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Geology 106 Laboratory

Studies in the geologic history of deep ocean basins

Sedimentary Rocks and Sedimentary Rock Classification

Sedimentary rocks are highly varied and include several major types that form in different settings. These major types are the:

Rocks of the conglomerate-shale siliciclastic series, carbonate series, and evaporite group are composed of minerals listed in Unit 1.
Volcaniclastic rocks are composed primarily of fragments of volcanic glass and rocks.
Organic deposits (peat, coal) are composed of the fragmentary remains of plants.
Diagenetic rocks (including concretions) are composed of minerals that form within sediments, especially as cements that fill pore spaces between grains. Common cements include calcite, quartz, and clay. Less common cements include hematite (limonite), pyrite, and apatite.

Compositional classification of siliciclastic sediments

In this classification, composition is plotted in terms of the percentage of three categories of grains: 1) quartz, 2) feldspar (all varieties), and 3) polycrystalline grains called lithic rock fragments. For visual display, the percentages are plotted on a ternary diagram, which has three scales of 0% to 100% that can be fit into the three corners of the diagram. A simplified classification is shown here, containing only three fields and omitting transitional categories. Because the mineral quartz is the dominant mineral in most sands, for a sand to be called a quartzose sand it most be composed of 90% or more quartz. If the non-quartz fraction contains more feldspar grains than rock fragment grains it is called a feldspathic sandstone (or arkose) and if it contains more rock fragment grains than feldspar grains it is called a lithic sandstone (or wacke).

Compositional classification of carbonate sediments.

Carbonate sediments have a smaller range in mineralogic composition, so are classified differently from siliciclastic sediments. The main components of carbonate sediment are 1) grains (mostly skeletal bioclasts—pieces of shell), 2) carbonate mud (micrite), which is also produced by organisms, and 3) cement, which forms after the grains and mud are deposited. Other distinctive types of carbonate sediment are composed of non-biogenic carbonate grains that form in special environments (ooids—-grains formed of concentric layers of carbonate formed when waters precipitate carbonate under conditions of regular agitation; intraclasts—pieces of carbonate mud broken up and moved by storm waves or strong water currents). Examples of carbonate sediments containing bioclasts and ooids are included in the lab material; examine them to see the nature of loose, unconsolidated carbonate sediment.

Textural classification of carbonate sediments

This classification uses different criteria for boundaries between different rock types (mud content vs. grain content; mud support vs. grain support of grains; mud filling of pore space vs. cement filling; loose accumulation vs. intergrown nature of skeletons). Thus, there is a genetic component to the classification of carbonate sediments, but an interpretation of the texture is required to properly classify deposits. Packstone and grainstone both consist mostly of carbonate grains, but differ in that packstone has a carbonate mud matrix while grainstone has carbonate cement between the grains. It is difficult to consistently distinguish between wackestone and packstone and is not important to make the distinction, so lump these two rock types together as wackestone-packstone.

Textural characteristics of sediments

The primary textural features of sediments are 1) the size of grains, 2) sorting of the sediment, and 3) the roundness and sphericity of the grains. Use the Wentworth grain size scale to describe sediment grain size; use terms of poorly sorted, moderately sorted, and well sorted to describe sorting. The sphericity of grains is not important for this exercise.

Bedding characters of sediments

Sedimentary deposits occur in beds, intervals of similar sediment type which are bounded by distinct surfaces. Within a bed, most sediments show finer scale layering called lamination—produced in response to small variations in energy of the currents during deposition. Sands and well sorted sediments are nearly always laminated when deposited. As sediment particles are moved over a surface at relatively low velocities (lower flow regime), they may move as a migrating dune or ripple, with a depositional surface that is not horizontal. This produces cross-bedding, lamination which is inclined to the bed surfaces at angles up to 30°-35°, the maximum angle of stability for a pile of loose grains. Another commonly observed type of bedding is wavy bedding, produced variously by irregular flow conditions, oscillating flow directions, or by relatively high velocity flow (upper flow regime) conditions. Sediments which show no noticable layering or lamination are simply described as being massive. Large scale wavy bedding may be produced by large storm waves, generating a bedding type called hummocky cross-bedding.

Depositional environments

Sediments are deposited under a variety of environmental conditions. Although there is a continuum of depositional environments from mountain slope to ocean deep, characteristic groups of sediments are deposited in the distinct depositional environments listed below. Each of these environments is characterized by physical conditions and depositional processes specific to that site, that produce a distinctive suite of sediment deposits and sedimentary structures in the sediments. These deposits also contain fossils of organisms restricted to that environment.

The progression of depositional environments from mountain front to deep basin can be readily divided into fairly distinctive settings. Those are: alluvial fan, braided stream, meandering stream, swamp, beach, open shelf, reef, submarine fan, and abyssal plains. Illustrations of the suite of sediments deposited in each of these are shown below. Obviously, depositional conditions vary within an environment and the suite of sediments can vary or be incompletely developed, but where the sediment suite is present it provides a reliable indicator of depositional environment. An extended list of depositional environments, with brief comments on depositional processes and sedimentary deposits is appended at the end of this exercise. A schematic illustration of the progression of depositional environments from sourceland to deep marine basin is shown below.

Primary sedimentary structures

These are produced by water or air currents at the time of deposition. A series of distinctive sedimentary structures is produced under conditions of increasing energy of flow by unidirectional currents (cross-lamination, cross-bedding, planar lamination). Other primary structures are produced by oscillatory motion of waves in standing water (oscillation ripples, hummocky cross stratification), and scour and winnowing of sediments (flute marks, gutter casts, imbrication of pebbles).

Secondary sedimentary structures

These are produced soon after deposition, when the sedimentary layer has been covered with other sediment. These include deformational structures (convolute bedding, load structures, ball and pillow). Load structures and ball & pillow structure are the result of density differences in the sediment, when heavier sediments (sands) sink down into lighter sediments (watery clay-rich mud). Convolute bedding develops when fluids rise upwards through overlying sediments, distorting them, or when the upper levels of a sediment pile shift relative to lower levels (slumping). Bioturbation (biogenic disturbance by organisms) also produces secondary structures (trace fossils, root traces), by animals living in sediments or plants growing on soil.



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