Lithic fragment (geology)

Lithic fragments, or lithics, are pieces of other rocks that have been eroded down to sand size and now are sand grains in a sedimentary rock. They were first described and named (in their modern definitions) by Bill Dickinson in 1970.[1] Lithic fragments can be derived from sedimentary, igneous or metamorphic rocks. A lithic fragment is defined using the Gazzi-Dickinson point-counting method and being in the sand-size fraction. Sand grains in sedimentary rocks that are fragments of larger rocks that are not identified using the Gazzi-Dickinson method are usually called rock fragments instead of lithic fragments. Sandstones rich in lithic fragments are called lithic sandstones.

Types

Igneous (Lv)

These can include granular (~rhyolitic), microlitic (~andesitic), lathwork (~basaltic), and vitric (glassy). These correlations between composition and volcanic lithic fragment type are approximate, at best.[2][3] By definition, intrusive igneous rock fragments can not be considered lithic fragments.

Sedimentary (Ls)

These can include shale siltstone fragments, and (at times) chert.

Metamorphic (Lm)

These can include fine-grained schist and phyllite fragments, among others.


  • Granular volcanic lithic fragment, scale in millimeters Top picture in plane-polarized light, bottom picture in cross-polarized light.
    Granular volcanic lithic fragment, scale in millimeters Top picture in plane-polarized light, bottom picture in cross-polarized light.
  • Microlitic volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
    Microlitic volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
  • Lathwork volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
    Lathwork volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
  • Vitric volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
    Vitric volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.

References
  1. Dickinson, W. R. (1970). "Interpreting detrital modes of graywacke and arkose". Journal of Sedimentary Petrology. 40: 695–707.
  2. Affolter, M.D. and Hendrix, M. S., 2004, Correlations between volcanic lithic fragments and volcanic rock, Geological Society of America Abstracts with Programs, Vol. 36, No. 5, p. 370
  3. Mathew D. Affolter, Raymond V. Ingersoll; Quantitative Analysis of Volcanic Lithic Fragments. Journal of Sedimentary Research ; 89 (6): 479–486. doi: https://doi.org/10.2110/jsr.2019.30
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