Original mineralogy of ordovician stromatoporoids

Abstract

Stromatoporoids in the Middle Ordovician Yeongheung Formation show delicate reticulate structures of persistent laminae and discontinuous pillars. Abundant microdolomite inclusions are found in the stromatoporoid skeletons. Coexisting aragonite precursor grains such as gastropod and cephalopod preserve no internal microstructures due to extensive dissolution and subsequent filling by low-magnesian calcite. Brachiopods (low-magnesian calcite precursor) show well preserved skeletal microstructures, but do not contain any microdolomite inclusions. Stromatoporoids were not dissolved as completely as original aragonitic shells, but recrystallized more than shells with a low-magnesian calcite precursor. The similarity of the stromatoporoid preservation pattern to that of echinoderm grains (high-magnesian calcite precursor) and the widespread occurrences of microdolomite rhombs in the stromatoporoid skeletons indicate that the Middle Ordovician stromatoporoids were originally composed of high-magnesian calcite.