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SINET: Ethiop. J. Sci., 37(1):31–42, 2014
© College of Natural Sciences, Addis Ababa University, 2014 ISSN: 0379–2897
PALEOMAGNETIC DATING OF THE ENTICHO SANDSTONE AT NEGASH
LOCALITY (TIGRAI REGION, NORTHERN ETHIOPIA): IMPLICATION
FOR QUATERNARY REMAGNETIZATION
Tesfaye Kidane
School of Earth Sciences, College of Natural Sciences, Addis Ababa University, PO Box 1176, Addis
Ababa, Ethiopia. E-mail: tesfaye.kidane@aau.edu.et; tesfayek@yahoo.com
ABSTRACT: New paleomagnetic result is reported from the Enticho Sandstone (Late Paleozoic age)
at Negash locality in Northern Ethiopia. Twenty-three paleomagnetic core samples were collected from
three sites for paleomagnetic investigations. Specimens were subjected either to progressive alternating
field (AF) or thermal (TH) demagnetization techniques. Rock magnetic experiments revealed major
magnetization carriers to be titano-magnetite and titano-hematite. Well-defined viscous remanent
magnetizations (VRM) components are removed by intermediate AF fields of between 20–30 mT and
heating above 600ºC. These magnetizations defining straight-line segments are directed towards the
origin and interpreted as the Characteristic Remanent Magnetization (ChRM). Directions of
magnetizations and site-mean directions in the in-situ coordinate results in Dec = 356.7°, Inc = 24.9º
(N=23, K = 43, 95 = 4.7°). Paleomagnetic stability tests confirmed that the ChRMs identified are
secondary and postdate age of deposition and tectonic tilting. The paleomagnetic pole position Long =
296.6ºE, Lat = 86.7ºN (A95 = 5.0º, N = 23) obtained from these data when plotted with the Apparent
Polar Wander Path (APWP) of Africa (Besse and Courtillot, 1991, 2003; Cogné, 2003) gives a Quaternary
age for the magnetization of Enticho Sandstone at Negash locality. Comparison of this result with that
of Enticho Sandstone at Enticho locality, which had primary magnetization fingerprints (Tesfaye
Kidane et al., 2013) with ages of between 260 Ma and 270 Ma (Late Carboniferous – Early Permian)
implies that the Quaternary age for the Enticho Sandstone at Negash is a recent remagnetization.
Key words/phrases: African Permian paleogeography, Ethiopia, paleomagnetism, primary
magnetization, remagnetization
INTRODUCTION
Background
For more than four decades now, the fundamen-
tal question in Late Palaeozoic Era has been the
paleogeographic configuration of continents
within Pangaea supercontinet, (e.g., Van der Voo,
1993; McElhinny and McFadden, 2000; Muttoni et
al., 2003; Domeier et al., 2012). Although there is a
broad consensus on the paleogeographic confi-
guration of Pangaea in the Early Jurassic to be
that of Pangaea ‘A’ (Van der Voo, 1993), there is
ongoing debate regarding the exact configuration
of Pangaea ‘A’ during Late Carboniferous and
Late Triassic since Pangaea ‘B’ was introduced by
Irving (1977). Reconstruction of type ‘A’ (Bullard
et al., 1965; Van der Voo and French, 1974) is
minor modification of Wegener’s (1915) original
reconstruction and represents a model accepted
by most geoscientists as the likely configuration,
just before the opening of the Atlantic Ocean in
the Early Jurassic (Van der Voo, 1993). However,
it is incompatible with available paleomagnetic
data triggering alternative reconstructions. Re-
construction model ‘B’ was then proposed and
follow-up paleomagnetic studies supported this
reconstruction and asserted its validity for
Carboniferous to Triassic Periods (Kanasewich et
al., 1978; Morel and Irving, 1981). Pangea ‘B’
configuration encountered another problem; it
requires dextral – slip of Gondwana farther to the
east by the order of ~3000 km with respect to
Laurasia, on the basis of this paleomagnetic data.
This scenario was disputed for scarcity of geo-
logical evidences supporting such a mega shear.
