Smectite Illitization in Pliocene-Age Gulf of Mexico Mudrocks

Abstract

Abstract--Utilizing high-resolution transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques, we have studied the transition from shallower smectite-rich mudrocks to deeper illite-rich mudrocks in Pliocene-age turbidite sediments from 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 sidewall cores from an offshore Louisiana well. In previous studies of GOM sediments, the smectite-to-illite reaction has mainly been documented in considerably older, Miocene-age sediments.The results of this study elucidate the reaction mechanisms entailed in the transformation of clays in this sediment from smectitic to illitic. We found that illite formed at the expense of smectite in 2 ways: 1) growth of preexisting discrete illite flakes, and 2) creation of new illite layers within mixed-layer illitesmectite. Also, illitization apparently proceeded via a dissolution/precipitation, Al-conserving reaction rather than a solid-state, layer-conserving reaction. Smectite illitization is commonly believed to require input of K from feldspar dissolution. Our XRD results found little correlation between decreases in K-feldspar and increases 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 as a sink for Fe liberated upon smectite illitization.

Illite authigenesis in sandstones of the Guaritas Allogroup (Early Paleozoic): Implications for the depositional age, stratigraphy and evolution of the Camaquã Basin (Southern Brazil)

“…It suggests the entire succession was submitted to temperatures between 80 and 110°C during a geologically significant time span after deposition. These temperatures are in agreement with temperature estimates for illite authigenesis in mesodiagenetic conditions (between 70 and 100°C; Rask et al, 1997;Morad et al, 2000).…”

Environmental effect and genetic influence: a regional cancer predisposition survey in the Zonguldak region of Northwest Turkey

“…Such microenvironments may favor conversion of smectite to illite during diagenesis, as determined by the SEM analysis, which was also reported by Hower et al (1976), Robertson and Lahann (1981), Bethke and Altaner (1986) and Rask et al (1997). The main source of Fe and Mg for traces of chlorite is ferromagnesian minerals, such as pyroxene and amphibole.…”

Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins

“…in aeolian facies or due to eogenetic dissolution) and/or in the vicinity of sites of influx of acidic waters such as along faults or adjacent to organic‐rich mudrocks. Progressive, burial transformation of detrital and eogenetic smectite, first into poorly, and then at greater depths into well‐ordered, mixed‐layer illite/smectite (I/S) or chlorite/smectite (C/S), respectively, and of berthierine into chamosite. These alterations, which proceed via dissolution/precipitation, rather than by solid‐state, layer‐conserving reactions (Rask et al . 1997), are volumetrically more important in mudrocks than in sandstones.…”

“…3 Progressive, burial transformation of detrital and eogenetic smectite, ®rst into poorly, and then at greater depths into well-ordered, mixed-layer illite/smectite (I/S) or chlorite/smectite (C/S), respectively, and of berthierine into chamosite. These alterations, which proceed via dissolution/ precipitation, rather than by solid-state, layerconserving reactions (Rask et al, 1997), are volumetrically more important in mudrocks than in sandstones. In sandstones, I/S and C/S form by replacement of mechanically in®ltrated, graincoating smectite, mud intraclasts and volcanic fragments.…”