Continental Carbonate "Diagenesis"

Primary fabrics in continental carbonates consist of micrite, sparite or a combination of both. Post-formation processes may cause micritisation and sparitisation. This results in highly complex fabrics, where it is often... [ view full abstract ]

Primary fabrics in continental carbonates consist of micrite, sparite or a combination of both. Post-formation processes may cause micritisation and sparitisation. This results in highly complex fabrics, where it is often challenging to distinguish primary from secondary features. In addition, these alteration processes are starting syn-depositional, but, given the susceptible aragonite or calcite mineralogy, continue up to the moment the sample is studied under the microscope. “Diagenesis” is thus a very vague term in continental carbonate research. Rather than the distinction between “depositional” and “diagenetic” fabrics, focus should lie on understanding the paragenesis and processes behind the formation and alteration of the precipitates.
Due to the continuity of rock forming and modifying processes, the study of rock fabrics should not only be combined with geochemical properties, but requires a multi-method approach. Cathodoluminescence microscopy, a widely applied technique in carbonate diagenesis studies, so far was of limited application in continental carbonates. This is not surprising given that the oxidative meteoric setting of most travertines will result in non- or only a weak luminescence. Exceptions are for example lacustrine travertines, where the depositional environment can become quickly reduced. Remarkably, also stalactite-like features in primary cave systems in waterfall facies show cathodoluminescent fabrics. It are these features that possibly provide a unique glance in the early diagenetic alteration of continental carbonates.
Carbonates that precipitate around hanging plants became coated with sealing phreatic cements, resulting in small-scale closed systems. Decay of organic matter could consume oxygen inside the core of these stalactite-like features and lead to locally reduced environments for their cores. The redox changes in combination with processes like micritisation and sparitisation, would result in fabrics with different luminescence characteristics. Alternatively, concentration changes due to the difference in distribution coefficient of elements can be the sole cause of cathodoluminescence variation in a closed system. The closing of the system thus provided a reference point in the paragenesis of the travertine in the core of the stalactite-like structures, which likely can be extrapolated based on fabric similarities to the whole geobody.
Secondary alteration has important implications for petrophysical, geochemical and dating analyses. Understanding carbonate rock alteration from deposition onwards is thus primordial.