Tectonically-induced strontium isotope changes in ancient restricted seas: The case of the Ediacaran-Cambrian Bambuí foreland basin system, east Brazil

Strontium isotope stratigraphy and trace-metal geochemistry of the Ediacaran-Cambrian Bambuí Group, São Francisco Basin

“…First, this study shows that authigenic fractions recorded for Cd, Cu, Mo, U and Zn are associated with low Al content and presenting mass fractions above the PAAS concentrations. Second, other proxies applied to these rocks, such as Sr/Ca and 87 Sr/ 86 Sr ratios, REY and δ 15 N data support chemical disturbances at the basal regressive stage, instead of lithologic variation Guacaneme et al 2021;. Third, recent research in the Bambuí Group support major mayor paleoenvironmental changes in epeiric seawater on the core of Gondwana at the late Ediacaran Caxito et al 2021c).…”

“…The lowermost 2 nd -order sequence shows a basal transgressive system tract (TST) composed of glacial-related coarse-grained siliciclastics of the Carrancas Formation that grade upward into carbonate rocks and pelites of the Sete Lagoas Formation, defining a retrogradational stacking pattern . Bounded by a basal erosional surface and an upper maximum flooding surface, this interval displays a low average Sr/Ca ratio of 0.001, negative to positive δ 13 C excursions from -5 to c. 0‰, an increase of 87 Sr/ 86 Sr ratios from 0.7074 to 0.7084, and some layers with aragonite pseudomorph fans, associated to Neoproterozoic cap carbonates Guacaneme et al, 2021).…”

“…However, a basin-wide geochemical change in the Sr content and Sr/Ca ratios within the HST led to Caetano-Filho et al (2019) to subdivide it into a basal (EHST) early highstand systems tract and an upper (LHST) late highstand systems tract. The latter records a decrease on the 87 Sr/ 86 Sr ratios to ~0.7076 and an increase on the Sr/Ca ratios around 0.004, reflecting marine restriction and major basin-scale paleogeographic changes (Guacaneme et al, 2021) and probably enhanced seawater alkalinity (Paula- .…”

“…Bounded at the top by another maximum flooding surface, these strata are overlain by a HST that presents siliciclastics that grade upward into the carbonate-dominated successions of the Lagoa do Jacaré Formation Suss, 2016, Caetano-Filho et al, 2019). This interval displays an extreme positive δ 13 C excursion that reaches values as high as +16‰ (i.e., Caetano-Filho et al, 2021), an average Sr/Ca ratio of 0.002 and variable 87 Sr/ 86 Sr ratios between 0.7074 and 0.7084 Guacaneme et al, 2021). Some paleoenvironmental interpretations based on Sr isotope ratios and REY patterns suggest a significant change in the weathering regime on the marginal orogenic belts of the São Francisco craton with a preferential contribution from carbonates during the restricted stage of the basin (Paula- Guacaneme et al, 2021).…”

“…Although this scenario has been consistently described in sedimentary successions throughout the world, recent advances in the study of the stratigraphic, geochemical and paleontological record of the Ediacaran-Cambrian Bambuí Group, exposed on the São Francisco craton (east Brazil), have showed contrasting paleoenvironmental conditions. Allowing a significant step forward in understanding the ancient conditions under which the early metazoans started to inhabit benthic environments, the available data have pointed that much of the Bambuí strata was deposited in a restricted foreland marine basin surrounded by Brasiliano/Pan-African orogens in the core of West Gondwana (i. e., Guacaneme et al, 2021Caxito et al 2021c). This foreland basin system would have experienced intermittent periods of connection with contemporary seas (Paula- Caxito et al 2021c), which allowed the episodic spread of cosmopolitan organisms (e.g., Cloudina sp.)…”