Sequence stratigraphy of an Upper Pennsylvanian,Midcontinent cyclothem from north America (Iola Limestone,Kansas and Missouri,USA) |
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Authors: | Thomas Olszewski |
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Institution: | (1) Panstwowy Instytut Geologiczny, Rakowiecka 4, PL-00-975 Warszawa;(2) Present address: Department of Geosciences, Pennsylvania State University, PA16802 University, Park, USA |
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Abstract: | Summary The Iola Limestone is the open-marine, carbonate portion of a Midcontinent cyclothem. It represents the transgressive and
highstand systems tracts of a stratigraphic sequence.
The sequence begins with a type 1 sequence boundary at the base of the Chanute Shale, which underlies the Iola. This surface
can be recognized by the presence of a paleosol and as much as 15 m of incision. Part of the Chanute is probably an estuarine
valley-fill package and represents the lowstand systems tract.
The lowest member of the Iola Limestone is the Paola Limestone Member. Its base is sharp and locally has a thin shell lag.
This contact is the transgressive surface and represents the onset of open-marine, clear-water carbonate sedimentation. It
is a ravinement surface cut by a winnowing environment that appears as a ‘kink’ in the base-level surface. Landward of this
‘kink’, sediment accumulated to a subaerial base-level, but seaward of it sediment did not accumulate above a subtidal threshold
probably controlled by wave base. Facies formed during regression also show the influence of a ‘kink’ in base-level. The Paola
itself is the lower part of the transgressive facies tract.
Overlying the Paola is the Muncie Creck Shale Member; its lower part is black and fissile, contains phosphate nodules, no
unequivocally benthic fossils, and no discernible trace fossils. However, this facies is not present everywhere; in much of
southeastern Kansas and just north of Kansas City, Missouri, it has been removed by submarine erosion. The only clues that
this facies was ever deposited are reworked diagenetic phosphate nodules that occur as a lag on a hardground on top of the
Paola. Where black shale and the immediately overlying, relatively unfossiliferous gray shale have been preserved, they are
overlain by a shelly lag that incorporates bored and encrusted micritic cobbles-strong evidence of reworking. This erosional
surface is interpreted as the maximum flooding surface. Such an interpretation suggests that the black shale formed during
sea-level rise and coastal transgression rather than at sea-level highstand. This is further supported by geochemical properties
of Midcontinent black shales.
Overlying the maximum flooding surface are a thin gray shale (within the Muncie Creek) and the Raytown Lime-stone Member.
In southern Kansas, the limestone contains an extensive phylloid-algal mound. In the subsurface, the elongate Raytown mound
is perpendicular to the outcrop belt; it directly overlies a Precambrian structural element called the Bourbon arch, suggesting
subtle tectonic control. In northern Missouri and in Nebraska, the upper Raytown contains carbonate tidal-flat deposits. The
existence of these two facies suggests that the Raytown was deposited while base-level was stable rather than during base-level
drop. The aggradational algal mound built into a positive sea-floor feature and did not shift position through the duration
of the cycle. Progradational carbonate tidal flats had the chance to develop in the northern part of the field area; if base-level
had been dropping, rapid regression would likely not have permitted accumulation of this facies at the top of the limestone.
Coarse-grained deposits, an oncolite in southern Kansas and bioclastic packstones and grainstones in northern Kansas, indicate
that the mound aggraded up to a subtidal base-level, presumably controlled by wave base. The carbonate tidal flats, on the
other hand, filled accommodation space up to a subaerial base-level as they prograded out over this surface. There must have
been two base-levels—one subtidal and the other subaerial—connected by the previously mentioned base-level ‘kink’ at the coastline.
The Lane Shale, typically gray-green and silty, overlies the Iola. In a core from northern Missouri, the Lane Shale directly
overlying the Raytown contains clay slickensides and a breccia of ‘fitted’ carbonate clasts, indicating subaerial exposure
and qualifying the base of the Lane Shale as the top boundary of the Iola sequence.
In northwestern Missouri, the Iola changes from a relatively clay-free limestone to mostly dark-gray, calcareous mudrocks
with thin shell stringers. This represents a part of the basin dominated by clastic input.
Stratigraphic interpretation implies that the Muncie Creek black shale formed during accommodation space increase and shoreline
transgression. This is consistent with recent geochemical studies of Midcontinent black shales. This lithology formed as inland
peat swamps underwent ravinement and organic matter was flushed onto the shelf. An influx of nutrients and plant material
made available by sea-level rise and transgression while sediment was trapped in nearshore accommodation space was the primary
cause of sediment starvation and anoxia in the basin. |
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Keywords: | Cyclothem Sequence Stratigraphy Kansas Missouri Upper Pennsylvanian (Iola Limestone) |
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