U-Pb carbonate geochronology of the Early Miocene (ca. 21-19 Ma) marine transgression on eastern coast, Brazil

The work presents new U-Pb results in carbonatic rocks and fossils from units deposited along the Brazilian east coast. The analytical procedures involved sampling, preparation of mounts and imaging by Scanning Electron Microscope and subsequent U-Pb analysis by LA-MS-ICP. The imaging phase allows the choice of appropriate mineral phases (reservoirs) to improve statistical treatment in order to improve the dating of carbonates of sedimentary and diagenetic origin. The ages reported here contribute to the refinement of the temporal limits of carbonates from the Pirabas Formation and Barreiras Formation with values between 22 and 19 Ma. These results, when interpreted together with petrographic data, descriptions of facies and fossil features allow interpret them as representing the maximum levels of this transgressive episode along the Brazilian coast with sediments also recorded in the southern regions of South America.

In this sense, a sample of fossiliferous limestone (MCT 1314-LE) from the Itaborai Basin, corresponding to the filling material of dissolution channels that cut older layers of travertine and argillaceous limestone; and the specimen DGM 294-M containing a mandibular fragment with two associated teeth of Tetragonostylops apthomasi (Atrapotheria, Mammalia) and DGM 235-R a fragment of a maxilla with 8 teeth of Sebecus (Sebecidae, Crocodyliformes), were immersed in epoxy resin, to be sawn, polished and laminated for subsequent isotopic analysis. For the polished surface with fossil material, gold metallization was carried out. Samples DGM 235-R, DGM 294-M and MCT 1314-LE were dated by the U-Pb method using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS), carried out at MUltiLab (UERJ). In this sense, the reading of the U-Pb isotopes was carried out together with appropriate reference materials to analyze the isotopic ratios in carbonate material. The results obtained comprised of 51 shots taken on the vertebrate fossils are here reported. For Tetragonostylops apthomasi, we obtained an age of 19±5 Ma. Meanwhile, the MCT 1314-LE sample resulted in an age of 22±8 Ma. These fossils have original ages between 56-54 Ma, and the younger ages obtained are interpreted as due isotopic rehomogenization resulting from immersion in a column of seawater during Early Miocene.

The Pirabas carbonate (which occurs on the coast of the state of Pará) had U-Pb age results in fossiliferous rocks of 21 ± 6 Ma and, together with the other results reported here, are

interpreted as important markers of a marine transgression event that occurred in South America, indicating that they are representative of the maximum levels of transgressive episodes with the formation of sedimentary deposits in a marine paleoenvironment. According to the literature on similar rocks, the deposition environment may indicate an important deglaciation event that reached the entire planet. Carbonate U-Pb dating holds significant potential for sedimentary and diagenetic dating of geologically relevant events despite remaining largely underutilized.