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Nummulite biostratigraphy of the Eocene succession in the Bahariya Depression, Egypt: Implications for timing of iron...
Article in Journal of African Earth Sciences · April 2016 DOI: 10.1016/j.jafrearsci.2016.04.016
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Nummulite biostratigraphy of the Eocene succession in the Bahariya Depression, Egypt: Implications for timing of iron mineralization
A.M. Afify, J. Serra-Kiel, M.E. Sanz-Montero, J.P. Calvo, E.S. Sallam
PII: S1464-343X(16)30132-7 DOI: 10.1016/j.jafrearsci.2016.04.016 Reference: AES 2552
To appear in: Journal of African Earth Sciences
Received Date: 2 February 2016 Revised Date: 14 April 2016 Accepted Date: 19 April 2016
Please cite this article as: Afify, A.M., Serra-Kiel, J., Sanz-Montero, M.E., Calvo, J.P., Sallam, E.S., Nummulite biostratigraphy of the Eocene succession in the Bahariya Depression, Egypt: Implications for timing of iron mineralization, Journal of African Earth Sciences (2016), doi: 10.1016/ j.jafrearsci.2016.04.016.
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 1 Nummulite biostratigraphy of the Eocene succession in the Bahariya Depression,
2 Egypt: Implications for timing of iron mineralization
3 Afify, A.M. a,b,* , Serra-Kiel, J. c, Sanz-Montero, M.E. a, Calvo, J.P. a, Sallam, E.S. b
4 a) Petrology and Geochemistry Department, Faculty of Geological Sciences, Complutense
5 University, Madrid, C/ José Antonio Nováis, 2, 28040 Madrid, Spain
6 b) Geology Department, Faculty of Science, Benha University, 13518 Benha, Egypt
7 C) Stratigraphy, Paleontology and Marine Geosciences Department, University of Barcelona, C/
8 Martí i Franquès, s/n, 08028 Barcelona, Spain
9
10 * Corresponding author ([email protected])
11
12 ABSTRACT 13 In the northern part of the Bahariya DepressionMANUSCRIPT (Western Desert, Egypt) the 14 Eocene carbonate succession, unconformably overlying the Cretaceous deposits,
15 consists of three main stratigraphic units; the Naqb, Qazzun and El Hamra formations.
16 The Eocene carbonates are relevant as they locally host a large economic iron
17 mineralization. This work revises the stratigraphic attribution of the Eocene formations
18 on the basis of larger benthic foraminifers from both carbonate and ironstone beds.
19 Eight Nummulites species spanning the late Ypresian – early Bartonian (SBZ12 to
20 SBZ17) were identified, thus refining the chronostratigraphic framework of the Eocene
21 in that regionACCEPTED of Central Egypt. Moreover, additional sedimentological insight of the
22 Eocene carbonate rocks is presented. The carbonate deposits mainly represent shallow
23 marine facies characteristic of inner to mid ramp settings; though deposits interpreted as
24 intertidal to supratidal are locally recognized. ACCEPTED MANUSCRIPT 25 Dating of Nummulites assemblages from the youngest ironstone beds in the
26 mines as early Bartonian provides crucial information on the timing of the hydrothermal
27 and meteoric water processes resulting in the formation of the iron ore mineralization.
28 The new data strongly support a post-depositional, structurally-controlled formation
29 model for the ironstone mineralization of the Bahariya Depression.
30
31 Keywords: Nummulites, Eocene carbonates, ironstone, chronostratigraphy, Western
32 Desert, Central Egypt.
33
34 1. Introduction
35 The Bahariya Depression is located near the central part of the Western Desert
36 of Egypt (Fig. 1) where it shows elliptical geometry surrounded by a carbonate plateau 37 mainly formed of Eocene rock units in its northernMANUSCRIPT part. The Eocene stratigraphy, 38 especially of the Middle to Upper Eocene formations in the Bahariya region has been a
39 matter of dispute (Issawi et al., 2009). This was probably due to the lithostratigraphic
40 variations and facies changes of the Eocene formations with respect to their equivalents
41 outside the region as well as lack of agreement about the stratigraphic discontinuities
42 between the exposed rock units in the area. Three Eocene rock units, the Naqb, Qazzun
43 and El Hamra formations, were described exclusively for the Bahariya Depression by
44 Said and Issawi (1964). These deposits have economic significance since they represent
45 the host rockACCEPTED of the only ironstone mineralization currently exploited for steel industry
46 in Egypt. Moreover, these ore deposits are unique along the Caenozoic palaeo-Tethyan
47 shorelines in North Africa and South Europe (Salama et al., 2014) and can be
48 interpreted as an analog for banded iron formations (BIFs) (Afify et al., 2015a, b). The
49 origin of these deposits has also been a matter of scientific discussion for long time ACCEPTED MANUSCRIPT 50 (e.g., El Shazly, 1962; El Akkad and Issawi, 1963; Said and Issawi, 1964; Basta and
51 Amer, 1969; Dabous, 2002; Salama et al., 2013, 2014; Baioumy, 2014; Afify et al.,
52 2014, 2015a, b). Despite this fact no much work was focused on the facies architecture
53 and evolutionary pattern of the Eocene host rocks and their relation with the iron
54 mineralization. Likewise, there is lack of detailed chronostratigraphic framework of the
55 Eocene formations. In the classical papers on the geology of the region, e.g., Said and
56 Issawi (1964), the age of the Naqb Formation was loosely attributed to the early Middle
57 Eocene whereas the same rock unit was dated as upper Ypresian (middle Ilerdian-
58 Cuisian) by Boukhary et al. (2011) on the basis of larger benthic foraminifera. The
59 Qazzun Formation was attributed to the upper Middle Eocene without detailed
60 biostratigraphic basis (Said and Issawi, 1964). This was also the case for El Hamra
61 Formation, which was dated as Upper Eocene (Said and Issawi, 1964). In contrast, the
62 stratigraphic review by Issawi et al. (2009) considered that the Lutetian is missing in the 63 Bahariya Depression, which clearly points out MANUSCRIPT a controversy on the chronostratigraphy 64 of the Eocene rocks of the area.
65 This paper provides a scheme of the depositional and diagenetic features present
66 in the Eocene carbonate formations cropping out in the northern part of the Bahariya
67 Depression and aims to precise their chronostratigraphic framework. This is supported
68 by new biostratigraphic evidence from larger benthic foraminifers collected from the
69 carbonate and associated ironstone rocks. As a result, timing of the iron ore 70 mineralizationACCEPTED can be assessed more precisely. 71
72 2. Geologic setting
73 The sedimentary succession exposed at the northern part of the Bahariya area
74 comprises the Cenomanian Bahariya Formation that is unconformably overlain by a ACCEPTED MANUSCRIPT 75 carbonate plateau formed of Eocene sediments (Fig. 1). The Eocene succession
76 represented by the Naqb, Qazzun and El Hamra formations is truncated by Oligocene
77 fluvial sandstone of the Radwan Formation (El Akkad and Issawi, 1963, Said and
78 Issawi, 1964). The Bahariya Eocene rock units are equivalent to the Minia Fm. (= Naqb
79 and Qazzun formations), Mokattam Fm. (= Rayan Fm.; = lower unit of the El Hamra
80 Formation) and Maadi Fm. (= Qasr El Sagha Fm.; = upper unit of the El Hamra Fm.),
81 which extend mostly to the north and north east of Egypt in the Nile Valley and Faiyum
82 areas (Issawi et al., 1999). The Eocene carbonates in northern Bahariya are associated
83 with ironstone mineralization, which affects these carbonate units at three main areas,
84 i.e. El Gedida, Ghorabi and El Harra mines (Fig. 1) along two major fault systems
85 (Afify et al., 2015b).
