AMBIENTES DE SEDIMENTAÇÃO: CONTINENTAL

Prévia do material em texto

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NICHOLS, G. Sedimentology and Stratigraphy. 2ª ed. –UK: John Wiley & Sons Ltd., 2009. 
419pp. 
 
1. AMBIENTES DE SEDIMENTAÇÃO: CONTINENTAL 
 
1.1 RIVERS AND ALLUVIAL FANS 
Rivers are an important feature of most landscapes, acting as the principal mechanism for the 
transport of weathered debris away from upland areas and carrying it to lakes and seas, where 
much of the clastic sediment is deposited. River systems can also be depositional, accumulating 
sediment within channels and on floodplains. The grain size and the sedimentary structures in the 
river channel deposits are determined by the supply of detritus, the gradient of the river, the total 
discharge and seasonal variations in flow. Overbank deposition consists mainly of finer-grained 
sediment, and organic activity on alluvial plains contributes to the formation of soils, which can be 
recognized in the stratigraphic record as palaeosols. Water flows over the land surface also occur 
as unconfined sheet floods and debris flows that form alluvial fans at the edges of alluvial plains. 
Fluvial and alluvial deposits in the stratigraphic record provide evidence of tectonic activity and 
indications of the palaeoclimate at the time of deposition. Comparisons between modern and ancient 
river systems should be carried out with care because continental environments have changed 
dramatically through geological time as land plant and animal communities have evolved. 
 
Fluvial and alluvial fan Deposition: summary 
Fluvial environments are characterised by flow and deposition in river channels and associated 
overbank sedimentation. In the stratigraphic record the channel fills are represented by lenticular to 
sheet-like bodies with scoured bases and channel margins, although these margins are not always 
seen. The deposits of gravelly braided rivers are characterised by crossbedded conglomerate 
representing deposition on channel bars. Both sandy braided river and meandering river deposits 
typically consist of fining-upward successions from a sharp scoured base through beds of trough 
and planar cross-bedded, laminated and cross-laminated sandstone. Lateral accretion surfaces 
characterize meandering rivers that are also often associated with a relatively high proportion of 
overbank facies. Floodplain deposits are mainly alternating thin sandstone sheets and mudstones 
with palaeosols; small lenticular bodies of sandstone may represent crevasse splay deposition. 
Palaeocurrent data from within channel deposits are unidirectional, with a wider spread about the 
mean in meandering river deposits; palaeocurrents in overbank facies are highly variable. Alluvial 
fan deposits are located near to the margins of sedimentary basins and are limited in lateral extent 
to a few kilometres from the margin. The facies are dominantly conglomerates, and may include 
matrix-supported fabrics deposited by debris flows, well-stratified gravels and sands deposited by 
sheetflood processes and in channels that migrate laterally across the fan surface. Alluvial and 
fluvial deposits will interfinger with lacustrine and/or aeolian facies, depending on the palaeoclimate, 
and many (but notall) river systems feed into marine environments via coasts, estuaries and deltas. 
Other characteristics offluvial and alluvial facies include an absence of marine fauna, the presence 
of land plant fossils, trace fossils and palaeosol profiles in alluvial plain deposits. 
 
Characteristics of fluvial and alluvial fan deposits 
. lithologies – conglomerate, sandstone and mudstone 
. mineralogy – variable, often compositionally immature 
. texture – very poor in debris flows to moderate in river sands 
. bed geometry – sheets on fans, lens shaped river channel units 
. sedimentary structures – cross-bedding and lamination in channel deposits 
. palaeocurrents – indicate direction of flow and depositional slope 
. fossils – fauna uncommon, plant fossils may be common in floodplain facies 
. colour – yellow, red and brown due to oxidizing conditions 
. facies associations – alluvial fan deposits may be associated with ephemeral lake and aeolian 
dunes, rivers may be associated with lake, delta or estuarine facies. 
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1.2 GLACIAL ENVIRONMENTS 
Glaciers are important agents of erosion of bedrock and mechanisms of transport of detritus in 
mountain regions. Deposition of this material on land produces characteristic landforms and 
distinctive sediment character, but these continental glacial deposits generally have a low 
preservation potential in the long term and are rarely incorporated into the stratigraphic record. 
Glacial processes which bring sediment into the marine environment generate deposits that have a 
much higher chance of long-term preservation, and recognition of the characteristics of these 
sediments can provide important clues about past climates. The polar ice caps contain most of the 
world’s ice and any climate variations that result in changes in the volumes of the continental ice 
caps have a profound effect on global sea level. 
 
