Abstract
This paper presents for the first time, detailed stratigraphical and petrographical analyses of the Harrat Shama Volcanic Basin (HSVB) of Miocene age in SW Saudi Arabia. This basin is filled by abundant volcanics intercalated with fluvio-lacustrine sedimentary successions. These successions are characterized by a syneruptive-intereruptive cycle, which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision. Two depositional sequences, showing an overall fining-upward trend, are preserved within the HSVB. The first comprises hyaloclastites, zeolite-bearing bedded tuffs, and bedded accretionary-lapilli-tuffs, interpreted as deep lake whereas the second, is made up of extensive pumiceous pyroclastic flows, peperitic breccias, and basaltic lava flows, which are in turn capped by siliciclastic deposits of lacustrine deposits. The passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography. The fining-upward volcaniclastic deposits of the HSVB sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input, indicating well-established proximal-distal relationship. HSVB was evolved through three stages, including pre-, syn-, and post-caldera stages. The first two stages correspond to proximal aggradational syn-eruptive event, whereas the latter stage refers to distal degradational inter-eruptive event. These stages overlap in time as shown by the alternation of explosive, effusive, and their resedimented deposits, reflecting diverse eruptive sequences formed by different fragmentation processes. The difference in the latter coupled with the variability in the magma-water interaction and the nature of substrate are responsible for such diversity, resulting in progressive environmental changes from subaqueous to subaerial regime (from bottom to top). The development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the HSVB.