Significance of Laminated and Massive Diatomites in the Upper Part of the Monterey Formation, California

Alternating laminated and massive diatomite packets, ranging from one-half to several meters thick, occur in several places within the upper part of the Miocene Monterey Formation. We attribute these alternations of laminated and massive dia- tomite layers to oscillations in the boundaries of the oxygen-minimum zone. These fluctuations occurred at intervals of a few thousand years and were probably climatically induced. Laminated dia- tomites have generally drab colors, dysaerobic foraminiferal faunas, and platy grain alignment; they are generally unburrowed but in some places have a few burrows, mostly of Chondrites. Massive diatom- ites tend to have light colors, contain aerated foraminiferal faunas, may have prominent burrow systems of the Nereites ichnofacies or may be structureless, suggesting they were completely bioturbated. Tran- sitions from massive to laminated layers are abrupt, those from laminated to massive tend to be more gradational.