Amba Aradam Formation

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Amba Aradam Formation
Stratigraphic range: Early Aptian
~120  Ma
O
S
D
C
P
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K
Pg
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DingiletWaterfall.jpg
Cliff and waterfall at Dingilet, Mika’el Abiy tabia
Type Geological formation
Sub-unitsGraua Limestone member
UnderliesTertiary basalts
Overlies Agula Shale, Mugher Mudstone
Thickness200 m (660 ft)
Lithology
Primary Sandstone, siltstone
Other Claystone, conglomerates, iron oxide
Location
Coordinates 13°19′41″N39°26′17″E / 13.328°N 39.438°E / 13.328; 39.438
Approximate paleocoordinates 15°12′S28°30′E / 15.2°S 28.5°E / -15.2; 28.5
Region Tigray
CountryFlag of Ethiopia.svg  Ethiopia
Extentnorthern Ethiopian Highlands
Type section
Named for Imba Aradom mountain, near Hintalo
Named by William Thomas Blanford
Year defined1868
Ethiopia relief location map.jpg
Green pog.svg
Amba Aradam Formation (Ethiopia)

The Amba Aradam Formation is a Cretaceous sandstone formation in Ethiopia. It is up to 200 metres thick, for instance in the Degua Tembien district. [1] As fossils are absent, the age of the Amba Aradam Formation was interpreted based on the age of assumed corresponding sandstones elsewhere in Ethiopia: [1] the Debre Libanos Sandstones in the Blue Nile Basin, and the Upper Sandstone near Harrar in southeast Ethiopia, both of Late Cretaceous age (100–66 million years ago). [2] [3] [4] The lithology of the Amba Aradam Formation makes it less suitable for rock church excavation; caves have however been blasted in this formation to serve as headquarters for the TPLF during the Ethiopian Civil War of the 1980s. [1]

Contents

Name and definition

The name “Amba Aradam Sandstone” was coined by geologist William Thomas Blanford, who accompanied the British invading army in 1868. The formation is named after the Amba Aradam mountain, where the formation widely outcrops. [5] So far the nomenclature was not proposed for recognition to the International Commission on Stratigraphy.

Stratigraphic context

The Amba Aradam Formation overlies the Agula Shale at Imba Aradom [6] and in the eastern part of Dogu’a Tembien; further to the west, it rests on the Antalo Limestone and then on the Adigrat Sandstone. [1] After the deposition of the sands that were to become the Amba Aradam Formation, planation has occurred. [7] This was then followed by the deposition of the Tertiary Trap Volcanics. [8]

Lithology

The Amba Aradam Formation comprises white or red cross-bedded sandstones, and also mottled purple to violet siltstones and claystones. The sandstones are composed of quartz grains, cemented by clay minerals and iron oxides. The sediment was probably deposited in rivers. The iron oxide cementation (laterisation) makes the upper part of the formation harder, heavier and almost impervious for water. [1] [9] Locally, conglomerates occur. [10]

Cliff in Amba Aradam Sandstone at Gumuara, here, at its westernmost occurrence, it directly overlies the Adigrat Sandstone Gumuara village, on top of the Adigrat Sandstone cliff.jpg
Cliff in Amba Aradam Sandstone at Gumuara, here, at its westernmost occurrence, it directly overlies the Adigrat Sandstone

Geographical extent

Besides at Imba Aradom, the formation outcrops on the ridges north of Imba Alaje, and particularly in the Dogu'a Tembien district, all in the Tigray region of Ethiopia.

