Sedimentology, Palynology, and Sea Level Fluctuations Recorded From Two Pennsylvanian Cores From Northwestern Missouri.

Abstract: Two cores from Buchanan and Andrew Counties in the Forest City Basin, northwestern Missouri, yielded approximately 1,200 feet of sedimentary rock deposited during the Pennsylvanian Period (approximately 300 million years ago), a period characterized by rapid fluctuations in sea level. The cores were logged from the Excello Formation in the Upper Desmoinesian Series upward to the latan Formation at the top of the Missourian Series. Lithofacies were identified from rock composition, grain size, color, bedding thickness, sedimentary structures, and fossils. Petrographic thin sections were taken from selected rock types and the results were used to complement the visual descriptions. Limestones are the dominant lithofacies, but clastic rocks and coal beds are also present. A preliminary palynological investigation of dispersed organic matter palynodebris), spores, and pollen from organic-rich lithofacies provided additional information on the depositional environments. The two cores were correlated prim arily on the basis of the sedimentologic, and then integrated with palynologic results. Several formations are characterized by facies changes. This study also examined the possible influences of sea level fluctuations on the sedimentary rocks, and it appears that coal beds, underclays, and shales are correlative with sea level falls.

Key Words: Palynology, palynomorphs, sedimentology, Pennsylvanian

Introduction

Pennsylvanian age rocks are present in the surface and/or subsurface of more than two-thirds of Missouri counties surrounding the Ozark area and may have been continuous over much of the state in the geological past (Figure 1). These rock beds contain many important limestone and coal beds along with the clastic deposits and total approximately 2,000 feet thick (Unklesbay and Vineyard, 1992). During the Pennsylvanian, different parts of the North American continent rose and fell at varying rates, and sea level fluctuations were caused by periodic Gondwanan glaciations to the south (Unklesbay and Vineyard, 1992). Differential warping of the crust allowed for many areas to be invaded by the seas while intervening areas were dry land or swamps. Consequently, marine and nonmarine beds alternate and interfinger to form a pattern of transgressive and regressive rock deposition, which is commonly referred to as the classic Pennsylvanian cyclothem sequence. A typical cyclothem sequence consists of marine limestones a nd shales, fluvial channel sandstones, sub-aerial shales, and coals (Prothero, 1990).

Pennsylvanian deposits in the Forest City Basin, which encompasses northwestern Missouri and eastern Kansas, have been of economic interest and attracted several coal mines and limestone quarries to the region. Several thousand feet of cores have been drilled and logged in northwestern Missouri, many of which were subsequently donated to the Mary McCracken Core Library of the Missouri Department of Natural Resources in Rolla. Two of these cores have been investigated in this study (Figure 1): WM-8 in Buchanan County and WM-10 in Andrew County.

The objectives of this study were to: (1) describe the rock types, associated sedimentary structures, and fossils; (2) relate petrographic thin sections of common rock types to visual descriptions; (3) describe dispersed organic matter (palynodebris) and palynomorphs (spores and pollen) in selected rocks; (4) correlate the two cores and effects of sea level fluctuations; (5) refine formation boundaries; and (6) reconstruct a depositional model from the dataset. An integrative study such as this has not been attempted before for this area.

Materials and Methods

Core WM-8 was drilled to a total depth of 1,215 feet below the surface elevation of 805 feet above sea level. WM-10 is located 20 miles northeast of WM-8 and was drilled to a total depth of 1,420 feet below the surface elevation of 1,000 feet above sea level. Each core was described beginning in or at the base of the Cherokee Group of the Upper Desmoinesian Series, and continued through the top of the Missourian Series (approximately 302 to 307 million years ago). We logged upward from a depth of 618 feet in WM-8 and 734 feet in WM-10. Each core was described at decimeter scale with respect to rock types and their thicknesses, biogenic and depositional structures, composition, color, and fossil contents (Figure 5). Diluted hydrochloric acid was used to determine the presence of carbonate within a sample. Tap water was sprayed on the core to accentuate the structures and contacts between rock types. Relative differences in grain size were noted when apparent. Samples were taken at random from WM-10 from a few rock types and processed at the University of Missouri-Rolla for thin sections. Four thin sections were made from limestone horizons in the Stanton and Dennis Formations, a sandstone layer in the Pawnee Formation, and a siltstone layer in the Labette Formation (Table 1, Figure 5).