86 The Bahariya Depression was deformed by a NE-trending right-lateral wrench
87 fault system associated with several doubly plunging folds and extensional faults (Fig. 88 1; Sehim, 1993; Moustafa et al., 2003). The MANUSCRIPT strain regime in the Bahariya area was 89 transpressional, starting by the end of the Campanian and being rejuvenated after the
90 Eocene (Said and Issawi, 1964; Sehim, 1993; Moustafa et al., 2003). The post–
91 Campanian NE-SW doubly plunging anticline folds and ENE strike-slip faults were
92 continued throughout the Paleocene and early Eocene. Moreover, syndepositional
93 tectonic activity and seismic pulses took place during deposition of the Eocene
94 sediments (Said and Issawi, 1964). The depositional pattern of the Eocene rocks in the 95 study area wasACCEPTED controlled by a paleorelief sculpted in Late Cretaceous – Early 96 Paleogene times, ultimately related to the rejuvenation of the Syrian Arc System (Said
97 and Issawi, 1964). As a consequence, the carbonate succession was fractured and folded
98 along NE to ENE oriented right-stepped en-échelon folds (Fig. 1). The faulting pattern ACCEPTED MANUSCRIPT 99 shows major NE-SW dextral strike-slip faults, WNW left-stepped, en-échelon normal
100 faults, E-W normal faults and local thrusts (Fig. 1).
101
102 3. Materials and methods
103 Detailed fieldwork on the Eocene carbonate succession and the associated
104 ironstone deposits was supported by analysis of satellite imagery (Figs. 1 and 2). Field
105 observations and lithostratigraphic logging were complemented by collecting fossil
106 specimens, especially larger benthic foraminifers (Nummulites ), mainly from three
107 outcrops (El Behour, Gar El Hamra and Teetotum Hill) as well as from the central part
108 of El Gedida mine (Figs. 1 and 3). Altogether, we studied six samples collected from El
109 Behour section (Figs. 1 and 3A), nine samples collected from Gar El Hamra section
110 (four samples from the Qazzun Formation, five samples from the El Hamra Formation)
111 (Figs. 1 and 3B), four samples from the Teetotum Hill section (Figs. 1 and 3C) and one 112 sample from El Gedida mine section (Fig. MANUSCRIPT 1). The soft samples with isolated
113 Nummulites were disaggregated in a solution of Na 2CO 3, H2O2 and water and later
114 sieved through apertures of 1.0, 0.5 and 0.2 mm. All the Nummulites samples are
115 housed at the Stratigraphy, Paleontology and Marine Geosciences Department,
116 University of Barcelona, Spain.
117 About 160 samples of carbonate, ironstone, sandstone, claystone and other rocks
118 were collected during fieldwork. Indurated samples were prepared as thin sections and 119 polished slabs.ACCEPTED Petrography was carried out by using transmitted polarized and reflected 120 light microscopes. Staining with alizarin red-potassium ferricyanide was used to
121 differentiate the carbonate minerals. Scanning electron microscopic studies (SEM) were
122 carried out for high-resolution textural and morphometric analyses. Fresh broken pieces
123 were placed on sample holders supported by carbon conductive tape, followed by ACCEPTED MANUSCRIPT 124 sputter coating of gold and studied with a JEOL JSM-6400 operating at 20 kV and
125 equipped with an energy dispersive X-ray microanalyzer (SEM-EDAX). Mineralogy for
126 nearly all collected samples was verified by X-ray powder diffraction analyses (XRD)
127 using a Philips PW-1710 diffractometer under monochromatic Cu Kα radiation ( λ=
128 1.54060 Å) operating at 40kv and 30 mA.
129
130 4. Stratigraphy and sedimentology of the Eocene formations
131 The Eocene stratigraphic succession exposed at the northern Bahariya study area
132 is sculptured geomorphologically as a carbonate plateau where superimposed cycles of
133 weathering and erosion on these rock units resulted in formation of several geological
134 landforms, e.g., mesas, buttes and conical hills (Fig. 1). Outcrops of the Naqb
135 Formation can be traced along the western part of the study area (Fig. 1) where it is
136 characterized by pinkish shading due to iron pigmentation and staining (Fig. 2A). Two 137 sedimentary sequences separated by an irregular MANUSCRIPT paleokarst surface can be differentiated 138 in the Naqb Formation (Afify et al., 2015b) (Figs. 2B, C). The overlying Qazzun and El
139 Hamra formations are well exposed to the north and east of the study area (Figs. 1 and
140 2A). The Qazzun Formation looks homogenous whilst the El Hamra Formation can be
141 subdivided into two units; Lower Hamra and Upper Hamra (Figs. 2D and 3) (Issawi et
142 al., 2009). The boundary between the two units of El Hamra Formation is marked by a
143 brecciated, concretionary irregular thin calcareous bed cropping out at El Behour and 144 Gar El HamraACCEPTED sections (Figs. 3A, B). The Eocene rock units show features indicative of 145 slight syndepositional folding forming anticline and syncline structures, especially
146 around highly faulted areas (Fig. 1). In addition to the NW-SE fault systems affecting
147 the Eocene carbonates, a major extensional NE fault system affected the study area (Fig. ACCEPTED MANUSCRIPT 148 1). These faults, with exposed length of tens of kilometers, likely developed in response
149 to the Red Sea/Gulf of Suez rifting.
150 The Eocene marine carbonate formations are capped disconformably by an up to
151 20 m-thick succession of horizontally-bedded continental lacustrine carbonate deposits
152 (Figs. 2A and 3). This unit (Continental carbonate unit) is described for the first time in
153 the northern Bahariya area where it crops out with good exposure in the Teetotum and
154 nearby hills (Fig. 1). This rock unit may be equivalent to the Miocene unit described by
155 Pickford et al. (2010) in the center of the Bahariya Depression and to the post-Eocene
156 deposits described by Sanz-Montero et al. (2013) in the adjacent Farafra Depression.
157
158 4.1. Depositional and diagenetic features of the Eocene carbonate units
159 Depositional features and palaeoenvironmental interpretation of the Eocene rock
160 units are summarized in Table 1. Some additional insight about these formations is as 161 follows: MANUSCRIPT 162
163 4.1.1. The Naqb Formation
164 Description: The Naqb Formation overlies unconformably the Cenomanian Bahariya
165 Formation and is overlain with seeming disconformity by the Qazzun Formation (Figs.
166 1 and 2A). The Naqb Formation at the Ghorabi and El Harra sections is up to 13 m
167 thick. At those localities, the sedimentary succession consists mainly of dolostone and 168 siliceous dolostoneACCEPTED beds with few marlstone intercalations (Figs. 2B, C). The lower 169 sequence of the Naqb Formation is mainly composed of nummulitic dolostone at the
170 bottom (Fig. 4A), oolitic and fossiliferous dolostone at the middle part of the sequence
171 (Fig. 4B; Table 1) and it is capped by fine- to medium-grained dolostone rocks (Fig.
172 4C). The upper sequence is composed of stromatolite-like, laminated (Figs. 4D, E), ACCEPTED MANUSCRIPT 173 fine-grained dolostone with scarce fossils (small sized, reworked foraminiferal tests).
