J. Appl. Geophys., Vol. 3, No. 2, September 2004, 177-187
PALEOMAGNETISM OF THE UPPER CRETAEOUS
BAHARIYA FORMATION, BAHARIYA OASIS,
WESTERN DESERT, EGYPT
By
Hatem Odah
National Research Institute of Astronomy and Geophysics,
11722 Helwan, Cairo, Egypt
ABSTRACT
A total of 70 samples were collected at 14 sites from the Upper Cretaceous Nubia
Sandstones of Gebel El Dist, Bahariya Formation, Bahariya Oasis, Western Desert for
paleomagnetic and rock magnetic studies. Rock magnetic measurements indicated that the
main magnetic carrier is hematite. Magnetite and goethite are also present as subordinate
constituents. Careful thermal demagnetization was applied to the samples because of the
existance of the high coercive minerals. Most of these samples carry a weak but stable
remanant magnetization. The demagnetization yielded a mean magnetization of Dec =
197º and Inc = -32º (α95 = 7.1ο), corresponding to a pole position of Lat. = 71ο N and
Long. = 151ο E. The obtained pole position is inconsistent with the Upper Cretaceous
African reference pole, but falls closer to the Eocene (40 Ma age) pole indicating that the
studied rocks could have suffered a complete remagnetization during the late Eocene time.
these rocks became magnetized. The
interpretation of the magnetic record of
these sediments depends critically on
when and how the natural remnant
magnetization (NRM) was acquired and
what
postdepositional
sort
of
modifications occurred.
In this study, the Bahariya
Formation, Bahariya Oasis, Western
Desert is subjected to paleomagnetic and
rock magnetic investigations. Despite the
assigning Lower Cenomanian age for the
Bahariya Formation by many authors (e.
g. Morsy, 1987; Hanafy et al., 1996;
Abdel-Monem et al., 2003), the age of this
formation and the postdepositional
processes affected it still a matter of
controversy. Therefore, the purpose of
this study is to use paleomagnetism in
INTRODUCTION
The Nubia Sandstones have been
intensively used in paleomagnetic
investigations for their high content of
magnetic minerals and measurable
magnetism. The first paleomagnetic
investigation on the Egyptian Nubia
Sandstones was carried out by El Shazly
and Krs (1970) who examined some
Nubia Sandstones from Wadi Natash in
the Eastern Desert. Later, the Nubia
Sandstones were studied in several
localities, ex. El Shazly and Krs (1973);
Hussain et al. (1976); Schult et al. (1978,
1981); Hussain and Aziz (1983); Ibrahim
(1993); Kafafy et al. (1995) and Ibrahim
et al. (1998). Despite such investigations,
the paleomagnetic data of the Nubia
Sandstones of Egypt remain a subject of
controversy due to the incomplete
understanding of the processes by which
dating and analysis of post depositional
magnetization,
hence
dating
the
177
178
Hatem Odah
postdepositional
diagenetic
processes
affected this formation. In addition, the
tectonic movements affected the sampling
area is also tested as the Bahariya Oasis lies
on the hingline between the stable and
unstable shelves (Said, 1962) and has been
subjected to subsequent phases of folding
and faulting (Morsy, 1987; El Agami, 1989).
GEOLOGIC SETTING
The term Nubia Sandstone is well
established in the stratigraphic column of
Egypt and is used to describe the
extensive sandy formations (clastic series)
overlying the Precambrian basement rocks
and those conformably underlies the
marine sequence of Upper Cretaceous
variegated shale. Similar formations of the
Nubia Sandstones are also known in some
surrounded countries in North Africa and
Middle East. Because fossils are generally
absent or rare in these rocks, it is difficult
to assign them definite ages. The
questions of the age of the Nubia
Sandstones and their environments of
deposition have been discussed in several
publications (Said, 1962; Pomeyrol, 1968;
Soliman, 1971; El Shazly and Krs, 1973;
Klitzsch et al. 1979; Ward and McDonald
1979; Van Houten et al., 1984; El
Khoriby, 2003). The age of the Nubia
Sandstone of the Eastern Desert according
to Van Houten et al. (1984) ranges from
Early Cretaceous to Late TuronianConiacian time. The studied Bahariya
Formation, Bahariya Oasis, Western Desert
is assigned a Cretaceous age (Cenomanian)
by many authors (e.g. Morsy, 1987; Hanafy
et al., 1996, and Abdel-Monem et al., 2003).
