Conglomerate (geology)

• 1. Introduction

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  Conglomerate is a sedimentary rock built from rounded gravel-sized pieces of rock, surrounded and held together by finer material like sand, silt, or clay. At Montserrat, near Barcelona, centuries of erosion have carved vertical channels into conglomerate cliffs so dramatic that the mountain earned a name meaning "jagged mountain." The rock there proved sturdy enough to quarry for the Santa Maria de Montserrat Abbey. So what exactly is this ancient material, and what does it take for loose gravel scattered across a riverbed or glacier to harden into enduring stone? The answers lie in pressure, time, and the surprising variety of environments where conglomerate quietly forms, from the floors of ancient seas to the surface of Mars.

• 2. Anatomy of a Rock

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  Each piece of gravel locked inside conglomerate has a name: clast. To qualify as conglomerate, a rock must contain at least 30 percent of rounded to subangular clasts larger than 2 mm in diameter, a category that spans granules, pebbles, cobbles, and boulders. The finer sediment filling the spaces between clasts is called the matrix. Calcium carbonate, iron oxide, silica, and hardened clay are the most common cements that bind clasts and matrix into a single coherent unit.

  Roundness is the defining distinction between conglomerate and its close relative, breccia. Breccia holds angular, jagged clasts rather than rounded ones. When a rock contains a mixture of both rounded and angular gravel, geologists sometimes call it breccio-conglomerate. This single physical characteristic, the shape of the clasts, carries enormous information about how far the pieces traveled before they came to rest.

  The internal arrangement of clasts also divides conglomerates into two fundamental types. In an orthoconglomerate, the gravel clasts touch one another directly and are typically cross-bedded, well-cemented, and lithified by calcite, hematite, quartz, or clay. In a paraconglomerate, so much matrix separates the clasts that they float independently within it, never touching. Paraconglomerates are often unstratified and frequently originate as glacial tills or debris flow deposits, while orthoconglomerates form under flowing water.

• 3. A Taxonomy Built on Gravel

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  Geologists classify conglomerates along three different axes, and the method chosen depends on the type of research underway. The first axis is texture, which produces the orthoconglomerate and paraconglomerate distinction already noted. The second is clast composition. A conglomerate made from a single rock or mineral type is called monomict or oligomict. One built from two or more rock or mineral types is polymict or polymictic. If a polymictic conglomerate carries an assortment of metastable and unstable rocks and minerals, it earns the additional label petromict or petromictic.

  The third axis is source. Clasts that differ significantly in composition from the surrounding matrix, and therefore came from outside the basin of deposition, define an extraformational conglomerate. Clasts that match the surrounding matrix and formed within the same basin define an intraformational conglomerate. Two notable intraformational types are the shale-pebble conglomerate, built from rounded mud chips cemented by clay and shaped by erosion in river channels or along lake margins, and the flat-pebble conglomerate, whose relatively flat lime-mud clasts were torn loose by storms, tsunamis, or tidal currents eroding shallow sea floors and tidal flats.

  A fourth organizational layer simply names the rock after its dominant clast size: a granule conglomerate, a pebble conglomerate, or a cobble conglomerate. These labels let a geologist communicate at a glance what size material dominates a deposit and, by extension, how energetic the environment of deposition was.

• 4. Where Conglomerate Forms

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  Conglomerate is rare. Across all sedimentary rocks, it probably makes up less than 1 percent by weight. Yet it appears in every age of the geologic record and across a remarkable range of environments. In deep-water marine settings, conglomerate turns up at the base of turbidite beds, where coarse material settles first from a sediment-laden current. These deep-water conglomerates are often well sorted, well rounded, and show a strong A-axis imbrication, meaning the clasts are tilted in a consistent direction relative to the current.

  Shallow-marine conglomerates form at the base of sequences deposited when a rising sea advances over an older land surface, a process geologists call a marine transgression. These basal conglomerates mark where the shoreline stood at a specific moment and are described as diachronous, because the same type of deposit accumulated at different times along different stretches of coast.

  Fluvial conglomerates, deposited by rivers, tend to be well rounded but poorly sorted. Clasts this large move as bedload and only at times of high flow. Because clasts wear down and shrink through abrasion during transport, conglomerates are most characteristic of immature, steep river systems. In mature rivers with gentler gradients, gravel is mostly confined to the base of channel fills, where it forms pebble lags.

  Alluvial fans, which develop at mountain fronts where fast-moving streams lose energy and spread out, host some of the thickest conglomerate deposits in the geologic record. Individual fans can merge into broad braidplains. Most alluvial conglomerates are clast-supported with a strong AB-plane imbrication. Debris flows on these fans also produce matrix-supported conglomerates. When these accumulate in rapidly eroding desert environments, the result is a fanglomerate. Glacial settings produce their own variety: tillites, the direct deposits of glaciers, are typically poorly sorted, matrix-supported conglomerates with a fine-grained matrix of milled rock fragments. Meltwater deposits associated with glaciers, including eskers, are also commonly conglomeratic.

• 5. Fanglomerates and Special Types

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  Fanglomerates occupy a particular position in the geological record. Poorly sorted and rich in matrix, they originated as debris flows on alluvial fans and are thought to contain the largest accumulations of gravel anywhere in the geologic record. The Crestone Conglomerate, found in and near the town of Crestone at the foot of the Sangre de Cristo Range in Colorado's San Luis Valley, is a well-documented example of poorly sorted fanglomerates. These rocks accumulated in prehistoric alluvial fans and related fluvial systems, and some carry striking hues of red and green.

  In Pennsylvania, a thick layer of Pottsville conglomerate was recognized in the nineteenth century to underlie anthracite coal measures. Nagelfluh, a conglomerate of Miocene age, is found in the Swiss Alps. Conglomerate cliffs run along the east coast of Scotland from Arbroath northward through the coastlines of the former counties of Angus and Kincardineshire. Dunnottar Castle sits on a rugged promontory of conglomerate that juts into the North Sea just south of the town of Stonehaven.

  Copper Harbor Conglomerate appears both on the Keweenaw Peninsula and at Isle Royale National Park in Lake Superior. The domed hills of Kata Tjuta in Australia's Northern Territory and the Buda Hills in Hungary are further examples of how conglomerate shapes terrain across vastly different climates and latitudes.

  When conglomerate is subjected to metamorphism, it transforms into metaconglomerate, a denser and recrystallized version of the original rock.

• 6. Conglomerate on Mars

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  NASA's Mars rover Curiosity discovered slabs of conglomerate at an outcrop scientists named Hottah. Analysis of the gravels, which ranged from the size of sand particles to the size of golf balls, led researchers to conclude that a stream once flowed across the site. The rounded shape of the clasts points to transport by water over some distance before deposition.

  Scientists interpreted the flow as having moved at roughly walking pace and as having been ankle- to hip-deep. The discovery carries weight beyond geology: rounded gravel cemented into conglomerate is strong evidence for sustained liquid water on ancient Mars. On Earth, the same rounding process requires repeated collision and abrasion during transport through a moving stream. Finding the same signature in Martian rock extends a process understood from river valleys and glacier margins here to another planet entirely.