Annual Meeting of the Geological Society of America, Denver, 2016

Introduction

Methods

Discussion

Conclusion

• 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.

Acknowledgements

Scott Beard received travel funds from the USI Endeavor Program, Provost Programming Grant, and the Student Government Association to offset costs of travel to the Geological Society of America meeting. Many thanks to Travis Durham, Kevin Howard, and Cindy Mauro for assisting with field work. One of the petrified wood samples was donated to our study by Dr. Paul K. Doss.

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.

The petrified wood specimens were prepared by cutting billets of the petrified wood samples in the tangential and transverse directions (Fig. 3). The surface of the billets were prepared using silicon carbide (220-, 320-, 400-grit on a 350TSL Covington Deluxe Floor Lap. Billets were washed to remove excess grit and then dried by placing them on a hot plate. A mixture of A-B Hillquist epoxy was mixed and about 1 mL was placed on the heated billet. A glass slide was then mounted to the billet. After the billet was allowed to cool to room temperature, a Model 650 Ingram Thin Section Saw/Grinder was used to cut the billet from the glass slide (Fig. 4). The exposed surface was then ground to about 35 microns, and a final thickness of 30 microns was achieved by hand grinding the thin section using 400-grit silicon carbide on a glass plate. The final thickness was verified by observing a pale yellow birefringence of quartz grains under a petrographic microscope. After the slide was cleaned, a cover slip was mounted on the thin-section using Hillquist C-D epoxy.

The petrified wood was identified using tangential sections, and growth patterns using transverse sections. The specimens were examined in plain light and observations compiled in Table 1. Samples were identified by comparing cell shape, size, and cell structures to published research on cordaitalean wood, such as Mencl et al. (2009), Poole and Francis (1999), and Jenson (1982).

Figure 9. 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.

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 and transverse thin sections were prepared from silicified and hematite replaced specimens. Transverse sections exhibit uniseriate rays with one 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 (Figs. 5, 6, 8, 9, & 10), two specimens are Cordaites, and two specimen are coniferous (Figs. 7 & 11). 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 (Fig. 12). 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.

Sample Description Preservation Type Growth RingsNumber of Cells High

Identification

PF-1Uniseriate rays; uniform cell size and shape

Silicified Not Present 1 to 7 Cordaite

PF-2

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

Silicified Not Present 1 to 12 Dadoxylon

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

Silicified Not Present 1 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

Previous work by Falcon-Lang et al. (2009) challenges the idea that tropical lowlands were dominated by rainforest, but 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. 13). 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 13. 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.

Table 1. Descriptions of all the cut samples of petrified wood.

Figure 4. Grinding thin-sections using the Model 650 Ingram Thin-Section Saw/Grinder in the Geology Preparation laboratory at the University of Southern Indiana.

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 11. 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 7. Transverse cut of hematite-limonite preserved petrified wood (D-1) from the Inglefield Sandstone. The size of the cells, along with ray structures, are consistent with a conifer. Scale bar is one centimeter in length.

Figure 5. 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 6. Transverse cut of specimen PF-2 showing rows of cells that lack growth rings. Scale bar is one centimeter in length.

Figure 3. Illustration showing the orientation of transverse, radial, and tangential sections through wood used for identification and resolving growth rings. Figure Courtesy of University of Miami (2016).

Figure 12. Transverse section of specimen PF-6 in plain polarized light showing significant deformation likely caused by compaction. Scale bar is 1 mm in length.

Figure 8. 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 Inglefield Sandstone Photomicrographs of Petrified Wood from Inglefield Sandstone

Petrified wood from the Inglefield Sandstone of the Patoka Formation (Upper Pennsylvanian) in Vanderburgh County, Southwest IndianaScott C. Beard and William S. Elliott, Jr.

Department of Geology and Physics, University of Southern Indiana, 8600 University Blvd., Evansville, Indiana 47712; scbeard@eagles.usi.edu

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