Petrology of Lower Cretaceous carbonate mud mounds (Albian, N. Spain): insights into organomineralic deposits of the geological record

Abstract

The process of organomineralization is increasingly well understood with respect to modern carbonate sediments accumulating adjacent to tropical reef atolls and reef caves. Mineralization related to non-living organic substrates results in autochthonous micrite production (‘automicrites’). ‘Automicrites’ are the main constructive element of Lower Cretaceous (Albian) carbonate mud mounds in northern Spain. These slope mud mounds occur within transgressive and early highstand system tracts encompassing several macrobenthic ecological zones. They are clearly separated from the biocalcifying carbonate factory (Urgonian carbonate platforms), in both space and time. Within these build-ups, most ‘automicrites’ were initially indurated and accreted to form a medium-relief growth framework. ‘Automicrites’ have a uniform, presumably high-Mg-calcite precursor mineralogy. They show an inorganic stable-isotope signature (∂13C around +3·3‰) within the range of early marine cements, and skeletal compounds lacking major vital effects. Epifluorescence microscopy shows that they have facies-specific fluorescence, which is similar to skeletal compounds of Acanthochaetetes, but clearly different from allomicritic sediment and cements, which are mostly non-fluorescent. The EDTA-soluble intracrystalline organic fraction (SIOF) of Albian automicrites shows an amino acid spectrum that is similar to shallow subsurface samples from their modern counterparts. Gel electrophoresis of the SIOF demonstrates an exclusively acidic character, and a mean molecular size range between 20 and 30 kDa. Experiments in vitro (inhibition tests) indicate that the SIOF has a significant Ca2+-binding capacity. Fluorescence and chemical characteristics of SIOF point to a main substance class, such as humic and fulvic acids, compounds that form from pristine organic matter during early diagenesis. Biomarker analyses provide evidence for the crucial role of biodegradation by heterotrophic microorganisms, but no biomarker for cyanobacteria has been found. Primary sources of organic material should have been manifold, including major contributions by metazoans such as sponges. It is concluded that many carbonate mud mounds are essentially organomineralic in origin and that the resulting fabric of polygenetic muds (‘polymuds’) may represent ancestral metazoan reef ecosystems, which possibly originated during the Neoproterozoic.