Glaciomarine Carbonate Sedimentation--Berriedale Limestone (Permian), Hobart, Tasmania, Australia: ABSTRACT

Abstract

The Berriedale Limestone, an alternating sequence of impure limestone and calcareous shale with a few metabentonite layers, is associated with marine glacial sediments. There is less diversity of fauna (mostly bryozoans, brachiopods, and pelecypods) and allochems (only intraclasts) than in warm-water limestones. Lonestones (dropstones) commonly disrupt laminae. Quartz grains show glacial textures in SEM analysis. Well-developed rhombs (up to 100µ), scalenohedra, elongated rhombic plates (length slow), and zoned and poikilotopic rhombic plates indicate low-magnesian calcite precipitated from normal seawater at a temperature of <3°C. The saturation of calcium carbonate fluctuated, ranging from undersaturated (pits, etchings, rhombic pores, and dissolution features) to saturated. The early diagenetic fabrics were extensively recrystallized. The low-magnesium (<0.3%) and high-manganese (201 to 912 ppm) concentrations confirm original calcite precipitation. The Ga, Rb, Ba, and total Fe concentrations reflect normal-marine conditions during deposition of the limestone units and brackish conditions (caused by melting of retreating, continental ice sheets) during deposition of the shale beds. The source of detrital material changed as a result of melting of icebergs during waning phases of carbonate deposition. The dropstones are clustered around limestone-shale contacts. The vertical facies variation through shale-limestone-shale beds indicates that a gradual drop (~100 m) in sea level was followed by a rapid rise in sea level, corresponding to glacial and interglacial stages, respectively. The facies of shallowest water deposition occur slightly above the middle of most limestone beds. The limestone beds represent the coldest periods (glacial stages). End_of_Article - Last_Page 513------------