Chapter PB BIOSTRATIGRAPHY, POWDER RIVER BASIN By D.J. Nichols in U.S. Geological Survey Professional Paper 1625-A Contents Biostratigraphy....................................................................................................PB-1 References ..........................................................................................................PB-3 Figures PB-1. Biostratigraphic reference sections in the Powder River Basin. PB-2. Occurrences of Momipites and Caryapollenites pollen and the definition of palynostratigraphic zones P1-P6. PB-3. Coal zones and palynostratigraphy of the Paleocene in the Powder River Basin. PB-4. Summary of age data, Wyodak-Anderson coal zone, Powder River Basin. PB-5. Distribution of biostratigraphically significant species in the upper part of the Fort Union Formation. PB-6. Biostratigraphically significant species of fossil pollen in the upper part of the Fort Union Formation. PB-7. Relative abundances of major groups of spores and pollen in Wyodak- Anderson coal. PB-8. Ratio of relative abundances of selected pollen genera in Wyodak-Anderson coal. PB-ii BIOSTRATIGRAPHY · The most common fossils in coal and coal-bearing rocks are pollen grains and spores. Thus, biostratigraphy based on pollen and spores (palynostratigraphy) has been used to determine the age of the coal beds and coal zones in the Powder River Basin. · Palynostratigraphy in the Powder River Basin ties reference sections (fig. PB-1) in selected outcrops and cores to subsurface data. Such data are used to correlate coal beds and coal zones. · There are six palynostratigraphic zones (fig. PB-2) within the Paleocene of the Powder River Basin, designated (from lowest to highest) P1 to P6, which are defined by occurrences of certain species of fossil pollen. · The principal coal zone being mined in the Powder River Basin is the Wyodak-Anderson (fig. PB-3). This coal zone is within palynostratigraphic Zones P5 and P6, in the upper part of the Paleocene. · Palynostratigraphy reveals the areal distribution of coal deposits of late Paleocene age in the Powder River Basin (fig. PB-4). These deposits, which are all in palynostratigraphic Zones P5 and P6, are in the Wyodak- Anderson coal zone. PB-1 · Palynostratigraphy in the Gillette coalfield makes use of core from a USGS drill hole at the Black Thunder mine. Palynological samples from this core have been analyzed in detail, and a palynological database has been constructed from these data. Samples from other mines in the coalfield supplement this database. · These data and others from similar studies are the basis of detailed correlations in the upper part of the Fort Union Formation (fig. PB-5) within and beyond the Gillette coalfield. · Samples from the mines in the Decker, Montana, area yield fossil pollen that are characteristic of uppermost Paleocene Zone P6 (fig. PB-5). They indicate that the Anderson-Dietz and Dietz coal beds in this area are correlative with the upper part of the Wyodak-Anderson coal zone in the Gillette area. · Fossil pollen and spores (fig. PB-6) also can reveal much about the composition of plant communities that inhabited ancient coal-forming mires, and thus can contribute to studies on the origin of coal. · Detailed analyses of occurrences and relative abundances of fossil spores and pollen grains (fig. PB-7) in samples from Black Thunder core BT558 (USGS locality D7141) support studies on the origin and nature of the Wyodak-Anderson coal. Detailed analyses also help to refine the palynostratigraphic zonation (fig. PB-8). PB-2 REFERENCES Nichols, D.J., 1994, Palynostratigraphic correlation of Paleocene rocks in the Wind River, Bighorn, and Powder River basins, Wyoming, in Flores, R.M., Mehring, K.T., Jones, R.W., and Beck, T.L., eds., Organics and the Rockies Field Guide: Wyoming State Geological Survey Public Information Circular No. 33, p. 17-29. _____ 1995, The role of palynology in paleoecological analyses of Tertiary coals: International Journal of Coal Geology, v. 28, p. 139-159. _____ 1996, Palynological zonation of the Paleocene of Rocky Mountain intermontane basins [abs.]: Geological Society of America, 1996 Annual Meeting, Abstracts with Programs, v. 28, no. 7, p. 372. Nichols, D.J., and Ott, H.L., 1978, Biostratigraphy and evolution of the Momipites-Caryapollenites lineage in the early Tertiary in the Wind River Basin, Wyoming: Palynology, v. 2, p. 93-112. Pocknall, D.T., 1987, Palynomorph biozones for the Fort Union and Wasatch Formations (upper Paleocene-lower Eocene), Powder River Basin, Wyoming and Montana: Palynology, v. 11, p. 23-36. Pocknall, D.T., and Nichols, D.J., 1996, Palynology of coal zones of the Tongue River Member (upper