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Paleomagnetic dating of Enticho Sandstone at Negash locality (Tigrai region, Northern Ethiopia), Implication for Quaternary Remagnetization.

SINET: Ethiop. J. Sci., 37(1):31–42, 2014
© College of Natural Sciences, Addis Ababa University, 2014 ISSN: 0379–2897
Tesfaye Kidane
School of Earth Sciences, College of Natural Sciences, Addis Ababa University, PO Box 1176, Addis
Ababa, Ethiopia. E-mail:;
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
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).
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
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
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
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).
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
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
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.
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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The landscape of Eritrea is highly variable and reflects the complex geological history of the area, which is only partially shared with the other regions of the Horn of Africa. The structural geomorphology of Eritrea was investigated through field surveys, literature reviews and a few topographic profiles oriented east–west across the country. The information collected led to the production of a new schematic geological map. The older crustal deformations controlled the orientation of the main fluvial systems, whereas later tectonic events affected their upstream drainage networks. The geological history of Eritrea is very long as it started in the Neoproterozoic, though it was punctuated by a few, more or less long intervals of quiescence. The modern landforms derive from the combined effects of the powerful uplift, to which the whole Horn of Africa was subjected throughout the Cenozoic, and the present arid climate. Fluvial erosion resulted in the accumulation of clastic deposits, a few thousands of meters thick, which gave rise to the coastal belt. In spite of an average denudation rate of 30 mm/ka (a value similar to those inferred for the upper Blue Nile and other areas with a similar structural setting), hard rocks such as the laterites, formed on top of old peneplain surfaces, are still preserved and their present-day elevation along east–west profiles witnesses an impressive upwards dislocation of about 2500 m associated with the impingement of the Afar Plume. In Eritrea, the emplacement of Trap basalts is spatially rather limited, especially if compared with the impressive expansion all across the neighboring Ethiopia. Most of volcanic activity of Eritrea is recent (Quaternary) and associated with the very last phases of the Danakil depression formation. Presently, arid conditions and a volcanic morphology provide the Eritrean Danakil with a unique and fascinating landscape.
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The pre-drift Wegenerian model of Pangea is almost universally accepted, but debate exists on its pre-Jurassic configurationsinceTedIrvingintroducedPangea‘B’byplacingGondwanafarthertotheeastbyV3000kmwith respect to Laurasia on the basis of paleomagnetic data. New paleomagnetic data from radiometrically dated Early Permian volcanic rocks from parts of Adria that are tectonically coherent with Africa (Gondwana), integrated with published coeval data from Gondwana and Laurasia, again only from igneous rocks, fully support a Pangea ‘B’ configuration in the Early Permian. The use of paleomagnetic data strictly from igneous rocks excludes artifacts from sedimentary inclination error as a contributing explanation for Pangea ‘B’. The ultimate option to reject Pangea ‘B’ is to abandon the geocentric axial dipole hypothesis by introducing a significant non-dipole (zonal octupole) component in the Late Paleozoic time-averaged geomagnetic field. We demonstrate, however, by using a dataset consisting entirely of paleomagnetic directions with low inclinations from sampling sites confined to one hemisphere from Gondwana as well as Laurasia that the effects of a zonal octupole field contribution would not explain away the paleomagnetic evidence for Pangea ‘B’ in the Early Permian. We therefore regard the paleomagnetic evidence for an Early Permian Pangea ‘B’ as robust. The transformation from Pangea ‘B’ to Pangea ‘A’ took place during the Permian because Late Permian paleomagnetic data allow a Pangea ‘A’ configuration. We therefore review geological evidence from the literature in support of an intra-Pangea dextral megashear system. The transformation occurred afterthecoolingoftheVariscanmega-sutureandlastedV20Myr.Inthisinterval,theNeotethysOceanopened between India/Arabia and the Cimmerian microcontinents in the east, while widespread lithospheric wrenching and magmatism took place in the west around the Adriatic promontory. The general distribution of plate boundaries and resulting driving forces are qualitatively consistent with a right-lateral shear couple between Gondwana and Laurasia during the Permian. Transcurrent plate boundaries associated with the Pangea transformation reactivated Variscan shearzonesandweresubsequentlyexploitedbytheopeningofwesternNeotethyanseawaysintheJurassic.
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Until now, only a schematic sedimentary interpretation for the Horn of Africa has been accepted. The history of the sedimentary section around Mekele--previously called Adigrat Sandstone, Antalo Limestone, and Upper Sandstone--was not so simple, and it already had begun either in the Ordovician or in the Carboniferous. Three basins of deposition were formed successively. In the first one, the lower Enticho Sandstone, which is exposed only east and north of Mekele, was deposited. The second basin was channel-like, trending north-south and filled by Edaga Arbi glacial deposits and the glacial facies of the Enticho Sandstone. This basin probably was coincident with a system of troughs trending from north to south in the Horn of Africa, eastern Africa, and southern Arabia. Th basin of deposition for the Adigrat Sandstone, Antalo Limestone, Agula Shale, and Amba Aradam Formation was the third to be formed; its history coincides with the regional sedimentary history of the Horn of Africa, from Triassic to Cretaceous. Sedimentation during this period was controlled by the widespread transgression covering the continent from the east and south. It involved two major structural highs, as well as two main basins, one of which, in central west Ethiopia, contains the Mekele outlier as a subbasin. These partly new aspects of the sedimentary history of the area may contribute to new ideas for oil exploration.
