Monthly Archives: July 2019

The provenance of detrital zircon

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detrital zircon

This post is part of the How To…series – using zircon geochronology to decipher provenance

Zircon is a common accessory mineral in igneous and metamorphic rocks so it’s not surprising that it is also a common constituent of sedimentary heavy mineral suites. Detrital zircon has assumed a remarkable popularity over the last 2-3 decades as a provenance indicator because:

  • crystals contain measurable amounts of uranium (U), lead (Pb) and thorium (Th) isotopes and can therefore be dated radiometrically,
  • zircon is resistant to chemical and mechanical change – crystals can survive multiple sedimentary cycles (i.e. episodes of erosion from source rocks, deposition, burial and uplift, whereupon the whole process begins anew), and
  • they commonly contain multiple stages of crystal growth that record magmatic, metamorphic and depositional episodes.

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Provenance and plate tectonics

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This post is part of the How To…series – Provenance, sedimentary basins and plate tectonics

Deciphering the history of sedimentary basins is one of the more exciting tasks geologists can undertake; the provenance of sandstones plays an important part in this adventure.

Sedimentary basins are crustal structures. They are regions of long-term subsidence, responding to tectonic and sediment loads, cooling in the crust and upper mantle, and tectonic dislocation along crustal structures like transform faults. The processes that create sedimentary basins and the sediments that fill them are inextricably linked to plate tectonics.

The idea that the composition of sedimentary rocks was related to large-scale crustal processes was acknowledged by Charles Darwin, Charles Lyell, Henry Sorby and others, but it wasn’t until the 1950s – 60s that sedimentologists began to develop empirical models of sandstone provenance. Robert Folk, Francis Pettijohn, Robert Dott and contemporaries observed links between sediment composition and tectonic domains (or provinces), such as stable cratons and geosynclines – this was the era before plate tectonics.

For geoscientists, the discovery and development of plate tectonic theory changed everything. This is where William R. Dickinson comes into the picture. Dickinson recognized the fundamental link between sedimentary basins and plate tectonics, particularly at plate boundaries. He developed models that relate the modal composition of sandstones to plate tectonic provinces such as collision orogens, magmatic arcs, forearc basins and stable cratons. It is important to remember that these models are based on empirical evidence – analysis of 1000s of thin sections that he and many others had recorded from diverse locations.

The models are based on ternary plots like those used by Dott to classify sandstones. Dickinson and his co-workers used different combinations of the quartz, feldspar and lithics end-members to emphasize certain characteristics of the sediment and the source rocks. Both models shown here use the full suite of minerals. Other plots used only the lithic components, or polycrystalline quartz and lithics.

The Qt-F-L plot combines all varieties of quartz (mono- and polycrystalline quartz, including chert) as a single category and as such emphasizes the maturity of the sediment. Deposits with greater volumes of quartz are generally considered more mature, where mechanical and chemical weathering during sediment transport and deposition have removed less- stable components like feldspar and lithics.

In the Qm-F-L plot, polycrystalline quartz is shifted to the lithic field and in so doing emphasizes the source rocks and production of rock fragments (the quartz component consists only of monocrystalline varieties). Lithics here are key indicators of reworked orogenic provinces along continent-continent and continent-magmatic arc collision provinces. Here, erosion of sedimentary cover and volcanic rocks tends to produce greater proportions of rock fragments.

The second set of diagrams shows typical plate tectonic configurations that correspond to the various QFL fields. The diagrams are highly simplified. In addition to pigeonholing sandstone compositions, the plots provide a useful means of documenting systematic changes as uplift and erosion expose deeper crustal rocks.

 

 

For example, unroofing a fold and thrust belt along a collision margin will yield an initial rush of lithics derived from the deformed sedimentary cover. Gradual exposure of a metamorphic core will yield increasing volumes of quartz (mostly polycrystalline) and a new suite of heavy minerals. Likewise, unroofing a magmatic arc complex (the “dissected arc” field in these plots) will provide abundant volcanic lithics followed by more felsic sediment from the deeper intrusive rocks. This is shown schematically in the cartoon below.

The Dickinson plots, like any scientific model, are highly simplified versions of the real world. No two collisional orogens are alike, no two magmatic arcs are exact duplicates. Finding exceptions to any of the models does not indicate their failure – quite the opposite. Their value lies in providing a direction for investigation.

