Some basic ideas.

Primary structures

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Beds in Sicily.In order to understand deformed rocks we need to have some idea of their organisation - their architecture - before they were deformed. We can start on a grain scale.

Many rocks show layering. This might be defined by systematic variations in grain size. One layer could have predominantly large grains while other layers might be dominated by smaller grains. Another type of layering might result from systematic variations in the distribution of the composition (mineralogy) of grains. This type of organisation has a general term: LOCATION FABRIC.

BeddingIn sedimentary rocks the systematic development of location fabric is known as BEDDING. For example, a sandstone may display bedding that is defined by systematic variations in grain size through a section. Graded bedsIn a rock sequence bedding might also be defined by alternations of layers that are predominantly composed of grains of the mineral calcite - in which case the rock layer would be called a limestone, and layers composed of clay minerals - forming a layer of clay or mudstone. The bedding then would be an example of a location fabric defined by variations in composition.

Dipping beds.Location fabric can be displayed by igneous rocks - especially those that crystallised gradually on the sides of magma chambers. In these situations subtle variations in the chemistry or physical conditions in the magma chamber can cause particular minerals to crystallise from the melt. Changing these conditions may cause a different mineral to crystallise out. So variations in crystallisation can be manifest by layering defined by alternations of different composition (different minerals). Chill margin.The general term COMPOSITIONAL BANDING is used to describe the layering. Igneous rocks can also show location fabrics defined by systematic variations in grain size - particularly when a volume of magma cools at different rates. Chilled margins.Fine-grained, chilled margins on the flanks of dykes and sills intruded at shallow crustal levels are examples of such location fabrics.

For both sedimentary and igneous rocks, the location fabrics are formed by the same processes that make the original rocks themselves. So in this sense they can be considered as primary structures. This is useful for structural geologists because we have a marker of the organisation of rocks from before deformation. For sedimentary rocks bedding is especially useful. Horizontal rocks in the Grand Canyon.Sedimentary layers are commonly laid down horizontally with bedding level, planar and continuous in the original undeformed state. So if we find beds that are strongly inclined, folded and cut and offset we are able to recognise deformation and begin to analyse it. In general the location fabrics in igneous rocks are less useful simply because we can be less certain about the original organisation and scale of the features. Compositional banding will reflect some aspect of the geometry of the magma chamber within which the igneous rock crystallised - we can assume that it was originally level, planar or even continuous. But there are still uses to igneous rocks in structural geology - as we'll see later.

An aside: bedding in sedimentary rocks

Gneissic banding.Metamorphic rocks can show location fabrics too. In general these are defined by the distribution of different minerals forming compositional banding. In coarse-grained (grains readily visible with the naked eye) metamorphic rocks such structures are generally termed GNEISSIC BANDING. But this is not a primary structure - because metamorphic rocks themselves originate as either sedimentary or igneous rocks. Gneissic banding is generally formed by the re-organisation of the original rock's chemical constituents into layers within which particularly minerals are concentrated. Only rarely will this banding mimic any primary layering (igneous compositional banding or sedimentary bedding). In general gneissic banding is new. Nevertheless in some regional metamorphic belts gneissic banding can have remarkably uniform orientations (leading early geologists to mis-interpret as mimicking bedding). So if we have reason to believe that gneissic banding had a reasonably simple, planar and parallel form prior to a particular deformation episode we can use it to recognise later folds and folds.

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