Imbrication (sedimentology)
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In sedimentology
Sedimentology
Sedimentology encompasses the study of modern sediments such as sand, mud , and clay, and the processes that result in their deposition. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary...

 imbrication refers to a primary depositional fabric
Fabric (geology)
In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up.-Types of fabric:* Primary fabric — a fabric created during the original formation of the rock, e.g...

 consisting of a preferred orientation of clasts such that they overlap one another in a consistent fashion, rather like a run of toppled dominoes. Imbrication is observed in conglomerates
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...

 and some volcaniclastic deposits.

Types of imbrication

N.B. In conglomerates the shape of many clasts can be approximated to an ellipsoid, with a long axis (A), an intermediate axis (B) and a short axis (C).
  • A-axis imbrication where the long axes of the clasts are orientated parallel to the flow direction. This fabric is characteristic of clasts carried in suspension and this is only preserved in the case of a fast-waning flow in which the clasts are deposited without any significant rolling. This fabric is typical of conglomerates at the base of turbidite
    Turbidite
    Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...

     beds but is also occasionally observed in alluvial fan
    Alluvial fan
    An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...

     deposits reworked by flash floods

  • AB-plane imbrication where the long axes of the clasts are orientated perpendicular to the flow direction and the imbrication is of the flattened AB plane, with the intermediate axes in the flow direction. This fabric is characteristic of clasts carried as bedload. This imbrication forms as the clasts are rolled along the base of a channel. The flat clasts roll onto those downstream and come to rest with their downstream end propped up by the downstream pebbles and can be used to diagnose original flow directions.

External links

  • http://meguma.earthsciences.dal.ca/classes/2203/2203-Conglomerates.ppt (Slides 24 & 25)
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