Compaction history of Upper Cretaceous shale and related tectonic framework, Arabian Plate, Eastern Oman Mountains

Shale of the Upper Cretaceous Al-Khod Formation intruded younger conglomerates of the same formation. Intrusion followed a preexisting fault that had been widened by extension. The fissility of the shale mimics the contact contours of the conglomeratic host rocks. Sandstone and conglomerate clasts are “floating” in the shale. Vertical postintrusive calcite veins are ptygmatically folded by compaction. These ptygmatically folded compaction veins display horizontal to gently dipping axial planes. Shortening amounts to ~ 40%, indicating that shale intrusion ensued with high water content. It also shows that an estimated amount of 35 to 45% of water content was expelled by compaction after vein formation. Countless, randomly oriented calcite veins in the conglomerate at the shale contact point to fluid expulsion from the shale into the conglomerate. Shale intrusion postdates the late Cretaceous obduction of the Semail Ophiolite. Intrusion most likely occurred during the Oligocene for which extension of the nearby Frontal Range Fault was due to gravitational collapse, associated with isostatic/elastic rebound. Shale dike formation is related to a widened fault within a sinistral negative flower structure with a minimum width of 100 m. Intrusion ensued at an overburden of approximately 100 m of the upper part of the Al-Khod Formation and approximately 900 m of the Paleogene limestone. Folding of calcite veins and the significant water loss was caused by corresponding compaction. Nontronite is the red shale’s main clay mineral. It derived from a source area of exposed and weathered mafic to ultramafic rocks.