Late Tournaisian Waulsortian-like carbonate mud banks from South China (Longdianshan Hill, central Guangxi): preliminary investigations

For the first time Waulsortian-like carbonate mud banks are described from lower Mississippian strata of South China. The carbonate mud banks occur in the upper member of the Long'an Formation (late Tournaisian in age) at Longdianshan Hill (near Liuzhou, central Guangxi). They are domal to tabular in shape, and locally form aggregates. Horizontal and vertical differentiation of the banks into core and flank facies was not observed. Onlapping thin-bedded, grainy off-bank facies indicates that the banks had depositional relief. The carbonate mud banks are composed of a mixture of two basic microfacies types: (1) spiculitic wackestone with diverse fauna and (2) fine-grained bioclastic wackestone and packstone. Colour changes indicate the presence of several mud generations, but syndepositional cavities are limited in volume. More common are larger spar-filled cavities representing matrix dissolution. The common reddish colour of the bank facies might be related to hematite-rich microaerophilic iron bacteria. Formation of the carbonate mud banks took place in an outer platform setting in several tens of metres water depth under dysphotic conditions during the relative sea-level rise of a third-order sequence. The carbonate mud banks at Longdianshan Hill share some characteristics of classical Waulsortian banks in Europe, notably the presence of bryozoans, crinoids and sponges, different mud generations and spar-filled cavities but, so far, polymuds with distinctive syndepositional cavities and a vertical zonation into grain assemblage zones is not documented. The Longdianshan Hill Waulsortian-like carbonate mud banks significantly enlarge the spectrum of Mississippian buildups in South China, which on a global scale is still relatively poor. Copyright © 2012 John Wiley & Sons, Ltd.