ABSTRACT

In highly agitated, shallow-marine environments, infestation of carbonate grains by endolithic organisms undermines and weakens grain surfaces which collapse due to impact with other grains. In most environments this is an irreversible process leading to grain diminution and eventual destruction. However, in environments where ooids are currently forming this process is reversible, and pits formed by collapse are filled with debris and carbonate cement. This process is observed to be taking place in ooids from Joulters Cays, Bahamas; the northeast coast of the Yucatan Peninsula: Baffin Bay, Texas; and the Arabian Gulf. Although the composition and crystal form of pit-filling cements varies somewhat from one locality to another, these cements always resemble the carbonate making up the primary ooid cortex.

Pit formation and infilling lead to alteration of the ooid cortex, forming irregularly shaped patches of poorly or randomly oriented crystals which cut across primary laminae. Under the petrographic microscope, infilled pits may superficially resemble the "micritized" products formed by infilling of internal borings. However, with SEM the crystal forms are seen to differ from intragranular cements in form and size. The entrapment of foreign material such as clays, diatoms, and shell fragments in the pits and their subsequent cementation in place provide a mechanism for incorporation of foreign material observed in ooids by previous workers.

The similarity between pit-filling cements and primary ooid cortical material suggests that their mode of precipitation is similar. Some pit-filling cements, columnar crystals of high-Mg calcite and aragonite needles and laths, are precipitated physicochemically. However, the most common pit-filling cements, aragonite nannograins and rods or batons, are invariably associated with algal and/or mucillagenous filaments and films, and are formed by their calcification. Because ooids forming today have cortices composed in large part of aragonite rods and nannograins, these observations are in good agreement with the conclusions of Fabricius (1977) and Kahle (1977, 1981) that ooid cortices are formed primarily by the biochemically or biologically mediated process(es) associated with calici ication of filaments and films formed primarily by algae.