21-08-2012, 10:42 AM
PETROFACIES
PETROFACIES.doc (Size: 28.5 KB / Downloads: 32)
The concept of sedimentologic “petrofacies” facies defined in terms of sediment composition rather than lithology- was introduced in early 1970’s by workers studying the great valley group in California (Dickinson and Rich, 1972). Petrofacies as defined by Dickinson and Rich (1972) complies detrital composition of sandstone and tectono-provenance significance .The relationship between plate tectonics and sandstone composition has been the subject of intensive pointed to an intimate relationship between detrital sand composition and tectonic setting (Crook, 1979; Dickinson and Suzeck, 1979; Ingersoll and Suzeck, 1979; Dickinson and Valloni, 1980; Schwab, 1981; Bhatia, 1985; Dickinson, 1985; De Celles and Hertel, 1989; Akhtar and Ahmad, 1991).
Petrofacies may reflect not only similarities in source rock, but in any of factor influencing sediment composition (Suttner,1974; Suttner and Dutta, 1986). Furthermore, petrologically defined facies do not necessarily imply common provenance. Johnsson et al., (1991), for example, defined a quartz-arenite petrofacies among modern sand of the Orinoco river drainage basin that was the result of diverse source rock and chemical weathering controls on sand composition: compositional similarity in these sands did not reflect similar provenance tectonic setting, or transportation history.
The proportions of detrital framework grains plotted on triangular diagram provides effective discrimination of a variety of plate tectonic setting and have been used as a powerful tool for determination the origin and tectonic reconstruction of terrigenous deposits (Dickinson,1985). But sometimes, correlation between tectonic setting and sandstone petrofacies does not hold good due to other factor that influence the detrital mineralogy of sandstone (Mack, 1984). Johnsson (1993) gave a detailed account of various sedimentary processes that control evolution of detrital composition and their role in the modification original composition of detritus. Climate and relief play most important role in this regard. The sandstone petrofacies analysis can furnish vital clues regarding the provenance and its tectonic setup, source rock composition, role of climate and transport. In turn these clues can be applied to interpret correctly the tectono-sedimentary evolution of geoprovenance and its sedimentary cover (Dickinson el al., 1983; Mack, 1984; Schwab, 1991; Graham et al., 1993; Cox and Lowe, 1995).
The necessity of low relief to allow prolong weathering also deserves emphasis as requirement for the development of first-cycle quartzose sand. Even where the climatic potential for intense equatorial weathering exist, Tropical high land developed quartz which regolith only on restricted interfluves with gentle local relief; both fluvial and littoral Holocene sand derived from drainage basin in tropical high lands with relief are quartz- poor. In evaluating the combined effect of climate and relief on the production of quartzose sand, however weathering during temporary storage on low-line flood plain along the continental dispersal path.