174 Laminated fenestral fabrics, stromatolite-like structure, evaporite pseudomorphs,
175 desiccation cracks and rhizoliths are common features in the upper part of the Naqb
176 Formation. This rock unit shows pervasive diagenetic features resulting from
177 micritization (Fig. 4A), dolomitization (Fig. 4), silicification (Fig. 4B), stylolitization
178 (Fig. 4C), dissolution (Fig. 4B) and cementation with pseudospherulitic, fibro-radiating
179 carbonates (Fig. 4F). The latter process was mainly related to development of the
180 paleokarst system that separates the two stratigraphic sequences.
181 Interpretation: The carbonate deposits of the Naqb Formation were accumulated in
182 shallow depositional environments, from shallow subtidal to intertidal-supratidal (Afify
183 et al., 2015b). The low diversity of fossil assemblages (e.g., nummulitids, alveolinids,
184 textulariids, and dasycladacean algae) in the lower sequence could be interpreted as
185 characteristic of oligophotic inner- to mid-ramp environments, preferably at water depth 186 ranging from 40 to 80 m (Hottinger, 1997; Beavingto MANUSCRIPTn-Penney and Racey, 2004) with 187 random scatter of nutrients where the nummulitids preferred lower nutrient levels (Bassi
188 et al., 2013). The association of nummulitids and alveolinids characterizes the shoals
189 and banks of inner-ramp settings (Buxton and Pedley, 1989). In contrast, abundance of
190 evaporite pseudomorphs, scarcity of fossils and calcretization in the upper sequence are
191 indicative of high salinity, very shallow and calm conditions in intertidal-supratidal
192 environments disturbed by current action (Warren, 2006; Ortí, 2010; Afify et al., 193 2015b). ACCEPTED 194
195 4.1.2. The Qazzun Formation
196 Description: The Qazzun Formation can be easily differentiated from the underlying
197 and overlying Eocene rock units because of its distinctive chalky nature and bright ACCEPTED MANUSCRIPT 198 white color (Fig. 2A). Thickness of the Qazzun Formation ranges from 4 m at El Gedida
199 area to 32.5 m at Gar El Hamra section (type section; Figs. 2D, 3B). The carbonate
200 deposits of the Qazzun Formation consist mainly of massive, bright white, nummulite-
201 rich chalky limestone forming tabular, meter-thick banks alternated with soft, slope-
202 forming mud-supported chalky limestones. The carbonate rocks of the Qazzun
203 Formation exhibit nummulitic wackestone and packstone fabrics where the nummulites
204 tests occur usually scattered in the micrite matrix (Fig. 5A) although closely packed
205 nummulites test are locally observed. Micritization and dissolution features are
206 common.
207 Interpretation: The occurrence of nummulitic wackestone/packstone facies in banks
208 with un-oriented fabrics reflects shallow water, high energy conditions in a middle ramp
209 setting (Hottinger, 1997). The homogeneity of the facies forming the Qazzun Formation
210 suggests low variation of sea-level through time. 211 MANUSCRIPT 212 4.1.3. The El Hamra Formation
213 Description: Similar to the Qazzun Formation, the thickness of the El Hamra Formation
214 increases towards the north where it reaches up to 65 m at Gar El Hamra section. The
215 carbonate deposits of the El Hamra Formation are composed of soft to slightly
216 indurated, fossil-rich limestone with sandy limestone, marlstone, glauconitic limestone,
217 and siltstone intercalations. The formation is subdivided into two units separated by a 218 thin discontinuousACCEPTED brecciated limestone bed (Lower Hamra and Upper Hamra; Figs. 2D, 219 3, Table 1). The lower unit is composed of yellow, massive, nummulite-rich limestone-
220 sandy limestone beds with gradual upwards increase of oyster and gastropod banks (Fig.
221 3). Nummulitic packstone/rudstone, fossiliferous wackestone-packstone (Fig. 5B) and
222 oyster rudstone (Fig. 5C) are the dominant facies in the Lower Hamra unit. Nummulitid ACCEPTED MANUSCRIPT 223 tests, miliolids, oyster and gastropod shells, coralline debris and echinoid spines are the
224 most abundant fossil skeletons in the Lower Hamra unit (Table 1; Figs. 5B, C). The
225 Upper Hamra unit is characterized by dominance of glauconitic fossiliferous sandy
226 limestone, siltstone/sandy clays with gradual upward increase of clastics and gastropod
227 and oyster banks (Figs. 2D, 3). These lithofacies are rich in silt-sized quartz grains and
228 the terrigenous content can exceed 50% of the volume of the rock thus resulting in
229 fossiliferous siltstone. The dominant facies of the Upper Hamra are the fossiliferous
230 glauconitic packstone-grainstone (Fig. 5D), oyster rudstone and fossiliferous siltstone
231 where the main skeletal particles are of oysters, gastropods, bryozoans and miliolids.
232 Calcitization of skeletal particles, micritization and dissolution features are the main
233 diagenetic features of the El Hamra Formation (Fig. 5B–D).
234 Interpretation: The carbonate and terrigenous facies of the El Hamra Formation are
235 indicative of deep to shallow subtidal environments. The common occurrence of 236 nummulite-rich banks in the lower unit reflects MANUSCRIPT deep subtidal environment. The relative 237 abundance of bioclasts in the Lower Hamra reflects different sub-environments, i.e.
238 miliolids, gastropods and oysters live in the inner ramp environment whereas the
239 nummulitids and echinoids are characteristic of deeper marine areas (Flügel, 2004). The
240 Lower Hamra unit shows shallowing upward conditions enhanced by occurrence of
241 oyster and gastropod banks in its upper part. The occurrence of brecciated deposits on
242 the top of the lower unit points to an episode of subaerial exposure. The dominance of 243 gastropod andACCEPTED oyster banks in the Upper Hamra unit indicates moderate to high energy 244 conditions, lesser than 50 m deep, in nutrient-rich waters of inner ramp settings (Flügel,
245 2004). The abundance of silt-sized quartz grains and glauconitic grains in the packstone
246 facies of the Upper Hamra unit typically reflects low sedimentation rate and deposition
247 from suspension nearby a clastic source in shallow subtidal environments (Flügel, ACCEPTED MANUSCRIPT 248 2004). Prevalent shallow conditions in the Upper Hamra unit are consistent with a sea-
249 level regression, as suggested by El Habaak et al. (2016).
250
251 4.2. Sedimentary features of the ironstone mineralization
252 The Bahariya ironstone rocks are mainly located at three areas; i.e. the Ghorabi,
253 El Harra and El Gedida mines. These locations are coincident with two major fault
254 systems oriented NE-SW (Fig. 1). In the Ghorabi and El Harra areas, thickness of the
255 ironstone succession ranges from 7 to 13 m, which is similar to the thickness of the
256 carbonate deposits of the Naqb Formation in nearby areas. Likewise, the ironstone
257 succession is formed of two sequences (Fig. 6A), where the iron-rich rocks show clear
258 evidence for replacement and/or cementation of the carbonate fabrics by Fe-Mn
259 minerals (Afify et al., 2015b). Some unaltered thin clay/marl beds occur intercalated
260 with the ironstone rocks (Fig. 6A). At El Gedida mine, the ore mineralization consists of 261 up to 30 m-thick black, indurated ironstone that MANUSCRIPT comprehensively replaced the carbonate 262 succession of the Eocene Naqb, Qazzun and El Hamra formations (Figs. 6B, 7). Most
263 facies and fabrics occurring in the carbonate units were recognized in the ironstone.
