AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Biomarker Evidence for Paleoenvironmental Conditions in the Upper Guadalupian (Mid-Permian) Bell Canyon Formation, Delaware Basin, West Texas
(1) Geological Sciences and Engineering, University Nevada Reno, Reno, NV.
(2) Earth and Planetary Sciences, Kyushu Univerisity, Fukuoka, Japan.
Organic biomarkers of the Reef Trail Member of the Bell Canyon Formation (Middle Permian) exposed in the Patterson Hills, Guadalupe National Park, West Texas, show fluctuating paleoenvironmental conditions coinciding with changes in radiolarian fauna in the uppermost Guadalupian, prior to the onset of evaporitic conditions. The Reef Trail Member is a basinal marine carbonate capped by the Castile Formation evaporites, and represents the uppermost stratigraphic unit in the Guadalupian series in the Delaware Basin. The Guadalupian-Lopingian boundary is interpreted to occur at or just above the Reef Trail-Castile contact based on the presence of conodont Clarkina postbitteri hongshuiensis. Radiolarian faunal shifts observed in the upper meters of Reef Trail Member are interpreted to be paleoenvironmentally driven, and associated with regional basinal events, but are also contemporaneous with the end Guadalupian event. Thirteen samples from the uppermost units in the Reef Trail Member were analyzed for organic biomarkers using GC/MS (gas chromatography-mass spectrometry) and GC/C/IRMS (gas chromatography-combustion-isotope ratio mass spectrometry). Biomarker results show fluctuating environmental conditions. The pristane to phytane ratio (Pr/Ph), a proxy for paleo-redox conditions, indicates anoxia at the Reef Trail-Castile contact. The dibenzothiophene to phenanthrene ratio (DBT/PHEN), a proxy for source material, fluctuates between marine and terrigenous source material, with increased terrigenous source material at the Reef trail-Castile contact. Compound-specific isotopes were also examined and δ13C(C17 alkane)/δ13C(C27 alkane), which indicates marine versus terrestrial contribution, supports the DBT/PHENdata, with increasing terrestrial contribution near the Reef Trail-Castile contact. The increasing terrestrial signature may be associated with marine regression, resulting in basin restriction accompanied by local anoxia. However, anoxia may be associated with the end Guadalupian event, because anoxia has been documented in other end Guadalupian sections throughout the world.