Dolomite cements vary from pore-filling microcrystalline rhombs to coarse anhedral
mosaics and large poikilotopic crystals. Ferroan (iron-rich) dolomite is also common in
sandstones that underwent reducing conditions.
1.9. Depositional environments of sandstone and conglomerate
The term facies should be defined before starting discussing the sedimentary
environments. A facies is a body or packet of sedimentary rocks with features that
distinguishes it from other facies. The features include lithology, texture, structure, and
fossil content. Therefore, usually two types of facies are distinguished; lithofacies and
biofacies.
Depositional environments of siliciclastic sediments can be classified into three major
environments with each subdivided further into several ones. The first environment is the
continental environment that includes: alluvial fans, rivers, deserts, lakes and glaciers.
The second environment is the marginal marine (or marine shorelines) environment that
includes: deltas, beaches/barrier bars, lagoons and tidal flats. The third environment is the
marine environment that includes: shallow marine shelves and epeiric seas, and
continental margins and deep-water basins.
1.9.1. Continental environments
Usually the term fluvial is used to indicate together alluvial fans and rivers. Fluvial
environments are complex systems of erosion, sediment transportation and deposition
which give rise to different landforms. At present time fluvial systems range from alluvial
fans, through braided and low sinuosity stream networks to meandering (high sinuosity)
rivers. Their sediments range from the coarsest conglomerate through sandstones to
mudrocks. Fluvial conglomerates are typically lenticular, often with cross-bedding, with
both extra-and intra-formational clasts and have a pebble-support fabric. Fluvial
sandstones are lenticular or laterally more persistent, sharp-based and cross-bedded,
almost free off fossils but with plant debris, and may contain soil horizons.
Alluvial fans are aprons of sediment occurring adjacent to upland areas, particularly
those bounded by faults, with the fan apex located at the mouth of a canyon or wadi.
They grade downslope into braided rivers, or playas or lakes, coastal plain, or even
directly into sea forming fan-deltas. Fans have usually a radius of 5 to 15 km, with their
size depending on the size of catchment’s basin. The surface of a fan is dissected by a
network of channels radiating out from the fan apex. Sediments of alluvial fans are the
coarsest of the other fluvial environments, consisting of gravels and coarse sand.
Alluvial fans are common in semi-arid regions where there is periodic or infrequent
heavy rainfall. Deposition takes place from debris flows, stream floods, and sheet floods.
Debris flows (mud flows) are high density; high viscosity flows consisting of much fine
sediment, together with clasts up to boulder size. Therefore, debris flow deposits
conglomerate with a matrix–support fabric. Stream floods are low viscosity flows,
usually confined to channels that give rise to cross-bedded pebbly sand deposits and
lenticular gravel with a pebble-support fabric. Sheet floods are shallow but extensive
surface flows that deposit thin bed of sand and gravel with planar and cross stratification.
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Alluvial fans can be divided into three parts: proximal (or upper) fan, mid-fan, and distal
(or lower) fan (Fig. 1.56).
Fig. 1.56: Facies diagram of an alluvial fan showing gravel and coarse sand in the upper
fan that pass down to pebbly sand and finer sediments of the mid to lower fan, which in
turn grade into playa muds.
Proximal fan has the steepest slope, and coarsest sediment, that are extremely poorly
sorted, matrix rich conglomerate, with poorly developed sedimentary structures. Mid-fan
has a gentler slope and sediments of intermediate size, giving rise to sheet-like deposits
of sand and gravel that show planar and trough cross-bedding and imbrication. Distal fan
has the gentlest slope, finest sediment giving rise to better sorted sands with thin gravel
interlayers.
Braided rivers are developed in areas of high slope, coarse sediments, high and variable
water discharge, and scarce vegetation. Actually, all these factors are controlled by
climate, tectonics and source-area geology. Braided or low sinuosity rivers are
characterized by presence of sand and gravel bars, sand flats and islands that divide water
flow into a number of smaller channels (Fig. 1.57).
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Fig. 1.57: Depositional model illustrating the braided river system.
Sediments deposited in braided rivers are mainly sands, with gravels in higher reaches.
As a result of the dominant channel and bar deposition, with little lateral migration,
braided river sequences form major elongate sand bodies consisting of lenticular crossstratified units (Fig. 1.57). Planar cross-bedding, formed by downstream migration of
bars and dunes (two-dimensional), and internal erosion surfaces between units are
common. Channel fills may show some fining-upwards of grain size. On a large scale,
braided streams give rise to multistory sand bodies with an elongate to sheet geometry.
