Most efforts in the study of sea-marginal sabkhas have concentrated on the Persian Gulf, but little is known about the sediments and mineralogy of sabkhas marginal to other seas. The purpose of this paper was to present some geochemical and mineralogical observations in a recent sabkha on the coast of Sinai along the Gulf of Suez. The sabkha is composed of coarse clastic sediments with marine-derived groundwater at depth of about 1 m. The general morphology, climate and water salinity of the Gulf of Suez resemble those of the Persian Gulf, despite the fact that the content of authigenic evaporites in this sabkha is more sparse. The evaporite minerals accumulated only in the upper 30–40 cm of the sabkha, below that and down to the groundwater table, there is no accumulation of evaporites. Laterally, the salinity of the groundwater in the sabkha and the concentration of evaporites in the sediments above it increase constantly with distance from the shore. In contrast to the Persian Gulf where anhydrite is a major evaporite mineral, in Belayim gypsum is the only calcium sulphate mineral in the recent sabkha. Anhydrite is found only in an old elevated sabkha where it recrystallized from gypsum. The gypsum occurs as interstitial crystal concentrations or lithified horizons almost exclusively at the depth of 20–40 cm below the sabkha surface. Above that, in the uppermost horizons, there is in situ accumulation of interstitial halite crystals. The total concentrations of gypsum and halite are almost equal in this sabkha. The sea water recharge in El Belayim is almost exclusively by seepage through the sabkha sediments and not by flooding. The groundwater under this sabkha is only slightly more saline than the Gulf water, thus, not heavy enough for extensive downward refluxing. The major hydrodynamic process must be upward migration of the brines from the groundwater, precipitating on the way gypsum and later halite with some magnesite. Since the sediments of the sabkha are too coarse to support extensive capillary movement, the brines must, therefore, migrate upwards due to ‘evaporative pumping’.
ABSTRACT A shelly sandstone containing a modern microfauna, recovered from the outer shelf off Delaware Bay at a depth of 43 fathoms (79 meters), had been cemented by cryptocrystalline aragonite and clear fibrous aragonite. Numerous organic borings, some algal, penetrate both cement and shells. The size-distribution and excellent sorting of the insoluble residue, and the textural relationships of the cement and algal borings, are identical with those of some kinds of beachrock. Beachrock forms today only in the intertidal zones of subtropical and tropical climates. However, faunal and geochemical data indicate that the sandstone is not a beachrock. All species of pelecypods, benthonic Foraminifera, and bryozoa are those whose modern representatives live variable distances north of New Jersey, in waters cooler than those found off the present coast of New Jersey. No tropical or subtropical species were found as would be expected if the sandstone were a submerged beachrock. The cement is greatly enriched in the light stable carbon isotope (12)C, suggesting that the carbon in t e aragonite comes from methane. A radiocarbon date of 4,390 = 120 years on molluscan shells contrasts with a date of 15,600 ± 250 years for the cement. Assuming that both dates are accurate, then the cementation of young shells by old cement can be explained only by postulating that the carbon of the cement originated earlier than the date of cementation, but was not introduced immediately into the sandstone. Methane originating in a submerged marsh is the most probable mechanism for creating a cement having a greater radiocarbon age than the shells cemented. On the basis of the radiocarbon-controlled sea level curve and on the supposition that the age of the shells indicates time of cementation, we conclude that cementation occurred on the sea floor under a cover of 75 m of seawater. Aragonite-cemented sandstone is associated with morphological evidence of a drowned shoreline which is a regional feature that extends all along the outer edge of the continental terrace of the eastern United States. Underwater television of cemented rock off Florida at the same depth as in the Delaware Bay sandstone, and other reports of rock near the shelf edge, suggest that the submerged littoral sand may have been cemented on a large scale. This cementation may have been controlled by methane originating in adjacent tidal marshes, now submerged. If this is true, a mechanism of lithification has been discovered by which quartz and carbonate sands can be cemented on the sea floor on a large scale without requiring unusual salinities of the sea water.
ABSTRACT Mineralogical analysis of calcite and Mg‐calcite by X‐ray diffraction requires that the samples be ground to a powder. Such grinding determines the particle size of the powder and the structural damage of the minerals. Both of these in turn affect the peak intensities recorded by the X‐ray machine. Most carbonate sediments are inhomogeneous; they contain both calcite and Mg‐calcite which are affected differently by grinding. Such differences cause quantitative analytical results to be inconsistent with the true mineralogical abundance. The two acceptable methods of analysis—(1) measurement of peak height from the base and (2) measurement of the area under the peak—were compared to determine if sample preparation affects the quantitative results. In samples with variable and relatively small amounts of calcite and Mg‐calcite the measurement of peak height yields more reproducible results than does the measurement of peak areas. Different proportions of particle size of the mineralogical components in a sample powder, affect proportionally more the peak areas than the peak heights. Extensive grinding causes structural damage of the component minerals which affects much more the peak areas than the peak heights. Thus for quantitative analyses of calcite and Mg‐calcite in inhomogeneous carbonate samples which require differing grinding times and have greatly variable amounts of calcite and Mg‐calcite, the peak height measurement seems to be a better method than peak area measurement.
