Sauk Megasequence Research Articles

Ordovician paleokarst and quartz sand: Evidence of volcanically triggered extreme climates?

In western Laurentia a thick Paleozoic succession is exposed that shows repeated paleokarst development. Karst events culminate within the Ordovician towards the top of the Sauk megasequence. Concomitantly, there is a systematic increase in depth of erosion and intensity of weathering from the base of the system towards the Mohawkian. After deposition of the Ninemile Shale, the amount of fine-grained detrital material is drastically reduced at the expense of quartz sand. Sand deposition culminated with the formation of the Eureka quartzite (Mohawkian/Cincinnatian). In the southern Great Basin, it is the only prominent (∼ 150 m thick) pure quartz sand unit within a thick (several 1000 m) Cambrian through Devonian dominantly carbonate platform succession. This paper discusses the reasons for the drastic changes in karst typology and the origin of the Eureka quartz sands. It is argued that Ordovician volcanism was the ultimate trigger for increasing kartsification and increasing quartz sand production through the formation of acid rain.