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Chão das Pias Formation

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Chão das Pias Formation
Stratigraphic range: Bajocian 170–168 Ma
↓
TypeGeological formation
Sub-units
  • Calcários de Vale da Serra
  • Dolomitos de Furadouro
UnderliesSerra de Aire Formation
OverliesBarranco do Zambujal Formation
AreaLusitanian Basin
Thickness50-60 m
Lithology
PrimarySlightly clayey or marly limestones
Location
LocationMaciço Estremenho
Coordinates39°32'38.0", N 8°47'57.0"W
RegionLusitanian Basin
CountryPortugal
Type section
Named forThe village of Chão das Pias, Santo António plateau
Named byAna Cristina Azerêdo
Year defined2007
Thickness at type section~55 m (180 ft)

The Chão das Pias Formation is a geological formation of Bajocian (Middle Jurassic) age in the Lusitanian Basin of Portugal.[1] Named after the village of Chão das Pias, situated slightly south of Serro Ventoso in the Santo António plateau.[1][2] This formation overlies the Barranco do Zambujal Formation in sedimentary continuity and is attributed to the transition from the Lower Bajocian to the Upper Bajocian, with its upper limit likely reaching the top of the Upper Bajocian and possibly the base of the Bathonian locally.[1] It comprises a sequence of limestones and dolomites, reflecting varied depositional conditions, and is subdivided into two main members: the Calcários de Vale da Serra and the Dolomitos de Furadouro.[1]

Description

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Geological setting of the Formation at the Cabeço da Ladeira paleontological site

The Chão das Pias Formation has an average thickness of 50-60 meters, occasionally exceeding 80 meters, particularly in the eastern sector where dolomitic facies are prominent.[1] It shows lateral and vertical variations, transitioning upward to dolomitic limestones and dolomites that link to overlying formations without clear paleontological boundaries.[1] It´s lithology is mostly and primarily made of slightly clayey or marly limestones, yellowish-gray, in decimetric beds, lacking notable macroscopic sedimentary structures. Siliceous nodules are common at the base, limestones dominate in the west, while calco-dolomitic and dolomitic facies prevail in the east, reflecting regional facies changes.[1] Is best exposed near the top of Fórnea/Barranco do Zambujal, extending through the area between Chão das Pias and Cabeço do Carvalho, slightly northeast of Serro Ventoso.[1] Lower and middle parts are visible near Chaínça hill and mill (southeast of Chão das Pias) and toward S. Bento-Covão do Sabugueiro. Upper dolomitic facies (Dolomitos de Furadouro) are exposed in quarries around Lagoa do Furadouro, Serra dos Candeeiros.[1]

The Calcários de Vale da Serra is the lower member, named after the Vale da Serra area and geodetic vertex, southeast of Minde, on the southern edge of the Serra de Aire. It´s made mostly of clayey or marly limestones, yellowish-gray, in decimetric beds, occasionally with centimetric foliated interbeds. The base features micrites and biomicrites (Mudstone and Wackestone) with fine bioclasts of crinoids, echinoids, sponge spicules, ostracods, lagenids, and calcispheres. Siliceous nodules, millimetric to decimetric, occur in the first 15 meters, with a notable level of larger, botryoidal nodules about 40 meters from the top. Upward, limestones become more compact, transitioning to intrapelmicritic and biopelmicritic limestones with ostracods, filaments, and bioturbation, followed by intra-biomicrites with resedimented ooids. At the top, centimetric laminar beds and calciclastic limestones, sometimes dolomitized, are present.[1]

The Dolomitos de Furadouro is the upper level, named after Lagoa do Furadouro, on the northeastern edge of the Serra de Aire.[1] Is predominantly made of massive dolomites, with interbeds of calcitic dolomites and dolomitic limestones. Dolomites are dolomicrosparites and dolosparites, with planar subhedral to euhedral crystalline textures, exhibiting intense moldic or vuggy porosity and karstification. Interbeds include intraclastic and oobioclastic limestones with varying dolomitization (“packstone-grainstone”), containing rare textulariids. Oncolitic and fenestral dolomicrites are frequent toward the top, with the upper part richer in microfossils like foraminifera.[1]

