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

  • F. MatternEmail author
  • A. Scharf
  • M. Al-Sarmi
  • B. Pracejus
  • A.-S. Al-Hinaai
  • A. Al-Mamari
Original Paper


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.


Shale compaction history Shale dike Ptygmatically folded compaction veins Al-Khod Formation Upper Cretaceous/Tertiary Negative flower structure Hajar Mountains 



We are thankful for the constructive review of the manuscript by Mohammed Al-Wardi (Sultan Qaboos University, Muscat) and three anonymous reviewers. The following acknowledgment is based on oral communication with Mohammed Al-Wardi. According to Mohammed Al-Wardi, the term “Frontal Range Fault” was coined by Samir S. Hanna (formerly of Sultan Qaboos University, Muscat) in discussions within the geoscientific community. He had identified the persistent occurrence of normal/extensional faults along the frontal range of the northern Oman Mountains, thus representing a system of faults rather than a single fault. He had prepared a manuscript on the FRF shortly before he withdrew from geology, but his manuscript was left unsubmitted.

The shale outcrops have been studied by Al Mamari (2016) and Al Hinaai (2016) as part of their Final Year Projects at Sultan Qaboos University (SQU, Muscat, Oman). Hamdan Al-Zidi’s (SQU, Earth Science Department) support in the sample preparation and Saif Al-Mamari’s (SQU, College of Science, CAARU center) X-ray diffraction work in the mineral analysis are thankfully acknowledged. We appreciate fruitful field discussions of the overall structural interpretation (flower structure) with Eugenio Carminati (University of Rome – La Sapienza, Italy), which greatly improved the manuscript. We thank the anonymous reviewers for their constructive comments.


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Copyright information

© Saudi Society for Geosciences 2018

Authors and Affiliations

  • F. Mattern
    • 1
    Email author
  • A. Scharf
    • 1
  • M. Al-Sarmi
    • 1
  • B. Pracejus
    • 1
  • A.-S. Al-Hinaai
    • 1
  • A. Al-Mamari
    • 1
  1. 1.Earth Sciences DepartmentSultan Qaboos UniversityMuscatOman

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