Paleolimnological reconstruction of a marginal area of Jurassic Capianga Lake, Jatobá Basin, northeast Brazil
The Alianca Formation, within the Brazilian Reconcavo–Tucano–Jatoba Rift System, is a succession of Late Jurassic sedimentary rocks that record the maximum flooding surface of shallow Capianga Lake in the Afro-Brazilian Depression, Gondwana. Several aulacogens and drift-marginal Mesozoic basins of eastern Brazil and western Africa recorded the great extensional event that occurred during the early stage of Gondwana break-up. Lithologic, petrographic, micropaleontological and geochemical analyses of lacustrine carbonates collected near the city of Ibimirim, Pernambuco State, were carried out to establish the paleolimnological evolution of a marginal area of Capianga Lake, in the Jatoba Basin. The ostracode fauna, obtained from grainstone and calcareous quartzarenite, is constituted by Theriosynoecum pricei, Theriosynoecum uninodosa, Theriosynoecum quadrinodosum, Reconcavona? jatobaensis and Alicenula? spp. Carbon and oxygen stable isotopes in bulk rock, Theriosynoecum pricei and Alicenula? spp. were measured. Theriosynoecum pricei displays the most negative δ13C values, between − 2.1 and − 1.3‰. The δ13C values obtained from Alicenula? spp. and bulk rock were also negative. The δ18O values for Theriosynoecum pricei range from − 10.1 to + 4.1‰. Alicenula? spp. show δ18O values between − 4.6 and + 5.8‰. The bioclastic character of the carbonates and the presence of cross-bedding and ripple marks suggest deposition in a low-energy, low-gradient, ramp-type littoral zone of a shallow, balanced-fill lake. The ostracode association, characteristic of permanent, alkaline, freshwater bodies, corresponds to the Theriosynoecum pricei ostracode biozone, which is of Tithonian age. The lithofacies, along with covariation of δ13C and δ18O, and large δ18O fluctuations, suggest that this marginal area of Capianga Lake was characterized by stable hydrological conditions in a closed basin. The δ13C values display a slightly narrower range, suggesting oligotrophic to slightly mesotrophic conditions, and holomixis in this marginal area of Capianga Lake.