Summary of Glacial Environments 
Glacial deposits are compositionally immature and tills are typically composed of detritus that simply 
represents broken up and powdered bedrock from beneath the glacier. Reworked glacial deposits 
on outwash plains may show a slightly higher compositional and textural maturity. There is a paucity 
of clay minerals in the fine-grained fraction because of the absence of chemical weathering 
processes in cold regions. Continental glacial deposits have a relatively low preservation potential 
in the stratigraphic record, but erosion by ice in mountainous areas is an important process in 
supplying detritus to other depositional environments. Glaciomarine deposits are more commonly 
preserved, including dropstones which may provide a record of periods of glaciation in the past. The 
volume of continental ice in polar areas is closely linked to global sea level, so the history of past 
glaciations is an important key to understanding variations in the global climate. 
 
Characteristics of glacial deposits 
. lithologies – conglomerate, sandstone and mudstone 
. mineralogy – variable, compositionally immature 
. texture – extremely poorly sorted in till to poorly sorted in fluvio-glacial facies 
. bed geometry – bedding absent to indistinct in many continental deposits, glaciomarine deposits 
may be laminated 
. sedimentary structures – usually none in tills, crossbedding in fluvio-glacial facies 
. palaeocurrents – orientation of clasts can indicate ice flow direction 
. fossils – normally absent in continental deposits, may be present in glaciomarine facies 
. colour – variable, but deposits are not usually oxidised 
. facies associations – may be associated with fluvial facies or with shallow-marine deposits 
 
 
 
1.3 AEOLIAN ENVIRONMENTS 
Aeolian sedimentary processes are those involving transport and deposition of material by the wind. 
The whole of the surface of the globe is affected by the wind to varying degrees, but aeolian deposits 
are only dominant in a relatively restricted range of settings. The most obvious aeolian environments 
are the large sandy deserts in hot, dry areas of continents, but there are significant accumulations 
of wind-borne material associated with sandy beaches and periglacial sand flats. Almost all 
depositional environments include a component of material that has been blown in as airborne dust, 
including the deep marine environments, and thick accumulations of wind-blown dust are known 
from Quaternary strata. Aeolian sands deposited in desert environments have distinctive 
characteristics that range from the microscopic grain morphology to the scale of cross-stratification. 
Recognition of these features provides important palaeoenvironmental information that can be used 
in subsurface exploration because aeolian sandstones are good hydrocarbon reservoirs and 
aquifers. 
 
 
 
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Summary of aeolian environments 
 
Aeolian deposits occurmainly in arid environments where surface water is intermittent and there is 
little plant cover. Sands deposited in these desert areas are characteristically both compositionally 
and mineralogically mature with large-scale cross-bedding formed by the migration of dune 
bedforms. Oxidising conditions in deserts preclude the preservation of much fossil material, and 
sediments are typically red–yellow colours. Associated facies in arid regions are mud and evaporites 
deposited in ephemeral lakes and poorly sorted fluvial and alluvial fan deposits. Aeolian deposits 
are less common outside of desert environments, occurring as local sandy facies associated with 
beaches and glaciers, and as dust distributed over large distances into many different environments, 
but, apart from Quaternary loess, rarely in significant quantities. 
 
