Sedimentary Environments - Geosciences
Sedimentary Environments - Geosciences
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Influences of Sedimentary Environments and Volcanic Sources on Diagenetic Alteration of Volcanic Tuffs in South China
Permian-Triassic (P-Tr) altered volcanic ashes (tuffs) are widely distributed within the P-Tr boundary successions in South China. Volcanic altered ashes from terrestrial section-Chahe (CH) and marine section-Shangsi (SS) are selected to further understand the influence of sedimentary environments and volcanic sources on diagenetic alterarion on volcanic tuffs. The zircon 206 Pb/ 238 U ages of the corresponding beds between two sections are almost synchronous. Sedimentary environment of the altered tuffs was characterized by a low pH and did not experience a hydrothermal process. The dominant clay minerals of all the tuff beds are illite-smectite (I-S) minerals, with minor chlorite and kaolinite. I-S minerals of CH (R3) are more ordered than SS (R1), suggesting that CH also shows a higher diagenetic grade and more intensive chemical weathering. Besides, the nature of the volcanism of the tuff beds studied is derived from different magma sources. The clay mineral compositions of tuffs have little relation with the types of source volcanism and the depositional environments. Instead, the degree of the mixed-layer clay minerals and the REE distribution are mainly dependent upon the sedimentary environments. Thus, the mixed-layer clay minerals ratio and their geochemical index can be used as the paleoenvironmental indicator.
Conflict of interest statement
The authors declare no competing interests.
Lithological columns of the correlation…
Lithological columns of the correlation and sampling horizons of the studied sections. Meishan…
U-Pb Concordia diagrams for (…
U-Pb Concordia diagrams for ( a ) CH-1, ( b ) CH-2, (…
XRD profiles from bulk samples…
XRD profiles from bulk samples of Chahe and Shangsi altered ash samples.
XRD profiles from Air-dried (AD)…
XRD profiles from Air-dried (AD) and EG-saturated (EG) of the I/S (illite-smectite) mixed…
Experimental and calculated XRD patterns…
Experimental and calculated XRD patterns of the oriented, air-dry specimens from studied clay…
( a ) Chondrite-normalized REE…
( a ) Chondrite-normalized REE patterns. ( b ) Primitive mantle-normalized trace element…
( a ) Zr versus Ti diagram (after Pearce (1982)). ( b )…
The evolution from the volcanic…
The evolution from the volcanic ashes to the I-S layer mineral in marine…
Using Sedimentary Structures to Interpret Ancient Environments
Rocks to a geologist are like books and ancient manuscripts are to an historian. Rocks contain a record of past events and places. The rock record is incomplete. So part of the puzzle is to also try and figure out what is missing. Is it time? Is it other rocks that have since disappeared or perhaps been moved to another location? These are the scientific puzzles that motivate geologists.
The formation of sediment and sedimentary rock involves many physical, chemical and biological processes, sometimes operating separately but more commonly in concert. The journey from loose sediment to hammer-ringing rock is one of the marvels of the geological world. Deciphering this journey requires us to delve into the rock record.
Imagine that a local geologist tells you that the rocks in your backyard were originally deposited as sand and mud in shallow seas, where beaches and broad coastal tidal flats passed seawards to deeper waters, and landwards to marshes and scrubby coastal plains across which rivers and streams coursed. How did our geologist figure this out? What is it that geologists see in the rocks that helps them paint this picture of a world that existed so many millions of years ago?
Tools at a geologist’s disposal
Sedimentary rocks, whether terrigenous, carbonate, chemical, or volcaniclastic, contain a wealth of information that relate how they formed at or close to Earth’s surface, and the various physical and chemical processes that affected them as they were buried. This article deals with the first part of the journey:
- How sediment moves across a substrate,
- The language of bedforms,
- The hydrodynamic significance of bedforms, and
- An atlas of common sedimentary structures
Transport of sediment bedload and suspension load
Movement of sediment creates beds, structures within beds (e.g., laminations, crossbedding), and entire depositional systems like deltas and submarine fans. The processes by which sediment moves determine what the deposit will look like: a train of ripples, turbidites, a layer of mud, or Martian sand dunes.
In this section we will examine four of these processes:
- The case where a fluid, (water, air) interacts with a bed of loose sand – i.e., bedload transport
- Where sediment accumulates from a suspension of water and fine particles, and
- Movement of sediment by surface waves
- Bedforms in relation to fluid flow – the Flow Regime.
As the name suggests, this element of sediment movement consists of loose, granular particles at the sediment-water interface (such as a stream bed or tidal flat): in other words, at the top of a bed. Air or water that moves across the bed will begin to move grains if the flow velocity is great enough to overcome the force of gravity and any resistance at grain contacts. This is the threshold velocity.
The bedload contains two main components:
The various components of force involved in initiation of grain movement are shown here. Fluid flowing over a sediment bed produces shear stresses that can be resolved into a component of drag (parallel to the bed) and a lift component (perpendicular to the bed). At the threshold velocity when the resultant fluid force on grains becomes greater than gravity, grains begin to roll, slide and jostle along the bed like a moving carpet – the traction carpet.
The short video is of wind ripples where sand moves in a traction carpet (viewed best in full screen).
As flow velocities increase, so too will the lift component of fluid forces and grains may temporarily leave the traction carpet, bouncing along with the flow this process is called saltation. The saltation load is included in bedload because grain excursions into the fluid do not last long. Saltating grains tend to travel much farther in air than water because the contrast in density between the grain and the fluid is much greater. Stand on any sandy beach on a blustery day and you will witness first-hand the effects of grain saltation. Saltation produces many collisions, and not just against your bare legs. Saltation collisions also result in grain abrasion.
The short video shows a saltation load across a windy, black sand beach (sand consists of magnetite, ilmenite and pyroxenes). Saltation also tends to winnow the lighter pyroxenes, separating them from the denser iron oxides.
The ease with which grains move also depends on their density and shape. Quartz grains will move more easily than heavier minerals of the same size. Because they are shaped like little hang-gliders, platy grains like micas, even though they are denser than quartz, will tend to respond hydraulically as lighter particles.
Hjulström’s graph showing the relationship between grain size and flow velocity. Both scales are logarithmic. The graph defines three regions of response: erosion, transport, and deposition of loose sediment. The size of sand sized grains is highlighted in yellow.
The Hjulström curve plots two domains: the erosion curve shows the critical flow velocity required to move a sedimentary particle across a bed, and the deposition curve the flow velocity at which the particle will be deposited (cease to move). Both scales are logarithmic. For reference, the common size range for sand is 0.0625 mm to 2 mm, highlighted in yellow.
In considering these processes, we also neglect grain cohesion. For most sand and silt-sized particles, cohesive forces will be negligible. This is not the case in fine-grained sediment that contain clay. In this situation, clay particles tend to adhere to each other because of electrical charges on the surface of their crystal structures. Once clays have been deposited, it is difficult to remobilize them as individual particles because of electrically induced cohesion. When erosion does occur, it tends to produce lumps of clay, also referred to as rip-up clasts.
