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.