264 Towards the upper part of the ironstone succession in El Gedida, a 3 m-thick
265 fossiliferous ironstone bed (Fig. 6B) furnished rich, well-preserved Nummulites
266 assemblages. This bed is overlain by pisolithic ironstone rocks occurring at the topmost
267 part of the ironstone succession. This is in turn is capped by up to 10 m-thick, greenish 268 glauconitic claystoneACCEPTED and up to 15 m-thick ferruginous black sandstone (the Radwan 269 Formation) respectively (Fig. 6B). The green glauconitic beds are stratigraphically and
270 sedimentologically correlatable with the upper unit of El Hamra Formation, despite it is
271 barren of fossils. ACCEPTED MANUSCRIPT 272 A variety of minerals was determined in the ironstones, i.e. iron oxyhydroxides,
273 quartz, manganese minerals, apatite, dolomite, clay minerals, and sulfate minerals
274 (Afify et al., 2015b). Goethite and hematite are the main iron-bearing minerals.
275 Petrography of the ironstone reveals that the ore deposits exhibit the main textures and
276 structures of their precursor host carbonates (Fig. 8). The main textures and fabrics of
277 the carbonates of the Naqb Formation are preserved in the ore deposits where
278 nummulitic mud-wacke ironstone, oolitic and fossiliferous ironstone, massive to
279 brecciated non-fossiliferous ironstone and stromatolite-like fabrics can be recognized
280 (Fig. 8A–D). Likewise, the nummulitic wackestone-packstone facies characteristic of
281 the Qazzun Formation and the fossiliferous packstone of the El Hamra Formation are
282 recognizable in the ironstones (Figs. 8E, F). The topmost part of the ironstone
283 succession includes large pisoids that show vadose cements (Figs. 8G, H) and some
284 rhizoliths replaced by iron oxides (Fig. 8I). The association of vadose cements, pisoliths 285 and root structures is clearly indicative of subaer MANUSCRIPTial exposure. 286 Both the stratigraphic and structural features analyzed in the northern Bahariya
287 area point to a close relationship between the distribution of the ironstone deposits and
288 major faults (Figs. 1 and 7). Extensive replacement of the Eocene carbonate formations
289 by Fe and Mn oxides and other associated minerals occurs in localized areas near major
290 fault lineaments where the ore deposits retain largely many stratigraphic and
291 sedimentary features of their host carbonate rocks, i.e. thickness, bedding, lateral and 292 vertical sequentialACCEPTED arrangement and fossil content. On basis of petrography, mineralogy 293 and geochemistry as well as ironstone associations and distributions, Afify et al. (2014;
294 2015a, b) suggested that the iron oxyhydroxides were deposited in the carbonate rocks
295 by hydrothermal solutions related to regional magmatic activity in the region and
296 moved upwards through the NE-SW major faults, fractures and discontinuities. ACCEPTED MANUSCRIPT 297 4.3. Biostratigraphy of the Eocene formations and ironstones
298 The new biostratigraphic data provided in this work are based on the
299 assemblages of Nummulites , a group of larger foraminifers well described and
300 illustrated in monographs such as those by Schaub (1981, 1995) and Racey (1995). The
301 biostratigraphic range of each species was determined according to zones defined by
302 Schaub (1981) and the Shallow Benthic Zones (SBZ) characterized by Serra-Kiel et al.
303 (1998). The species identified in this work are illustrated in Plate 1 and their
304 biostratigraphic ranges are given in Figure 9. All data about the intervals and
305 distribution of Nummulites in the studied sections are summarized in Figure 3, where
306 the range chart of the 19 fossil samples is shown. All the Nummulites specimens were
307 collected from the Qazzun and El Hamra formations. It was difficult to find well
308 preserved specimens from the Naqb Formation in the studied sections due to the strong
309 replacement of the fossil grains by silica. The sample collected from the upper part of 310 the ironstone succession in the El Gedida mine MANUSCRIPT (Fig. 6B) shows good preservation of 311 Nummulites specimens with slight replacement by iron.
312 In the carbonate beds of the Qazzun Formation, at its type locality of Gar El
313 Hamra section (Figs. 1 and 3B), three larger benthic foraminifers were identified, i.e.
314 Nummulites syrticus SCHAUB , 1981 (Pl. 1, Fig. 26) , N. praelorioli HERB & SCHAUB ,
315 1963 (Pl. 1, Figs. 10–14) and N. migiurtinus AZZAROLI , 1952 (Pl. 1, Figs. 15–18).
316 Likewise, four Nummulites species were identified from the carbonate deposits of the 317 lower unit ofACCEPTED the El Hamra Formation in the three studied sections (Fig. 3), i.e. N. 318 migiurtinus, N. gizehensis (FORSKÅL, 1795) (Pl. 1, Figs. 1–9) , N. discorbinus
319 (SCHLOTHEIM , 1820) (Pl. 1, Figs. 27, 28) and N. beaumonti D’A RCHIAC & HAIME ,
320 1853 (Pl. 1, Figs. 20, 21). ACCEPTED MANUSCRIPT 321 The carbonate beds of the upper unit of the El Hamra Formation are barren of
322 Nummulites (Fig. 3), but contain diverse macrofossils such as Ostrea clotbeyi, O.
323 multicostata, Carolia placunoides and Turritella sp.
324 A fossil sample collected from the uppermost part of the ironstone succession in
325 the central hill of El Gedida mine (Figs. 1, 6B) yielded Nummulites gizehensis , N.
326 biarritzensis D’A RCHIAC & HAIME , 1853 (Pl. 1, Fig. 25), N. beaumonti and N. lyelli
327 D’A RCHIAC & HAIME , 1853 (Pl. 1, Figs. 29, 30).
328 The eight identified Nummulites species allow us to reassess the age of the
329 Eocene rock units, especially the Qazzun and El Hamra formations. The distribution and
330 time-span of these Nummulites species are shown in Figures 9 and 10. The presence of
331 Nummulites syrticus and N. praelorioli in the lower part of the Qazzun Formation
332 indicates an early Lutetian age (SBZ13) (Schaub, 1981; Serra-Kiel et al., 1998) whilst
333 presence of Nummulites migiurtinus in the upper part of the Qazzun Formation and the 334 lowermost part of the El Hamra Formation indicatesMANUSCRIPT an early to middle Lutetian age 335 (SBZ13/SBZ14) (Schaub, 1981; Serra-Kiel et al., 1998). The dominance of Nummulites
336 gizehensis , associated with N. beaumonti and N. discorbinus in the lower unit of the El
337 Hamra Formation at Gar El Hamra and El Behour sections yields a middle-late Lutetian
338 age (SBZ15/SBZ16) (Schaub, 1981; Serra-Kiel et al., 1998). The occurrence of
339 Nummulites beaumonti in the upper part of the lower unit of the El Hamra Formation at
340 the Teetotum Hill section suggests middle Lutetian (SBZ15) to Bartonian (SBZ17). 341 Summarizing,ACCEPTED the Naqb Formation is considered to be middle-late Ilerdian in 342 age because two Nummulites species, i.e. Nummulites fraasi DE LA HARPE , 1883 and
343 Nummulites pernotus SCHAUB , 1951 (SBZ6 – SBZ9; Schaub, 1981; Serra-Kiel et al.,
344 1998), were recorded in it by Boukhary et al. (2011) (Fig. 10). The Nummulites species
345 studied from the Qazzun Formation assigned an early Lutetian age or SBZ13 for this ACCEPTED MANUSCRIPT 346 rock unit (Fig. 10). The lower unit of the El Hamra Formation is considered to be
347 middle-late Lutetian/early Bartonian in age or SBZ14-SBZ17 (Fig. 10). Although no
348 larger benthic foraminifers were collected by us from the upper unit of El Hamra
349 Formation, it contains Nummulites striatus (BRUGUIÉRE , 1792) according to Issawi et al.
350 (2009), indicating the Bartonian-early Priabonian or SBZ18-SBZ19 (Fig. 10).