The suspension load
Most natural flows in rivers, shallow marine settings and air are turbulent. Even at low-flow velocities, the speed and trajectories of flow can vary considerably. Even seemingly tranquil streams show turbulent eddies and boils on their surface. Very fine particulate sediment (particularly particles of clay) can be kept in suspension for long periods by turbulence the stresses generated by turbulent flow balance or overcome the gravitational force acting on the particles.
If turbulence decreases significantly, for example when a river empties into a lake, then most particles will gradually settle to the sediment bed. The rate at which a particle settles out of suspension is called the settling velocity, where the force of gravity (downwards) exceeds the combined effects of upward-directed buoyancy forces acting on a grain and the drag on a particle caused by fluid (viscous) resistance. Thus, the rate of settling depends on the size, shape and density of particles, and the viscosity of the surrounding fluid. In general, settling through air is much more rapid than through water.
Both bedload and suspension load are important processes in the generation of sedimentary structures. In particular, bedload transport of loose sand is the critical process for growth of bedforms and their internal cross-stratification (crossbedding). The description of bedforms (crossbeds) and the flow conditions (flow regime) under which they form are described in following section.
Sea and lake surface waves are generated by wind. The wind provides the energy which is transferred to surface waters. As a general rule, the stronger the wind, the greater the wave amplitude, wavelength, and speed. Surface waves are basically pulses of energy. As such, a water mass does not move in concert with the wave. Instead, water particles beneath waves have a circular or elliptical motion (referred to as an orbital). The largest orbitals occur immediately below the wave crest, decreasing in size to a depth that equates to about half the wavelength. This depth where wave action ceases is called the wave base (or wavebase). If the wave base is deeper than the sea floor, then the waves will interact with the sediment there. Conversely, in deep water, the seafloor sediment is beyond the reach of the wave base, and so waves do not interact with the sea floor.
/> Diagram illustrating the orbital movement of water beneath surface waves. The water depth where wave orbitals interact with the sea floor is called wavebase – in this region waves can move sediment to form ripples and larger dunes.
In normal weather, the wave base is at a given depth, though that actual depth from place to place is variable, depending on wave amplitude, prevailing wind direction and speed, and the shape of the coastline. But during storms, there is more energy transferred from the wind to the sea surface, and so the waves are larger, and wave base is pushed deeper, where it may be able to interact with sediment that was previously “out of reach.”
Landward of wave base, the orbitals move sediment and generate bedforms, most commonly symmetric ripples where the lee and stoss faces have similar inclinations (because of the back-and-forth action of wave orbitals).
As waves approach a beach some of the energy is transferred to sediment on the sea floor, wave speed decreases (because of friction and loss of momentum),and wave amplitude increases. At a certain amplitude, waves become unstable and break breaking waves transfer the water as a turbulent, aerated mass that rushes up the beach (swash) and then recedes (backwash), and in the process stirs surface sediment across the breaker zone and beach.
Crossbedding – some common terminology
Crossbeds are nearly ubiquitous in sedimentary rocks. They can be found on the deep ocean floor, the driest desert, and pretty well any depositional environment in between. They are most common in sandy deposits. They are less common —but no less important— in gravels (e.g., low sinuosity settings such as braided rivers). Crossbeds form where air and water flow across a bed of loose sediment, so long as the individual sediment grains have low cohesion between the particles: in other words, they are not “sticky.” Mud crossbeds are rare because individual clay particles tend to stick to one another (a result of the electrical charges on the surface of these tiny grains).
A cartoon cross-section showing the different crossbed/bed relationships at the upper and lower portion of the bed. Image by Callan Bentley (2020).
Crossbeds in the rock record are visible in bed cross-sections, or as exhumed 3D ripples and dunes on exposed bedding planes. The term crossbed refers to their internal structure i.e., laminations that are usually inclined in the down-flow, or down-stream direction. Crossbeds are therefore useful in interpreting paleocurrent flow direction. Because the laminations often show a tangential relationship to the main bed at the bottom of the main bed and a truncated relationship at the top, they are also useful as geopetal (“way-up”) indicators.
Terminology for common forms of subaqueous crossbeds (Ripples, dunes). The inspiration for this diagram is from McKee and Weir, 1953
The laminae are called foresets. In a 2D cross-section view, a single crossbed consists of any number of foresets bound above and below by flat or curved boundaries. The geometrical arrangement of foresets, their bounding surfaces and their size or amplitude gives us the information needed to decipher:
- the kind of crossbed,
- the hydraulic conditions under which the crossbed formed, and to some extent
- the paleoenvironment in which they formed keep in mind that most crossbeds can be found in a range of paleoenvironments but used in conjunction with other criteria such as body and trace fossils, sediment composition and stratigraphic trends (e.g., fining upward) will help pin-point specific depositional settings.
Our interpretations can be advanced further if we are lucky enough to see exhumed structures on bedding, such that we can define:
- the shape of the ripple or dune crest line (is it straight or sinuous?),
- the wavelength between successive ripple or dunes, and
- a relatively unambiguous measure of ripple-dune migration across the bed (i.e., paleocurrents).
Most of our knowledge about ripples and dunes (collectively referred to as bedforms) and how they form has been garnered from studies of modern environments. After all, if on your walks across a tidal flat or open-air dune field you see ripples that look identical to those preserved in rocks, it is quite reasonable to infer that the ancient bedforms developed in ways similar to the modern analogues (this is the Uniformitarian Principle at work).
In fact, they have also been seen and recorded forming in real time on Mars.
A slice through modern ripples in soft river sand. Current flow indicated. Compare the example with the diagram above for details of terminology and mechanism of formation.
The hydrodynamic significance of bedforms – Flow Regime
In this section we introduce some basic hydraulics of sediment movement, bedforms and the concept of Flow Regime.
Ripples and dunes form when a fluid (usually water or air on Earth, but the same concepts apply to lava flows, crystal mushes in a magma chamber, or even the Martian atmosphere) flows across a sediment surface. Structures formed by air flow are called subaerial ripples or dunes those in water have the qualifier subaqueous. These structures are given the general name bedform. The construction of bedforms requires certain conditions:
- The sand must be cohesionless (i.e., the grains do not stick together).
- Flow across the sediment surface must overcome the forces of gravity and friction, and
- As noted above and illustrated in the Hjulström curve, there is a critical flow velocity at which grain movement will begin this also depends on the mass of individual grains, and to some extent their shape. For example, flat, platy minerals like mica are easier to move than a grain of quartz with the same volume but a chunkier shape.
Ripples and dunes form under a relatively limited range of flow conditions. We can illustrate this in a graph of flow velocity against grain size, plotting areas on the graph that correspond to bedform growth. Most of the data for plots like this are derived from experimental flumes where flow conditions can be monitored closely. The plotted distribution shows that bedforms can be categorized according to flow and sediment conditions.