This Cenomanian age ranges from 98 to 93
Ma according to the Geological Time Scale
for the Phanerozoic (Gradstein and Ogg,
1996).
Amer (1973), Khalifa (1977) and
Franks (1982) assumed that the tectonism
in the Bahariya Oasis was essentially due
to the Syrian arc belt deformations which
continued in the post Upper Cretaceous
and pre-Lower Eocene. The most
important tectonic event occurred during
the late Cretaceous and early Tertiary was
probably related to the movement of
North Africa plate towards Europe. This
resulted in the elevation and folding of the
north Western Desert along ENE and
WSW and development of faults.
Moreover, by end of the Oligocene, the
Bahariya Oasis underwent faulting by
ruptures producing E –W fractures and
conjugate shear fractures. Among these
fractures, thermal and volcanic activities
took place (Morsy, 1987). Issawi et al.
(1999) stated that there were periods of
heavy rain fall and activity of surface
water during Oligocene. Therefore, after
the NE – SW anticline of the Bahariya
Oasis has formed and fractured by faults,
both running water and wind excavated
the center of this anticline to form a great
anticlinal depression (reverse topography,
El Agami, 1989). The Cenomanian
Bahariya Formation at the Bahariya Oasis
was therefore subjected to diagenetic
changes by meteoric water and thermal
activity after their deposition and until the
Miocene. One of these diagenetic phases
occurred during the Late Eocene and
Early Oligocene.
The Bahariya Formation forms most
of the cliffs and slops surrounding the
Bahariya depression in the Western
Desert. It also forms the southern and
northern parts of depression floor and
covers most of Farafra depression (Morsy,
1987). The stratigraphy and structure of
this formation have been discussed by
several authors (e.g. Ball and Beadnell,
1903; Morsy, 1987). The lithostratigraphic section of Bahariya Formation
given by most authors is nearly similar
where the section of Gabal El Dist is
considered as the type locality of this
formation. In general, the formation consists
of cross-bedded, variegated sandstone
alternating with clay and shale and could be
P a le o m a g n e tis m o f T h e U p p e r Cr e ta e o u s B a h a r iy a F o r m a tio n
subdivided into two members, from base to
top, the Mandisha Member and El Dist
Member (Morsy, 1987).
SAMPLING AND METHODOLOGY
In this study, 70 specimens out of 14
sites were obtained from Bahariya
Formation at Gebel El Dist. The sampling
site (lat. 28º 55´ N and long. 28º 25´ E) is
located to the west of El Gidida mine (Fig.
1). All samples were collected as hand
samples that were oriented using a magnetic
compass.
28° 40'
28° 50'
29° 00'
29° 10'
29° 20'
Study area
El Gidida Mine
28°
30'
Sampling Section
G. El Dist
0
5
10 Km.
Quaternary Dep.
Qatrani Fm.
El Heiz Fm.
Iron ore Dep.
Eocene Dep.
Bahariya Fm.
Haddadin Basalt
Duwi Fm.
Sabaya Fm.
28°
20'
Fig. 1. Location map showing the
sampling locality.
Cores of 2.5 cm diameters were later
drilled in the laboratory and sliced into
specimens of 2.2 cm length to suite
paleomagnetic measurements. The structural
attitude of the studied formation was
carefully measured and was found to be
strike/dip = 270/25 for the structural
correction of the magnetic vectors back to
their pre-tilting position.
All the magnetic measurements were
carried out at the Paleomagnetic Laboratory
of the Department of Earth and Planetary
Science, University of Tokyo, Japan. The
natural
remnant
magnetization
was
measured using the fully shielded 2GSQUID Cryogenic magnetometer. The
present geomagnetic field direction at the
179
study area is Dec.= 2.863° and Inc.= +
41.867° calculated using the 9th generation
of the International Geomagnetic Reference
Field program (http://www.geomag.bgs.ac.
uk / gifs / igrf_form.shtml).