A total of ten samples were selected from all the rock types in the two cores (Table 1, Figure 5) and sent to Global Geolab in Alberta, Canada where they were processed by standard palynological procedure (Traverse, 1988). Following maceration the samples were treated in dilute hydrochloric acid to remove carbonates, treated in 48% hydrofluoric acid to break down silica and silicate minerals, and underwent heavy liquid separation in Zinc bromide (SG = 2.2). Kerogen slides were mounted before oxidation with nitric acid and sieving with 10 [micro]m meshes because: (1) oxidation affects the natural colors of organic matter which are crucial in the determination of thermal maturation and their classification, and (2) sieving removes amorphous organic material which is one of the categories used for interpretation. A system for differentiating between the various types of detrital organic material (palynodebris) was determined from literature survey (e.g., Traverse, 1988; Oboh, 1992; Turner and Spinner, 1993; Oboh -Ikuenobe et al., 1997). Nine palynodebris categories were identified using a transmitted light microscope: amorphous organic matter, marine palynomorphs, black debris, yellow-brown debris, brown-black debris, cuticles, plant tissues, wood, and sporomorphs (spores and pollen) (Table 2). Three hundred palynodebris particles were point counted on each kerogen slide. The relative distributions of the palynodebris were determined and correlated with rock types.

Identification of pollen and spores (Figures 2A, 2F, 21) in ten oxidized and sieved slides was done with the aid of references such as Wilson (1962), Turner and Spinner (1993), Owens (1996), and the index cards of Jansonius and Hills (1976+supplements). These palynomorphs are characterized by openings or scars (apertures) through which the pollen tube and prothallus emerged.

Results and Discussions

Lithofacies and palynological data have yielded information about the depositional and post-depositional conditions of the rocks. Palynofacies, which refers to the distribution of palynodebris in sedimentary rocks, provided added information about thermal maturation. Additional observation about the colors of the organic clasts, which can be used as thermal alteration indices (TAI: scale of 1 to 5), was made in this study. The palynodebris in the samples near the top of each core have yellow to brown colors (thermal alteration index, TAI 2, 2+, 3-; Traverse, 1988), while those near the bottom which are much darker (3+, 4-, 4, 5) because the former have undergone shallower burial and lesser degree of maturation (Figure 2A to D).

Lithofacies: Figures 3 and 4 illustrate the diverse lithologies and structures of the rocks drilled by cores WM-8 and WM 10. Sandstones, siltstones, shales, and underclays constitute approximately 40% of the core thickness in WM-8, and 45% in WM-10. Common sedimentary structures in these rocks, except underclays, include flaser bedding (Figure 3B), lenticular bedding, low-angle cross lamination, and convolute bedding. Few macrofossils (e.g., crinoid stems, brachiopods) occur in the clastics. Limestones account for 55-60% of rock thickness recovered in both cores. Using Dunham's (1962) classification, the varieties identified included lime mudstones (very fine-grained), wackestones (fine-grained with few grains, Figure 4D), and packstones (grain-supported with some mud, Figure 3F), which sometimes intergrade (Figure 4A). The wackestones and packstones are very fossiliferous, and contain abundant brachiopods (Figure 4C), bivalves, some bryozoans, and fusulinid foraminifera (e.g., Triticites). Oncolitic algal st ructures (Figure 4B) also commonly occur. Coals overlie underclays, are usually very thin, and sometimes not recovered with the core.

Inferences can be made about depositional conditions and diagenetic effects on the basis of lithologic observations from the cores and thin sections. The macrofossils, in particular, fusulinids, crinoids, bryozoans suggest a shallow marine environment for the limestones. In addition, the calcareous nature of some sandstones and marine fossils suggest marine influence. Differences in composition and the presence of impurities in the sediments have imparted greenish to bluish, brownish, reddish, various shades of gray, and black coloration on the sediments. For example, iron nodules have imparted a reddish color in the clastic sediments, while abundant organic matter resulted in dark gray to black color (Figure 4H). Black coloration in shales can be attributed to the presence of phosphate and organic matter (Figure 3E). Coal seams formed from the accumulation of plant debris, and the underclays represent the swampy environment behind the coal swamps (Figure 6). In the limestones, pressure solution has resulted in the formation of stylolites (Figure 4D), while fenestral porosity (Figure 3C) formed as a result of the decay of organic material.