A schematic of bedform transitions and surface water configurations for Lower and Upper Flow Regime sedimentation – significantly modified from Harms and Fahnstock (1965), Plate 1. Note that the antidune foresets dip upstream.
This partitioning of bedforms was used to construct the Flow Regime hydraulic model, first published in the now classic 1965 paper by J.C. Harms and R.K. Fahnestock and used widely ever since.
The Flow Regime model considers three fundamentally different states of flow:
- No bed movement where there is too little energy in the system to initiate and maintain sand grain movement,
- A Lower Flow regime in which all common bedforms develop. Here, plane bed (basically parallel, planar laminae with no ripples) represents the lowest velocity, or energy conditions where sediment movement is initiated. It has been observed in flumes and in natural channels that the size of bedforms increases from ripples to large subaqueous dunes as flow velocity increases. Dune type also changes from two dimensional structures (straight crests and planar crossbed bounding surfaces), to three dimensional structures that have sinuous, arcuate and lunate outlines and spoon or scour-shaped bounding surfaces (commonly seen as trough crossbeds).
- An Upper Flow Regime where the power of stream flow washes out ripples and dunes, replacing them with plane bed (this kind of plane bed commonly has parting lineations), plus antidunes, and erosional chutes and pools.
- As stream flow increases the transition from Lower to Upper flow regime produces one of the more interesting bedforms – antidunes. They are mostly found in shallow channels (e.g., fluvial and tidal channels). You can recognize that this transition has taken place when you see standing surface waves – watch closely and you will see the waves migrate upstream. Antidunes are the bedforms that develop immediately below standing waves (the two are in-phase). If high flow is maintained, the antidunes will also migrate upstream. However, once flow slackens they tend to wash out. This means that the preservation potential of antidunes is low.
- Hydraulic jumps: The transition from Lower to Upper flow regime passes with a change in bedform, in particular washing out of subaqueous dunes, but there is no sudden break in surface flow – the transition is reasonably smooth. This is not the case for an Upper to Lower flow regime transition that is marked by an abrupt increase in water level and turbulence: a hydraulic jump. Hydraulic jumps can be thought of as standing waves. They are caused by a reduction in Upper Flow Regime velocity, a change in stream-bed gradient or water depth, or combinations of all three.
You can generate a hydraulic jump in your kitchen or laundry sink, so long as the sink floor is reasonably flat. Turn on the tap until you see a flow pattern like the one in the photograph at left. Flat laminar flow is generated by the downward force of tap water – this is the plane bed. The hydraulic jump is located at the circular ridge where the flow changes abruptly. Beyond the jump is lower flow regime flow.
We can use the Flow Regime concept in the field as a qualitative indicator of changes in flow conditions with time (i.e., stratigraphically) and space (laterally). For example, a succession of strata that contains a bed of ripples overlain by larger trough crossbeds indicates that flow velocities, and hence stream power increased abruptly. What kind of paleoenvironment could this have occurred in? This is one of the central questions for any sedimentological analysis.
Some common sedimentary structures, their hydraulic significance, and the environments in which they typically are found, are illustrated below.
A short movie of dune-ripple formation
The link here takes you to a short movie of subaqueous dunes forming in a flume (a flume is a narrow tank containing a sand bed, where the velocity of water flow that can be controlled – they are used in experiments to observe the formation of bedforms, and for modelling engineering problems such as the flow of water around bridge foundations). Flow in this experiment is Lower Flow Regime. The sequence begins by showing water flow in front of the migrating lee face look carefully and you will notice (movie presented by Michael Calzi, SUNY Genseo Dept. of Geological Science):
- sand grains being carried along the stoss slope bed
- when grains reach the dune crest, they avalanche down the lee face – forming crossbed foresets,
- water flow downstream of the lee face appears to flow backwards – this is the backflow shown schematically in the Ripple formation diagram above. LINK TO THIS DIAGRAM
Some common sedimentary structures
Identifying sedimentary structures and deciphering how they formed are two of the most important tasks for any sedimentological study.They provide clues, in some situations the only clues, to interpreting ancient environments. The utility of sedimentary structures to unravel the past becomes even more powerful when used in conjunction with other rock properties such as fossil content and geochemistry.
Any examination of sedimentary rocks begins with descriptions of bed geometry, sediment texture, fabric, color and fossil content sedimentary structures are part of this descriptive vocabulary.Some basic descriptors that are used for outcrop, core or hand sample are listed below. Here is a JPEG version of the same table.
Bedding Thickness, Geometry (e.g. parallel bedded, lenticular, lensoidal) Weathered attributes – is lithology resistant, recessive, cliff-forming? Colour Note the colour is it uniform, variable? Texture/ Fabric Grain size – The range of sizes (sorting) Grain size – Maximum clast size (e.g. in conglomerates) Clast framework e.g. clast-supported, matrix-supported, variable Clast roundness Clast angularity Clast shape (e.g. spherical, platy) Is there preferred clast orientation (e.g. alignment, imbrication)? Is there preferred clast distribution (e.g. graded, bimodal) Sediment/rock classification: e.g. sandstone/grainstone, mudstone/lutite, conglomerate/breccia/rudite Composition Begin with the most basic classification: – is it a carbonate, siliciclastic, volcaniclastic? Main clast types: quartz, feldspar, lithics, bioclasts, ooids Cements: carbonate, silicic, iron oxides Induration: soft, hard does it ring when you hit it with a hammer, or make a dull thud? Fossils Body fossils, trace fossils, casts or moulds? Are there preferred faunal or sedimentary associations? Shells intact, broken Have shells been transported or are they in the same position when living? Relationship to bedding (e.g. infaunal, epifaunal) Degree of preservation
In this section, we present examples of:
- Crossbedding (cross stratification)
- Structures formed at the soles of beds
- Structures formed by desiccation
- Structures associated with sediment texture and fabric
- Structures formed by sediment gravity flows (mainly turbidites and debris flows)
- Structures formed by deformation and mass movement soon after deposition
- Structures formed by organisms crawling, burrowing, or grazing through sediment in search of food, a place to live, or shelter from predators (these are trace fossils) ( link to ? )
- Structures formed by chemical precipitation (evaporites, iron formations, phosphates)
For each structure, or set of structures, there is:
- An annotated image from outcrop describing the defining attributes
- Where possible an annotated image of a modern analogue
- A brief description of each structure and its formation
- A list of common depositional environments in which it may be found.
Bedding is a sedimentary structure. Sediment moved by currents, or falling from suspension through water or air, will accumulate in a layer, or bed, a process that continues until the supply of sediment is terminated. The plane that defines the top of the bed (bedding plane) represents the termination of that depositional event. Renewed deposition will see the accumulation of a new bed. A bedding plane thus represents a period of time in which no, or very little sediment was deposited – the duration of the hiatus might be minutes, weeks, or centuries.