Characteristics of aeolian deposits 
. lithologies – sand and silt only 
. mineralogy – mainly quartz, with rare examples of carbonate or other grains 
. texture – well- to very well-sorted silt to medium sand 
. fossils – rare in desert dune deposits, occasional vertebrate bones 
. bed geometry – sheets or lenses of sand 
. sedimentary structures – large-scale dune crossbedding and parallel stratification in sands 
. palaeocurrents – dune orientations reconstructed from cross-bedding indicate wind direction 
. colour – yellow to red due to iron hydroxides and oxides 
. facies associations – occur with alluvial fans, ephemeral river and lake facies in deserts, also with 
beach deposits or glacial outwash facies 
 
1.4 Outros: LAKES 
Lakes form where there is a supply of water to a topographic low on the land surface.They are fed 
mainly by rivers and lose water by flow out into a river and/or evaporationfrom the surface. The 
balance between inflow and outflow and the rate at which evaporation occurs control the level of 
water in the lake and the water chemistry. Under conditions of high inflow the water level in the lake 
may be constant, governed by the spill point of the outflow, and the water remains fresh. Low water 
input coupled with high evaporation rates in an enclosed basin results in the concentration of 
dissolved ions, which maybe precipitated as evaporites in a perennial saline lake or when an 
ephemeral lake dries out. Lakes are therefore very sensitive to climate and climate change. Many 
of the processes that occur in seas also occur in lakes: deltas form where rivers enter the lake, 
beaches form along the margins, density currents flow down to the water bottom and waves act on 
the surface. There are, however, important differences with marine settings: the fauna and flora are 
distinct, the chemistry of lake waters varies from lake to lake and certain physical processes of 
temperature and density stratification are unique to lacustrine environments. 
 
Characteristics of lake deposits 
. lithologies – sandstone, mudstone, fine-grained limestones and evaporites 
. mineralogy – variable 
. texture – sands moderately well sorted 
. bed geometry – often very thin-bedded 
. sedimentary structures – wave ripples and very fine parallel lamination 
. palaeocurrents – few with palaeoenvironmental significance 
. fossils – algal and microbial plus uncommon shells 
. colour – variable, but may be dark grey in deep lake deposits 
. facies associations – commonly occur with fluvial deposits, evaporites and associated with aeolian 
facies. 
 
 
 
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2. AMBIENTES DE SEDIMENTAÇÃO: TRANSICIONAL 
 
THE MARINE REALM: MORPHOLOGY AND PROCESSES 
The oceans and seas of the world cover almost three-quarters of the surface of the planet and are 
very important areas of sediment accumulation. The oceans are underlain by oceanic crust, but at 
their margins are areas of continental crust that may be flooded by seawater: these are the 
continental shelves. The extent of marine flooding of these continental margins has varied through 
time due to plate movements and the rise and fall in global sea level related to climate changes. 
The sedimentary successions in these shallow shelf areas provide us with a record of global and 
local tectonic and climatic variations. There is considerable variety in the sedimentation that occurs 
in the marine realm, but there are a number of physical, chemical and biological processes that are 
common to many of the marine environments. Physical processes include the formation of currents 
driven by winds, water density, temperate and salinity variations and tidal forces: these have a strong 
effect on the transport and deposition of sediment in the seas. Chemical reactions in seawater lead 
to the formation of new minerals and the modification of detrital sediment. The seas also team with 
life: long before there was life on land organisms evolved in the marine realm and continue to occupy 
many habitats within the waters and on the sea floor. The remains of these organisms and the 
evidence for their existence provide important clues in the understanding of palaeoenvironments. 
 
Marine environments: Summary 
The physical processes of tides, waves and storms in the marine realm define regions bounded by 
water depth changes. The beach foreshore is the highest energy depositional environment where 
waves break and tides regularly expose and cover the sea bed. At this interface between the land 
and sea storms can periodically inundate low-lying coastal plains with seawater. Across the 
submerged shelf, waves, storms and tidal currents affect the sea bed to different depths, varying 
according to the range of the tides, the fetch of the waves and the intensity of the storms. 
Sedimentary structures can be used as indicators of the effects of tidal currents, waves in shallow 
water and storms in the offshore transition zone. Further clues about the environment of deposition 
are available from body fossils and trace fossils found in shelf sediments. More details of the coastal, 
shelf and deepwater environments are presented in the following chapters. 
 
