Application of organic facies and biomarkers in characterization of paleoenvironmental conditions and maturity of sediments from the Codó Formation in the west-central part of the São Luís Basin, Brazil

Abstract

Paleoenvironmental conditions of the sediments of the Cretaceous Codó Formation in Brazil are still being debated. The main point of controversy is whether they were deposited in a strictly marine environment or under lacustrine conditions of varying salinity. In this study, organic facies and biomarker analyses were performed on samples from the Codó Formation in the São Luís Basin, northwest Brazil, to reconstruct the paleoenvironmental conditions, the origin, and the thermal maturity of the organic matter. Four types of palynofacies associations (PI – amorphous organic matter, PII – amorphous organic matter+sporomorphs, PIII – phytoclasts, and IV – marine palynomorphs) were identified. The total organic carbon (TOC) content exhibits a strong correlation (ρ = 0.75, p < .05) with amorphous organic matter (AOM) content suggesting that AOM is the main factor controlling TOC in the stratigraphic section, and that increases in the AOM contribution were associated with an increase in the water column and expansion of reducing conditions that favored organic matter preservation. The TOC and total sulfur (TS) plots suggest that there were salinity variations in the depositional environment. The identification of marine palynomorphs (Subtilisphaera genus) are indicative of several local marine incursions. The n-alkane distribution (unimodal and bimodal) confirmed the presence of strong input of higher plant components (phytoclast-dominated Palynofacies III) and the bacterially-reworked organic material (Palynofacies I/II). The aquatic terrestrial (ATR), terrigenous/aquatic (TAR), and proxy for aquatic macrophytes (Paq) ratios suggest autochthonous (Palynofacies I and II) and allochthonous (Palynofacies III) organic matter, and eutrophic and submerged-floating plants. Based on palynofacies associations and biomarkers we conclude that the depositional environment ranged from an anoxic/reducing to dysoxic/oxic within a closed-lacustrine system having a stratified water column. High evaporation rates favored a decrease in the water column and an increase in salinity, allowing the precipitation of salt by evaporation (CaCO3 > 80%). The thermal maturity indicators including vitrinite reflectance (%Ro), spore color index (SCI), Ts/(Ts + Tm) ratios, and C29 20S/(20S + 20R) and C29 ββ/(αα + ββ) indicate that the organic matter is immature to marginally mature.