Magadi-type chert, indicator of a lacustrine environment in the Middle Eocene McBean formation, South Carolina.

Magadi-type chert (chert derived from magadiite) occurs in the basal part of the formation at three localities in Orangeburg and Calhoun Counties. These occurrences imply that the basal part of the McBean formation was deposited here in an alkaline lake with a high silica content. Formation and filling of the lacustrine depression could have resulted from a timely combination of the following factors: an initial nearshore to estuarine depositional environment; falling sea-level; minor tectonic uplift of the Cape Fear Arch and/or the northern boundary of the south Georgia rift basin; intense chemical weathering in a tropical climate; and seasonal rainfall. The mineralogy, chemistry and genesis of the chert are considered.-R.S.M.