Sedimentology and Petrography of the Ghayathi Formation (Quaternary, Abu Dhabi)

Abstract

In the central and western UAE, Quaternary outcrops of loose or moderately to well-cemented, carbonate-dominated palaeodune sandstones occur as isolated areas draping older Miocene or Madinat Zayed Formation bedrock. These deposits have been named Ghayathi Formation, after a type section at Jabal Marban, near Ghayathi in Abu Dhabi Emirate. The Ghayathi Formation consists of aeolianites deposited before and after the last interglacial period. The shallow marine Fuwayrit Formation and recent aeolianites overlie the Ghayathi Formation. The Quaternary deposits in UAE provide information of the sea-level change since LGM (the last glacial maximum), and the understanding of the relationship between the Ghayathi and Fuwayrit formations will contribute to establish accurate relative dates of these formations. Most of the previous studies on Quaternary sediments do not include detailed research of the sedimentology of the Ghayathi Formation even though it was indicated in several locations in the UAE, Oman, Saudi Arabia, and India. A comprehensive review of the available numerical ages for the Ghayathi Formation using radiocarbon and optically stimulated luminescence (OSL) resulted in several age groups from 160 ka to 9 ka. The Fuwayrit Formation ranges from 87 to 130.4 ka in some articles while others dated it to more recent ages. Age dating for both formations went through a long debate and many factors potentially affected the uncertainty in the age dating of the Ghayathi Formation such as the water content and cosmic dose history of the samples. The paleogeography of the Arabian Gulf is important for the genesis of the Ghayathi Formation as exposed Arabian Gulf seafloor was the source of sediments deflated by shamal wind in interglacial periods. The main objective of this research is to investigate and better understand the composition and sedimentation processes for the Ghayathi aeolianite, and its relationship with the Fuwayrit Formation through logging, sampling of several sections of the Ghayathi and Fuwayrit formations and chemical analysis. Forty-nine samples from 18 locations, all located in Abu Dhabi emirate, have been collected. Thin sections have been prepared and all samples were analyzed by XRF for their bulk chemical composition. The lithology of the Ghayathi Formation shows alternation of siliciclastic and carbonate-rich laminae; with a pattern of increasing carbonate content and grain size as heading from the land to the shore. The Ghayathi Formation shows various types of lamination and crosses bedding, and these structures are absent in the Fuwayrit Formation. The shallow marine Fuwayrit Formation is characterized by sedimentary structures such as bioturbation and the presence of intraclasts. The Ghayathi Formation was strongly affected by meteoric diagenesis. Nevertheless, meniscus cement as indicated in previous studies was not found. The petrographic analysis illustrates a high porosity, enhanced by the diagenetic dissolution of aragonite grains. Micritization and isopachous cement had a crucial role in keeping the original shape of originally aragonite bioclastic grains after deposition and during diagenesis. The cementation pattern is variable with generally stronger cementation closer to the shore. The geochemical data are broadly consistent with the petrographic interpretations and show that Si and Ca concentrations from XRF are consistent with percentages of siliciclastics and calcite estimated from the thin sections. The XRF analysis illustrates different relationships between different elements, as shown in scatter plots. By quantifying the correlation coefficient, it shows trends and positive patterns between siliciclastic elements and negative correlation with calcite. An inland outcrop of very weakly cemented aeolianites is proposed to represent an older period of deposition and cementations, as its composition and diagenesis do not match with other outcrops of the Ghayathi Formation.

Date of Award	May 2020
Original language	American English