This alternative model is, therefore, discarded on
the suspicion that the paleomagnetic interpreta-
tion relied on poor quality data included from
sediments with known geomagnetic inclination
error and poor age control inserting latitudinal
artifacts (Rochette and Vandamme, 2001; Muttoni
et al., 2003). Domeier et al. (2012), after detailed
re-analyses of late Paleozoic early Mesozoic
paleomagnetic data concluded that, existing pa-
32 Tesfaye Kidane
leomagnetic data could be reconciled with
Pangaea during early Mesozoic and late Permian.
Recent high-resolution paleomagnetic study from
the Paleozoic Tillite of Northern Ethiopia
brought new hopes of rectifying this on-going
debate (Tesfaye Kidane et al., 2013) about
Pangaea paleogeography. This work underlined
the need for more paleomagnetic study on rocks
of similar ages from this part of Africa. It also
invites more comprehensive Paleozoic paleomag-
netic investigations to be made in different
outcrops and discriminate rock types at various
localities recording primary and secondary
magnetizations. Accordingly the paleomagnetic
results from the glacial sandstone named Enticho
Sandstone from the Negash locality, northern
Ethiopia (Fig. 1A) are presented here.
Geological setting
The Paleozoic sediments in northern Ethiopia
were first described by Dow et al. (1971) and Beyth
(1972a and b). The two works differentiated two
facies: Glacigenic Sandstone and tillite, described
and named “Enticho Sandstone” and “Edaga Arbi
Tillite”, respectively.
The Enticho Sandstone is the lowermost sedi-
mentary unit exposed in different areas of Tigrai
Region: along the margins of the Mekele Basin,
along the Adigrat – Adwa ridge, and also along the
Astbi horst; it unconformably overlies the Precam-
brian basement rocks. This unit has a variable
thickness in different places but is believed to be
less than 160 meters. It generally has white color
and of medium grain size with silt laminations and
thin flat-lying bedding. In the sampling site;
Belessa area of Negash locality (Fig. 1), the
sandstone, outcrops along a major E–W oriented
fault and is coarse-grained, white colored, cross
bedded (120º/12ºE) and steeply tilted
(095º/16ºSW). In this particular outcrop, iron
encrustations and ferruginous layers are present
along the bedding plane (Fig. 1B).
Attempts have been made to determine the exact
age of these sediments. However, because age
diagnostic fossils (see Tesfaye Kidane et al., 2013
and references therein) are missing, no precise age
could be determined. Bussert and Schrank (2007)
have extracted Palynomorphs from these sedi-
ments at the upper part and had assigned an
Upper Ordovician age for the lower part of the
Enticho sandstone and concluded this later part to
be equivalent to those in Eritrea. Recent and
reliable paleomagnetic age estimation for the
Edaga Arbi Glacials (Tesfaye Kidane et al., 2013)
and for the Enticho sandstone at Enticho yielded a
Late Carboniferous – Early Permian age. However,
more robust spatially distributed paleomagnetic
data are required in order to constrain the paleo-
geographic position of Pangaea configurations. For
this sustained effort, it has become imperative to
collect samples of the sandstones from different
places. In this study, samples of Enticho sand-
stones from Negash locality are analyzed in detail
and results are then compared with those of the
Enticho Sandstone from Enticho locality reported
in the work of Tesfaye Kidane et al., (2013).
MATERIALS AND METHODS
Paleomagnetism
Sampling
A total of 23 paleomagnetic core samples from the
Enticho Sandstone outcrops at Negash locality
(Fig. 1) were collected at three paleomagnetic sites
using a pomeroy portable drill. Core samples were
oriented with an orienting fixture mounted with
standard magnetic compass following the routine
of paleomagnetism. Each paleomagnetic site is
defined as a different stratigraphic level within the
Enticho Sandstone layers. Exposures of the Enticho
Sandstone are observed north of a prominent E-W
trending normal fault overlying the basement
rocks. The existence of the fault is recognized
mainly because of a creek that developed later
along this fault line that juxtaposes sediments to
the south and basement to the north. The Enticho
Sandstone exposures here are characterized by
being cross-bedded (120º/12ºNE) medium to
coarse-grained whitish colored and tilted
(095º/16ºSW).
Laboratory analyses
At least one specimen per core sample drilled
was used to measure directional behavior while 5
additional specimens were used to characterize
their magnetic properties. In most of the cases both
thermal (TH) or alternating field (AF) demag-
netization techniques were used in order to resolve
the directional spectrum in these samples. For
most of the samples TH technique was used more
than the AF. The paleomagnetic and rock–magnetic
experiments were done in the paleomagnetic
laboratory facility at Ludwig–Maximilians–
Universität München, Germany.
