ABSTRACT

Arenites contain several types of non-carbonate intrabasinal (NCI) grains which are seldom given specific attention in petrographic studies. However, recognition and analysis of these particles may be of invaluable help for stratigraphic, paleogeographic and even metamorphic reconstructions.

Rip-up clasts are the most common NCI grains in various depositional environments. Concentration of other NCI types, such as green peloids, ferruginous ooids, reworked phosphates, chert, soil fragments, black pebbles or pyrite clasts, occurs at times of reduced sedimentation rates. During major transgressions, a wealth of iron, phosphorus, silica and organic matter of both terrestrial and marine origin is provided by rapid flooding of coastal plains, erosion of tropical soils developed during previous lowstand stages, and plankton blooms in nutrient-rich waters. Prolonged periods of starved detrital supply foster authigenesis and formation of sediments rich in iron, phosphorus and silica in shallow encroaching seas. Eventually, a variety of NCI grains from terrestrial, paralic and mar ne sources are reworked during stepwise coastal retreat and left as lags on ravinement surfaces or deposited in accreting sandwaves by storm or tidal currents. On passive margins, NCI-rich arenite intervals are typically found in the basal transgressive tract of depositional supersequences, when a rapid short-term eustatic rise is superimposed on regional thermo-tectonic subsidence associated with waning rift volcanism, break-up and reorganization of plate motion. Ironstone deposition is also favored by global "polytaxic" conditions and expanded oxygen-minimum layer in the oceans. Various types of NCI grains are also found in active margins and foreland basins, where they are generally diluted due to higher sedimentation rates.

Other NCI grains, including plant debris, siliceous tests, clastic gypsum and even rarer particles may occur as framework components in arenites.