EXPLOSIVE VOLCANISM AND ITS CONTRIBUTION TO MUDROCK GENESIS

Clay minerals are the most abundant minerals in mudrocks, making up over 60% of all shales and related mud-dominated rocks. Tertiary and younger mudrocks consist mostly of smectite and mixed layer clays whereas in older mudrocks illite starts to become the dominant clay mineral. As noted some years ago by Tourtelot (1974) "... the accumulation of nearly all the highly montmorillonitic, thick, and widespread shale units that are of recognized engineering, industrial, or agricultural significance has resulted from the deposition of volcanic ash in ocean basins.” Marine smectitic clays are more abundant and extensive than those of fresh-water origin because throughout geologic history ocean basins have been the largest repositories of detrital sediments and probably have been the largest areas in which the environment was favorable for the formation of smectite. Smectite is formed by the alteration of silica-bearing rocks; the altering solutions are alkaline and magnesium-rich. Silica-bearing rocks as varied as granite, basalt, serpentine, and graywacke sandstone may alter to smectite under the appropriate conditions. The smectite species formed depends on the conditions of alteration and the chemistry of the parent rock and altering solutions. Glassy rhyolitic volcanic ash is especially susceptible to alteration and is known to be the parent material of many smectite deposits in the form of bentonite and K-bentonite. While bentonites are generally smectite-rich, the clay mineralogy of K-bentonites is typically dominated by a regularly interstratified illite-smectite (I/S) in which the swelling component accounts for 20-40 percent of the total structure. Accessory clays are commonly either kaolinite or chlorite. Burial diagenesis is seen as one in a series of transformations between a smectite starting material and pure illite. The application of this model to the interpretation of clay mineral assemblages in both young and old sediments is made more attractive by the agreement between field evidence (Roberts and Merriman, 1990; Srodon, 1999) and experimental studies (Eberl and Hower, 1976; Eberl et al., 1990; Velde and Vasseur, 1992) which supply answers for some of the important thermodynamic and kinetic questions involved.