The Field Description of Igneous Rocks Richard Thorpe & Geoff Brown Sedimentary Rocks in the Field Third Edition Maurice Tucker. At the outset, this book describes how the characteristics of sedimentary rocks can be recorded in the field, particularly through the construction of graphic logs.
ACKNOWLEDGEMENTS
INTRODUCTION
- Tools of the Trade
- Other Tools for the Field
- Use of GPS in Sedimentary Studies
- Safety in the Field and General Guidance for Fieldwork Working in the field should be a safe, enjoyable and very rewarding experi-
A compass usually has a millimeter – centimeter scale which can be useful for measuring the size of small objects such as pebbles and fossils. Epoxy-resin cloth peels can be made in the field from vertical cuts through soft sandy sediments.
FIELD TECHNIQUES
What to Look for
Examine the texture of the rock: grain size, shape and roundness, sorting, fabric and color; see Chapter 4. Look for sedimentary structures on and below layers and within layers; see Chapter 5.
The Approach
Record information about place and succession with notes and sketches in a field notebook and photographs; if appropriate, make a graphic journal; if the rocks are folded, check the path of the layers upwards; see Chapter 2. Consider, perhaps later, lithofacies, cycles, sequences, depositional processes, environmental interpretations, and paleogeography; see Chapter 8.
Field Notes
If so, it is best to record the continuity in the form of a graphic log (Section 2.4). If the exposure is not good enough for a log, then field notebook notes and sketches will have to suffice.
Graphic Logs
Ornament for the lithology and/or sedimentary structures can be added to this texture column. If fossils make up much of the rock (as in some limestones), then the symbol(s) of the major group(s) can be used in the lithology column.
The Logging of Cores
This can be used for particular properties of the bedrock or rock unit, such as degree of weathering (see Section 4.7) and presence of authigenic minerals (pyrite, glauconite, etc.) and for supplementary data on sedimentary structures, texture or lithology. It is often very useful if the wireline logs are also available for the well and can be viewed alongside the core itself or the core log.
Lithofacies Codes
In carbonates (Table 2.4), the initials of grainstone (G), packstone (P), wackestone (W), mudstone (M), boundstone (B), etc. may be used together with appropriate qualifiers such as s (stromatolitic), f (fenestral), o (ewe eel), c (coral), q (quartzite), etc. The lithofacies code approach can be useful when you need to document a very thick sequence of sediments.
Collecting Specimens
Specimens can be collected for the extraction of microfossils, such as foraminifera from Mesozoic-Cainozoic mudrocks and conodonts from Paleozoic limestones. Many fossils can be sufficiently identified in the field if research focuses on sedimentology and paleoenvironment, and do not need to be removed.
Presentation of Results
Maps showing the distribution of lithofacies of laterally equivalent layers over an area can be very useful. Statistical analysis can be performed on bed thickness data, paleocurrent data, and other measurements.
The Way-Up of Sedimentary Strata
Graded litter: coarser grains at the bottom of the bed (although consider the possibility of reverse grading, especially in conglomerates and very coarse sandstones); see Fig. Geopetal structures in limestone: internal sediment in the lower part and calcite cement in the upper part of the cavity; see figures 5.39 and 5.40.
Stratigraphic Practice
Apart from the sequence boundary (sb), other key surfaces are the transgressive surface (ts), which may coincide with the sequence boundary in more proximal (landward) parts of a basin, at the base of the TST, and the maximum flood surface (mfs) which separates the TST from the HST (see Figs. 2.6 and 2.7). In more distal parts of the basin there is usually condensed section (cs) equal to the.
SEDIMENTARY ROCK TYPES
Principal Lithological Groups
Limestone (section 3.5) is composed of more than 50% CaCO3, so the standard test is to use dilute hydrochloric acid (HCl); the rock will. Cherts (Section 3.8) are mostly cryptocrystalline to microcrystalline siliceous rocks that occur as very hard-bedded units or nodules in other lithologies (especially limestone).
Sandstones
Used to give a rough estimate of the percentage of grains or bioclasts (fossils) or crystals (etc.) present in the rock. Note the light reflected from some grain surfaces; these are crystal planes of the overgrowth cement.
Conglomerates and Breccias
Broadly speaking, the composition of sandstones is related to the plate tectonics of the depositional basin. For interpretations of the depositional mechanisms of sedimentary rocks with pebbles and boulders, the texture is important: clast-supported conglomerates (so-called orthoconglomerates) must be distinguished from matrix-supported conglomerates (paraconglomerates, also called diamictites or diamicton when uncemented) (see section 4.4). .
Mudrocks
Some specific types of breccia include collapsing breccia, formed by the dissolution of limestone as in a karstic breccia or evaporite (see Sections 4.6 and 5.4.1.5), meteorite impact breccia, volcanic breccia (see Section 3.11), and tectonic breccia. Note the color, degree of fissibility, sedimentary structures, and mineral, organic, or fossil content (Table 3.2).
Limestones
Carbonate skeletons have different original mineralogies and the preservation of bioclasts in limestone depends on this. Most dolomites, especially those of the Phanerozoic, were formed by the replacement of limestones.
Evaporites
Anhydrite and gypsum nodules can be replaced by a variety of minerals, particularly calcite, quartz and dolomite. The exterior of such joints usually has a cauliflower-like appearance (Fig. 3.19). Geodesms can form where crystals, especially of calcite.
Ironstones
It usually has a boxy texture, from the dissolution of evaporite and other clasts, and is yellow/buff/cream in color. The French term cargneule (there are other pronunciations) is often used for this particular tectono-sedimentary rock.
