Hesham M. ELAsmar and G.M. Attia - Volume 4, January, 1996

Diagenetic trends in Quaternary coral reef terraces, Ras Mohammed-Sharm ElSheikh Coast, Southern Sinai, Egypt
Hesham M. ELAsmar and G. M. Attia^¹

Dept. Geology, Damietta Fac. Sciences, Egypt.
^¹Dept. Geology, Fac. Sciences, Menoufia Univ., Egypt.

The diagenetic history, minerals and geochemistry of trace elements and stable oxygen and carbon isotopes for five coral reef terraces at Ras Mohammed Sharm ElSheikh coast reveals the presence of three diagenetic stages in relationship to the geologic age.

The first stage is cement - free aragonitic skeleton detected in the younger terrace "V" of 4000 B.P. with characteristic high concentration of unstable carbonate minerals (aragonite and HMC) and the associated trace elements (Sr^²⁺, Mg^²⁺). The second stage is marine - cemented aragonitic skeleton detected in terrace "IV" of 125 ka. In this stage, both aragonite and HMC show different fabrics of cementation in marine phreatic (subtidal to intertidal) conditions. The unstable carbonate minerals are still enriched with their associated trace elements. The third stage is meteoric - cemented calcitic skeleton detected in samples from the other three terraces (i.e. terraces III, II and I" dated as 200- > 300 ka). In this stage, there are several evidences of meteoric diagenesis including transformation and cementation with the development of different fabrics characteristic for freshwater phreatic and vadose. The unstable carbonate minerals are completely eliminated; instead, low Mg-calcite (LMC) and dolomite were developed.

Both oxygen and carbon isotopes show values of marine influence in the younger terrace "V" and show progressive enrichment of the negative lighter values with age indicating meteoric diagenesis.