Dickinson’s models have been through several iterations, but their basic structure has survived 40 years of intense scrutiny by geoscientists. They are still useful starting points for unravelling the links among sediment composition, sedimentary basins and plate tectonics.

Check out the companion article – Provenance of sandstones

 

Some useful texts and papers:

Petrology of Sedimentary Rocks, Sam Boggs Jr. 2012

Dickinson, W. R. and C. A. Suczek, 1979, Plate tectonics and sandstone compositions: American Association of Petroleum Geologists Bulletin, v. 63, p. 2164–2182.

W. R. Dickinson, 1988. Provenance and Sediment Dispersal in Relation to Paleotectonics and Paleogeography of Sedimentary Basins. In New Perspectives in Basin Analysis, Editors, Karen L. Kleinspehn & Chris Paola,  Springer-Verlag, pp 3-25.

R.V. Ingersoll, T. F. Lawton, and S.A. Graham, 2018. Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson. Geological Society of America, Special Paper v.540, 757 pages

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The provenance of sandstones

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Provenance – the origin of sand

This post is part of the How To… series

See the companion posts – Provenance and plate tectonics

The provenance of zircon

Provenance is an investigative process that attempts to find out where things originated; a piece of art, an ancient manuscript, a sandstone. In geology, provenance usually applies to sediments and sedimentary rocks – where did they come from? At its most basic, provenance determines the kind of source rock that supplied loose sediment. But provenance can also provide information about:

  • Paleoclimate, particularly weathering of rock where the physical agents of erosion and chemical agents of mineral alteration, determine the volumes of sediment produced and its composition. For example, warm humid climates favour the formation of deep soil profiles where minerals like feldspar, and ferromagnesians like amphiboles and pyroxenes are susceptible to dissolution and alteration.
  • Paleotopography – was there mountainous relief (e.g. orogenic belts or volcanic arcs), or low relief across an ancient craton?
  • Ancient sediment transportation corridors such as terrestrial drainage systems or sediment distribution across a marine shelf or platform to the deep ocean,
  • Possible tectonic dislocation between source areas and sites of sedimentation (sedimentary basins), for example shortening across a mountain thrust belt, or lateral displacement along strike slip or transcurrent faults? Such dislocations may even involve tectonic plates or terranes that have moved 100s of kilometres, separating source rocks from the sedimentary basin.

  • Changes in mineralogy resulting from unroofing. For example, the upper crustal levels of magmatic arcs (including volcanoes) will produce abundant lithics and plagioclase, and lesser amounts of quartz. As uplift and erosion exposes (unroofs) deeper intrusive rocks, there may be a change to lithic-poor sediment with concomitant increases in quartz and or feldspar. Likewise, uplift and erosion of an orogenic belt may produce an initial pulse of sedimentary lithics and recycled quartz, that with gradual unroofing of a metamorphic core produces sediment laden with polycrystalline quartz and a new suite of heavy minerals, particularly micas.

It is instructive to look at suites of sedimentary rocks. This is important because there may be an opportunity to gauge regional trends in composition and texture in relation to changes in the source area (e.g. uplift), drainage, and depositional paleoslope (i.e. the regional dip of sedimentary basins). Your first task is to identify the mineralogy and textural properties like grain sorting and angularity.

Some of the problems associated with the determination of provenance are nicely illustrated using quartz arenites as an example. Quartz arenites contain 95% and more detrital quartz grains. They tend to be well sorted; grains are well rounded. Whence all that quartz? Some useful questions and considerations to begin your determination might be (the questions apply to sedimentary rocks of any composition):

