Abstract

Compilation of porosity data from 11 Ocean Drilling Program legs and 53 sites show that compaction in deep marine to upper-slope sediments is, at least in part, a function of both burial depth and lithology. Overall porosities range from 4 to 94%. Despite the broad range of porosities, there is an exponential trend toward decreasing porosity with depth. Comparison among samples dominated (90 to 100%) by each of six lithologies, based on grain size and composition, reveals the lithology dependence of compaction, except for biogenic silt-size sediments and clastic sands. In the former case, this may, in part, be due to mixing of calcareous and siliceous nannofossils. Clay-size clastic sediments show a clear change from rapid porosity reduction in the top 172 meters (porosity = 82.7 e^−depth/430) to less pronounced loss of porosity below (porosity = 61.4 e^{−depth/1671}). The remainder of the lithologies, silt (porosity = 75.5 e^{−depth/1091}), biogenic sand (porosity = 88.5 e^{−depth/1338}) and micrite (porosity = 69.5 e^{−depth/1235}) show a good to strong exponential reduction of porosity with depth. These relations compare well with literature studies, taking into account the less pure nature of the lithologies used in this work. The silt compaction curve is unique; no previous studies of porosity separated silt from other grain sizes.

Most sediment samples are of mixed grain sizes and types. The lithology-based porosity-depth relations were used to test four approaches to decompaction in mixed-lithology sediment used in backstripping. All methods generated porosity estimates that correlated strongly to the observed values, with correlation coefficients ranging from 0.629 to 0.686. These methods that have been used for years are, for the first time, shown to be valid.