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The geometrical fit of the continents now separated by oceans has long been discussed in relation to continental drift. This paper describes fits made by numerical methods, with a `least squares' criterion of fit, for the continents around the Atlantic ocean. The best fit is found to be at the 500 fm. contour which lies on the steep part of the continental edge. The root-mean-square errors for fitting Africa to South America, Greenland to Europe and North America to Greenland and Europe are 30 to 90 km. These fits are thought not to be due to chance, though no reliable statistical criteria are available. The fit of the block assembled from South America and Africa to that formed from Europe, North America and Greenland is much poorer. The root-mean-square misfit is about 130 km. These geometrical fits are regarded as a preliminary to a comparison of the stratigraphy, structures, ages and palaeomagnetic results across the joins.
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One hundred fourteen oriented palaeomagnetic core samples were collected from 13 palaeomagnetic sites on subhorizontal to tilted glacial sediments at five localities of Northern Ethiopia. Combined alternating field (AF) and stepwise thermal demagnetization techniques were successfully applied to resolve the complete directional spectrum. A viscous remagnetization (VRM) and one stable component of magnetization were identified in most of the specimens. The VRM is removed between a temperature range of 120-350 °C and AF of up to 30 mT. Further heating until ˜650 °C results in smooth decay of the natural remanent magnetization (NRM) intensity to about 50 per cent and the rest of the NRM is efficiently removed by heating to 690 °C, while only 30-50 per cent of NRM is removed by the maximum AF available suggesting haematite as remanence carrier. Results of the magnetization decay curve plots and rock magnetic analyses using the variable field translation balance indicated the presence of magnetite with minor goethite, pyrrhotite as well. The high stability component defining a straightline segment, starting 350 °C and/or 30 mT is mostly directed towards the origin and interpreted as the characteristic remanent magnetizations (ChRMs). The direction of magnetization is determined both by best-fitting line using the least-square technique of Kirschvink and remagnetization circles of Halls for few unresolved overlapping components. The site mean directions of the sediments from two sites are normal polarity and are close to present-day field directions at the sample site. The site mean directions from 11 sites, on the other hand, are reversed in polarity with better grouping in the tilt-corrected coordinate and pass the McFadden fold test. This overall site mean direction is Dec = 143.4º, Inc = 58.8º (N = 11, α95 = 9.7º) with a corresponding mean pole position of Lat = 26.0º, Lon = 249.5º (N = 11, A95 = 13.1º). This geomagnetic pole position is later rotated into West Africa coordinates to allow for extensional rifting in the Benue Trough about an Euler pole position, at 19.2ºN, 352.6ºE through an angle -6.3º (clockwise). The resulting pole position is located at φs = 246.6ºE, λs = 31.8ºS (N = 11, A95 = 13.1º), this pole with its 95 per cent confidence circle intersects the 270-310 Ma, segment of the APW path for West Africa consistent with ages of between late Carboniferous and early Permian. The result also implies that the Late Carboniferous Dwyka land ice sheet had probably extended more than 1000 km further north to Ethiopia than previously known.
Critical assessment of Paleozoic paleomagnetic results from Australia shows that paleopoles from locations on the main craton and in the various terranes of the Tasman Fold Belt of eastern Australia follow the same path since 400 Ma for the Lachlan and Thomson superterranes, but not until 250 Ma or younger for the New England superterrane. Most of the paleopoles from the Tasman Fold Belt are derived from the Lolworth-Ravenswood terrane of the Thomson superterrane and the Molong-Monaro terrane of the Lachlan superterrane. Consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic Australia by the late Early Devonian. The Lolworth-Ravenswood terrane is interpreted to have undergone a 90° clockwise rotation between 425 and 380 Ma. Although the Tamworth terrane of the western New England superterrane is thought to have amalgamated with the Lachlan superterrane by the Late Carboniferous, geological syntheses suggest that movements between these regions may have persisted until the Middle Triassic. This view is supported by the available paleomagnetic data. With these constraints, an apparent polar wander path for Gondwana during the Paleozoic has been constructed after review of the Gondwana paleomagnetic data. The drift history of Gondwana with respect to Laurentia and Baltica during the Paleozoic is shown in a series of paleogeographic maps.
This book makes the common techniques of paleomagnetism understandable to students of geology and explains how paleomagnetism is used to map the movement of major portions of the Earth's surface through time. It contains an extensive catalog of paleomagnetic results, which is used to describe and analyze paleogeography and tectonic movements and to place them in the context of prevailing hypotheses. A critical assessment of the data base in terms of quality and reliability is also made, and geographically, the entire Phanerozoic world except for the Pacific Ocean (and its rims) is covered. This book is unique for being results-based and for containing a paleopole catalog.