Accretionary Mechanisms and Temporal Sequence of Formation of the Boda Limestone Mud-Mounds (Upper Ordovician), Siljan District, Sweden

Late Ordovician mud-mounds in the Boda Limestone, Siljan area, central Sweden, consist mainly of massive spiculitic stromatactis limestone similar to other well-known Paleozoic mud-mounds. The mound cores are predominantly formed by spiculite and ostracod-rich mudstone and/or wackestone, and networks of spar-filled syndepositional cavities. The mounds of the Boda Limestone are specific in two ways: (1) large parts of the lower mound cores are formed by a Palaeoporella framestone and/or bafflestone with signs of early recrystallization and cementation; and (2) the stromatactis facies is accompanied by masses of limestone with non-stromatactis cryptocrystalline patches. Cryptocrystalline patches are filled with massive, fibrous low-Mg calcite, have cloudy, transitional margins, contain randomly spaced skeletal grains and dolomitic micro-inclusions, and are interpreted as recrystallized patches of the original host lime mud. Models of formation of Paleozoic mud-mounds generally hypothesize accretionary processes that are based on sponge biomass degradation and associated diagenetic processes. In the Boda Limestone Palaeoporella skeleton disintegration, early recrystallization, and additionally, cementation were essential for the mound formation. The Boda mounds exemplify that formation of Paleozoic mud-mounds reflects a spectrum of different essentially diagenetic accretionary mechanisms. This diversity should be taken into account in future discussions about Paleozoic mud-mound formation. The new name Osmundsberget Formation is given for a Hirnantian succession of brachiopod-rich limestone above the Boda Limestone. The Osmundsberget Formation occurs only in direct geographical vicinity to the mud-mounds.