Tremp Formation

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. 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 . 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. The basin is subdivided into four synclinal areas, from east to west Vallcebre, Coll de Nargó, Tremp and Àger. While in Benabarre, the Tremp Formation overlies the Arén Formation, in Fontllonga the formation rests on top of the Les Serres Limestone. The formation is partly laterally equivalent with the Arén Formation. The Tremp Formation is stratigraphically overlain by the late Paleogene, locally called Ilerdiense, Àger Formation and the Alveolina Limestone, 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. 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. while the top of the Tremp Formation, in the western portion of the basin overlain by the Alveolina Limestone, named due to the abundance of Alveolina, is set at 56 Ma. 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. Subdivisions Studies performed in the 1990s described the Tremp Formation, also called Garumnian (), as a group with a subdivision into: Claret Formation • Etymology - Claret • Type section - along the road 1311 • 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 Esplugafreda Formation • Etymology - Esplugafreda canyon • Type section - Barranco de Esplugafreda, in the valley of the Ribagorçana River east of Areny de Noguera • 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 - 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 Basturs • 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 • Type section - Isona anticlinal • 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. The Vallcebre limestone is laterally equivalent with another described unit, the Suterranya Limestone. Pujalte and Schmitz in 2005 defined another member, the Claret Conglomerate, as representative of a conglomeratic bed inside the Claret Formation. == Tectonic evolution ==