Paleoenvironment

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Paleoenvironment reconstruction of Cabeço da Ladeira beach

This unit was deposited in the Lusitanian Basin during the Middle Jurassic, likely within an inner carbonate platform setting. Micritic and biomicritic limestones with fine bioclasts (crinoids, echinoids, sponge spicules, ostracods, etc.) indicate a calm, low-energy marine environment, typical of lagoonal or platform interior zones.[1][2] Siliceous nodules suggest episodic silica input, possibly from volcanic activity or oceanic circulation. Calciclastic limestones and resedimented ooids reflect higher-energy episodes, such as storms or currents, in areas closer to agitated zones, likely platform margins or shoals. Dolomites, with moldic and vuggy porosity and karstification, point to diagenesis in subaerial or hypersaline fluid settings, characteristic of sabkhas or reflux zones on carbonate platforms. Lateral variation indicate a shift from open carbonate deposition to more restricted, evaporitic conditions eastward. The age, spanning the Lower to Upper Bajocian and possibly the base of the Bathonian, reflects dynamic sedimentation during a period of rift-related basin evolution.[1]

Cabeço da Ladeira Lagerstätte

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Example of the exceptional preservation of echinoderms at the Cabeço da Ladeira site

At Cabeço da Ladeira, a 50-60 m sequence of Calcários de Vale da Serra starts with greyish-yellow marly and clayish limestones (mudstone–wackestone micrites and biomicrites), grading to calciclastic and dolomitic limestones. The cm-scale beds exhibit ripple marks, microbial mat structures, and bioturbation, with clay layers filling epichnial trails. The sequence is dated to the Humphriesianum ammonite subzone (top of lower Bajocian) based on ammonite finds.[2] This sequence is known as the "Jurassic beach", as it became known for its exceptional preservation of coastal invertebrate faunas.[3] Some famous finds are the potential brachyuran Laterigradus lusitanica, possibly the longest invertebrate trackway ever recorded, as well as a crinoid crawling trail (Krinodromos bentou) a rare example of autotomy-based movement in the fossil record, rhinobatid(?) ray resting imprints, feeding pits, and possible regurgitates.[3]

Paleobiota

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The Chão das Pias Formation preserves a diverse Middle Jurassic biota, primarily from the Calcários de Vale da Serra and sparsely from the Dolomitos de Furadouro.[2] Microbial mats are prominent in the Calcários de Vale da Serra, evidenced by wrinkle structures, Kinneya- and flat-topped ripples, ripple patches, mud chips, blisters, and shrinkage cracks, indicating a tidal mudflat environment with episodic subaerial exposure. Foraminifera are present in both members, with rare agglutinated Textulariida in Furadouro and unspecified foraminifera more abundant in its upper part, while occurring without specific genera in Vale da Serra.[2]

