温帯浅海域の炭酸塩堆積システム
オーストラリア, ヴィクトリア州南部, 第三系上部漸新統・下部中新
1994 年 103 巻 1 号 p. 30-48
詳細
1994 年 103 巻 1 号 p. 30-48
With the purpose to envisage depositional systems of the temperate shelf carbonates with respect to sea level changes in a tectonically inactive region, eight sedimentary facies (facies A to H) are differentiated in a sedimentary sequence which occurs within the Jan Juc Formation (Late Oligocene to Early Miocene) along the sea cliff at Torquay area, southern Australia. The sequence ranges from 1 to 6m in thickness and over 3.5 m.y. in duration and their spatio-temporal relationship is restored.
While the lower sequence boundary (25.5Ma) formed during a lowstand period varies basinward from karstified surface into a conformable surface within the heavily burrowed beds, the upper sequence boundary (22Ma) is traced along just below the well-indurated “Bird Rock Cap (BRC)”, which was deposited during the subsequent lowstand period.
The sequence is unevenly divided into two packages of sediments by MFS (maximum flooding surface), which is traced within the interval less than 1m above the shell bed (facies E); the thick TST (transgressive systems tract) below MFS and the less developed HST (highstand systems tract) above it. The TST sediments of especially very fine packstone-mudstone of which fossils are largely made up of benthic foraminiferal tests and shell fragments (facies C) are best developed, onlapping onto the lower sequence boundary. The transgressive lag characterized with profusion of glauconitic minerals and shell fragments of gravel size (facies A) occurs basinward just above the sequence boundary. Sediments of the HST are mainly composed of fine packstone with bivalve fragments and benthic foraminiferal tests (facies F) and relatively thin as progradation took place only landward.
It is concluded, therefore, that the most salient characteristic of the temperate carbonate depositional systems for a complete cycle of sea level change is low influx of sediments at early transgression and highstand periods, as evidenced by grain composition, especially abundance of glauconitic minerals, and difference in thickness between the TST and HST.