Palaeocurrents of low sinuosity rivers are unimodal with a low degree of variance than
meandering streams.
Meandering or high sinuosity streams possess distinct channel and overbank
subenvironments and processes (Fig. 1.58).
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Fig. 1.58: Depositional model for a meandering stream.
The channel occupies only a small part of the alluvial plain but it migrates laterally
through bank erosion and point bar deposition. Sand is moved as dunes on the channel
floor and lower part of the point bar and as ripples on the upper part. Trough crossbedding is produced by the downstream migration of the dunes, whereas cross-lamination
results from the migration of ripples at the point bar top. The lateral accretion of point
bars produces erosive-based sandstone showing an upward decrease in grain size and
scale of cross-stratification. The presence of a large scale cross-bedding called epsilon or
lateral accretion surfaces signifies this periodic lateral accretion.
During floods, water flows over the banks of the river covering an area adjacent to the
river called floodplain. From the suspended load of the flooding water, silt and clay are
deposited on the floodplain by settling out, a process called vertical accretion. This
results in a fine-grained member overlying the channel point bar sandstone (Fig. 1.58,
above) which both constitute a fining upward sequence characteristic of meandering
rivers. The floodplain could be also a site of soil formation, marches, swamps under
humid climate, and salt precipitation or caliche (calcareous deposit) under arid climate.
Some sand could be deposited on levees adjacent to the channel when the river overtops
its banks; sand is also deposited on floodplains as a result of crevassing, when a river
breaches its banks. These sands occur as intercalations within the floodplain muds. The
fining-upward sedimentary cycle produced through meandering stream deposition can be
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repeated many times in a fluvial sequence. Palaeocurrents for meandering river
sandstones have a greater dispersion than those from braided stream deposits.
Deserts are areas of intense aridity, generally located in the subtropical belts (latitudes of
20 to 30º). Deserts are not only covered by sand, but also alluvial fans, ephemeral
streams, and desert lakes (playas), bare rocks all could occur in desert regions. The
aeolian desert sands vary from a thin, impersistent cover to extensive sand seas, termed
ergs.
Desert dune sands are typified by a grain size of fine to coarse sand, good sorting and a
positive skew. Grains are well rounded with a frosted surface. This textural maturity,
produced by frequent grain collisions, is matched by compositional maturity: desert sands
are mature to supermature, many being quartz arenites. Many desert sandstones are red
trough hematite pigmentation, and fossils are absent apart from occasional vertebrate
bones and footprints.
The characteristic sedimentary structure of aeolian sands is large –scale, high angle crossbedding. Associated with the aeolian sand may occur playa-lake sediments: mudrocks
and evaporites.
In Lakes sand and coarser sediments are being deposited along their shorelines, in their
deltas where rivers drain into them, and on their deep basin floor. Beach sands and
gravels of lakes are moderately sorted and subrounded due to low wave activity and
absence of tides if compared with marine beaches. Wave-formed ripples, polygonal
desiccation and syneresis cracks are common in finer-grained lake shoreline sediments.
On deep floors of lakes, river underflows deposit graded beds with scoured bases. Also,
finely laminated sediments (rhythmites) are deposited in lake-basin centers from dilute
density currents and settling out of clay from suspension. The recognition of silisiclastic
lacustrine (lake) sediments is based on the absence of marine fauna and an association
with certain minerals (like sepiolite, palygorskite and corrensite clay minerals) and rocks
that are almost restricted to lakes.
Glacial environments include a wide range of depositional settings from continental to
marine, subgalcial to supraglacial, and glaciofluvial to galciolacustrine. Sediment
deposited directly from a glacier is referred to as till, or tillite if indurated. Subglacial
(lower part of the glacier in contact with the bed) till and tillite: are mostly massive; have
an extensive lateral continuity with a thickness of several to tens of meters; lack
stratification; have much matrix which supports the clasts, some of which have striations
and facets; contain clasts of both local and exotic origin; and have a matrix that is largely
comminuted (ground) rock fragments. Other evidence for glaciation include: cracked
boulders, impression if ice crystals, and striated pavements on bedrock.
