Burrow-mediated carbonate dissolution in rudist biostromes (Aurisina, Italy): implications for taphonomy in tropical, shallow subtidal carbonate environments

In Upper Cretaceous rudist biostromes at Aurisina, Italy, marked taphonomic loss of radiolitids by burrow-mediated dissolution and mechanical disinteg ration illustrates the relation between synecology and early diagenesis in the fossilization of tropical shallow-water bioconstructions. The studied p latform succession accumulated in a prevalently open lagoon with bioturbate d carbonate sand, rudist thickets, bioclastic dunes, and areas with shelly lime ooze. Rudist biostromes consist of radiolitids and hippuritids, or of radiolitids only, and have an open, parautochthonous rudist fabric with a m atrix of bioclastic, bioturbated wackestone. "Swirly" disorientation of bio clasts records early softground bioturbation. Later firmground burrows comp rise an irregular network of tunnels and chambers filled with bioclastic pa ckstone to grainstone. The size, geometry and sediment fill of the firmgrou nd burrows suggest that they were produced by crustaceans. Radiolitid prese rvation ranges from complete to relictic. Radiolitid relicts formed by (a) spalling and/or dissolution of the cellular boxwork ostracum of the attache d valve, leaving "calcite-tubes' built by a distinct, thin ostracal shell l ayer, or (b) dissolution of the entire shell, leaving the sedimentary fills of the intertabular spaces of the attached valve as a diagnostic vestige, or (c) shell dissolution within the firm sediment, with subsequent filling of the biomould by bioclastic wackestone to packstone to grainstone. Loss o f radiolitids produced a taphonomic bias towards rudists with non-cellular ostracum. Locally, taphonomic loss produced "ghost biostromes" composed nea rly entirely of faint radiolitid relicts. Shell dissolution resulted from c hemical gradients in the sediment within and near the burrows, and from enh anced microboring and microbial infestation. Dissolution of radiolitids was favoured by the combined effects of chemical instability of hypostracal ar agonite and by the structure of the calcitic boxwork ostracum of thin-walle d cells. Some biostromes are intercalated with, or are capped or overlain w ithin a short vertical distance, by a hardly recognizable emersion surface, as a consequence of the shallow depth of biostrome accumulation. Taphonomi c loss by dissolution is widespread in open, parautochthonous rudist fabric s, and confirms actuogeological results of other authors that bioturbation mediates carbonate dissolution also under shallow tropical waters supersatu rated for calcium carbonate. The amount of carbonate dissolved upon burrowi ng of the biostrome matrix is hardly quantifiable but, by analogy to Recent carbonate environments, may have been large. Within bioconstructions, deep -tiered burrowing occurred at least since the Carboniferous. Taphonomic los s by dissolution thus may have been active in the fossilization of tropical shallow-water mounds and biostromes over much of Phanerozoic times. (C) 20 01 Elsevier Science B.V. All rights reserved.