Conglomerate
Introduction
Conglomerates occupy a paradoxical position in sedimentary geology: they are volumetrically minor - probably less than 1 percent by weight of the entire sedimentary rock mass - yet they are disproportionately informative. A conglomerate records, in its clasts, a direct sample of the source terrain. Its fabric, whether clast-supported or matrix-supported, records the energy and rheology of the transport mechanism. And its position in a sedimentary succession - at the base of a fining-upward sequence, for example - can define an unconformity or a tectonic event. No other sedimentary rock carries so much geological information per grain.
Definition and Abundance
The term conglomerate is used as a general class name for sedimentary rocks that contain a substantial fraction - at least 30 percent - of gravel-size (>2 mm) particles. Breccias, composed of very angular gravel-size fragments, are not distinguished from conglomerates in the following discussion. [1]
Conglomerates are common in stratigraphic successions of all ages but probably make up less than 1 percent by weight of the total sedimentary rock mass. They are closely related to sandstones in terms of origin and depositional mechanisms, and they contain some of the same sedimentary structures, including tabular and trough cross-bedding and graded bedding. [1]
Particle Composition
Conglomerates may contain gravel-size pieces of individual minerals such as quartz; however, most gravel-size framework grains are rock fragments (clasts). Individual sand- or mud-size mineral grains are commonly present as a matrix. Clasts of any kind of igneous, metamorphic, or sedimentary rock may be present, depending upon source rocks and depositional conditions. [1]
The matrix of conglomerates commonly consists of clay minerals and fine micas and/or silt- or sand-size quartz, feldspars, rock fragments, and heavy minerals. The matrix may be cemented with quartz, calcite, hematite, clay, or other cements. [1]
Clast Stability Classification
Some conglomerates are composed only of the most stable and durable clast types - quartzite, chert, vein quartz. Stable conglomerates composed mainly of a single clast type are referred to as oligomict conglomerates. Most oligomict conglomerates were probably derived from mixed parent-rock sources. Continued recycling of mixed stable and unstable clasts through several generations of conglomerates ultimately led to selective destruction of the less stable clasts and concentration of the stable ones. [1]
Conglomerates that contain an assortment of many kinds of clasts are polymict conglomerates. Polymict conglomerates made up of a mixture of largely unstable or metastable clasts such as basalt, limestone, shale, and metamorphic phyllite are commonly called petromict conglomerates. [1]
Clast-Supported vs. Matrix-Supported Fabric
Epiclastic conglomerates in which gravel-size framework grains touch and form a supporting framework are called clast-supported conglomerates. Clast-poor conglomerates consisting of sparse gravels supported in a mud or sand matrix are called matrix-supported conglomerates. [1]
This fabric distinction carries genetic weight. Clast-supported fabrics indicate high-concentration flows in which clasts are the dominant particle - typical of stream gravels, beach gravels, and debris flows in which the solid fraction is high. Matrix-supported fabrics indicate flows where the clasts were dispersed within a fine-grained slurry - characteristic of debris flows and cohesive mass-flow deposits where the matrix has sufficient strength to support coarse clasts without them touching one another.
Genetic Classification
Conglomerates can originate by several processes. The most geologically important is the epiclastic type, formed by breakdown of older rocks through weathering and erosion. Epiclastic conglomerates include both extraformational varieties (clasts derived from outside the depositional basin) and intraformational varieties (formed by penecontemporaneous fragmentation of weakly consolidated sedimentary beds within the basin). Other genetic types include volcanic breccias (formed by explosive eruptions, autobreccia from lava movement, or hyaloclastic shattering at water contacts) and cataclastic breccias (from landslides, tectonic faulting, collapse into solution voids, or meteorite impact). [1]
Diamictite and the Fabric-Based Classification
Matrix-supported conglomerates are referred to as diamictites. Although the term diamictite is commonly applied to poorly sorted glacial deposits, it is a nongenetic term that applies to any nonsorted or poorly sorted siliciclastic sedimentary rock containing larger particles of any size in a muddy matrix. [1]
Both conglomerates and diamictites can be subdivided based on clast stability. Those containing more than 90 percent ultrastable clasts are quartzose (oligomict) conglomerates or diamictites, while those containing fewer than 90 percent ultrastable clasts are petromict conglomerates or diamictites. Further subdivision by clast lithology - igneous, metamorphic, or sedimentary - can be applied where required. [1]
This two-tier system - first by fabric support (clast vs. matrix), then by clast stability - provides a classification that is simultaneously descriptive and genetically informative.
