Conglomerates

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

Bedding

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.

Structures

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.

Fabric

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 Texture

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.

Composition

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 based on clast type and matrix content. Note that within each of the principal types (metamorphic, igneous and sedi mentary) it is possible to have monomict, oligomict and polymict varieties. Breccia is the term used where the dominant clast shape is angular.
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).

Fig. 1 – conglomerate (breccia) showing clast alignment and reverse grading; part of deep-water turbidite succession.
Cretaceous, Dana Point, California, USA.
Occurrence

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.

Monomict limestone conglomerate (breccia) with random clast arrangement in sparry calcite cement; part of proximal rockfall scree deposit. Wine pouch 15cm wide. Plio–Pleistocene, near Benidorm, SE Spain
Sandy matrix-supported polymict conglomerate, poorly sorted with clasts up to 50cm diameter in 1m thick bed; sandy debrite within deep-water turbidite succession. Hammer 45cm. Eocene, Tabernas Basin, SE Spain.
Polymict clast-supported conglomerate, very poorly sorted with clasts over 1.2m in diameter, bedding indistinct to absent, crude stratification due to subparallel clast alignment (tabular clasts), indicate bedding dips at approximately 35° from upper right to lower left; part of coarse-grained alluvial fan system.
Neogene, NW Crete, Greece.
Monomict limestone conglomerate, very poorly sorted with clasts over 1.5m diameter, crude subparallel clast alignment but no clear bedding; part of debris flow/flow–slide deposit on subaerial alluvial fan.
Bedding is approximately horizontal, based on basal contact of this unit (out of view). Hammer 30cm. Plio–Pleistocene, near Benidorm, SE Spain.
Monomict limestone conglomerate, poorly defined bedding, crude stratification due to subparallel clast alignment, but note clast imbrication (dashed lines) in parts (flow towards left); poorly sorted and clast supported; part of coarsegrained fluvial system.
Scale bar 20cm. Plio–Pleistocene, near Benidorm, SE Spain.
Sandy oligomict conglomerate in sandstone/pebbly sandstone sequence, poorly defined bedding, crude stratification in conglomerates, parallel and cross-lamination (bottom left) in sandstones; part of raised beach deposit with rounded limestone clasts mixed with locally derived dolerite clasts. Hammer 25cm. Plio–Pleistocene, near Benidorm, SE Spain.
Conglomerates, pebbly sandstones and sandstones; part of resedimented facies on subaqueous portion of fan delta; note slight unconformity (dashed line) probably due to active synsedimentary tectonics. Miocene, Pohang basin, SE Korea.
Polymict conglomerates, pebbly sandstones and sandstones; part of resedimented facies (coarse grained turbidites) from deep-water slope apron/ submarine fan succession; distinct beds with gradational contacts, individual turbidites are coupled conglomerate– sandstone units (arrows); top to left. Width of view 1.5m. Paleogene, central California, USA
Oligomict conglomerate, with crude bedding stratification, from subaerial part of alluvial fan/fan delta succession; clast-supported near base, matrix supported in parts near top. Crude stratification (dashed lines) shows slight divergence of bedding from left to right (i.e. wedge geometry). Carboniferous, Quebrada de las Lajas, NW Argentina.
Pebbly mudstone (diamictite or tillite), beddingplane view. Ancient glacial till deposit, very poorly sorted with clasts of mixed composition in a fine-grained matrix. Note several groove marks oriented top left to bottom right, and ‘bulldozer’ ruck marks in front of large clast 6cm left of the scale bar (in centimetres). Late Carboniferous, Wynard foreshore, N Tasmania, Australia.
Shale-clast conglomerate, part of resedimented deep water massive sandstone succession. Bedding approximately horizontal. Hammer 45cm. Miocene, Urbanian Basin, central Italy.
Sandy muddy conglomerate, small part of over 50m thick Gordo Megabed, a tripartite resedimented unit (slidedebrite/ slurry bed-turbidite) in small marginal marine deepwater basin succession, showing distinct bed-parallel alignment of tabular metamorphic clasts, and grain-size oscillation typical throughout megabed. Top to right, no overall grading evident. Hammer 45cm. Miocene, Tabernas Basin, SE Spain.
Chaotic oligomict breccia/conglomerate (limestone and volcanic clasts), part of subaqueous debris avalanche into small marginal marine basin. Hammer 45cm. Late Triassic–Jurassic, Dolomites, N Italy.
Oligomict conglomerate with large rounded volcanic clasts and finer shelly debris in sandy matrix; part of debris avalanche deposit downflank from bioherm capped submarine volcano. Width of view 60cm. Miocene, Cabo de Gata, near Carboneras, SE Spain.

Dorrik A.V. Stow Ph.D, School of Ocean and Earth Science Southampton Oceanography Centre University of Southampton

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