COLUMBUS LIMESTONE
& DELAWARE
LIMESTONE
The bedrock in the vicinity
of Columbus (Franklin County, central Ohio, USA) is Middle to Late Devonian in
age. In the eastern portions of Franklin County, the Devonian is
dominated by shales and some siltstones & sandstones. In western
Franklin County, the Devonian is dominated by limestones.
Marblecliff Quarry is
located in western Franklin County, Ohio, just off the eastern side of Dublin
Road, a little west of the Scioto River.
The cut shown below is along
an access road at the northern edge of Marblecliff Quarry. It has a nice
section of the Columbus Limestone-Delaware Limestone boundary interval.
Looking N.
Looking N.
The thick-bedded limestones
in the lower part of the cut represent the uppermost Columbus Limestone.
The thin-bedded limestones in the upper part of the cut represent the lowermost
Delaware Limestone. The Columbus Limestone seen here is Eifelian
in age (early Middle Devonian). The overlying Delaware Limestone is also
Eifelian, but it does get up into the Givetian in places (late Middle
Devonian).
The Columbus-Delaware
contact is fairly obvious here, and is represented by an unconformity
(disconformity; a type-2 sequence boundary). Not much missing time is
represented by the unconformity. Biostratigraphic studies have shown that
one conodont biozone is missing in the section here, probably representing ~1
to 3 million years.
The lowermost Delaware
Limestone at this section is light-colored, soft, thin-bedded, argillaceous
lime mudstone. The uppermost Columbus Limestone at this section is light
gray to light brown fossiliferous wackestone and some packstone & lime
mudstone with lenses of mostly silicified fossil hash. Fossils include
favositid corals, rugose corals, spiriferid brachipod shells, crinoid stems,
and charophyte oogonia (see photo below).
Fossiliferous limestones of
the upper Columbus Limestone at Marblecliff Quarry. Favositid coral, crinoid
columnal, and brachiopod material is abundant here (see arrows).
Chert nodule horizon in the
upper Columbus Limestone (~41 cm below the Columbus-Delaware contact),
extending horizontally from the level of the geology hammer's blue handle.
A bone bed occurs near the
Columbus-Delaware contact here (& at other localities). The bone bed
is dominated by phosphatic fish bone pieces.