351 Unfortunately, the low diversity of Nummulites -only eight species have been
352 identified- does not permit greater precision between the boundaries of the Shallow
353 Benthic Zones (SBZs).
354
355 5. Discussion on timing of ore mineralization processes
356 The close similarities of both lithostratigraphic and sedimentary features
357 between the ironstone and the Eocene carbonate formations in which the ironstone is
358 hosted strongly support replacement and cementation of the carbonates by silica, iron 359 oxyhydroxides, manganese-rich and other subordinate MANUSCRIPT minerals as a result of post- 360 depositional and structurally-controlled processes. This was clearly demonstrated by
361 Afify et al. (2015b) for the dolostones of the Naqb Formation. Moreover, recognition of
362 replacement and/or cementation of the carbonate deposits of the Qazzun and El Hamra
363 formations by the same assemblage of Fe-Mn minerals strongly suggests that the whole
364 set of Eocene deposits were affected by a unique hydrothermal event sourcing iron-rich
365 fluids. These fluids moved throughout the main fault systems that affected the Eocene 366 carbonate formationsACCEPTED (Fig. 7) and mixed with meteoric groundwater (Afify et al., 367 2015a) so that relative timing for the precipitation of the iron ore minerals must be
368 linked to the deformational stages of the Eocene carbonate plateau. Fault zones played a
369 crucial role in focusing fluid migration into the basin, as can be inferred from the study
370 of many hydrothermal, sediment-hosted ore deposits worldwide (Ceriani et al., 2011). ACCEPTED MANUSCRIPT 371 The lateral association of the ironstone mineralization with volcanic rocks south of the
372 study area argues for the relationship between the formation of the ironstone and
373 magmatism (Afify et al., 2014; 2015a, b).
374 The ironstone deposits were formed through dissolution-corrosion of the host
375 carbonate rocks with no replacement of the associated clay intercalations. The
376 widespread red pigmentation of the carbonate deposits of the Naqb Formation by
377 comparison to those of the Qazzun and El Hamra formations was favored by the more
378 porous fabrics of the dolostones, i.e. dolostone is more porous and susceptible to
379 fracturing and brecciation than limestone (Budd and Vacher, 2004). The replacement
380 was most probably syngenetic to the formation of the pisolithic fabrics on the topmost
381 part of the ironstone succession and before the deposition of the glauconitic claystone
382 overburden that was not affected by iron mineralization. Altogether, the aforementioned
383 deformational and karstic features determined the morphology and extent of alteration 384 and mineralization exposed in the area and MANUSCRIPT enhanced by permeability of carbonate 385 rocks.
386 The new biostratigraphic data acquired in this paper allows integration of the
387 post-depositional genetic model proposed previously by Afify et al. (2015a, b) for the
388 ironstone deposits with the chronology of the ore-forming processes. The presence of
389 ferruginized specimens of N. gizehensis (SBZ14–SBZ16), N. beaumonti (SBZ15–
390 SBZ17), N. lyelli (SBZ17) and N. biarritzensis (SBZ17) of middle-late 391 Lutetian/BartonianACCEPTED age in the upper part of the ironstone succession at El Gedida mine 392 area along with the presence of alveolinids and nummulitids of the Naqb Formation and
393 the nummulitids of the Qazzun Formation indicates that the carbonates replaced by the
394 ore deposits span late Ypresian – early Bartonian. The formation of the ore deposits can
395 be dated later than early Bartonian, most probably during the Priabonian and before the ACCEPTED MANUSCRIPT 396 Oligocene which is supported by the fact that neither the Priabonian glauconitic
397 claystones related to the upper unit of the El Hamra Formation nor the Oligocene
398 sandstones show iron replacement. This statement is consistent with the Late Eocene
399 magnetization assigned for the Bahariya area by Odah (2004) and confirms previous
400 interpretation by Afify et al. (2015b) that the ironstone was a post-depositional,
401 structurally-controlled ore deposit.
402
403 6. Concluding remarks
404 The chronostratigraphy of the Eocene rock units forming the carbonate plateau
405 to the north of the Bahariya Depression (Western Desert) has been precised by
406 analyzing new samples of Nummulites species collected in the study area. The Naqb
407 Formation is dated as middle to late Ilerdian (late Ypresian; SBZ6 to SBZ9) whilst the
408 Qazzun Formation contains fauna attributable to the early Lutetian (SBZ13). On the 409 basis of larger benthic foraminifers, the lower MANUSCRIPTunit of the El Hamra Formation is dated 410 as middle to late Lutetian (SBZ14 to SBZ16) reaching up to the early Bartonian
411 (SBZ17) whereas the age of the upper unit is attributed to the late Bartonian and part of
412 the Priabonian as indicated by mollusk assemblages. According to this data, the
413 Lutetian stage is identified for the first time in the region. Moreover, the succession
414 exposed in the northern Bahariya shows a rather long record of Eocene strata.
415 Dating of Nummulites assemblages from the youngest ironstone beds as early 416 Bartonian givesACCEPTED light on the relative timing for the hydrothermal and meteoric 417 groundwater processes that led to the formation of the ore body. These processes
418 probably took place throughout the Priabonian. At that time, sea level regression
419 resulted in subaerial exposure ultimately related to tectonic deformation of the region. ACCEPTED MANUSCRIPT 420 Ascending hydrothermal fluids leading to the formation of the ore minerals followed the
421 pathways created by the fault systems affecting the area.
422
423 Acknowledgements
424 We would like to thank Dr. Juan Pablo Rodríguez-Aranda for helping in drawing
425 figures and revising the manuscript. The authors are indebted to Editor-In-Chief of
426 Journal of African Earth Sciences, for reviewing and editing of the manuscript. Special
427 thanks to Prof. Dr. Johannes Pignatti, Roma University, and an anonymous reviewer for
428 their encouraging comments and annotations that greatly improved an earlier version of
429 the manuscript. This project was financially supported by the Egyptian Government in a
430 full fellowship to the first author at Complutense University of Madrid, Spain. This
431 work is a part of the activities of Research Groups BSHC UCM-910404 and BSHC
432 UCM-910607 and part of the project CGL2015-60805-P on the biostratigraphy of the 433 Paleogene led by Carles Martin Closas, Barcelona MANUSCRIPT University. 434
435 References
436 Afify, A.M., Arroyo, X., Sanz-Montero, M.E., Calvo, J.P., 2014. Clay mineralogy in Bahariya area, 437 Egypt: Hydrothermal implications on fault-related iron ore deposits, 7th Mid European Clay 438 Conference, Germany, Abstract, pp. 137.
439 Afify, A.M., González-Acebrón, L., Sanz-Montero, M.E., Calvo, J.P., 2015a. Unravelling the origin of 440 Bahariya ironstone of Egypt. ECROFI XXIII Conference (The Sorby Conference on Fluid and 441 Melt Inclusions), United Kingdom, Abstract, pp. 136–137.
