections exhibit uniseriate rays with two to twelve cells high. Most cells are circular in shape, while others are angular or distorted. This distortion is most likely caused by compaction. Tangential sections show narrow rays in contrast to broad tracheids. Four specimens are identified as Dadoxylon, two specimens are Cordaites, and two specimen are coniferous. All petrified wood samples from the Inglefield lack growth rings, suggesting an absence of seasonality.

Furthermore, specimens studied from the Inglefield exhibited deformational features including compaction, quartz-filled fractures, contortion, and shearing. These deformation features attest to significant burial and diagenetic compaction of the petrified wood.

In summary, the presence of cordaitalean and conifer petrified wood in lowstand deposits is consistent with the expansion of dry upland vegetation during glacial periods in the Late Pennsylvanian. Finally, petrified wood from the Inglefield provides another avenue to further study ancient ecosystems and provide insights into seasonality and paleoclimate.

Previous work by Falcon-Lang et al. (2009) challenges the idea that tropical lowlands were dominated by rainforests in the Pennsylvanian, but instead were dominated by seasonally dry vegetation during periods of glaciation. Falcon-Lang et al. (2009) discusses fossil plants found within incised channels in paleovalleys beneath the Pennsylvanian Baker Coal of Illinois, Indiana, and Kentucky. Furthermore, DiMichele (2013) discusses two qualitatively distinct kinds of seasonally dry assemblages that appeared periodically in Pennsylvanian lowlands. Theses assemblages include cordaitalean dominated and conifer dominated with overlap between the two environments.

Traditionally, the coal measures of the Pennsylvanian have been interpreted to represent vast tropical wetlands and coal swamps (Fig. 12). Over the past 40 years, researchers have modified their focus on the paleobotany of Pennsylvanian sediments, focusing on intervals related to lowstands of sea level. From these studies, researchers discovered a diversity of plants during glacial intervals, including conifers and cordaites, attesting to seasonally dry upland ecosystems in the Pennsylvanian (Falcon-Lang et al. 2009; DeMichele, 2013).

Figure 12. A. Artists reconstruction of the traditional view of Pennsylvanian swamps dominated by tree ferns and lycopsids. B. Cordaitalean forest dominated by large trees with undergrowth of ferns and seed ferns consistent with dryland landscapes. Modified from DiMichele (2013).

Periodic regression of continental seas due to Gondwana glaciation resulted in periodic exposure of the Illinois Basin and lowstand deposits during the mid- to Late Pennsylvanian. In particular, the Inglefield Sandstone is interpreted as an incised paleovalley fill associated with backfilling following a lowstand. Thus, the petrified wood in the Inglefield provides a unique opportunity to learn more about the paleobotany of dry upland environments in the Pennsylvanian.

Conclusions

• Specimens studied from the Inglefield Sandstone (Pennsylvanian) exhibit deformational features including compaction, quartz-filled fractures, contortion, and shearing that attest to significant burial and diagenetic alteration of the petrified wood.

• Presence of cordaitalean and conifer petrified wood in lowstand deposits of the Inglefield Sandstone is consistent with the expansion of dry upland vegetation during glacial periods in the Late Pennsylvanian of southwest Indiana.

• Petrified wood of the Inglefield Sandstone (Pennsylvanian) provides another avenue to study ancient ecosystems, and specifically the paleobotany of dry upland environments of the Late Pennsylvanian.

AcknowledgementsMany thanks to Travis Durham, Kevin Howard, and Cindy Mauro for assisting with field work. One of the petrified wood samples (D-1) was donated to our study by Dr. Paul K. Doss.

Sample Description Preservation Type Growth RingsNumber of Cells High Identification

PF-1Uniseriate rays; uniform cell size and shape

Silicified Not Present 2 to 7 Cordaite

PF-2Uniseriate rays; minimal distortion from compaction, cells are mostly circular and uniform in shape

Silicified Not Present 2 to 12 Dadoxylon

PF-3Uniseriate rays; deformed sections, cells not uniform in size because of compaction

Silicified Not Present 2 to 7 Dadoxylon

PF-4Uniseriate rays; cells are rectangular; many cells merge to form larger cells

Silicified Not Present 3 to 10 Dadoxylon

PF-5Uniseriate rays; uniform cell size and shape

Silicified Not Present N/A Cordaite

PF-6Uniseriate rays; cells are rectangular with rounded cells and rays deformed

Silicified Not Present 1 to 6 Dadoxylon

D-1

Uniseriate rays with circular cells of approximately the same size;sample has been degraded by weathering

Hematite/Limonite Not PresentToo degraded to determine

Conifer-Like

D-2Uniseriate rays; sample has been degraded by weathering

Hematite/Limonite Not PresentToo degraded to determine

Conifer-Like

Table 1. Description and Characteristics of Inglefield Petrified Wood.

Figure 8. Longitudinal section of PF-2 highlighting structure of Dadoxylon in plane polarized light.

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Figure 9. Transverse section of specimen PF-5 in plain polarized light showing the lack of growth rings.

March 24, 2018