Antalo Limestone

Antalo Limestone
Stratigraphic range: Oxfordian-Kimmeridgian ~163–153  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Formation at Kurkura village (Dogua Tembien), holding St. Mika'el's cave
Type	Geological formation
Underlies	Mugher Mudstone
Overlies	Adigrat Sandstone
Thickness	800 m (2,600 ft)
Lithology
Primary	Limestone, marl, calcareous sandstone
Other	Grainstone, wackestone
Location
Coordinates	13°35′52″N39°16′38″E / 13.5977°N 39.2772°E / 13.5977; 39.2772 Coordinates: 13°35′52″N39°16′38″E / 13.5977°N 39.2772°E / 13.5977; 39.2772
Approximate paleocoordinates	7°30′S26°36′E / 7.5°S 26.6°E / -7.5; 26.6
Region	Tigray
Country	Ethiopia   Eritrea
Extent	Mekelle Outlier, Danakil Alps
Type section
Named for	Antalo town
Named by	William Thomas Blanford
Year defined	1868
Antalo Limestone (Ethiopia)

The Antalo Limestone, also known as the Antalo Sequence, is a geological formation in Ethiopia. It is between 300 and 800 metres thick and comprises fossiliferous limestones and marls that were deposited in a reef. Marine microfossils have shown an age between 165 and 150 million years. ^{[1] [2]}

Name and definition

The Antalo Supersequence includes two main stratigraphic units: the Antalo Sequence and the Agula Group. ^{[2] [3]} The Antalo Sequence, or Antalo Limestone has been named after the town of Hintalo in Tigray, Ethiopia. The name of the formation was coined by geologist William Thomas Blanford, who accompanied the British Expedition to Abyssinia in 1868. ^[4] At that time, Hintalo was a major town on the route of the invading British army. ^[5] So far the nomenclature has not been proposed for recognition to the International Commission on Stratigraphy.

Geographical extent

The sedimentary succession is found in Ethiopia, in the Mekelle Outlier, in the Blue Nile gorge, in the Harrar Plateau and around Dire Dawa. ^[2]

Stratigraphic context

The Antalo Limestone overlies the Adigrat Sandstone, 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, it was below the sea level. At that time, what would become Ethiopia was positioned just south of the equator. ^[6]

Lithology

Quarry in the third member of Antalo Limestone at May Qarano in Dogu’a Tembien

The limestones and marls of the Antalo Sequence also hold shale and calcareous sandstone layers. The Antalo Limestone comprises four members: (1) a basal member with grainstone and wackestone lithologies, with marly interlayers and in the upper part stromatoporoid coral-like level; (2) sandy limestone deposited in estuaries and lagoons; (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. ^{[2] [3]}

Rock sample of sandy limestone (second member), collected in Addi Idaga

Rock sample from the fourth (upper) member, collected in Miheno

Fossil content

Rock sample of Antalo Limestone with mollusks, collected in 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, with sand bars and spits near river mouths. ^[6] This sea bed hosted many invertebrate animals: echinoderms, crustaceans, bivalves and gastropods 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 mollusc, a giant nautilus with a characteristic spiral shell. ^[6]

Invertebrates

Bivalves [7]
Genus	Species	Presence	Material	Notes	Images
Actinostreon	A. solitarium	10 specimens from AL1, 65 from AL2.	75 specimens.	A palaeolophid.
Arcomytilus	A. laitmairensis	AL1 and AL2.	3 left valves, 1 right valve and 4 articulated specimens.	A mytilid.
Eopecten	E. velatus	AL1.	1 left valve.	A pectinid.
Gryphaea	G.? (Bilobissa?) balli	AL1.	1 left valve.	A gryphaeid.
Integricardium	I. (Integricardium) cf. bannesianum	7 specimens from AL1, 1 from AL2.	4 left valves, 1 right valves and 3 double-valved specimens.	A cardiid.
Liostrea	L. sp.	AL1 and AL2.	2 specimens.	An ostreid.
"Lucina"	"L." cf. cecchii	AL2.	4 double-valved specimens.	A lucinid.
Modiolus	M. (Modiolus) imbricatus	2 specimens from AL1, 1 from AL2.	3 double-valved specimens.	A mytilid.
Musculus	M. (Musculus) somaliensis	AL1 and AL2.	2 articulated specimens.	A mytilid.
Nanogyra	N. nana	AL2.	2 articulated specimens.	A gryphaeid.
Pholadomya	P. (Bucardiomya) somaliensis	1 specimen from AL1, 6 from AL2.	7 double-valved specimens.	A pholadomyid.
	P. (Bucardiomya) lirata	AL2.	1 double-valved specimen.	A pholadomyid.
Plagiostoma	P. harronis	AL1 and AL2.	3 left valves, 5 right valves and 2 articulated specimens.	A limid.
	P. sublaeviusculum	AL2.	1 left valve 1 right valve and 1 articulated specimen.	A limid.
Seebachia	S. ("Eoseebachia") sowerbyana	AL2.	2 left valves.	An astartid.
Spondylopecten	S. (Spondylopecten) palinurus	AL1.	2 left valves.	A pectinid.
Stegoconcha	S. gmuelleri	AL2.	2 articulated specimens.	A pinnid.