limestones on top of the younger Maastrichtian Tremp Formation 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, with extensional tectonics starting in the Early Jurassic with the opening of the Neotethys ocean between southwestern Europe and Africa. During this period, evaporites were deposited in the rift basins, later in the tectonic history becoming important décollement surfaces for the compressional movements. The phase of extension continued into the Early Cretaceous when the Iberian plate started to move counterclockwise to converge with the Eurasian plate. Back-arc basin Approximately from the late Berriasian to late Albian (120 to 100 Ma), the Iberian plate was an isolated island, separated from current southern France by a mostly shallow sea with a deeper pelagic channel in between the southwestern Eurasian and northeastern Iberian coasts. The present-day area of the Pyrenees with an area of in those times was much larger due to the various episodes of compressional tectonic forces and resulting shortening afterwards. The Tremp Basin, alternatively called Organyà Basin, was the depocenter of sedimentation during the late Early Cretaceous, showing an estimated vertical sedimentary thickness of comprising mostly hemipelagic marls and limestones, deposited in a back-arc basin setting with normal faults parallel to the Pyrenean axis, and cross-cut by transverse faults, separating the various west-to-east minibasins. These minibasins showed a deepening trend from the Gulf of Biscay to the Mediterranean. At the end of formation of the back-arc basin, around 95 Ma, high temperature metamorphism developed as a result of crustal thinning synchronously or immediately after the Albian to Cenomanian basin formation. Lower crustal granulitic rocks, as well as ultramafic upper mantle rocks (lherzolites) were emplaced along the prominent North Pyrenean Fault (NPF) crustal feature. The North Pyrenean Fault developed during the sinistral (left-lateral) displacement of the Iberian plate, which age is determined by the age of flysch pull-apart basins formed synchronously with the strike-slip movement along the NPF from Middle Albian to Early Cenomanian. This period is characterized by a local unconformity in the Tremp Basin, while this is not registered farther to the west of the Pre-Pyrenean minibasins near Pont de Suert. Tectonic inversion The previous phase was followed by a tectonically more quiet setting in the basins surrounding the slowly rising Pyrenees. Research published in 2014 has revealed a renewed phase of evaporitic deposition from the Coniacian to Santonian in the Cotiella Basin, west of the Tremp Basin. The relative tectonic quiescence lasted until the late Santonian, approximately around 85 Ma, At this time, continental subduction and back-arc basin inversion commenced, As is common in inverted tectonic regimes, the normal faults of the early Mesozoic were reactivated into reverse faults at the end of the Cretaceous and continuing into the Paleogene. due to the large thickness and poor seismic resolution, but later analysis using tomography has identified this feature below the Pre-Pyrenean chain. The presence of lithospheric subduction is a common feature in other Alpine orogenic chains as the Alps and Himalayas. Piggyback basin From the late Santonian to the late Maastrichtian, on the different thrust sheets of the southward compressional Pre-Pyrenees, a series of piggyback basins were formed, one of which was the Tremp Basin. The bathymetry of these basins show a general deepening towards the west, with major turbidite deposition in the Ainsa Basin and farther west. In the Jaca Basin, to the west of the Ainsa and Tremp Basins, during the Middle Eocene, flysch was deposited in an underfilled basin setting, while in the western Tremp Basin thick conglomerates, known as the Collegats Formation, were deposited, sourced by the various thrust sheets in the hinterland. Boixols and Montsec thrusting The Boixols–Cotiella thrust sheet was emplaced since the Late Cretaceous, placing late Santonian rocks on top of the northernmost Tremp Formation, found in the subsurface underneath the Sant Corneli anticline. This was followed by the tectonic movement of the Montsec–Peña Montañesa thrust sheet during the Early Eocene and the western Sierras Exteriores thrust sheet from the Mid-Eocene to Early Miocene. The dating of the Montsec Thrust has been established on the basis of the stratigraphies of the overlying hanging wall (Triassic to Cretaceous) onto the Lutetian (locally called Cuisian) fluvial sediments of the Àger Basin to the south of the Montsec. These tectonic movements are indicative of the main uplift phase of the Pyrenees. offshore Tunisia, the Zagros Mountains of Iraq and Iran, northern Carpathians in Poland, western, and eastern Colombian, along the Eastern Frontal Fault System of the Eastern Ranges of the Andes, the Al Hajar Mountains of Oman, Dnieper-Donets Basin in Ukraine, the Sivas Basin in Turkey, the Kohat-Potwar fold and thrust belt of Pakistan, the Flinders Ranges in South Australia, during the Eurekan orogeny in the Sverdrup Basin of northeastern Canada and western Greenland, and many more. In the western Cotiella Basin, salt inflation and withdrawal played a major role in the differential sedimentary thicknesses, facies changes and tectonic movements. Eocene to recent After the Middle Eocene, thick conglomerates were deposited in the western Tremp Basin and the thrust sheets reached their maximum displacement, this led to a shift of the depocenter from the Pre-Pyrenees towards the Ebro Basin. Paleomagnetic data show that the Iberian plate went through another phase of counterclockwise rotation, though not as fast as in the Santonian. Between 25 and 20 Ma, in the late Oligocene and early Miocene, a rotation of 7 degrees has been noted. This phase of rotation correlated with the thrusting in the westernmost areas of the southern Pre-Pyrenees, the Sierras Marginales, leading to continental conditions in that area from the early Miocene (Burdigalian) onwards. == Depositional history ==

The depositional environment of the Tremp Formation varies between continental, lacustrine, fluvial, and marginally marine (estuarine to deltaic and coastal). The continental deposits in the east of the basin have been interpreted as the distal part of alluvial fans, while the presence of cyanobacteria Girvanella in the lacustrine limestones indicates variability in salinity in the lacustrine areas and a possible lateral relation with transitional environments. The presence of great quantities of the fungus Microcodium indicates traces of rootlets. The top of the Tremp Formation is close to the Paleocene–Eocene Thermal Maximum, which could explain the relative lack of diversity in mammal genera. • Formation of an estuarine regime near the end of a Cretaceous regression in the Pyrenean basins, characterized by coastal plains where thick clays were deposited, cut by sporadic fluvial channels. At the margins of the basin, swampy conditions existed with sedimentation of carbonates. In these zones, the last dinosaurs inhabiting the area before the Cretaceous-Paleogene boundary left their marks in tracks, eggs and bones. These areas were accompanied by marshes, as evidenced by the many plant remains that produced the lignite deposits found in the lower part of the Tremp Formation. During this first phase in the sedimentary sequence of the formation, the Montsec was already a slightly elevated area in the south and along the submerged slopes of that hill, lacustrine limestones were deposited. • At the end of the Cretaceous, a geologically sudden drop of sea level happened, giving rise to a wide fluvial-dominated basin. In this environment, river channels deposited sandstones and abundant overbank clays with numerous paleosols in the basin. On the southern side of the rising Montsec, the Àger Basin, a similar fluvial system developed with a far more coarse-grained sandy character than in its northern counterpart around Tremp. The paleocurrents in the Àger Basin were towards the north and northwest. The enclosed continental basin turned into a more coastal environment at a transgressional phase with smaller channels where oncolites were laid down. The river systems on both sides of the Montsec were sourced by the easternmost parts of the present Pyrenees, with the Empordà High as provenance area. This east-to-west fluvial system, contrary to the present-day west–east flowing direction of the Ebro Basin, persisted until the Late Eocene. The uppermost unit of the Maastrichtian sequence, the coarse-grained Reptile Sandstone, has been interpreted as a fast-flowing braided river channel. The latest phase of depositional evolution is noted in a wider area in the Pre-Pyrenees and to the south in the Ebro Basin, that began its formation during the Eocene, building up to its present shape in Oligocene and Miocene times. == Cretaceous-Paleogene boundary ==

