Clay Water Diagenesis During Burial: How Mud Becomes Gneiss

Mineral and chemical studies of muds samples (sidewall cores) from 4233 ft to 16,450 ft deep in the Gulf of Mexico (courtesy Chevron Oil Company) and from the surface to 24,003 ft in the Anadarko Basin, Oklahoma (courtesy Shell Oil Company) were studied in order to obtain information on the diagenesis of muds during deep burial. With depth the montmorillonite in these montmorillonite-rich shales is altered to mixed-layer illite-chlorite-montmorillonite; the regularity of the mixed-layer phase tends to increase with depth. Much of the interlayer hydroxy material is A1 and Fe, acquired before deposition. K is acquired from the river and seawater, and after deposition, from K-feldspar. Increased lattice charge in the montmorillonitic layers (largely beidellite) is due to the reduction of iron in the octahedral layer and the incorporation of additional Al in the tetrahedral layer. It is suggested that the latter phenomenon is caused by the migration of interlayer Al into the hexagonal holes in the oxygen sheet. Most of the loss of expandable layers occurs by 12,000 ft. As depth and temperature increase, kaolinite is destroyed, and the Al is deposited between the expanded layers as hydroxy-Al to produce layers of dioctahedral chlorite. Both discrete dioctahedral chlorite and mixed-layer illite-dioctahedral chlorite-montmorillonite (ultimately mixed-layer illite-chlorite) are formed. During regional metamorphism the sequence is kaolinite and mixed-layer illite-montmorillonite→mixed-layer illite-dioctahedral chlorite $K , Mg , Fe →$ muscovite + chlorite. The Al₂O₃ content of the bulk samples ranges from 9.94 to 17.47 percent. This is equivalent to approximately 55 to 75 percent clay minerals. The Al₂O₃/K₂O, Al₂O₃/MgO, and Al₂O₃/Fe₂O₃ values of the Chevron and Mississippian (except for Al₂O₃/MgO) samples indicate a deficiency of K, Mg, and Fe with respect to the Paleozoic and Precambrian shales. There is an indication that these cations increase with depth and may have migrated upward from the interval where granitization is taking place. These montmorillonite-rich clays cannot be converted to the typical illite-chlorite clay suite of the older shales without the addition of K, Mg, and Fe from external sources. Dioctahedral chlorite, both as discrete chlorite and as mixed-layer illite-chlorite, is present in most Paleozoic and Precambrian shales and will be even more abundant in deeply buried Tertiary shales unless cations are added from outside the system. The data indicate that, in areas where the conversion of montmorillonite-rich clays to illite-chlorite clays occurs, the geothermal gradient is relatively high, and K, Mg, and Fe are added from below. The possibility exists that geothermal gradients were higher in the Paleozoic and Precambrian, particulary preceding the middle Carboniferous break-up of the continents. The total exchange cations (Na + K + Mg + Ca) average 30 meq/100 g down to approximately 8000 ft; deeper samples average 20 meq/100 g. The C.E.C./Al₂O₃ values decrease to 10,000 ft, then remain constant, confirming the X-ray interpretation. The more mobile Na replaces Mg to become the dominant exchangeable cation in the shallow samples. As the expandable layers are converted to nonexpanded layers, the weakly bonded Na is released to the pores, and Ca is selectively retained. Most of the exchangeable Mg is used to form dolomite or is flushed into the sands where it combines with montmorillonite to make chlorite. The pore water content decrease to 10,300 ft abruptly increases, as high pressures are encountered, and then systematically decreases. The cation concentration in these waters is two to three times that of seawater. The cation concentration increases to 10,000 ft, abruptly decreases by 20 percent, then remains relatively constant. Na and K are more concentrated than seawater; Ca and Mg are less concentrated. K and Mg increase with depth; this is presumably a function of the increase in temperature. In the shallow samples the Na concentration increases as the pore water decreases. In deeper samples the ratio remains relatively constant. The anion concentration is HCO₃ > SO₄ > Cl. Cl systematically decreases with depth and is one-fourth the concentration of seawater by 10,000 ft. Presumably, this is due to selective flushing. Over the same depth interval, SO₄ increases by a factor of 6 to 7, comparable to the decrease in pore water, suggesting concentration by selective filtering. HCO₃ increases in concentration to 10,000 ft and then remains constant. The high HCO₃ values are due to the decomposition of organic matter and calcite. Functional organic groups are released from the clay minerals as the temperature and Na concentration increase. During burial a physical permeability barrier is formed by the rapid dewatering of the top of the thick water-rich mud section. Upward migrating Ca precipitates as calcite increasing the effectiveness of the buried mud. Na diffuses through the barrier more easily than water; Cl diffuses through more easily than SO₄ and HCO₃.