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Beds and bedding planes

Definition of bedding

Beds are sedimentary layers. They usually have observable boundaries top and bottom, referred to as bedding planes. These bounding surfaces are either abrupt where the bedding plane is well defined (you can put your finger on it), or gradational where the compositional or textural change from one bed to the next occurs over some thickness of sediment. Bedding planes demark changes in sediment texture, structure, and/or composition that signify a change in the depositional conditions. The upper bedding plane, if preserved intact, represents a depositional surface – a sediment-air or sediment-water interface. Beds form in all types of sediment: carbonate, siliciclastic, volcaniclastic, chemical. Deposition takes place within a broad spectrum of environmental conditions.

Original orientation

Nicholas Steno (1669) introduced the concept of ‘original horizontality’ where the deposition of sediment at its inception (and therefore bed formation) is approximately horizontal. In reality, the original orientation of a bed will be determined by depositional slope, or paleoslope. This orientation may change during burial compaction, disruption and displacement during soft-sediment deformation, or later tectonism.

Measurable quantities of beds

Thickness: Bed thickness is measured between and at right angles to bedding planes. Thickness can vary from millimetres to many 10s of metres depending on the depositional conditions, such as he continuity of sediment supply. For example, slow deposition from suspension in a lake or deep sea can produce millimetre thick laminae, whereas deposition from a debris flow or pyroclastic density current may be metres thick.

Bed geometry: This is usually identified by the 2D and 3D shape of the bedding planes. The scale of observation, particularly in terms of lateral extent, is not codified but is commonly taken to be at least at outcrop scale. As a general rule, bedding is most easily identified at distance from an outcrop – the closer you get, the more complicated it becomes; the point bar deposits shown below illustrate this problem. Common geometric forms include:

Parallel bedding where bedding planes are parallel at outcrop scale and beyond. Laterally extensive parallel bedding can often be observed in cliff and mountain side exposures. Classic examples occur in flysch-turbidite successions where parallel beds are stacked 100s of metres thick.

Well-developed parallel bedding in an Early Miocene turbidite-debris flow succession. There is a huge range of bed thicknesses here, from 1-2 cm to about 300 cm. Individual beds can be trace laterally for a few hundred metres. Goat Island Marine Reserve, New Zealand.

Wedge-shaped bedding where bedding planes are not parallel and meet at a pinch out. Theoretically, all beds are probably wedge shaped. At a local scale, examples of this type of bedding include sandstone wedges on a fluvial point bar, and gravel bars in flood-dominated channels on the active portion of an alluvial fan.
Scour shaped bedding: This type is analogous to wedge-shaped beds, but the lower bounding surface is concave upwards. Examples of this type are commonly attributed to channels and channel-forming processes.

Lateral accretion in this Carboniferous point bar is characterised by discontinuous wedge-shaped sand beds interleaved with siltstone-mudstone layers. From a distance, (left) the inclined lateral accretion beds look quasi-continuous, but their complexity becomes apparent on closer inspection (right). Kentucky, Highway I64.

An amalgamation of sandstone beds filling a fluvial channel – the lowest bed has a distinctive concave-upward bedding plane. Dunvegan Formation (Cretaceous), Alberta.

Internal organization:

Massive bedding – relatively homogenous and structureless throughout.
Graded bedding – a change in grain size from bottom to top (e.g., normal, reverse).
Crossbedded – a variety of bedforms are possible depending on sediment grade and current velocity.
Event beds: Within any succession, there may be beds that stand out because of an abrupt change in thickness, geometry, or composition – they signify a unique event. For example, a succession of bedded sandstone may be interrupted by a bed containing large mud rip-up clasts, signifying an unusual event such as a storm deposit, or beds that record slumping and soft-sediment deformation, or clasts of different composition that may indicate a change in sediment source (provenance).
Marker beds: A bit like event beds except they can be traced over large distances across a sedimentary basin. Common examples are volcanic ash beds that represent single eruptions. Minerals in the ash are also potentially useful for radiometric dating the event. Beds like these are important because they approximate chronostratigraphic surfaces and can be used to correlate widely distributed successions.
Crystal size grading: This applies to chemical sediments. Notable examples include bottom-precipitated evaporite minerals like gypsum and halite, and bedded chert.
The internal organization of all bed types can be modified by bioturbation, compaction, and post-depositional soft-sediment deformation.

A very distinctive event bed in the Lower Miocene Waitemata Basin, Auckland, consisting of slumped, pulled-apart, folded, and partially liquified sandy turbidite beds. The basal contact is an undeformed glide-plane – a surface over which the entire mass transport deposit moved. The upper bedding plane is irregular, reflecting the relief on top of the slump package, and over which the next sediment gravity flow was deposited.

Thin wavy, undulating and discontinuous beds of gypsum that precipitated at the interface between a salt lake (salar) floor and the overlying brine. Each bed is about 20 mm thick. The gypsum crystals grew vertically from the salar floor. Chilean Altiplano. Probably Late Pleistocene.

