SEDIMENTARY ROCK TYPES
C: Bog-iron ores: rarely preserved in rock record D: Placer deposits, especially with magnetite and ilmenite
3.11 Volcaniclastic Deposits
This class of sedimentary rock includes the pyroclastic deposits and epiclas- tic deposits (reworked ‘secondary’ volcanic sediments).Tephrais a general term used for pyroclastic deposits, the material fragmented by explosive vol- canic activity. Tephra consist of (a) pyroclasts, i.e., pyroclastic fragments that include juvenile (fresh) lumps of lava and glass (pumice or scoria) which fragment to give glass shards; (b) crystals (phenocrysts), especially of quartz and feldspar; and (c) lithic fragments consisting of pieces of lava from earlier eruptions (non-juvenile) and of country rock. The components of volcaniclastic deposits are given in Table 3.5.
Pyroclastic deposits are often emplaced at high temperatures. Recogni- tion features include carbonised wood, pink/red coloration due to thermal oxidation, dark coloration due to finely disseminated microlites of magnetite, radial cooling joints, gas-escape structures (including fossil fumarole pipes, vertical structures a few centimetres in width filled with coarse ash), welding together of grains, and streaked-out and flattened pumice fragments.
Table 3.5 Main types of pyroclastic material occurring in volcaniclastic deposits.
Pumice: light-coloured, low-density (if modern), vesiculated lava fragments, acid magma, mm– dm in size. Vesicles may contain calcite, zeolite or clay minerals
Scoria:dark-coloured equivalent of pumice
Glass shards: small grains of solidified glass, sub-mm in size, formed by vesiculation and fragmentation
Glassy matrix: plastic deformation textures around hard clasts, hard and splintery, welded if hot and soft when deposited
Fiamme: compressed pumice fragments, hot and soft when deposited, mm– cm in size
Accretionary lapilli: concentrically laminated spheres of ash, 2 – 20 mm in diameter
Lithic clasts: non-juvenile lava fragments and country rock clasts, solid when deposited, resistant to deformation, mm– m in size
Phenocrysts: crystals that grew in the magma, mm-size
Table 3.6 Classification of volcaniclastic grains and sediments on grain-size.
Volcaniclastic grains
Size Volcaniclastic sediment terms
(tephra) bombs – ejected fluid coarse agglomerate blocks – ejected solid – 256 mm – volcanic breccia
fine 64 mm
coarse – 16 mm –
lapilli medium lapillistone
– 4 mm – fine 2 mm
very coarse – 1 mm –
coarse
ash – 0.5 mm –
medium – 0.06 mm –
fine
tuff
vitric lithic crystal
Figure 3.23 Accretionary lapilli and ash in lapilli-tuff deposit. Cen- timetre scale. Ordovician, N.W.
England.
On the basis of grain-size, the pyroclasts are divided into ash, lapilli, blocks and bombs (Table 3.6). The term pumice refers to light-coloured vesicular glassy rock of rhyolitic composition, and scoria is used for darker pieces, still vesicular, generally of andesitic or basaltic composition.
Pumice has a low density and can float on water. The vesicles in these lava fragments may be filled with calcite (clear), zeolite (white) or clay minerals (green).
Accretionary lapilli are small, con centrically laminated spheres, 2 – 20 mm in diameter (like ooids), of fine volcanic ash, in some cases around a
nucleus of a coarse ash grain (Fig. 3.23). They commonly formed in wet (phreatic or phreatomagmatic) eruption columns and by fallout from steam- rich plumes.
Volcanic bombs are common in volcaniclastic successions and consist of large, usually rounded, randomly distributed ‘blobs’ of lava, which may depress or rupture the bedding (bomb sags), or be deformed into strange shapes if they were still soft when they landed. Based largely on shape, spindle, bread-crust and cow-dung bombs can be distinguished.
3.11.1 Pyroclastic deposits
Three types are distinguished in terms of origin (Table 3.7).
