The fossil diatoms


The diatomites

The centric diatoms have appeared 150 millons years (MA) ago, the pennate diatoms without raphe 70 MA ago and those with raphe 20 MA later.
Their intense proliferation in waters rich in silica and the accumulation of their rotproof siliceous frustules during long geological periods are at the origin of deposits of rocks named diatomites. Diatomites are constituted at least at 80 % by frustules. One cubic centimeter of some diatomites contains around 2.5 millions frustules.
The most important deposits of diatomites are in California. In France, diatomites are exploited open air in Cantal (Virargues-Foufouilloux, thickness of diatomites : 18 meters) and in Ardèche (Montagne d'Andance, St. Bauzille, thickness : 40 meters). A deposit was exploited in a subterranean quarry in Cantal (La Bade). These diatomites have formed either in crater lakes (maars) or in lake of dam due to a basalt casting. At Virargues, the diatomites are about 5.3 MA old and at the Montagne d'Andance they are between 8 and 6 MA old.

About 120 taxa have been listed in the diatomites from the Montagne d'Andance (Serieyssol, 1993) and at least forty at Virargues. Several of these taxa are always present in the fresh waters.

The gallery of pictures:

Most of the frustules/valves present in the diatomites are brooken. However some ones are in a perfect state of preservation although they are about 5-6 MA old.
The preparation of the frustules/valves from the diatomites is more complex and longer than for diatoms of the fresh waters. It needs to dry the stone and to use hydrogen peroxide and hydrochloric acid in hot conditions and ammoniac. The obtained cleaned frustules/valves were
permanently mounted in Melmount© (ND = 1,704 ; Cargille labs, USA) and coloured micrographs were taken.

The works of Germain (1981) and of Serieyssol (1993) were used to determine the species.

Measures are indicated in micrometers (µm) ; one micrometer is equal to one thousandth of millimeter.
The taxa which are also present  in the present fresh waters are indicated by (A).