Bedding plane geometry: How a bedding plane is described depends on the resolution of our observations. From a distance, a bedding plane may appear relatively flat or featureless. On closer inspection of the same plane, we might observe undulations that result from large-scale variations in thickness, for example the upper surface of dune bedforms, or large clasts that protrude into the overlying bed; in both cases the departures from ‘flatness’ are produced during the underlying event. However, in many depositional settings, bedding planes are scoured – in this case the scouring is usually associated with the succeeding event. Indeed, erosion and scouring can remove entire beds. Bedding plane irregularities can also result from post-depositional compaction.

The channel-like, lower bounding surface of a crossbedded fluvial sandstone bed has eroded the underlying shelf deposits, in places removing several beds. The fluvial sandstone was deposited during a sea level lowstand when terrestrial drainage extended across the exposed shelf. Jurassic Bowser Basin, northern British Columbia.

Andesite boulders up to 40 cm across protrude through the upper bedding plane of a lahar where they are draped by later, airfall ash and lapilli (white to red-brown layered beds near the top of the exposure). Pliocene Karioi volcano, Raglan, New Zealand.

The significance of bedding planes

Chronostratigraphic significance of a bed: Every bed represents a period of mechanical or chemical deposition; they are depositional events. The duration of an event can be measured in seconds through millennia. Except under controlled experimental conditions (e.g., flumes), we do not know the duration of these events. We can attempt to find an average duration for a succession, by dividing the number of events (a bed count) by the total time (assuming we can measure the total time represented by the succession), but even this method is woefully inadequate because of…
Bedding planes as hiatal surfaces: Bedding planes represent the cessation of depositional events. What is unknown is the length of time between the end of one event and the beginning of the next event. A couple of examples: A bed deposited during a river flood is overlain abruptly by a second, similar bed. Was the second bed deposited during a different period of river flooding, or does it represent a very short- duration shift in depositional locus during the same event (maybe the channel axis shifted laterally, or perhaps there was a surge in flow)? In comparison, turbidity currents leave a well-defined and identifiable depositional record, such as the Bouma sequence. Deposition of coarse-grained intervals (A and B) is probably rapid (minutes, hours, days) but the finer-grained parts of the depositional event may take years to complete. The hiatus between this event and the next could well be measured in 100s or 1000s of years.
Dip and strike: Any description of beds requires us to position them geographically (e.g., latitude-longitude, UTM grids) and to orient them in 3D space. Dip and strike provide unique measures of bed attitude.

A tilted bedding plane covered with current ripples. The strike, true dip, and apparent dips of are indicated. Paleocene, Ellesmere Island, Arctic Canada.

Things that are not beds

Metamorphic layering: Original bedding can be preserved in low grade metasedimentary and metavolcanic rocks (e.g. subgreenschist to low greenschist grade). High grade rocks (upper greenschist, amphibolite) commonly present compositional layering and through-going foliation that result from alignment of recrystallized sheet silicates like muscovite and biotite. In most cases, original sedimentary bedding has been obliterated.
Igneous dykes (dikes) and sills: Both represent intrusion of igneous melts into an existing pile of rock. They are not beds.
Sedimentary dykes: These too are intrusive bodies (of fluid sediment) that are insinuated into an existing pile of sedimentary beds.
Stratiform iron pans: Bands of nodular limonite and goethite commonly precipitate within existing beds of porous sediment, in response to groundwater infiltration and watertable fluctuations. The iron bands commonly mimic bedding because of the permeability advantage, but can also cross-cut bedding.

Discontinuous limonite iron pans superficially mimic the left-dipping bedding in these Pleistocene sand dune deposits, but the pans also crosscut the dune bedding (arrows). The iron pans post-date dune deposition and formed as subsurface precipitates of iron oxide during older, fluctuating watertables. Kariotahi, New Zealand.

Stereographic projection – the basics

Plotting the orientation of a plane on a stereonet; finding strike and dip from two apparent dips

This post is part of the How To… series

My first exposure to stereographic projection methods of structural analysis was one of my more baffling moments of undergraduate geological learning. Looking around the class I could see I wasn’t the only one. Our tutor, a master at pointing out the obvious, suggested we try thinking in three dimensions. In retrospect it was probably some of the best advice I ever received. After all, geology is nothing if not an exercise in visualizing the 3- and 4-dimensional world around us.

Visualize a sphere; cut the top half off – we don’t need it. If you look directly down the vertical axis of the hemisphere it will appear to be two-dimensional. When you do this, you are basically projecting the 3-D hemisphere onto a 2-D circle. Once this projection has become embedded in your mind’s eye, everything else will fall into place.

The brief animation below shows a hemisphere rotated from an initial oblique view to the point where you are looking directly down the axis (or radius). At this point the hemisphere is projected as a circle that can then be overlaid by a stereonet grid like the example shown above. Each point and curve on the grid represents a point and curve on the hemisphere.

