Sedimentology of Quaternary perched springline and paludal tufas: criteria for recognition, with examples from Guadalajara Province, Spain

Abstract

Outcrop-scale geometries and bed relationships of ambient temperature freshwater carbonates are poorly understood because many described tufas have been dismantled by erosion and present only part of a particular depositional model. At the field scale, four end-member models encompass the tufa continuum: (1) perched springline; (2) paludal; (3) fluvial; and (4) lacustrine tufas. Individual bed types can occur with variable dominance within several of these models, but one or more beds are characteristically dominant only within a single tufa model, so it can be differentiated from relatively isolated outcrop fragments. Two models (perched springline tufas and paludal tufas) are known in outline only in the literature despite being present within the Quaternary deposits of most karstic regions. Perched springline tufas generally form lobate, fan-shaped mound morphologies on hillslopes and develop from single or multiple spring resurgences. Mature deposits show a subhorizontal top and a steep face on the downflow side. The steep outer zones of tufa mounds may be developed into cascades with moss curtains or can be dominated by shallow rimstone pools according to face angle. Tufa deposits lying downslope of the mounds are usually detrital in nature, especially if some dismantling of the mound has occurred. The relatively thin subhorizontal lobe-top deposits commonly contain organic-rich deposits. Paludal tufas develop predominantly in waterlogged valley bottom situations, where line-sourced waters emerge from valley side and bottom aquifers. Lime mud precipitation predominates in these sites. Mud is deposited as subhorizontal laminites that thin towards the valley axis and downstream of resurgences. Tufa spring-mounds may form where lesser volumes of water are involved. Individual tussocks (phytoherm cushions) of grasses and rushes are the most diagnostic feature of the model, but sapropels and peats may be intercalated. Diagenesis in both models is rapid. Lithification of individual beds is virtually instantaneous and always occurs before the decay of the associated living vegetation. The resulting highly porous and permeable fabrics remain fresh in Holocene tufas, but aggrading neomorphism and partial spar infill of vegetation moulds is common in older deposits. Dissolution in many perched springline tufas is small scale. Many large cavities are primary, but with later coatings of speleothems. Early removal of Sedimentology (2003) 50, 23-44 Ó 2003 International Association of Sedimentologists 23 organics from paludal tufas is responsible for autobrecciation and differential compaction.

Holocene tufa in the Slovak Karst: facies, sedimentary environments and depositional history

“…The tufa stud ied at Hrhov clearly cor re sponds to the "perched springline model" de fined by Pedley (1990), Ford and Pedley (1996) and Pedley et al (2003) or the "slope trav er tine sys tem" dis tin guished by Violante et al (1994). The tufa was fed by a perched spring on the side of the val ley, the lo ca tion of which re sulted from the oc cur rence of im per me able Lower Trias sic clastic strata (Figs.…”

“…Be ing sub ject to encrustation, moss grows par tic u larly ef fi ciently (Pen te cost, 1998). Mosses con struct phytoherms of var i ous size, from small moss cush ions to large bar rages (Pedley, 1990;Pedley et al, 2003;Vázquez-Urbez et al, 2012).…”

“…Intraclastic tufa is formed as a re worked prod uct of the dis in te gra tion of older tufas of other types (e.g., Pedley et al, 2003). Coarse-grained clasts de rived from re sistant tufa fa cies whereas silt-sized com po nents may have resulted from dis mem ber ing of cal cite-en crusted al gal fil a ments (e.g., Gradziñski, 2010).…”

“…Many tufas of perched springline type have lim ited ar eas (Pedley et al, 2003). The ex cep tions in clude the Mat lock Bath de pos its (Derbyshire, UK) with an area of about 6 ha (Pen te cost, 1999) and Rocchetta a Volturno (south ern It aly) with an area of around 10 km 2 (Violante et al, 1994).…”

“…Pen te cost and Viles (1994) and Pen te cost (2005: p. 49-76) de scribed and clas si fied the ge om e try of grow ing tufa struc tures. Pedley (1990), Ford and Pedley (1996) and Pedley et al ( , 2003 stud ied fa cies sys tems (mod els) and in ter preted their sed i men tary en vi ron ment. Pedley et al (1996) pro posed two dif fer ent fa cies mod els for cool tem per ate and warm semi-arid flu vial tufas.…”