CEMENTATION OF HOLOCENE BEACHROCK IN THE AQABA AND THE ARABIAN GULFS: COMPARATIVE STUDY

摘要

Extensive precipitation of aragonite and high-Mg calcite (12-14 percent MgCO_3) cements in the intertidal sediments of the Gulf of Aqaba, Egypt and the Arabian Gulf, Qatar results in the formation of dominant beachrock exposures. The 20-60 cm thick beachrocks in both areas are parallel to the shoreline and slope gently seaward. The 14 C dating values show that the cement of the Gulf of Aqaba beachrock (2470 + - 60y) are rather older than those of the Arabian Gulf (1360 + - 45y). Framework grains in the Gulf of Aqaba beachrock are moderate to unsorted coarse terrigenous rock fragments, which differ than the unsorted carbonate particles of the Arabian Gulf beachrock. Carbonate cements in both the Aqaba and the Arabian Gulfs display the same architecture, which comprises: 1) thin isopachous crust made up of high-Mg calcite mosaics and/or aragonite needles that surround grains and 2) intergranular cryptocrystalline high-Mg calcites, which fill the rest of the pores.' Minor dolomite mosaics may associate with the intergranular cement. The co-existence of the aragonite needles, of the isopachous crust, with the micritized grains and micritic envelopes is evidence that marine phreatic processes are dominant in the intertidal zone and that lithification has started in this' zone. The bi-mineralic composition of the isopachous crust in the Aqaba beachrock is attributed mainly to kinetic factors (i.e. the rate of supply of carbonate ions) and to the composition of the substrate and/or organic control in the beachrock of the Arabian Gulf. Some physico-chemical, kinetic, hydrologic and biologic factors are believed to be effective in controlling the precipitation rates of the isopachous cement. The oxygen and carbon isotopic composition of the intergranular high-Mg calcite cement of the Aqaba Gulf (+2.0 to -1.6 and +2.9 to +4.4 per thousand PDB respectively) is in accord with their precipitation in equilibrium with marine water. However, the relatively depleted delta ~(18)O (-0.5 to -3 per thousand PDB) and delta ~(13)C (+0.3 to +2.2 per thousand PDB) values of the intergranular high-Mg calcite cement of the beachrock of the Arabian Gulf is attributed to extraneous source of bicarbonate ions. The minor dolomite rhombs are formed directly from seawater within microenvironments created in response to the release of Mg~(2+) ions to the pore water following the partial dissolution of some high-Mg calcite carbonate particles.