Multi-Proxy Constraints on the Thermal Maturity of a Mesoproterozoic Succession from the Alto Tapajós Basin (Amazonian Craton): Illite Crystallinity, Raman Spectroscopy of Carbonaceous Material, and the Acritarch Alteration Index

Thermal maturity assessment in Precambrian sedimentary successions is commonly hindered by the inapplicability of vitrinite reflectance and by the limited calibration of individual proxies. Here, we constrain the thermal maturity of a Mesoproterozoic interval from the L4F3 drill core (Alto Tapajós basin, Amazonian craton) using an integrated approach that combines the Kübler Index (KI), Raman spectroscopy of carbonaceous material measured directly on organic-walled microfossils (RSCM), and the Acritarch Alteration Index (AAI). Illite crystallinity was evaluated from the FWHM of the 10 Å (001) reflection on oriented <2 µm mounts under air-dried and ethylene glycol conditions, with peak decomposition (DecompXR) used for internal quality control. RSCM thermometry was applied to Leiosphaeridia tenuissima wall material, and AAI classes were assigned by standardized transmitted light color comparison. KI proxy values indicate conditions near the late diagenesis-to-incipient anchizone transition, with eight horizons approaching anchizone reference values. RSCM yields peak temperature estimates of 213–263 °C with an uncertainty of approximately ±30 °C, while AAI results are uniform (class 3.0), providing qualitative support consistent with advanced maturation. Together, these indicators place the studied succession broadly within late diagenesis to incipient anchizone conditions and provide constraints relevant to thermal history reconstruction and basin modeling in ancient cratonic settings.