3.11.1.1 Pyroclastic fall deposits
These include subaerial and subaqueous (submarine or sublacustrine) fallout tephra. They are characterised by a gradual decrease in both bed thickness and grain-size away from the site of eruption. Beds are typically well sorted and normally graded. However, they are frequently reworked by currents and waves if deposited in water, or wind if subaerial, and thus may show cross- or planar lamination (strictly these would then be epiclastic deposits). Larger fragments of pumice may occur on top of the beds if they floated before being deposited. These deposits can be spread over wide areas and are useful for stratigraphic correlation, forming marker beds. Pyroclastic fall deposits mantle the topography, with layers of roughly constant thickness over both hills and valleys (Fig. 3.24).
3.11.1.2 Pyroclastic flow deposits
These are the product of hot gas/solid high-concentration density currents, which may travel at velocities of 20 – 100 m s−1. One common pyroclastic flow deposit is anignimbrite, produced by a violent plinian eruption, which generally occurs in subaerial situations, although the flows may continue into the sea or a lake. Ignimbrites are characterised by their homogeneous Table 3.7 Main types of volcaniclastic sediment (and depositional process).
A pyroclastic fall deposit: graded tuff (subaqueous/subaerial fallout) B pyroclastic flow deposit: includes ignimbrite (pumice and ash flow) C pyroclastic surge deposit (base-surge deposit)
D lahar deposit (volcanic mudflow)
E hyaloclastite (fragmentation of basaltic lava through contact with water)
pyroclastic fall deposit
pyroclastic surge deposit
pyroclastic flow deposit
Figure 3.24 Different geometries of pyroclastic deposits.
appearance with little sorting of the finer ash particles, so they lack internal stratification. Coarse lithic clasts in the bed may be normally graded (size decreasing upward) whereas large pumice clasts (which are very light at the time of eruption) may show reverse grading (size increasing upward), or be concentrated at the top of the bed. Flattened and stretched fragments of pumice (termedfiamme) and glass shards indicate the soft (and hot) nature of the vesiculated and fragmented melt during transport (Fig. 3.25). Many ign- imbrites show welding in the central-to-lower part; here ash particles merge to form a denser, less porous rock compared with the upper and lower parts of the bed. A subplanar foliation, termedeutaxitic texture, may develop here from the aligned fiamme. In its extreme, the rock is entirely glassy (vitro- phyric). Lithic fragments in the deposit resist deformation and the hot plastic glassy material is deformed around them. Some ignimbrites have a colum- nar jointing, also indicating that they were still hot on deposition. Typical thicknesses of an ignimbrite deposit are 1 m to 10 m or more. The flows are topographically controlled and so the deposits fill valleys and depres- sions (Fig. 3.24).
3.11.1.3 Pyroclastic surge deposits
These result from highly expanded turbulent gas – solid density currents with low particle concentrations. Phreatomagmatic and phreatic eruptions
Figure 3.25 Ignimbrite showing streaked-out pumice fragments (fiamme).
Millimetre scale. Tertiary, New Zealand.
Figure 3.26 Base-surge and pyroclastic fall deposits. The base-surge tuff occurs in the upper part and shows well-developed bedding (antidune cross- bedding) with flow to the right. The pyroclastic air-fall deposits occur in the lower part. Metre staff. Quaternary, Germany.
involve steam and generate base-surge deposits. They are characterised by well-developed unidirectional sedimentary bedforms (dunes) giving cross- stratification (Fig. 3.26), pinch-and-swell features and antidune cross-bedding (Section 5.3.3.15), since they are deposited by very fast-flowing ash-laden steam flows. Individual laminae are generally well sorted. These deposits tend to blanket the topography, although they do thicken into the depressions (Fig. 3.24). There is a complete gradation between high-particle concentration pyroclastic flows and low-particle concentration pyroclastic surges.
3.11.2 Epiclastic deposits
Specific types of these reworked primary volcanic sediments include lahar deposits and hyaloclastites.
