Most earth scientists thinking about geochemical sediments envision stratigraphic sequences, not natural rock exposures.
Yet, rarely do we see the true coloration and appearance of natural rock faces without some masking by biogeochemical curtains.
Geochemical sediments known as rock coatings (Table 8.1) control the hue and chroma of bare-rock landscapes.
Tufa and travertine (Chapter 6), beachrock (Chapter 11), and nitrate efflorescences (Chapter 12) exemplify circumstances where geochemical sediments can cover rocks.
Perhaps because its ability to alter a landscape’s appearance dramatically (Fig.
Upon examination with secondary and backscattered electron microscopy, the accretionary nature of rock varnish becomes obvious, as does its basic layered texture imposed by clay minerals (Dorn &Oberlander 1982).
Manganese enhancement, two orders of magnitude above crustal values, remains the geochemical anomaly of rock varnish and a key to understanding its genesis.
Field observations have resulted in a number of informal classifications.
Early field geochemists recognized that varnish on stones in deserts differs from varnish on rock surfaces intermittently flooded (Lucas 1905). (2001) Stabilization of friable sandstone surfaces in a desiccating, wind-abraded environment of south-central Utah by rock surface microorganisms.
Another example of differentiating varnish involves position on a desert pavement clast (Mabbutt 1979): black varnish rests on the upper parts of a pavement clast; a shiny ground-line band varnish at the soil-rock-atmosphere interface; and an orange coating on the underside of cobbles in contact with the soil (Engel & Sharp 1958).