Stereographic projection is all about representing planes (e.g. bedding, foliation, faults, crystal faces) and lines (e.g. dip and plunge directions, fold axes, lineations) onto the 2-D circle. In geology, we overlay the 2-D projection with a grid of meridians, or great circles (analogous to longitudes), and parallels or small circles (analogous to latitudes). Thus, all compass points are represented. The net-like grid is called a Wulff Net, or stereonet. It is an equal angle grid where meridians and parallels intersect at right angles.

Note the two extreme projection conditions: the circumference is actually the great circle of a horizontal plane; the opposite is a straight line passing through the centre that is the projection of a vertically dipping plane. Thus, shallowing dips tend toward the circumference.

The following animation shows how the great circle changes as the plane rotates from vertical to horizontal dip (in increments of 15°). The great circle corresponding to the dip is in red. The line drawn from the two points on the circumference through the centre is the strike.

In practice we use a transparent overlay pinned to the centre of the stereonet; we rotate the overlay according to the orientation of lines and planes.

Plotting a dipping plane

Follow the next exercise with the short animation. It was made from still images: use the pause and play buttons as you work through the exercise.

Imagine a plane striking 060^o (N60E), dipping 60^o SE; the plane passes through the centre of the sphere. In our vertical 2-D view the intersection of the plane with the hemisphere appears as a curved line, or great circle. Rotate the overlay counter-clockwise 60^o; mark this point on the circumference. The great circle representing the angle of dip is found by counting along a line at right angles to the strike, i.e. along the E-W axis, counting 60^o from the horizontal (the circumference).

Note that in the 2-D view, the great circle meets the circumference at 060^o and 240^o that are two points on a horizontal line across the plane; the great circle is therefore the strike of the plane.

If a plane projects onto the stereonet as a great circle, then a line or lineation on the plane will plot as a point on the same great circle. To illustrate, we will find the true dip and strike of a plane for which we have measured two apparent dips: (1) 30^o at 315^o (N45W) and (2) 32^o at 014^o (N14E). On a transparent overlay, mark the positions of the N-S and E-W axes, and the stereonet centre. For (1), rotate the overlay 45^o clockwise, mark the position at the N pole, then count 30^o from N along the N-S axis. This point is the projection of apparent dip (1). Likewise, repeat for apparent dip (2). Rotate the overlay until the two apparent dip projections lie on the same great circle – read the bearing from N and the true dip along the E-W axis.

Follow this exercise with the short animation. It was made from still images: use the pause and play buttons as you work through the exercise.

Stereographic projection is a powerful method, not just to solve relatively simple (but important) problems of dip and strike, but as an analytical tool for more complex structural geology. There are several good software programs and Apps to automate projections for large data sets. But before you dive into these digital tools, try the simple overlay first – there’s nothing like a hands-on approach to help galvanise an understanding of basic methodology.

Some other useful posts in this series:

Measuring dip and strike

Solving the three-point problem

Stereographic projection – unfolding folds

Stereographic projection – poles to planes

The Rule of Vs in geological mapping

Folded rock; some terminology

Here are some good website links:

Visible Geology

Rick Allmendinger’s Stereo 10 – there’s also a mobile App version

Innstereo (open source)

Links to several programs in The Structural Geology Page

Solving the three-point problem

Measuring dip and strike

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How to measure dip and strike

This post is part of the How To… series

If we had to designate one set of measurements that is fundamental to all geology, it would have to be Dip and Strike. These simple measures define uniquely the orientation (compass bearings and angles) of a planar surface – any plane: bedding, faults, axial planes, mineralized veins, dykes and sills. Armed with dips and strikes, a geologist can project planes and the rocks they encompass across valleys, through mountains and deep beneath the Earth’s surface. They are fundamental to deciphering Earth structures.

Strike: The compass bearing of an imagined horizontal line across a plane. If the plane is flat there is an infinite number of strike lines, all having the same dip (zero) but different bearings. If the plane is curved (e.g. a plunging fold) the bearing may change systematically over the fold.

Azimuth, or compass bearing is recorded as either (for example) 035^o or N35E, or its counterpart 215^o and S35W.

Dip: Dip is the angle of inclination measured from a horizontal line at right angles to strike. The angle is measured by placing a compass on the line of dip and rotating the inclinometer to the point where a spirit level indicates horizontal. The direction of dip need not be measured (it can be calculated directly from the strike bearing), but an approximate direction should always be recorded to avoid ambiguity, as in 48^oNW.

The inclination measured at right angles to strike is the true dip. Inclinations measured at other angles on the plane will always be less than true dip – these are called apparent dips.

The animation below was made from still images: use the pause and play buttons as you work through the exercise.

Dip and strike indicate the orientation of a plane at a specific location. Dips and strikes of folded strata will tend to show systematic changes at different locations. In the example below the fold axis is horizontal and axial plane vertical. Strikes at any point on the fold limbs will all have the same azimuth, but dips will change progressively from one limb to the other. Dips and strikes recorded on geological maps can be used to reconstruct the 2- and 3-dimensional structure of deformed strata.