Abstract:

Broad uplift of the margins of Red Sea has extensively eroded the pre-rift sedimentary section and exhumed the Proterozoic basement in the Arabian and Nubian Shields. However, some pre-rift sedimentary rocks are preserved within rift grabens along the coast, and also on top of the Arabian plateau beneath syn-rift basalts of Harrat Hadan. Previous studies of outcrops of pre- and syn-rift sedimentary rocks near Jeddah assigned them various ages and Formations leading to confusion. Moreover, no attempts were made to correlate them to the section sitting on top of the Arabian plateau. This study redefines the stratigraphy pre-rift sedimentary rocks in the Jeddah area (Usfan and Shumaysi Formations) and correlates them with similar rocks located 200 km east over the Arabian Shield (Khurma and Umm Himar Formations).

Field work, petrographic investigation and satellite image mapping data from the central Red Sea are used in this study to reveal new stratigraphic correlations for the pre-rift section and new insights about the uplift of the Arabian plate.

The pre-rift sedimentary rocks rest uncomfortably on the Precambrian basement. They consist of sandstones, oolitic ironstone, shale, and a bioclastic limestone. Their distinguishing characteristic is the textural and compositional maturity of the sandstones, which consist of quartz arenite with well-rounded vein quartz pebbles near the base of the section, and the total absence of basement-derived lithic pebbles.

The pre-rift sedimentary rocks are disconformably overlain by a syn-rift section, up to several kilometers thick, which consists of immature continental redbeds that are composed of basement-derived conglomerates, sandstones, and mudstones. They are distinguished by their brick-red color, poor sorting, and compositional immaturity.

Prior to the start of the Red Sea rifting, the Arabian Peninsula was a contiguous part of the African plate. The presence of pre-rift marine sediments in Harrat Hadan over the Arabian shield and in the coastal plain of the Red Sea indicates that it was at or below sea level during the early Cenozoic. Subsequently, rifting which started during the Oligocene caused uplift of the margins of Red Sea and erosion of the pre-rift sedimentary rocks which exhumed the basement in the Arabian Shield. However, the hanging walls of normal faults subsided into half grabens, preserving the pre-rift sediments within the Red Sea rift. The rift was filled with immature continental syn-rift sediments (red beds) eroded from the margins.

Some key markers, particularly oolitic ironstones, allowed us to define correlative units throughout the study area. Furthermore, the presence 28 MA old basaltic lava flows at the base of the syn-rift section in both the Jeddah and Harrat Hadan areas provides, for the first time, a reliable date for the start of rifting in the central Red Sea, and clear separation of pre-rift from syn-rift sedimentary rocks