Detailed Sedimentological Study of the West Franklin Limestone Member (Desmoinesian to Missourian) of the Shelburn Formation (Upper Pennsylvanian) of southwestern Indiana Grace L. Stone and William S. Elliott, Jr. Department of Geology and Physics, University of Southern Indiana, 8600 University Blvd., Evansville, Indiana 47712; [email protected] Introduction Results Discussion The West Franklin Limestone is a key marker bed in correlating Middle to Upper Southwest Indiana is underlain by gently, westward dipping (2° to 3°) Middle to Upper Photomicrograph of skeletal 76.4 to 76.9 feet depth: tan to gray, Pennsylvanian strata in the Illinois Basin (Wier & Ault, 1986; Brown & Rexroad, 2009). It is Pennsylvanian strata on the southern margin of the Illinois Basin consisting of recurring packstone showing broken bioturbated, skeletal packstone with typically identified by two limestone benches separated by an interval of siliciclastic deposits of limestone, sandstone, shale, and coal (Fig. 1). USI 1-32, an exploratory USI 1-32 Core Upper Bench of skeletal fragments that include, broken crinoid, brachiopod, sediment, but the number of benches is variable from the absence of the limestone to four coalbed methane well drilled in 2009, is located at 37.951° N and 87.670°W south of 75 bryozoans, gastropods, forams, West Franklin bryozoan, and gastropod fossils with discrete limestone intervals (Anderson, 1956; King, 1994; Heckel et al., 1998; Rexroad et al., the campus of the University of Southern Indiana in Vanderburgh County, Indiana (Fig. and algae, with a micrite mud traces of disseminated pyrite. 2008; Gray, 2011). 2). The total depth of the well is 780 feet with a core recovered from the West Franklin Limestone matrix from 76.45 feet depth. py In this study, the core contains three limestone intervals separated by siliciclastic intervals. Limestone from 76 to 100 feet. 1 mm The lower bench of the West Franklin Limestone (0.6 feet) is a skeletal packstone The West Franklin Limestone Member, Desmoinesian to Missourian in age, is the containing broken and abraded brachiopods, crinoids, and fusulinids. The geophysical well uppermost section of the Shelburn Formation. It is exposed at several localities in 78.8 to 79.0 feet depth: light greenish gray, bioturbated and Photomicrograph of log from USI 1-32 suggests a thickness of 1.7 feet for this limestone. This basal unit is Vanderburgh County, along with its type locality in West Franklin, Indiana. In most ggy py fractured claystone with overlain by 1.4 feet of dark gray, laminated to bioturbated claystone. locations, including Vanderburgh County, the West Franklin consists of two to four Upper Claystone of fractured silty claystone, with 80 M organic stringers and The middle bench of the West Franklin (16.4 feet) is made up of 5.1 feet of brecciated, distinct carbonate benches interbedded with siliciclastic sediment. West Franklin sparse organic stringers, burrows, and secondary pyrite. Note the charcoal fragments from bioturbated, skeletal wackestone containing broken brachiopod, bryozoan, rugose coral, fractures are filled with drilling 78.9 feet depth. and gastropod fauna with secondary pyrite; 4.9 feet of brecciated and fractured packstone Y Limestone S P G 1 mm E O U muds (M). with brachiopods, corals, along with stylolites and secondary pyrite; 2.5 feet of mottled, I L O R FORMATION MEMBER O e bioturbated, skeletal packstone containing brachiopods, bryozoans, crinoids, gastropods, R E H T S G I n L Photomicrograph of and rugose corals with microstylolites; and 3.9 feet of mottled, intraclastic, skeletal 0 o N Inglefied t 81.4 to 82.0 feet depth: intraclastic skeletal grainstone containing crinoid columns, brachiopods, and corals with silica nodules. The ) A s I t skeletal grainstone with R Sandstone e packstone with middle bench is overlain by 4.4 feet of light greenish gray, bioturbated silty claystone with e ) U t 85 e broken skeletal fragments O Patoka e microstylolites and charcoal and carbonized organic stringers. m f S unnamed shale e 82.4 to 82.7 feet 50 f i ( o S ( including crinoid columns, r I rip-up clasts with o s R R depth: skeletal The upper bench, 1.2 feet, is a skeletal packstone with broken and abraded algae, L s M Ditney Coal b s brachiopods, and corals s s e py grainstone with secondary quartz and brachiopods, bryozoans, crinoids, forams, and gastropods. Another core from the USI n e N n West Franklin n a k overlain by an intraclastic i A n pyrite from 81.7 feet. I c e l Limestone i abraded skeletal 1 mm Groundwater Lab (less than 3 km from USI 1-32) provides a thickness of 7.9 feet for the L 100 S k h packstone with abundant c k E T c fragments including upper carbonate bench of the West Franklin Limestone. M i N n microstylolites. I Shelburn h crinoid columns, a O Photomicrograph of skeletal