The Tremp Formation spans the latest stage of the Cretaceous (Maastrichtian) and the earliest stages of the Paleocene (Danian and Thanetian). This has made the formation one of a few European unique localities to study the K/T boundary. In the Tremp Basin, the boundary is registered at Coll de Nargó, Isona and Fontllonga and established on the basis of paleomagnetism and a strong decrease of ∂13C and ∂18O isotopes. The typical iridium layer, found in other sites where the Cretaceous-Paleogene boundary has been noted, as Gubbio in Italy and Caravaca in Spain, has not been registered in the Tremp Formation. == Paleontology ==

The Tremp Formation provided many fossilized dinosaur eggs. The dinosaur eggs of Basturs are contained in the formation bordering the Arén Formation and the area where eggs are found stretches out for . A great number of nests are visible as well as numerous fragments of egg shells. The presence of wave ripples indicates a beach-like environment where dinosaurs laid their eggs for a long time. The eggs are subcircular with diameters of approximately and egg shell thicknesses between . Many eggs are found in groups of between four and seven gatherings, indicating the in situ preservation of the nests. Also, remains of several genera of dinosaurs are described from the Tremp Formation. The Tremp and underlying Arén Formations are the richest sites for dinosaur fossils in the Pyrenees, with only at Basturs more than 1000 bone fragments found. The dinosaur paleofauna has been compared to Hațeg in Romania, famous for the pterosaur Hatzegopteryx named after the location. Furthermore, a rich variety of other reptiles, among which the new species and youngest fossil record of the Cretaceous turtle Polysternon; Polysternon isonae, as well as amphibians, lizards, fish, and mammals, have been registered, showing a unique faunal assemblage for the Cretaceous-Paleogene boundary, not found elsewhere in Europe. The holes found on the dip slope at Ermita La Posa were initially interpreted as tracks produced by sauropod dinosaurs. Later investigations and interpretations of the depositional environment of the Maastrichtian; the coastal origin of the trackbed with plenty of marine invertebrates, have led researchers to interpret part of the ichnofossils as feeding traces of rays in the intertidal zones. During their feeding activity, the rays produce holes in the top sedimentary layers, when they feed on marine invertebrates buried in the top sediment. Bothremydidae, crocodile teeth, theropod limbs, and hadrosaur femurs. This paper is the most comprehensive and up to date review of the fossils and stratigraphy of the formation. Sauropod nesting sites A detailed analysis of the nesting sites of Coll de Nargó, at the Pinyes locality, has been performed in 2010 by Vilat et al. The eggs were found in the lower portion of the Lower Red Garumnian, with local facies comprising calcareous silty mudstones, very fine to fine-grained sand bodies, and medium to coarse-grained sandstones. The rocks, in a thick interval, are interpreted as sedimentary deposits of a fluvial environment located some distance away from an active stream channel. Most eggs exposed at the Pinyes locality were incompletely preserved because of recent erosion; however, excavation occasionally revealed relatively intact specimens in the subsurface. Some eggs exposed in cross-section revealed numerous eggshell fragments, predominantly oriented concave up within the mudstone matrix that filled the egg interior. Analysis of the eggshells at Pinyes provided a range of in shell thickness, with a mean range of . Radial thin sections and SEM images of the eggshells showed a single structural layer of calcite. The eggshell surfaces displayed abundant elliptical pore openings that varied from 65 to 120 microns in width. The eggs, in clusters or "clutches" of up to 28 individual eggs, were described as Megaloolithus siruguei, an oospecies well documented from various localities in northern Catalonia and southern France. The description was done on the basis of egg size, shape, eggshell microstructure, tuberculate ornamentation, and the presence of transversal canals in a tubocanaliculate pore system, an unequivocal feature of this oospecies. The egg horizons within the Tremp Formation were continuous before the tectonic inversion phase of the basin. The compressional tectonic regime produced structural deformation of the egg-bearing strata. The dip of the beds in the mountainous region can contribute to misinterpretation of reproductive behavior, hence the analysis of the eggs in combination with tectonic stresses gives a more complete picture of the shapes of the eggs. An interpretation of the nest excavation at Pinyes was made and compared to other nesting sites of sauropods found all over the world, in particular in the Aix Basin of southern France, the Allen and Anacleto Formations of Argentina, and the Lameta Formation of India. The nest sizes and shapes of Pinyes show great similarities with the other analyzed sites. Research conducted in 2015 by Hechenleitner et al. include a comparison with the Cretaceous Sanpetru Formation of Hațeg paleo-island in Romania, the Los Llanos Formation at in Argentina, and the Boseong Formation of the Gyeongsang Basin in South Korea. A common nest size of 25 eggs has been suggested for the Pinyes locality. Small egg clusters that display linear or grouped egg arrangements reported at Pinyes and other localities likely reflect recent erosion. The distinct clutch geometry reported at Pinyes and other megaloolithid localities worldwide, strongly suggests a common reproductive behavior that resulted from the use of the hind foot for scratch-digging during nest excavation. Due to their size and weight, the titanosaurs could not heat the eggs by direct body contact, so must have relied on external environmental heat for incubating their eggs. However, modern megapode birds as the maleo (Macrocephalon maleo), the Moluccan megapode (Eulipoa wallacei) and scrubfowls (Megapodius spp.) in Southeast Asia and Australia, burrow their eggs using the heat in the top soil to incubate them and provide protection from predators. The egg spatial distribution, in small clusters linearly to compactly grouped, but contained in round shaped areas of up to would either support burrow- or mound-nesting at Pinyes. Hadrosaur ichnofossils Over 45 fossil localities yielded hadrosaurid fossils in the Lower Red Garumnian of the eastern Tremp Syncline. Furthermore, many hadrosaur ichnofossils have been found in the Tremp Formation and were analyzed in great detail by Vila et al. in 2013. The most abundant track types in fluvial settings are the pedal prints of hadrosaurs, while titanosaur ichnofossils and a single theropod track were found in lagoonal environments. The authors concluded: • The fluvial lower red unit of the Tremp Formation exhibits meandering and braided fluvial systems with favorable conditions for track production and preservation, like those of North America and Asia. • The dinosaurs mainly produced the tracks on the floodplain, within the channels, and on and within crevasse splay deposits in low water stage conditions, and the footprints were infilled by sands during high water stage (stream reactivation). • The track record is composed of abundant hadrosaur and scarce sauropod and theropod tracks. The hadrosaur tracks are significantly smaller in size but morphologically similar to comparable records in North America and Asia. They are attributable to the ichnogenus Hadrosauropodus. • A rich track succession composed of more than 40 distinct track levels indicates that hadrosaur footprints are found above the early Maastrichtian–late Maastrichtian boundary and most noticeably in the late Maastrichtian, with tracks occurring abundantly in the Mesozoic part of the C29r magnetochron, during the last 300,000 years of the Cretaceous. • The occurrence of hadrosaur tracks in the Ibero-Armorican island seems to be characteristic of the late Maastrichtian time interval and thus they are important biochronostratigraphic markers in the faunal successions of the Late Cretaceous in southwestern Europe. == Paleofauna ==