Palynology: Palynodebris are organic clasts which behave like sedimentary particles, and therefore give information about rock facies and depositional conditions. Marine limestones are richest in amorphous organic material, although they contain different proportions of other organic components. The organic debris recovered in the lime mudstones appeared as very fine-grained, unidentifiable specks under the microscope (palynofacies 1), while the wackestones and packstones contained identifiable, larger palynodebris in addition to amorphous organic matter (palynofacies 2). Coal contained large amounts of dark brown wood fragments and black debris, indicating nonmarine sediments that underwent intense thermal alteration during the process of coal formation (Palynofacies 3). Palynodebris found in shales and siltstones typically consisted of a distribution of all the various types of debris, including spores and pollen, amorphous organic material, wood, black-brown debris, yellow-brown debris, plant tissues, and cuticles (palynofacies 4). These are summarized in Table 3.

Shales and siltstones yielded higher numbers of pollen and spores than limestones. Sandstones were not sampled for palynological analysis because they generally contain very few playnomorphs. The identified pollen and spores represented a typical Late Pennsylvanian assemblage. The alete, densely clavate spore Clavatasporites irregularis (Figure 2F) and the trilete spore Spilaeotriletes (Figure 2H) was identified in the Weston shale Formation. Within the limestone of the Dennis Formation, two periporate pollen grains were identified as Chenopodipollis. An elongate folded grain of Schopfipollenites and smooth trilete spores with spines were found in the Pleasanton shale.

Correlation and Sea Level Fluctuations: Figure 5 illustrates the correlation between cores WM-8 and WM-10, and the eustatic sea level curve for the Late Pennsylvanian (Ross and Ross, 1988). Facies changes are evident between the two cores. Within the Pawnee Formation, a facies change occurs in the middle of the formation where sandstone (not present in WM-8) is deposited within the siltstone in WM-10. Further, a thin carbonate mudstone (not present in WM-10) appears in WM-8 below the Mine Creek shale member. Upon initially using a straight-line correlation method connecting formations between the two cores, several inconsistencies became apparent. Formation boundaries previously determined by geologists at the Missouri Department of Natural Resources were reexamined (consulting Thompson, 1995) and some were changed based on our results. Of particular interest was the top of the Kansas City Group where the boundaries between the Iola, Lane, and Wyandotte Formations have been considerably revised. The Marmaton Group also had multiple discrepancies which have been addressed.

Situated to the right of the cores in Figure 5 is a column illustrating periods of deposition and nondeposition (Ross and Ross, 1988). Periods of sea level falls were represented by gaps in the geologic record (nondeposition and /or erosion), and were marked by a shift from marine deposition to more brackish and nonmarine deposition. The latter group of sediments are characterized by an abundance of wood and black debris, unlike the marine sediments which have abundant amorphous organic matter (Figure 6). Unconformities occur within the Labette Formation with the appearance of coal (indicating swampy, nonmarine conditions), appearing to correlate with the drop in sea level in the column. The coal seams have the highest amounts of wood, black debris and degraded debris. At the base of the Pleasanton Group, between the Desmoinesian and the Missourian Series, a major unconformity exists where underclay would seem to correlate with a drop in sea level approximately 305 million years ago (Ross and Ross, 1988). Th e entire Pedee Group (Weston and latan Formations) at the top of the Missourian Series is missing in WM-8, due to erosion.

Prothero (1990) and Prothero and Schwab (1996) discussed two models of interpretation for the classic Pennsylvanian cyclothem: the traditional model and a modem model. The traditional model treated a disconformity at the base of the channel sandstone as a rapid regression followed by a slow transgression. The more modem interpretation from Friedman and Sanders (1978) placed a rapid transgression below the first marine limestone or shale, and considered the rest of the sequence as regressive, progradational delta front. The latter is now accepted as the correct model. Our interpretation fits well with this revised model because we are interpreting coals and underclays as representing periods of sea level lowstand marked by the onset of transgression.

Depositional Model: The Absaroka transgressive event began in the North American craton near the beginning of the Middle Pennsylvanian time, and deposited repetitive alternating nonmarine and marine sediments in the Midcontinent (Heckel, 1977, 1980; Levin, 1991). However, the eastern mountains in the Appalachian region experienced predominantly nonmarmne deposition. In northwestern Missouri, the cyclic alternations were more frequent during the Desmoinesian Epoch than at other times during the Pennsylvanian. The Missourian Epoch was characterized by fewer coal beds and alternating, thicker marine limestones and shales (Figure 5). This suggests a more stable depositional setting during the Missourian (Unklesbay and Vineyard, 1992). Although sea level fluctuations occurred during this time, they were less frequent and formed fewer swampy conditions.