Brachiopods [7]
Genus	Species	Presence	Material	Notes	Images
cf. Amydroptychus	cf. A. sp.	Base of AL2.	10 individuals.	A rhynchonellid.
Cererithyris	C. sp.	1 from AL1, 105 from AL2 .	106 individuals.	A terebratulid.
Cymatorhynchia	C. sp.	AL2.	16 individuals.	A rhynchonellid.
Daghanirhynchia	D. sp.	3 individuals from AL1, 51 from AL2.	54 individuals.	A rhynchonellid.
Monsardithyris	M. sp.	AL2.	20 individuals.	A terebratulid.
Somalirhynchia	S. africana	Mostly from AL2.	More than 100 individuals.	A rhynchonellid.

Cnidarians [7]
Genus	Species	Presence	Material	Notes	Images
Actinastrea	A. crassoramosa	47 specimens from AL1, 13 from AL2.	60 larger fragments of colonies.	A stony coral.
Coenastraea	C. arabica	AL2.	1 specimen.	A stony coral.
Collignonastraea	C. cf. grossouvrei	AL2.	1 specimen.	A stony coral.
Columnocoenia	C. gemmans	AL1.	2 specimens.	A stony coral.
Comoseris	C. meandrinoides	AL3.	1 complete colony.	A stony coral.
Cladophyllia	C. excelsa	AL3.	1 specimen.	A stony coral.
Cryptocoenia	C. slovenica	7 specimens from AL1, 4 specimens from AL2.	11 specimens.	A stony coral.
Ironella	I. arabica	AL1.	12 specimens.	A stony coral.
Isastrea	I. bernensis	20 specimens from AL1, 22 specimens from AL2.	42 specimens.	A stony coral.
Kobyastraea	K. lomontiana	AL1.	1 specimen.	A stony coral.
Latiastrea	L. greppini	2 from AL1, 3 from AL2.	5 specimens.	A stony coral.
Lochmaeosmilia	L. trapeziformis	AL1.	16 karger fragments of colonies.	A stony coral.
Ovalastrea	O. michelini	AL2.	3 specimens (including 1 complete colony).	A stony coral.

Echinoderms
Genus	Species	Presence	Material	Notes	Images
Ausichicrinites	A. zelenskyyi		Nearly complete specimen. [8]	A comatulid.
Pygurus	P. meslei	Upper part of the formation. [9]		A sea urchin.

Molluscs [10]
Genus	Species	Presence	Material	Notes	Images
Orthosphinctes	O. aff. tiziani	Top part of sub-unit II.	5 fragmentary specimens.	An ataxioceratid ammonite.
Pachyceras	P. cf. lalandeanum	From the middle part of sub-unit II, GPS location 10°02′39.7″N, 38°13′53.7″E.	One poorly preserved specimen (no. AF020).	A pachyceratid ammonite.
Paracenoceras	P. cf. ennianus	Top part of sub-unit II.	1 specimen (no. AF012).	A nautiloid.
	P. aff. prohexagonum	Lower Limestone Member. [11]		A nautiloid.
	P. cf. kumagunense	Top part of sub-unit II.	1 specimen (no. AF004).	A nautiloid.
	P. cf. giganteum	Middle part of sub-unit II.	1 specimen (no. AF005).	A nautiloid.
Purpuroidea	P. aff. gigas	One specimen (AF025) from the top part of sub-unit I; One specimen (AF026) from the top part of sub-unit II.	2 specimen (AF025 and AF026).	A gastropod.

