The Chão das Pias Formation is a geological formation of Bajocian (Middle Jurassic) age in the Lusitanian Basin of Portugal. Named after the village of Chão das Pias, situated slightly south of Serro Ventoso in the Santo António plateau. 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. 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.
Description
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. It shows lateral and vertical variations, transitioning upward to dolomitic limestones and dolomites that link to overlying formations without clear paleontological boundaries. Its 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. 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. 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.
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.
The Dolomitos de Furadouro is the upper level, named after Lagoa do Furadouro, on the northeastern edge of the Serra de Aire. 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.
Paleoenvironment
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. 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.
Cabeço da Ladeira Lagerstätte
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. This sequence is known as the "Jurassic beach", as it became known for its exceptional preservation of coastal invertebrate faunas. 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.
Paleobiota 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. 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.
Body Fossils GenusSpeciesStratigraphic PositionMaterialNotesImagesActinostreongregareumCabeço da LadeiraShellsA bivalve of the family OstreidaeCamptonectescf. obscurusCabeço da LadeiraShellsA bivalve of the family PectinidaeCtenostreonrugosumCabeço da LadeiraShellsA bivalve of the family LimidaeComatulida indet.IndeterminateCabeço da LadeiraSemi-articulated skeletonsCrinoidEntoliumcorneolumCabeço da LadeiraShellsA bivalve of the family PectinidaeEopectenspondyloidesCabeço da LadeiraShellsA bivalve of the family PectinidaeGoniasteridae indet.IndeterminateCabeço da LadeiraSemi-articulated skeletonsStarfishGymnodiademahessiCabeço da LadeiraSemi-articulated skeletonsAn echinoid of the family DiadematidaeGymnocidariscf. guerangeriCabeço da LadeiraSemi-articulated skeletonsAn echinoid of the family CidaridaeHeterocidarissolaris; sp. novCabeço da LadeiraSemi-articulated skeletonsAn echinoid of the family CidaridaeIsocrinida indet.IndeterminateCabeço da LadeiraSemi-articulated skeletonsCrinoidLaevitomariasp.Cabeço da LadeiraShellsA gastropod of the family PleurotomariidaeLitoceratidaegen. et sp. indet.Cabeço da LadeiraPhragmoconesAn ammonite of the family LitoceratidaeNoviastersp. nov.Cabeço da LadeiraSemi-articulated skeletonsAn asteroid of the family AsteriidaeParacidarisspinulosaCabeço da LadeiraSemi-articulated skeletonsAn echinoid of the family CidaridaePleurotomariasp.Cabeço da LadeiraShellsA gastropod of the family PleurotomariidaePraechlamystextoriaCabeço da LadeiraShellsA bivalve of the family PectinidaePropeamussiumaff. pumilum, laevirudiatumCabeço da LadeiraShellsA bivalve of the family PropeamussiidaePseudopectenbarbatus, dentatusCabeço da LadeiraShellsA bivalve of the family PectinidaeRhabdocidarissp. A, sp. BCabeço da LadeiraSemi-articulated skeletonsAn echinoid of the family CidaridaeSpondylopectencardinatus, subspinosus, palinurusCabeço da LadeiraShellsA bivalve of the family PectinidaeStephanocerasmutabileCabeço da LadeiraShellsAn ammonite of the family StephanoceratidaeStomechinusbigranularis, sp.Cabeço da LadeiraSemi-articulated skeletonsAn echinoid of the family Stomechinidae
Ichnofossils Genus Species Location Material Made by ImagesArchaeonassafossulataCabeço da LadeiraTrackwayGastropod Asterosoma ludwigae Cabeço da Ladeira Burrow Worm-like invertebrate Diplopodichnus isp. Cabeço da Ladeira Trackway Myriapod or similar arthropod (Axiidae?) Gyrochorte comosa Cabeço da Ladeira Burrow Deposit-feeding invertebrate, worm or arthropod Haplotichnus indianensis Cabeço da Ladeira Trackway Diptera larvae (tentative) Krinodromos bentou Cabeço da Ladeira Trail Isocrinid crinoid Laterigradus lusitanica Cabeço da Ladeira Trackway Brachyuran crab Rhizocorallium commune Cabeço da Ladeira Burrow (spreite) Crustaceans (possibly Axiidae)Piscichnusisp.Cabeço da LadeiraFeeding burrowRajiformes Thalassinoides suevicus Cabeço da Ladeira Burrow Ghost shrimp or similar decapod Undichna isp. Cabeço da Ladeira Swimming trail Fish (fin traces)
See also List of dinosaur-bearing rock formations
Blue Lias, England Charmouth Mudstone Formation, England Zagaje Formation, Poland Drzewica Formation, Poland Ciechocinek Formation, Poland Borucice Formation, Poland Rotzo Formation, Italy Saltrio Formation, Italy Moltrasio Formation, Italy Marne di Monte Serrone, Italy Calcare di Sogno, Italy Podpeč Limestone, Slovenia El Pedregal Formation, Spain Aganane Formation, Morocco Tafraout Group, Morocco Azilal Formation, Morocco El Mers Group, Morocco Anoual Formation, Morocco Budoš Limestone, Montenegro Kota Formation, India Cañadón Asfalto Formation, Argentina Los Molles Formation, Argentina Kandreho Formation, Madagascar Elliot Formation, South Africa Clarens Formation, South Africa Evergreen Formation, Australia Cattamarra Coal Measures, Australia Hanson Formation, Antarctica Mawson Formation, Antarctica
References