Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf successions

The graptolitic Early Ordovician succession of the Mount Hunneberg locality, southern Sweden, shows the response of graptolite faunas to sea-level changes. The exposed interval consists of intercalated carbonates and shales at the base, grading into pure black shales in its central and upper part. This facies trend records a deepening of the depositional environment due to an overall sea-level rise. The Mount Hunneberg graptolite fauna is dominated by nearshore, shallow-marine forms found in most layers throughout the succession. Deeper-water pandemic species occur only rarely together with shallowwater graptolites but are dominant at four distinct traceable levels at Mount Hunneberg. This change in graptolite faunal composition is interpreted to indicate sea-level fluctuations. With rising sea level, shallow-water endemics and pandemics migrated landward, and deeper-water pandemic graptolites became increasingly frequent on the shelf. During the peak of the transgression, deep-water forms dominated. With failing sea level, shallow-water forms again appeared and replaced the deep-water graptolites. The four levels with deep-water graptolites at Mount Hunneberg are, therefore, interpreted to represent maximum flooding surfaces. This study demonstrates that changes in the faunal composition of such planktic organisms as graptolites provide a promising tool for recognizing maximum flooding surfaces. As these faunal turnovers are also detectable in monotonous parts of the Mount Hunneberg succession, this biofacies-based approach enables the recognition of maximum flooding surfaces even when no lithologic changes are present, enhancing the applicability of sequence stratigraphic interpretations tomonotonous outer shelf strata.