WERE OMAN’S GIANT CAVERNS FORMED DURING PLUVIAL CONDITIONS OR BY HYPOGENIC WATERS?

The 11.5 km long, 385 m deep Selma Cave System on the Selma Plateau in northern Oman, seems incongruent in its arid setting within an ~11 km² surface catchment basin. Large diameter pits in the bottom of normally dry wadis drop between 250-300 m from the 4 distinctly separated entrance areas: Arch Cave, Funnel Cave, 7th Hole, and 3-Window Cave. The 4 entrance rooms constitute the 4 largest (up to ~80x200 m and 70 m high) chambers in the system. Dramatically more constricted passages depart each entrance chambers at ~1000 m msl, following regional dip (~5-10^o) towards the east. These lower passages connect into one system that merges into a larger trunk passage, which leads down dip to the intermittent resurgence cave, Kahf Tahry. The entire system carries water during uncommon storms.

The nearby Majlis Al Jinn consists of a single, huge (3.9 x 10⁶ m³) chamber and may reveal important insight into the genesis of the Selma System. The smooth walls of the broad, dome-shape Majlis Al Jinn room are coated with a ubiquitous veneer of calcite, possibly a product of sub-aerial dissolution in a CO²-rich environment. The floor is mostly covered with insoluble residue from the clastic-rich, Middle Eocene Seeb Formation bedrock, as well as surface material, secondary calcite floor deposits, calcite flakes from the walls, and breakdown boulders. The cave's 3 entrances are near the top of a hill and there is no evidence that the huge room has ever received significant surface inflow. Majlis Al Jinn appears to have formed under phreatic conditions, probably from hypogenic, acidic waters in a model similar to the giant caverns of the Guadalupe Mountains, New Mexico (Carlsbad Cavern, Lechuguilla Cave, etc.), and shares morphologic characteristics (rounded ceiling, blind alcoves, etc.) with those caves. However, no evidence of sulfuric acid involvement has been noted in Majlis Al Jinn.

Hypogenic waters may have also dissolved the large Selma System chambers before surface entrances formed and without the benefit of concentrated surface inflow. After uplift and lowering of the water table, pluvial surface streams incised today’s wadis, intersected the paleo-chambers, and focused the inflow of groundwater into the four older caves. The younger, vadose canyon passages then formed. Direct evidence of the Selma System’s hypogenic origin was obliterated by the later, large rivers flowing through the cave.