Influence of mechanical compaction and clay mineral diagenesis on the microfabric and pore-scale properties of deep-water Gulf of Mexico mudstones

We report on how the effects of mechanical compaction and clay mineral diagenesis have affected the alignment of phyllosilicates in a suite of Miocene-Pliocene mudstones buried to sub-seabed depths of between 1.8 and 5.8 km in the deep-water Gulf of Mexico. Mechanical compaction has reduced the porosity of the samples to 15% at 5 km, with modal pore sizes between 10 and 20 nm. High-resolution X-ray texture goniometry data show that the intense mechanical compaction has not resulted in a strongly aligned phyllosilicate fabric. The muds were apparently deposited with a weak or isotropic phyllosilicate fabric which was not substantially realigned by mechanical compaction. Unusually, X-ray diffraction of < 0.2 mu m separates shows that: (1) there is no illitization trend between 90 and 120 degrees C; and (2) discrete smectite persists to similar to 120 degrees C, coexisting with R1 I-S or R0 I-S with 30-40% expandable layers. Between 120 and 130 degrees C, discrete smectite disappears and the expandability of I-S decreases to similar to 25-30%. We propose a two-stage diagenetic process involving (1) the alteration of volcanic glass to smectite and (2) the illitization of smectite and I-S; the alteration of glass results in smectite without a preferred orientation and retards the illitization reaction. We suggest that the lack of a strongly aligned phyllosilicate fabric reflects the apparently limited extent of illitization, and thus recrystallization, to which these mudstones have been subjected.