Microfabric and origin of cambrian carbonate ooids—examples from the cambrian oolite of carteret (Northeastern Armorican Massif, France)

Abstract The oolite at Carteret, Cambrian (Atdabanian) in age, is composed of purely radial, radial-concentric and composite ooids. The close resemblance between the cortex of purely radial ooids and the innermost cortex of radial-concentric ones suggests that the latter ooids may originate from purely radial ones with an accretion of envelopes with a radial fabric. The appearance of a concentric texture is the result of phenomena of abrasion undergone by purely radial ooids with diameters over 0.600–0.680 mm when transported as bed-load. The abrasion resulting from this transport stops the radial growth of crystals formerly developing in a low-energy environment. Then in a high-energy environment laminae of small equant crystals are formed. It is due to an alternation between a low-energy environment and a high-energy environment that ooids acquire their concentric texture. When compared to similar present-day ooids, it appears that radial ooids may have been formed in algal mats interbedded with Carteret oolite.