Howard R. Feldman1, Evan K. Franseen2, R. Matthew Joeckel3, Philip H. Heckel4, Allen W. Archer5
(1) ExxonMobil Upstream Research Co, Houston, TX
(2) University of Kansas, Lawrence, KS
(3) University of Nebraska-Lincoln, Lincoln, NE
(4) University of Iowa, Iowa City, IA
(5) Kansas State University, Manhattan, KS
ABSTRACT: Longer-term Climate Cyclicity in the Midcontinent Pennsylvanian Succession of High-frequency Sequences (Glacial-eustatic Cyclothems): Impact on Sequence Architecture
A succession of eight high-frequency Pennsylvanian sequences in the northern Midcontinent reveal a pronounced wet-seasonal to dry-seasonal climate cyclicity at the scale of 3-4 sequences. The primary evidence for climate change comes from lowstand valley fills and sequence-boundary paleosols. Wetter climates are indicated by low-chroma paleosols and common coals; associated incised valley fills are large, dominated by sandstone from distant source areas, and contain plant assemblages consisting mostly of ferns, seed ferns, and calamites. Drier climate indicators include high-chroma paleosols, typically with pedogenic carbonates; associated valleys are small, filled with locally-derived clasts, and contain plant assemblages dominated by conifers. Transgressive systems tracts contain thin, extensive limestones and shales, largely unaffected by climate, probably because rapid sea-level rises sequestered siliciclastics in updip positions. Highstand systems tracts above the thin condensed-section phosphatic shales show a distinct range from thick marine shales in the wetter sequences to thinner marine limestones in the drier sequences.
A test of the climate model is the relationship of sequence climatic expression to magnitude of sea-level change of individual sequences. Although the wetter climate interval is dominated by lower-magnitude sea-level oscillations, two of the high-magnitude sea-level cycles (based on landward extent of the most offshore marine facies in the condensed section) are within the wetter interval, whereas two others are within the drier intervals. Thus, climate plays a role distinct from that of sea-level change, and our results indicate that modest climate shifts have a clear impact on sediment dispersal patterns and sequence architecture.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado