gravel and conglomerate

gravel and conglomerate Gravel and its indurated (i.e. hardened) equivalent conglomerate is a sediment (or sedimentary rock) composed of more than 50 per cent of rounded fragments (clasts) with diameters greater than 2.0 mm. The name is, however, commonly applied to deposits with as little as 10 per cent clasts.

When deposited by currents and waves the clasts form a supportive framework enclosing voids (open-work gravel) which when infilled with fine sand and then cemented is termed orthoconglomerate. The clasts can be of one lithological type (oligomict conglomerate) or of several types (petromict conglomerate). When the clasts have been derived from immediately underlying rocks in the basin of conglomerate deposition the term intra-formational conglomerate is used; when the clasts have come from distant sources the deposit is called an extraformational conglomerate. Paraconglomerate is a term used for deposits which are not true conglomerates but contain gravel clasts enclosed in a matrix of sand and mud and were not deposited by traction currents but by sedimentary gravity flows and other processes. The terms diamictite or pebbly-mudstone have also been used for this type of deposit. Glacial tills (boulder clays) and their ancient equivalents tillites are of this type. Tilloid is a term used for till-like deposits of doubtful but probably non-glacial origin. Although widely used in provenance studies, these deposits are quickly modified by transport: clasts are rounded and the softer rock fragments are lost after several kilometres of transport. They thus contain relatively larger proportions of hard components, such as vein quartz and quartzite, than the original source area. Conglomerates and gravels are generally only crudely stratified and commonly occur as lenses or as channel-fills in finer deposits. The individual clasts are commonly imbricated (i.e. they lie parallel to each other) and they dip upstream. This relationship is used in palaeocurrent studies for determining the directions of former currents. Conglomerates undergo relatively minor diagenetic changes, such as recrystallization of any clay matrix and precipitation of mineral cement. Pitting at the points where clasts are in contact can be observed in some conglomerates when high pressures have caused solution to occur at their contacts.

Petromict orthoconglomerates and subsidiary paraconglomerates are the principal components of huge alluvial fans along the feet of mountain chains. Thick wedges of such ancient fan deposits mark the locations of modern or old mountain belts. They usually wedge out laterally over short distances into fluvial, lacustrine, or marine deposits. The repeated occurrence of conglomerate wedges is often taken as evidence of tectonic movements in the source area.

Gravels are common in rivers, they cover old river terraces, and they infill buried channels beneath many river valleys in ancient fluvial sedimentary sequences. Conglomerates commonly occur as thin basal beds within cyclic sedimentary sequences, within which they represent the lateral migration of channels. Gravel beaches are common features of many coastlines, and thin layers of gravel commonly cover raised beaches and other surfaces of marine planation. They form relict deposits on continental shelves and sometimes thin contemporary deposits—gravel lags where currents are sufficiently strong. Conglomerates form thin layers above unconformities, where they represent the beginning of a new cycle of sedimentation; that is, they are basal conglomerate. Matrix-supported gravels form slumped masses along modern continental margins and occur interbedded with ancient slope and basin sediments. Matrix-supported gravels also form extensive sheets of till which blanket vast areas in former Quaternary glaciated regions. Similar deposits that accumulated during earlier glaciations occur at various levels in the geological column.

Both gravels and conglomerates are valuable raw materials for the construction industry. They are also important aquifers and are hosts to some mineral deposits, especially gold and tin.

G. Evans

Bibliography

Pettijohn, E. J. (1975) Sedimentary rocks. Harper and Row, New York

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