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6.2 Illy Formation - ILL (old: Rv3)

Author: Beugnies, 1960.
Description: Black slate and silty slate (called “quartzophyllade” in the older literature), micaceous with not frequently thin sericite laminae; the middle and upper part of the formation show frequent alternations of black slate and quartzite beds never exceeding 0.50 to 1 m in thickness. In the slate beds often microfolds are observed and more rarely millimetric chloritoid crystals (Beugnies, 1960).
Stratotype: not yet defined; type area in the outcrops along the Ruisseau de la Hatrelle, east of Illy.
Area: Givonne Massif.
Thickness: 850 m (Beugnies, 1960).
Age: The acritarchs in the lower part of the formation belong to the Zone 4b, indicating a Mid to Late Cambrian age (Vanguestaine, 1992). This result confirms at least certain lithostratigraphic correlations of Beugnies (1960) and contradicts those of Graulich (1954) and of Fourmarier (1960) who put the Givonne massif formations at a level below the Deville Group.