(Table 1) Age and description, showing diagenetic changes with depth at DSDP Site 80-550

Leg 80 of the Deep Sea Drilling Project drilled four coreholes through syn- and postrift sediments on the seaward edge of the Goban Spur in the Bay of Biscay. A relatively thin sedimentary veneer composed of nannofossil chalk and calcareous and siliceous mudstone was cored, providing the opportunity to document the effects of burial depth and other factors on the porosity of chalks by means of the scanning electron microscope. Four diagenetic "fades" were described from SEM analysis of the chalks; in order of increasing depth they are (1) mechanical compaction facies, (2) a transitional facies with both mechanical and chemical compaction features, (3) a chemical compaction facies, and (4) a cement-inhibited facies. It appears that advanced stages of cementation and lithification can occur in pure chalks at relatively shallow depths of burial (100-700 m) but can be inhibited by the presence of clays and coarser grained detritus.