RAD-ICAL DIAGENESIS: RECOGNIZING COMPLEX REPLACEMENT AND INFILLING OF RADIOLARIAN TESTS

Radiolarians are an important intrabasinal component and source of silica in the sediments that formed many Phanerozoic carbonate and siliciclastic mudrock units. The biogenic silica that composed radiolarian tests made them susceptible to replacement by minerals, dissolution, or both during diagenesis. The intraparticle pores within radiolarian tests also made them subject to infilling mineralization which can be difficult to distinguish from replacement. Radiolarian tests come in a variety of shapes and sizes, which complicates identification after alteration and replacement. To preserve the overall radiolarian structure relatively intact, replacement and infilling must start to take place during shallow burial, otherwise the radiolarian test will be crushed during compaction.

Differing chemical environments lead to differences in alteration and infilling of the tests. For example, in the Cretaceous Eagle Ford Group of South Texas, radiolarians are commonly replaced by pyrite and quartz and have infilling of kaolinite and secondary organic matter (bitumen). In the Permian Wolfcamp Shale of west Texas, radiolarians are commonly replaced or infilled by quartz and chlorite in some beds and calcite in others. Some mudrocks beds that were initially siliceous can become calcareous because of the calcitization of radiolarians. Wholesale replacement and infilling by calcite can cause altered radiolarians to be easily mistaken for originally calcareous allochems. This is particularly notable in the Jurassic-Cretaceous Vaca Muerta Shale of Argentina. Cathodoluminescence imaging can reveal the original structure of some calcitized radiolarians. Minor amounts of quartz replacement within the calcite also offer clues to pre-alteration provenance. However, care must be taken in respect to identifying partially calcitized radiolarians, as partially silicified calcareous fossils occur in mudrocks as well. To further complicate textural interpretation, calcitized radiolarians are susceptible to secondary replacement by dolomite and/or albite feldspar. Varying degrees of alteration and preservation within a sample are common and can provide important clues for identifying completely replaced or partially destroyed examples as having originally been radiolarians.