Audio magnetotelluric surveys to constrain the origin of a network of narrow synclines in Eocene limestone, Western Desert, Egypt Accepted Manuscript Audio magnetotelluric surveys to constrain the ori[.]
Accepted Manuscript Audio-magnetotelluric surveys to constrain the origin of a network of narrow synclines in Eocene limestone, Western Desert, Egypt Elhamy A Tarabees, Barbara J Tewksbury, Charlotte J Mehrtens, Abdellatif Younis PII: S1464-343X(17)30105-X DOI: 10.1016/j.jafrearsci.2017.03.001 Reference: AES 2837 To appear in: Journal of African Earth Sciences Received Date: 30 November 2016 Revised Date: 17 February 2017 Accepted Date: March 2017 Please cite this article as: Tarabees, E.A., Tewksbury, B.J., Mehrtens, C.J., Younis, A., Audiomagnetotelluric surveys to constrain the origin of a network of narrow synclines in Eocene limestone, Western Desert, Egypt, Journal of African Earth Sciences (2017), doi: 10.1016/j.jafrearsci.2017.03.001 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain ACCEPTED MANUSCRIPT Elhamy A Tarabeesa**, Barbara J Tewksburyb*, Charlotte J Mehrtensc, and Abdellatif Younisd a SC Department of Geology, Damanhour University, Damanhour, 22516 Egypt; etarabees@yahoo.com b Department of Geosciences, Hamilton College, Clinton, NY 13328 USA; btewksbu@hamilton.edu c Department of Geology, University of Vermont, Burlington, VT 05405 USA ; Charlotte.Mehrtens@uvm.edu d National Research Institute of Astronomy and Geophysics, Cairo, 11421 Egypt; abdougeoman@yahoo.com M AN U 10 11 12 13 14 15 16 17 18 19 20 21 22 *Corresponding author during refereeing and publication Tel.: 315 859 4713 E-mail address: btewksbu@hamilton.edu (B Tewksbury) **Corresponding author post-publication Tel.: +2 01095400111 E-mail address: etarabees@yahoo.com (E Tarabees) Abstract TE D RI PT Audio-magnetotelluric surveys to constrain the origin of a network of narrow synclines in Eocene limestone, Western Desert, Egypt Recent work with high resolution satellite imagery has revealed a network of 24 narrow synclines developed during the Oligocene or Miocene over tens of thousands of 25 square kilometers in Eocene limestone of the Thebes Group in the Western Desert of 26 Egypt The synclines are non-tectonic, and their scale and geometry strongly resemble 27 sag synclines in Qatar that were produced by dissolution of subsurface evaporites and 28 resulting sag of overlying layers Evaporite dissolution cannot explain the Egypt 29 synclines, because subsurface evaporites of any significance have never been reported 30 in this part of Egypt In this study, we use audio-magnetotelluric surveys to illuminate AC C EP 23 page AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT the subsurface under the synclines in order to constrain possible models for their 32 formation We suspected karst dissolution at depth, and, given a modern water table 33 depth of over 400 m, we expected that dry fracture networks and void spaces under the 34 synclines might result in higher electrical resistivities than surrounding coherent 35 limestone We also anticipated a significant change from high to low resistivity at the 36 contact between the Thebes Group and the underlying Esna Shale at depths of 400 m or 37 more Instead, we found localized low resistivity zones extending from about 50-100 m 38 below the surface to depths of more than 400 m that are strongly correlated with 39 synclines We suggest that these localized low resistivity zones are filled with artesian 40 groundwater that has insufficient hydraulic head to rise to the modern topographic 41 surface and that is localized in subsurface voids and collapse breccias produced by 42 dissolution Sag of overlying limestone layers is a reasonable model for syncline 43 formation but, given the Oligocene/Miocene age of the synclines, dissolution and sag 44 would be unrelated to young groundwater processes TE D M AN U SC RI PT 31 47 48 49 Keywords: geophysical survey, satellite image analysis, karst, Western Desert, Egypt AC C 46 EP 45 Introduction Satellite imagery of the area west of the Nile in central Egypt displays striking color 50 patterning (Fig 1A) Recent work combining both field work and analysis of high 51 resolution satellite imagery (Tewksbury et al., this issue) has established that much of 52 this patterning is due to an extensive network of synclines in Eocene limestone (Figs 1B- page AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT D) Tewksbury et al (this issue) have argued that syncline geometries and scales are not 54 typical of tectonic fold systems and have proposed a non-tectonic sag origin for the 55 syncline network RI PT 