Common descriptive elements of bedding are listed in the table above. Keep in mind that descriptive words like tabular bedding (where bedding planes are basically parallel), or lenticular (where beds are lens-shaped) depend on the scale of your observations. For example, beds that are tabular at the outcrop scale, may pinch out (as at the outer limits of a lens), or thicken in exposures farther afield. Note too, that outcrop views are commonly two-dimensional, so what appears to be a “tabular” bed may be lenticular in the third dimension (i.e., deeper into the rock beyond the outcrop face).
Sedimentary structure Primary attributes Common environments Tabular bedding can form in many environments. This example is from a turbidite succession. Beds occur in packages that are also tabular. Can form in almost any sediment type, from conglomerate to mudstone siliciclastic, carbonate, volcaniclastic. Almost any marine, lacustrine or terrestrial environment. Lenticular bedding is common in many environments. This example is from Carboniferous fluvial deposits.It can form in any sediment type, from conglomerate to mudstone siliciclastic, carbonate, volcaniclastic. Almost any marine, lacustrine or terrestrial environment. Channelized bedding downcutting the underlying lithology. This example is a fluvial channel that has eroded slightly older floodplain mudrocks. Common in coarse-grained lithologies (sandstone, conglomerate). The lower contact is erosional. Many kinds of fluvial, tidal flat, estuarine, alluvial fan, delta, and submarine fan channels.
Refer to the text and diagrams above for an outline of fundamental processes of crossbed formation, and crossbed terminology.
Sole structures are formed during deposition of sediment. The most common types are structures formed by scouring erosion of the substrate by a flowing current, by objects dragged across the sea or lake floor (e.g., bits of wood, shells, or pebbles), and by objects that bounce along the substrate. The depressions thus formed are filled by new sediment such that they are part of the basal contact of the overlying bed – i.e., they are casts of the depressions and are located on the bottom of the overlying bed. As the sole of a shoe is on its underside, sole structures are also formed on the underside of beds. They are sometimes called sole casts. Sole structures are most easily identified on exposed bedding. Flute, groove, and bounce casts are common in turbidites, but can form on shallow continental shelves and platforms, particularly during the passage of storms. Examples include gutter casts, narrow, elongate scours in the sea floor 15-20 cm deep, that form during coastal storm surges.
Sedimentary structure Primary attributes Common environments Large flute casts at the base of a turbidite bed. Current flow (indicated) determined from flutes is generally unique. The groove casts parallel the flutes but inferred flow is ambiguous. Flute casts are most commonly observed at the base of turbidites in submarine fan, submarine canyon, and prodelta deposits. Close-spaced flute casts in an Eocene shelf sandstone flow direction indicated. Flute casts are less common under normal conditions on a shelf or platform, but can form from offshore flow of storm surges. A nice collection of skip, groove and flute casts at the base of a turbidite bed. The flutes indicate flow to the right. There are also some load casts, but these formed after deposition was completed. The utility of flute casts for determining paleoflow directions is nicely illustrated here. Groove casts at the base of a sandstone (shelf environment). Arrow indicates the ambiguity of paleoflow – possible flow directions are 180°apart. Gutter casts (outlined) in Jurassic shelf sandstone probably the result of a coastal storm surge. The same bed also has some pebbles and fossil debris that were transported offshore by the surge. These structures known mainly from shelf and platform deposits, formed by bottom-hugging return flows generated by storm surges
Structures formed by desiccation
Desiccation means “drying out.” Desiccation of subaerially exposed surface sediment or soil drives off ambient moisture, reducing its volume, and resulting in shrinkage. Shrinkage cracks can extend many centimeters below the surface. Propagation of shrinkage cracks, or mud cracks, across a sediment surface commonly produce 5 and 6-sided polygons. If desiccation continues, the polygon margins will begin to curl upward. Mudcracks are common on river floodplains, the inactive parts of alluvial fans, and supratidal environments that are exposed for long periods. Desiccated sediment is prone to reworking during river flooding, and periodic king tides or storm surges across intertidal and supratidal flats.
Mud cracks preserved in the rock record are excellent indicators of subaerial exposure and may indicate:
- falling sea levels,
- changes in the location of river channels and adjacent floodplains,
- arid climate
Structures formed by sediment textures and fabrics
This category of structures is based on textural properties of sediments, such as clast shape and orientation. We often think of sedimentary grains as approximating spheres. However, clasts commonly deviate from this ideal shape – they may be blocky, flattened or plate-like, rod-like, or tapered cones (fossil groups such as gastropods commonly conform to the latter shape). During transport in flowing currents (air, water), clasts like these will tend to be aligned according to their most stable hydrodynamic orientation. Identification of preferred clast orientations provides another useful tool for measuring paleoflow directions.
Sedimentary structure Primary attributes Common environments Imbrication (preferred alignment) of flat pebbles and cobbles in a recent river channel. The flat clasts are consistently inclined upstream flow was to the right. Common in gravelly rivers, particularly braided rivers, but can occur in any channel containing flattened clasts or shells. Imbrication of oblong or platy gravel clasts can serve as an indicator of paleo-current flow direction. This example comes from Death Valley, California. A similar effect can be seen in leaves shed from deciduous trees, as they “shingle up” on on another when washed along by rainstorm currents. Current alignment of Permian fusulinids (foraminifera) on bedding. The sense of flow direction is ambiguous in this case, but might be confirmed by other structures in the rocks. Current alignment of fossils is common in channels, or over broader substrates like the sea floor where strong tidal currents are present. Bedding view of parting lineation in laminated sandstone each parting is one or two grains thick. This fabric develops under Upper Flow Regime Plane Bed conditions. Paleocurrent directions are ambiguous. Most common in fluvial or tidal channels where high flow velocities are sustained.
Structures formed by sediment gravity flows
This section contains images and brief descriptions of turbidites and debris flows.
Turbidites are mostly found in deep marine basins and lakes, but are also known to occur in shallow settings like continental shelves. They are deposited from turbulent, bottom-hugging flows of water carrying (mostly) sand and mud. Turbidity currents can travel many tens of kilometers across the sea floor. Turbidity current deposits, called “turbidites,” are the principal components of submarine fans.
The classic descriptive model of a turbidite deposit was developed by Arnold Bouma, and is appropriately called a Bouma Sequence. The sedimentary components of a typical Bouma Sequence are shown below. A Bouma sequence represents a single turbulent flow.
A complete turbidite contains intervals A through E.
A Interval: This interval contains the coarsest sandstones, and may include pebbles or chunks of mudstone that were ripped from the sea floor during passage of the turbulent current – not surprisingly these chunks are called mudstone rip-ups. Contact between the A interval and the underlying deposit is abrupt and may be scoured the scours may be filled with pebbles.
B Interval: The B interval is also sandy, but is distinguished from the A interval by two common attributes: (1) the sandstones are graded (coarser framework grains at the bottom of the interval, becoming finer towards the top), and (2) the sandstones are commonly laminated.