442 Afify, A.M., Sanz-Montero, M.E., Calvo, J.P., 2015b. Ironstone deposits hosted in Eocene carbonates 443 from Bahariya (Egypt) - New perspective on cherty ironstone occurrences. Sedimentary Geology 444 329, 81–97.
445 Baioumy, H.M.,ACCEPTED Ahmed H. H., Mohamed Z. K., 2014. A mixed hydrogeneous and hydrothermal origin of 446 the Bahariya iron ores, Egypt: Evidences from the trace and rare earth elements geochemistry. 447 Journal of Geochemical Exploration 146, 149–162.
448 Bassi, D., Nebelsick, J.H., Puga-Bernabéu, Á., Luciani, V., 2013. Middle Eocene Nummulites and their 449 offshore re-deposition: A case study from the Middle Eocene of the Venetian area, northeastern 450 Italy. Sedimentary Geology 297, 1-5.
451 Basta, E.Z., Amer, H.I., 1969. El Gedida iron ores and their origin, Bahariya oasis, Western Desert, 452 U.A.R. Economic Geology 64, 424–444. ACCEPTED MANUSCRIPT 453 Beavington-Penney, S.J., Racey, A., 2004. Ecology of extant nummulitids and other larger benthic 454 foraminifera: applications in palaeoenvironmental analysis. Earth-Science Reviews 67, 219–265.
455 Boukhary, M., Hussein-Kamel, Y., Abdelmalik, W. Besada, M., 2011. Ypresian nummulites from the 456 Nile Valley and the Western Desert of Egypt: their systematic and biostratigraphic significance. 457 Micropaleontology 571, 1–35.
458 Budd, D.A., Vacher, H.L. 2004. Matrix permeability of the confined Floridan Aquifer, Florida, U.S.A. 459 Hydrogeology Journal 12, 531–549.
460 Buxton, M.W.N., Pedley, H.M., 1989. Short paper: A standardized model for Tethyan Tertiary carbonate 461 ramps. Journal of the Geological Society, London 146, 746–748.
462 Ceriani, A., Calabró, R., Di Giulio, A., Buonaguro, R., 2011. Diagenetic and thermal history of the 463 Jurassic-Tertiary succession of the Zagros Mountains in the Dezful Embayment (SW Iran): 464 constraints from fluid inclusions. International Journal of Earth Sciences 100, 1265–1281.
465 Dabous, A.A., 2002. Uranium isotopic evidence for the origin of the Bahariya iron deposits, Egypt. Ore 466 Geology Reviews 19, 165–186.
467 El Akkad, S.E., Issawi, B., 1963. Geology and iron ore deposits of Bahariya Oasis. Geological Survey of 468 Egypt 18, 300 pp.
469 El Habaak, G., Askalany, M., Galal, M., Abdel-Hakeem, M., 2016. Upper Eocene glauconites from the 470 Bahariya depression: An evidence for the marine regression in Egypt. Journal of African Earth 471 Sciences 117, 1–11.
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474 Flügel, E., 2004. Microfacies of Carbonate Rocks: Analysis, Interpretation and Application. Springer, 475 Berlin, 976 p.
476 Hottinger, L., 1997. Shallow benthic foraminiferal assemblagesMANUSCRIPT as signals for depth of their deposition 477 and their limitations. Bulletin de la Societé géolo gique de France 168, 491–505.
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485 Odah, H., 2004. Paleomagnetism of the Upper Cretaceous Bahariya Formation, Bahariya Oasis, Western 486 Desert, Egypt. Journal of Applied Geophysics 3(2), 177–187.
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509 Schaub, H. 1995. Nummulites of Israel. In: The Biostratigraphy of the Eocene of Israel H. Schaub, C. 510 Benjamini & S. Moshkovitz eds. Mémoires suisses de Paléontologie, 11, 19–32, 12 plates.
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516 Warren, J.K., 2006. Evaporites: Sediments, Resources and Hydrocarbons: Springer, Berlin, 1035p.
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518 MANUSCRIPT 519
520
521
522
523
524 ACCEPTED 525
526
527 ACCEPTED MANUSCRIPT 528 TABLE CAPTIONS
529 Table 1. Summary of the sedimentary features, fossil content and palaeoenvironments
530 for the three Eocene formations in the northern Bahariya region.
531
532
533
534
535
536
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538 MANUSCRIPT 539
540
541
542
543
544 ACCEPTED
545
546
547 ACCEPTED MANUSCRIPT 548 FIGURE CAPTIONS
549 Fig. 1. Geologic map of the northern Bahariya area (modified after Said and Issawi,
550 1964). See locations of the measured sections.
551 Fig. 2. A. Landsat image showing the distribution of the Eocene rock units through the
552 northern part of the Bahariya area (1- Naqb Formation, 2- Qazzun Formation and 3- El
553 Hamra Formation). Note that the Eocene formations are overlain disconformably by
554 nearly horizontal continental carbonate unit (4). B. Columnar section showing the two
555 sequences of the Naqb Formation at the section studied at Ghorabi area. C. Outcrop
556 view showing the two sequences (1: lower sequence, 2: upper sequence) of the Naqb
557 Formation separated by paleokarst features. D. Field photograph showing the type
558 section of the Qazzun and El Hamra formations at Gar El Hamra area.
559 Fig. 3. Stratigraphic cross-section showing the distribution of Nummulites along the El- 560 Behour section (A), the Gar El Hamra section (B)MANUSCRIPT and the Teetotum Hill section (C). 561 Fig. 4. Petrographic features of the Naqb Formation. A. Nummulitic dolostone with
562 medium-grained dolomite rhombs. Note the partial dissolution of the dolomite crystals
563 as well as the micritization of the nummulitid tests. B. Oolitic fossiliferous dolostone
564 showing dissolution pores cemented by quartz. C. Fine to medium grained dolostone.
565 Note a vertical stylolite cemented by calcite (arrow). D, E. Stromatolite-like dolostone.
566 Note the white laminae of quartz in-between the fine laminae of dolomites (D) with
567 desiccation cracks (E). F. Pseudospherulitic and fibro-radiating dolomite after calcite. 568 All microphotographsACCEPTED in crossed nicols (C.N.). 569 Fig. 5. Textures of the Qazzun and El Hamra formations. A. Nummulitic
570 wackestone/packstone facies with nummulitid tests scattered in the micritic matrix
571 (C.N.). B. Fossiliferous packstone with nummulitid tests, gastropods, oysters and
572 miliolids closely packed together. Note the calcitization of the skeletal particles (C.N.). ACCEPTED MANUSCRIPT 573 C. Oyster rudstone with shells commonly bored (C.N.). D. Glauconitic fossiliferous
574 packstone with bivalve shell fragments, gastropods, miliolids and oxidized glauconitic
575 grains (PPL). (C.N. = crossed nicols, PPL = plane polarized light).
576 Fig. 6. A. Panoramic view of the Naqb Formation showing ironstone beds and clay
577 intercalations (white arrows) arranged in two sequences. B. Outcrop view of the
578 ironstone succession exposed at the central part of El Gedida mine (X is the location of
579 the collected fossil sample).
580 Fig. 7. Landsat image showing the main exploited ore mine of El Gedida and the
581 surrounding carbonates. The geologic profile shows the structural and stratigraphic
582 relationships of the Eocene units and the ore deposits in the same area.
