Evolution of some depositional models in late Carboniferous rocks of the Appalachian coal fields

  • Published on
    21-Jun-2016

  • View
    214

  • Download
    2

Embed Size (px)

Transcript

<ul><li><p>International Journal of Coal Geology, 12 (1989) 259-292 259 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands </p><p>Evolut ion of some deposit ional models in Late Carboniferous rocks of the Appalachian coal fields </p><p>J.C. FERM and G.A. WEISENFLUH </p><p>Department of Geological Sciences, University of Kentucky, Lexington, KY 40506-0059, U.S.A. </p><p>(Received March 1, 1988; revised and accepted August 1, 1988) </p><p>ABSTRACT </p><p>Ferm, J.C. and Weisenfluh, G.A., 1989. Evolution of some depositional models in Late Carboni- ferous rocks of the Appalachian coal fields. In: P.C. Lyons and B. Alpern (Editors), Peat and Coal: Origin, Facies, and Depositional Models. Int. J. Coal Geol., 12: 259-292. </p><p>Models for Carboniferous coal-bearing strata developed early in this century and consisted of vertical columns that described commonly occurring sequences of coal beds and associated rocks. These were followed by other models that expressed patterns of both vertical and lateral variation but, in those, as well as earlier models, depositional interpretation did not extend much beyond a basic marine-nonmarine dichotomy. Subsequently, depositional processes were linked to lithic components and showed that the areal distribution of these components formed a lateral, geo- morphically controlled continuum ranging from alluvial plains seaward to barrier islands. Evalu- ation of this model within a tectonic setting of rapid and differential subsidence produced by growth faulting in the southern part of the Appalachian coal fields has shown that, while deposi- tional processes are substantially the same as those suggested by the model, the form and spatial distribution of resulting lithic components are profoundly affected by movements on contempor- aneously active subsurface structures. This leads to the conclusion that previous models are a product of depositional processes strongly controlled by their tectonic setting and that further attempts at model generation will require careful assessment of the relationships of lithic distri- bution to subsurface structures and amounts of data necessary to recognize them. </p><p>INTRODUCTION </p><p>The history of development of depositional models for coal-bearing Carbon- iferous rocks in the Appalachian Basin is short, spanning about 60 years. The 'coal measures' as a rock unit were recognized and mapped in the 19th century, but documentation of individual coal beds did not begin until the early 1900's. The major effort at that time, which marked intensive mining development in the bituminous coal fields, was naming, correlating, and mapping individual coal seams as well as some of the strata between them. Unfortunately, systems </p><p>0166-5162/89/$03.50 1989 Elsevier Science Publishers B.V. </p></li><li><p>260 </p><p>of coal-bed nomenclature developed with a strong local flavor and the number of synonyms ibr correlative beds multiplied. This, combined with uncertainty about correlations, led to a tangle of stratigraphic names that, at least in some cases, has yet to be resolved. Given the effort required in resolving these prob- lems plus the view of some authorities of the time that variation within the coal measures was basically unsystematic {e.g., Ashley, 1931 ), it is small won- der that little was accomplished toward interpreting modes of sedimem deposition. </p><p>But beginnings were made and it is the objective of this paper to trace the development and growth of some concepts of depositional environments for parts of the Appalachian coal fields and to indicate directions for future study. The starting point will be the essentially descriptive models of vertical se- quences that were developed in the Illinois Basin. It will be shown that these were ancestors for a later model for the Allegheny Formation in the northern </p><p>Fig. 1. Distribution of Carboniferous rocks in eastern United States showing the major coal-bear- ing basins. Cross sections A-B and X- Y are on Figures 7 and 8, respectively. Modified from the Geologic Map of the U.S., King ( 1961 ) and King and Beikman { 1974 ). </p></li><li><p>261 </p><p>Monongahela Formation </p><p>Conemaugh Group Casselman Fm. Glenshaw Fm. </p><p>Allegheny Formation </p><p>n- Kanawha Fm. (W) Breathitt Fro. (K) </p><p>I l l 0 </p><p>14. Q </p><p>Z &gt; </p><p>0 New River Fm. ON) o rn ,1~. </p><p>Lee Fm. (K) </p><p>Pocahontas Fm, (W~ </p><p>Hazard No. 9 Coal Bed (K) </p><p>Hazard NO. 8 Coal Bed (K) </p><p>Hazard No. 7 Coal Bed (K) </p><p>Hazard No. 5 and No. 6 Coal Beds (K) </p><p>Magoffin Shale (K) </p><p>Hazard No, 4 Coal Bed (K) </p><p>Mauch Chunk Fm. (W) Pennington Fm. (K) </p><p>Lower Carboniferous Limestone : Greenbriar Limestone (W) - Newman Limestone (K) </p><p>Fig. 2. Stratigraphic names of rock units used in this paper. (W) =West Virginia nomenclature; (K) = eastern Kentucky nomenclature. Outcrop areas of major units shown on Figure 1. </p><p>Appalachian Plateau which, although descriptive, stressed the relationship of rock units to