Ipubi Formation | |
---|---|
Stratigraphic range: Aptian-Albian ~ | |
Type | Geological formation |
Unit of | Santana Group |
Underlies | Romualdo Formation |
Overlies | Crato Formation |
Thickness | up to 40 m (130 ft) average 15 m (49 ft) |
Lithology | |
Primary | Shale, anhydrite |
Other | Sandstone |
Location | |
Coordinates | 5°36′S64°18′W / 5.6°S 64.3°W |
Approximate paleocoordinates | 9°12′S34°00′W / 9.2°S 34.0°W |
Region | Pernambuco, Piauí & Ceará |
Country | Brazil |
Extent | Araripe Basin |
Type section | |
Named for | Ipubi |
Named by | Beurlen |
Year defined | 1971 |
The Ipubi Formation is found in the southeastern part of the Araripe Basin, near Ipubi |
The Ipubi Formation is the middle geological formation of the Santana Group, the middle part of the Araripe Group, in the Araripe Basin of northeastern Brazil. The formation is dated to the Aptian to Albian stages of the Early Cretaceous, unconformably overlying the Crato Formation and unconformably overlain by the Romualdo Formation, formerly known as the Romualdo Member of the Santana Formation. The averaging 15 metres (49 ft) thick Ipubi Formation comprises shales and sandstones in the lower section and evaporites in the upper part of the formation, deposited in a transgressive to highstand lacustrine environment in the Araripe rift basin.
The Ipubi Formation contains little fossils, apart from an indeterminate Pelomedusoides turtle, but contains high levels of TOC and the formation is identified as a potential target for shale gas development.
The Ipubi Formation was described as a member of the Santana Formation by Beurlen in 1971. [1] Later revision of the stratigraphy elevated the members of the former Santana Formation to separate formations; Crato, Ipubi and Romualdo from base to top. The formations were subsequently included in the Santana Group, representing the middle part of the formerly described Araripe Group. [2]
The tectono-sedimentary evolution of the Araripe Basin, located in the Borborema Geologic Province, encompasses four stages, with five tectonostratigraphic phases: [3]
1) Syneclise phase - Silurian to Devonian - characterized by tectonic quiescence in the Borborema Province. It is represented by the deposits of the Cariri Formation, that include medium to coarse-grained quartz sandstones, locally conglomeratic, deposited in large braided fluvial systems
2) Pre-rift phase - Tithonian - characterized by the mechanical subsidence due to lithosphere thinning that preceded the rift. It is represented by the Brejo Santo Formation, that comprises red shales and claystones, and the Missão Velha Formation, constituted by medium to coarse-grained quartz-feldspathic sandstones, locally conglomeratic, that contains entire trunks and fragments of silicified wood ( Dadoxilon benderi ) conifer
3) Rift phase - Berriasian to Hauterivian - characterized by increasing mechanical subsidence that created a system of grabens and half grabens. It is represented by the Abaiara Formation, that includes shales, siltstones, sandstones and conglomerates
4) Post-Rift I phase - Aptian to Albian - characterized by thermal subsidence. The lowermost unit Barbalha Formation, represents a fluviolacustrine phase and is composed of red and gray shales, siltstones and claystones.
The Santana Group was formed during this stage and comprises three stratigraphic units:
5) Post-rift II phase - Albian to Cenomanian - characterized by a major sag phase, and is formed by two stratigraphic units:
Palynomorph associations showed that the lacustrine system evolved in an arid–semiarid tropical area where the most characteristic flora consisted of conifers, Gnetales , Bennettitales and early angiosperms. The Ipubi Formation is characterized by containing abundant algal/bacterial-derived well-preserved organic matter. The pattern of organic matter accumulation and preservation corresponds well with cyclical paleoenvironmental changes (from anoxic to oxic bottom conditions and from a fresh to saline upper water layer), which affected the shallow lacustrine system. The deposition of thin, organic matter-rich mudstones probably occurred during higher water levels, which resulted in relatively low salinity waters and low detrital contribution to the inner lacustrine zones. [4]
Data collected from outcrops and boreholes indicate that the Ipubi Formation is separated from the underlying Crato Formation and the overlying Romualdo Formation by regional unconformities in proximal domains. The lower unconformity separates the top of the Crato Formation, which is mainly represented by its uppermost interval of laminated limestones (C6), from the basal black shales and claystone deposits of the Ipubi Formation. These basal deposits formed due to the transgressive event that expanded the relative lake level beyond the previous extension of the Crato Formation over the adjacent basement areas. During the Ipubi transgression, anoxic conditions influenced the formation of basal black shales with high levels of TOC. [5]
The carbonaceous pyrobitumen shales of the Ipubi Formation occur as a 0.5 to 2 metres (1.6 to 6.6 ft) layer in most of the mines in the region and have a potential for shale gas development. [6]
The deposition of the Ipubi Formation represents a transition from a transgressive systems tract to a maximum flooding surface. Six stages in the depositional cycle were described by Fabin et al. in 2018: [7]
While the underlying Crato and overlying Romualdo Formations are particularly rich in fossils, comprising flora, fish, arthropods, insects, snakes, turtles, dinosaurs and pterosaurs, [8] [9] [10] [11] [12] [13] the Ipubi Formation contains little fossil material. Fossils of a Pelomedusoides turtle were found in the formation. The material, archived as CPCA 3560, consists of crushed fragments of the skull, a partial lower jaw and the carapace. The fossil was described in 2011 by Ribeiro Oliveira et al., who were unable to definitively assign the specimen to a specific genus. [14] The same specimen was mentioned by Fielding et al. in 2005 and reported as Araripemys . [15]
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
The Crato Formation is a geologic formation of Early Cretaceous (Aptian) age in northeastern Brazil's Araripe Basin. It is an important Lagerstätte for palaeontologists. The strata were laid down mostly during the Aptian age, about 113 million years ago. It thought to have been deposited in a semi-arid lacustrine wetland environment.
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The Romualdo Formation is a geologic Konservat-Lagerstätte in northeastern Brazil's Araripe Basin where the states of Pernambuco, Piauí and Ceará come together. The geological formation, previously designated as the Romualdo Member of the Santana Formation, named after the village of Santana do Cariri, lies at the base of the Araripe Plateau. It was discovered by Johann Baptist von Spix in 1819. The strata were deposited during the Aptian stage of the Early Cretaceous in a lacustrine rift basin with shallow marine incursions of the proto-Atlantic. At that time, the South Atlantic was opening up in a long narrow shallow sea.
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The Santana Group is a geologic group, formerly included as the middle part of the Araripe Group, in the Araripe Basin of northeastern Brazil. The group comprises the Crato, Ipubi and Romualdo Formations and is dated to the Aptian to Albian stages of the Early Cretaceous. The formations of the group were deposited in a lacustrine to subtidal shallow marine environment in the Araripe rift basin.
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