SINET: Ethiop. J. Sci., 37(1), 2014 33
Figure 1. (A) Location general geological map of the studied area in Tigrai Region Ethiopia. (B) A picture showing outcrop of the
sampled Enticho Sandstone at Negash locality.
Rock magnetic properties
Two representative specimens from the Enticho
Sandstone at Negash locality were chosen for
Isothermal Remanent Magnetization (IRM) experi-
ments (Fig. 2A). An initial steep rise in IRM up to an
applied field of 300 mT was observed in both
samples. Further, the samples show a gentle slope
(300 mT – 500 mT) followed by gradual increase in
magnetization in fields up to 2000 mT. The
magnetization of one of the samples (NPSST3–9)
couldn’t attain saturation at the highest applied
field of 2250 mT. The steep increase in
magnetization in fields up to 300 mT followed by a
34 Tesfaye Kidane
more gradual increase without reaching saturation
is diagnostic of magnetic assemblages containing
both magnetically soft (e.g., titano-magnetite) and
hard (e.g., titano-hematite) minerals. The
corresponding AF demagnetizations of these IRM
results show that ~90% of the IRM could not be
demagnetized (Fig. 2B) using the maximum
available laboratory alternating field (100 mT),
indicating predominance of magnetically hard
(high coercive) materials. The NRM intensity decay
curves show the contribution from the low
coercive or soft magnetic materials could go as
high as 50% (Figs 3A and B). These generally
indicate the Enticho sandstone at Negash is
characterized by ferromagnetic assemblages with
both magnetically soft (titano-magnetite) and hard
materials (titano-hematite) and probably with
minor contribution from iron oxyhydroxides or
ironsulfides (Fig. 2B).
Figure 2. (A) IRM acquisition experiment for representative specimens from Enticho Sandstone at Negash. (B) AF
demagnetization curve of the IRM experiment in A; the corresponding specimen names are given.
SINET: Ethiop. J. Sci., 37(1), 2014 35
RESULTS
Paleomagnetic directions
The paleomagnetic directions obtained from the
Enticho Sandstone at Negash using the AF and
thermal techniques are different. When AF tech-
nique is used more than 50 percent of the total
Natural Remanent Magnetization (NRM) remains
after the maximum field available in the laboratory
is applied (Figs 3A and B). The component of mag-
netization directions obtained at intermediate AF
fields between 20–30 mT, in the in–situ coordinates
are subparallel to the present geomagnetic field at
Negash (Fig. 3B). For AF fields below and above the
given interval paleomagnetic directions are erratic.
This progressive AF demagnetization removed
only 40–50 percent of the total NRM indicating that
magnetic materials of high coercivity are
predominant.
Figures 3 A and B. Examples of Zijderveld diagrams for specimens from the Enticho Sandstone at Negash area one treated by
Alternating Field (AF) and another by thermal demagnetization techniques. A represents the in-situ coordinate; B
represents the tectonic corrected coordinates. (The magnetic polarity is down and pointing north consistent with normal
configuration for the location).
36 Tesfaye Kidane
The results of progressive TH demagnetizations
of the Enticho Sandstone at Negash village indicate
an erratic behavior until a temperature of 625ºC
and further stepwise heating to 650ºC removed all
the NRM components. The high stability compo-
nent defines a straight-line segment directed to-
wards the origin and is considered as the Charac-
teristic Remanent Magnetization (ChRM) direc-
tions (Figs 3C and D). But the directions in the in-
situ coordinate, like the AF equivalent, are sub
parallel to the present geomagnetic field at Negash
(Fig. 3D) suggesting probably it is a Viscous
Remanent Magnetization (VRM). The AF and TH
progressive demagnetization results are consistent
and agree very well. The directions of magnetiza-
tion for specimens resulting in stable straight line
segments is determined by the best-fit line using
the least square technique of Kirschvink (1980) for
specimens with overlapping spectra and unblock-
ing temperatures, direction of magnetization is
determined by remagnetization circles of Halls
(1976; 1978).
