Bajada formation by monsoonal erosion of a subaerial forebulge, Sultanate of Oman

Abstract

A vast, low gradient bajada is present in the interior of northern Oman. With an area of ∼40,000 km 2 and longitudinal gradients less than 0.3°, this Late Cenozoic bajada is 10 times larger and 10 times flatter than classical bajadas. The tectonic and climatic setting that favored aerially extensive bajada formation is investigated in this paper. A tectonic model is proposed that involves forebulge development associated with subduction of the Arabian plate beneath the Asian plate. The model identifies the southern Gulf of Oman as a foredeep, the Oman Mountains as a subaerial forebulge, the vast bajada as a deposit that blankets the distal forebulge, and the Umm as Samim sabkha as a backbulge basin deposit. The model is supported by an excellent match between modern topography and a theoretical flexural profile of the Omani lithosphere, by the lateral distribution of Pliocene–Pleistocene sediments, by the inferred Late Miocene to Pleistocene age span of the vast bajada, and by evidence of active surface uplift of the Oman Mountains. A second mechanism, post-orogenic exhumation of the pre-existing Oman Mountains, is likely to have accompanied and perhaps slightly preceded forebulge growth, facilitating erosion and transport of sediment across the forebulge. Evidence for a palaeoclimatic model is provided by previous sedimentological studies and stable isotope data of cements in channel gravels as well as proxy data from ODP Leg 117 (Oman Margin). Transport of coarse-grained sediment across the bajada is interpreted to have occurred mainly during the Late Miocene to Early Pleistocene when a humid-wet climate prevailed across Arabia. High water tables and accompanying cementation of bajada gravels may have aided long-distance transport. The evolution of surficial bajada sediments is correlated with changing climatic and hydrological conditions—from seasonal/perennial flows during the Pliocene (and before) to mainly ephemeral and finally flash flood flows (signifying increasing aridity) during the Pleistocene. These data imply that the Indian Ocean monsoon, represented by rainfall frequency and intensity, had a more northerly range in Arabia in the past compared to the present.