Schematic view of graded bedding, with coarsest clasts at the base, becoming finer toward the top of the bed. The matrix is commonly muddy (clay plus silt particles).
C Interval: Mostly fine-grained muddy sandstone typically containing sandy ripples and climbing ripples, and ripple crossbedding that has been deformed into convoluted patterns (see the examples below).
D Interval: Mostly laminated, graded, muddy siltstone. This interval represents the late stages of deposition from the turbidite.
E Interval: This interval contains the mud component of a turbidity current, most of which falls out of suspension from the cloudy plume above the main flow. It is generally mixed with background suspended sediment from the water column above it can be difficult to distinguish these two sediment components.
F Interval: represents a return to ‘normal’ background sedimentation which in many cases is a mix of pelagic (e.g., skeletal remains of micro-organisms) and suspended particles in the water column (the latter is also referred to as hemipelagic mud).
In thick turbidite successions, it is common to find individual flow units that are incomplete Bouma sequences thus one flow unit may preserve only A and B interval sands, where others present B, C, D and E, or C, D, and E intervals. Variations like these generally reflect proximity to the source of the turbidity current, as well as the availability of sediment. For example, sandy turbidites containing thick A and B intervals are more likely to be deposited in the proximal parts of submarine fans (i.e., closer to the sediment source and the head of the fan) finer-grained turbidites lacking thick A or B intervals will tend to accumulate in the more distal parts of submarine fans (i.e., further to the sediment source and closer to the outer fringes of the fan).
Debris flows are also mixtures of mud, water, and coarse debris, but unlike turbidites they lack fluid turbulence during flow. The capacity of a debris flow to carry material, including house-sized blocks, lies in the viscosity and mechanical strength of its mud matrix. Debris flows are mobile, commonly destructive phenomena. In terrestrial settings, they can evolve from landslides, aided by high precipitation or snow melt. The equivalent phenomena in volcanic terrains are called lahars – debris flows consisting almost entirely of volcanic debris – that develop both during and after eruptions.
Subaqueous debris flows are commonly generated during slope failures (submarine landslides) that are triggered by seismic events or gravitational instability. Turbidites and debris flows are often found together in the rock record.
Sedimentary structure Primary attributes Common environments Two coarse-grained, sandy turbidite flow units – the low unit has a single, graded B interval the upper unit begins with a poorly graded A interval (remainder of this unit out of the picture). Contact between the two units is planar. Thick, coarse-grained turbidites tend to accumulate on the proximal parts of submarine fans. Three turbidite flow units: A lower B-C-D-E unit with excellent examples of ripples and climbing ripples in the C interval a middle B unit, the top of which has been eroded by the upper B-C-D-E unit. Units 1 and 3 are graded. A lag of mudchips formed during scouring of the middle unit. Probably mid-submarine fan A fine-grained, muddy turbidite with well developed C and D intervals. The convoluted bedding (small fold structures) formed after deposition during the early stage of compaction Mud-dominated turbidites (lacking sand) tend to accumulate on the outer, more distal parts of submarine fans. A pebbly mudstone, or muddy debris flow, showing typical high mud content that supported the larger, denser clasts during flow – i.e., a matrix-supported texture, that indicates viscous flows (generally lacking the turbulence of a turbidite). The bed in this images dips left. Commonly associated with turbidites on submarine fans. Can be initiated by collapse of submarine slopes or landslides. Good examples of matrix-supported debris flows in the San Onofre breccia. Most clasts here are angular indicating little or no mechanical abrasion during flow. Clasts include glaucophane schist derived from subducted rocks in a nearby metamorphic terrane. Clasts in debris flows tend to be pebble through boulder size, and are extremely useful for identifying ancient sediment sources (i.e., provenance) An Early Miocene debris flow that carried megablocks – in this case, a large piece of jointed basalt. Clearly demonstrates the mechanical strength of the mud matrix and the mobility of such flows. A succession of submarine debris flows initiated on the submerged flanks of early Miocene volcanoes. Image right shows typical textural variations: very poor sorting, a range in clast shapes, and variable clast supporting characteristics ranging from matrix- to clast-supported. The general category of debris flows includes lahars – flows generated on the subaerial or submerged flanks of volcanoes, and consisting of volcanic debris.
Structures formed by mass movement (MTDs)
Mass Transport Deposits, or MTDs is the term given to sedimentary slumps and slides, mostly generated on relatively high-angle slopes between the shelf or platform margin, and deep-water settings at the base-of-slope and beyond. The term is generally reserved for sediment packages that move and deform en masse under the influence of gravity.
There is a close association between MTDs and autochthonous (undisturbed) muddy slope deposits, and turbidites in submarine fans. MTD packages commonly overlie undisturbed turbidite assemblages, and in turn are overlain or draped by non-deformed strata. The slumps, and slides themselves generally consist of deformed turbidites and related deposits.
MTDs develop via a range of emplacement mechanisms and mechanical processes most sediments will be ‘soft’, unconsolidated or only mildly so, and have high interstitial fluid contents (usually seawater). Sedimentary layers may bend and fold, or break like brittle materials. Liquefaction is also common where sediment becomes fluid-like. All these mechanisms may occur in the same structure. The deforming slump or slide package may also generate debris flows and turbidites.
Sedimentary structure Primary attributes Common environments Turbidite beds, while still relatively soft, have been folded and overturned into a recumbent anticline. This structure is part of a much larger package of deformed strata that moved down the ancient Miocene submarine slope Most common in Slope and deeper basin environments, associated with submarine fans. A smaller-scale folded and faulted sandstone bed within a succession of turbidites. Folding and sliding of soft sediment also formed small faults, and squishing of water-logged that liquefied the muddy sediment. Early Miocene, Auckland. Deformation of soft sediment can involve brittle failure (small faults), plastic-like folding, and flow from liquefied sediment The slump package, or MTD, (Miocene, Auckland) is separated from undeformed turbidites by a relatively smooth glide plane (a plane of movement). The MTD is bound on the right by a fault – the fault terminates at the glide plane. The fault formed during the slumping and deformation event. MTDs commonly overlie undeformed strata, separated by a planar surface that is also undeformed. Large-scale folds and faults in Pliocene turbidites, Ridge Basin, California. Some of this deformation may have been triggered by seismic activity on the ancestral San Andreas fault. Slumping and sliding can be triggered by oversteepening of the sea floor as sediment accumulates (gravitational), by seismic events, or large storm surges. Small-scale folds and breakage of thin beds of carbonate mudstone (lutite). This example formed in lime mudstones on a slope adjacent to a Paleoproterozoic carbonate platform (Belcher Islands). Sedimentary slumps and slides can occur in siliciclastic, carbonate, and volcaniclastic deposits
Structures formed by sediment compaction and dewatering
The term syn-sedimentary deformation tends to be used rather loosely, as deformation that takes place during or soon after deposition the ‘soon’ is the loose part of this broad definition. Sediment begins to compact almost immediately following deposition, where framework grains move closer together. As compaction progresses, interstitial water (i.e. the water between grains at the time of deposition) is expelled, and this process in itself can deform the sediment. Water that is expelled from sediment can escape to the surface (e.g. the sea or lake floor), or it can be prevented from flowing by low permeability layers – particularly those containing mud. When this happens, local pore pressures increase, and this, in turn, reduces sediment shear strength, promoting deformation.
