The Tremp Formation (, ), alternatively described as Tremp Group (), is a geological formation in the comarca Pallars Jussà, Lleida, Spain. The formation is restricted to the Tremp or Tremp-Graus Basin (), a piggyback foreland basin in the Catalonian Pre-Pyrenees. The formation dates to the Maastrichtian to Thanetian,Pujalte & Schmitz, 2005, p.82 thus the formation includes the Cretaceous-Paleogene boundary that has been well studied in the area, using paleomagnetism and carbon and oxygen isotopes. The formation comprises several lithologies, from sandstone, conglomerates and shales to marls, siltstones, limestones and lignite and gypsum beds and ranges between in thickness. The Tremp Formation was deposited in a continental to marginally marine fluvial-lacustrine environment characterized by estuarine to deltaic settings.
The Tremp Basin evolved into a sedimentary depression with the break-up of Pangea and the spreading of the North American and Eurasian plates in the Early Jurassic. Rifting between Africa and Europe in the Early Cretaceous created the isolated Iberian microplate, where the Tremp Basin was located in the northeastern corner in a back-arc basin tectonic regime. Between the middle Albian and early Cenomanian, a series of pull-apart basins developed, producing a local unconformity in the Tremp Basin. A first phase of tectonic compression commenced in the Cenomanian, lasting until the late Santonian, around 85 Ma, when Iberia started to rotate counterclockwise towards Europe, producing a series of piggyback basins in the southern Pre-Pyrenees. A more tectonically quiet posterior phase provided the Tremp Basin with a shallowing-upward sequence of marine carbonates until the moment of deposition of the Tremp Formation, in the lower section still marginally marine, but becoming more continental and lagoonal towards the top.
Shortly after deposition of the Tremp Formation, the Boixols Thrust, active to the north of the Tremp Basin and represented by the Sant Corneli anticline, started a phase of tectonic inversion, placing upper Santonian rocks on top of the northern Tremp Formation. The main phase of movement of another major thrust fault, the Montsec to the south of the Tremp Basin, happened not before the Early Eocene. Subsequently, the western Tremp Basin was covered by thick layers of conglomerates, creating a purely continental foreland basin, a trend observed going westward in the neighboring foreland basins of Ainsa and Jaca.
A rich and diverse assemblage of fossils has been reported from the formation, among which more than 1000 dinosaur bones, tracks dating up to just 300,000 years before the Cretaceous-Paleogene boundary, and many well-preserved eggs and nesting sites in situ, spread out over an area of . Multiple specimens and newly described genera and species of crocodylians, mammals, turtles, lizards, amphibians and fish complete the rich vertebrate faunal assemblage of the Tremp Formation. Additionally, fresh-to-brackish water clams as Corbicula laletana, bivalves of Hippurites castroi, gastropods, plant remains and cyanobacteria as Girvanella were found in the Tremp Formation. The unique paleoenvironment, well-exposed geology, and importance as national heritage has sparked proposals to designate the Tremp Formation and its region as a protected geological site of interest since 2004, much like the Aliaga geological park and others in Spain.Bosch Lacalle, 2004, p.40
Due to the exposure, the interaction of tectonics and sedimentation and access, the formation is among the best studied stratigraphic units in Europe, with many universities performing geological fieldwork and professional geologists studying the different lithologies of the Tremp Formation. The abundant paleontological finds are displayed in the local natural science museums of Tremp and Isona, where educational programs have been established explaining the geology and paleobiology of the area. In 2016, the Tremp Basin and surrounding areas were filed to become a Global Geopark, and on April 17, 2018, UNESCO accepted this proposal and designated the site Conca de Tremp-Montsec Global Geopark.Conca de Tremp-Montsec Global Geopark - UNESCO.org Spain hosts the second-most Global Geoparks in the world, after China.Global Geoparks Network - Members list
Etymology The Tremp Formation was defined and named in 1968 by Mey et al., just as the Tremp Basin after the Pre-Pyrenean town of Tremp.Rosell et al., 2013, p.19 The various subdivisions of the formation or alternatively called group, are named after the villages, rivers, canyons and hills in the basin.
Description
The Tremp Formation is a marginally marine to fluvial to lacustrine and continental sedimentary unit with a thickness varying between .Arribas et al., 1996, p.11 The formation is found in the Tremp-Graus Basin, a piggyback basin enclosed by the Sant Corneli anticline in the north, the Boixols Thrust in the northeast, the Montsec Thrust in the south and the Collegats Formation in the west.Bosch Lacalle, 2004, p.18 The Tremp-Graus Basin is bordering the Ainsa Basin to the west, and the Àger Basin to the south.Bosch Lacalle, 2004, p.23 The basin is subdivided into four synclinal areas, from east to west Vallcebre, Coll de Nargó, Tremp and Àger.Blanco et al., 2014, p.3 While in Benabarre, the Tremp Formation overlies the Arén Formation, in Fontllonga the formation rests on top of the Les Serres Limestone.López Martínez et al., 1996, p.63 The formation is partly laterally equivalent with the Arén Formation.Prieto Márquez et al., 2013, p.2 The Tremp Formation is stratigraphically overlain by the late Paleogene, locally called Ilerdiense, Àger Formation and the Alveolina Limestone,De Renzi, 1996, p.205 though in many parts of the Tremp Basin the formation is exposed and covered by alluvium.
