The Antalo Limestone, also known as the Antalo Sequence, is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

Ethiopia Ethiopia, , om, Itiyoophiyaa, so, Itoobiya, ti, ኢትዮጵያ, Ítiyop'iya, aa, Itiyoppiya officially the Federal Democratic Republic of Ethiopia, is a landlocked country in the Horn of Africa. It shares borders with Eritrea to the ...

. It is between 300 and 800 metres thick and comprises

fossiliferous A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

s and

marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part o ...

s that were deposited in a reef. Marine microfossils have shown an age between 165 and 150 million years.

Name and definition

The Antalo Supersequence includes two main stratigraphic units: the Antalo Sequence and the Agula Group. The Antalo Sequence, or Antalo Limestone has been named after the town of

Hintalo Hintalo ( ti, ሕንጣሎ), also called Antalo, was Administrative Center of Enderta’s historical wereda of Gabat Melash, is a small town located in the Debub Misraqawi (Southeastern) Zone of the Tigray Region of Ethiopia. It lies on a platea ...

in Tigray, Ethiopia. The name of the formation was coined by geologist

William Thomas Blanford William Thomas Blanford (7 October 183223 June 1905) was an English geologist and naturalist. He is best remembered as the editor of a major series on '' The Fauna of British India, Including Ceylon and Burma''. Biography Blanford was born ...

, who accompanied the

British Expedition to Abyssinia The British Expedition to Abyssinia was a rescue mission and punitive expedition carried out in 1868 by the armed forces of the British Empire against the Ethiopian Empire (also known at the time as Abyssinia). Emperor Tewodros II of Ethiopia, t ...

in 1868. At that time, Hintalo was a major town on the route of the invading British army. So far the nomenclature has not been proposed for recognition to the

International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigra ...

Geographical extent

The sedimentary succession is found in Ethiopia, in the

Mekelle Mekelle ( ti, መቐለ, am, መቀሌ, mäqälle, mek’elē) or Mekele is a special zone and capital of the Tigray Region of Ethiopia. Mekelle was formerly the capital of Enderta awraja in Tigray. It is located around north of the Ethiopi ...

Outlier, in the

Blue Nile The Blue Nile (; ) is a river originating at Lake Tana in Ethiopia. It travels for approximately through Ethiopia and Sudan. Along with the White Nile, it is one of the two major tributaries of the Nile and supplies about 85.6% of the water ...

gorge, in the

Harrar Harar ( amh, ሐረር; Harari: ሀረር; om, Adare Biyyo; so, Herer; ar, هرر) known historically by the indigenous as Gey (Harari: ጌይ ''Gēy'', ) is a walled city in eastern Ethiopia. It is also known in Arabic as the City of Sai ...

Plateau and around

Dire Dawa Dire Dawa ( am, ድሬዳዋ, om, Dirree Dhawaa, 3=Place of Remedy; so, Diridhaba, meaning "where Dir hit his spear into the ground" or "The true Dir", ar, ديري داوا,) is a city in eastern Ethiopia near the Oromia and Somali Re ...

Stratigraphic context

The Antalo Limestone overlies the

Adigrat Sandstone 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 ...

, and is covered by the Agula Group or Agula Shale and the Mugher Mudstone.

Environment

The Antalo Limestone comprises sediment that was deposited in a shallow tropical sea in the upper Jurassic. As the region had undergone a

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

, it was below the sea level. At that time, what would become Ethiopia was positioned just south of the equator.

Lithology

The limestones and marls of the Antalo Sequence also hold

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

and

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

sandstone layers. The Antalo Limestone comprises four members: (1) a basal member with

grainstone Under the Dunham classification (Dunham, 1962) system of limestones, a grainstone is defined as a grain-supported carbonate rock that contains less than 1% mud-grade material. This definition has recently been clarified as ''a carbonate-dominated ...

and

wackestone Under the Dunham classification (Dunham, 1962Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir. Amer ...

lithologies, with marly interlayers and in the upper part stromatoporoid coral-like level; (2) sandy limestone deposited in

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...

and

lagoons A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...

