DEPOSITIONAL ENVIRONMENTS
Continental Fluvial Alluvial Fan, Braided River, Meandering River Desert/Aeolian, Lacustrine Glacial Deltaic Delta Plain, Delta Front, Pro-delta Marginal Marine Beach, Barrier Island, Estuary, Tidal Flat Marine Continental Shelf Carbonate Reefs and associated environments, Siliciclastic Shelves. Continental Slope and Ocean Floor
An Environment: a distinctive kind of geographic setting characterized by a distinctive set of physical and/or chemical and/or biological conditions. Characteristics of Depositional Environment: Here’s a list of what you can look for in a sediment rock or a sedimentary bed that might tell you something about depositional environment: grain size grain shape grain surface texture grain fabric sedimentary structures composition (siliciclastic; carbonate, evaporite, coal, chert) fossils (body fossils, trace fossils) stratification sequence sediment-body geometry/architecture
Continental depositional environments & Characteristic Deposits: Glacial: Till, moraines, deltas, lakes, outwash, kames, drumlins Desert: Arroyo (wadi) deposits, eolian sands, sand dunes, dust, loess Lake: clastic deposits (bedded), chemical (salts, carbonates) Fluvial: Channels, banks, flood basins
FLUVIAL - RIVERS Make up a very large fraction of terrestrial sedimentary rocks Rivers are largely erosive, but some relatively continuous (pointbar) and periodic (flood) deposition Braided streams - horizontal beds of conglomerates and sands with unidirectional crossbeds , almost no mud. Meandering streams Produce very distinctive sediments:
Channel conglomerates Cross-bedded channel sandstones, well-sorted Pointbar sands, somewhat finer than channel sands Levee sands, ripple marks and finer grains Backswamp, non-laminated mudstones This package of sediments produces a sequence that fines upwards. Also associated with meandering streams Cut-off meanders (ox-bow lakes) with lacustrine sediments
Additionally, meandering stream sequences surrounded by flood-plain deposits of paleosols interrupted by layers of flood muds. The whole package of stream deposits, paleosols, and ox-bow lakes form fluviolacustrine deposits . In the above, each particular subenvironment produces rocks of a particular facies (particular combination of sedimentological, structural, and biological features).Walther's Law: As depositional environments migrate laterally , the sediments of one environment come to lie on top of sediments of the adjacent environment. Identification of Fluvial Environment : absence of marine fossils presence of plant fossils red beds scoured channels unidirectional-flow cross-stratification broadly unidirectional paleocurrents paleosols desiccation cracks plant fossils
FLUVIAL ENVIRONMENT TYPES Alluvial fans Alluvial fans are fan-shaped deposits of water-transported material (alluvium). They typically form at the base of topographic features where there is a marked break in slope. Consequently, alluvial fans tend to be coarse-grained, especially at their mouths. At their edges, however, they can be relatively fine-grained. Gravel Deposit --on an alluvial fan in Death Valley, California. Note how the upper half of the deposit is coarser than the lower half. It probably marks a former channel that migrated over the non-channelled part of the fan. It is now stranded above the active channel and is being eroded. Channel Deposits When seen in cross-section, channels (red arrow) have elliptical profiles and are filled with sediment that is generally coarser than the surrounding rock. They also cut into (by erosion) the underlying rock. Hammer (circled) shows scale. Channel deposits indicate running water: they form on alluvial fans, in rivers, on deltas, even in submarine fans. Both of these photos are of channels in alluvial fan deposits (note how coarse the rock is). The red dashed line on the right shows the channel. the orange lines show bedding beneath the channel. Notice how it is cut by the channel. Braided River Some rivers have lots of small channels that continually split and join. These are called braided rivers. Braided rivers are usually wide but shallow. They form on fairly steep slopes and where the river bank is easily eroded. Braided Rivers exhibit numerous channels that split off and rejoin each other to give a braided appearance. They typically carry fairly coarse-grained sediment down a fairly steep gradient. Additionally, the water discharge tends to be highly variable. Consequently, braided rivers usually exist near mountainous regions, especially those with glaciers. When it rains really hard you can find little braided rivers flowing across building sites and paths made of sand or fine gravel. Braided rivers have a lot of rock, gravel and sand that is carried along the river bottom. When the amount of water flowing through individual channels deceases the river dumps this material creating islands and bars. Braided-river systems Characteristics: •moderately steep grade •fairly straight •many channels, bars, and islands •coarse-grained sediments Meandering rivers Rivers flowing over gently sloping ground begin to curve back and forth across the landscape. These are called meandering rivers. Meandering rivers erode sediment from the outer curve of each meander bend and deposit it on an inner curve further down stream. This causes individual meanders to grow larger and larger over time. Meandering river channels are asymmetrical. The deepest part of the channel is on the outside of each bend. The water flows faster in these deeper sec tions and erodes material from the
river bank. The water flows more slowly in the shallow areas near the inside of each bend. The slower water can't carry as much sediment and deposits its load on a series of point bars. Oxbow lakes form when a meander grows so big and loopy that two bends of the river join together. Once the meander bends join, the flow of water reduces and sediment begins to build up. Over time oxbow lakes will fill with sediment and can even disappear. The point where the two bends intersect is called a meander cut-off. The low-lying area on either side of a river is called a floodplain. The floodplain is covered with water when the river overflows it banks during spring floods or periods of heavy rain. Sediment is deposited on the floodplain each time the river floods. Mud deposited on the floodplain can make the soil really good for agriculture
AEOLIAN ENVIRONMENTS Aeolian sedimentary processes are those involving transport and deposition of material by the wind. 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
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 rec ord. Processes in glacial environments:
Glaciers radically transform landscapes because they carry out four types of geomorphological work. Glacial erosion: erosion of rock by the direct action of moving glaciers. Glacial transportation: movement of debris that is either frozen within the glacier or lying on the surface. Glacial deposition: occurs when rock debris, either frozen within the glacier or lying on the surface, is deposited onto the ground by melting of ice. Glacio-fluvial activity: processes (erosion, transportation, deposition) caused action of meltwater streams.