ROCK MAGNETISM
The Fe oxides which contribute to the
NRM are identified using blocking
temperature spectra (Irving and Opdyke,
1964),
and
isothermal
remnant
magnetization IRM (Dunlop, 1972). The
timing of the NRM acquision of the studied
rocks was obtained by comparing the
obtained pole position with the previously
published pole positions. Some pilot samples
representing the fourteen sampled sites were
subjected to rock magnetic measurements to
identify the main magnetic carriers using a
vibrating sample magnetometer (VSM,
MicroMag
3900)
of
Princeton
Measurements Corporation.
Thermomagnetic curves (Js-T curves)
of the pilot samples were measured in
helium atmosphere (using the MicroMag).
Figure 2a is an example of the Js-T curves
showing three Curie temperatures at about
216 οC, that may reveal the presence of
goethite, at about 575 οC, indicating the
presence of magnetite, and at 700 οC at the
end of the experiment indicating the
presence of hematite.
Hysteresis loops of the studied
samples show wasp tail shape without
saturation even in field of 800 mT. Figure 2b
shows an example of the studied hysteresis
loops indicating the presence of low
coercivity phase (“magnetite-like” phase)
and high coercivity phase (“hematite-like”
phase) and/or (“goethite-like” phase)
confirming
the
results
of
the
thermomagnetic curves. The calculated
mean hysteresis parameters are: the
saturation magnetization Ms = 1.34 Am2/kg;
the saturation remanence Mrs = 2.15 Am2
/kg; the ratio of Mrs / Ms = 1.6 and the
coercive force Hc = 180.38 mT.
180
Hatem Odah
14
6
A
12
4
10
Ms = 1 34 *
Mrs = 2 15 *
Mrs / Ms =
Hc = 180 4
B
-2
-2
2
8
0
6
-
4
-
2
0
0
100
200
300
400
Temperature (
500
o
600
700
800
C)
-
-
0
Applied Field
400
800
Fig. 2: Rock magnetic measurements of the studied samples: a) Thermomagnetic curve of
sample 1 from site 3 with Curie temperatures showing the presence of hematite, magnetite and
goethite, and b) Hysteresis loop of sample 1 from site 3 shows the presence of hematite.
PALEOMAGNETISM
All the studied samples showed
weak magnetization with initial NRM
intensities ranging from 0.5 to 0.9 mA/m,
which was well above the noise of the
squid. It was noticed that the intensity of
magnetization for the studied sandstone is
directly related to the degree of red
pigmentation i.e. the more reddish the
sample, the higher its intensity.
In order to isolate the characteristic
remnant magnetization, all the collected
samples were subjected to a full range of
thermal demagnetization in steps up to
690 OC. Thermal demagnetization was
effective and yielded well resolved
paleomagnetic components. Most of the
treated samples showed erratic behavior
during the beginning of the demagnetzation that was explained by the removal
of a soft overprint that may be carried by
goethite. This soft component was
removed after heating to about 100 OC in
most of the samples. Direction and
intensity of the demagnetization data were
plotted on the orthogonal projection
(Zijderveld, 1967) and the linear segment
was selected to perform the principle
component analysis (Kirschvink, 1980)
which is based on the determination of
the demagnetization trajectories heading
towards the origin. Typical examples of
the magnetic behavior during the thermal
demagnetization are shown in Figure 3.
Most samples showed stable magnetization,
intensity decreasing continuously and
directions changed apart from the first few
steps, linearly heading toward the origin of
the orthogonal plot. Most of the NRM
intensity was lost by 600 οC (Fig. 3).
The unit weigh is given to each site
and the site mean directions were
calculated and combined together for
calculating the overall mean direction
using Fisher statistics (Fisher, 1953). The
obtained results before and after bedding
correction are listed in Table 1 and
plotted in Figure 4 (a and b).
DISCUSSION AND CONCLUSIONS
All sites show negative inclination
with consistent directions, except sites 8 and
14 with a small deviation (Table 1; Fig. 4).
