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    Sedimentary environments

DIAGRAM - SEDIMENTARY ENVIRONMENTS

                     Continental

    Eolian Deserts (Dune fields); Fluvial (Rivers); Lacustrine (Lakes); Glacial (Glaciers)

             Marginal - Marine

     Estuaries (inlet of the sea reaching into a river valley);
    Tidal Flats(low relief, low wave energy; subtidal, intertidal, supratidal salt marsh flooded by Spring or storm tides);
    Lagoons (isolated or partly isolated stretch of coastal seawater - fresh water, brackish, sea-water, hyperhaline zones);
    Deltas; Beach and Barrier Islands; (Cross-beds)

                          Marine

    Neritic (shallow water areas shoreward of the shelf break at depths of 18 to 915 m; ave - 125 m; pericontinental/marginal (east coast of North America); epicontinental/epeiric (Hudson's bay); benthonic faunas)
                -  inner shelf dominated by tidal, wind, and storm wave processes;
             - outer shelf affected by intruding major ocean currents (e.g. Labrador current);
                -  Reefs (Great Barrier reef of Australia).

    Oceanic
                - slope; (Debris flows)
                - rise; (Proximal turbidites)
                - deep abyssal plains (pelagic suspended particles; oozes) (Graded beds.)

    The sediment deposited in each of these environments exhibits characteristic structures (e.g. cross bedding), textures (e.g. grain size), and mineralogical and chemical compositions (e.g. carbonate, quartzite) that provide a 'fingerprint' for that environment.

    Sedimentary Structures

    Beddding is the most characteristic feature of sedimentary rocks. Nevertheless, the form and internal structure of the beds may be quite different depending on the environment of deposition, e.g. cross bedding in the case of shallow water deposition and graded bedding (Bouma sequence) in the case of deep water deposition. Other structures include dunes and ripple marks; mud cracks; slump folds; dish structures and sand volcanoes;load casts, 'ball and pillow' / flame structures, and sandstone dikes;   stromatolitic lamination; scour and fill structures, sole markings, and flute clasts;bioturbation, trace fossil burrows and  trails.

    Sedimentary Composition

    The principal constituents of sedimentary rocks are:

    1) terrigenous grains - representing weathering residues (quartz, feldspar, rock fragments)

    2) pyroclastic particles - from volcanic explosion

    3) secondary minerals - produced at the weathering site (clay, iron oxides; secondary quartz);

    Plagioclase breaks down to form the clay mineral kaolinite - if the plagioclase is associated with basaltic rocks, the clay mineral will be smectite ( = kaolinite plus Mg [Mg-Al coupled substitution for Al-Si]), whereas if the the plagioclase is associated with granite the clay mineral will be illite ( = kaolinite plus K [K-Al substitution for Si]). K-feldspar also breaks down to kaolinite/illite but is in general more resistant to weathering than plagioclase. Ferrous iron in olivine and pyroxenes is oxydized to ferric iron and precipitates as haematite or limonite. The Ca and Mg in olivine and pyroxene, minerals which are unstable and break down easily (thet are said to be labile), and Ca, Na, and K in feldspars mostly enter into solution and are carried to the sea by rivers. Si in riverine waters is soluble as silicic acid H4SiO4.

    4) chemical and biochemical precipitates - as single crystals (carbonates, phosphorites, evaporites, BIF) or as composite gains of precipitated material and oolites, pellets, fossil shells and limeclasts.

    5) organic residues - derived from the partial decomposition of plant and animal matter ( oil, coal).

    On the basis of the dominant presence of one of these types of material, sedimentary rocks are subdivided into three major groups :

    Clastic rocks formed by the deposition of material derived from the weathering of pre-existing rocks (rock fragments, quartz, feldspars, clay minerals).

                    a) shallow  water environments

                    b) deep  water environments

    Chemical sediments formed by precipitation from aqueous solutions:

                 a) carbonates

                   b) cherts

                    c) iron formations

                    d) evaporites and phosphates

    Carbonaceous rocks (peat, lignite, coal) formed from decayed vegetation

       CLASTIC ROCKS

    Clastic rocks are usually composed of a framework of terrigenous grains, and a matrix of finer grained material (clay) formed either by a) weathering, b)  in situ chemical precipitation of cements (quartz, calcite), and/or c)  the breakdown of labile (unstable) components (basalt; mafic minerals) to form new authigenic material (chlorite, clay, iron oxides, pyrite). Rocks with a high proportion of quartz to labile grains are considered to be compositionaly mature. On the other hand, if the grains are also well rounded and smooth the rocks are considered to be texturally supermature. (Immature - poorly sorted with much clay; submature - not well sorted; mature - well sorted and little clay, but grains not rounded; supermature - well sorted with little clay; grains are rounded).

    The variation in grain size of clastic sedimentary rocks forms the basis for their subdivision into conglomerates, sandstones, and shales. Within each of these major categories, the rocks are further subdivided according to their composition and texture.