Lenticular and stratified conglomerates and sandstones could be interbedded within
tillites. These are water-laid deposits resulting from glaciofluvial (a river that receives
glacial meltwater) processes. Glaciolacustrine (a lake that receives glacial meltwater)
sediments include sands and gravels deposited by streams entering and constructing
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deltas; and rhythmically laminated mudrocks (or also called varves), with some scattered
clasts dropped from rafted ice, deposited in deeper parts of the lake.
Where ice sheets reach the sea, till is deposited on the sea floor and might be reworked by
marine processes. This galciomarine till may contain fossils, and dropstones which are
clasts released from icebergs calved from the ice sheet margin.
1.9.2. Marginal marine environments
Marginal marine (or marine shorelines) environments include: deltas, beaches/barrier
bars, lagoons and tidal flats.
Deltas form where rivers enter the sea, and represent one of the main sites of siliciclastic
deposition. Deltas are complex environments where the main factors controlling sediment
distribution and delta morphology are the interplay between river regime, tides and wave
action. Climate, water depth and subsidence rate are also important.
Deltas consist of three parts: delta plain, delta front, and prodelta (Fig. 1.59).
Fig. 1.59: Subenvironments of lobate delta (left) and birdfoot delta (right).
The delta plain or delta top refers to the area landward of the shoreline. It can be divided
into upper delta plain, dominated by river processes, and lower delta plain where there is
some marine influence. Subenvironments of the delta plain include: the distirbutary
channels, interdistributary bays, floodplain’s marches and swamps. The delta front
includes the subaqueous mouth sand bars and distal silt to fine sand bars in front of the
distributary channels. The muddy prodelta is the part of delta in the deepest offshore
region.
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There are two common types of deltas: the lobate delta (Fig. 1.59 left) with numerous
radiating and dividing distributary channels, and the birdfoot delta (Fig. 1.59 right) with
just one or several major distributaries and well-developed interdistributary bays.
The characteristic appearance of deltaic sediments is a vertical sequence of coarseningupward units (Fig. 1.60), produced by progradation (building out) of the delta front.
Thickness of these sequences depends on the size and type of delta, water depth, and
subsidence rate, but is generally more than 30 m.
Fig. 1.60: Sketch graphic log and interpretation of typical coarsening-upward sequence
produced by delta progradation.
Sands deposited at distributary mouths gradually build out over prodelta muds. Delta
plain deposits in the form of seatearths (clays below coal) and coals, and possibly thin
coarsening-upward units formed by the infilling of interdistributary bays, occur above the
delta front sequence. Mouth bar sands and channel sands show cross-stratification and
planar bedding while flaser and wavy cross-bedding is common in distal bar and prodelta
deposits.
Marine shoreline environments include beaches, barrier islands, tidal flats and
estuaries. They are sites of deposition of much siliciclastic sediments. Sediment supply,
tidal range, wave action, sea level and tectonic history, and climate all affect the
sedimentation. Beaches and barriers are best developed in areas of moderate to high wave
action, whereas tidal flats are more characteristic of areas with high tidal range.
Beaches are linear belts of sand along the coast whereas barrier islands are ridges of
sand separated from the land by a lagoon which in turn could be connected to the sea by
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tidal inlets or tidal channels (Fig. 1.61). Mud flat sand marches are common around the
lagoon.
Fig. 1.61: Subenvironments of a beach-barrier island and lagoon shoreline system.
Going from land through the silisiclastic shoreline to the deeper part of the sea, the
following zones can be distinguished: backshore, foreshore, shoreface, transitional area,
offshore (deep sea) (Fig. 1.62).
Also from beaches and barrier islands to the offshore there is a decrease in grain size and
succession of bedforms reflecting the changing wave conditions (Fig. 1.62).
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Fig. 1.62: Wave zones, subenvironments, sediments and sedimentary structures along
siliciclastic shoreline.
In the backshore (land area covered only by water during storms and highs) wind dunes
are active that give rise to large-scale aeolian cross-bedded sand, landward oriented, with
root traces and some land organism burrows.
The foreshore zone is characterized by wave action towards the shore (swash) and
backward return of water towards the sea (backwash) causing deposition of flat bedded
and planar laminated fine sand. The foreshore zone corresponds to the intertidal zone (sea
bottom depth from high tide level to low tide level) in shores with high tidal activity.
