SILTSTONE COMPACTION – INSIGHTS FROM DEEP SEA SEDIMENTS

While silts are abundant in terrestrial clastic environments, they are usually mixed with particles of sand size, clay size, or both. Thus, assessing compaction of silts has been neglected. However, in some pelagic marine sediment silt-sized particulates are abundant and occasionally quite pure due to the dominance of coccoliths. We have examined cores from ODP (Ocean Drilling Project) legs 208, 207, 198, 183, 182 and 181, all of which contain 50 % to 100 % silt-oozes based on reported smear slide data. Twenty eight porosity vs. depth data points from sites 181, 182, 198 and 208 were interpreted as 100% silt-sized ooze and define a fairly tight porosity vs. depth relation: Porosity (%) = 67.6 exp(-z/2052 m). Mixing the calcisiltstones with other lithologies results in a broad scatter of porosities and a general trend towards lower porosities. For example, fitting the 462 data points that included 90-99.9% calcisiltstone, resulted in the best-fit porosity vs. depth relation: Porosity (%) = 57.4 exp(-z/2478 m). The data show a strong dependence of porosity on burial rate in the case of one leg, ODP 198, on the Shatsky Rise. In all other cases, burial depth is the dominant control on compaction.