; (3) micritic (very fine grained) limestone with intercalations of wackestone and coquina beds deposited in relatively deep water; and (4) a succession of marls and limestone, with cherty limestone at the base. Rock sample 11 Sandy limestone Addi Idaga

Rock sample 11 Sandy limestone Addi Idaga

Rock sample 7 Upper limestone formation Miheno

Fossil content

Rock sample 6 Limestone with mollusks Azef

The Antalo Limestone sediments were deposited at the time of dinosaurs and primitive birds. Well away from coasts, coral reefs formed the edge of the continental shelf. At shallow depth, the sea bottom was made of large

mudflats Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal fl ...

, with sand bars and spits near river mouths. This sea bed hosted many invertebrate animals:

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea ...

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

and

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

s were common. There was also fish. As it was not a nutrient-rich ecosystem, larger predators were rare, maybe some marine reptiles like crocodiles. A striking scavenger in this fauna was a

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

mollusc, a giant

nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in ...

with a characteristic spiral shell.

Invertebrates

Limestone and karst geomorphology

The layering is sub-horizontal, the same as that of the underlying sedimentary formations. This gives rise to a structural sub-horizontal

relief Relief is a sculptural method in which the sculpted pieces are bonded to a solid background of the same material. The term ''relief'' is from the Latin verb ''relevo'', to raise. To create a sculpture in relief is to give the impression that the ...

, with alternating cliffs and flats. Dissolution processes in limestone lead to the occurrence of caves. Most described caves in Mesozoic limestone in Ethiopia are located in the

region (Sof Omar cave) and in the Dogu’a Tembien district of Tigray.

Traditional uses of Antalo Limestone

Antalo_Limestone_as_building_material_-_house_in_Addi_Ateroman_in_Dogu'a_Tembien

Given its nearly rectangular shape and its strength, the hard layers of Antalo Limestone are used for :* House building. Traditionally, fermented mud will be used as mortar :* Fencing of homesteads, generally in dry masonry :* Milling stone: for this purpose plucked-bedrock pits, small rock-cut basins that naturally occur in rivers with kolks, are excavated from the river bed and further shaped. Milling is done at home using an elongated small boulder (

mano Mano may refer to: People * Mano people, an ethnic group in Liberia * Mano (name), a list of people with either the given name or surname * Mano (Mozambican footballer) (born 1984), real name Celso Halilo de Abdul * Mano (Portuguese footballer) ...

) :* Door and window

lintels A lintel or lintol is a type of beam (a horizontal structural element) that spans openings such as portals, doors, windows and fireplaces. It can be a decorative architectural element, or a combined ornamented structural item. In the case of ...

:* Footpath paving :* Stone bunds or ''gedeba'': terrace walls in dry masonry, typically laid out along the contour for sake of soil and water conservation :*

Check dam A steel check dam A check dam is a small, sometimes temporary, dam constructed across a swale, drainage ditch, or waterway to counteract erosion by reducing water flow velocity. Check dams themselves are not a type of new technology; rather, th ...

s in

gullies A gully is a landform created by running water, mass movement, or commonly a combination of both eroding sharply into soil or other relatively erodible material, typically on a hillside or in river floodplains or terraces. Gullies resemble lar ...

for sake of

gully erosion A gully is a landform created by running water, mass movement, or commonly a combination of both eroding sharply into soil or other relatively erodible material, typically on a hillside or in river floodplains or terraces. Gullies resemble la ...

control :*

Cobble stone Cobblestone is a natural building material based on cobble-sized stones, and is used for pavement roads, streets, and buildings. Setts, also called Belgian blocks, are often casually referred to as "cobbles", although a sett is distinct fr ...

s, sold to the towns for paving secondary streets

References

{{Reflist Geologic formations of Eritrea Geologic formations of Ethiopia Jurassic System of Africa Oxfordian Stage Kimmeridgian Stage Limestone formations Marl formations Shallow marine deposits Paleontology in Eritrea Paleontology in Ethiopia Formations