Abstract

In a Quaternary siliciclastic tidal-bar deposit, crustacean burrows have been selectively dolomitized, giving two distinct zones of cement fabrics. The inner, central part of the burrow has a microdolomite fabric. In contrast, the outer wall shows a variety of coarse dolomites ranging from zoned rhombic or quasi-rhombic crystal fabrics to rounded, spherulitic fabrics comprising concentric layers and radial fibrous crystallites. The latter fabric is derived from other, much simpler arrangements, but evidence of replacive features is lacking. The boundary between the two fabric zones is sharp. All evidence points to a primary origin for the spherulitic dolomite. The composition of the dolomite varies from Ca 52 Mg 48 to Ca 57 Mg 43 , and both fabric zones in a given burrow show variable dolomite compositions along its length. These observations suggest that the composition of the dolomitizing fluids and the kinetics of crystal growth were not uniform during the growth of these cements. It is surmised that fluid-flow conditions and organic matter (very likely related to bacterial activity) within the burrows were probably significant controls of selective dolomitization and cement fabrics.

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