 
2.1 DELTAS 
The mouths of rivers may be places where the accumulation of detritus brought down by the flow 
forms a sediment body that builds out into the sea or a lake. In marine settings the interaction of 
subaerial processes with wave and tide action results in complex sedimentary environments that 
vary in form and deposition according to the relative importance of a range of factors. Delta form 
and facies are influenced by the size and discharge of the rivers, the energy associated with waves, 
tidal currents and longshore drift, the grain size of the sediment supplied and the depth of the water. 
They are almost exclusively sites of clastic deposition ranging from fine muds to coarse gravels. 
Deposits formed in deltaic environments are important in the stratigraphic record as sites for the 
formation and accumulation of fossil fuels. 
 
Characteristics of deltaic deposits 
- lithologies – conglomerate, sandstone and mudstone 
- mineralogy – variable, delta-front facies may be compositionally mature 
-texture – moderately mature in delta-top sands and gravels, mature in wave-reworked delta-front 
deposits 
- bed geometry – lens-shaped delta channels, mouth bar lenses variably elongate, prodelta deposits 
thin bedded 
- sedimentary structures – cross-bedding and laminationin delta-top and mouth-bar facies 
- palaeocurrents – topset facies indicate direction of progradation, wave and tidal reworking variable 
on delta front 
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- fossils – association of terrestrial plants and animals of the delta top with marine fauna of the delta 
front 
- colour – not diagnostic, delta-top deposits may be oxidised 
- facies associations – typically occur overlying shallow-marine facies and overlain by fluvial facies 
in an overall progradational pattern. 
 
 
 
 
2.2 CLASTIC COASTS AND ESTUARIES 
The morphology of coastlines is very variable, ranging from cliffs of bedrock to gravelly or sandy 
beaches to lower energy settings where there are lagoons or tidal mudflats.Wave and tidal 
processes exert a strong control on the morphology of coastlines and the distribution of different 
depositional facies. Wave-dominated coasts have well developed constructional beaches that may 
either fringe the coastal plain or form a barrier behind which lies a protected lagoon. Barrier systems 
are less well developed where there is a larger tidal range and the deposits of intertidal settings, 
such as tidal mudflats, become important. A very wide range of sediment types can be deposited in 
these coastal depositional systems and in this chapter only terrigenous clastic environments are 
considered: carbonate and evaporite coastal systems are covered in the following chapter. Estuaries 
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are coastal features where water and sediment are supplied by a river, but, unlike deltas, the 
deposition is confined to a drowned river valley. 
 
Recognition of estuarine deposits: summary 
There are many features in common between the deposits of deltas and estuaries in the stratigraphic 
record. Both are sedimentary bodies formed at the interface between marine and continental 
environments and consequently display evidence of physical, chemical and biological processes 
that are active in both settings (e.g. an association of beds containing a marine shelly fauna with 
other units containing rootlets).The key difference is that a delta is a progradational sediment body, 
that is, it builds out into the sea and will show a coarsening-up succession produced by this 
progradation. In contrast, estuaries are mainly aggradational, building up within a drowned river 
channel. The base of an estuarine succession is therefore commonly an erosion surface scoured at 
the mouth of the river, for example, in response to sea level fall. It may be difficult to distinguish 
between the deposits of a tidal estuary and a tide-dominated delta if there is limited information and 
it is difficult to establish whether the succession is aggradational and valley-filling or progradational. 
 
Characteristics of coastal and estuarine systems 
These complex, heterogeneous depositional environments are divided into four elements for the 
purposes of summarising their characteristics. 
 
 
A wave-dominated coastline with a beach-barrier bar protecting a lagoon. 
 
