While mapping the lower Hinton Formation (the middle red member of Miller,
1974) of the Mauch Chunk Group (Upper Mississippian, Chesterian) near Bluestone
Lake in Summers County, West Virginia, a pale blue green fossiliferous bed was
encountered The pale blue green bed, roughly 20 cm thick, appears to be a calcareous,
argillaceous siltstone. This bed is roughly 65 meters above the Stony Gap Sandstone and
roughly 175 meters below the Avis Limestone (Little Stone Gap Member).
The lower contact with a red siltstone is fairly sharp. It is possible that some of
the calcium carbonate from the pale blue green bed provided the cement for the basal red
siltstone as well as the material for small (to 2 cm) calcareous nodules at the top of the
basal red bed as well as the bottom of the pale blue green bed. The upper contact with a
friable red siltstone is not as sharp as the bottom contact. It appears as if bioturbation
may have mixed the two sediments at the contact surface. The color also changes at and
above the upper contact over a vertical distance of about 10 cm as if oxidation of the
sediments was gradually beginning, or as if the reducing effects of the pale blue green
bed were diffusing upwards. There were no fossils in this transition zone.
The pale blue green (5BG 7/2) bed itself is composed of a significant fraction (to
25 % at least) of septimyalinid shells. Ostracodes are also scattered throughout the pale
blue green bed. No sign of plant debris was observed; however, small (<0.1 mm on
edge) cubes of pyrite were seen sparsely scattered throughout the bed. The
septimyalinids appear from the base of the pale blue green bed intermixed with
calcareous nodules and continue to be distributed nearly to the top of the bed. They
appear to be a new species of Septimyalina Newell and are being described in another
paper (Peck, in prep). The septimyalinids are present in a range of sizes, from spat to
fully mature specimens. In some exposures of the outcrop, the septimyalinid shells
appear to comprise a major portion of the calcareous siltstone. Ostracodes are also
present in the pale blue green bed, but not nearly as numerous as the septimyalinids.
The lower Hinton Formation (middle red member) extends from the Stony Gap
Sandstone Member to the Avis Limestone (Little Stone Gap Member) roughly 240
meters. Very few if any fossils are found in the red mudstones of this interval. Fossils
(mostly Spirorbis, ostracodes, bivalves, and gastropods) that do occur are found in thin
beds and mud drapes. Very infrequently, horizons exposing plant fossils occur. More
frequently are dark gray to brown micaceous siltstones with small, unidentifiable
carbonized plant fragments. When invertebrate fossils are found, they are invariably of
limited diversity appearing mostly as monocultures, a single species of bivalve with or
without ostracodes and Spirorbis at that specific horizon. The pale blue green bed is
typical with a limited diversity but atypical being marine, calcareous, and abundantly
populated. The septimyalinids in this report were used for biostratigraphic correlation in
the study area. They have been found only in the pale blue green bed where they seem to
thrive. The pale blue green bed could be used locally as a marker bed since it extends
over 4000 meters along one axis in the study area.
The underlying and overlying red beds are suggestive of a fine grained overbank
flood plain deposits on a low-relief topographic surface. A rise in sea level occurred
(Brezinski, 1989; Miller and Eriksson, 1999, 2000; Swann, 1964), perhaps from local
subsidence to accommodate previous sediment deposits or possibly a minor eustatic rise,
resulting in near coastal marine conditions suitable for the ostracode and bivalve.
Apparently sea level rose fairly rapidly, covered the basal red bed sediment
without much disturbance, and leveled off at a depth making conditions ideal for the
septimyalinid to flourish. Whether the septimyalinid was an opportunist that fit the niche
and conditions or whether reduced access gave this more mobile bivalve the opportunity
to flourish is open to question. However, as the sea level rose, this marine bivalve did
flourish to such an extent that its shells added significantly to the sediment deposition.
Then sea level began to drop. Clastic sediment deposition increased, allowing for
bioturbation or intermixing at the contact of the two sediments while still underwater.
Subsequent subaerial exposure allowed oxidation to begin on the upper deposits. This
would also go far in explaining the graded change in color of the overlying mudstone
from pale blue green through dark red gray to red as oxidizing conditions begin to prevail
with subaerial exposure. The coastal flood plain emerges above sea level, oxidized
sediments are deposited again (due to the new proximity to alluvial deposits as well as
subaerial exposure), and the pale blue green bed is history.
Initially it was believed that the sediments were deposited in a reducing
environment, suspecting the pale blue green color was due to a low ratio of ferric/ferrous
iron. The low dissolved oxygen of the reducing environment could have been
responsible for the reduced diversity. There did not appear to be any organic debris
(Potter et al, 1980). Furthermore, small cubes of pyrite were found sparsely distributed
throughout the bed. However, the presence of the septimyalinids in vast numbers as well
as the coexisting ostracodes suggested an environment with sufficient dissolved oxygen
to support their metabolic requirements. Also, the fact that the environment was marine
albeit near shore should provide some circulation, thus maintaining sufficient dissolved
oxygen levels to support the abundant fauna. The limited diversity suggests a restricted
rather than open marine conditions (Miller & Erikkson, 2000).
Conceivably, a large erosional event occurred, rapidly burying the pale blue green
sediments and the septimyalinids with over 20 cm of muddy sediments, thus sealing the
organic matter from the dead bivalves into the pale blue green sediment. Unoxidized
organic matter from dead septimyalinids probably consumed remnant oxygen and
produced the conditions (Eh, pH) necessary for the pyrite production as well as for the
low ratio of ferric/ferrous iron and resulting pale blue green color of the bed. The
carbonate from the large quantity of septimyalinid shells probably also assisted in
providing a favorable pH, contributing to the pale blue green color. It is suggested that
the reducing conditions occurred after deposition of the pale blue green bed sediments
and after clastic sediments had rapidly covered the bed to a depth of several centimeters.
At this point the pale blue green bed had in place all the necessary elements to make the
transition from an oxidizing environment to a reducing environment. This reducing
effect may have diffused upward in the overlying red mudstone, producing the graded
change in color at the upper contact.
Brezinski, David K. 1989. Late Mississippian depositional patterns in the northcentral Appalachian basin, and their implications to Chesterian hierarchal stratigraphy.
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Miller, Marshall S. 1974. Stratigraphy and coal beds of upper Mississippian and
lower Pennsylvanian rocks in southwestern Virginia. Virginia Division of Mineral
Resources Bulletin 84.
Miller, J. Daniel, and Kenneth A. Eriksson. 1999. Linked sequence development
and global climate change: The Upper Mississippian record in the Appalachian basin.
Geology, vol. 27, 35-38.
Miller, Daniel J., and Kenneth A. Eriksson. 2000. Sequence stratigraphy of
Upper Mississippian strata in the central Appalachians: A record of glacioeustasy and
tectonoeustasy in a foreland basin setting. A. A. P. G. Bulletin, V. 84, No. 2 (Feb.), p.
210-233.
Potter, Paul E., J. Barry Maynard, and Wayne A. Pryor. 1980. Sedimentology of
Shale: Study Guide and Reference Source. New York: Springer-Verlag.
Swann, David H. 1964. Late Mississippian Rhythmic Sediments of Mississippi
Valley. Bull. A.A.P.G, Vol. 48, No. 5, pp. 637-658.