Diatoms

Diatoms are autotrophic, unicellular planktic and benthic marine freshwater algae with lacy, "pill box" opaline skeletons of elongate, circular, hemicircular, triangular or quadrangular shape, 40 - 60 µm in diameter. They occur in marine sediments of Cretaceous to Recent age and in non-marine sediments of Eocene to Recent age. Marine diatoms can usually provide precise biostratigraphy in the Neogene and Quaternary (time resolution of 1 my or better). In the Paleogene the time resolution offered by diatoms is about 3 my; in the Cretaceous the biostratigraphic interpretations are approximate but resoultion has improved. Freshwater diatoms can be used as stratigraphic indicators in the Neogene and Quaternary and normally provides ages with a precision of about 2 - 3 my. Diatom assemblages provide palaeoenvironmental information such as temperature, water depth, salinity and upwelling. Diatoms occur in softer sedimnets such as clay, shales, marls and ashes and are unlikely to be found in rocks such as chert, sands, sandstones and limestones.