Definition and range of types
CONGLOMERATES are all those coarse-grained sedimentary rocks that consist dominantly of gravel-sized (>2mm) clasts. They are also known as rudites. Strictly speaking, conglomerates should contain >50% clasts over 2mm in diameter; anything less than this and they are more correctly termed pebbly sandstones or pebbly mudstones as appropriate. Most melanges, olistostro Esad, and many debrites fall into this category and are therefore discussed.
Poorly sorted or non-sorted sediments that contain a wide range of clast sizes (pebbles, cobbles, boulders) in a muddy matrix are also called diamictites. Some authors reserve this term for use with glacially deposited pebbly mudstones and muddy conglomerates (also known as till or tillites).
Conglomerates, in which the clasts are separated by finer-grained sediment, are known as matrix-supported, whereas those in which the clasts are in contact with one another are termed clast-supported. Conglomerates with a dominance of angular rather than rounded clasts are known as angular conglomerates, or breccias.
On the basis of clast origin, intraformational and extraformational conglomerates can be distinguished. Intraformational clasts are those derived from within the basin of deposition, and typically include shales or micritic limestones.
Extraformational clasts are those derived from outside the basin and can include a wide range of types. In many cases, of course, it is not possible to determine whether the clasts are intraformational or extraformational, so neither prefix should be used.
Polymict conglomerates are those with many different types of clast, oligomict and monomict conglomerates have, respectively, few and just one type of clast. Where the dominant clast type is limestone or dolomite the rock is known as a carbonate conglomerate or calcirudite. Likewise, where volcanic clasts are dominant the rock is a volcaniclastic conglomerate.
In some cases, angular conglomerates or breccias are formed in situ by breakage, collapse or solution. Such rocks are termed cataclastic breccias and solution breccias. Where the clast size is extremely large, then the term megabreccia or mega-conglomerate can be used, whatever the origin.
The fundamental genetic types of conglomerates are shown in Table 1.
Principal sedimentary characteristics
Thickness: variable, may be very thick (massive); bedding commonly indistinct or absent. Bed thickness may vary systematically through a succession, often in association with sandstone beds, where they are referred to as thinning-up or thickening-up sequences. Conglomerates are also prone to showing proximal to distal decrease in bed thickness over relatively short distances (e.g. tens to hundreds of metres).
Shape: irregular and lenticular beds common, in some cases with channel-like geometry.
Boundaries: top and bottom boundaries typically irregular or gradational; bottom may be erosive and sharp.
Large clast size and poor sorting commonly make it difficult to observe primary structures in conglomerates. Many beds may appear structureless (or massive) initially but closer inspection can reveal crude (or subtle) stratification– look carefully for parallel-alignment of elongate clasts. Both parallel and crossstratification occur, with the latter in some cases only slightly inclined.
Normal and reverse grading occur through distinct beds, but an irregular oscillation of grain size is often observed in an unbedded or poorly bedded unit. In some conglomerates the larger clasts have been rafted towards the middle or top of the bed due to buoyancy forces acting during transport (e.g. in debris flows). Soft and semi-consolidated sediment clasts typically show deformation structures. Water-escape features are rare and bioturbation generally absent.
Matrix-supported and clast-supported fabrics both occur in conglomerates, with clastsupport more typical in fluvial, beach, reeftalus and many volcaniclastic deposits, whereas matrix-support is more common in debrites (subaerial or subaqueous) and glacial diamictites. Thin-bedded clast-supported conglomerates, showing evidence of extensive reworking and abrasion, may be due to rapid marine transgression over a low-lying continental shelf.
Such basal conglomerates occur at the base of a transgressive system tract. Other fabric types are noted by the disposition of tabular and blade-shaped clasts: e.g. random, bed-parallel or sub-parallel, and imbricated. In fluvial and shallow-marine current-deposited conglomerates, long axes are generally oriented normal-to-current as the result of a rolling action of pebbles over the bed surface. In glacial diamictites, a parallel-to-flow orientation results from a sliding action, while in debrites and coarsegrained turbidites a parallel-to-current orientation results from very rapid deposition and flow freezing. In-situ brecciation processes in limestones (e.g. karst collapse, hardground fragmentation, and calcretization of soil horizons), autobrecciation in volcaniclastic deposits (e.g. autoclastites and hyaloclastites), and cataclastic processes (e.g. various tectonic breccias), all tend to produce random clast fabrics.
Conglomerates have a dominant mean size 2mm, but include a wide range of size and sorting characteristics. Some of the largest boulders or clasts may be the size of a car or house! Modal size is generally easier to determine than mean size in the field and many conglomerates are, in fact, bimodal or polymodal in their grain size distribution.
The maximum clast size is also a good indication of flow strength or velocity. With some depositional processes there is a positive correlation between maximum clast size and bed thickness – this is true of muddy debris flows and stream floods. Most other processes, including normal braided river flow, do not yield this relationship. Depositional porosity and permeability are generally very high, except where there is an abundant muddy matrix. Both decrease markedly with compaction and cementation.
Conglomerates, like sandstones, can include almost any pre-existing mineral or rock fragment, the least stable ones only being preserved where deposited close to source. Field observations should include the type, variety and approximate proportions of different rock types present as clasts. These data give very good information on the provenance (source area) and likely transport distance. Different levels or beds may yield different compositions, suggesting change of source area or multiple sources.
Classification of conglomerates
THERE ARE relatively few classification schemes for conglomerates, apart from those encompassed in the definition of types listed above. Clast size can be used as a descriptor term such as cobble-rich or boulder-rich con – glomerate, but a more systematic compositional classification is probably most helpful.
Terms for the range of different clast types have already been given; terms for the relative clast stability also exist. Conglomerates made up of framework grains that consist dominantly of ultrastable clasts (i.e. >90% quartzite, chert, and vein quartz) are quartzose conglomerates. Those with abundant metastable or unstable clasts are petromict conglomerates.
The classification scheme proposed by Boggs (1992) is currently the most comprehensive, and this has been modified herein (Fig. 1).
CONGLOMERATES and breccias are deposited in a range of environments by high-energy processes. They are typical of continental environments, such as those of alluvial fan and fluvial systems, where they may occur as part of a red-bed succession. They also occur in glacial deposits, typically as matrix-supported conglomerates and pebbly mudstones, or on fan-deltas just fringing into a lacustrine or marine setting.
Thinner deposits occur in beach and shallow marine settings, where they are associated with shallow water fossils, calcareous encrustations and borings. In deeper water, slope apron, and submarine fan systems, they are common deposits of debris flows and high-concentration turbidity currents, especially in submarine channels.
Dorrik A.V. Stow Ph.D, School of Ocean and Earth Science Southampton Oceanography Centre University of Southampton