Invertebrates Bivalves } Gastropods Ostracods Verterbrates Amphibians Crocodilians Dinosaurs ; Ornithischians ; Sauropods ; Theropods Fish } Pterosauria Remains of Giant Azhdarchids have been found in the Tremp Formation. They possibly share affinities with another giant Azdarchid from the coveal Marnes d'auza Formation Mammals Squamates Turtles Oogenus Ichnofossils == Paleoflora ==

Flora Algae } ;Pollen Additionally, many pollen have been described from the Tremp Formation, east of Isona and east of Tremp: • Polypodiaceoisporites gracicingulis, P. maximus, P. tatabanyensis, P. vitiosus • Leiotriletes adriennis, L. dorogensis, L. microadriennis • Cycadopites kyushuensis, C. minar • Monocolpopollenites dorogensis, M. tranquillus • Semioculopollis croxtonae, S. praedicatus • Cicatricosisporites cf. triangulus • Cupressacites insulipapillatus • Cupuliferoipollenites pusillus • Cyrillaceaepollenites barghoorniacus • Granulatisporites palaeogenicus • Inaperturopollenites giganteus • Labraferoidaepollenites menatensis • Laevigatosporites haardti • Minorpollis hojstrupensis • Nudopollis minutus • Oculopollis cf. minoris • Pityosporites insignis • Plicapollis serta • Punctatisporites luteticus • Retitricolporites andreanszkyi • Rugulitriporites pflugi • Subtriporopollenites constans • Suemigipollis cf. triangulus • Tetracolporopollenites halimbaense • Trilobosporites (Tuberosisporites) • Vacuopollis cf. concavux • Granomonocolpites • Patellasporites • Platycaryapollenites • Polyporites • Retimonocolpites == Microbes ==

== Research and exhibitions ==

Every year, over 800 geologists visit El Pallars Jussà and more than 1500 university students from all over Europe come to the Tremp-Graus Basin to carry out their geological fieldwork. The basin is also regarded by petroleum companies as a perfect place to study the interplay of tectonic movements with the different types of lithologies. The Museu Comarcal de Ciències Naturals ("Local District Natural Science Museum") in Tremp, built attached to the Torre de Soldevila in the center of town, is a popular destination for school visits. It houses a permanent fossil exhibition with a wide variety of remains, ranging from dinosaurs to fossilized invertebrates such as corals, bivalves, gastropods, and more. The Museu de la Conca Dellà of Isona houses replicas of bone remains, restorations of dinosaurs and an authentic nest of eggs, left behind by the last dinosaurs to have lived in the valley during the Cretaceous period. The museum also contains numerous other archaeological remains from the Roman settlement of Isona. In recent years, the Consell Comarcal (Regional Council) has promoted several new initiatives, including the creation of a geological program especially adapted to local schools and a series of guided visits to the main archaeological sites of the region. The unique paleoenvironment, well-exposed geology, and importance as national heritage have sparked proposals to designate the Tremp Formation and its region as a protected geological site of interest, much like the Aliaga geological park and others in Spain. and included in the Global Geoparks Network. On April 17, 2018, UNESCO accepted the proposal and designated the site as Conca de Tremp-Montsec Global Geopark, stating: "This area is internationally recognized as a natural laboratory for sedimentology, tectonics, external geodynamics, palaeontology, ore deposits and pedology. In addition, other natural and cultural heritage is also remarkable including astronomy and archaeological sites." == Panoramas ==

Notes References Bibliography Regional geology • • • • • • • • • • • • • • • • Local geology • • • • • • • • • • Salt tectonics • • • • • • • • • • • • • • Paleontology publications Dinosaurs • • • • • • • • • • • Other groups • • • • • • • • • • • • • • == Further reading ==