Jm. Malinky et Ph. Heckel, PALEOECOLOGY AND TAPHONOMY OF FAUNAL ASSEMBLAGES IN GRAY CORE (OFFSHORE) SHALES IN MIDCONTINENT PENNSYLVANIAN CYCLOTHEMS, Palaios, 13(4), 1998, pp. 311-334
Faunal composition and diversity of macroinvertebrate assemblages in t
he gray facies of nineteen ''core'' shales of Midcontinent Pennsylvani
an cyclothems confirm the offshore marine origin previously establishe
d for those shales. Diverse assemblages of only calcitic fossils that
show grain corrosion at northern. Midcontinent localities (Nebraska/Io
wa) indicate deposition under oxic to slightly dysoxic conditions with
in the calcite lysocline but below the aragonite compensation depth fo
r this particular sea. Southward (Kansas) sparser calcitic faunas indi
cate that ''core'' shales there were deposited farther from the northe
rn shoreline in water of greater depth and less bottom oxygen, closer
to or locally below the calcite compensation depth. Farther southward
near the Ouachita detrital source (Oklahoma), greater influx of sedime
nt permitted rapid burial of shelled organisms to overprint the effect
s of lysoclines and compensation depths. Both the originally aragoniti
c molluscs as well as calcitic organisms are preserved in those shales
. Fine detrital grain, size, radiolarian-bearing nonskeletal phosphate
, and pelagic fauna (ammonoids and conodonts) further substantiate an
offshore marine origin for the black ''core'' shale facies that is sur
rounded by the gray facies, and which was deposited in a probably deep
er more continually anoxic environment where most benthos was excluded
.