  • Have the distribution and age of sedimentary units and potential source rocks been mapped in sufficient detail?
  • Stratigraphic-sedimentologic evidence may help pin-point potential sediment sources? Particularly useful are any mineralogical changes that coincide with down-dip facies changes or vertical stratigraphy.
  • Can regional paleocurrent trends be linked to drainage of potential source rock areas?
  • Do the potential source rocks contain sufficient amounts of quartz; do they contain the right kinds of quartz?
  • What is the mineralogy of the quartz? You will need to determine the proportions of monocrystalline and polycrystalline varieties, and whether the quartz has strained (i.e. deformed crystal lattice) or unstrained extinction. Is there any volcanic quartz?
  • The suite of heavy minerals can be very instructive. Common minerals include the ferromagnesian groups of like pyroxenes, amphiboles, micas, and olivine, plus iron oxides such as magnetite and ilmenite, and tourmalines, garnets and zircon. Most of these minerals occur in several different rock types (intrusive, metamorphic, volcanic), and on their own may not be diagnostic. Ferromagnesians are prone to alteration by weathering and during diagenesis. But with careful observation you should be able to tease useful provenance information from the suite. Of the micas, muscovite is probably the most useful in that it does not occur in extrusive volcanics but is common in metamorphic and intrusive rocks. Minerals like sillimanite and kyanite are almost exclusively metamorphic. Olivine (although uncommon as a heavy mineral), is particularly prone to alteration and in most cases will be a first cycle product that pinpoints mafic volcanic and intrusive source rocks.
  • Zircon is extremely durable and can survive two or more cycles of deposition, burial, metamorphism and uplift. Over the last 2 decades, Zircon has assumed a position of importance because technology now allows the determination of radiometric dates not just from whole single crystals, but from different parts of a crystal. Knowing zircon ages helps  pinpoint source candidates.

  • Are there any trends in textural properties or mineralogy along depositional dip or stratigraphically? Are there down-stream changes in grain size and angularity with distance from source? Has the proportion of less stable minerals been reduced? For example, if the primary source rock is granite there will be an initial mix of monocrystalline quartz, potassium feldspar and plagioclase, and heavy minerals like biotite and muscovite. Mechanical abrasion and chemical weathering will preferentially reduce the feldspar population.  The micas, even though they are denser than quartz or feldspar, will be preferentially removed because they behave like hydraulically light minerals. There will be a tendency for quartz to become dominant even though it originally was volumetrically subordinate to the feldspars.From your thin section observations and having considered some of the above, what are the potential source rock types? In answering this question, you need to keep in mind the likelihood that the sedimentary rock under the microscope may look nothing like its progenitor.

Having decided on likely source rocks and source areas, it is now time to consider provenance in relation to tectono-stratigraphic domains associated with plate tectonics – are the source rocks-source terrain and adjacent sedimentary basin part of an orogenic belt and foreland basin, an accretionary prism, a magmatic arc-trench complex?  A companion post examines the rationale behind these questions.

 

 

Some useful texts and papers:

Petrology of Sedimentary Rocks, Sam Boggs Jr. 2012

R.V. Ingersoll, 1978, Petrofacies and Petrologic Evolution of the Late Cretaceous Fore-Arc Basin, Northern and Central California. The Journal of Geology Vol. 86, pp. 335-352

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The mineralogy of sandstones – matrix & cement

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SEM of quartz overgrowths

This post is part of the How To… series – How to identify matrix and cement in arenites

The primary architectural element of a sandstone is its framework of sand grains. Spaces between grains will be filled, to varying degrees by much finer sediment – or matrix. The amount of matrix initially deposited is strongly dependent on the energy of the depositional system and the degree of sediment reworking. Wind-blown dune sands and beach sands will have very little matrix at the time of deposition; those deposited farther out to sea will tend to accumulate more.

The proportion of matrix to framework is an important determinant of sandstone classification.  Arenites have less than 15% matrix, wackes have more than 15%. Sandstones with little matrix (commonly referred to as clean sands) have the potential to preserve some of their original porosity.

Detrital matrix generally consists of clay minerals mixed with silt-sized quartz and feldspar.  At the point of deposition there is a significant volume of interstitial water, most commonly seawater or freshwater.  The mix of solid and liquid phases means that matrix is highly reactive in both mechanical and chemical contexts. Compaction that begins soon after deposition and continues during burial, will physically compress the matrix and at the same time drive off some of the interstitial fluid. As burial proceeds, the increase in temperature and accompanying fluid flow will promote chemical reactions involving dissolution of some detrital minerals (particularly clays and feldspar) and precipitation of new minerals.  Thus, the matrix gradually changes from purely detrital to a mix of detrital components and diagenetic products. At some point in this transformation there may be no recognisable detrital matrix. Continue reading

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