Stress diagenesis

Bradley and Powley (1994) suggest, The primary cause of either pressure or flow is epirogenic movement, with erosion or deposition, which changes the temperature, stress, and pore pressure in the system, which in turn, affect the diagenesis in the system. ... 

The distribution of vanadyl vs. nickel porphyrins is noteworthy (Table IV). An overview of the data from Site 368 shows that normal porphyrin diagenesis has progressed to the nickel DPEP porphyrin stage at + 15.87 m (IS). At greater depth, the normal process is interrupted by high thermal stress with concomitant formation of vanadyl porph)u is and decomposition of the native nickel porphyrins. Further below the sill (—8.39 m) only slightly altered nickel porph)rins, again predominantly... 

Bjdrlykke, K. and H0egh, K. 1997. Effects of burial diagenesis on stresses, compaction and fluid flow in sedimentary basins. Mar. Pet. Geol., in press. 

In summary, the new technique developed at BP uses a proprietary transformation that relates velocity directly to effective stress, temperature and gross lithology, takes account of the major causes of overpressure in clastic basins (namely, undercompaction, clay dehydration and diagenesis, buoyancy and charging of fluids in dipping, permeable beds), and predicts effective stress directly, which is the most fundamental quantity for pressure prediction. 

There is thus no clear indication from the DSDP analyzed chert 6 B record that ancient cherts can be used to reconstruct seawater paleo-pH or paleo-8 B values, in agreement with previous statements on the importance of diagenesis in the control of chert fi B values (Ishikawa and Nakamura, 1993). However, it must be stressed that this is probably not the case for silica having undergone limited diagenesis such as in opal-A or opal-CT. 

---