Figures 3 C and D. Examples of Zijderveld diagrams for specimens from the Enticho Sandstone at Negash area one treated by
Alternating Field (AF) and another by thermal demagnetization techniques. C represents the in-situ coordinate where as
D represents the tectonic corrected coordinates. (The magnetic polarity is down and pointing north consistent with
normal configuration for the location)
SINET: Ethiop. J. Sci., 37(1), 2014 37
Site mean directions
The directions of magnetization determined in
either best-fit line or remagnetization circles tech-
niques are plotted on stereogram (Fig. 4). The
distribution of ChRM for all specimens of the
Enticho Sandstone at Negash village is given both
for in-situ and tectonic corrected coordinates (Fig.
4). A mean direction is then calculated for all
specimens in the in-situ and tectonic corrected
coordinates resulting in Dec = 356.7°, Inc = 24.9º
(N=23, K = 43, 95 = 4.7°), and Dec = 352.6°, Inc =
37.3° (N=23, K = 37.3, 95 = 5.0°) respectively. The
in-situ and tectonic corrected coordinates differ
slightly with the distribution being more clustered
in the in-situ coordinates and showing a bit of
scatter after the correction. All specimens of
Enticho Sandstone at Negash village have ChRM
directions of normal polarity configuration that is
not related and not antipodal to the reversed
polarities described for Enticho area above.
Figure 4. Streographic projection showing the Characteristic Remanent Directions (ChRM) as determined by both
remagnetization circles of Halls (1976; 1978) and least–square technique of Kirschvink (1980); A) in-situ coordinates
(prior to tilt correction); B) after tectonic correction and restoration to the pre-tilting position. In both cases the
corresponding overall mean directions with 95 percent confidence circles are shown by star symbols and circles.
38 Tesfaye Kidane
Site mean directions are calculated by using
Fisher (1953) statistics for those having stable end-
points and McFadden and McElhinny (1988)
statistics for combined stable endpoints and great
circles employing the PaleoMac software package
of Cogné (2003). These site mean directions of
Enticho Sandstone in the in-situ and tectonic cor-
rected coordinates are compared (Fig. 5). The site
mean directions in both coordinates have positive
inclinations with the tectonic corrected coordinate
value steeper than the in-situ one and declinations
being sub-parallel to the Earth’s axis of rotation,
consistent with current geomagnetic field direction
at sampling locality. This suggests that the ChRM
direction is VRM acquired after sedimentation or a
recent remagnetization. The mean ChRM direction
becomes consistent with the current geomagnetic
field only with in-situ coordinates and diverges
from it when tectonic correction is applied, indicat-
ing ChRM postdating the tilting of the sandstone at
Negash (Fig. 6).
DISCUSSION
Age estimation
Paleomagnetic pole positions were calculated from
the Virtual Geomagnetic Pole (VGP) of the
specimens of Enticho Sandstone at Negash locality.
This resulted in: Lon = 296.6ºE, Lat = 86.7ºN (A95 =
5.0º, N = 23) and Lon = 355.4ºE, Lat = 80.0ºN (A95 =
5.9º, N = 23), respectively for the in-situ and
tectonic corrected coordinates (Fig. 6). The
paleopole position of the Enticho Sandstone at
Negash locality was then compared with the
Apparent Polar Wander Path (APWP) of Africa (Fig.
6; Besse and Courtillot, 1991; 2003; Cogné, 2003).
The full star and full diamond symbols
respectively represent the in-situ and tectonic
corrected coordinates. Evidently the in-situ pole
position coincides with the North geographic Pole
coordinate where as the tectonic corrected
coordinates has no relation to the APWP. This
observation, together with the better grouping
Figure 5. Stereographic projection of the overall mean directions of the Enticho sandstone at Negash with the corresponding 95%
confidence circles indicated as circles around the star in in-situ and restored or tectonic corrected coordinates.
SINET: Ethiop. J. Sci., 37(1), 2014 39
Figure 6. Spherical projections with the major continents in their present day configurations and the Apparent Polar Wander Path
(APWP) curve of Africa in West African coordinates (McElhinny et al., 2003; Besse and Courtillot, 1993; 2003). The
remagnetized pole position for the Enticho Sandstone at Negash is shown by star symbol in the in-situ coordinate and
by a full diamond symbol in the tectonic corrected coordinates; the corresponding 95% confidence circle is given as
darken shades. The in-situ coordinate has smaller 95 percent confidence circle and the in-situ mean pole position is
consistent with the current geomagnetic field position (geographic north). In the tectonic corrected coordinates, the pole
gets removed away both from the north geographic pole position and the APWP curve.
of the data in the in-situ coordinate (smaller 95
percent confidence circle) suggest that
magnetization postdates the tilting of the
sediments and that the age of magnetization is in
the Quaternary Period.