The Marmaton Group displays a typical Desmoinesian pattern of alternating marine limestones, brackish to marine shales and siltstones, and nonmarmne coals, and underclays (Figure 5). Within the Pleasanton Group of the lower Missourian, the Middle Unnamed Formation in WM-10 indicates continuous sand deposition within a channel fill, but the correlative WM-8 consists of a repetitive sequence of limestone, sandstone, and shale indicating sea level fluctuations at that locality. Figure 6 shows our reconstruction of upper Pennsylvanian depositional model in the study area. This model is slightly different from the classic cyclothem for the illinois Basin (Shaw, 1964; Prothero, 1990), in that nonmarine limestone is missing, and deep marine shales are absent. Note that all the units of this model do not occur throughout our study area because of the changing environments associated with fluctuating shorelines.

Conclusions

Sedimentological and palynological data have provided important correlative and dating evidences for rocks from the Pennsylvanian time period in northwestern Missouri. The study has resulted in a refinement of some formation boundaries within the Missourian Series of the Missouri geological column. The cyclical sediments have been correlated with sea level curves for the Pennsylvanian Period and related to palynofacies. A depositional model has also been reconstructed for the rock sequences. This study contributes toward the understanding of the depositional history of Upper Pennsylvanian sedimentary rocks.

Acknowledgements

We would like to thank Dave Smith and Hairl Dayton of the Missouri Department of Natural Resources, Rolla for access to the cores and encouragement to carry Out this study. Sarah De La Rue, formerly of the Geology and Geophysics Department, University of Missouri-Rolla, and now at Louisiana State University reviewed the initial draft of the manuscript. The manuscript benefited immensely from constructive suggestions made by anonymous reviewers. Financial support for this study was provided by the University of Missouri-Rolla's Opportunities for Undergraduate Research Experience (OURE), for which the senior author is grateful.

Literature Cited

Dunham, R. J., 1962. Classification of carbonate rocks according to depositional texture. In: Ham, W. E. (Editor), Classification of Carbonate Rocks. American Association of Petroleum Geologists, Tulsa, Oklahoma, pp. 108-121.

Friedman, G. M., and Sanders, J. E., 1978. Principles of Sedimentology. John Wiley, New York, 792 p.

Heckel, P. H., 1977. Origin of phosphatic black shale facies in Pennsylvanian cyclothems of Midcontinent North America. American Association of Petroleum Geologists Bulletin, v. 61, p. 1045-1068.

Heckel, P. H., 1980. Paleogeography of eustatic model for deposition of Midcontinent Upper Pennsylvanian cyclothems. In: Fouch, T. D. and Magathan, E. R. (Editors), Paleozoic Paleogeography of the West-Central United States. Society of Economic Paleontologists and Mineralogists, Rocky Mountain Section, pp. 197-215.

Jansonius, J., and Hills, L. V., 1976. Genera File of Fossil Spores and Pollen. Special, Publication Department of Geology, University of Calgary (several supplement have been added since the initial publication).

Levin, H. L., 1991. The Earth Through Time, Fourth Edition. Saunders College Publishing, 651 p.

Oboh, F. E., 1992. Multivariate statistical analyses of palynodebris from the Middle Miocene of the Niger Delta and their environmental significance: Palaios, v. 7, p. 559-573.

Oboh-Ikuenobe F. E., Yepes, O, and ODP Leg 159 Scientific Party, 1997. Palynofacies analysis of sediments from the Cote d'Ivoire-Ghana Transform Margin: preliminary correlation with some regional events in the eastern Equatorial Atlantic. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 129, p. 291-314.

Owens, B., 1996. Upper Carboniferous spores and pollen. In: Jansonius, J., and McGregor, D. C. (Editors), Palynology: Principles and Applications, American Association of Stratigraphic Palynologists Foundation, v. 2, pp. 597-606.

Prothero, D. R., 1990. Interpreting the Stratigraphic Record. W. H. Freeman and Company, 410 p.

Prothero, D. R., and Schwab, F., 1996. Sedimentary Geology. W. H. Freeman and Company, 575 p.

Ross, C. A., and Ross, J. R., 1988. Late Paleozoic transgressive-regressive deposition. In: Wilgus, C. K., Hastings, B. S., Kendall, C. G. St. C., Posamentier, H. W., Ross, C. A., and Van Wagoner, J. C. (Editors), Sea-Level Changes-An Integrated Approach, SEPM Special Publication No. 42, pp. 227-247.

