Septarian carbonate concretions in the Permian Rio do Rasto Formation: Birth, growth and implications for the early diagenetic history of southwestern Gondwana succession

Abstract

a b s t r a c tBetween the Late Carboniferous and Early Triassic, the southwestern Gondwana supercontinent was characterized by the development of a huge intracratonic basin. A large confined epeiric sea and the accumulation of a transgressive-regressive sequence were formed by continuous subsidence related to tectonic effects caused by the Sanrafaelic Orogeny and the consequent generation of accommodation space. The Permian Rio do Rasto Formation documents the last progradational cycle related to the complete continentalization of this epeiric sea. The basal member of the Rio do Rasto Formation (Serrinha) is believed to have been deposited in a shallow epicontinental water body subjected to storms and influenced by episodic deltaic incursions. One of the most remarkable characteristics of the Serrinha Member is the presence of carbonate concretions hosted in mudstones and very fine sandstones. Here, we combine sedimentological and petrographic descriptions coupled with geochemical and stable carbon and oxygen isotopic data to elucidate the nature of these carbonate concretions. The nondeformed internal structure, decreasing proportion of carbonate cements relative to detrital grains toward the concretion edges, core-to-rim isotopic variations, and perhaps most importantly, the preservation of a welldeveloped cardhouse fabric support an early diagenetic origin for these structures at shallow burial depths of tens of meters. Stable isotope analyses of micritic calcite cements and calcites filling the septarian fractures reveal major negative excursions in both δ 18 O and δ 13 C values. Oxygen isotope ratios obtained for the micritic calcite cements vary between −12.1 and −2.6‰. The calcite filling septarian fractures also exhibit negative values of δ 18 O (− 14.2 to −13.8‰), with an average of − 14‰. The δ 13 C values of micritic calcite cements range from − 5.0-0.2‰. The carbon isotopic data from the calcite-filling septarian fractures are also negative (− 4.4 to −3.3‰). The δ 18 O signatures suggest that the early diagenetic carbonate concretions precipitated in a shallow freshwater environment rather than in a marine setting. The δ 13 C values suggest that the carbon isotopes were derived from a source with slightly depleted 13 C, supporting at least a partial organogenic contribution with weak sulfate reduction rates typical of freshwater systems. Sedimentological analysis shows that the epicontinental water body in which the Serrinha Member was deposited was constantly supplied by rivers and meteoric waters, which suggests that an enormous freshwater basin with restricted marine connections to the Panthalassa Ocean once existed.

A new Permian bivalve-dominated assemblage in the Rio do Rasto Formation, Paraná Basin, Brazil: Faunal turnover driven by regional-scale environmental changes in a vast epeiric sea

“…Among the recorded conchostracan species, Monoleaia unicostata is one of the most abundant, showing a wide stratigraphic range (Rohn, 1988(Rohn, , 1994Ferreira-Oliveira and Rohn, 2010) and occurring in direct association with the studied bivalves. The tendency of the water towards increased freshening at the end of the Permian Period in the Paran a Basin is also attested by the oxygen and carbon stable isotope values of eodiagenetic concretions of the Serrinha Member (Alessandretti et al, 2015), nearly 10 m above the contact with the underlying Teresina Formation. Additionally, d 13 C data reveal low rates of primary productivity at the surface of the freshwater body (Alessandretti et al, 2015).…”

“…The tendency of the water towards increased freshening at the end of the Permian Period in the Paran a Basin is also attested by the oxygen and carbon stable isotope values of eodiagenetic concretions of the Serrinha Member (Alessandretti et al, 2015), nearly 10 m above the contact with the underlying Teresina Formation. Additionally, d 13 C data reveal low rates of primary productivity at the surface of the freshwater body (Alessandretti et al, 2015). In the studied interval, an abrupt change of the plant assemblages also occurs, including the disappearance of lycopod plants and expansion of glossopterids, ferns, and sphenopsids (Rohn, 2007; but see Neregato et al, 2008; for the possible taphonomic control on plant preservation at the Teresina/Rio do Rasto transition).…”

“…The facies association of the Serrinha Member suggests a scenario characterized by the episodic deposition of tempestites in an offshore setting marked by oxygenated bottom conditions (Warren et al, 2008;Alessandretti et al, 2015). The sporadic occurrences of mud-crack-like structures in the basal portion of the unit were interpreted as seismites by Warren (2005).…”

“…Greenish sandstones, siltstones and mudstones characterize the Serrinha Member and are generally organized in tabular strata. These rocks were deposited in a large and shallow epeiric water body influenced by storms (Warren et al, 2008;Alessandretti et al, 2015). However, reddish fine sandstone and mudstones that compose the Morro Pelado Member were interpreted as deposited under proximal deltaic and eolian settings (Rohn, 1994;Warren et al, 2008;Holz et al, 2010;Schemiko et al, 2014).…”

“…The faunal turnover coincides with an interval of pronounced freshwater influence that could account for the disappearance of the pre-existing dominant bivalve groups, such as the Pinzonellinae. Freshening of the marginal settings of the Paran a Basin during the deposition of the Serrinha Member is attributed to the prevalence of more humid conditions (Rohn, 1994;Simões et al, 2012;Alessandretti et al, 2015). Indeed, Wesselingh (2007) noticed that extreme faunal turnovers in molluscan faunas that thrived in epeiric seas or long-lived lakes might be partially linked to climate change and periods of pronounced freshening as well as food-web changes (see also Todd et al, 2002;Pfuhl and McCave, 2007).…”