The formation comprises several different lithologies, as sandstones, shales, limestones, marls, lignites, gypsum beds, conglomerates and siltstones have been registered.Arribas et al., 1996, p.17
The start age of the Tremp Formation has been established on the basis of the presence of Abathomphalus mayaroensis, a planktonic foraminiferan indicative of the latest Maastrichtian age of the formation. The lower section of the formation at the Elías site has been dated at 67.6 Ma,Puértolas et al., 2011, p.2 while the top of the Tremp Formation, in the western portion of the basin overlain by the Alveolina Limestone,Serra Kiel et al., 1994, p.276 named due to the abundance of Alveolina, is set at 56 Ma.Barnolas & Gil Peña, 2001, p.24
On the northern side of the axial zone of the Pyrenees, in the French sub-Pyrenean zone and Aquitaine Platform of the foreland basin bordering the mountain range, the time-equivalent stratigraphic units of the Tremp Formation are the Mas d'Azil Formation and Marnes d'Auzas Formation for the latest Maastrichtian, the Entonnoir Formation for the Danian and the Rieubach Group correlating with the Thanetian portion of the Tremp Formation.Ford et al., 1967, p.434
Subdivisions Studies performed in the 1990s described the Tremp Formation, also called Garumnian (),Cuevas, 1992, p.97Arribas et al., 1996, p.10 as a group with a subdivision into:Cuevas, 1992, p.96
Claret Formation Etymology - Claret Type section - along the road 1311Cuevas, 1992, p.103 Thickness - up to Lithologies - ochre to red shales, gypsum beds and intercalated sandstones and conglomerates Depositional environment - transitional marine to continental
La Guixera Member Etymology - La Guixera Type section - Mongai Thickness - Lithologies - gypsum beds alternating with shales, sandstones and conglomerates Depositional environment - evaporitic lacustrine deposits at times of retrogradation of alluvial fansCuevas, 1992, p.106
Esplugafreda Formation
Etymology - Esplugafreda canyon Type section - Barranco de Esplugafreda, in the valley of the Ribagorçana River east of Areny de NogueraCuevas, 1992, p.102 Thickness - Lithologies - continental red beds; shales, sandstones and conglomerates Depositional environment - alluvial fans
Sant Salvador de Toló Formation Etymology - Sant Salvador de Toló Type section - Conquès River Thickness - Lithologies - micritic limestones and greenish shales Depositional environment - lacustrine to coastal
Talarn Formation
Etymology - Talarn Type section - Barranco de La MataCuevas, 1992, p.101 Thickness - Lithologies - fining-upward sequence of sandstones and conglomerates at the base, grading into siltstones and shales near the top Depositional environment - alluvial channel and overbank deposits
Conquès Formation Etymology - Conquès River Type section - Barranco de BastursCuevas, 1992, p.100 Thickness - Lithologies - greenish shales, sandstone lenses and conglomerates at the base Depositional environment - perilagoonal
Tossal d'Obà Member
Etymology - Tossal d'Obà Type section - Tossal d'Obà Hill Thickness - Lithologies - micritic limestones and marls Depositional environment - distal fluvial to lagoonal-barrier island
Basturs Member Etymology - Basturs Type section - Barranco de Basturs Thickness - Lithologies - micritic limestones, greenish shales and bioturbated fine sandstones Depositional environment - perilagoonal
Posa Formation
Etymology - Ermita La PosaMuseu de la Conca Dellà - La Posa Type section - Isona anticlinalCuevas, 1992, p.99 Thickness - Lithologies - grey shales, limestones, marls, lignite and sandstones Depositional environment - lagoonal to barrier island
Alternative subdivisions An alternative subdivision uses Grey Garumnian at the base, overlain by Lower Red Garumnian and Vallcebre Limestone at the top.Bravo et al., 2005, p.51 The Vallcebre limestone is laterally equivalent with another described unit, the Suterranya Limestone.Díez Canseco, 2016, p.53 Pujalte and Schmitz in 2005 defined another member, the Claret Conglomerate, as representative of a conglomeratic bed inside the Claret Formation.
In 2015, a new unit was allocated to the uppermost Cretaceous section of the Tremp Group, near the top of the Lower Red Garumnian. The thick series of lithologically mature coarse-grained sandstones and microconglomerates rich in feldspars is positioned below the Danian Vallcebre Limestone and was called the Reptile Sandstone.Blanco et al., 2015b, p.148
Tectonic evolution
The Tremp Basin was formed in the northeastern corner of the Iberian plate, a microplate that existed as a separate tectonic block between the Eurasian and African plates since the Hercynian orogeny that formed the supercontinent Pangea. Progressive opening of the Atlantic Ocean between the Americas and at first Africa, later Iberia and finally Europe, caused large differential motions between these continents,Andeweg, 2002, Ch.1 p.1 with extensional tectonics starting in the Early Jurassic with the opening of the Neotethys ocean between southwestern Europe and Africa.Sibuet et al., 2004, p.3 During this period, evaporites were deposited in the rift basins,García Senz, 2002, p.264 later in the tectonic history becoming important décollement surfaces for the compressional movements.López Mir et al., 2014, p.15 The phase of extension continued into the Early Cretaceous when the Iberian plate started to move counterclockwise to converge with the Eurasian plate.Rushlow et al., 2013, p.844
Back-arc basin