Another important process that operates within glacial areas is frost weathering or freeze-thaw action. Frost weathering causes the disintegration of rock masses into fragmented debris due to the growth and expansion of ice in joints and pores. Once a rock mass has been weakened by frost weathering it collapses under the force of gravity. If this occurs on a steep slope the loosened debris moves down the slope as rock fall. Frost weathering of rock slopes on the flanks of glacial valleys supplies debris to the surface of a glacier. This debris may eventually be deposited as till or may be transported down to the base of a glacier to become to the tools of erosion. The action of these glacial environment processes is shaping and has shaped the landscapes in contemporary glacial areas and regions that were glaciated during the Ice Ages 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.
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 evaporation from 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 evaporates. 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
DELTAIC ENVIRONMENTS 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. Deltas 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. Delta-top sub-environments: Deltas are fed by a river or an alluvial fan and there is a transition between the area that is considered part of the fluvial/alluvial environment and the region that is considered to be the delta top or delta plain. Delta-front sub-environments: At the mouth of the channels the flow velocity is abruptly reduced as the water enters the standing water of the lake or sea. The delta front immediately forward of the channel mouth is the site of deposition of bedload material as a subaqueous mouth bar. Pro-Delta sub-environment The furthest offshore portion of a delta, lying at the toe of the delta front, and characterized by a relatively slow rate of fine-grained deposition. 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, mouthbar lenses variably elongate, prodelta deposits thin bedded Sedimentary structures: cross-bedding and lamination in delta-top and mouth-bar facies. Palaeo-currents: topset facies indicate direction of progradation, wave and tidal reworking variable on delta front 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
M A R G I N A L - M A RI N E E N V I R O N M E N T S 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 coastal plain, 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 crosslamination and inclined heterolithic stratification . palaeocurrents – bimodal in tidal estuaries . fossils – shallow marine . colour – not diagnostic . facies associations – may be overlain by fluvial, shallow marine, continental or delta 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 Depositional characteristics for marginal-marine environments River, wave, and tidal processes are dominant
sedimentary processes Salinity varies significantly Some environments are of intermittent to nearly constant subaerial exposure, others are continuously covered by shallow water. ØWide variety of sediment types including conglomerates, sandstones, shales, carbonates, and evaporites.
MARINE ENVIRONMENTS The Shallow Marine Environment and Continental Shelf: It is a region of shallow water which may stretch tens to hundreds of kilometers out to sea before the water deepens down to the abyssal depths of ocean basins, which is c alled the continental shelf. Shallow marine environments are areas of accumulation of substantial amounts of terrigenous clastic material brought in by rivers from the continental realm., Limestones are also common and widespread sedimentary rocks that are mainly formed in shallow marine depositional environments. 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 evaporite successions, so-called ‘saline giants’, that have no modern equivalents.
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 cross stratification 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 underlain by coastal, deltaic, estuarine or deeper marine facies. Characteristics of shallow marine carbonates lithology – limestone mineralogy – calcite and aragonite texture – variable, biogenic structures in reefs, well sorted 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 mate rial 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
Deep Marine Environments The deep oceans are the largest areas of sediment accumulation on Earth but they ar e also the least understood. Around the edges of oc ean basins sediment shed from land areas and the continental shelves is carried te ns 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. 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 and pale pelagic limestones facies associations – may be overlain or underlain by shelf facies.
