TUFA in Pontburn Valley
Introduction to Tufa
TUFA is a type of limestone that forms in many freshwater settings, these can include streams, lakes, springs and marshes.
In Britain Tufa refers to deposits formed at ambient (cool-water) temperature, mostly in water derived from rain, near-surface ground-water or run-off that has been through the soil and near-surface bedrock.
The term travertine is used for calcareous deposits precipitated from warm or hot-water (above 37oC) springs, although the water may have cooled down to ambient temperature at the site of precipitation. This water may come from deep within the Earth’s surface, and is often related to volcanism or faults. Tufa is normally composed of calcite, and typically forms in areas of limestone.
In Britain, classic areas for Tufa are the Yorkshire Dales (e.g. Malham) and the Peak District, e.g. Matlock, but there are numerous smaller localised sites too. One well-known site of spring tufa in NE England is at Mother Shipton’s Cave in Knaresborough.
Our discovery of Tufa in several side streams in the Derwent Valley by members of Pont Valley Network and ecologist John Durkin during bank vole and botanical surveys is extremely exciting.
Tufa would not be expected in an area of coal measures where there is little or no limestone, like our valley. Our Tufa deposits have been described as really spectacular – in terms of their field occurrence, variety, facies, microstructure, nanostructure and geochemistry!!
Description
Tufa commonly forms alongside plants, usually moss, green algae, reeds, leaves, and twigs/branches. One of the current controversies in tufa studies is how the calcite is precipitated. An initial requirement is that the water is saturated. This is usually the case where the bedrock is limestone, hence the association of Tufa with limestone.
Calcite precipitation may simply be the result of inorganic, chemical processes, and most commonly this is through degassing of CO2 from water (which is how stalactites form). This would occur where there is a waterfall along a stream through water agitation or where a spring emerges from the ground.
At the other end of the spectrum, calcite may be precipitated purely by the biochemical processes of the plants and microbes; photosynthesis utilizes CO2 and this would cause calcite precipitation.
Tufa has distinctive textures and fabrics varying from vacuolar (very porous and ‘holey’) to dendrolitic (calcified fanning microbes) to laminated and pustular.
The microstructure of tufa consists of very fine calcite crystals (called micrite, less than 4 microns), often forming clots or rounded grains (peloids), and coarser crystals (microspar, up to 62 microns) and much coarser elongate fibrous crystals of calcite, up to several mm in length.
The much coarser sparry calcite is commonly regarded as an inorganic product.
Two common types of autochthonous tufa are: Phytoherm framestone and Phytoherm boundstone. Phytoherm framestone is characterized by a primary framework constituted by living hydrophytes and semi-aquatic macrophytes. A diverse fauna inhabits these structures and commonly includes annelids, ostracodes, insect larvae and molluscs. Phytoherm boundstone is formed through in-situ calcification of a microbial biofilm. This Tufa type is dominated by skeletal stromatolite and consists entirely of cement fringes formed in intimate association with cyanobacterial layers.
TUFA geology in the Derwent Valley
The local area geologically has a younger Upper Carboniferous strata that crop out in the lower Derwent Valley area and are represented by the Pennine Coal Measures Formation, consisting of a succession of interbedded grey mudrock, siltstone, sandstone and coal. There is no limestone but there may locally be calcareous shales. The mudrocks contain siderite (FeCO3) and pyrite (FeS2) nodules. The depth beneath the lower Derwent Valley to the Great Limestone, the first thick limestone beneath the dominantly clastic succession, is at around 400 metres. This may be the cause of the barrage Tufa found in side-streams of the River Derwent occurring at five main sites, all in the area of Coal Measures (Upper Carboniferous) bedrock:
4 in the lower Derwent Valley near Rowlands Gill-Dipton at Thornley Wood Burn, Leapmill Burn Gibside, Lily Burn and Pikewell Burn, and there is one site near Allensford by Consett in Dene Burn.
There are smaller occurrences of spring tufa where water is emerging from rocky cliffs, as at Chopwell Crags on the River Derwent itself near Lintzford Bridge.
Specialists have been into the valley over the last few months and examined, recorded and removed samples of these Tufa deposits. We are hoping this will help us to understand why they have appeared in our valley and how we can help to preserve them. Temperature, conductivity and pH of stream waters have been measured in the field, and in the laboratory. Tufa samples have been analyzed for their carbon and oxygen isotopic composition, and for their trace elements. They have also been examined in thin-section using a petrographic microscope for the microfabrics, and with a luminoscope for their cathodoluminescence. The nano-structures of the tufa were examined with an SEM, and a combination of fixation and dehydration techniques was used to avoid alteration or destruction of the organic tissues and possible modification of the delicate mineral ultra-structures. In order to understand better the growth processes of tufa, glass slides were placed in the tufa-depositing streams at Thornley and the precipitates monitored, weighed and examined with the SEM.
TUFA is a type of limestone that forms in many freshwater settings, these can include streams, lakes, springs and marshes.
