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

References

^ Dickinson, W. R. (1970). "Interpreting detrital modes of graywacke and arkose". Journal of Sedimentary Petrology. 40: 695–707.
^ 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
^ 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