 
Morphological features of a coastline influenced by wave processes and tidal currents. 
 
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Beach/barrier systems 
- lithology – sand and conglomerate 
- mineralogy – mature quartz sands and shelly sands 
- texture – well sorted, well rounded clasts 
- bed geometry – elongate lenses 
- sedimentary structures – low-angle stratification and wave reworking 
- palaeocurrents – mainly wave-formed structures 
- fossils – robust shelly debris 
- colour – not diagnostic 
- facies associations – may be associated with coastalplain, lagoonal or shallow-marine facies. 
 
 
Lagoons 
- lithology – mainly mud with some sand 
- mineralogy – variable 
- texture – fine-grained, moderately to poorly sorted 
- bed geometry – thinly bedded mud with thin sheets and lenses of sand 
- sedimentary structures – may be laminated and wave rippled 
- palaeocurrents – rare, not diagnostic 
- fossils – often monospecific assemblages of hypersaline or brackish tolerant organisms 
- colour – may be dark due to anaerobic conditions 
- facies associations – may be associated with coastal plain or beach barrier deposits 
 
Tidal channel systems 
- lithology – mud, sand and less commonly conglomerate 
- mineralogy – variable 
- texture – may be well sorted in high energy settings 
- bed geometry – lenses with erosional bases 
- sedimentary structures – cross-bedding and cross lamination and inclined heterolithic 
stratification 
- palaeocurrents – bimodal in tidal estuaries 
- fossils – shallow marine 
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- colour – not diagnostic 
- facies associations – may be overlain by fluvial,shallow marine, continental or delta facies 
 
Tidal mudflats 
- lithology – mud and sand 
- mineralogy – clay and shelly sand 
- texture – fine-grained, not diagnostic 
- bed geometry – tabular muds with thin sheets and lenses of sand 
- sedimentary structures – ripple cross-lamination and flaser/lenticular bedding 
- palaeocurrents – bimodal in tidal estuaries 
- fossils – shallow marine fauna and salt marsh vegetation 
- colour – often dark due to anaerobic conditions 
- facies associations – may be overlain by shallow marine or continental facies 
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3. SHALLOW SANDY SEAS 
 
Shallow marine environments are areas of accumulation of substantial amounts of terrigenous 
clastic material brought in by rivers from the continental realm. Off shore from most coastlines there 
is a region of shallow water, the continental shelf, which may stretch tens to hundreds of kilometres 
out to sea before the water deepens down to the abyssal depths of ocean basins. Not all land areas 
are separated by ocean basins, but instead have shallow, epicontinental seas between them. 
Terrigenous clastic material is distributed on shelves and epicontinental seas by tides, waves, 
storms and ocean currents: these processes sort the material by grain size and deposit areas of 
sand and mud, which form thick, extensive sandstone and mudstone bodies in the stratigraphic 
record. Characteristic facies can be recognised as the products of transport and deposition by tides 
and storm/wave processes. Deposition in shallow marine environments is sensitive to changes in 
sea level and the stratigraphic record of sea-level changes is recorded within sediments formed in 
these settings. 
 
Characteristics of deposits of shallow sandy seas 
. lithology – mainly sand and mud, with some gravel 
. mineralogy: – mature quartz sands, shelly sands 
. texture – generally moderately to well sorted 
. bed geometry – sheets of variable thickness, large lenses formed by ridges and bars 
. sedimentary structures – cross-bedding, cross-and horizontal lamination, hummocky and swaley 
crossstratification 
. palaeocurrents – flow directions very variable, reflecting tidal currents, longshore drift, etc. 
. fossils – often diverse and abundant, benthic forms are characteristic 
. colour – often pale yellow-brown sands or grey sands and muds 
. facies associations – may be overlain or underlainby coastal, deltaic, estuarine or deeper marine 
facies. 
 
