9:30 AM - 9:45 AM
[BCG07-03] Provisional diatom stratigraphy of Miocene–Pliocene lacustrine sediments from Western Snake River Basin, NE USA
During the late Miocene, the planktonic diatom flora in freshwater lakes underwent a complete turnover, with the disk-shaped family Stephanodiscaceae becoming dominant besides continuous dominance of the cylinder-shaped genus Aulacoseira. The turnover was known as a synchronous event at least in the northern hemisphere, may have global extent. Among Stephanodiscaceae in from the late Miocene–Pliocene are diverse, with extinct genera and species in addition to extant ones. Establishing highly reliable diatom biostratigraphy by the occurrences of these taxa with detail ages is expected to offer the age controls for lacustrine sediments which had been difficult to estimate the ages.
In the Western Snake River Basin, northwestern U.S., lacustrine sediments, Idaho Group, distribute widely. Radiometric ages of volcanic lavas or ashes intercalated them showed the ages from the middle Miocene to the early Pleistocene. The sedimentary sequence is known for one of the most continuous one in the world, however the several unconformities exist in the sediments, with sedimentary structures showing lake shallowing or dried-up. An extinct diatom genus, Mesodictyon, was described from the late Miocene Chalk Hills Formation, Idaho Group, which is the earliest Stephanodiscaceae. The authors performed field works in the Western Snake River Basin including the type locality of Mesodictyon to take sediment samples for diatom analysis, magnetic analysis and radiometric dating. The first occurrences of Stephanodiscaceae, such as Mesodictyon, Cyclotella, Lindavia, Stephanodiscus and Discostella should be dated by our studies. The provisional results of diatom biostratigraphy and its significance and prospectives will be presented.
In the Western Snake River Basin, northwestern U.S., lacustrine sediments, Idaho Group, distribute widely. Radiometric ages of volcanic lavas or ashes intercalated them showed the ages from the middle Miocene to the early Pleistocene. The sedimentary sequence is known for one of the most continuous one in the world, however the several unconformities exist in the sediments, with sedimentary structures showing lake shallowing or dried-up. An extinct diatom genus, Mesodictyon, was described from the late Miocene Chalk Hills Formation, Idaho Group, which is the earliest Stephanodiscaceae. The authors performed field works in the Western Snake River Basin including the type locality of Mesodictyon to take sediment samples for diatom analysis, magnetic analysis and radiometric dating. The first occurrences of Stephanodiscaceae, such as Mesodictyon, Cyclotella, Lindavia, Stephanodiscus and Discostella should be dated by our studies. The provisional results of diatom biostratigraphy and its significance and prospectives will be presented.