Body Fossils

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Genus Species Stratigraphic Position Material Notes Images
Actinostreon[2] gregareum Cabeço da Ladeira Shells A bivalve of the family Ostreidae
Camptonectes[2][3] cf. obscurus Cabeço da Ladeira Shells A bivalve of the family Pectinidae
Ctenostreon[2] rugosum Cabeço da Ladeira Shells A bivalve of the family Limidae
Entolium[2] corneolum Cabeço da Ladeira Shells A bivalve of the family Pectinidae
Entolium specimens
Eopecten[2][3] spondyloides Cabeço da Ladeira Shells A bivalve of the family Pectinidae
Gymnodiadema[2][3] hessi Cabeço da Ladeira Semi-articulated skeletons An echinoid of the family Diadematidae
Gymnocidaris[2] cf. guerangeri Cabeço da Ladeira Semi-articulated skeletons An echinoid of the family Cidaridae
Heterocidaris[2][3] solaris Cabeço da Ladeira Semi-articulated skeletons An echinoid of the family Cidaridae
Laevitomaria[2] sp. Cabeço da Ladeira Shells A gastropod of the family Pleurotomariidae
Litoceratidae[2] gen. et sp. indet. Cabeço da Ladeira Phragmocones An ammonite of the family Litoceratidae
Noviaster[2] sp. nov. Cabeço da Ladeira Semi-articulated skeletons An asteroid of the family Asteriidae
Paracidaris[2] spinulosa Cabeço da Ladeira Semi-articulated skeletons An echinoid of the family Cidaridae
Pleurotomaria[2][3] sp. Cabeço da Ladeira Shells A gastropod of the family Pleurotomariidae
Pleurotomaria specimens
Praechlamys[2] textoria Cabeço da Ladeira Shells A bivalve of the family Pectinidae
Propeamussium[2][3] aff. pumilum, laevirudiatum Cabeço da Ladeira Shells A bivalve of the family Propeamussiidae
Pseudopecten[2] barbatus, dentatus Cabeço da Ladeira Shells A bivalve of the family Pectinidae
Rhabdocidaris[2] sp. A, sp. B Cabeço da Ladeira Semi-articulated skeletons An echinoid of the family Cidaridae
Spondylopecten[2][3] cardinatus, subspinosus, palinurus Cabeço da Ladeira Shells A bivalve of the family Pectinidae
Stephanoceras[2] mutabile Cabeço da Ladeira Phragmocones An ammonite of the family Stephanoceratidae
Stephanoceras specimen
Stomechinus[2] bigranularis, sp. Cabeço da Ladeira Semi-articulated skeletons An echinoid of the family Stomechinidae

Ichnofossils

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Genus Species Location Material Made by Images
Archaeonassa[2] isp. Cabeço da Ladeira Trackway Gastropod
Asterosoma[3] ludwigae Cabeço da Ladeira Burrow Worm-like invertebrate
Diplopodichnus[3] isp. Cabeço da Ladeira Trackway Myriapod or similar arthropod
Gyrochorte[3] comosa Cabeço da Ladeira Burrow Deposit-feeding invertebrate
Haplotichnus[3] indianensis Cabeço da Ladeira Trackway Arthropod (tentative)
Krinodromos[3] bentou Cabeço da Ladeira Trail Isocrinid crinoid
Laterigradus[3] lusitanica Cabeço da Ladeira Trackway Brachyuran crab
Rhizocorallium[3] R. commune Cabeço da Ladeira Burrow (spreite) Crustaceans (possibly Protaxius)
Piscichnus[2] isp. Cabeço da Ladeira Feeding burrow Rajiform
Thalassinoides[3] suevicus Cabeço da Ladeira Burrow Ghost shrimp or similar decapod
Undichna[3] isp. Cabeço da Ladeira Swimming trail Fish (fin traces)

See also

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References

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  1. ^ a b c d e f g h i j k l m n "Formalização da litostratigrafia do Jurássico Inferior e Médio do Maciço Calcário Estremenho (Bacia Lusitânica)". Universidade de Lisboa (in Portuguese). Retrieved 2025-06-04.
  2. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa Machado, Susana; Mergulhão, Lia; Pereira, Bruno Claro; Pereira, Pedro; Carvalho, Jorge; Anacleto, José António; Neto de Carvalho, Carlos; Belo, João; Paredes, Ricardo; Baucon, Andrea (2021-08-30). "Geoconservation in the Cabeço da Ladeira Paleontological Site (Serras de Aire e Candeeiros Nature Park, Portugal): Exquisite Preservation of Animals and Their Behavioral Activities in a Middle Jurassic Carbonate Tidal Flat". Geosciences. 11 (9): 366. Bibcode:2021Geosc..11..366M. doi:10.3390/geosciences11090366. hdl:10316/95938. ISSN 2076-3263.
  3. ^ a b c d e f g h i j k l m n o p q r Carvalho, Carlos Neto de; Pereira, Bruno Claro; Klompmaker, Adiel; Baucon, Andrea; Moita, José António; Pereira, Pedro; Machado, Susana; Belo, João; Carvalho, Jorge M. F.; Mergulhão, Lia (3 May 2016). "Running crabs, walking crinoids, grazing gastropods: behavioral diversity and evolutionary implications in the Cabeço da Ladeira lagerstätte (Middle Jurassic, Portugal)". Comunicações Geológicas: 39–54. ISSN 0873-948X. Retrieved 8 June 2024.
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