Limestone and karst geomorphology

A spring in Antalo Limestone at Santarfa

The layering is sub-horizontal, the same as that of the underlying sedimentary formations. ^[3] This gives rise to a structural sub-horizontal relief, 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 Harrar region (Sof Omar cave) and in the Dogu’a Tembien district of Tigray. ^[12]

Traditional uses of Antalo Limestone

Antalo Limestone is commonly used for house building; here a homestead 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) ^{[13] [14]}
• Door and window lintels
• 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 dams in gullies for sake of gully erosion control
• Cobble stones, sold to the towns for paving secondary streets

See also

• Ausichicrinites

References

1. 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.
2. Bosellini, A.; Russo, A.; Fantozzi, P.L.; Assefa, G.; Tadesse, S. (1997). "The Mesozoic succession of the Mekelle Outlier (Tigrai Province, Ethiopia)". Mem. Sci. Geol. 49: 95–116.
3. Dramis, F.; Fubelli, G. (2019). "Limestone, Karst and Tufa Dams in the Western Part of the Mekelle Outlier". Geo-trekking in Ethiopia's Tropical Mountains. GeoGuide. Cham (CH): SpringerNature. pp. 107–119. doi:10.1007/978-3-030-04955-3_7. ISBN   978-3-030-04954-6. S2CID   199102740.
4. 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.
5. Markham, C.; Prideaux, W. (1869). A history of the Abyssinian expedition. Macmillan.
6. Lerouge, F.; Aerts, R. (2019). Fossil evidence of Dogu'a Tembien's environmental past. In: Geo-Trekking in Ethiopia's Tropical Mountains, the Dogu'a Tembien District. SpringerNature. ISBN   978-3-030-04954-6.
7. Kiessling, Wolfgang; Pandey, Dhirendra Kumar; Schemm-Gregory, Mena; Mewis, Heike; Aberhan, Martin (2011). "Marine benthic invertebrates from the Upper Jurassic of northern Ethiopia and their biogeographic affinities". Journal of African Earth Sciences. 59 (2–3): 195. doi:10.1016/j.jafrearsci.2010.10.006. ISSN   1464-343X.
8. Salamon, Mariusz A.; Jain, Sreepat; Brachaniec, Tomasz; Duda, Piotr; Płachno, Bartosz J.; Gorzelak, Przemysław (2022). "Ausichicrinites zelenskyyi gen. et sp. nov., a first nearly complete feather star (Crinoidea) from the Upper Jurassic of Africa". Royal Society Open Science. 9 (7): 220345. doi:10.1098/rsos.220345. PMC   9297031 . PMID   35875469.
9. Radwańska, Urszula; Jain, Sreepat (2020-10-01). "First Late Jurassic echinoid record of Pygurus meslei Gauthier from the Antalo Limestone Formation (Blue Nile Basin, Ethiopia)". Journal of African Earth Sciences. 170: 103898. doi:10.1016/j.jafrearsci.2020.103898. ISSN   1464-343X. S2CID   219739555.
10. Jain, Sreepat; Schmerold, Roland (2021). "Callovian and Kimmeridgian fossils and stratigraphy of the Blue Nile Basin (central western Ethiopia)". Annales Societatis Geologorum Poloniae. doi: 10.14241/asgp.2021.11 . ISSN   0208-9068. S2CID   239211851.
11. Jain, Sreepat (2019-01-01). "First Bathonian (Middle Jurassic) nautiloid Paracenoceras Spath from Ethiopia". Journal of African Earth Sciences. 149: 84–96. doi:10.1016/J.JAFREARSCI.2018.07.027. ISSN   1464-343X. S2CID   134655647.
12. Catlin, D; Largen, M; Monod, T; Morton, W (1973). "The caves of Ethiopia". Transactions of the Cave Research Group of Great Britain. 15: 107–168.
13. Nixon-Darcus, L.A. (2014). The cultural context of food grinding equipment in Northern Ethiopia: an ethnoarchaeological approach. PhD thesis. Canada: Simon Frazer University.
14. Gebre Teklu (2012). Ethnoarchaeological study of grind stones at Lakia'a in Adwa, Tigray Regional State, Ethiopia. PhD diss (PDF). Addis Ababa University.