(a) Detailed photograph of sample T displaying numerous clasts and a large void, about 1 by 2 cm, in the centre of the tube. (b) and (c) Thin-section natural and polarized light illuminations showing the five previously identified calcites C1 to C5. The calcite C1 is replacing shell fragments whereas the calcites C2 to C5 are concentrically arranged from the periphery to the centre of the void. The white equant calcite C5 locally contains traces of hydrocarbons or bitumen in veins as previously shown by Peckmann et al. (1999) in the area. Isotopic analyzes (C, O and Sr) were conducted on four samples (see text for details and Tab. 1 for values).