Abstract

The depositional environment of the Ostracode Zone and equivalent strata have been investigated from a paleohydrological perspective using O, C and Sr isotopic compositions in representative fossil shells. Using a mass balance approach, the isotopic water budgets are used to estimate paleosalinities for the Ostracode Zone depositional waters. Although it is widely believed that the Ostracode Zone was deposited under brackish water conditions, a fresh-water environment is deduced for the deposit at least for the carbonate lithologies and clastic sediments containing shells. The narrow range in ∂¹⁸O values for shell material from the Ostracode Zone and equivalent strata (20.9 ± 0.7‰) indicates either a single well-mixed lake or a series of hydrologically connected lakes. The 4-7‰ ¹⁸O-enrichment of Lake Ostracode waters (−10.1‰) relative to fluviatile inputs (−14‰ to −17‰) indicates long water residence times and therefore slow through-flow of fluviatile waters to the Moosebar-Clearwater sea. To account for the presence of marine indicators such as dinoflagellates in some Ostracode Zone strata, a restricted marine connection is postulated, similar in conception to modern Lake Maracaibo, Venezuela, where low uniform salinities (3.3‰) are maintained in a hydrological balance between fluvial and marine inputs. Therefore, both climatic (wet and dry periods) and tectonic factors may have influenced water levels and salinities in ancient Lake Ostracode. Provided suitable material is available, the reconstructed Sr and O isotope paleohydrologies may be used to gain insight into other Mannville Group paleoenvironments through isotopically derived constraints of paleosalinity.