About 16,360 km2 of an area covering the coastal plain of Umm Lajj City and the adjacent mountainous area of the Arabian shield has been mapped at scale 1: 50,000 and based on twenty four sheets of topographic maps and field work. The detailed geologic mapping of the Cenozoic sediments alongside the Red Sea coast aimed to identify the Cenozoic faults for consideration in the seismic hazard assessment.
The Cenozoic rocks are mostly Oligocene to Eocene, in part, Miocene sediments and formed as a result of probably fault-controlled trough covered extensively by Miocene alkali olivine basalt flows (Harrats). Later, the Harrats were eroded forming up-arching alongside the Red Sea coast. The coastal plain is a flat, arid, and sandy desert area with rare hills few tens of meters high. It is underlain by predominantly detrital, organo-detrital, carbonate, evaporitic marine sedimentary rocks, and the youngest fault system that is poorly exposed at the surface. To the east, the Harrats and, partially, sediments rest unconformably over undifferentiated Precambrian basement rocks. Just inland of the coastal plains are the foothills of these basement rocks that rise steadily to the top of the Red Sea escarpment and were abruptly bounded against the coastal plains by a master normal listric fault. In the south, the master fault is striking NNW-SSE and changes, in the middle of western part, abruptly N-S, NNW-SSE, N-S, then turns farther north along the NNW-SSE trend.
Generally, the area has been affected by two systems of Cenozoic faults; pre-rift foothills normal faults and syn- to present-day coastal plain normal faults. Both systems were formed in response to crustal extension and local failure of blocks during Tertiary. Originally, the first faults were most probably formed due to the reactivation via main factor of normal dip-slip motion on the expense of sinistral shear in the Najd fault system, while the second ones are related to the opening and afterward local sedimentary collapse in the Red Sea region. The first faults are clearly identified and plotted, to the east, in the Precambrian rocks. They are dipping east and, in places, to the west and produced horst-graben topography in the Precambrian rocks. Also, formation of the Samnah trough in the southwestern corner of the area is related to the movement along these faults. The aeromagnetic data of the Arabian shield showed, in the present area, intrusions of unexposed gabbroic dike-like bodies along these faults as an association of a newly oceanic crust comparable with those noted in the Tihama Asir Copmlex farther south at the Jizan area. The second system of faults are syn- to post-rift and most of them are poorly exposed west of the master normal listric fault. They have produced resistant horsts of Precambrian rocks that stand out of Miocene - quaternary sediments at Jabal Dhaylan in the middle of western part.
Kadi, K.A. and Al-Rimi, N.N., 2015, Cenozoic faulting and uplifting along the coastline of Umm Lajj area and environs, northwestern Saudi Arabia: Saudi Geological Survey Technical Report SGS-TR-2014-6, 17 p., 11 figs., 2 pls.