CONGLOMERATES AND BRECCIAS
 
 

Epiclastic

Extra- formational

Orthoconglomerates   matrix < 15%       ----------------------     Paraconglomerates or   diamictites   matrix 15

Metastable <10%   orthoquartzitic   (Oligomict)   conglomerate   ---------------------   Metastable 10%   (Petromict)   conglomerate   --------------------   laminated matrix;   laminated conglo-    meratic mudstone   ---------------------   non-laminated matrix   tillite (glacial)   tilloid (non-glacial)

('Extraformational' implies derivation from outside the basin of deposition; 'metastable' means
clasts of relatively unstable material such as gabbro, granite, amphibolite; basalt).
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                        Intrafor-               Intraformational conglomerates
                        mational                and breccias
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Pyroclastic             Volcanic breccias and agglomerates
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                               Landslide and slump breccias
Cataclastic             Fault and fold breccias; "tectonic moraines"
                              Collapse and solution breccias
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Meteorite               Impact breccias
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SANDSTONES

SHALE

Def. shale = more than 50% grains less than 0.062 mm ( 0.062 mm = 1/16th inch = fine grained sandstone)

Non-indurated Beds          Bedded          Bedded           Bedded
                        > 1 cm       silt                  mud                claymud

                        Laminae     Laminated     Laminated     Laminated
                        < 1 cm         silt                  mud                claymud

Indurated         Beds          Bedded          Mudstone       Claystone
                       > 1 cm        siltstone

                         Laminae    Laminated     Mudshale       Clayshale
                        < 1 cm         siltstone

Metamorphosed                          Quartz         |              Argillite
                                                      argillite       |
                                                     Quartz         |              Slate
                                                      slate            |
                                                                          |              Phyllite
                                                                  Mica schist

     RELATIVELY PURE LIMESTONES

  Limestones are essentially monomineralic rocks composed of calcite in the form of carbonate grains (allochems) made of clasts, skeletal particles, oolites, peloids, and lumps; microcrystalline carbonate matrix mud called micrite; and recrystallized coarse grained translucent carbonate called sparry calcite. The following is a classification based on whether the carbonate grains were (as would be the case in a reef or stromatolite bed) or were not organically bound together during deposition; the clast size; and the proportion of micrite.

    1. Original components not organically bound during deposition
                 Finer                                  |     Coarser
Of the allochems                                        |Of the allochems 
less than 10%  2 mm                                     |more than 10%  2 mm

        Contains carbonate mud  |Mud absent             |Matrix         |Grain
     (particles < 0.03 mm diam.)|                       |supported      |supported
        Mud supported           |Grain supported        |               |
Less than       |More than      |                       |               |
10% grains      |10% grains     |                       |               |
                |               |                       |               |
Mudstone        |Wackestone     |Packstone   |Grainstone|Floatstone     |Rudstone
(The term 'Allochem' refers to carbonate grains that have undergone mechanical transport.)

    2. Original components organically bound during deposition
                               Boundstones
|Organism acting as     |Organism encrusting    |Organisms building
|baffles                |and binding            |a rigid framework
|Bafflestone            |Bindstone              |Framestone

    Dolomites or dolostones are carbonate rocks in which calcium has been partly replaced by Mg. Dolomitization is a syn-sedimentation or diagenetic phenomena that takes place in the near-shore environment. Dolomites tend to weather to an orange colour.

EVAPORITES

Marine evaporites are dominated by the presence of gypsum and anhydrite, rock salt (halite), potassium salts, and magnesium sulphate. On the other hand continental evaporites are characterised by the presence of hydrated sodium bicarbonate, sodium sulphates and borax.

    SOURCE MATERIAL
 

Relevant sections in Boggs, Jr., S. 1987. Principles of Sedimentology and Stratigraphy

3). Transport and deposition of Siliciclastic Sediment

4) Deposition of Nonsiliciclastic Sediments - dolomite p. 82; Sabkha p. 85; evaporites 86; cherts p. 90; iron p. 94; phophorites p. 98.

5) Sedimentary Textures - grain size

6) Sedimentary Structures

7) Composition and classification of Siliciclastic Sedimentary rocks- sandstones p. 200; conglomerates 210; shale p. 213

8) Carbonates - classification p. 227; evaporites p. 234; phosphorites p. 248.

9) Lithification and diagenesis - authigenesis p. 273

10) Principles of Environmental Interpretation and Classification

11) Continental environments

Fluvial; Eolian Desert systems; Lacustrine; Glacial

12) Marginal - Marine Environments

Deltas; Beach and Barrier Island systems; Estuarine and Lagoonal systems; Tidal Flat systems.

13) The Marine Environment

Neritic, Shelf, Reefs; Oceanic; kinds p. 502; ancient p. 513

FIGURES

Structural Provinces of North America.

Sedimentary environments

Classification of sandstones.

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