Paleocene) of the Fort Union Formation, Powder River Basin, Montana and Wyoming: American Association of Stratigraphic Palynologists Contributions Series, No. 32, 58 p., 7 pls. PB-3 Biostratigraphic age determination of the Wyodak-Anderson coal zone is based on palynologic analyses of samples from seven reference sections: MONTANA 1 2 (1) two mines near Decker, Montana WYOMING (2) cores and outcrop sections in the Powder River area (3) outcrop sections in the TA Hills area, south of Buffalo, Wyoming 3 4 (4) core from the Big George coal bed (5) outcrops near Linch, Wyoming 5 6 (6) core from the Black Thunder mine (7) the Dave Johnston mine near Glenrock, Wyoming 7 Figure PB-1. Biostratigraphic reference sections in the Powder River Basin. P6 U P5 M. anellus M. actinus P4 M. leffingwellii M M. waltmanensis M. wyomingensis P3 C. veripites M. inaequalis P2 C. wodehousei L M. ventifluminis P1 M. triorbicularis Figure PB-2. Occurrences of Momipites and Caryapollenites pollen and the definition of palynostratigraphic zones P1-P6. L, M, and U designate lower, middle, and upper Paleocene. P6 Tongue River Mbr P5 The palynostratigraphic zonation for the Powder River Basin is based on ranges of fossil pollen and spores P4 composited from reference sections. These sections include outcrops and Lebo cores. The economically important Mbr deposits of the Wyodak-Anderson coal zone (uppermost three black lines in diagram) are in the upper P3 Paleocene palynostratigraphic Zones P5 and P6. Other coal beds in Fort Union Formation the Tongue River Member are in Zones P4 or P5. Tullock P2 Mbr 150 Meters P1 Figure PB-3. Coal zones and palynostratigraphy of the Paleocene in the Powder River Basin. Age determinations of the Wyodak-Anderson coal zone are based on palynologic analyses of samples from seven reference sections: (A) two mines near Decker, Montana MONTANA A P6 (B) cores and outcrop sections in the B Powder River area, Montana and Wyoming WYOMING P5-P6 (C) outcrop sections in the TA Hills area (D) core from the Big George coal bed (E) outcrop sections near Linch, Wyoming P5-P6 (F) core from the Black Thunder mine (G) Dave Johnston mine near Glenrock, C D P5 Wyoming P5-P6 F Zones P5 and P6 are late Paleocene in age. E P5-P6 G P6 Figure PB-4. Summary of age data, Wyodak-Anderson coal zone, Powder River Basin. Scale 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 (Meters) D6707-U, -Z Smith 400 D6707-O, -Q, -T P6 D6707-K, -M, -N Anderson D6707-A, -E, -I, D6708-D 300 D6708-A, -B D6709-H, -I, -J, -K Canyon D6709-E, -F, -G P5 200 D6709-A, -B, -C, -D, D6710-A Wall D6711-E, -F Pawnee D6711-C, -D O'Dell 100 D6711-A, D6768, D6711-B Local 2 D6766-A, D6767 Blue numbers refer to D6764, D6765 Macklin... names of species shown P4 D6762, D6763 in the next graphic display. Sawyer Figure PB-5. Distribution of biostratigraphically significant species in the upper part of the Fort Union Formation. 1. Momipites ventifluminis 2. Momipites wyomingensis 3. Caryapollenites wodehousei 4. Triatriopollenites granulatus 1. 5. Aquilapollenites spinulosus 6. Polyatriopollenites vermontensis 7. Caryapollenites inelegans 8. 8. Caryapollenites veripites 9. Tilia vescipites illustrations of 10. Insulapollenites rugulatus selected specimens 11. Talisiipites pulvifluminus 10. 12. Alnipollenites scoticus 13. Periporopollenites sp. 14. Pistillipollenites mcgregorii 15. Paraalnipollenites sp. cf. P. confusus 14. Figure PB-6. Biostratigraphically significant species of fossil pollen in the upper part of the Fort Union Formation. 1 2 9 10 11 12 13 Fern spores 14 15 16 Conifer pollen 17 18 Angiosperm pollen 19 20 21 22 23 As indicated by the abundance 24 25 of pollen, conifers (trees of the 26 family Taxodiaceae) were the 27 28 most common kind of plant that 29 contributed to development of Sample numbers in Core BT558 (D7141) 30 this coal bed, which is 70 ft thick 31 32 in USGS core BT558. 33 34 35 0 20 40 60 80 100% Figure PB-7. Relative abundances of major groups of spores and pollen in Wyodak-Anderson coal. 1 2 9 10 11 Detailed palynostratigraphic 13 zonation of the interval is 14 15 possible on the basis of the 16 ratio between kinds of fossil 17 pollen present in the coal. 18 19 The ratio of the total number 20 of specimens of all species 21 22 of the genus Momipites to that 23 P6 of specimens of all species of 24 25 the genus Caryapollenites is 26 P5 consistently less than 1.0 in 27 28 uppermost Paleocene Zone P6. 29 These data can be used to place Sample numbers in Core BT558 (D7141) 30 31 the Zone P5-P6 boundary. 32 33 34 35 0 0.5 1.0 1.5 2.0 2.5 3.0 Ratio of Momipites to Caryapollenites Figure PB-8. Ratio of relative abundances of selected pollen genera in Wyodak-Anderson coal..