The shoreface (subtidal) zone extends from the low tide level to the fair-weather wave
base (FWWB) which is the depth of sea bottom where normal waves begin to affect the
sea bed. This shoreface is characterized by deposition of symmetrical and asymmetrical
wave-formed ripples and lunate dunes that give rise to planar and trough cross-bedding
and cross-lamination. Burrow structures are common in shroface sediments. Also, swaley
cross-stratification my form here (SCS, Fig. 1.62).
Offshore from the shoreface, sands with hummocky cross-stratification (HCS, Fig. 1.62)
generated by storm waves may occur between fair-weather and storm wave-base (depth
of sea bottom where storms begin to affect sea bed)
Going into deeper water hummocky cross-stratification then passes into muddy sands
with ripples and bioturbation, and graded sand layers (‘tempestites’) and shell lags
produced by storm currents.
Barrier islands and beaches may migrate both seawards and landwards, depending on the
state of sealevel (whether it is static or rising), the supply of sand and the rate of
subsidence. Coarsening-upward sequences are produced by seaward progradation.
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Tidal flats are developed in shorelines characterized by a high range between the low
tide and the high tide. They could reach several kilometers in width and occur around
lagoons, behind barriers, and in estuaries. There is usually a decrease in sediment grain
size across the flat from sand in the lower intertidal zone to silt and clay in the higher
part.
Cross-bedded sands with herringbone structure are deposited in the low tidal flat.
Burrows and grazing trace fossils are common. In the mid-upper tidal flat various types
of ripples, showing interference patterns, are formed giving rise to flaser, wavy, and
lenticular bedding. Bioturbation and different types of trace fossils are common.
Desiccation cracks form in the high intertidal to supratidal zone, and depending on the
climate, marshes or sabkhas will occupy the supratidal area. Progradation of tidal flat
sediments typically forms a fining-upward sequence, with the thickness is determined by
the paleotidal range.
1.9.3. Marine environment
Marine environment includes: shallow marine shelves and epeiric seas, and continental
margins and deep-water basins.
Shallow marine shelves and epeiric seas
In the shallow marine environment away from the coastline, where water depths range
from 10 to 200 m, sands are deposited in continental shelf seas, such as on the presentday astern shelf of North America, and in epeiric, epicontinental seas, such as the North
Sea. In some shelf and epeiric seas tidal currents dominate, while others are storm or
wave dominated. Deposits of tide-dominated seas and shelves are the tidal sand bodies,
but muds are more extensive because large areas of these seas have relatively weak
currents. The bedforms developed in tidal sand bodies include ripples, dunes, and sand
waves (or megaripples) that give rise to cross stratification and planar bedding.
Ancient shallow marine sandstones vary from thin bedded, sharp based sandstones with
graded bedding and cross lamination, deposited by storm-generated currents, and
interbedded with mudrocks, to thick sequences of planar and trough cross-bedded
sandstone where sand waves existed. Marine body fossils and marine trace fossils are
common. Due to reworking many shallow sandstones are texturally and compositionally
mature; many are quartz arenites.
Continental margin and deep water basin
Continental margins and deep-water basins are the depositional sites of sandstones and
conglomerates derived from adjacent slopes and shelves. Transport downslope is through
sliding, slumping, and through sediment gravity flows such as turbidity currents and
debris flows.
Apart from these resedimentation processes, deeper-water environments are primary
pelagic and hemipelagic deposits such as cherts, pelagic limestone and muudrocks.
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Turbidity currents are density currents of sediment and water in which sediment is kept in
suspension through the fluid turbulence. Some coarse sediment is transported as bed load.
Turbidites, deposited by decelerating turbidity currents, are the most common type of
deep water sandstone. They typically range from a few centimeters to a meter or more
thick, and form sequences many hundreds or thousands meters thick consisting of
sandstones regularly alternating with hemipelagic mudrocks. Many turbidite sandstones
possess well-developed sole structures: flutes, and other tool marks, as well as trace
fossils and load casts. Graded bedding is a characteristic internal structure of turbidites,
and horizontal and cross laminations are common.
Some turbidites show a definite sequence of internal structures, referred to as Bouma
sequence (Fig. 1.63). It comprises: a graded A division; a lower horizontally laminated B
Fig. 1.63: Sequence of structures and Bouma divisions of a siliciclastic turbidite bed.
division; a cross-laminated C division; an upper horizontally-laminated D division and a
pelite E division. This sequence can be interpreted in terms of deposition from a waning
flow: division A and B represent upper flow regime, C indicates lower flow regime , and
D and E deposition from suspension.
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