'BIG MAC CALCARENITE CONCRETIONS (LOWER OLIGOCENE, NW-BELGIUM): CONCEPTUAL GROWTH MODEL DERIVED FROM STRATIGRAPHY, PETROGRAPHY AND GEOCHEMISTRY

摘要

'Big Mac' calcarenite concretions were dredged at Wintham (Province of Antwerp, NW-Belgium). Small concretions are mostly spherical, larger ones circular but slightly flattened. They can coalesce, forming club-shaped bodies. Several large concretions show a subtle central indentation corresponding with a strongly bioturbated zone, providing them a 'hamburgerlike' shape (holotype). Grain-size composition and microfossil biostratigraphy of the enveloping sediment sampled in a nearby borehole, suggest that the concretions appear in the top of the Niel Formation, of which the upper(most) portion could have been (partly) eroded. This is evidenced by the occurrence of a level of reworked concretions, which is interpreted as a lag deposit. Most ichnofossils in the concretions belong to the Cruziana ichno-facies typical for a shallow-marine environment. Variations in petrographical and geochemical concretion characteristics suggest a concentric growth that initiated at the core (nucleus). Their constituting sediment originally consisted of a fine-grained, slightly clayey sand, containing mainly quartz, feldspar, glauconite, mica and heavy minerals. The sediment was bioturbated, and pyrite framboids formed during early diagenesis in a reducing environment, in particular in bioturbations and microfossil tests. Cementation matrix consists mainly of micritic calcite, replacing particles originally present in the sediment. δ~(18)O -values in the calcite matrix of the concretions decrease from the core to the rim (with extreme values up to -3.3 per thousand PDB). Compared to the mean δ~(18)O -value for Oligocene ocean water, these results suggest that the calcite precipitated from seawater with a slightly depleted composition. The more negative values at the outer zones might result from water-sediment interaction and/or recrystallisation by meteoric water. Calcite δ~(13)C -values increase from the cores to the margins of the concretions, and suggest that calcite precipitated from pore waters with active bacterial sulphate reduction. The slightly less depleted values at the concretion margin are interpreted in terms of contribution of bicarbonate derived from methanogenesis. However, recent surface alteration might also play a role.