COMPARATIVE OOID SEDIMENTOLOGY: A CAUTIONARY TALE

Comparative sedimentology is recognized as one of the most powerful intellectual tools in the earth sciences. It is universally applicable to siliciclastic sedimentary rocks and, with the caveat of biotic and associated chemical evolution, is a cornerstone for the interpretation of carbonate rocks. Comparative sedimentology of autochthonous marine ooids in modern tropical settings and in Phanerozoic carbonates, where associated skeletal biota allow paleoenvironmental verification, confirms that these particles pinpoint shallow, warm-water, typically tidal conditions. This is now almost an axiom of carbonate sedimentology and is used throughout geological history to interpret Precambrian and Phanerozoic carbonate rocks. Some ooids in Neoproterozoic successions related to global glacial conditions, however, are interbedded with sediments containing glendonites, indicating very cold-water neritic environments of formation. Such conditions imply that ooids in the Proterozoic may form across the neritic paleoenvironmental spectrum, from polar to tropical settings. This is likely the result of higher marine carbonate saturation before the skeletal carbonate biota evolved and began to extract large amounts of carbonate from the seawater, lowering overall saturation and localizing ooids to warm-water neritic settings. Thus, cautionary thinking is required when interpreting Proterozoic ooids. Comparative sedimentology does not fail in this example, but, on the contrary, by forcing comparison with other rocks and modern sediments, it illuminates the differences and thus increases our understanding of ancient oceans.