Imbrication (sedimentology)

Imbricated clasts, with an implied flow direction of left to right.
Note how the clasts are oriented at an angle to the base of the channel (blue line), which was originally approximately horizontal.

In sedimentology imbrication refers to a primary depositional fabric 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 and some volcaniclastic deposits.[1][2]

Types of imbrication

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).

Relation to paleocurrent

The type of imbrication is generally related to paleoflow direction.[4] Wadell[5] found the long axis aligned with paleocurrent, and dipping basinward in glacial sediments, whereas deltaic gravels may be oppositely inclined.[6]

References

  1. Karatson,D., Sztano,O. & Telbisz,T. (2002). "Preferred Clast Orientation In Volcaniclastic Mass-Flow Deposits: Application Of A New Photo-Statistical Method" (PDF). Journal of Sedimentary Research. 72 (6): 823. Bibcode:2002JSedR..72..823K. doi:10.1306/040402720823.
  2. Pettijohn F. J., 1957, Sedimentary Rocks, Harper and Row, Second Edition p. 78
  3. Sinclair,H.D. & Jaffey,N. 2001. "Sedimentology of the Indus group, Ladakh, northern India: Implications for the timing of initiation of the palaeo-Indus River". Journal Geological Society. 158: 151–162.
  4. Becker, G.F., 1893, Finite homogenous strain, flow, and rupture of rock, Bull. Geol. Soc. of Amer., 4, pp. 13-90; cite in Pettijohn, 1957.
  5. Wadell, H., 1936, Shape and position of rock fragments, Geografiska Annaler, pp. 74-92, cite in Pettijohn, 1957.
  6. Krumbein, W. C., 1940, Flood gravel of San Gabriel Canyon, Bull. Geol. Soc. of Amer., vol. 51, pp. 636-676.
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