Origin and Occurrence
Quartzose (Oligomict) Conglomerates
Quartzose conglomerates require that all less-stable rock types be destroyed by weathering, erosion, and transport - perhaps through multiple sedimentary cycles - leaving only a residuum of stable clasts. Because those stable clasts represent only a small fraction of a much larger original rock mass, the total volume of quartzose conglomerates is small. They occur as thin pebbly layers or lenses within dominantly sandstone units, and they may be either clast-supported or matrix-supported. Despite their low volume, quartzose conglomerates are common throughout the geologic record from the Precambrian to the Tertiary. Most are of fluvial origin, deposited mainly in braided streams; marine, wave-worked quartzose conglomerates deposited in littoral (beach) environments are also known. [1]
Petromict Conglomerates
Most petromict conglomerates are polymict, containing a variety of metastable clasts derived from plutonic igneous, volcanic, metamorphic, and sedimentary sources. A given body may be dominated by one clast type - limestone, basalt, schist - but the group is compositionally diverse. Plutonic-clast-dominated conglomerates are uncommon because granitic rocks tend to disintegrate into sand-size particles rather than forming larger blocks. The volume of ancient petromict conglomerates far exceeds that of quartzose conglomerates; they form the great conglomerate bodies of the geologic record and may reach thicknesses of thousands of metres, implying rapid erosion from sharply elevated highlands or active volcanic terrains. These bodies are deposited in environments ranging from fluvial through shallow-marine to deep-marine. Deep-marine examples - resedimented conglomerates - were retransported from nearshore areas by turbidity currents or other gravity-flow processes. Truly thick bodies (>20 m) were likely deposited in nonmarine (alluvial fan or braided river) or deep-sea fan settings. [1]
Intraformational Conglomerates
Intraformational conglomerates differ fundamentally from extraformational varieties: their clasts were generated within the depositional basin itself. They originate by penecontemporaneous deformation of semiconsolidated sediment, with the fragments redeposited close to the site of breakup. The breakup may occur subaerially - such as mud drying and cracking on a tidal flat - or underwater, driven by tidal currents, storm waves, or sediment-gravity flows ripping up semiconsolidated muds. Sedimentation is interrupted for only a short time. The most common clast types are siliciclastic mud clasts and lime clasts. Clasts are commonly angular or only slightly rounded, reflecting minimal transport. Where wave or current agitation is especially strong, flattened clasts may be stacked on edge to form edgewise conglomerates. Intraformational conglomerates typically form thin beds - a few centimetres to a metre thick - that may be laterally extensive. Flat-pebble conglomerates composed of carbonate or limy siltstone clasts are particularly characteristic of Cambrian-age rocks in North America and of early Palaeozoic limestones in the Appalachian region. Shale rip-up clast conglomerates at the bases of sandstone beds are a hallmark of sediment gravity-flow successions. [1]
Related Topics
Sandstone
Sandstones make up 20-25 percent of all sedimentary rocks. They are common in geologic systems of all ages and are distributed throughout the continents. They occur in beds ranging in thickness...
Sedimentary Structures
Sedimentary structures are large-scale features of sedimentary rocks - including parallel bedding, cross-bedding, ripples, and mudcracks - that form as a direct result of depositional or...
Heavy Minerals
Heavy minerals are a small but disproportionately informative fraction of sandstone framework grains. Despite their low abundance - typically well below 1 percent - they carry provenance...
Graded Bedding
Graded bedding is one of the most distinctive and informative primary sedimentary structures. It records a systematic change in the energy of the depositing agent - from high to low as a...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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