Shaw, A. B., 1964. Time in Stratigraphy. McGraw-Hill, New York, 365 p.

Thompson, T. L., 1995. The Stratigraphic Succession in Missouri (Revised-1995). Missouri Department of Natural Resources, vol. 40 (2nd Series) Revised, 190 p.

Turner, N., and Spinner, E., 1993. A palynostratigraphic study of Namurian -- Westphalian deltaic sequences of the southern central Pennine Basin, Derbyshire, England. Review of Palaeobotany and Palynology, v. 77, p. 23-43.

Traverse, A., 1988. Paleopalynology. Unwin Hyman, 600 p.

Unklesbay, A. G., and Vineyard, J. D., 1992. Missouri Geology: Three Billion Years of Volcanoes, Seas, Sediments, and Erosion. University of Missouri Press, 189 p.

Wilson, L. R., 1962. Permian plant microfossils from the Flowerpot Formation, Greer County, Oklahoma. Oklahoma Geological Survey Circular 49, p. 1-50.

                       List of samples processed
                          for analyses. Their
                    horizons are shown in Figure 5.
Core   Depth in feet  Lithology                 Type of analysis
WM-8   246            Shale                     Palynology
WM-8   321            Limestone                 Palynology
WM-8   422            Shale                     Palynology
WM-8   523            Underclay                 Palynology
WM-10  175            Limestone                 Palynology
WM-10  258            Limestone                 Thin section
WM-10  330            Shale                     Palynology
WM-10  450            Limestone                 Palynology
WM-10  452            Limestone                 Thin section
WM-10  537            Siltstone                 Palynology
WM-10  627            Shale                     Palynology
WM-10  683            Sandstone                 Thin section
WM-10  704            Siltstone with interbeds  Thin section
                      of very fine sandstone
                       Descriptions of dispersed
                     organic matter in the samples.
Palynodebris              Characteristics
Amorphous organic matter  Unstructured, irregularly-shaped,
                          light brown to
                          medium brown masses; usually gel-
                          like (Figure 2A)
Marine palynomorphs       Scolecodonts (chitinous mouth parts
                          of marine annelid
                          worms) and acritarchs (cysts of
                          marine algal
                          phytoplankton, Figure 2G)
Black debris              Opaque particles with sharp angular
                          outlines; lath-shaped,
                          sometimes more equidimensional
                          (Figure 2E)
Yellow-brown debris       Structureless particles of yellow
                          to light brown color;
                          attributable to highly degraded
                          herbaceous material
                          (Figure 2A)
Black-brown debris        Unstructured dark brown to nearly
                          black particles;
                          attributable to highly degraded
                          woody material
Cuticles                  Platy epidermal fragments of
                          leaves, roots, etc.; deep
                          yellow to light brown in color
                          (Figure 2B)
Plant tissue              All other herbaceous material
                          including tubes and
                          parenchyma (spherical to polygonal
                          cells, Figure 2B)
                          and other non-woody debris
Wood                      Light to dark brown particles with
                          sharp angular edges
                          and/or discernible cellular
                          structure; mainly lath-shaped
                          (Figure 2A)
Sporomorphs               Land plant spores and pollen
                          dispersed by water and
                          wind into continental and marine
                          environments
                          (Figures 2A, 2F, 2I)
Palynodebris              Size ([micro]m)
Amorphous organic matter  Variable
Marine palynomorphs       30-100
Black debris              20-[greater than]200
Yellow-brown debris       5-80
Black-brown debris        Variable
Cuticles                  30-[greater than]150
Plant tissue              Variable
Wood                      30-[greater than]200
Sporomorphs               10-80
                    Characteristics of palynofacies
                  assemblages. Sandstone horizons were
                 not sampled for analysis because they
                contain small amounts of organic matter.
Palynofacies  Lithology
1             Lime mudstone
2             Wackestone, Packstone
3             Coal
4             Siltstone, shale
Palynofacies  Characteristics
1             Predominantly amorphous organic matter
              ([greater than]90%), few marine
              microfossils and wood
2             Dominated by amorphous organic matter
              (50%), followed by black debris,
              brown-black debris, marine
              microfossils, wood, cuticles, and
              Sporomorphs in that order
3             Brown-black debris dominant (45%),
              followed by wood and black debris and
              sporomorphs
4             Variable numbers of all types of
              Palynodebris