Glendonite-Like Carbonate Aggregates from the Lower Ordovician Koporye Formation (Russian Part of the Baltic Klint): Detailed Mineralogical and Geochemical Data and Paleogeographic Implications

Abstract

Stellate and plate-like carbonate bodies, traditionally called anthraconites, are found throughout the Baltic-Ladoga Klint in bituminous shale of the Koporye Formation (Tremadocian, Lower Ordovician). Although this time interval is usually considered as a greenhouse, there is some evidence for the existence of at least temporary cold conditions during the Cambrian–Ordovician. However, the origin of anthraconites is still strongly debated. We studied the mineralogical, petrographic, cathodoluminescence, geochemical, and isotopic characteristics of anthraconites from five sections of the Russian part of the Baltic paleobasin. A close similarity between the morphological, petrographic, cathodoluminescence, and isotopic characteristics of the studied anthraconites with those of glendonites allow us to suggest that these bodies formed in a similar paleo-environment and should be considered as pseudomorphs of the mineral ikaite. The oxygen and carbon isotope ratios reveal that ikaite precipitation occurred in low-temperature conditions on the seafloor. The carbon isotopic values reveal influence of inorganic seawater carbon along with organic matter decomposition and/or methane oxidation during ikaite-glendonite transformations. The oxygen isotopic composition significantly changed after deposition due to meteoric diagenesis. We propose that the studied Tremadocian anthraconites formed under a region of upwelling, where cold phosphate-rich deep waters rose to the relatively shallow part of the Baltic paleobasin, providing favorable conditions for ikaite precipitation. Based on our cathodoluminescence study, we suggest that ikaite was transformed to calcite over several stages during diagenesis. Mineralogical studies also reveal that primary calcite was transformed to sulfate (gypsum) or dolomite during late superimposed processes.

Database of global glendonite and ikaite records throughout the Phanerozoic

“…However, the primary morphology of ikaite can be preserved due to fast seawater cementation (Huggett et al, 2005;Selleck et al, 2007). Seawater diagenesis of ikaite takes place in the sulphate-reduction zone and glendonite can be subsequently altered during burial diagenesis, where it can be replaced by younger carbonate generations or non-carbonate minerals such as silica (Wang et al, 2017), dolomite (Loog, 1980) or gypsum (Mikhailova et al, 2019).…”

“…There is a notable absence of glendonite occurrences for a prolonged period of time below the Carboniferous, spanning the Devonian, Silurian and Middle−Upper Ordovician. Lower Ordovician glendonites have only recently been discovered and described (Popov et al, 2019;Mikhailova et al, 2019), as until recently, they were known as a kind of 'anthraconite' and were not considered as glendonites. Their findings are restricted to Baltoscandia.…”

“…δ 13 С values of ikaite-derived calcite range from −27.7 to −14.3 ‰PDB, while δ 18 O values range from −5.1 to +1.3 ‰PDB(Frank et al, 2008). Lastly, δ 13 С values of secondary carbonate cements range from −20.2 to −3.9 ‰PDB, while δ 18 O values range from −23.1 to −6.2 ‰PDB(Frank et al, 2008;Vasileva et al, 2019; seefig. 3).…”

Marine diagenesis of ikaite: Implications from the isotopic and geochemical composition of glendonites and host concretions (Palaeogene–Neogene sediments, Sakhalin Island)

“…They are found all over the world in sediments of different ages, ranging from the Mesoproterozoic to the Recent. Glendonites are predominantly composed of calcite (Kaplan, 1979; Carr et al., 2006; Frank et al., 2008) and, to a lesser extent, dolomite (James et al., 2005; Mikhailova et al., 2019), quartz (Wang et al., 2016) and gypsum (Mikhailova et al., 2019). They are occasionally found within nodules composed of micritic limestone (Muramiya et al., 2018) or cemented sandstone (Geptner et al., 2014).…”

“…Glendonite cementation occurs after ikaite dehydration (Vickers et al., 2018). During burial diagenesis, partially altered ikaite or its calcite pseudomorph can occasionally be replaced by non‐carbonate minerals such as gypsum or quartz (James et al., 2005; Wang et al., 2016; Mikhailova et al., 2019).…”