Note that this kind of deformation is an integral part of the Mass Transport Deposits pictured above, but it also takes place in beds where there is no wholesale slumping or sliding. Soft sediment deformation is common in conditions of rapid deposition where interstitial water is trapped, as in turbidity currents and sandy fluvial channels.
Some of the more common structures illustrated here include:
Load structures (also called load casts, or ball-and-pillow structures): Common in newly deposited beds where a higher density layer, like sand, overlies a lower density mud. Differential loading cause bulbous shaped sand bodies to project into the mud. Laminae within the load cast are also deformed. Load casts may detach completely and appear to float in the mud layer.
Flame structures: Closely spaced load casts will force relatively fluid mud upwards, forming tapered, flame-like mud wisps that protrude into the overlying sand.
Dewatering pillars and sheets: During compaction, interstitial water may escape along preferred pathways these escape route may be tube-like, or form as vertical sheets. Dewatering pillars and sheets that extend to the sediment surface can carry suspended muddy sediment that will accumulate around the vent as mud blisters or small mud volcanoes.
Dish structures: Dewatering of saturated sands can truncate primary laminae, forming concave-upward, or dish-shaped structures.
Sedimentary structure Primary attributes Common environments Detached load casts of volcanic ash appear to float in blue-grey mudstone. Laminae within the casts are intricately folded in the upper layer (lens cap). Load casts probably formed at about the same time as the dewatering pillars. Most of the primary bedding here was deformed soon after deposition. Paleoproterozoic, Belcher Islands. Common in turbidites and interbedded lithologies where there are density and permeability contrasts. They indicate instability between the two layers during early compaction. Numerous load casts at the base of the top-most sandstone (coin for scale) have protruded into the underlying dark grey mudstone. Wispy mudstone flame structures are sandwiched between the casts. Paleoproterozoic, Belcher Islands. Flame structures are commonly associated with load casts. Bed cross-section view of dewatering pillars in the laminated (Bouma) B interval of a sandy turbidite. The pillars are white because fine matrix was removed during water expulsion. Paleoproterozoic, Belcher Islands. Indicative of rapid deposition of water saturated sediment. Sheets are layered because of permeability contrasts in the sandstone. Bedding exposure of small blisters, or mud volcanoes at the exits of dewatering pillars. Paleoproterozoic, Belcher Islands. Dish structures in Rosario Group sandstone, San Diego. The dewatering pillars occur between adjacent dish-shaped laminae. Indicative of rapid deposition of water saturated sediment.
The tracks of a booted biped meandering across a tidal mud flat. Low mangroves in the background, and Auckland city beyond.
Trace fossils have the privilege of being two things at once: sedimentary structures, and fossils. They occur in sediment, are made of sediment, but represent the activities of creeping, crawling or burrowing critters, mostly at or immediately below the sediment water interface (marine, lacustrine, estuarine, swamp), or subaerial environments such as dune fields. As such, trace fossils represent the range of activities that critters are normally occupied with – grazing or foraging for food, home construction and house-keeping, predating or escaping predators, wandering aimlessly, or taking a nap after an exhausting day. Some critters like to rough it, preferring the tumble of waves or strong currents, while others like the peace and quiet of deeper realms. Lives are frequently interrupted by storms or violent, turbulent flows of sand and mud their traces, or lack of them, also reflect these events.
Most animals produce more than one kind of trace depending on what they are doing, which means that in most cases, traces reflect animal activity and biometrics, rather than the specific critter species. Most traces do not contain any remnants of the animal that made them (there are a few exceptions) finding a trilobite body fossil at the end of its scampering trail is pretty rare.
Trace fossils provide valuable information on benthic communities, environmental conditions such as wave or current energy, redox conditions (presence or absence of oxygen), rates of sedimentation, or periods of time when sedimentation slowed (e.g. hiatuses, disconformities, omission surfaces).
Intense bioturbation can also obliterate other kinds of sedimentary structures for a geologist, this may be an annoyance or a happy circumstance. Most Precambrian successions are free of trace fossils and bioturbation this changed during the Ediacaran, the period that appears to have been a kind of precursor to the Cambrian invertebrate explosion. Most Phanerozoic sedimentary successions (since 540 million years ago) have enjoyed the munching-burrowing efforts of a myriad nameless critters.
The average continental shelf is about 45 mi (75 km) wide. Shelf sediments generally decrease in grain size with increasing distance from shore. This occurs for two reasons: (1) greater distance from sediment sources and (2) decreasing sediment movement (transport) with increasing water depth.
Shelf sediments vary significantly with latitude. At high latitudes, glacial ice flowing into coastal water generates icebergs, which transport large sediment loads of various sizes out onto the shelf. As icebergs melt, they drop their load. These glaciomarine sediments are generally less sorted and coarser grained than lower latitude deposits. In fact, boulders known as dropstones occur on the sea floor in deep water, hundreds of miles from shore.
Rivers deliver most of the sediments to mid-latitude shelves. Therefore, grain size routinely decreases with distance from shore sediment sorting also tends to be good. Shallow water, nearshore sediments form thick sand blankets with abundant ripple marks. As depth increases and water movement decreases, average grain size decreases, and sand, silt, and clay occur interbedded. In water depths greater than 150 to 200 ft (45-60 m), even storm waves do not stir the bottom consequently, silts and clays predominate. Scattered sand deposits are also located on outer shelf margins. During periods of lower sea level, rivers flowing across what is now the inner shelf deposited these so-called relict sediments.
At low latitudes, bottom-dwelling plants and animals secrete large volumes of calcium carbonate, producing thick blankets of carbonate sediment. Perhaps the best known carbonate environment is the coral reef. Corals produce a rigid framework of carbonate rock (limestone), which is also a major source of sediment of various grain sizes. Where stream input is great, terrigenous sediments discourage habitation by carbonate-producing organisms and dilute any carbonate sediment that is produced.
The School of Geosciences has a growing group of faculty and students studying a broad range of environmental geoscience questions using a wide array of tools. Research in the environmental geosciences includes applications of hydrology, environmental geochemistry, biogeochemistry, hydro geophysics, and natural hazards to improve our understanding of geological processes and human perturbations that occur in the critical zone which extends from the top of the bedrock that lies beneath of feet, through the soil, and up to the top of the biosphere (cityscapes and tree canopy). Current research focuses on water quality impacts as a result of both human and natural processes, fate and transport of metals, near-surface geophysics applications in hydrology, and water-mineral interactions in groundwater. Environmental geosciences research provides a wide breadth of field and laboratory experiences from the undergraduate to graduate levels.
