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The ~ 5.5 km sized Jabal Rayah ring structure located at 28° 390 N/37° 120 E in Saudi-Arabia has been classified as a possible complex impact structure located in flat-lying-Paleozoic clastic sediments. Previous, detailed mapping showed that erosional processes led-to a relief inversion, with displaced, folded, and faulted blocks of Silurian to Early-Devonian strata, interpreted to form a ring syncline, now forming a topographically-outstanding, 150 m high ring crest. The drainage toward the center of the structure seems-controlled by a set of radial faults. This central part is eroded to the level of the-surrounding plateau and partially covered with gravel. Analysis of 28 Qusaiba Formation-sandstones showed that at the present outcrop level, the sediments seem devoid of shock-features. Measurement of fold axes in the central part of the structure shows radially-outward plunging fold axes, becoming steeper toward the center, and also fold axes of other-orientation, and folded folds. This fold axis pattern is interpreted as an upward-pointing,-kilometer-sized sheath fold. Assuming an impact scenario and using the present size of the-structure, the minimum central structural uplift is estimated at ~ 500 m, which is consistent-with Qusaiba Formation occupying the center of the ring structure.

Meteorite searches in Saudi Arabia between 2008 and 2014 yielded 46 meteorites from the Yabrin area (23°N 49°E), 35 meteorites from the Rub’ al-Khali sand desert (19°–20°N, 48°–51°E), and 1 meteorite from Al Haddar. No meteorites were found near Hafar al Batin (29°N 45°E). The 82 new meteorites represent ~57 falls comprising 43 ordinary chondrites, 4 carbonaceous chondrites, 2 enstatite chondrites, 3 ureilites, 3 eucrites, 1 acapulcoite, and 1 lunar meteorite. The median of 31 14C terrestrial ages is 6.2 ka, significantly younger than the Oman population (19.5 ka, n = 128). A further assessment of terrestrial 14C contamination is advised by a 11–15 ka 14C terrestrial age of heavily weathered meteorite Khawr al Fazra 014, geology indicating a terrestrial age >100 ka. Find densities of 0.4–2.8 km-2 for Yabrin and the western Rub’ al-Khali are similar to ~0.5 km-2 observed in Oman. Higher find densities of ~135 km-2 (29 km-2 for masses >10 g) exist on small Pleistocene outcrops in blowouts in the south-central Rub’ al-Khali: 21 unpaired meteorites (four >10 g) were found in 11 blowouts with a combined area of 0.14 km2. The Rub’ al-Khali meteorites show a relatively high degree of weathering (median W 3.6; 2.5 for Yabrin), low median mass (4.3/138 g), and a high H/L ratio (2.3/1.1). The high density of small meteorites is explained by prolonged sand protection and recent deflation. The high meteorite density and relatively high proportion of rare meteorite types render the Rub’ al-Khali blowouts an interesting target for future exploration.

Abar al' Uj (AaU) 012 is a clast-rich, vesicular impact-melt (IM) breccia, composed of lithic and mineral clasts set in a very fine-grained and well-crystallized matrix. It is a typical feldspathic lunar meteorite, most likely originating from the lunar farside. Bulk composition (31.0 wt% Al2O3, 3.85wt% FeO) is close to the mean of feldspathic lunar meteorites and Apollo FAN-suite rocks. The low concentration of incompatible trace elements (0.39ppmTh, 0.13ppm U) reflects the absence of a significant KREEP component. Plagioclase is highly anorthitic with a mean of An(96.9)Ab(3.0)Or(0.1). Bulk rock Mg# is 63 and molar FeO/MnO is 76. The terrestrial age of the meteorite is 33.4 +/- 5.2kyr. AaU 012 contains a similar to 1.4x1.5mm(2) exotic clast different from the lithic clast population which is dominated by clasts of anorthosite breccias. Bulk composition and presence of relatively large vesicles indicate that the clast was most probably formed by an impact into a precursor having nonmare igneous origin most likely related to the rare alkali-suite rocks. The IM clast is mainly composed of clinopyroxenes, contains a significant amount of cristobalite (9.0 vol%), and has a microcrystalline mesostasis. Although the clast shows similarities in texture and modal mineral abundances with some Apollo pigeonite basalts, it has lower FeO and higher SiO2 than any mare basalt. It also has higher FeO and lower Al2O3 than rocks from the FAN- or Mg-suite. Its lower Mg# (59) compared to Mg-suite rocks also excludes a relationship with these types of lunar material.

Shallow lakes and sabkha deposits are evidence of past periods of higher groundwater table than today, and hence reflect increased humidity in the Rub' al-Khali, the large desert in the southern part of the Arabian Peninsula. Reported here are the observations made during four expeditions into this remote area in combination with luminescence dating constraining the age of the deposits. The results presented extend the spatial coverage of evidence and show that most of the deposits correlate to MIS 5 (ca. 130–70 ka) and the Early/Mid Holocene (ca. 11–5.5 ka). A single sample points towards potentially more humid conditions during the transition MIS 4/3 (ca. 65–55 ka). The presence of the mussel Unio in some of the deposits attributed to MIS 5 is indirect evidence for the presence of fish in the lakes, which must have supported a rich and diverse fauna (and probably also flora). Together with other evidence, this demonstrates that a number of small persistent lakes were spread across the Rub' al-Khali for at least some of the time. These lakes would not only have been a potential source of freshwater but could also have provided an additional and easy accessible food source via the fish for humans migrating through the area. The presented palaeoenvironmental data underlines that parts of MIS 5 represent a time window of opportunity for a potential expansion of modern humans across Arabia.

We have investigated the Ash Shutbah circular structure in central Saudi Arabia (21 degrees 37N 45 degrees 39E) using satellite imagery, field mapping, thin-section petrography, and X-ray diffraction of collected samples. The approximately 2.1km sized structure located in flat-lying Jurassic Tuwaiq Mountain Limestone has been nearly peneplained by erosional processes. Satellite and structural data show a central area consisting of Dhruma Formation sandstones with steep bedding and tight folds plunging radially outward. Open folding occurs in displaced, younger Tuwaiq Mountain Limestone Formation blocks surrounding the central area, but is absent outside the circular structure. An approximately 60cm thick, unique folded and disrupted orthoquartzitic sandstone marker bed occurring in the central area of the structure is found 140m deeper in undisturbed escarpment outcrops located a few hundred meters west of the structure. With exception of a possible concave shatter cone found in the orthoquartzite of the central area, other diagnostic shock features are lacking. Some quartz-rich sandstones from the central area show pervasive fracturing of quartz grains with common concussion fractures. This deformation was followed by an event of quartz dissolution and calcite precipitation consistent with local sea- or groundwater heating. The combination of central stratigraphic uplift of 140m, concussion features in discolored sandstone, outward-dipping concentric folds in the central area, deformation restricted to the rocks of the ring structure, a complex circular structure of 2.1km diameter that appears broadly consistent with what one would expect from an impact structure in sedimentary targets, and a possible shatter cone all point to an impact origin of the Ash Shutbah structure. In fact, the Ash Shutbah structure appears to be a textbook example of an eroded, complex impact crater located in flat-lying sedimentary rocks, where the undisturbed stratigraphic section can be studied in escarpment outcrops in the vicinity of the structure. (Less)