t The fossil assemblages in each of the three limestone intervals of the West Franklin unnamed shale r M brachiopods, grainstone showing sorted S 150 Limestone are consistent with normal marine conditions during deposition. The alteration F Middle Bench E bryozoans, algae, and t 90 py skeletal materials from 83.6 D 83.7 to 84.0 feet depth: of siliciclastic and carbonate sediments in the West Franklin Limestone Member may be s of West rugosan (R) corals. feet depth. due to: (1) the complexity of shallow transitional marine settings; (2) interruption of e mottled skeletal Figure 1. Stratigraphic column of the Pennsylvanian Figure 2. Geologic map of the grainstone with rip-up carbonate deposition by avulsion and subsequent delta migration with increasing influx of rocks located near the University of Southern Indiana, southeast part of the Illinois W Franklin clasts, washed, sorted 1 mm detrital sediment; and/or (3) localized variation in depositional environments due to Vanderburgh County, Southwest Indiana Basin and location of USI 1-32. 88.3 to 88.8 feet Limestone skeletal debris including Photomicrograph of penecontemporaneous grabens that formed from reactivated faults of the Rough creek- depth: brecciated Shawneetown Fault Zone. The latter is further supported by thickness variation and limited crinoid columns, skeletal packstone skeletal wackestone brachiopods, and corals. with burrows lateral extent of the overlying Inglefield Sandstone. Finally, there is no evidence of deep Methods with broken water deposition in this part of the Illinois Basin during the late Desmoinesian to early 95 py brachiopod, bryozoan, replaced by silica from 88.7 feet depth. Missourian. The rock core of the West Franklin Limestone from USI 1-32 was prepared in the and gastropod fauna with stylolites, silica Geological Preparation Laboratory at the University of Southern Indiana. The core was 93.6 to 94 feet depth: 1 mm nodules, and Conclusion first cut in half using a trim saw. Limestone slabs were prepared using silicon carbide grit brecciated and fractured r secondary pyrite. Photomicrograph of (120, 220, 320, and 400) on a lap wheel, and then polished using 1200-grit abrasive e skeletal wackestones to • t USI 1-32, a exploratory coalbed methane well in southwest Indiana, contains 24 brecciated skeletal disks on a wet-lapidary wheel. The claystone intervals were dry-sanded by hand with e packstone with broken gy py wackestone with feet of the West Franklin Limestone Member of the Shelburn Formation (Upper sandpaper ranging from 400- to 1200-grit. The prepared core was described with the m brachiopod, bryozoan, Pennsylvanian); the cored part of the West Franklin is made up of 3 limestone assistance of a hand lens and a stereoscopic microscope. The three carbonate benches brachiopods and other shell gastropod, and benches separated by two siliciclastic intervals. 100 m w p g fragments from 93.7 feet of the West Franklin Limestone were described and lithofacies were identified from echinoderm fauna. a r a u a • Fossil assemblages in each of the three limestone intervals of the West Franklin c 1 mm c depth. d each interval. Photographs of each lithofacies were captured using a iPhone 7 camera. i k k n s s e Limestone are consistent with normal marine conditions during deposition. s t t o s t o o Petrographic thin sections were prepared from the other half of the core for each t n • Alteration of siliciclastic and carbonate sediments is most likely due to localized o n n e Photomicrograph of n e identified lithofacies. Billets were cut from the core and surfaces prepared using a lap e Lower e 98.8 to 99.0 feet depth: dark gray, claystone showing variation in depositional environments due to penecontemporaneous grabens as wheel and silicon carbide grit, washed, and dried on a hot plate. A mixture of A-B laminated to bioturbated fractures, burrows, a result of reactivated faults of the Rough Creek-Shawneetown Fault Zone. Hillquist epoxy was made and approximately 1 mL was placed on each of the heated Claystone of LEGEND claystone with sparse charcoal and organics billets. Next, a glass slide was mounted to the billet and then cooled to room FOSSILS SEDIMENTARY STRUCTURES West Franklin and/or organics with pyrite filled stringers from 98.9 temperature. The billet was cut from the glass slide using a Model 650 Ingram Thin AND FEATURES burrows (Chondrites). feet depth. Scale bar Acknowledgements Section Saw/Grinder. The surface exposed on the glass slide was ground to a thickness Echinoderm Limestone Brachiopod & Bryozoan Planar stratification is 1 mm. of 30-microns starting with the Ingram Grinder and finishing with a 400-grit slurry on a Burrowed Grace Stone received travel funds from the USI Endeavor Rugose Coral Program and the Provost Student Travel Grant to offset the glass plate.