Below are some of the specific sub-disciplines we are focused on. Please visit the linked faculty pages for specific projects and contact them for more information.
The San Luis Valley, located in southern Colorado, geological past includes a lake that once encompassed the entire valley floor, the largest land volcanic caldera in the western San Juan Mountain Range, and the up-trust mountains to the east, the Sangre de Cristo Mountain Range, with continues to grow the length of a pencil width every year and includes Mt. Blanca, one of Colorado’s 14teener. Mining discoveries during the early settlements met with gold, silver, and turquoise, among other minerals.
This rich geological history makes for the best location to study the geosciences. Adams State University, located in the largest community in the SLV, Alamosa, will prepare students for future careers in the industry. Facilities include a soils research laboratory, excellent teaching faculty, small classes, opportunities for world travel, and Advanced Geographic Information Systems (GIS) laboratory, and opportunities for professional software training. GIS and remote sensing technology is incorporated into course work.
Sedimentary Environments - Geosciences
Page and figure numbers refer to the textbook: Stanley, Earth System History
Understanding modern environments of deposition allows geologists to understand the environments in which ancient sedimentary rocks were deposited and thereby help us recreate past conditions on the Earth.
glacial deposits (p. 126-128)
Glaciers are flowing streams of ice. They may be huge continental ice sheets or small alpine (mountain) glaciers . All glaciers scrape up sediments and incorporate them into the base of the ice sheet. Sand, gravel, and large boulders polish and gouge the surface of the bedrock that they are dragged over leaving glacial striations (Fig 5-4) . Glaciers do not sort sediments as flowing water and wind do. Poorly sorted glacial sediments are known as till . Large boulders often lie in a matrix of sand and silt (matrix-supported conglomerate) (Fig 5-5) . At the end of a glacier, where ice is melting as fast as it is being supplied from upstream, the sediments are deposited in a terminal moraine , a ridge of poorly-sorted glacial till. Thinner depostits of glacial sediments called a ground moraine or till plain are found behind the terminal moraine. Sorted sediments carried by networks of braided streams out from the terminal moraine form an outwash plain .
deserts and aeolian (wind) deposition (p.128-131)
In the desert belts centered around 10 to 20 degrees north and south of the equator there is very little rainfall. Because of this there is only sparse vegetation. The soil is exposed. The soil is also dry because of lack of rain so the particles have no cohesion as it does when moist. Here, the principle medium for sedimentary transport is not streams, it is the wind. Because the soil and sediments are not protected by a covering of vegetation or held together by roots and cohesion, the wind is free to pick up and carry sediments.
Fine particles, clay and silt are picked up as windblown dust (analogous to the suspended load in stream transport). The dust will eventually settle in an area adjacent to the desert in a more humid area with sufficient vegetation to protect sediments from further wind transport. The resulting deposits are called loess.
sand dunes (Fig. 5-12)
Sand is transported by a means called saltation. The sand grains tumble and bounce along the desert floor close to the ground or perhaps as high is several feet in a strong wind. Where they encounter an obstacle they may settle behind it, protected from the wind. Sand may build up here eventually forming a dune. The sand is blown up a gentle slope facing the wind and is deposited on a steep slope opposite to the wind. Over time layers of sand dune deposits may be preserved as large scale cross-bedded sandstone .
Desert sands are typically well sorted and rounded. The sand grains appear frosted under a micoscope because of constant collisions with other grains during wind transport.
alluvial fans (p. 129-130, Fig. 5-9)
Where a steep mountain stream flows out into a valley the reduction in gradient and stream velocity causes the stream to deposit its coarser sediments. A pile of coarse sediments (sand and gravel) builds up at the base of the mountains. The sediments are piled in a semicircular, fan-shaped body that is tallest at the base of the mountains. These coarse sediments when lithified are preserved as conglomerates. Stream deposited conglomerates are typically clast-supported (the large clasts lie against one another, and finer sediments fill in the voids between them) in contrast to typically matrix-supported glacial deposits. An apron of overlapping alluvial fans deposited adjacent to a tectonic escarpment or uplift is often called a "fanglomerate."
fluvio-lacustrine (stream and lake) deposition (Figs. 5-13, 5-15, 5-16)
In humid climates on the continents, streams are the primary medium for transport and deposition of sediments. Streams carry sediments from uplifted (mountains) source areas, eventually to the sea. Lowlands are often sites of stream deposition. Meandering streams deposit point bar sands that may be preserved as a sheet of sandstone as a result of the migration of the point bar across the stream valley. Occasional flooding carries suspended silt and clay out of the stream channel and onto the flood plain. As the flood waters recede, the fine sediments are deposited in sheets in the backswamp area. These so-called overbank deposits may be preserved as layers of shale. The sandstones and shales formed in a stream valley will contain terrestrial and fresh water fossils, not marine fossils. The overbank shales often contain mudcracks that form when the floods recede and the clay dries out (Fig. 5-10) and raindrop impressions . Assymetric or current ripple marks also indicate deposition in a stream. Lakes normally have muddy bottoms and perhaps a narrow shoreline of sand and gravel. Shales with fossils of fresh-water organisms are commonly formed in lakes.
Sediments that reach the ocean may be deposited in a delta (Figs. 5-17, 5-18, 5-19) , which is in many ways like an underwater alluvial fan. Sediments are distributed in a fan-shaped body that grows outward (seaward) with time. Longshore currents will transport sediments along the coast.
All along a coast, sediments derived from longshore drift and sediments formed in place from wave action are distributed by wave energy. Wave action is strongest at the ocean surface and decreases with depth in the water down to a depth of half the wavelength (L/2). Because of this, in shallow water near the shore the fine sediments are washed away as suspended load. Only coarse sediments are deposited in shallow water. As the depth to the bottom increases, the bottom is stirred less and less by wave action progressively finer sediments can be deposited in increasingly deeper water. Deposited sediments progress from sands near the shore to silts and clays farther offshore. In cool or turbid (murky) water, fine sediments will dominate to the edge of the continental shelf.
In warm tropical waters, if most of the fine sediments have already been deposited, coral reefs will grow in shallow water on the continental shelf (p. 140-145) . Modern coral reefs do not form in the deep ocean abyss where there is no light because symbiotic algae that lives in the coral needs light to grow. Coral reefs also do not grow in turbid nearshore waters where terrestrial sediments have not yet been deposited.