 
Characteristics of a storm-dominated shelf environment. 
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3. AMBIENTES DE SEDIMENTAÇÃO: MARINHO 
 
 
3.1 SHALLOW MARINE CARBONATE AND EVAPORITE ENVIRONMENTS 
Limestones are common and widespread sedimentary rocks that are mainly formed in shallow 
marine depositional environments. Most of the calcium carbonate that makes up limestone comes 
from biological sources, ranging from the hard, shelly parts of invertebrates such as molluscs to very 
fine particles of calcite and aragonite formed by algae. The accumulation of sediment in carbonate-
forming environments is largely controlled by factors that influence the types and abundances of 
organisms that live in them. Water depth, temperature, salinity, nutrient availability and the supply 
of terrigenous clastic material all influence carbonate deposition and the build up of successions of 
limestones. Some depositional environments are created by organisms, for example, reefs built up 
by sedentary colonial organisms such as corals. Changes in biota through geological time have also 
played an important role in determining the characteristics of shallow-marine sediments through the 
stratigraphic record. In arid settings carbonate sedimentation may be associated with evaporite 
successions formed by the chemical precipitation of gypsum, anhydrite and halite from the 
evaporation of seawater. Shallow marine environments can be sites for the formation of 
exceptionally thick evaporate successions, so-called ‘saline giants’, that have no modern 
equivalents. 
 
Characteristics of shallow marine carbonates 
. lithology – limestone 
. mineralogy – calcite and aragonite 
. texture – variable, biogenic structures in reefs, wellsorted in shallow water 
. bed geometry – massive reef build-ups on rimmed shelves and extensive sheet units on ramps 
. sedimentary structures – cross-bedding in oolite shoals 
. palaeocurrents – not usually diagnostic, with tide, wave and storm driven currents 
. fossils – usually abundant, shallow marine fauna most common 
. colour – usually pale white, cream or grey 
. facies associations – may occur with evaporites, associations with terrigenous clastic material may 
occur 
 
Characteristics of marine evaporites 
. lithology – gypsum, anhydrite and halite 
. mineralogy – evaporite minerals 
. texture – crystalline or amorphous 
. bed geometry – sheets in lagoons and barred basins, nodular in sabkhas 
. sedimentary structures – intrastratal solution breccias and deformation 
. palaeocurrents – rare 
. fossils – rare 
. colour – typically white, but may be coloured by impurities 
. facies associations – often with shallow marine carbonates 
 
 
3.2 DEEP MARINE ENVIRONMENTS 
The deep oceans are the largest areas of sediment accumulation on Earth but they are also the 
least understood. Around the edges of ocean basins sediment shed from land areas and the 
continental shelves is carried tens to hundreds of kilometres out into the basin by gravity-driven 
mass flows. Turbidity currents and debris flows transport sediment down the continental slope and 
out on to the ocean floor to form aprons and fans of deposits. Towards the basin centre terrigenous 
clastic detritus is limited to wind-blown dust, including volcanic ash and fine particulate matter held 
in temporary suspension in ocean currents. The surface waters are rich in life but below the photic 
zone organisms are rarer and on the deep sea floor life is relatively sparse, apart from strange 
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creatures around hydrothermal vents. Organisms that live floating or swimming in the oceans 
provide a source of sediment in the form of their shells and skeletons when they die.These sources 
of pelagic detritus are present throughout the oceans, varying in quantity according to the surface 
climate and related biogenic productivity. 
 
Recognition of deep ocean deposits: summary 
Our knowledge of the deep oceans today is very poor compared with other depositional 
environments and considering the sizes of these areas of sedimentas ‘deep water’ it should be 
remembered that the actual palaeo water depth of deposition might have been anything below 200 
m. 
 
