Postglacial transgressive shales of Upper Devonian–Lower Carboniferous boundary of the Parnaíba Basin

Abstract

Upper Devonian-Lower Carboniferous deposits in north Brazil show a postglacial transgressive event characterized by the formation of thick layers of gray to black shales. In the Parnaíba Basin, the shales of the Longá Formation overlap the Upper Famennian non-marine glacial deposits of the Cabeças Formation marked by a ravinement surface. The Longá shales exhibit wavy-lenticular beddings in their base, grading to gray-black shales locally interbedded with thin layers of sandstones with hummocky cross-stratification and megaripple bedding. These deposits were interpreted as having formed in shallow marine upper offshore to lower shoreface with episodic storm events. The organic matter analysis of the outcropping Longá shales using Total Organic Carbon (TOC), Rock-Eval pyrolysis, and biomarkers provide several geochemical parameters for the interpretation of the paleoenvironment, the origin of the organic matter and its thermal maturation. The Longá shales presented low TOC contents (less than 1%), very low numbers of free hydrocarbons (S1), and potential generators (S2). The maximum pyrolysis temperature (Tmax), as well as the thermal maturation biomarker parameters, indicate that the Longá shales are immature. The values of pristane/n-C17, phytane/n-C18, Hydrogen Index (HI) and Oxygen Index (OI) suggest kerogen types III and IV, which are formed from conical land leaves, needles, and plant waxes; this interpretation is also supported by total sulfur (TS) < 0.2 wt%, terrigenous/aquatic ratio (TAR), C29-sterol, hopane/sterane and Methylphenanthrene (MPs). This organic matter was deposited in an epeiric sea with shallow water, oxidizing conditions, normal salinity, and low temperatures prevalence.