AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
An Ocean Record of Atmospheric Dust from Pennsylvanian Carbonates of the Akiyoshi Atoll, Japan
(1) Geology and Geophysics, University of Oklahoma, Norman, OK.
(2) Dept. of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan.
(3) Dept. of Geosciences, Boise State University, Boise, ID.
Atmospheric dust is an important archive and agent of climate change, and records of dust are well studied for the Cenozoic, from the continental realm as well as ocean core. Dust records from pre-Jurassic oceanic settings, however, are unknown. To address this, we studied a true oceanic record: the Carboniferous-Permian Akiyoshi Limestone is interpreted as a paleo-atoll from the Panthalassic Ocean incorporated into a Late Permian subduction-accretion-terrane (Akiyoshi Terrane) of southwest Japan. As an atoll, this system was completely isolated from any siliciclastic input, excepting that delivered via eolian processes. Hence, we can use the silicate mineral fraction contained in this system as an indicator of atmospheric dust. We logged and sampled, at cm-scale resolution, two ~20 m sections of the Akiyoshi Limestone recording Middle (Moscovian) and Late (Gzhelian) Pennsylvanian time. Litho- and bio-facies associations were described, bulk gamma-ray radiation, magnetic susceptibility, and samples were taken along these sections. Consistent with previous studies, we observed lithofacies variations (e.g., mudstone through grainstone), that reflect a stratigraphic cyclicity (sequence framework) on the 10-m scale and which reflect shifts in base level. Within the middle Pennsylvanian (Moscovian) section, Microcodium structure occurs commonly, recording exposure or near-exposure conditions. We hypothesize that atmospheric dustiness increased during glacial stages; if so, then we expect to find a correlation between facies and diagenetic signs of relative sea-level (ultimately reflecting glacioeustasy), and amount of the silicate mineral fraction (i.e., dust) preserved within the limestone. We will also be testing how porosity types change within the sequence stratigraphic framework of this ancient mid-oceanic atoll.