specific depositional environments. This model will be applied to rocks of the Pocahontas and Warrior Basins, two very thick wedges of Carbon- iferous rocks in the southern Appalachian region. The results of these tests will demonstrate the effects of movements of deep-seated structures on depo- sitional processes described for the Allegheny and will suggest a scale of obser- vation required to detect these effects. </p><p>Figure 1 shows the outcrop of the major rock units in the Appalachian and Illinois Basins and Figure 2 shows the stratigraphic names of some of the rock units or coal beds within the Pocahontas Basin. </p><p>THE CYCLOTHEM AND OTHER MORPHOLOGICAL MODELS </p><p>Early models that led to recognition of depositional environments in the United States were devoted primarily to the establishment of the vertical se- quential arrangement of rock units. (See Merriam, 1964; Duff et al., 1967; Heckel, 1984 for excellent summaries.) The first was that of Udden (1912) who recognized a commonly recurring sequence in the coal-bearing strata in northwestern Illinois (Fig. 3A). Examination of Udden's 'cycle' shows that it consists of what is now commonly recognized as a 'coarsening-upward se- quence' which resulted from progradation of sediment into a body of open water whose volume far exceeded that of sediment-bearing streams entering it. Fine- grained, further transported sediments make up the lower part of the sequence </p></li><li><p>262 </p><p>A B </p><p>- - - : - - - - - - - - " . . . _~ _- - - - - ._ .~-- </p><p>.~__--_ </p><p>iII ~_-,,- </p><p>~ Member I0. Gray shale, sandy at topl marine fossils and ironstone concretions In lower part. </p><p>MARINE </p><p>Y </p><p>Member 9. Limestone; marine fossils </p><p>Member 8. Black, laminated shale; large con- cretions t marine fossils. </p><p>Member 7. Limestone; marine fostil|, Member 6. Gray shale; pyritic nodules, iron - </p><p>s tone concretions at baseimarine fossils rare. </p><p>Member 5. Coa l </p><p>~ . - "~- Member 4 . Underclay,medium tolight gray; </p><p>. . . . . . i </p><p> / / . . .J . . . . . . i . . . . . . i . . . . . . ! " . ' - . " ," , ' . i </p><p>lower port calcareous. Member 3. Freshwater limestonetnodules or ~discontinuous bedsl usually nonfossiliferoul. Member 2.Gray sandy shale. </p><p>Member I. Fine groined,micaceous sandstone, and siitetone,moeeive to thin-bedded; plant remains. </p><p>Erosional base </p><p>Fig. 3. Column models for coal-bearing rocks in western Illinois. A = Udden (1912); B = Weller (1930). Symbols on A are the same as on B. Modified from Krumbein and Sloss ( 1963, p. 536). </p><p>and provide a platform over which coarser sediment prograded. Marine fossils were found only in the fine-grained rocks in the lower part of the sequence indicating its basically regressive character. Moreover, because coal beds and underclays form the top and bottom of the sequence and the intervening rocks were probably subaqueously deposited, the total thickness of the sequence pro- vided a rough, postcompactional depth estimate of the water body being filled. Not all of this was evident to Udden, but both he and, in Ohio, Stout (1931) clearly make the point that the process involved subsidence followed by or concurrent with sediment in-filling. </p><p>The next major attempt at characterizing depositional sequences was that of Weller (1930) and Wanless and Weller (1932). This model (Fig. 3B), also developed in western Illinois, was much more detailed than Udden's model and especially stressed the erosional surface at the base of the sandstones. These sandstones with erosional bases had been noted previously, especially in coal mines where they cut into and removed the coal, and Weller believed that they represented unconformities of regional extent. Although both Udden's 'cycle' </p></li><li><p>263 </p><p>and Weller's 'cyclothem' were described from the same area, Weller's idea re- ceived greater acceptance and was later applied in the Appalachian Basin, (e.g., Cross and Arkle, 1952; Sturgeon et al., 1958; Beerbower, 1961; Gray, 1961; Branson, 1962). </p><p>Although the regional extent of the erosional surfaces proposed by Weller have been very difficult to prove, the main contribution of the cyclothem to depositional interpretations is that it described and, in a general way, inter- preted correctly what is now the well-established 'fining-upward' point-bar model for channel deposits of meandering streams. Although features such as lateral accretion beds and levee deposits were not noted, major descriptive ele- </p><p>A KANSAS ILLINOIS WEST VIRGINIA </p><p>B </p><p>Limest one'~r </p><p>Fig. 4. Models for lateral variation in coal-bearing rocks. A. Regional model by Wanless (1950). B. Model for the Allegheny Formation in the northern Appalachian Basin by Ferm and Williams (1963). Outcrop area of the Allegheny Formation is shown on Figure 1. </p></li><li><p>264 </p><p>ments were presented and, as will be shown, became incorporated into later models. </p><p>These early models were essentially single columns with strong emphasis placed on vertical sequences of rock types but with only general reference to lateral variation. Two subsequent models treat this subject. The first was