Chão das Pias Formation

File:Cabeço da Ladeira geology.png

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. 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. 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.

File:Cabeço da Ladeira tidal flat.png

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.

File:Cabeço da Ladeira echinoderms.png

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 know as the "Jurassic beach", as it became known for its exceptional preservation of coastal invertebrate faunas.{{Cite journal |last1=Carvalho |first1=Carlos Neto de |last2=Pereira |first2=Bruno Claro |last3=Klompmaker |first3=Adiel |last4=Baucon |first4=Andrea |last5=Moita |first5=José António |last6=Pereira |first6=Pedro |last7=Machado |first7=Susana |last8=Belo |first8=João |last9=Carvalho |first9=Jorge M. F. |last10=Mergulhão |first10=Lia |date=3 May 2016 |title=Running crabs, walking crinoids, grazing gastropods: behavioral diversity and evolutionary implications in the Cabeço da Ladeira lagerstätte (Middle Jurassic, Portugal) |url=https://repositorioaberto.uab.pt/handle/10400.2/5285 |journal=Comunicações Geológicas |pages=39–54 |issn=0873-948X |access-date=8 June 2024}} 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.

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.

• List of dinosaur-bearing rock formations

{{div col|colwidth=29em}}

• 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

{{div col end}}

{{Reflist}}

{{DEFAULTSORT:Chao das Pias Formation}}

Category:Geologic formations of Portugal

Category:Jurassic System of Europe

Category:Limestone groups

Category:Paleontology in Portugal

Category:Bajocian Stage

Category:Jurassic Europe