Ipubi Formation

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 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.
Description.
The Ipubi Formation was described as a member of the Santana Formation by Beurlen in 1971. 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.
Basin history.
The tectono-sedimentary evolution of the Araripe Basin, located in the Borborema Geologic Province, encompasses four stages, with five tectonostratigraphic phases:
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:
Lithologies and deposition.
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.
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.
The carbonaceous pyrobitumen shales of the Ipubi Formation occur as a layer in most of the mines in the region and have a potential for shale gas development.
Sequence stratigraphy.
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:
Fossil content.
While the underlying Crato and overlying Romualdo Formations are particularly rich in fossils, comprising flora, fish, arthropods, insects, snakes, turtles, dinosaurs and pterosaurs, 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. The same specimen was mentioned by Fielding et al. in 2005 and reported as "Araripemys".