Abstract
Consolidation of sediment is the main cause of porosity reduction with depth in the upper 1000 m of the sediment column. The consolidation of high-porosity sediment is mostly mechanical: the weight of the overlying sediment drives the rearrangement of individual particles and groups of sedimentary particles and domains. The mechanics of particle reorientation may be understood best through an examination of the sediment microfabric. A clay-rich sediment section 318 m thick, recovered during ODP Leg 113 from the South Orkney Microcontinental Margin, Site 697 in the Weddell Sea, was examined by transmission electron microscopy of ultrathin sections. Reorientation of randomly arranged par ticles of this fine-grained, high-porosity (70%-75%) sediment occurs very gradually: porosity decreases to only about 50% at a depth of 318 m, because of the very fine-grained nature of the sediment and the presence of extremely fine grained smectite, which imparts a very low permeability even at porosities of 50%.
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