ISSUE16 | FEB2010 www.gso.org.om Page 02...President’s Message Page 02...Note from the Editor Page 03...A World-class Exposure Page 08...Structural Evolution Page 12...Fault Geometries in North Oman Page 14...Sealing of Faults Page 19...Neogene Compressional Structures Page 22...Field Trip Report Page 25...International News Page 28...AGM Report Page 29...Publications 2009 Page 33...Upcoming Events Page 2 President’s Message Dear GSO Members, On behalf of the GSO Executive proactive approach to geological 2010 season and I would like you to Committee I welcome you to the 2010 projects that serve preservation and come forward with your contributions GSO activity season. knowledge dissemination of our to GSO activities. The Society is This year is going to be very special geological heritage in Oman. For the best platform for discussion and as GSO will be celebrating its 10th this, the Society needs your input and sharing of your ideas and projects with Anniversary which will coincide with support. the wider geoscientist community. the 40th National Day for Oman. Geophysics and hydrogeology In summary, I would like to thank Thus, please join us to make it a very are two fields that we would like to all of you for your commitment and special geological year for the Society see more contribution from in our contribution, and to encourage you to and Oman. activities. I urge all geophysicists and stay connected to the Society though Since its inception, GSO has achieved hydrogeologists to contribute to GSO its activities and programs. much but many milestones are yet activities for the coming season and to be met. 2010 is going to be very share their knowledge and experience Regards, challenging, as we need to diversify with Society members. Dr. Mahmood Saif Al Mahrooqi our activities and have a more This al Hajar issue is the first for the GSO – President Note from the Editor Hi All, Welcome to the 16th Edition contributed an International News Manager for the Middle East Learning of Al Hajar. This edition focuses on section and our upcoming events Hub for Shell Development Oman. the structural aspects of Oman’s section can be found on the last page. He brings an infectious enthusiasm geology with excellent contributions As the temperature starts to rise the for Omani geology to the role and an from GUTech, PDO and Shell. We field trip season draws to an end undeniable passion for life!! I’m sure would, as always, be delighted to but we will still continue with talks. you will join me in extending a warm hear your feedback on these articles. Upcoming talks include Measuring welcome to Ru as Editor. Drop a line to ‘The Ed.’ and we will Climate Change in the Arctic & in Just remains for me to say publish any thoughts you wish to April, one outlining EOR techniques thank you for all of your support – share. We also include a publications applicable in Oman. I look forward to I have thoroughly enjoyed the last two list detailing research in the Sultanate seeing you there. years as Editor and look forward to for 2009. I’m sure you will join me in This is my last edition as seeing the GSO go from strength to thanking John Aitken for diligently Editor as shortly Carlos & I will leave strength. providing this over the last few years. Oman. Ru Smith will now take over As always, IHS have generously this role. Ru is currently Programme Very best regards, Caroline Front Page Al Hajar 16th edition Feb 2010 Page 3 A World-class Exposure of a Fossil High Pressure Cell on the Southern Flank of Jabal Shams in the Oman Mountains Max Arndt, Simon Virgo, Zoe Soebisch, Marc Holland, Christoph Hilgers, Janos L. Urai, Geological Institute, RWTH Aachen University and Department of Applied Geoscience, German University of Technology GUTech, Muscat, Oman. Abstract The exhumed Cretaceous Our study area is located on the of which we find the vein density carbonates on the southern flank of Jabal southwest flank of Jabal Shams, throughout the entire field area to Shams in the Oman Mountains offers the highest peak of the Al Jabal al be very high (Holland et al., 2009). a world-class, ultra high resolution look Akhdar domal structure (Figure 1a, The detailed field into the inner workings of high pressure e.g. Glennie et al., 1974; Beurrier observations in excellent exposures cells which are common in sedimentary et al., 1986; Loosveld et al., 1996; provide the basis for a model of basins and contain large oil and gas Breton et al., 2004; Glennie, 2005; the multiphase evolution of the Jabal Shams high-pressure cell in deposits. This more than 2 km thick Al-Wardi, 2006; Hilgers et al., 2006a; Searle, 2007). accordance with the work of Hilgers sedimentary pile develops a complex Several deep wadis cut the et al. (2006a). This evolution is and rapidly changing set of continuously dip slope and offer impressive