3.11.2.1 Lahar deposits
The flows that deposit these sediments form a continuum between high- temperature flows in which hot pyroclastic material is mixed with water (such as streams or snowmelt) during an eruption, and low-temperature flows representing the remobilisation by water of already deposited cool pyroclas- tic material. Lahars are mudflows containing principally volcanic material which typically deposit large volcanic fragments in a fine ashy matrix. They are characterised by a matrix-support fabric with large ‘floating’ clasts of volcanic material (Section 4.4). Lahars usually have a larger range of blocks and boulders, many of which are lithic clasts, rather than juvenile material, and an absence of any indications of very hot temperatures, compared with the hot pyroclastic flow deposits described above. Lahar deposits, by virtue of their muddy matrix, tend to be more consolidated than pyroclastic flow deposits with their ashy matrices.
3.11.2.2 Hyaloclastites
These form where lava is extruded into water and the rapid chilling and quenching cause fragmentation of the lava. These deposits typically consist of lava chips and flakes, a few millimetres to a few centimetres across.
The chilled glassy lava is commonly altered by hydration to a yellow-green material called palagonite. Hyaloclastites lack any sorting or stratification close to the site of eruption, but they can be reworked and resedimented to show sedimentary structures as with any other clastic sediment. Hyaloclastites are typical of submarine basaltic volcanism.
3.11.3 Studying young volcaniclastic successions
It is generally easier to study young volcaniclastic deposits, since it will be more obvious initially that are volcanic and they are then likely to preserve many of the original features, such as the local and regional variations in bed thickness, bed geometry and internal structures. If they are poorly lithi- fied, then the grain-size distributions can be studied (by sieving back in the laboratory) to provide useful information on the depositional processes (see textbooks). The different types of volcaniclastic deposit noted above may then be readily identified. There may well be lava flows interbedded with all their typical features (see below). Needless to say, working in areas of recent volcanic activity can be very dangerous.
3.11.4 Studying ancient volcaniclastic sediments
In more ancient strata, where erosion after deposition may have removed much of the volcanic edifice, or tectonic and metamorphic processes have obscured primary features or caused recrystallisation, it may be necessary to use a basic lithological approach to document the features, i.e., noting the grain-size, composition, degree of welding, bed thickness, sedimentary struc- tures, colour, etc., as in any other sedimentary rock, before the depositional processes can be unravelled.
In ancient volcanic successions, it can be difficult to distinguish tuffs from lavas; brecciated lavas may be confused with agglomerates, and flow- banded lavas with ignimbrites. Typical features of lava flows are (a) columnar jointing in the central part (as with some ignimbrites!), (b) a blocky texture, (c) brecciated basal parts and tops, (d) vesicles concentrated towards the tops, and (e) weathered, reddened and/or rubbly upper surfaces. Ignimbrites do not usually have basal breccias.
3.11.5 Volcaniclastic successions
It is useful to document the upward changes through a volcaniclastic suc- cession; make graphic logs and look for long-term changes in volcaniclastic deposit type.
• Are there changes in the proportion of pyroclastic fallout versus flow deposits? Are there systematic upward bed thickness changes in the tuffs, reflecting increasing or decreasing volcanic activity?
• Are there long-term changes in the composition of the volcanic mate- rial – more acid to more basic, for example (examine the glass fragments for colour changes, the degree of vesiculation and phenocryst composi- tion)?
• Is there any evidence for compositional zonation or stratification within the magma chamber, increasing phenocryst content within a single flow deposit, for example? There may be an upward increase (or decrease) in the proportion of non-volcanic interbeds.
In some volcaniclastic successions there is a packaging of the fallout and flow deposits, like cycles in other sedimentary strata. In a complete ‘classic’
pyroclastic eruption episode, the succession deposited begins with air-fall tuff, overlain by a pyroclastic surge deposit, and then a pyroclastic flow deposit, before more air-fall tuff at the top of the unit (Fig. 3.27).
fine tuff
pyroclastic flow deposit
pyroclastic surge deposit
air-fall tuff
Figure 3.27 A complete ‘classic’ pyroclastic succession from one major erup- tion. Fumarole pipes filled with coarse ash are shown towards the top of the flow deposits, from escaping gas.