53 The synclines of the Western Desert bear a striking resemblance to structures that 57 are developed in the Eocene Dammam and Miocene Dam Formations in Qatar and that 58 are also easily visible in high resolution satellite imagery (Fig 1E) Like the Western 59 Desert synclines, the structures in Qatar are synclinal and have shallow limb dips, 60 branching and cuspate geometries, and shallow, but doubly plunging, hinges that define 61 multiple basin closures Also like the Western Desert synclines, the Qatar structures are 62 synclinal structures in otherwise flat-lying limestone layers Previous workers have 63 established that the Qatar structures are non-tectonic synclines formed as a result of 64 dissolution of underlying Eocene evaporites of the Rus Formation, with accompanying 65 sag in the overlying limestone layers (Prost, 2014; Sadiq and Nasir, 2002; Cavelier, 66 1970) Stewart (2015) describes similar structures in eastern Saudi Arabia resulting from 67 subsurface dissolution of Rus Formation evaporites and sag in overlying layers 68 Structures of similar geometry and scale also occur in the Pecos Valley of New Mexico 69 and West Texas, USA, and were formed by dissolution of subsurface evaporites in the 70 Castile and Salado Formations (e.g., Stafford et al., 2008; Land and Love, 2006; Motts, 71 1962) AC C EP TE D M AN U SC 56 72 Despite similarities in surface expression between these structures and the 73 Western Desert synclines, evaporites of any significance have never been reported in 74 the subsurface in the east central Western Desert of Egypt (e.g., Barakat and Asaad, page AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT 1965; Issawi et al., 2009) Knowledge of what lies in the subsurface beneath the 76 Western Desert synclines is critical for evaluating whether sag is a reasonable model for 77 the origin of the synclines and, if so, what mechanism might have caused the sag We 78 conducted a series of audio-magnetotelluric surveys designed to illuminate the bedrock 79 down to about 500 m depth In this paper, we present evidence for the presence of a 80 locally elevated water table beneath the synclines, and we address the constraints this 81 places on possible models for origin of the synclines 82 Geologic setting M AN U SC RI PT 75 The east-central portion of the Western Desert is part of what is known as the 84 Limestone Plateau, a gently north-sloping and little-dissected region that sits 200-250 m 85 above the Nile Valley to the east and nearly 300 m above the Kharga Valley to the 86 southwest (Figs 1A and 2A) The Plateau has little topographic relief except where 87 wadis dissect the area immediately along the Nile escarpment The area is traversed by 88 only two roads, the Assiut-El Kharga Road and the Western Desert Road (Figs 1A and 89 2A), and much of the Limestone Plateau is essentially inaccessible and has been little 90 studied except in satellite imagery EP The Plateau is capped by Eocene limestone, with underlying less resistant AC C 91 TE D 83 92 Cretaceous through earliest Eocene shales, marls, and chalk exposed in the escarpments 93 bordering the Plateau along the Nile and Kharga Valleys (Figs 2B and 3) Upper Jurassic 94 and Cretaceous fluvial and shallow marine clastic sedimentary rocks lie unconformably 95 on Precambrian basement at the bottom of the section and are exposed in the Kharga 96 Valley Although this general stratigraphy is well-established, thicknesses of individual page AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT units as reported in the literature vary widely, and stratigraphic nomenclature, 98 particularly for the Eocene limestones, has not been standardized Because rock types in 99 the stratigraphic sequence, as well as thicknesses of specific units, are critical for 100 RI PT 97 interpreting our geophysical profiles, we address each of these issues Determining a reasonable picture of the stratigraphy and thicknesses of units 101 beneath our study area is not a straightforward task Because the Plateau is essentially 103 undissected, there are no places where a representative stratigraphic column can be 104 measured in the field We have used data from measured sections to the southwest in 105 the Kharga escarpment by El Azabi and Farouk (2011) and to the southeast at Gebel 106 Gurnah (Fig 2B) in the Nile escarpment by King et al (this issue) in addition to data from 107 Said (1960, 1962, 1990), El Hinnawi et al (1978), Issawi (1972), Issawi et al (2009), 108 Keheila and El-Ayyat (1990), Khalil and El-Younsy (2003), Keheila et al (1990), and El 109 Hinnawi et al (2005) A well drilled in the 1960s approximately