vertical illustrated by the schematic drawing forming and re-sealing fractures, leading profiles of which the tallest section shown in Figure 2. The earliest to a complex mechanical stratigraphy at Wadi Nakhr offers a continuous structures (V1) are a series of and producing several generations of vertical exposure of approximately 1 anticlockwise rotating veins (Figures pervasive regional fault and vein sets. km. The strata expose the Sahtan, 3, 4 and 5). Burial extension and the formation of Kahmah and Wasia groups of the The first of these formed overpressures led to the formation of Hajar Supergroup (Figure 7). in a north-south trending direction numerous fracture generations in an The emphasis of this study is (Figure 2b), followed by a set striking anticlockwise rotating stress field. This the characterization of the structural approximately 130º (Figure 2c), was followed by bedding-parallel shear evolution (Figure 2). These are 090º (Figure 2d) and 045º (Figure under lithostatic fluid-pressure conditions predominantly brittle deformation 2e). All vein sets are perpendicular at a minimum temperature of 134–221°C. fractures and faults (Figure 1b) to the bedding, have large apertures The high pressure cell was drained along dilatant normal faults that were also repeatedly cemented and reactivated. Introduction The southern flank of Jabal Shams in the Oman Mountains offers a world-class outcrop of high pressure cells. The Structural Geology group of RWTH Aachen and the Department of Applied Geoscience of GUtech in Muscat has been studying these unique outcrops for the past 5 years, in projects funded by Shell International and more recently by DGMK. Figure 1a: Landsat overlay on DEM showing the field area Front Page Al Hajar 16th edition Feb 2010 Page 4 Figure 1b: Simplified map showing study area (rectangle) together with interpreted faults. The lithology of interest spans primarily the Wasia Group with the Nahr Umr and Natih formations as well as the Kahmah Group. After Holland et al., 2008 0 0 with blocky calcite cement. The geometry of the fracture shows no signs for interaction between the fracture sets in abutting or curving. Rapid sealing of the fractures and thereby a restored tensile strength is interpreted to be the major cause for the dense spacing of this pattern. The tensile effective stresses required for the formation of this regional vein system may have been formed in response to overpressure build-up during burial, perhaps in combination with outer- arc extension during emplacement of the Hawasina and Semail nappes. The joints in the ramp are normal to bedding, suggesting that the ramp postdates the jointing process. The south to southwest vergence of the ramp could indicate its relation to the emplacement of the Hawasina Figure 2: The evolution of the regional fracture network is interpreted to result from multiphase de- and Semail nappes. formation: (a) sets of veins with prominent apertures. (b, c, d, e) These fractures are formed perpen- The next stage is bedding dicular to the bedding probably as a response to high fluid pressures. The open-mode fractures are parallel shear (Figure 6), which cemented with white calcite. (f) An isolated ramp structure is interpreted to have formed next with a indicates a major change of the top-to-south-southwest movement. (g) Bedding parallel shear with a top-to-north and northeast move- effective stress tensor. Bedding- ment postdates the bedding-perpendicular veins forming layer-parallel veins and shear zones. Normal parallel veins indicate fluid faults (h) develop in the next stage, and nucleate partly along the anisotropy of the striking veins. (i) Exhumation, neotectonics and weathering lead to the opening of joints (simplified sketch, not to scale; pressures close to lithostatic. and arrow points north). Front Page Al Hajar 16th edition Feb 2010 Page 5 Figure 3: Exposed carbonate bed with different sets of veins and joints. Cementation, spac- Figure 4: Complex vein network on polished surface with mul- ing and apertures differ between the different sets. tiple strike directions. system (Hilgers et al., 2006a). The faults nucleated as en-échelon vein sets or along the preexisting veins in the 090º and 130º strike direction. This means that these faults cannot be simply used to infer the principle stress directions, because of the anisotropy. A detailed geological map and profile of the SE-part of the area is shown in Figs. 9 and 10. The uncemented joints exposed in plane and profile views all strike into the directions of the first vein group perpendicular to Figure 5: Example of an overprinting relationship: A thin vein striking approximately 045º offsets two the bedding (Figures 2i and 8). 130º striking veins. The uncemented joints generally cementation repeatedly restored the rock strength during deformation.