Preservation of fossils in clay minerals; a unique example from the Upper Ordovician Soom Shale, South Africa

In the Soom Shale, labile soft tissues have been replaced rapidly after dea th by authigenic clay minerals which now have an illitic composition. There are two possible pathways for this mineralization: (1) initial replacement of soft tissue by kaolinite which was then diagenetically transformed to i llite; or (2) direct replacement of soft tissue by illite. Comparison of th e compositions of the authigenic illites that replace the soft tissues and of the detrital illites in the host rock show that the former have higher m agnesium numbers and enhanced potassium contents. These compositional diffe rences are better explained if illite directly replaced the soft tissues. K aolinite and/or illite stability are controlled by the pH and the ratio of K+/H+ in the fluid, expressed as: 2(KAl3Si3O10(OH)(2)) + 3H(2)O + 2H(+) <-- --> 3(Al2Si2O5(OH)(4)) + 2K(+). Although the porewaters of the Soom Shale w ere anoxic and, at times, euxinic, and so might well have favoured kaolinit e authigenesis: the conditions within the local environment of the carcasse s could have been very different. Ammonia, produced by protein decay, and p otassium, concentrated within the tissues, would have been elevated relativ e to the surrounding pore waters, so that the K+/H+ ratio would have favour ed illite authigenesis. Authigenic illites have higher magnesium numbers th an detrital sedimentary illites owing to extensive and early pyrite precipi tation, indicated by small pyrite framboid diameters (average 4.3 mu), whic h left the porewaters depleted in iron. The magnesium number in authigenic illites could also have been enhanced by the inclusion of magnesium from se awater.