In a sedimentary sequence, alternating sandstone, shale, and limestone generally indicates a marine environment. Almost all limestone is deposited in the ocean. The sandstones and shale would contain fossils of marine organisms. The shales would almost certainly have no mudcracks.
off the continental shelf (Figs. 5-33, 5-34)
The continental shelf is the shallow ocean surrounding the continent. The depth at the edge of the shelf is usually not more than 100 to 150 meters (the length of one to one-and-a-half football fields). Some sand and mud are carried to the edge of the continental shelf via submarine canyons which are like undersea river valleys. Sediments build up at the edge of the shelf and when too much has accumulated these flow down the continental slope and rise as turbidity currents (like underwater mud flows). The resulting deposits, called turbidites , contain some chaotic, poorly sorted coarse layers at their base and then finer layers on top. Repeated sequences of turbidites indicate deposition on the continental slope and continental rise.
deep abyssal plains (Fig. 5-35)
In the deep sea, out on the abyssal plains, the depth to the seafloor varies from about 2.5 to 6 km (2500 to 6000 meters) or more below sea level. The abyssal plains receive very little sediment from the continents. Pelagic clays from windblown dust from the continents and oceanic volcanoes form finely laminated (layered) shales. Biogenic oozes: Calcareous oozes from deposits of single-cell, microscopic organisms with calcite shells result in finely laminated limestone. Siliceous oozes from single-cell, microscopic organisms with silica shells form finely laminated chert (silica) layers. Furthermore, the limestones indicate warm water limestone dissolves in cold water. Chert indicates high biological productivity and cool water.
Morphological plasticity and survival thresholds of mangrove plants growing in active sedimentary environments
Alejandra G. Vovides , . Thorsten Balke , in Dynamic Sedimentary Environments of Mangrove Coasts , 2021
Intertidal sedimentary environments are highly dynamic due to frequent sediment redistribution, tidal flooding, and wind exposure, yet mangrove plants are able to colonize and shape (sub) tropical coastlines. Morphological plasticity, the ability of organisms to modify their anatomical traits independent of their genotype in response to environment variations, allows mangrove plants to adjust to such dynamic environments. First, mangrove establishment on bare tidal flats is largely dependent on pioneer traits and disturbance-free periods (Windows of Opportunity). As soon as the first seedlings root, morphological plasticity, as a response to abiotic forcing, determines plant survival (e.g., through differential allocation of biomass). This unidirectional control on vegetation survival shifts to a bidirectional feedback system between abiotic processes and vegetation growth, once mangrove cover surpasses a critical density threshold to attenuate hydrodynamic energy. Fully grown mangrove trees, however, still show morphological plasticity, resulting from both tree-to-tree interactions and the abiotic environment. While crown displacement toward light available spaces is a fundamental adaptive plastic trait that reduces competition, it is also influenced by wind forcing, and as growth is limited by higher salinities, with greater salt stress, tree crowns also become more susceptible to wind dynamic loading. The mechanical imbalance caused by crown displacement might increase the risk of felling but might also be compensated by plastic responses of the root systems, a topic largely unexplored within mangroves. In this chapter, we aim to provide an overview of the current understanding of plastic traits in mangrove trees, with a focus on biophysical interactions in dynamic sedimentary environments, discussing ecological implications and current gaps of knowledge that are detrimental to achieve successful adaptive management and conservation strategies.
Geology on Mars: Using stratigraphic columns to tell the story of Gale Crater
Stratigraphic columns are a basic tool in geology, used for everything from understanding the geologic history of a location to determining relationships between different features.
But what are stratigraphic columns really?
Stratigraphic columns are a sketch representing a sequence of actual geologic layers. Most commonly, stratigraphic columns are used for sedimentary rocks. Unless disturbed by tectonic activity, sedimentary rocks generally form on an essentially flat surface—such as from material deposited by wind on the surface of a planet, or stuff that has fallen out of suspension in water from a lake or ocean. This means that sedimentary layers typically form a sequence such that the bottommost layer is the oldest and the topmost layer is the youngest.
Each layer in the stratigraphic column is designated by symbols showing the specific (usually) sedimentary rock. Sedimentary rocks are classified by their origins: whether they are formed by physical processes (clastic) or by chemical and biological processes. Clastic sedimentary rocks are classified by the size of the grains that form them. This classification gets very complex for sedimentologists, but for our purposes we will use the chart below. For each particle size category, there are corresponding sedimentary rocks. Chemical and biological sedimentary rocks are rocks formed by chemical or biological processes.
Types of sedimentary rocks Image: Modified from Azcolvin429 via Wikimedia (CC-BY-SA-3.0 license)
Below is an example of a stratigraphic column from the Grand Canyon, showing how the column relates to the actual rock layers exposed in the canyon wall. Each of these rocks has its own symbol in the stratigraphic column as shown in the figure. Each sedimentary rock corresponds to a different environment in which it was deposited. These "depositional environments" can give us some idea of the past environmental conditions (climate, etc.). Other features within the rock layers can give us other clues as to the geologic history of the area, such as tectonic events.
Stratigraphic column of the Grand Canyon Image: <a href="http://www.pri.org">Paleontological Research Institute</a>
Stratigraphic columns give us a lot information about the geology of the area they represent: First, the symbols show us what kind of rock makes up the layer. Second, the darker divisions show us different geologic formations, usually distinguished by age of the rock and depositional environment. In the example shown, the Kaibab Limestone is a formation composed almost entirely of one rock type: limestone. The Toroweap Formation is composed of limestone, sandstone and mudstone. Finally, in this example, the width of the each rectangle representing a geologic formation relative to the other formations indicates average relative grain size. In the example shown, the Coconino Sandstone has a larger average grain size that either of the other two formations.
Stratigraphic sections may also contain features called "unconformities." The three kinds of unconformities are shown below. Angular unconformities are when a tectonic event (such as plates colliding to form mountains) tilts the older layers and then younger layers are deposited horizontally on top of the tilted layers. Disconformities occur any time there was erosion or no deposition, resulting in an indication that there is some period of time not recorded in the sedimentary rocks as they are preserved today. Nonconformities are when igneous rocks or metamorphic rocks are overlain by sedimentary rocks, indicating a passage of time not recorded in the sedimentary rock record.
So what does this have to do with Mars? Well, like Earth, Mars has sedimentary rocks, deposited in layers. When they are exposed in a sequence, we can construct stratigraphic columns to help us understand the geologic history and past environments.
In a recent paper on sandstones within the Gale Crater, Thompson et al., (2016) constructed a stratigraphic column for all sedimentary rocks encountered within Gale Crater to sol 1200 of the Curiosity rover's mission. The stratigraphic columns in the figure below show not only the sequence of sedimentary deposits by elevation, but also the width of the individual rectangles in the column indicate the relative grain size.
Stratigraphy in Gale Crater Image: <a href="http://onlinelibrary.wiley.com/doi/10.1002/2016JE005055/abstract">From Thompson et al. (2016)</a>
The stratigraphy at Kimberley is expanded to show a layer of material called "breccia-conglomerate" at the base of that sequence, and mostly sandstones with various sedimentary structures. These sedimentary structures such as cross-stratification indicate the level of energy in the depositional environment. In the case of cross stratification, this indicates a high energy (fast moving) environment.
Watch the video: Sedimentary environments video 2 0