The Lourinhã Formation is a geological formation in western Portugal, outstanding for its abundant fossilized fauna and flora, including dinosaur bones, dinosaur eggs and nests, and fossil tracks, to name but some examples. At its upper limit, the formation includes some Early Cretaceous (Berriasian) strata, but it mostly consists of Late Jurassic (Kimmeridgian/Tithonian) rocks. The stratigraphy of the formation is generally complex and controversial, with the classification and grouping of its lithostratigraphic units varying between different stratigraphers. The fossil biota present there is very similar to that of the Morrison Formation in the United States and, to a lesser extent, the Tendaguru Formation in Tanzania. There are also similarities to the Villar del Arzobispo Formation in Spain, and the neardy Alcobaça Formation. The formation is named after the municipality of Lourinhã, the geographic area it mostly occupies.

Lithology and depositional history

Depositional history

The Lourinhã Formation is located within the Lusitanian Basin, a mostly onshore North South orientated rift basin within western Portugal, formed during the Opening of the North Atlantic Ocean, with sediment deposition beginning during the Late Triassic-Early Jurassic. It primarily consists of syn-rift near-coastal continental siliciclastic sediments, with several marine intercalations. The primary flow direction was North to South, originating from Galicia and flowing between the Iberian landmass to the east and the now largely submerged Berlengas horst, a north–south oriented ridge, to the west.

Stratigraphy

The unit was first formally proposed by Hill in 1988. The stratigraphy of the Lourinhã Formation is complex and varies between sub-basins with several competing stratigraphic proposals and there is currently no consensus on the matter, one of the most recent stratigraphies, divides the formation into three members which are from oldest to youngest the Praia da Amoreira-Porto Novo Member, Praia Azul Member, and the Assenta Member.

Praia da Amoreira-Porto Novo Member

The Praia da Amoreira-Porto Novo Member is composed of the Priaia de Amoreira Member, which consists of massive mudrock-sand with metre thick sandstone lenses, with massive mudrock with calcrete. The overlying Poto Novo Mb. consists of massive bodies of sandstone, often cross bedded. The environment of deposition is interpreted as a meandering fluvial system, while the Porto Novo Mb is interpreted as a deltaic deposit. It is interpreted to be latest Kimmeridgian in age, and overlies the Consolacao Unit at the top of the Aulacostephanus eudoxus ammonite zone.

Praia Azul Member

The Praia Azul Member, formerly known as the Sobral unit/member is 80 to 130 metres thick and consists of tabular marls and mudstones, with rare sandstones bodies. There are three distinct laterally extensive (>20 km) thin shelly carbonate horizons within this member, indicating brief marine transgressions. South of Santa Cruz primarily consists of sandstone with rare conglomerate. The age is considered to be latest Kimmeridgian to earliest Tithonian, correlated to the ammonite zones of Hybonoticeras beckeri and Hybonoticeras hybonotum.

Santa Rita Member + Assenta Member

The Santa Rita Member in the Consolação sub-basin and its lateral equivalent in the Turcifal Basin the Assenta Member is around 300 metres thick and predominantly consists of mudstones with frequent layers of caliche. Near the top of the member several layers of tens of metres thick nodular and marly bioclastic limestones are present, containing marine benthic forams, the nodularity is derived from intense Thalassinoides burrowing. It is predominately late Tithonian in age, with the last few metres probably being earliest Berriasian, with the top of the formation roughly correlative with the base of the magnetochron M18n. (~144.7 Ma) The environment of deposition is interpreted as being an upper fluvial-dominated delta to meandering fluvial systems flowing on a paralic plain.

Fauna

Dinosaurs

In a 2003 study, an analysis of all Portuguese dinosaurs was published. The study created a cladogram showing the possible relations of all Portuguese dinosaurs, including those at the time known from the Lourinhã Formation.

Ornithischia

Ornithopods

Thyreophorans

Other Ornithiscians

Saurischia

Sauropods

Theropods

Pterosaurs

Mammaliaformes

Docodonta

Cladotheria

Multituberculata

Amphibans

Squamates

Crocodyliformes

Fish

Flora

Correlation