Current geomorphology of the Amba Aradam Formation

The mesas of the Amba Aradam Formation, locally with trap volcanic cover, have an alternance of steep slopes and short moderate sections due to the alternance of beds of sandstone and layers of less resistant claystone or siltstone. The formation does not have high vertical cliffs like the Adigrat Sandstone. The iron-cemented upper part of the formation forms however steep cliffs, which can be more than 10 metres (33 ft) high. These layers are also impermeable, what results in a spring line at the edge of the Amba Aradam sandstone cliff. [1] [11]

Cliff in Amba Aradam Sandstone at Addi Geza'iti; the previous TPLF headquarters were hewn in this cliff, at right AddiGezaetiCliff.jpg
Cliff in Amba Aradam Sandstone at Addi Geza'iti; the previous TPLF headquarters were hewn in this cliff, at right

Caves used as headquarters during the Ethiopian Civil War

In the 1980s, during the civil war, the Tigray People's Liberation Front (TPLF) excavated a cave in the sandstones of the Amba Aradam Formation, using dynamite. The cave is located near the Addi Geza'iti village, to the west of Hagere Selam, and served as offices for its leaders, including Meles Zenawi and Siye Abraha. The allied Ethiopian People’s Democratic Movement (EPDM) installed its HQ in a nearby cave at Melfa. Major military operations were coordinated from these caves almost until their victory against the Derg government in 1991. Both caves can be visited on appointment (contact at the Dogu'a Tembien district office, Hagere Selam, Tigray). [1]

Related Research Articles

<span class="mw-page-title-main">Dogu'a Tembien</span> District in Tigray Region, Ethiopia

Dogu'a Tembien is a woreda in Tigray Region, Ethiopia. It is named in part after the former province of Tembien. Nowadays, the mountainous district is part of the Southeastern Tigray Zone. The administrative centre of this woreda is Hagere Selam.

Amba Aradam is a table mountain in northern Ethiopia. Located in the Debub Misraqawi (Southeastern) Zone of the Tigray Region, between Mek'ele and Addis Abeba, it has a latitude and longitude of 13°20′N39°31′E and an elevation of 2,756 metres (9,042 ft).

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<span class="mw-page-title-main">Ashangi Basalts</span>

The Ashangi Basalts are the earliest Tertiary volcanic rocks in north Ethiopia, hence they are in the lowest position. These dark porphyritic basalts are separated from the Mesozoic formations below it by basal conglomerates. The basalts hold phenocrysts that developed before the magma reached the earth surface. These basalts are weathered, partially eroded and have a sub-horizontal stratification, particularly at the lower part. This series was created during the first period of the flood basalt eruptions in north Ethiopia, in the Oligocene.

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References

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  2. Sembroni, A.; Molin, P.; Dramis, F. (2019). Regional geology of the Dogu'a Tembien massif. In: Geo-trekking in Ethiopia's Tropical Mountains - The Dogu'a Tembien District. SpringerNature. ISBN   978-3-030-04954-6.
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  5. Blanford, W.T. (1870). Observations on the geology and zoology of Abyssinia, made during the progress of the British expedition to that country in 1867-68. London: Macmillan and Co.
  6. Coltorti, M.; Pieruccini, P.; Ogbagabriel Berakhi; Dramis, F.; Asfawossen Asrat (2009). "The Geomorphological Map of Mt. Amba Aradam Southern Slope (Tigray, Ethiopia)" (PDF). Journal of Maps. 5 (1): 56–65. Bibcode:2009JMaps...5...56C. doi: 10.4113/jom.2009.1043 . S2CID   129046585.
  7. Coltorti, M.; Dramis, F.; Ollier, C. (2007). "Planation surfaces in northern Ethiopia". Geomorphology. 89 (3–4): 287–296. Bibcode:2007Geomo..89..287C. doi:10.1016/j.geomorph.2006.12.007.
  8. Justin-Visentin, E. (1974). "Petrografia, chimismo e petrogenesi dei corpi subvulcanici di Macallè (Tigrai Etiopia)". Memorie dell'Istituto di Geologia e Mineralogia, Universita di Padova. 31: 1–33.
  9. Beyth, M. (1972). To the Geology of Central-Western Tigre. Bonn: Dissertation Rheinische Friedrichs-Wilhelms-Universität Bonn. p. 159.
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  11. Vandecasteele, I. and colleagues (2011). "Hydrogeology and groundwater flow in a basalt-capped Mesozoic sedimentary series of the Ethiopian highlands". Hydrogeology Journal. 19 (3): 641–650. Bibcode:2011HydJ...19..641V. doi:10.1007/s10040-010-0667-0. S2CID   129417175.