Characteristics of deep marine deposits 
. lithology – mud, sand and gravel, fine-grained limestones 
. mineralogy – arenites may be lithic or arkosic; carbonate and chert 
. texture – variable, some turbidites poorly sorted 
. bed geometry – mainly thin sheet beds, except in submarine fan channels 
. sedimentary structures – graded turbidite beds with some horizontal and ripple lamination 
. palaeocurrents – bottom structures and ripple lamination in turbidites show flow direction 
. fossils – pelagic, free swimming and floating organisms 
. colour – variable with red pelagic clays, typically dark turbidites 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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4. AMBIENTES DE SEDIMENTAÇÃO: MISTO 
 
4.1. VOLCANIC ROCKS AND SEDIMENTS 
The study of volcanic processes is normally considered to lie within the realm of igneous geology 
as the origins of the magmatism lie within the crust and mantle. However, the volcanic material is 
transported and deposited by sedimentary processes when it is particulate matter ejected from a 
vent as volcanic ash or coarser debris. Furthermore, both ashes and lavas can contribute to 
sedimentary successions, and in some places the stratigraphic record is dominated by the products 
of volcanism. Transport and deposition by primary volcanic mechanisms involve processes that are 
not encountered in other settings, including air fall of large quantities of ash particles that have been 
ejected into the atmosphere by explosive volcanic activity, and flows made up of mixtures of hot 
particulate matter and gases that may travel at very high velocities away from the vent and rapidly 
form a layer of volcanic detritus. Volcanic activity can create depositional environments and it can 
also contribute material to all other settings, both on land and in the oceans. The record of volcanic 
activity preserved within stratigraphic successions provides important information about the history 
of the Earth and the presence of volcanic rocks in strata offers a means for radiometric dating of 
these successions. 
 
Recognition of volcanic deposits: summary 
The single most important criterion for the recognition of volcanigenic deposits is the composition of 
the material. Lavas and primary volcaniclastic detritus rarely contain any material other than the 
products of the eruption, the nature of which depends on the chemical composition of the magma 
and the nature of the eruption. Recognition of the volcaniclastic origin of rocks in the stratigraphic 
record becomes more difficult if the material is fine-grained, altered or both. In hand specimen a 
fine-grained volcaniclastic rock can be confused with a terrigenous clastic rock of similar grain size. 
Microscopic examination of a thin-section usually resolves the problem by making it possible to 
distinguish the crystalline forms within the volcaniclastic deposit from the eroded, detrital grains of 
terrigenous clastic material. Alteration can destroy the original volcanic fabric of the rock principally 
by breakdown of feldspars and other minerals to clays: rocks of basaltic composition are particularly 
susceptible to alteration. Complete alteration may mean that the original nature of the material can 
be determined only from relict fabrics, such as the outlines of the shapes of feldspar crystals 
remaining despite total alteration to clay minerals, and the chemistry of the clays as determined by 
XRD analysis. 
 
Characteristics of volcaniclastic deposits 
. lithology – basaltic to rhyolitic composition with lithic, crystal and glass fragments 
. mineralogy – feldspar, other silicate minerals, some quartz 
. texture – poorly to moderately sorted 
. bed geometry – may mantle or fill topography 
. sedimentary structures – parallel bedding, dune and antidune cross-bedding in pyroclastic flows 
. palaeocurrents – cross-bedding may indicate pyroclastic flow direction 
. fossils – rare except for plants and animals trapped during ash falls and flows 
. colour – from black in basaltic deposits to pale grey rhyolitic material 
. facies associations – pyroclastic deposits may occur associated with any continental and shallow-
marine facies. 
 
 
 
 
 
 
 
 
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5. POST-DEPOSITIONAL STRUCTURES AND DIAGENESIS 
 
A sediment body deposited on land or in the sea normally undergoes significant modification before 
it becomes a sedimentary rock. Physical, chemical and, to some extent, biological processes act on 
the sediment at scales that range from the molecular to basin-wide. Generally these processes 
change sediment into sedimentary rocks by compacting loose detritus and adding material to create 
cements that bind the sediment together. Chemical changes occur to form new minerals and organic 
substances, and physical processes affect the layers on large and small scales. An important 
product of these post-depositional processes is the formation and concentration of fossil fuels: coal, 
oil and natural gas are all products of processes within sedimentary strata that occur after deposition.