Tufa

Abstract: Investigation of travertine accumulations throughout central Italy and the west-central U.S. has shown that the carbonate is precipitated in response to both inorganic and organic processes. Individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent; all of the carbonate is low-magnesian calcite. Water chemistry, temperature, and morphology of the accumulation greatly influence the constituents comprising these deposits. Harsh environmental situations favor inorganic deposits, while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material. Morphological variations of travertine deposits recognized include 1) waterfall or cascade, 2) lake-fill, 3) sloping mound, fan, or cone, 4) terraced ound, and 5) fissure ridge. Gross morphology, internal stratification, and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters. Bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations, exceeding 90% of the framework grains comprising some of the lake-fill deposits. Bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents. The bacteria are primarily rods, generally 0.2 µm in diameter and less than 1.0 µm in length. The rods readily decay resulting in calcite crystals loaded with micropores. The basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 µm in diameter enclosed in a single crystal of calcite. Aggregates of these crystals produce a variety of deposits including 1) crudely laminated carbonate mud, 2) finely laminat d layers of mud, 3) intraclasts, 4) foam rock, and 5) shrubs. The shrubs are most striking, commonly forming layers 1-3 cm thick but also producing bacterial pisoids. At some locales, bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit. The shrub layers are the result of flourishing summer growth of bacteria and, furthermore, show remarkable daily laminae 0.1-0.5 mm thick. The importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits.

Abstract: Traditionally, fresh water carbonate research has focused on the sedimentology and palaeontology of ancient lacustrine deposits. Lithofacies in such low-energy deposits are typically fine-grained, developed uniformly in a generally concentric distribution (‘bulls-eye’ pattern) and are predictable even when preserved imperfectly. In contrast, because of their local lithofacies and palaeontological complexities, fluvial carbonates were either delegated to a status of ‘minor geomorphological features’ or barely considered prior to the 1970s. This viewpoint was based on the depositional record of fluvial and spring-fed fresh water carbonates, which were considered to be restricted generally to localized karstic areas. Such deposits are often preserved as scattered patches of ambient temperature tufa. Occasionally, however, in active tectonic areas, localized travertine deposits are also developed from deeply circulating hydrothermal waters. With a few exceptions (for example, basins with high subsidence rates or in arid climate zones), these fresh water carbonates are prone to erosion from continuing river incision and thus may not be preserved in the geological record. A partial record of fluvial and spring-deposited carbonates is often preserved in Quaternary deposits, but the record in older deposits is typically fragmentary and often diagenetically modified. Yet once their unique facies architecture (and specialized nomenclature) is understood, these carbonates provide an important record of past sedimentological cycles of great value in palaeoenvironmental landscape modelling. The emphasis of modern research is to acquire information that explains how active systems function. In this respect, tufas reveal much of how carbonate precipitation is a shared product of physico-chemical and microbiological biomediation processes. Likewise, travertines not only show an intimate interrelation with active tectonism but also hold great potential as monitors of past volcanic carbon dioxide emissions. In addition, both tufas and travertines contain palynological records that can be used as proxy indicators of climate change. Perhaps no other field of sedimentology has witnessed more developments and applications over such a brief period of study.