583 Fig. 8. Microphotographs showing different ironstone fabrics: A. Nummulitic ironstone
584 with quartz cementing moldic porosity (C.N.). B. Oolitic, fossiliferous ironstone where
585 all the grains are partly replaced/cemented by iron and quartz after dolomite (C.N.). C. 586 Highly crenulated, colloform ferromanganese MANUSCRIPToxides replacing speleogenic carbonates 587 (PPL). D. Laminated iron with high porosity in-between the laminae (PPL). E. Ghosts
588 of nummulitid tests scattered in iron oxide groundmass, Qazzun Formation (PPL). F.
589 Fossiliferous ironstone with nummulitic tests and some fragmented bivalve shells
590 replaced by iron as well as quartz, El Hamra Formation (C.N.). G, H. Pisolithic
591 ironstone with irregular pisoliths packed together and cemented by fibrous and
592 microcrystalline iron cement (G. PPL, H. reflected light). I. SEM photo showing tabular 593 hematite replacingACCEPTED rootlets in the pisolithic ironstone. 594 Fig. 9. Nummulites species identified in the Qazzun and El Hamra formations and their
595 biostratigraphic range according to Schaub (1981) and Serra-Kiel et al. (1998).
596 Fig. 10. A composite section (not at scale) of the main Eocene lithostratigraphic and
597 chronostrigraphic units and shallow benthic foraminiferal zones (SBZs) after Serra-Kiel ACCEPTED MANUSCRIPT 598 et al. (1998). The shallow benthic zones written in red color are re-interpreted after
599 previous dating by Boukhary et al. (2011) and Said and Issawi (1964). The violet
600 shaded rectangle is the relative timing proposed for the iron mineralization.
601
602
603
604
605
606
607
608
609
610 611 MANUSCRIPT 612
613
614
615
616
617 618 ACCEPTED 619
620
621
622 ACCEPTED MANUSCRIPT 623 PLATE CAPTIONS
624 Plate 1.
625 1–9: Nummulites gizehensis (FORSKÅL, 1795) 1–7 megalospheric forms; 8–9
626 microspheric forms; 1, 3, 5, 6 and 8 equatorial sections; 2, 4, 7 and 9 external views.
627 Specimens 1, 2, 5-9 from sample LF 6B; 3 and 4 from sample LF 2A. 10–14:
628 Nummulites praelorioli HERB & SCHAUB , 1963 ; 10–13 microspheric forms; 14
629 megalospheric form. All specimens are equatorial sections. Specimens 10, 11 and 14
630 from sample LF 3B; 12 and 13 from sample LF 2B. 15–18: Nummulites migiurtinus
631 AZZAROLI , 1952 15–17 microspheric forms; 18 megalospheric form; 15, 16 and 18
632 equatorial sections; 17 external view. All specimens from sample LF 4B. 19–24:
633 Nummulites beaumonti D’A RCHIAC & HAIME , 1853 ; 19–21 microspheric forms; 22–
634 24 megalospheric forms; 19, 21–24 equatorial sections; 20 external view. Specimens 635 19–21 from sample LF 1C; 22–23 from sampleMANUSCRIPT LF 8B and 24 from sample LF 7B. 25: 636 Nummulites biarritzensis D’A RCHIAC & HAIME , 1853 ; microspheric form; equatorial
637 section. Specimen from El Gedida ironstone sample. 26: Nummulites syrticus SCHAUB ,
638 1981 ; microspheric form; equatorial section. Specimen from sample LF 1B. 27–28:
639 Nummulites discorbinus (S CHLOTHEIM , 1820) ; microspheric forms, 27 equatorial
640 section; 28 external view. Specimens from sample LF 9B. 29-30: Nummulites lyelli
641 D’A RCHIAC & HAIME , 1853 ; megalospheric forms; 29 equatorial section; 30 external
642 view. Specimen from El Gedida ironstone sample. ACCEPTED ACCEPTED MANUSCRIPT
Lithostratigraphic units Sedimentary facies Petrography Palaeoenvironments Slightly-indurated sandy limestone and Sandy glauconitc packstone, Abundance of sandy and silty glauconitic limestone beds are intercalated fossiliferous siltstone and oyster detrital grains as well as Upper with fossiliferous siltstone/claystone beds. rudstone are common. occurrence of glauconite grains Hamra Sandy and silty grains in the limestone beds altogether with the fossil grains are more abundant to the top. The carbonate reflects shallow water near the beds form banks of macrofossils. clastic source in restricted lagoonal Fossil conten t – barren of Nummulites , but environments. rich in macrofossils, e.g. Ostrea clotbeyi, O. multicostata, Carolia placunoides , and El Hamra Turritella sp. Formation Bedded to massive limestone locally Variety of carbonate fabrics with Deposition in inner to middle ramp showing flat to concave-up, reef-like variable skeletal grains including settings of moderate to high Lower structure. Indurated to friable limestone fossiliferous (namely energy, as indicated by occurrence Hamra beds include few marl interbeds. Coralline nummulitic) packstone, bivalve of banks of abraded nummulitid debris and burrows are common. (namely oysters) rudstone, tests, coralline debris, gastropod Fossil content - high faunal diversity, i.e. fossiliferousMANUSCRIPT shells, and oysters showing grain Nummulites (N. migiurtinus, N. gizehensis, packstone/grainstone. (but matrix-bearing)-supported N. discorbinus, N. beaumonti ), oysters carbonate fabrics. (Ostrea clotbeyi, O. multicostata, Carolia placunoides ), gastropods (e.g. Turritella sp.), and echinoid spines. Homogeneous chalky limestone showing Carbonate fabrics are Deposition in a low-energy middle characteristic snow whitish color at large homogeneous and comprise ramp setting as indicated by lack Qazzun Formation outcrop scale. Boundaries between the mainly nummulitic wackestone of tractive sedimentary structures, chalky limestone beds are not well defined with minor occurrence of grain- low abrasion of the nummulitid and internal structure is mostly massive. supported carbonate. tests, and abundance of mud- Fossil content - nummulitidsACCEPTED ( Nummulites supported carbonate fabrics. praelorioli, N. syrticus and N. migiurtinus ). Thick-bedded dolostone overlain by Stromatolite-like laminated Deposition in intertidal to ACCEPTED MANUSCRIPT
stromatolite-like laminated dolostone. dolostone formed of mudstone supratidal environments Cross-bedded bivalve dolostone at the top. to wackestone with scattered undergoing episodic desiccation Upper Local occurrence of desiccation cracks, bioclasts. Local occurrence of and high salinity conditions, which sequence rhizoliths and bioturbation tubes. pseudo-morphs of evaporite resulted in formation of evaporite Fossil content - scarcity of fossils, mainly crystals replaced by quartz. minerals and development of small reworked nummulitid tests. Bivalves Bivalve-rich beds show exposure features. These are abundant in the uppermost part of the wackestone to packstone fabrics, conditions are also indicated by sequence where they occur as densely- the bivalve shell being silicified very low faunal diversity together Naqb packed bivalve shells. and/or calcitized. with small size of the foraminifer Formation tests. Variety of sedimentary facies including Carbonate fabrics are varied Deposition of the carbonate thinly-bedded dolostone/marly dolostone, concomitantly with the variety sediments of the lower sequence cross-bedded oolitic and fossiliferous of sedimentary facies. Bioclastic took place in a lagoon – inner shelf Lower dolostone, and massive non-fossiliferous packstone and oolitic grainstone environment with development of sequence dolostone. abound in the middle part of the moderate-to-high energy bioclastic Fossil content - nummulitids, bivalve and sequence. and oolitic shoals. This gastropods dominant at the base of the Silicification,MANUSCRIPT i.e. replacement environment became progressively sequence whilst dasyclads, nummulitids, and/or cementation by quartz of shallow until complete exposure and alveolinids are abundant in the middle/ the carbonate grains is common. and further development of a upper part of the sequence. paleokarst.
