PALEOECOLOGY AND TAPHONOMY OF FAUNAL ASSEMBLAGES IN GRAY CORE (OFFSHORE) SHALES IN MIDCONTINENT PENNSYLVANIAN CYCLOTHEMS

Citation
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
Citations number
103
Categorie Soggetti
Geology,Paleontology
Journal title
ISSN journal
08831351
Volume
13
Issue
4
Year of publication
1998
Pages
311 - 334
Database
ISI
SICI code
0883-1351(1998)13:4<311:PATOFA>2.0.ZU;2-J
Abstract
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 .