Textural and mineralogical heterogeneities in coastal lagoon sediments : al-Mejarma lagoon, Red Sea, Saudi Arabia

الملخص EN

The possible source(s) of alongshore variations in facies, textural and mineralogical characteristics of Al-Mejarma coastal lagoon sediments were studied.

Two shallow push manual cores, A and B were collected from the shallow subtidal zone of the lagoon.

The sediments of core A are carbonate rich with 73 % average carbonate content.

They are homogeneous, massive, medium to coarse sands moderately to poorly sorted and mostly positively skewed.

The sediments of core B, on the other hand, are carbonate poor (average carbonate content ~ 15 %).

They display upward coarsening character, from fine to coarse sand; however, fine sands are the dominant.

The degree of sorting varies between poorly to well sorted and the majority of samples display negative skewness.

The mineral composition of the mud fraction in the two cores includes carbonate and non-carbonate minerals.

Sediments of core A are dominated by Mg-calcite and quartz with relatively low contents of feldspars, aragonite, phyllosilicates, hornblende and anhydrite.

The sediments of core B, on the other hand, are dominated by dolomite and quartz with relatively low contents of feldspar and hornblende and rare occurrences of phyllosilicates, gypsum and Mg-calcite.

Although the two cores are close to each other, their textural and mineralogical heterogeneities are attributed to variations in sediments sources, nonuniform supply and sediment distribution and alongshore variations in depositional conditions.

The core A was extracted from a channel which is incised by relatively strong flood current and dominated by biogenic sediments sourced from the sea together with rare detrital sediments transported by wind action.

The upward coarsening character of core B suggests prograding offshore-shoreface deposits with multiple sources and transporting agents including biogenic sediments of marine sources, detrital carbonates (mostly dolomite) and non-carbonate sediments transported by wind and ebb currents.