ACCEPTED ACCEPTED MANUSCRIPT
29° 00` E 29° 10` E 29°2` 0 E
Ghorabi El-Behour area X mine 210 Gar El Hamra N 28° 30` N X 316 Teetotum Hill
221
260 X
El Gedida mine El Harra 250
mine 26°E 28°E 30°E 32°E 34°E MEDITERRANEAN SEA N
140 252 30°N Qattara Cairo SINAI Depression Faiyum
28°N Bahariya Farafra RED SEA 26°N
Bahariya Depression 4 km 24°N 150 km 28° MANUSCRIPT 20` 22°N N
X
Bahariya Fm. Naqb Fm. Qazzun Fm. El Hamra Fm. Continental Radwan Fm. Iron ore deposits Syncline Anticline Faults & Studied Conical Roads carbonate unit folds folds thrusts sections hills
ACCEPTED ACCEPTED MANUSCRIPT
A B N 3 Dolostone Fm. Seq. Fossils Lithology 28° 29´ 42 ´´ N Silicified Fe dolostone Fe Limestone 2 Fe 4 Sandy 4 4 3 L.S 3 Chalky 1 3 Fe L.S 3 Fe Argillaceous limestone 28° 29´ 30 ´´ N Marlstone Fe 200 m Fe 29° 21´ 55´´ E 29° 22´ 04´´ E Siltstone/ Fe claystone C Upper Sequence Fe Sandstone 2 Paleokarst Breccia
1 10 m Fe Iron
Naqb Fm. Paleokarst
Bahariya Fm. Unconformity
Naqb Formation Fe Lithology Fe MANUSCRIPTFe Fe Gastropods D Radwan Fm. Fe Upper Hamra Fe Echinoids Fe 2.0 m Larger benthic
Lower Sequence foraminifera
Algae Lower Hamra 1.0 m Fe El Hamra Fm. ≈100 m Fe Bivalves 0.0 m Fossils Bahariya Formation
Qazzun Fm. ACCEPTED Fm.Seq.ACCEPTEDLithology MANUSCRIPTSamplesNummulites species Radwan Fm.
N. migiurtinus
N. syrticus
N. praelorioli
N. beaumonti
N. gizehensis N. discorbinus Seq. Lithology Samples N. sp.
80 m
Seq. Lithology Samples N. sp.
N. beaumonti
N. gizehensis 40 40 m
Continental unit
m Upper Hamra
N. migiurtinus
N. gizehensis N. discorbinus 60
LF 9B
Upper Hamra
Upper Hamra 20 LF 4C 20 El Hamra Formation LF 8B LF 6A LF 3C LF 5A LF 4A LFMANUSCRIPT 7B LF 3A 40 LF 2C
LF 2A Lower Hamra LF 6B LF 1C
LF 1A 0 LF 5B Lower Hamra 0 Qazzun LowerBase Hamra Fm. unexposed LF 4B
20 (C) (A)
ACCEPTEDLF 3B
LF 2B
Qazzun Formation LF 1B 0 Naqb Fm. (B) A BACCEPTED MANUSCRIPT C
500 µm 500 µm 200 µm
D E F
500 µm 200 µm
MANUSCRIPT
ACCEPTED A ACCEPTEDB MANUSCRIPT
500 µm 500 µm C D
1 mm 500 µm
MANUSCRIPT
ACCEPTED A 10 m Stromatolite-like ACCEPTED MANUSCRIPT ironstone
Brecciated, UpperSeq. non-fossiliferous ironstone Oolitic, fossiliferous
Naqb Fm. ironstone
Manganiferous Lower Seq. ironstone
Naqb Fm. 0 ()Unaltered clays Sandstone and shale intercalations Bahariya Fm. Fm.
Bahariya
B Sandstone Radwan Fm. G G G Glauconitic clays Upper Hamra Pisolitic ironstone Lower Fossiliferous Hamra ironstone (X) Nummulitic Qazzun El Hamra Fm. ironstone Fm. Stromatolite-like Upper
45 m ironstone Seq. Brecciated, non- fossiliferous ironstone Lower Oolitic, fossiliferous Naqb Fm. ironstone Seq. Manganiferous ironstone
Sandstone and shale Bahariya intercalations Fm.
MANUSCRIPT
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29 10´ 34 ´´ E 29 17´ 07 ´´ E
N 28 28´ 03 ´´ N
A
B
28 25´ 23 ´´ N
A El Gedida mine B 250 5 m 4 3 6 3 2 2 4 200 1 m 0 km 2 4 6 8 10 km 1 2 3 4 MANUSCRIPT5 6 Bahariya Fm. Naqb Fm. Qazzun Fm. El Hamra Fm. Radwan Fm. Continental carbonate unit Ironstones
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A B C
500 µm 500 µm 500 µm D E F
500 µm 500 µm 500 µm G H I
500 µm 500 µm 25 µm MANUSCRIPT
ACCEPTED ACCEPTED MANUSCRIPT Nummulites Species SBZ Time Unit Rock Unit
Upper Hamra
Priabonian
late N. biarritzensis N. lyelli SBZ
17 Bartonian
early
N. beaumonti SBZ 16 late Lower N. gizehensis N. discorbinus Hamra
SBZ El Hamra Formation 15 SBZ
14 middle N. migiurtinus
Lutetian Qazzun N. praelorioli N. syrticus SBZ 13 Formation
early SBZ 12
late
Ypresian MANUSCRIPT
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Age Rock unit SBZ Section Main lithologies Radwan Oligocene Formation Sandstone Fossiliferous siltstone SBZ1/ 9 Fossiliferous glauconitic SBZ18 packstone Fossiliferous siltstone
Bartonian/
Upper Hamra
Priabonian Fossiliferous wacke stone/ SBZ1/ 7 packstone SBZ15 Nummulitic wacke stone/
El Hamra Formation packstone Oyster rudstone m
Lutetian Lower Hamra SBZ1/ 4 Fossiliferous wacke stone/
middle-late (60 ) SBZ13 packstone
Lutetian Qazzun SBZ Nummulitic wacke stone/
early Formation 13 packstone Lutetian (30m )
MANUSCRIPTFe Bivalve dolostone Fe Fe Stromatolite -like laminated dolostone Fe Fe Fine laminated non-fossiliferous dolostone
Upper Sequence
Naqb Formation Brecciated non - fossiliferous SBZ9/
Ypresian dolostone SBZ6 Fe Oolitic and fossiliferous Fe dolostone
13 m
middle-late Ilerdian
() Nummulitic dolostone
Lower Sequence Cenomanian Bahariya Formation Siliciclastic rocks ACCEPTED ACCEPTED MANUSCRIPT
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ACCEPTED ACCEPTED MANUSCRIPT Highlights
• The chronostratigraphic framework of Eocene succession in Central Egypt is
updated.
• The carbonate succession represents a long record of Eocene strata.
• Eight Nummulites species spanning the late Ypresian-early Bartonian are
identified.
• Ironstone formation took place later than the early Bartonian, mostly in the
Priabonian.
• Age dating of iron mineralization is crucial to explain its genesis.
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