Paleohydrology of an Upper Aptian lacustrine system from northeastern Brazil: Integration of facies and isotopic geochemistry
Abstract The Codo Formation records the initial evolutionary stages of an intracontinental rift system formed along the Brazilian equatorial margin in the late Aptian. Deposits of this unit exposed on the eastern margin of the Grajau Basin include gypsum, bituminous black shales and limestones. These lithologies were formed in a low energy, well stratified, anoxic and hypersaline lake system developed in a dominantly arid/semi-arid climate. This lacustrine succession is internally organized into three categories of shallowing-upward cycles, with the first- and second-order cycles being related to seismic activity associated with fault reactivations, and the third-order cycles recording climatic fluctuations. Studies emphasizing petrography and analysis of the geochemical tracers Fe, Mg, Sr, Mn, Na and Ca helped to identify the sedimentary facies that kept a primary signal, which were thus appropriate for isotopic investigations aiming paleoenvironmental and paleohydrologic reconstructions. The results of this study revealed a wide distribution of dominantly low carbon and oxygen isotope values in carbonates, ranging from − 5.69‰ to − 13.02‰ and from − 2.71‰ to − 10.80‰, respectively. This paper demonstrates that at least in the particular case of oxygen, the isotope ratios vary according to seismically-induced shallowing-upward cycles, with values in general lower at their bases, where central lake deposits dominate, and progressively higher upward, where marginal lake deposits are more widespread. In addition to confirming a depositional signature for the analysed samples, this behavior allowed the development of a seismic-induced isotope model. The lighter isotope ratios appear to be related to flooding events promoted by subsidence, which resulted in the development of a perennial lake system, while heavier isotope values are related to ephemeral lake phases favored by uplift and/or increased stability. Furthermore, the results show that a closed lake system dominated, as indicated by the overall good positive covariance (i.e., + 0.42 to + 0.43) between the carbon and oxygen isotopes, though open phases are also recorded by negative covariance values of − 0.36. During closed phases, the δ 18 O displayed the highest range of variation (i.e., − 3.63‰ to − 4.89‰) due to increased residence time, while this variation was low (i.e., − 0.09‰ to − 1.87‰) during open lake phases, when there was a balance in the water isotope composition maintained by continuous basin inflow.