Deposition, palaeoecology and diagenesis of the Silurian reef-like limestones on Gotland

Sammanfattning: Several Silurian (Wenlockian-Ludlovian) reef-like limestones on Gotland wereselected for studies of depositional, palaeoecological, and diagenetic conditions. Thereef-like limestones were deposited on a warm, shallow and open marine shelf. Theywere often exposed above the sea level in their depositional time as indicated byoccurrences of karstic features and vadose cement. The constituents of the reef-likelimestones were quantitatively analysed. The most important reef-building organismswere stromatoporoids for all the investigated limestones. However, the limestones in theupper part of the Gotland sequence (Sundre and Hemse Beds) show stronger dominationby stromatoporoids and higher stromatoporoid density on the rock surfaces than thelimestones in the lower sequence (Slite and Högklint Beds). The stromatoporoids ofGotland were subdivided into three ecomorphotypes: ambitopic, enveloping, andintermediate species. Ambitopic species were capable of growing extensively on loosebottom, whereas enveloping species preferred to encrust hard substrate. Intermediatespecies developed both ambitopic and enveloping characteristics. The most importantidentifiable sediment producers in the Gotland reef-like limestones were echinoderms,with the exception of one of the Högklint Beds in which calcareous algae were dominant.The investigated reef-like limestones vary as to stages of maturity. Only one of theinvestigated limestones (Holmhällar 1; Sundre Beds) can be strictly regarded as a reeflimestone. The limestone shows obvious zones consisting of boundstones and crinoidaldebris. The boundstones were formed by large laminar stromatoporoids rigidly bound toeach other. The other investigated limestones exhibit no or poorly developedframeworks. Important conditions for the development of the Gotland reefs were theformation of stable bottom associated with karstic processes, low sedimentation stress,and domination by stromatoporoids as a condition for faunal capacity in constructingframeworks. This last condition may have been dependent on the stages of globalevolution of the Palaeozoic reef community.