Comparison with Enticho sandstone at Enticho
locality
A similar paleomagnetic investigation was
carried out recently on Paleozoic glacial sediments
of Northern Ethiopia (Tesfaye Kidane et al., 2013).
In that work, detailed rock magnetic, optical micro-
scopy and demagnetization behaviors showed that
rocks of the Edaga Arbi tillites and Enticho
Sandstone at Enticho locality retained original
magnetization of high quality (Tesfaye Kidane et
al., 2013). The age of deposition of the Edaga Arbi
Glacials in Northern Ethiopia and Enticho Sand-
stone at Enticho had both been determined to be
between late Carboniferous and early Permian
(Tesfaye Kidane et al., 2013). For the purpose of
comparison with the present results, the paleomag-
netic data of the Enticho Sandstone from Enticho
locality was recalculated. Fig. 7 shows the com-
parison of this recalculated pole position with the
Apparent Polar Wander (APW) path curve for
Africa in Western African coordinates (Besse and
Courtillot, 1991; 2003; Cogné, 2003). This pole is
shown in full diamond symbol and the star symbol
shows the pole position after the pole is rotated to
the co-ordinate of West Africa to allow for
extensional rift system from the Benue Trough
about a Euler pole position, at 19.2ºN, 352.6ºE
through an angle -6.3º (clockwise) (Lottes and
Rowley, 1990; McElhinny et al., 2003). The final
transferred pole position is located at Lon =
238.6ºE, Lat = 50.3ºS (A95 = 5.5º, N=43). This pole
40 Tesfaye Kidane
with its 95 percent confidence circle intersects the
APW path at pole positions corresponding to be-
tween 260 Ma and 270 Ma, consistent with the
recent age estimation (Tesfaye Kidane et al., 2013)
and hence indicating primary magnetization
fingerprints from the Enticho Sandstone at Enticho.
Figure 7. Spherical projection with the major continents in their present day plate tectonic configurations and Apparent Polar
Wander Path (APWP) of Africa in West African coordinates (Besse and Courtillot, 1993; 2003; McElhinny et al., 2003), in
which (A) The Enticho Sandstone (at Enticho) pole position shown as diamond and the rotated pole in West African
coordinate about Euler pole position as star symbol for comparison. The rotated pole is consistent with ages of 270 – 260
Ma. (B) Blowup portion of the spherical projection to show the details of the pole position clearly. Coordinates of the
corners are given latitude, longitude subdivisions are at intervals of 10 degree.
SINET: Ethiop. J. Sci., 37(1), 2014 41
CONCLUSIONS
Rock magnetic studies were carried out in order
to characterize the magnetic mineralogy carrying
the ChRM of the Enticho Sandstone at Negash
area northern Ethiopia and to determine the
range of magnetic grain sizes. These studies have
shown that the sampled Enticho Sandstone at
Enticho locality preserved original magnetization
of high quality which are carried by detrital
hematite (Tesfaye Kidane et al., 2013). The
Enticho Sandstone at Negash area, Belesa
locality, on the other hand, is characterized by
coarser grained sand with magnetization carried
by magnetically hard and fine-grained titano-
hematite and soft materials dominantly coarse
titanomagnetite whose magnetization is known
to relax quickly with time.
Comparison of the paleopole position from
these sediments at the two localities with the
current APWP curve for West Africa gives a
Quaternary remagnetization age for the Enticho
Sandstone at Negash area and a primary magne-
tization age range of ≈260–270 Ma ( Early
Permian and Late Carboniferous) for the same
sandstone at Enticho locality.
ACKNOWLEDGEMENTS
I am very grateful for the fellowship support from
Alexander von Humboldt Fellowship that made this
investigation possible. I am indebted to Mulugeta Alene
for his support in the field and the Research and
Technology Transfer office for Addis Ababa University
for financial support of the fieldwork.
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