# Antalo Limestone

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.

## 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 in Tigray, Ethiopia. The name of the formation was coined by geologist William Thomas Blanford, who accompanied the British Expedition to Abyssinia 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.

## 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.

## 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.

## Lithology

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.

## Fossil content

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. 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.

### Invertebrates
#### Bivalves

Bivalves of the Antalo Limestone. :
| Genus | Species | Presence | Material | Notes |
| 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. |
| Pholadomya | 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. |
| Plagiostoma | 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

Brachiopods of the Antalo Limestone :
| Genus | Species | Presence | Material | Notes |
| 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. |

#### Cnidarian

Cnidarians of the Antalo Limestone :
| Genus | Species | Presence | Material | Notes |
| 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

Echinoderms of the Antalo Limestone :
| Genus | Species | Presence | Material | Notes | Images |
| Ausichicrinites | A. zelenskyyi | | Nearly complete specimen. | A comatulid. | |
| Pygurus | P. meslei | Upper part of the formation. | | A sea urchin. | |

#### Molluscs

Molluscs of the Antalo Limestone :
| Genus | Species | Presence | Material | Notes |
| 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. |
| Paracenoceras | P. aff. prohexagonum | Lower Limestone Member. | | A nautiloid. |
| Paracenoceras | P. cf. kumagunense | Top part of sub-unit II. | 1 specimen (no. AF004). | A nautiloid. |
| Paracenoceras | 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

The layering is sub-horizontal, the same as that of the underlying sedimentary formations. 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.

## Traditional uses of Antalo Limestone

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)
   * 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