Abstract
Even being the more studied of the interior basins of Northeast Brazil, the Araripe Basin still lacks research in organic geochemistry designed to support interpretations of depositional systems and conditions of formation. This work aims to investigate the organic behavior of evaporites and shales from the Santana Group (Lower Cretaceous), as well as discuss their role in the evolution of its depositional systems. A total of 23 samples, 17 shales and six evaporites, were collected in outcrops and quarries. Analyses of Total Organic Carbon (TOC), Total Sulfur (TS), Rock Eval pyrolysis, and the δ34S isotope ratio were performed. The TOC results revealed high organic content for seven intervals, of which only five had high TS content. From the Rock Eval pyrolysis, dominance of the Type I kerogen was verified, thus corresponding to the best type of organic matter (mainly algal) for the generation of liquid and gaseous hydrocarbons. The Lower Cretaceous (probably Aptian) response to the progressive evolution in redox conditions is linked to a remarked Oceanic Anoxic Event (OAE-1a). The TOC/TS ratio suggests variable palaeosalinity, indicating most of the shales were formed under brackish waters with saline influence, yet tending to increase the salinity upwards where hypersaline conditions dominate in the Ipubi Formation. The isotope data also suggest the occurrence of marine ingressions in the depositional systems even prior to the well-documented event of the Romualdo Formation.
Highlights
IntroductionEvaporites can record climatic, physiographic, environmental, and hydrochemistry changes
In ancient sedimentary basins, evaporites can record climatic, physiographic, environmental, and hydrochemistry changes
The location and vertical position of the collected samples of the Santana Group are shown in Figures 1 and 2, while Table 1 exhibits the analytical results of geochemical and isotopic analysis of them
Summary
Evaporites can record climatic, physiographic, environmental, and hydrochemistry changes. Some evaporites show evidence of having formed from a mixture of open-ocean water and waters that record a continental influence (cf [6]) In such a transitional environment, evaporites may be associated with a semi-enclosed basin that becomes progressively isolated and were deposited under brackish-saline conditions, such that its sedimentological and geochemical characteristics become increasingly influenced by oceanic inputs [7,8,9]. In this environment, some geochemical evidence suggests that salinity conditions may be due to influxes of seawater during brief marine connections and lake transgression [10]. The geochemical research suggests that the sulfur isotopic composition could reconstruct fluctuations in the base level of sedimentary environments [11,21,22]
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