SMECTITE ILLITIZATION IN PLIOCENE-AGE GULF-OF-MEXICO MUDROCKS

Utilizing high-resolution transmission electron microscopy (TEM), ener gy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) technique s, we have studied the transition from shallower smectite-rich mudrock s to deeper illite-rich mudrocks in Pliocene-age turbidite sediments f rom the northern Gulf of Mexico (GOM). Our objective in this work was to better understand how the smectite-illite transition may affect the onset of geopressuring in GOM sediments. The samples studied were sid ewall cores from an offshore Louisiana well. In previous studies of GO M sediments, the smectite-to-illite reaction has mainly been documente d in considerably older, Miocene-age sediments. The results of this st udy elucidate the reaction mechanisms entailed in the transformation o f clays in this sediment from smectitic to illitic. We found that illi te formed at the expense of smectite in 2 ways: 1) growth of preexisti ng discrete illite flakes, and 2) creation of new illite layers within mixed-layer illite-smectite. Also, illitization apparently proceeded via a dissolution/precipitation, Al-conserving reaction rather than a solid-state, layer-conserving reaction. Smectite illitization is commo nly believed to require input of K from feldspar dissolution. Our XRD results found little correlation between decreases in K-feldspar and i ncreases of illite. However, in 1 instance TEM/EDS analyses indicated the presence of high-charge smectite, which suggests that insufficient K was available for illitization. TEM images also show small packets of authigenic chlorite in illite-rich mudrock. This chlorite may act a s a sink for Fe liberated upon smectite illitization.