Abstract

Radial and concentric textures in ooids from the upper Cambrian Warrior Formation of Pennsylvania are primary features whose development may have been controlled by the hydrodynamics in the environment of formation. The ooids grew with carbonate in radial fibrous orientation to a maximum diameter of approximately 0.6 mm. Beyond that diameter the ooids grew by the addition of concentric layers, giving an onion-skin texture. The first model considers the role of the threshold of motion, the 0.6 mm size corresponding to the minimum flow velocity needed to initiate ooid transport. The second and more probable model relates the 0.6 mm ooid diameter to the critical size in the transition from a predominance of transport in suspension to bed-load transport. Our model suggests that these ooids may have grown radially by direct precipitation while small enough to remain in suspension. After reaching 0.6 mm diameter they moved as bed load, and the ensuing abrasion resulted in the development of a concentric texture. Calculated paleocurrents of 40 to 120 cm/sec agree with observed current velocities in modern environments of ooid formation. Thus ooid textural development may be utilized to estimate paleocurrents.

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