PLANTS AMD EARLY DIAGENESIS:INFILTRATED ROCKY TUBULES AND THEIR FORMATION MECHANISM

A kind of rock- tubules(3-25 mm in diameter and 30-50cm in length)occurs in the late Pleistocene biosparites on the Shidao Island of lisha Islands，China.These tubules, whcih had been misunderstood as colony of corals species Acropora, or colony of warm pipes, have concentric stratoid structures.Its inner stratu m, having acentral tubule 5一8 mm in diameter, is made of dark brown micrite with corrosional cements but the light gray outerstratum is made of biosparites cemented by finely crystalline sparry calcites(0. 03-0. 04mm)，and locally filled up the intergranular space.Another kind of rocky tubules occurs in the Pleistocene fine sandy-silty sediments interbedded with stratoid and lenticular dolomites is the Paipu area of northwestern Hainan Island,China.These tubules have mainly vertical and horizontal occurrences with circle and oval cross section.They- also have stratoid structures.The loose and rough central area ,about 4 mm in diameter. consists of 65一75% quartz and other elastic materials and 10-15% crystalline dolomite.The fine and dense intensely dolomitized inner stratum( 5 -10 mm in thickness)consists of 90% dolomite,only 5% clastic materials and 5 % pore space.This dolomite is a euhedral rhombohedron,medium to very finely crwstalline with porphwritic testure.The silty clastic particles have relict structures caused by the infiltration and replacement.The loose cemented outer stratum consists of 40% elastic particles(medium to coarse grain size)，40%euhedral finely一medium crystalline dolomite and about 20% pore.The physiological action of plants controls the formation of these rocky tubules around the plant roots. Their formation mechanism is as follows:The transpiratio and respiration or metabolism of plants suck up moisture from below.drain，off the CO₂ and organic acid on the one hand. and the vadose watr moves down along the roots on the other.Thus. in the microen}ironment near the root, the dissolution.precipitation and metasomatism of carbonate take place re-etpeatedly around the root system.and primary carbonate tubules are formed during their living time。After their death.the surface water may infiltrate along the formed tubules, the dissolution of the sediments and precipitation of crystalline calcite within the pore space mac continuously- take place.Carbonate tubules formed in such a way are called infiltration rocky tubules by the authors. In the vadose zone with fresh water, calcite rocky tubule is formed, whereas in the intertidal zone of the coast.the dolomitized infiltration rock3- tubule is formed under the influence of schizahaline condition produced by sea water alternating with fresh ground water.This unusual process of dolomite formation may- be called dolomitization of plants.The formation of infiltrated rocky tubules as a process of early diagenesis is of great sigricance for the romprehension of the direct effect of plants on the diagenesis