The formation and environmental significance of calcite rafts in tropical tufa‐depositing rivers of northern Australia

Abstract

Spring-fed rivers of the Barkly karst in tropical northern Australia form an array of tufa and related freshwater carbonate deposits. One of these deposits, calcite rafts, is precipitated at the water-air interface principally as a consequence of CO 2 degassing and evaporation. Calcite rafts have been reported in cave environments but have not been described in detail from fluvial systems. Observations using scanning electron microscopy coupled with water chemistry data reveal that they form by a combination of physical, chemical and biological processes. They grow downwards into the water column and form a dentate lower surface, while a flat upper surface occurs at the water-air interface. The rafts are readily inhabited by microorganisms, particularly diatoms, which frequently become entombed by calcite as the rafts develop. The decay of the biological material leaves voids, creating a pockmarked texture. The rafts are subject to secondary calcite growth along the crystal edges. Once they become submerged in the water column after disturbance of the water surface, they may become completely covered by this overgrowth, creating a homogeneous veneer. The rafts form in quiescent settings, principally behind tufa dams in large, lake-like water bodies along each river. Therefore, they can be used in conjunction with adjacent exposures of other tufa facies to decipher palaeohydrological conditions. Although the rafts are extremely thin and fragile, they are readily preserved within fossil waterhole facies, and their occurrence has been identified in rocks from the Quaternary to the Tertiary.

Depositional architecture of a mixed travertine-terrigenous system in a fault-controlled continental extensional basin (Messinian, Southern Tuscany, Central Italy)

“…Various mechanisms may cause the formation of raft rudstones. According to Jones (1989) and Taylor et al (2004), rafts can break when crystals, preferentially growing under the lower surface of the raft, increase in density, causing rafts to sink to the bottom of the pool. Other causes for raft breakage are water agitation due to wind, rain and current and the pressure exerted by gas bubbles under the raft surface (Folk et al, 1985;Guo and Riding, 1998;Gandin and Capezzuoli, 2014).…”

Genesis of folia in a non-thermal epigenic cave (Matanzas, Cuba)

“…Calcite rafts are formed at the surface of water bodies by strong CO 2 degassing or evaporation (Jones, 1989;Taylor et al, 2004). Tower cones in Santa Catalina Cave, formed by piles of rafts sunk by drip water, are evidence for efficient raft formation.…”

An environmental model of fluvial tufas in the monsoonal tropics, Barkly karst, northern Australia

“…The calcite raft facies consists of thin (b1 mm), floating calcite crusts, while the microdetrital facies comprises micritic mud that is probably produced by cyanobacteria (Pedley, 1990) but also contains sunken calcite rafts (Taylor et al, 2004). Both facies are restricted to zones of standing water in pools and waterholes.…”

“…The model proposed here is based on general features of existing schemes, especially those reported by Pedley (1990), Violante et al (1994) and Pedley et al (2003), but it has been adapted to suit tropical monsoonal fluvial tufas through detailed analysis of Barkly karst tufas. Fossil tufa facies and deposit morphologies observed in Barkly sequences are also presently forming in the modern environment and provide a vital link between fossil tufa characteristics and their environments of formation (Carthew et al, 2002;Drysdale et al, 2003;Taylor et al, 2004). The model contains the characteristic depositional environments, geomorphic units and facies that form in tufa-depositing rivers operating under this climatic regime.…”

“…Such organisms play an important role in tufa formation and the development of numerous distinctive tufa facies (Drysdale, 1999;Drysdale et al, 2003;Carthew et al, 2003b). High evaporation rates contribute to the formation of calcite rafts, which are unusual in more temperate locations (Taylor et al, 2004). The strongly seasonal precipitation and regular destructive floods may result in lamination, abrupt changes in tufa facies, dramatic shifts in stream position, abundant phytoclastic and intraclastic deposits, and a general lack of oncoids and thick sedimentary fills (Carthew et al, 2003a).…”