Chemical iron-rich sediments
Cherty iron-formation: iron minerals include hematite, magnetite, siderite, commonly in a fine lamination alternating with
Ironstone: textures similar to limestone with oolitic varieties typical; iron minerals include chamosite-berthierine, goethite,
- Iron-rich mudrocks
Pyritic mudrocks: pyritic nodules and laminae, often in black or bituminous shales, usually marine
Sideritic mudrocks: mostly nodules in organic-rich mudrocks;
Iron-rich laterites and soils: often developed at unconformities, on lavas
Bog-iron ores: rarely preserved in rock record D: Placer deposits, especially with magnetite and ilmenite
- Cherts
- Sedimentary Phosphate Deposits (Phosphorites)
- Organic-Rich Deposits
- Volcaniclastic Deposits
Coarse lithic clasts at the base may be commonly graded (size decreasing upward), while large pumice clasts (which are very light at the time of eruption) may have the reverse grade (size increasing upward) or be concentrated at the top of the layer. The basal-naval tuff occurs in the upper part and shows well-developed layering (antidyne cross-layering) with a rightward flow. There may be an increase (or decrease) in the proportion of non-volcanic inputs.
SEDIMENTARY ROCK TEXTURE
- Introduction
- Sediment Grain Size and Sorting
- Grain-size, sorting and size-grading
- Morphology of constituent grains
- Fabric
- Grain Morphology
- Sediment Fabric
- Textural Maturity
- Texture of Conglomerates and Breccias
- Induration and Degree of Weathering
- Colour of Sedimentary Rocks Colour can give useful information on
Pebble shape is primarily a reflection of composition and any degree of weakness, such as bedding/lamination, splitting or jointing in rock. An assessment of the textural maturity of a sandstone can be made in the field by close examination with a hand lens. The latter can give an indication of the composition of the rock, for example, in terms of iron content.
SEDIMENTARY STRUCTURES AND GEOMETRY OF SEDIMENTARY DEPOSITS
Introduction
Erosional Structures
Shrinkage cracks: Section 5.3.6 Structures confined to or predominant in limestone Hollow structures: usually filled with calcite: Section 5.4.1. Groove/trough castings indicate the trend of the flow and their orientation should be measured (Chapter 7). At fluvial, deltaic and tidal channels, check for signs of lateral accretion (section, i.e. low angle sloping surfaces, indicating lateral migration (meandering) of the channel (Fig. 5.32).
Depositional Structures
The crests of wave-shaped waves are generally straight and bifurcation of crests is common (Figure 5.9); the. Flaser bedding is where cross-laminated sand contains streaks of mud, usually in ripple troughs (Figure 5.21). Antidyne cross-strata are known from turbidites and fluvial sandstones (but are very rare) and pyroclastic waves (Fig. 3.26).
Depositional Structures of Limestones (Including Dolomites) This section describes sedimentary structures which are more commonly
These will normally occur on top of the limestone bed, although they may be cut by the dissolution surface. Elongated cavities (laminoid fenestrae, Section 5.4.1.2) are common in microbial laminites and thin grainstone beds (possibly of storm origin) may be interbedded. The laminae may be asymmetric and discontinuous in growth on the top of the ball while quiescent.
Post-Depositional Sedimentary Structures
A further special type of nodule is a composite of calcite exhibiting cone-in-cone structure (Fig. 5.59). As a result of the usual early diagenetic origin, lamination in the host muddy sediment is deflected around the nodules (Fig. 5.60) and the nodules may retain the original lamina thickness. The amount of compression that has taken place can be inferred from these nodes (Fig. 5.60).
Biogenic Sedimentary Structures
Vertebrates such as reptiles (especially the dinosaurs), amphibians and mammals leave footprints as trace fossils. Feeding structures are trace fossils developed in the sediment by sediment-feeding organisms searching for food. Some trace fossils (footprints, U-shaped and escape pits) can be used to show the way up layers.
Geometry of Sedimentary Deposits and Lateral Facies Changes
To ensure that sections are equivalent, it is necessary to have either a laterally continuous horizon in the succession (a marker bed), or the presence of the same zonal fossils. The nature of the overlapping surface itself (which may be an anomaly) may also change laterally; for example, it may become a more prominent paleokarst toward onlap. This would reflect the longer time for erosion of the rocks beneath the overlapping unit.
FOSSILS IN THE FIELD
- Introduction
- Distribution of fossils in sediment 1. Fossils largely in growth position
- Fossil assemblages and diversity
- Diagenesis of fossil skeletons
- Fossil Distribution and Preservation
- Fossil Associations and Diversity .1 Fossil assemblages
- Skeletal Diagenesis
Is the fossil assemblage the same in all layers of the section or are several different assemblages present. First, qualitatively determine the fossil assemblage by estimating the relative abundance of different fossil groups. However, some mortal assemblages consist of the skeletons of organisms that lived in the same general area.
PALAEOCURRENT ANALYSIS
- Introduction
- Palaeocurrent Measurements
- Structures for Palaeocurrent Measurement .1 Cross-bedding
- Interpretation of the Palaeocurrent Pattern
First measure the dip (or strike) direction and dip angle of the bed surface (or bottom surface); then measure the acute angle between the extension of the structure and the strike of the bed (this is the height or grade of the structure) and note the direction of dip of the structure. First measure the dip (or strike) direction and dip angle of the bed surface, and then the dip (or strike) direction and dip angle of the cross bed. The asymmetry of the corrugations (downstream steeper side) and dip direction of the cross sheets are easily measured.