half way along the desert 110 road between Assiut and El-Kharga (Fig 2B) provides the only information about 111 lithologies and unit thicknesses beneath the Plateau itself (Barakat and Asaad, 1965), 112 although correlating the well data with more recent measured stratigraphic columns is 113 complicated by the fact that stratigraphic nomenclature has changed M AN U TE D EP AC C 114 SC 102 We have compiled a stratigraphic column in Fig that we believe is as 115 representative as possible of the sequence underlying our study area A word is in order 116 about our choice of stratigraphic terminology Eocene limestones in Egypt were 117 originally divided into a number of formations exposed in different areas, and the term 118 “Thebes” was originally applied to one of these formations Klitzsch et al (1987) page AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT elevated the term Thebes to Group status when they created the 1:500,000 scale 120 geologic maps of Egypt and assigned a new formation name, the Serai, to what had 121 been the Thebes Formation In our area, the El Rufuf, Drunka, and Serai on these maps 122 are part of the Thebes Group The 2005 1:250,000 scale geologic maps (Riad el al., 2005) 123 return the Thebes to Formation status and deprecate the term “Serai” King et al (this 124 issue) advocate a “Thebes Limestone Formation” containing all of the various facies of 125 Eocene carbonate strata in the Western Desert SC RI PT 119 We have chosen to use “Thebes Group” terminology in part because of 127 uncertainty about specific formations and thicknesses in the subsurface under the 128 Plateau and in part because we have chosen to base the geologic map in Fig 2B 129 primarily on Klitzsch et al (1987) because the 2005 maps (Riad et al., 2005) not 130 extend east of the Nile nor to the northern edge of our study area Furthermore, 131 although the Eocene carbonate sequence is well-documented to consist of interlayered 132 limestones of various characters, marly limestones, and silicified/cherty limestone (e.g., 133 Khalifa et al., 2004 and 2014; Khalifa and Zaghloul, 1990; King et al., this issue; ), these 134 lithologies should all have high electrical resistivities, and grouping them together in the 135 stratigraphic column is reasonable for this study The main change in resistivity with 136 depth would be expected at the transition to the underlying Esna Shale TE D EP AC C 137 M AN U 126 We have indicated a thickness of approximately 400 m for the Thebes Group in our 138 study area This is a conservative estimate based on approximately 430 m measured in 139 the Assiut-Kharga well (Barakat and Asaad, 1965), 350 m measured by King et al (this 140 issue) in the Nile escarpment at Gebel Gurnah near Thebes, and thinner sections page AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT reported elsewhere in the literature The Thebes Group is capped by a significant 142 unconformity and is locally overlain by gravels of the Katkut Formation (Issawi et al., 143 2009; El-Hinnawi et al., 2005; Abu Seif, 2015) The original thickness of the Thebes 144 Group is uncertain, although Macgregor (2012) has suggested that as much as 200 m of 145 limestone may have been removed by post-Eocene erosion RI PT 141 The modern hydrologic setting is also relevant for interpretation of the resistivity SC 146 data The environment is hyperarid, rainfall is negligible, and the modern water table 148 lies 400-500 m below the Plateau surface (Salim, 2012) 149 Objectives M AN U 147 The purpose of this study is to determine what underlies synclines in the Eocene 150 limestones in order to constrain models for origin of the syncline network Sag of layers 152 caused by collapse of subsurface void space created by epigenic or hypogenic karst 153 dissolution in limestone is a reasonable model to test Given that the modern water 154 table lies at depths of at least 400 m beneath the Limestone Plateau, any such collapse 155 features would be relict of an earlier time period, and we would expect any void spaces 156 and collapse zones to be dry in the top several hundred meters of the limestone We 157 chose the AMT method for our surveys because it should be able to detect dry voids, 158 extensive fractures, and collapse breccias, which should have higher resistivity than the 159 solid host limestone AC C EP TE D 151 160 Our study area is located along the Western Desert Road approximately 40 km 161 SSW of Assiut (Figs 2A and B) We chose this area in part for accessibility and in part 162 because synclines in a variety of orientations are well-exposed and had been mapped in page AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT detail in high resolution satellite imagery (Fig 2C) as part of our main syncline mapping 164 project (Tewksbury et al., this issue) 165 Methodology 166 4.1 Audio-magnetotelluric (AMT) method RI PT 163 The audio-magnetotelluric method (AMT) is an important high resolution, non- 167 seismic geophysical technique that measures variations in the Earth’s natural 169 electromagnetic fields to detect electrical resistivity variations in the subsurface at 170 shallow to intermediate depths Resistivity values are controlled mainly by porosity, 171 fractures, water content, concentration of dissolved solids, and permeability The AMT 172 method provides detailed information about variations in subsurface electrical 173 resistivity values that can be used for interpretation of lithological and/or structural 174 differences along the profile line The method is capable of imaging the subsurface with 175 resolutions good enough to detect features a few meters across The shallow subsurface 176 is imaged from high frequency measurements, and the deeper subsurface is imaged 177 from low frequency measurements The depth of investigation can extend to km, 178 although the depth of investigation is reduced by the presence of near-surface 179 conductive sediments M AN U TE D EP AC C 180 SC 168 Our anticipated depth of investigation of 400-500 m is well within the range of the 181 capability of the AMT method, especially given that the hyperarid setting and limestone 182 bedrock are unlikely to result in highly conductive near-surface materials Additionally, 183 the AMT method is low cost, lightweight, accurate, and requires a minimum of field 184 personnel in comparison to seismic methods page AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT 185 4.2 Field survey and data acquisition Figs 2C, 4, 5, and show the locations of our audio-magnetotelluric (AMT) survey 187 lines All but one line crosses synclines at a high angle in order to investigate differences 188 between what underlies the narrow synclines and what underlies adjacent inter- 189 syncline areas The remaining line is oriented parallel to the axial surface trace of a 190 syncline and provides data along, rather than across, the trend of a syncline We 191 collected data at 25 stations along these four profiles SC RI PT 186 We used a Stratagem hybrid AMT instrument that both measures natural M AN U 192 magnetotelluric waves in a low frequency range and uses controlled-source 194 electromagnetic signals in a high frequency range to obtain a continuous electric 195 sounding beneath the measurement site The instrument uses electrode stakes with 196 dipole length of 50 m for measuring electric fields, and highly sensitive magnetic coils 197 for measuring magnetic fields We recorded natural magnetotelluric (MT) waves using 198 the Stratagem low frequency antenna and then repeated the measurement using an 199 artificial signal source for Controlled-Source AMT (CSAMT) in the high frequency range 200 The CSAMT method differs from the AMT method in that a grounded dipole transmitter 201 is used to generate the source fields We used a dual-loop antenna, located from 250 to 202 400 m away from the measured site, to provide a non-polarized electromagnetic field 203 source AC C EP TE D 193 We collected data in both frequency ranges because it permits investigation of 204 205 structures to depths of 500 m, and we are interested in potential karst voids and 206 collapse breccias lying in the top few hundred meters The distance between the MT page AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT Bostick, F X., 1977, A simple almost exact method of MT analysis: Workshop on 359 Electromagnetic Methods in Geothermal Exploration, Snowbird, Utah, U.S 360 Geological Survey, Contract no 14080001-8-359 361 RI PT 358 Bosworth, William, Stockli, Daniel F., and Helgeson, Daniel E., 2015, Integrated outcrop, 3D seismic, and geochronologic interpretation of Red Sea dike-related 363 deformation in the Western Desert, Egypt – The role of the 23 Ma Cairo “mini- 364 plume”: Journal of African Earth Sciences, v 109, p 107-119 Cavelier, Claude, 1970, Geological description of the Qatar Peninsula (Arabian Gulf): Bureau de Recherches Géologiques et Minières, Paris, France 366 367 M AN U 365 SC 362 Constable, S C., Parker, R L., and Constable, C G., 1987, Occam’s inversion: a practical algorithm for generating smooth models from electromagnetic sounding data: 369 Geophysics, v 52, p 289–300 TE D 368 de Groot-Hedlin, C and Constable, S C., 1990, Occam's inversion to generate smooth, 371 two-dimensional models from magnetotelluric