The calculated overall mean direction after
the bedding correction (Dec = 197º and Inc
= -32º) differs from the direction of the
present Earth's magnetic field at the study
area (Dec. = 2.863°, Inc. = 41.867°) i.e. the
magnetization is not related to the present
Earth’s magnetic field.
P a le o m a g n e tis m o f T h e U p p e r Cr e ta e o u s B a h a r iy a F o r m a tio n
181
N, UP
2
A
1.5
NRM
1
575
0.5
0
NRM
0
100
200
300
400
500
600
700
o
Temperature ( C)
N, UP
8
7
B
6
5
4
W, W
3
550
2
1
NRM
0
0
100
200
300
400
500
600
700
o
Temperature ( C)
N, UP
2
C
1.5
1
W, W
550
0.5
NRM
0
0
100
200
300
400
500
600
700
o
Temperature ( C)
Fig 3. Characteristic examples of the thermal demagnetization and intensity
behaviour of the samples.
182
Hatem Odah
Table 1. Site-mean directions of the studied sandstone samples with the overall
mean direction.
Site No.
α95 (ο)
Directions
Paleomag. Pole
Inc. (ο)
Dec. (ο)
Lat. (°N)
Long. (°E)
1
5.4
197
-17
65
166
2
4.8
201
-23
64
155
3
6.0
204
-20
61
153
4
5.3
220
-32
52
127
5
5.0
215
-33
57
129
6
6.2
202
-15
61
161
7
4.3
217
-21
51
138
8
8.1
239
-14
31
128
9
4.5
203
-24
64
152
10
5.3
197
-13
63
170
11
6.2
218
-21
51
138
12
6.2
208
-16
57
151
13
5.1
204
-16
60
157
14
7.9
199
-47
73
116
BBC
7.1
209
-23
58
145
Mean
ABC
7.1
197
-32
71
151
α95 = radius of the cone of the 95% confidence;
Dec and Inc. = declination and inclination, respectively;
Lat. = north latitude; Long. = east longitude.
BBC, ABC = Mean before and after bedding correction respectively.
N
N
Fig. 4. Site mean directions with the overall mean of the sites before and after
bedding correction.
P a le o m a g n e tis m o f T h e U p p e r Cr e ta e o u s B a h a r iy a F o r m a tio n
The obtained paleomagnetic pole
position from this study after bedding
correction lies at Lat. = 71° N and long. =
151° E (α95 = 7.1ο). Unfortunately, a fold
test could not be applied in the present study
as
the
sampled
formation
tilts
homogeneously in the same direction.
The resultant pole still deviates from
the poles of 90 - 100 Ma of the Cretaceous
African reference poles of Besse and
Courtillot (2002). These data were extracted
from
the
computerized
Global
Paleomagnetic Database (GPMD V3.3) of
McElhinny and Lock (1995). On the other
side, the resultant pole shows a good
consistency with pole positions of age
window 30 – 50 Ma (Fig. 5).
This inconsistency between the
resultant pole and Cretaceous African
reference pole (90 Ma) may refer to a
tectonic movement that could have affected
the sampling area. A trial was done to
calculate the amount of rotation with respect
to the 90 Ma reference pole of Africa of
Besse and Courtillot (2002). The mean of
pole positions for the ages from 82 -111 of
GPMD V3.3 of McElhinny and Lock (1995)
is calculated to lie at Lat. = 66.4° N and
long. = 247.2° E (α95 = 6.3ο).
The
calculated rotation value is found to be 16.3ο
± 6ο (Rot-flat program). This rotation angle
is too large to be expected in this area and
has no geological explanation. Therefore,
such tectonic interpretation of this inconsistency is excluded.
Therefore, another interpretation for
this inconsistency of the resultant pole with
the pole positions of 90 - 100 Ma and its
consistency with the pole positions of 30 –
50 Ma (Fig. 5) is that these sediments have
suffered post depositional remagnetization
that affected the Bahariya Formation and at
the age of 30 – 50 Ma.
