Coimbra Formation

Coimbra Formation
Coimbra Formation
Stratigraphic range: Early to Late Sinemurian 199–188 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	S. Pedro de Muel Beach, the main outcrop
Type	Geological formation
Underlies	Água de Madeiros Formation;
Overlies	Dagorda Formation;
Area	Lusitanian Basin
Thickness	120-150 m
Lithology
Primary	Limestones
Location
Location	Coimbra Region
Coordinates	39.8° N, 9.0° W
Region	Lusitanian Basin
Country	Portugal
Type section
Named for	The Village of Coimbra
Thickness at type section	~120 m (390 ft)
	Coimbra Formation (Portugal)

The Coimbra Formation (Also known as Camadas de Coimbra or Calcários de S. Miguel) is a geological formation of Sinemurian (Lower Jurassic) age in the Lusitanian Basin of Portugal.^[1] The unit represents a series of peritidal to intertidal facies of a Carbonate platform mostly of Obtusum–Oxynotum age, that gradually evolve to open marine/hemipelagic units Vale das Fontes Formation and Lemede formation.^[2]^[3] This unit is known for its fossil content, including Invertertebrate and vertebrate ichnofossils and fossils.^[4]^[5]

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The sediments in the Coimbra Formation represent the shallow sections of an inner carbonate platform, adjacent to the Paleozoic basement in the E.^[6] They are rich in organic matter and are found alongside marl layers in a shallow marine setting. The organic material is divided into three groups (A, B, and C), showing how it is spread across different layers.^[1] The middle section mainly consists of group B, with a lot of microbial mat particles and very few plant particles. This layer has the highest level of Total organic carbon (TOC), indicating it was deposited in a restricted and stagnant area with little water movement.^[7] In the upper section, all three groups (A, B, and C) are present. The lower part of this section contains more plant debris and fewer marine particles, suggesting a stronger influence from land and more oxygen in the environment, leading to lower TOC levels.^[6] As we move up, group B shows a reduced land influence. At the top, group A takes over, with more marine particles and fewer plant materials, suggesting a transition to a more open marine environment. Overall, the Coimbra Formation is made up of shallow marine deposits formed in a low-energy setting, with occasional stronger events like storms or floods.^[8] The area was a calm, shallow marine environment with limited water flow. Most sediments were fine-grained, showing low-energy conditions and a semi-arid climate. Microbialites formed during periods of slow sediment buildup, indicating low water movement. Occasional storms brought better oxygenated water, supporting brief marine life bursts. Fossils suggest shifts between restricted and open marine environments due to minor sea-level changes. Over time, open-marine conditions increased, with more typical marine fossils reflecting a significant rise in sea level.^[8] Individual Stromatolite mounds are clearly defined, either as separate structures lined up next to each other or as irregular, flat to wavy layers that merge at the base.^[8] Palynology includes Classopollis (Cheirolepidiaceae Conifer) and Botryococcus as the dominant continental components, while marine palynomorphs include the genera Tasmanites and Cymathiosphaera (Prasinophyte algae) as well Zygnemataceae.^[6] The recent discovery of Dinosaur tracks revelated the presence of supratidal environments located in a shallow, tropical coastal area, likely near the shore.^[4]

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Color key

Taxon	Reclassified taxon	Taxon falsely reported as present	Dubious taxon or junior synonym	Ichnotaxon	Ootaxon	Morphotaxon

Notes
Uncertain or tentative taxa are in small text; ~~crossed out~~ taxa are discredited.