data: Geophysics, v 55, p 1613– 372 1624 374 375 376 El-Azabi, Mounir H and Farouk, Sherif, 2011, High-resolution sequence stratigraphy of the Maastrichtian-Ypresian succession along the eastern scarp face of the Kharga AC C 373 EP 370 Oasis, southern Western Desert, Egypt: Sedimentology, v 58, p 579-617 El Hinnawi, Mohamed, Abdallah, Amin M., and Issawi, Bahay, 1978, Geology of Abu 377 Bayan-Bolaq stretch, Western Desert, Egypt: Annals of the Geological Survey of 378 Egypt, v 8, p 19-50 page 17 AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT 379 El Hinnawi, Mohamad E., Said, Mohamad M., El Kelani, Ali H., and Attiya, Mohamad N., 2005, Stratigraphic lexicon and explanatory notes to the geological map of the 381 south Western Desert, Egypt: The Egyptian Geological Survey and Mining 382 Authority (EGSMA) and The National Authority for Remote Sensing and Space 383 Sciences (NARSS), Cairo, Egypt RI PT 380 Issawi, Bahay, 1972, Review of Upper Cretaceous-Lower Tertiary stratigraphy in central 385 and southern Egypt: The American Association of Petroleum Geologists Bulletin, 386 v.56, no 8, p 1448-1463.Issawi, Bahay, Francis, Maher H., Youssef, El-Sayed 387 A.A., and Osman, Rifaat A., 2009, The Phanerozoic geology of Egypt: a 388 geodynamic approach, 2nd edition: Cairo, Ministry of Petroleum and The 389 Egyptian Mineral Resources Authority Special Publication 81, 571 p 390 Keheila, Esmat, A and El-Ayyat, Abd Alla M., 1990, Lower Eocene carbonate facies, TE D M AN U SC 384 environments and sedimentary cycles in Upper Egypt: evidence for global sea- 392 level changes: Paleogeography, Paleoclimatology, Paleoecology, v 81, p 33-47 393 Keheila, E A., Soliman, H A., and El-Ayyat, Abd Alla M., 1990, Litho-and biostratigraphy 394 of the Lower Eocene carbonate sequence in Upper Egypt: evidence for uplifting 395 and resedimentation of the Paleocene section: Journal of African Earth Sciences, 397 AC C 396 EP 391 v 11, no 1/2, p 151-168 Khalifa, M A and Zaghloul, E A., 1990, Carbonate lithofacies and depositional 398 environments of the lower Eocene Farafra Lomestone, Farafra Oasis, Western 399 Desert, Egypt: Journal of African Earth Sciences and the Middle East, v 11, no 3- 400 4, p 281-289 page 18 AES4509 (Tarabees et al.) Revision with tracked changes ACCEPTED MANUSCRIPT 401 Khalifa, M A., Abu El Ghar, M S., and Al Aasm, I., 2014, Linking carbonate cyclicity in platforms to depositional and diagenetic overprints: an example from the Lower 403 Eocene Drunka Formation, west of Assiut-Minia stretch, Western Desert, Egypt: 404 Arabian Journal of Geosciences, v 7, p 5159-5170 405 RI PT 402 Khalifa, M A., Abu El Ghar, M S., Helal, S., and Hussein, A W., 2004, Depositional history of the Lower Eocene drowned carbonate platform (Drunka Formation), 407 west of Assiut-Minia stretch, Western Desert, Egypt, in, 7th International 408 Conference on the Geology of the Arab World: Cairo University, p 233-254 M AN U 409 SC 406 Khalil, M and El-Younsy, A R M., 2003, Sedimentological approach to high resolution sequence stratigraphy of the Upper Cretaceous-lower Eocene succession, Farafra 411 Oasis, Western Desert, Egypt: United Arab Republic Journal of Geology, v 47, no 412 1, p 275-300 413 TE D 410 King, Christopher, Dupuis, Christian, Aubry, Marie-Pierre, Berggren, William A., Knox, Robert O’B., Fathi, Wael, and Baele, Jean-Marc, in review this issue, Anatomy of a 415 mountain: the Thebes Limestone Formation (Lower Eocene) at Gebel Gurnah, 416 Luxor, Nile Valley, Upper Egypt: this isssue 418 419 420 Klitzsch, Eberhard, List, Franz K., and Pöhlmann, Gerhard, 1987, Geologic maps of Egypt, AC C 417 EP 414 1:500,000 scale: The Egyptian General Petroleum Company (EGCP) and Conoco Coral, Bir Misaha, Dakhla, El-Saad El-Ali, and Luxor sheets Land, L and Love, D., 2006, Third day road log, in, Land, L., Leuth, V., Raatz, B., Boston, 421 P., and Love, D., eds., Caves and Karst of Southeastern New Mexico: New Mexico 422 Geological Society, Guidebook 57 page 19 AES4509 (Tarabees et al.) Revision with tracked changes ... what underlies the narrow synclines and what underlies adjacent inter- 189 syncline areas The remaining line is oriented parallel to the axial surface trace of a 190 syncline and provides data... Abstract TE D RI PT Audio- magnetotelluric surveys to constrain the origin of a network of narrow synclines in Eocene limestone, Western Desert, Egypt Recent work with high resolution satellite imagery... two synclines in Subarea are separated by a broad 264 inter-syncline area that has a gently plunging anticlinal nose in the west but is 265 characterized by flat-lying bedding where the synclines