Does climate control the morphological fabric of freshwater carbonates? A comparative study of Holocene barrage tufas from Spain and Britain

Abstract

Calcium carbonate production in most freshwater tufa systems is controlled by a combination of physico-chemical precipitation, and biomediation associated with procaryote-microphyte biofilms. Inorganic precipitation is dominant under turbulent flow regimes, whereas biomediation necessitates sluggish flow or static water. We show here that temperature is also likely to affect tufa formation rates and architecture, leading to the development of a clearly definable range of fabrics. Two contrasting models are proposed: Warm semi-arid tufas: well seen in the Ruidera Pools National Park, Albacete Province, Central Spain, where tufa barrages consist of vertical, narrow rimmed arcuate structures. Frequently these are bounded both upstream and downstream by deep lakes. Laterally accreting mammilate stromatolites dominate the upper two thirds of lake margin subaqueous overhangs, whereas conical growth-forms dominate shady basal parts of subaqueous overhangs. Dense reed stands grow in thin peats on top of the vertical lake margin walls. Resistivity survey and percussion augering show up to 20 m of earlier lake fill which is banded into dark and pale beds. The upper 10 m accumulated in 4600 years. There is little or no preservation of organic material within these deposits. Cool temperate tufas: typified by slowly accumulating barrage tufas at Caerwys (N. Wales) and Alport (N. Derbyshire, UK). Transverse barriers across valleys are dominated by arcuate buttress developments built up of downstream-facing tabular carbonate sheets. Pools upstream of the barriers are shallow, dominated by clotted lime mud and organic sapropels in their depocentres, and stromatolite-dominated (laminar to low domes, or oncoidal) facies in broad marginal zones. Marginal stands of semi-aquatic vegetation are rooted in thick peats but frequently are also encrusted around their exposed stems by oncoidal carbonate. Fabric preservation is good for the carbonate material, however, marginal peats rapidly oxidise and stem encrustations collapse to produce cylindrical oncoid beds. Depocentre sapropels are always preserved and contain important pollen records otherwise rare in carbonate regions. Carbonates in both models accumulate relatively rapidly. They have a moderate preservation potential within Late Cenozoic terrestrial successions. The distinctly different characteristics of their preserved fabrics should allow recognition of two contrasting “climatic” models for fluvial barrage tufas in ancient deposits.