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The low-latitude Marinoan glaciation (635 Ma) record in the southern Amazon Craton has been one of the most important sites with strong evidence for the Neoproterozoic Snowball Earth hypothesis. In Central Brazil, the 50-m-thick carbonate succession of the post-Marinoan cap carbonate exposed in the Tangará da Serra region, Central Brazil, represents a complementary section of the classical occurrence of the Puga cap carbonate in Mirassol d'Oeste region, that overlies glaciogenic deposits marking the well-preserved Cryogenian-Ediacaran boundary in South America. The 8-m-thick cap dolostone that represents the Mirassol d'Oeste Formation consists of pinkish microcrystalline peloidal dolomudstone, exhibiting even parallel to low-angle lamination locally tubestone, and megaripple bedded dolopackstone. The cap dolostone is succeeded upsection by the cap cementstone limestone (basal Guia Formation), comprising laminated lime mudstone rich in silt-sized terrigenous grains (quartz), and subordinately bituminous shale, dolomitic marls, intraformational breccias, and abundant calcite crystal fans and crusts. Crystal fans occur in isolated and decimetre-scale arrays and are considered aragonite pseudomorphs. Primary precipitation of dolomite, crystal fans, and micrite and the formation of tubestone are related to depositional processes. The cap dolostone was affected by the neomorphism of the dolomicrite, development of vug and intercrystalline porosity, euhedral dolomite precipitation, synsedimentary fracturing, calcite and quartz precipitation, chemical compaction, Fe-oxide substitution, and precipitation. The diagenetic processes of the cap cementstone limestone were pyrite precipitation, calcite cementation, neomorphism of micrite and crystal fan, partial crystal fan dissolution, dolomitization, chemical compaction, fracturing, ferrous dolomite and calcite precipitation, pyrite (pseudomorph) and micrite replacement with Fe-oxide and replacement of rhombohedral dolomite by Mn-oxides. Hydrocarbons are rare and its migration represents the last diagenetic event being found: 1) in fractures and interpeloidal, vug and intercrystalline pores (~2%) with low permeability and connectivity of the cap dolostone; and 2) in intercrystalline pores, fractures, and rare pores in crystal fans of the cap cementstone limestone. During the post-Marinoan sea-level rise occurred the establishment of a shallow to moderately deep microbially induced dolomitic platform locally influenced by storm-action. The progressive transgression led the implantation of a deep anoxic and CaCO3-oversaturated aragonitic platform. The similarities of cap carbonate microfacies among the post-Marinoan succession worldwide reflects an extraordinary and non-recurring global precipitation event in geological history. In contrast, besides the majority of cap carbonate have the same textural and compositional framework, the diagenetic history seems to be regional and completely dependent on the tectonic, thermal, and burial history of the depositional basins where they were deposited. [ABSTRACT FROM AUTHOR]

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-C 17 , phytane/n-C 18 , 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), C 29 -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. • The Longá shales record the postglacial transgressive event of D-C Boundary of South America. • Longá Formation is composed by wave- and storm-dominated marine systems. • The organic poor-shales were deposited in a marine environment under oxic and normal salinity with high terrestrial organic matter input. • The organic matter is immature forming kerogen type III and IV from land plant. • The Longá shales are correlated with other Devonian shales of Western Gondwana intracratonic basins. [ABSTRACT FROM AUTHOR]