Petrographic criteria for the recognition of marine, syntaxial overgrowths, and their distribution in geologic time

Abstract

Syntaxial calcite overgrowths on echinoderm grains are a common cement type in ancient carbonate rocks. Most workers have interpreted these to be of meteoric diagenetic origin. Many originated, however, in the marine realm. Even when an initial marine overgrowth zone is present, it is common for later meteoric stages to make up a greater volume of this cement. Because marine overgrowths are often so thin, it is frequently difficult to sample for geochemical analysis. None the less, the initial marine overgrowths provided significant amounts of initial cement. We provide twelve petrographic criteria by which the marine parts of overgrowths can be recognized.

Applying these criteria to echinoderm-rich Paleozoic rocks, we suggest that marine syntaxial overgrowths varied in abundance through that Era. They are rare in the Cambrian, abundant in the Ordovician, rare in the Devonian, but virtually absent in Carboniferous rocks. Literature review suggests that marine syntaxial overgrowths are present, though rare, in the lower Mesozoic and also in the Paleogene. This type of cement has not been reported from modern shallow-water carbonate environments. A curve of changing abundance of marine overgrowths through time mirrors the oscillatory trend in nonskeletal carbonate mineralogy reported by Sandberg (1983). However, the peaks in the formation of marine overgrowths, which probably consisted of high-magnesium calcite, occur during times of preferential low-magnesium calcite facilitation (Sandberg, 1983). Absence or rarity of aragonitic marine cement at these times may have reduced competition for nucleation sites. Our data suggest that overgrowth cement on echinoderm ossicles should be reevaluated, and that controls on nonskeletal mineralogy are more complex than previously thought. *** DIRECT SUPPORT *** A00QA009 00002