Diagenesis in the coastal quaternary carbonates in Qatar, Arabian Gulf

Abstract

Combined field studies, thin-section examination and scanning electron microscopy were utilized to study the diagenesis of the Quaternary carbonates in twelve stratigraphic sections along the coastline of Qatar. The sequence is dominated by calcarenites. It comprises three lithostratigraphic units representing two transgressive phases of the Pleistocene sea separated by a regressive phase. Diagenesis has modified the original textural and compositional characteristics of the Quaternary rocks considerably. Dissolution and cementation played major roles, whereas neomorphism and compaction were much less effective. Dissolution resulted in the development of fabric-and nonfabric-selective types of porosity. Cementation was mainly by calcite and, rarely, aragonite, gypsum and silica. Calcite crystals are nonferroan, mainly low-magnesian and have wide ranges of size, habit and distribution patterns. Aragonite cements are commonly crystals are nonferroan, mainly low-magnesian and have wide ranges of size, habit and distribution patterns. Aragonite cements are commonly isopachous and consist of fibrous and needle-like crystals. Massive and fibrous gypsum and cryptocrystalline and microcrystalline silica fill intergranular pores and microchannels. Aggrading and degrading neomorphism were selective. Compaction effects were more profound in calcarenites which were not subjected to early cementation. The diagenesis of the Quaternary carbonates occurred in a wide spectrum of settings which prevailed during the eogenetic, mesogenetic and telogenetic phases. The marine phreatic environment witnessed the commence of cementation with aragonite and high-magnesian calcite and the formation of micritic envelopes of fossil shells. The early phases of dissolution, micritization of other allochems and aggrading neomorphism occurred under mixed phreatic conditions. Most of the recorded diagenetic effects were developed in the meteoric vadose and phreatic environments. These include the completion of the dissolution events, the cementation with sparitic low-magnesian calcite, silica and gypsum, and the conversion of aragonite into low-magnesian calcite. There are remarkable variations in the nature and intensity of freshwater cementation throughout the sequence which indicate marked fluctuations in the groundwater levels.