In Britain Tufa refers to deposits formed at ambient (cool-water) temperature, mostly in water derived from rain, near-surface ground-water or run-off that has been through the soil and near-surface bedrock.
The term travertine is used for calcareous deposits precipitated from warm or hot-water (above 37oC) springs, although the water may have cooled down to ambient temperature at the site of precipitation. This water may come from deep within the Earth’s surface, and is often related to volcanism or faults. Tufa is normally composed of calcite, and typically forms in areas of limestone.
In Britain, classic areas for Tufa are the Yorkshire Dales (e.g. Malham) and the Peak District, e.g. Matlock, but there are numerous smaller localised sites too. One well-known site of spring tufa in NE England is at Mother Shipton’s Cave in Knaresborough.
Our discovery of Tufa in several side streams in the Derwent Valley by members of Pont Valley Network and ecologist John Durkin during bank vole and botanical surveys is extremely exciting.
Tufa would not be expected in an area of coal measures where there is little or no limestone, like our valley. Our Tufa deposits have been described as really spectacular – in terms of their field occurrence, variety, facies, microstructure, nanostructure and geochemistry!!
Description
Tufa commonly forms alongside plants, usually moss, green algae, reeds, leaves, and twigs/branches. One of the current controversies in tufa studies is how the calcite is precipitated. An initial requirement is that the water is saturated. This is usually the case where the bedrock is limestone, hence the association of Tufa with limestone.
Calcite precipitation may simply be the result of inorganic, chemical processes, and most commonly this is through degassing of CO2 from water (which is how stalactites form). This would occur where there is a waterfall along a stream through water agitation or where a spring emerges from the ground.
At the other end of the spectrum, calcite may be precipitated purely by the biochemical processes of the plants and microbes; photosynthesis utilizes CO2 and this would cause calcite precipitation.
Tufa has distinctive textures and fabrics varying from vacuolar (very porous and ‘holey’) to dendrolitic (calcified fanning microbes) to laminated and pustular.
The microstructure of tufa consists of very fine calcite crystals (called micrite, less than 4 microns), often forming clots or rounded grains (peloids), and coarser crystals (microspar, up to 62 microns) and much coarser elongate fibrous crystals of calcite, up to several mm in length.
The much coarser sparry calcite is commonly regarded as an inorganic product.
Two common types of autochthonous tufa are: Phytoherm framestone and Phytoherm boundstone. Phytoherm framestone is characterized by a primary framework constituted by living hydrophytes and semi-aquatic macrophytes. A diverse fauna inhabits these structures and commonly includes annelids, ostracodes, insect larvae and molluscs. Phytoherm boundstone is formed through in-situ calcification of a microbial biofilm. This Tufa type is dominated by skeletal stromatolite and consists entirely of cement fringes formed in intimate association with cyanobacterial layers.
TUFA geology in the Derwent Valley
The local area geologically has a younger Upper Carboniferous strata that crop out in the lower Derwent Valley area and are represented by the Pennine Coal Measures Formation, consisting of a succession of interbedded grey mudrock, siltstone, sandstone and coal. There is no limestone but there may locally be calcareous shales. The mudrocks contain siderite (FeCO3) and pyrite (FeS2) nodules. The depth beneath the lower Derwent Valley to the Great Limestone, the first thick limestone beneath the dominantly clastic succession, is at around 400 metres. This may be the cause of the barrage Tufa found in side-streams of the River Derwent occurring at five main sites, all in the area of Coal Measures (Upper Carboniferous) bedrock:
4 in the lower Derwent Valley near Rowlands Gill-Dipton at Thornley Wood Burn, Leapmill Burn Gibside, Lily Burn and Pikewell Burn, and there is one site near Allensford by Consett in Dene Burn.
There are smaller occurrences of spring tufa where water is emerging from rocky cliffs, as at Chopwell Crags on the River Derwent itself near Lintzford Bridge.
Specialists have been into the valley over the last few months and examined, recorded and removed samples of these Tufa deposits. We are hoping this will help us to understand why they have appeared in our valley and how we can help to preserve them. Temperature, conductivity and pH of stream waters have been measured in the field, and in the laboratory. Tufa samples have been analyzed for their carbon and oxygen isotopic composition, and for their trace elements. They have also been examined in thin-section using a petrographic microscope for the microfabrics, and with a luminoscope for their cathodoluminescence. The nano-structures of the tufa were examined with an SEM, and a combination of fixation and dehydration techniques was used to avoid alteration or destruction of the organic tissues and possible modification of the delicate mineral ultra-structures. In order to understand better the growth processes of tufa, glass slides were placed in the tufa-depositing streams at Thornley and the precipitates monitored, weighed and examined with the SEM.
This is our most recent correspondence from our specialist in Tufa formations, result from a walk taken a few weeks ago where samples where taken to be analysed and lots of excitement about the possibilities of a designated SSSI for our valley due to the findings.
Below you will find lots of information about TUFA found in and around the UK. These are scientific documents recording findings from some of the top specialists in the field and are very interesting to read.......