Abstract The Chuos Formation of Namibia is the sedimentary product of the Neoproterozoic Sturtian (c. 720–660 Ma) glaciation and contains massive diamictites intercalated with finely laminated iron formation. Similar Sturtian glacially associated iron formations are found globally. The iron formations are laminated and generally very pure. The diamictites are massive, contain abundant clasts and can be highly ferruginous. These two lithofacies are repeatedly interbedded with no facies transition. The iron formations preserve the rare earth element geochemistry of their contemporaneous seawater and contain rare Ce and Eu anomalies. The geochemistry does not implicate a hydrothermal influence. The Chuos iron formation is interpreted to have been deposited in an ice-proximal glacio-marine setting in a sub-ice shelf environment. Oxygenated fluids, such as sea ice brines and glacial meltwater, are invoked as a mechanism to precipitate iron oxides due to mixing with ferruginous seawater. The iron formation accumulates under an ice shelf with little clastic input. Episodic movement of the grounding line reworks the sediments into ferruginous diamict. Glaciogenic debris flows are intercalated with the iron formations. Palaeobathymetric depressions and accompanying brine pools increased the preservation potential of these iron formations. This model explains the relationship between glaciation and iron formation in the Neoproterozoic. Supplementary material: The full set of geochemical data is available at https://doi.org/10.6084/m9.figshare.c.4031125.v1

Abstract: The late Oligocene-Miocene subsurface Clifton Formation in the Otway Basin, southeastern Australia, consists predominantly of skeletal carbonate grains of originally calcite mineralogy deposited in a non-tropical shelf environment. Bryozoans are the dominant allochem, with echinoderms, foraminifers and siliciclastics as subordinate grain types. In the area of study, the Clifton Formation had an uncomplicated burial history and was subject to very little uplift (< 100 m). Present burial depths (ranging from 160 m to 670 m below present surface) equate approximately with maximum burial depths. Shallow-buried limestones are weakly cemented by calcite (<5% calcite cement down to 430-m depth) while more deeply buried limestones (>550 m) are almost completely cemented by calcite. Evidence for mechanical compaction (mainly grain breakage) is present throughout the unit. Products of pressure-dissolution in the Clifton Formation, which can only be observed microscopically, are interpenetration of grains (with smooth and sutured contacts), fitted fabrics, dissolution seams and microstylolites. Significantly, no macroscopic stylolites are present (to a maximum depth of 670 m), suggesting that in these limestones, stylolite formation requires burial depths greater than about 700 m. In grainstone intervals, interpenetration of grains is apparent in the shallowest samples (less than 200 m), whereas fitted fabrics and microstylolites first appear at depths of approximately 550 m. In clay-rich lithologies, dissolution seams occur at depths as shallow as 190 m, although they are better developed at and below 340 m. In the Clifton Formation, microstylolites appear to develop from an amalgamation of sutured interparticle contacts in grainstones with fitted fabrics. Further dissolution and amplification of these microstylolites could lead to the formation of stylolites. Such a model has implications in the timing of origin of particular diagenetic events such as hydrocarbon emplacement or mineral precipitation from metal-bearing fluids. The coincidence of a dramatic increase in cementation with an increase in pressure-dissolution features (in the form of fitted fabrics and microstylolites) between 430-m and 550-m depth suggests that much CaC0 3 cementation was caused by pressure-dissolution. Mechanical compaction, on the other hand, dominates pressure-dissolution in porosity loss.