Abdeldayem (1999) has constructed a
Cenozoic APWP for Africa. He has
computed the mean poles for the
appropriated time windows and plotted them
183
together with the 62 Ma pole of Besse and
Courtillot (1991). It was therefore decided
to plot the resultant pole against the recently
Cenozoic APWA of Abdeldayem (1999) to
try to test the date of this magnetization. As
Figure 6 shows, the present pole coincides
with the 40 Ma (Late Eocene) pole.
180
74
90
44
BNS
34
89 30
82
50
72
37
60
93
270
24
90
80
0
Fig. 5. Comparison between the
obtained Bahariya pole position with the
African reference poles from 24 - 90 Ma age
of Besse and Courtilliot (2002) extracted
from the GPMD V3.3 Database of
McElhinny and Lock (1995).
180
BNS
62
40
18 30
9
270
60
90
0
Fig. 6. Comparison between the obtained
Bahariya pole position with the APWP of
the African Cenozoic of Abdeldayem
(1999).
184
Hatem Odah
To confirm this age, the resultant
pole was further plotted with some
selected genuine poles representing the
Eocene age for Egypt (Fig. 7 and Table
2). As Figure 7 shows, there is a clear
consistency between the resultant pole
and the other poles of the Mokattam and
Maadi limestone of Kafafy et al., (1995)
and Mokattam limestone of Abdeldayem
(1999).
180
4
1
3
2
60
270
90
0
Fig. 7. Comparison between the
present studied pole position with some
selected Eocene poles from Egypt.
All these results confirm that the
studied formation was remagnetized
postdepositionally, most probably of 40 Ma
age. The hypothesis of the remagne-tization
processes for the studied rocks is more
accepted and came combatable with the
available geologic information which
indicates that the studied formation has been
subjected to subsequent of folding, faulting
and diagenetic changes by the act of
meteoric water and thermal activity (El
Kammar and El Kammar, 2002). One of
these phases of diagenetic changes, which
occurred during the Late Eocene and Early
Oligocene caused the oxidation and
dissolution of the primary iron bearing
minerals (magnetite and pyrite) and
precipitated the iron in the form of hematite
and goethite. In addition, the climate during
most of the Tertiary was humid, implying
that ample runoff water was available
(Bown et al., 1982). Moreover, Issawi et al.
(1999) stated that there were periods of
heavy rain fall and activity of surface water
during Oligocene that could led to addition
and precipitation of secondary magnetic
minerals, more probably hematite. This is in
agreement with the rock magnetic results
that indicate that hematite is the main carrier
of the magnetization. The studied formation
has therefore acquired a secondary chemical
remnant magnetization CRM that replaced
all pre-existing magnetization and therefore
bears no relationship to the depositional age
of the rock. The remagnetization hypothesis
is therefore more acceptable and more
realistic as it was assumed by similar
paleomagnetic studies carried out on Nubia
sandstones from other areas (e.g., Kafafy et
al., 1996; Ibrahim et al., 1998) which
proposed a regional remagnetization due to
their porous nature that allowed secondary
minerals to be deposited between and
around the rock grains diagenetically.
Additionally, El Khoriby (2003) stated that
the high porosity of the Nubia Sandstones
allowed the passage of groundwater that
made these rocks more susceptible to
extensive alteration. The calculated mean
inclination indicates that the magnetization
was acquired when the studied rock and the
sampling locality were at a paleo-latitude of
17.3ο north the equator (near equatorial
environment), confirming the warm and arid
weather which led to the deposition of the
hematite during the prevailing diagenetic
process. To sum up, it is concluded that the
Bahariya Formation carries a week but
stable remnant magnetization carried by
diagenitic hematite as the main magnetic
carrier while goethite and magnetite are
subordinates. A Late Eocene – Early
Oligocene (40 Ma) age is assigned for
this magnetization.
.
185
P a le o m a g n e tis m o f T h e U p p e r Cr e ta e o u s B a h a r iy a F o r m a tio n
Table 2. Some selected Eocene pole positions of Egypt.
No.
Locality
1
2
3
4
Mokattam limestone
Maadi limestone
Mokattam limestone
Bahariya Sandstone
Paleomagnetic Poles
Lat. ° N
Long. °
E
73
147
75
150
78
163
71
151
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