Geochemical and sedimentary facies study – Implication for driving mechanisms of organic matter enrichment in the lower Silurian fine-grained mudstones in the Baltic Basin (W Lithuania)

Abstract

More than 100 m-long core sections from Western Lithuania (Baltic Basin), presenting an almost continuous lower Silurian fine-grained siliciclastic organic matter-rich sedimentary record from Aeronian to Homerian, were investigated to evaluate the main forces governing the organic matter enrichment. The methods applied include organic and inorganic geochemistry, organic petrology, sedimentological and petrographic studies along with framboidal pyrite analysis, as well as analysis of stable nitrogen and organic carbon isotopes. The petrographic observations showed distinct mudstone diversity reflecting a variety of paleoenvironmental depositional conditions evolving against a backdrop of post-Ordovician climate, thus, sea level variations, and progressing development of foreland basin throughout the early Silurian. Reflectance measurements on vitrinite-like particles, supported by molecular organic geochemical and Rock-Eval data prove that kerogen from the studied section reached the oil window maturity stage. Rock-Eval pyrolysis, organic geochemical and petrographical as well as δ13Corg data clearly confirm the marine origin of the organic matter. Various geochemical proxies and sedimentary features indicate that in Aeronian a period of more humid climate favored enhanced nutrient input, which in turn intensified organic matter productivity resulting in large amounts of organic matter accumulation. Redox proxies (TOC/TS ratio, trace elements enrichment, molecular organic geochemical indices, framboidal pyrite analysis, TOC/P ratio) demonstrate a rather broad spectrum of bottom water oxygenation levels in the lower Silurian sediments accumulated under oxic - dysoxic – anoxic conditions interrupted by several episodes of euxinia. A highly condensed section of Aeronian black-grey mudstones manifests a very good organic matter preservation under sediment-starved and oxygen-depleted settings. Here, the oxygen-depleted water bottom conditions are well evidenced by low values of δ15N and δ13Corg pointing to the presence of diazotrophic N-fixing and chemoautotrophic organisms. The Sheinwoodian-Homerian section of monotonous grey calcareous mudstones evenly and moderately enriched in organic matter reflects the most steady environmental settings developed under more arid climate in comparison to Llandovery. Throughout the early Silurian sedimentation rates were overall low, although probably enhanced in Wenlock, and played a role in organic matter preservation. It was a critical factor during sedimentation of grey-greenish mudstone of the Telychian characterized by comparatively the lowest organic matter preservation efficiency. Variations in the intensity of the pycnocline, thus organic matter preservation, were probably controlled by climate-triggered sea level changes, although tectonically-driven paleoenvironmantal effects, bathymetry, and, at some point, intensified paleoproductivity inducing oxygen-restricted conditions, were likely contributing. Although MoEF/UEF ratios mainly indicate fully open marine settings during the entire early Silurian, Mo–TOC covariation clearly displays strict basinal restriction in the Aeronian. The profile of δ13Corg demonstrates six carbon isotope excursions along the lower Silurian section. The identified Late Aeronian and Early Sheinwoodian Carbon Isotope Excursions are linked to global sea-level changes, thus, changes in oceanic water circulation pattern. A high percentage of tiny pyrite framboids within the interval of Early Sheinwoodian CIE could reflect an increased pyrite burial under highly reducing conditions and might be associated with an Oceanic Anoxic Event. The three positive δ13Corg excursions within the Telychian seem to be related to an early diagenetic partial oxidation of organic matter.