Abstract
The Ipubi Formation represents the Aptian-Albian siliciclastic-evaporitic succession of Araripe Basin, NE Brazil. This succession comprises siliciclastic rocks (bituminous shales and claystones) and evaporites (gypsum and secondary anhydrite) and represents part of the lacustrine-shallow marine post-rift phase I. This study used sequence stratigraphy concepts to define the relations between changes in the relative lake level and the formation of Ipubi deposits. Results show that the organic-rich shales of the Ipubi Formation formed during a transgressive pulse that covered large areas of the proximal domains. These deposits overlie a regional unconformity that marks the end of the deposition of the underlying Crato Formation. A High Stand stage that followed the transgression influenced the formation of evaporitic deposits. Climate conditions played a major role in influencing the triggering and stopping of evaporite deposition. Thus, a new relative lake level fall event caused the exposure of the Ipubi Formation deposits, and created another regional subaerial unconformity accompanied by widespread karstification of evaporite beds. A posterior transgression caused the deposition of siliciclastic rocks of the Romualdo Formation over the Ipubi Formation strata, and also promoted a new event of karstification of the Ipubi upper evaporite beds.
Highlights
Evaporites represent sedimentary rocks that normally form in saline aqueous bodies under arid conditions with high evaporation rates
The results here presented indicate that the top of the Crato Formation is a regional unconformity caused by a relative lake level fall that exposed these deposits in proximal areas
This unconformity marks a major retreat in the relative lake level (Neumann 1999, Assine et al 2014), following a series of pulses of expansion that culminated with the formation of the uppermost interval of laminated limestones (C6 interval) (Fig. 3)
Summary
Evaporites represent sedimentary rocks that normally form in saline aqueous bodies under arid conditions with high evaporation rates. Such conditions, as well as a lack of terrigenous input, allow evaporites to form in saturated brines (Collinson and Thompson 1982, Cavalcante and Ramos 2010, Warren 2006, 2010, Mohriak et al 2008, Ortí et al 2017). Tectonic processes are important in forming and sustaining evaporitic systems due to the formation of restrictive conditions in lakes and epeiric seas. Abundant deposition of evaporites occurred in many marginal basins in Brazil during the Upper Aptian period (Hashimoto et al 1987, Uesugui 1987, Mohriak et al 2008, Szatmari and Milani 2016)
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