## 4 Discussion
Large impact events commonly fracture the ground surface to several kilometers deep and create pathways in a sedimentary basin for fluid migration from the hydrocarbon reservoirs at depth. The exact age of the proposed structure is yet to be estimated; however, a maximum age of the Rimaal structure is probably \(\leq\)65 Ma, as it most likely formed in sandstones of the Cretaceous age. In the Eastern Sahara, just west of the region where the Rimaal structure is found, lies the Al-Kufrah Basin, with a high potential hydrocarbon reservoir [35]. Since this reservoir is estimated as Late Triassic to Early Cretaceous in age [36], it can be assumed that the Rimaal structure postdates the hydrocarbon reservoir in this area. This suggests that the Rimaal structure might represent preferential conduits for fluid migration from such a potential reservoir. The Rimaal structure is situated entirely in a vast area of monotonous Mesozoic sediment and is far from the large volcanic province of Awaynat massif and any known volcanic features in the region. Therefore, the volcanic origin for the structure formation is unlikely. Likewise, Maar formation and sand volcanism processes, produced by gas bubbles erupting through moist sands, can be discarded by the large size of the Rimaal structure (~86.6 km\({}^{2}\)). No salt plug or gypsum- karst features are known in the study area. The Rimaal structure is entirely carved in sandstone; accordingly, a dissolution karst origin for the structure can be ruled out. Lastly, a glacial origin cannot be inferred for Rimaal since the structure is located in the Great Sahara and circular glacial features, like kettles, could only be formed in loose sediments and not in the firm Nubian Sandstone where the structure exists. Based on the above discussion, the endogenic processes for the formation of the Rimaal structure are improbable; therefore, an impact origin is proposed. To date, nine circular structures have been discovered in the Eastern Sahara, and all are excavated within the Nubian Sandstone Formation. Five of these structures have been confirmed as impact craters, which are Kamil in Egypt [37]; BP and Oasis in Libya [38; 39]; and Aorounga and Gweni-Fada in Chad [40]. None was discovered yet in Sudan. The morphology of the Rimaal structure bears a distinct morphological similarity to the Upheaval Dome impact complex crater in Utah, USA [41] (Figure 9). Both craters have central peaks that exhibit inner morphological depressions (appear as inner ring structures), and their outer rims are partially breached by river courses. The defined central inner ring of the Rimaal structure was also described for the Aorounga impact crater in the neighboring country of Chad [40] (Figure 9). In fact, other resemblances were found between the Rimaal and the Aorounga Crater, where both structures are situated in sandstone and are shielded by the Sahara sand and, thus, hardly visible by optical satellite images. The subsurface radar imaging capabilities facilitated the exposure of both structures. With a diameter of 10.5 km, if the Rimaal structure is confirmed to be of an impact origin, it would be among the very few large complex impact craters found in Africa and the fourth largest structure in North Africa after Oasis, Gweni-Fada, and Aorounga impact craters. Rimaal might be a smaller relic of a formally larger structure that has been deeply eroded.