Adigrat Sandstone
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| Adigrat Sandstone | |
|---|---|
| Stratigraphic range: | |
 Dabba Selama monastery in Dogu’a Tembien, established on a mesa in Adigrat Sandstone – one of the most inaccessible monasteries in the world[1] | |
| Type | Geological formation |
| Underlies | Antalo Limestone |
| Overlies | Enticho Sandstone, Edaga Arbi Glacials, Fincha Sandstone |
| Thickness | 600 m (2,000 ft) |
| Lithology | |
| Primary | Sandstone |
| Other | Siltstone, Claystone, Dolomite |
| Location | |
| Coordinates | 14°10′51″N 39°28′31″E / 14.1808°N 39.4752°E |
| Region | Tigray |
| Country | Ethiopia, Eritrea |
| Extent | Eritrea, Tigray |
| Type section | |
| Named for | Town of Adigrat |
| Named by | William Thomas Blanford |
  Adigrat Sandstone (Ethiopia) | |
The Adigrat Sandstone formation in north Ethiopia, in a wide array of reddish colours, comprises sandstones with coarse to fine grains, and locally conglomerates, silt- and claystones. Given the many lateritic palaeosols and locally fossil wood fragments, the formation is interpreted as a deposit in estuarine, lacustrine-deltaic or continental environments. The upper limit of Adigrat Sandstone is of Middle-Late Jurassic age (around 160 million years or Ma ago) whereas the lower boundary is Triassic (200 Ma).[2][3] There are numerous rock-hewn churches in this formation.
The name “Adigrat Sandstone” was coined by geologist William Thomas Blanford, who accompanied the British Expedition to Abyssinia in 1868. The formation is named after the town of Adigrat, on the route of the invading British army.[4] So far the nomenclature was not proposed for recognition to the International Commission on Stratigraphy.
Stratigraphic context
The Adigrat Sandstone has been deposited upon a Triassic planation surface.[2] The age of the Adigrat Sandstone is not well-defined and sedimentation has probably started at different times in different areas. Recent investigations using fossils determined a Late Triassic to Middle Jurassic age.[5][6]
Environment
A large part of the formation was deposited in a shallow sea, as indicated by the presence of fossils typical of brackish water and shallow marine environment (such as bivalves, foraminifera, and marine crocodiles). The sedimentary structures also suggest tidal environments and storm deposits. Other sediment and also sedimentary structures indicate that parts of the sands were deposited in a continental environment.[5]
Lithology
The Adigrat Sandstone comprises thick sequences of cross-bedded sandstone with minor intercalations of siltstone and claystone and sometimes dolomite. The sandstones consist essentially of quartz, but hold also some feldspar. The grains are cemented by silica and clay minerals.[5] The thickness of the Adigrat Sandstone increases from north to south: maximum values (600 metres) are west of the Mekelle Outlier at the edge of Kola Tembien and Dogu’a Tembien districts.[2]
Life
In the Adigrat Sandstone, fossil wood fragments and trace fossils have been recorded. In addition, remains of vertebrate animals, such as non-marine amphibians and reptiles, have also been found.[7][8]
Geographical extent
The formation outcrops widely in north Ethiopia, particularly in the gorges formed by the Tekezze River and its affluents. In extends further to the southeast towards Soqota and to the northeast towards Gheralta, Adigrat and central Eritrea.[2]
![](#/media/File:Kayeh_Be'ati,_"red_caves"_in_Adigrat_Sandstone,_near_Gelebeda.jpg)