Oncoids and groundwater calcrete in a continental siliciclastic succession in a fault-controlled basin (Early Permian, Northern Italy)

Abstract

Lower Permian continental deposits of the fault-controlled Orobic Basin (Central Southern Alps; Northern Italy) include alluvial fan facies interfingering with muddy basin-floor deposits, consisting of three facies associations: heterolithic finegrained siliciclastic facies, laminated sandstone facies, and oncoidal limestone facies. Besides oncoidal and microbial limestones, carbonates occur as nodules in sandy tabular beds within the laminated sandstone facies association. Microfacies analyses distinguish several types of oncoidal carbonate (consisting of an alternation of microbial carbonate and fibrous calcite) and carbonate nodules. Each type of carbonate has been characterized in terms of δ 18 O and δ 13 C. The two types of carbonate in the oncoids record a stable δ 18 O and a slightly varying δ 13 C, whereas the isotope composition of the calcite in nodules is completely different. Carbonate nodules in sandy beds of the laminated sandstone facies association have a diagenetic origin as indicated by cross-cutting relationships between nodules and lamination; the nodules are interpreted as groundwater calcrete, formed in the subsurface at the top of the unconfined water table. The exclusive sedimentation of oncoidal carbonate facies within siliciclastic deposits indicates that when oncoids were formed in ephemeral shallow ponds, siliciclastic input was minimal. The sedimentological and geochemical characteristics of the studied succession and the stable isotopic composition of the oncoids (the absence of covariance between δ 18 O and δ 13 C excludes deposition in evaporating basins) indicate persistent stable conditions for sufficient time to permit growth of centimeter-sized oncoids. Oncoids are interpreted to have formed in spring-fed ponds and outflow channels, with flowing, clean water, at the toe of major alluvial fans. Episodes of rapid delivery of sand and silt-sized sediments by flash floods, with an oscillating water table, caused the observed facies alternation. The precipitation of calcareous cements close to the water table surface produced nodules in sandy layers. Carbonate precipitation within laminated sandstone reduced porosity and permeability, causing a strong compartmentalization in the well-bedded continental succession.

A predictive model for the ichnological suitability of the Jezero crater, Mars: searching for fossilized traces of life-substrate interactions in the 2020 Rover Mission Landing Site

“…In addition, carbonate precipitation in aqueous environments is an excellent mechanism for biosignature preservation (Farmer & Marais, 1999;Horgan et al, 2020). This suitability scoring approach is supported by observations at the Collio Basin (Permian, Italy), which preserves lacustrine carbonates with abundant biostratification ichnofossils (Berra, Felletti & Tessarollo, 2019). In the Collio Basin, oncoids, stromatolites and MISS are associated with lacustrine palaeoenvironments dominated by carbonates, i.e., spring-fed ponds at the toe of alluvial fans (Fig.…”

“…The presence of lacustrine carbonates throughout the region and inside the Jezero crater make this palaeolake a landing site of great interest not only for in-situ studies but also for potential sample return (Ehlmann et al, 2008a;Ehlmann et al, 2008b;Mangold et al, 2020). In fact, lacustrine carbonates have a high potential of preserving morphologic, organic, and isotopic biosignatures (Berra, Felletti & Tessarollo, 2019;Horgan et al, 2020).…”