that of Wanless ( 1950 ) who attempted to develop a pattern of lateral variat;.on for Weller's cyclothem from the Western Interior Basin, through Illinois, to the Appalachian Plateau (Fig. 4A). The Appalachian portion was based on his studies in eastern Kentucky and southern West Virginia (Wanless, 1939, 1946 ). The principal feature of this model is a steady reduction of marine strata in an eastward (landward) direction and an increase in the number of coal beds arising either by splitting of a major coal bed or by generation entirely de novo below the split bed. </p><p>Perhaps the most important aspect of this model is the thickness variation arising from differential subsidence. The Illinois portion reflects less sediment accumulation than does the Appalachian portion, and hence, if sedimentation and subsidence were about in balance, it can be inferred that subsidence in the Appalachian Basin was greater than in Illinois. However, the lesser marine influence and fewer carbonate rocks in the Appalachian Basin also suggest that clastic sediment accumulation was probably greater than that in Illinois. The importance of the influence of subsidence on patterns of sediment distribution will become evident in following portions of this paper. </p><p>A second model describing lateral variation was suggested by Ferm and Wil- liams (1963) on the basis of their studies of the 100-m-thick Allegheny For- mation in western Pennsylvania (Figs. 1, 2). The most obvious feature of this model (Fig. 4B ) is the reconciliation of the coarsening-up and fining-up detri- tal sequences of the Udden and Weller models. These sequences occur side by side, and in a general way, the coarsening-upward portion was believed to rep- resent 'seaward' deposition and the fining-upward 'landward'. These interpre- tations about deposition were also reflected in the 'chemically' derived sedi- ment at the top and bottom of the model. Coal was found on the landward side, marine limestone on the seaward side, with beds of nodular, sideritic ironstone between them. The model, as drawn, showed neither transgression nor regres- sion; marine limestone, ironstone and coal occur directly above one another. In transgressive sequences, coal would be expected to be more widespread on the bottom and limestone and ironstone similarly developed on the top with a coarsening-upward sequence dominating the intervening detrital sequence. In a regressive sequence, the reverse arrangement would occur. Finally, given that one of these models would be overlain and underlain by others, a series of' 'regressive' models superimposed one upon the other would yield the common occurrence of marine limestone overlying the coal seams as in the Udden and Weller models. </p><p>The greatest contrast between this model and that of Wanless was the treat- </p></li><li><p>265 </p><p>ment of the landward and seaward extremes of the rock bodies (Fig. 4). Wan- less showed increasing thickness in both directions, whereas Ferm and Wil- liams (1963) visualized 'no sediment' points with soil (underclay) at the landward end and marine water at the seaward extreme. As will be shown, this proved to be a reflection of the tectonic setting of the groups of rocks from which each model was derived. </p><p>THE ALLEGHENY MODEL </p><p>The models described above included important elements for deducing de- positional environments but, for the most part, inferences at that time did not go much beyond 'peat swamp', 'marine mud', and 'channel sandstone'. In the 1960s, data concerning recent deposits of the modern Mississippi delta became available, (e.g., Fisk, 1960; Scruton, 1960; Coleman and Gagliano, 1964; Allen, 1965; Shirley, 1966; Coleman et al., 1969; Morgan, 1970; Saxena et al., 1972). Combining these data with a concurrent study of the Allegheny Formation led to a model that followed morphological lines, but placed stronger emphasis on interpretations of how the rock bodies were formed (Fig. 5). Hence, the coar- sening-upward sequence in this model (C, Fig. 5 ) does not differ substantially from that of the Udden model, but, in this case, it is interpreted as a deposit of an interdistributary bay or small delta front similar to those on the lower delta plain of the Mississippi River. Similarly, Weller's fining-upward channel model can be seen at H on Figure 5, but lateral accretion in the point-bar deposits are clearly indicated as are levee deposits on the upper flanks of the channel sandstones. </p><p>A major difference between this model and that on Figure 4B, is the quart- zose, barrier-island deposit at A on Figure 5. These quartz arenites occurred only at the northern limit of the Allegheny outcrop and, although the amount of information about them was limited, it could be shown that limestones and </p><p>I LOWER OELT i PLAIN I BARRIER SANDSTONE INTERDISTRIBUTARY MOUTH BAR SAND </p><p>BAY - DELTA FRONT </p><p>UPPER DELTA PLAIN J ALLUVIAL PLAIN </p><p>I SHALLOW LAKES I FLUVIALCHANNEL I SOIL AND PONDS </p><p>~ H 15M Fig. 5. The Allegheny model. Dots = lithic arenites; dots with "Q"- quartz arenites; triangles point toward diminishing grain size in siltstone and shale; brick=limestone; black=coal; wiggly lines = s...</p></li></ul>