Field Techniques Lecture 8 PDF

'old chisels • large and small C 1 notebook and pencil magnifying 1.ens compass newspapers labels colour for vegetation or to distinguish features orientation J clear- - - - 'Vs )i title \.;;_J c-::r_ l 'P "'' :,,.[~, \. simple -._:__ Q ornament ~\. lo help define lhe _;,.-...

'old chisels • large and small C 1 notebook and pencil magnifying 1.ens compass newspapers labels colour for vegetation or to distinguish features orientation J clear- - - - 'Vs )i title \.;;_J c-::r_ l 'P "'' :,,.[~, \. simple -._:__ Q ornament ~\. lo help define lhe _;,.- unit _ _. / ' 0---:.- -_ I ~ so they are not obscuring the Sk.elch ~ -- -shading to show darke coloured re -:-. ~~ ... ~ minimal labels positioned layers ) B small / ' - .,, - - ~~ amount of interpretation use of different ~ne weights I edge of sketch clearly defined \\ \ inset box to show position of higher resolution sketches or position Distinguishing foliations Spaoed Form of planes? Are matrix ,grains visible to naked eye? Mierofuld limbs Oissolunon seams RENULATI0N CLEAVAGE PRESSURE SOLUTION CLEAVAGE Fractures MY LONITE ,! Yes DISJUNCTIVE FOLIATION No I SLATY CLEAVAGE Pressure solution d @avage Upperr crust, outer zonGS of mountain belts Dissolution of soluble groins du& to directed stress Dork/ pale colour striping; partially dissolved fossi Is, dasts; stylolitic surfaces Slaty d0CJVoge Upperr crust, outer zonGS of mountain belts ; fi~rained rocks Alignment of platy grains by rotation, dissolution and recrystallizatioo during1 applied stress Fine fabric that rock deaves along; typically associated Fracture, d eovag,g Upper crust, outer zones of mountain belts; competent rocks Tensioool failure under high fluid pressure* in competent rock types Spaced crocb in competent rock lype Mylon itic foliation High strain faults and shear zones at all but shallowest depths Extr8me flattening and sirefching in narrow, high-strain zones of shearing Strongly planar fabric; other high-strain feature,s [see text for examples! Schistosity Middle crust, inner Mineral alignment under applied stress, during metamorphic crystallization Visibl-G mineral grojns; millimetre-, to centimetre-,scole folio, rougher than sloty zones of mountain befts; metamorpho~ rocks w ith folds deovoge Crenulation d @ovage Middle crust, inner zones of mountain betts; metomorphos;e,d rocks Microfolding of a preexisting planar fabric (tecfoo ic or sooi mentary) Microfold hinges, crenulotion ltneo:tion (Section 8.3.51 •Many of t'he rnecho ni!,ms fur cleavage formotion (e~ ially fracture cleavage) ore o matter of debate. tong sight htnge dinometer declination adjustment screw (a) clinometer arm clinometer scale magnetic needle aZimuth scale ~..-- pin to lock magnetic needle locl< for (c) adjuster for dinometer arm (arro1NS show movement) compass needle (arrow shows movement) Figure 2.3 Labelled photographs of the ports of two of the most commonly used types of compass-clinometer. These terms are refer.red lo in the text and in other figures. {a}-lcl The Brunton-type compass-clinometer: in this case the Brunton Geo. Views: {a) side; (bl top; (cl bottom. Field Equipments ► Compass-clinometer • Silva - type 1. Scales I Rulers - - - - - - - - - - r --:.,,- r - - - - 7 2. Direction of Travel - - - - - , 3. Orienting Arrow - - - - , 4. Declination Scale- - , /'l 'l~ /'l/ 1/,y;>'"V''l/ 5. Dial with Degrees (part of housing) - ~r-- -7. Magnetic Needle tt'lll--- - - 8. Orienting Lines r-- - - - - 9. Rotating Housing . - - - - - - - - 10. Base Plate www.WlldernessCamping.com Sampling ► Sample for thin section • A sample of about 10 cm thick x 5 cm x 5 cm is usually sufficient for producing one or more thin - sections provided the rock is not too coarse grained. If the rock is coarse - grained then a sample size that is appropriate to the grain size should be taken. The sample needs to be large enough to enable it to be clamped in the rock saw and care needs to be taken with sample position and shape if it needs to be orientated. In deformed rocks it is useful to cut two or even three thin - sections at right angles to each other, so a slightly larger sample may be required. Sampling ► Samples for geochemical analysis • A 200 g sample is usually sufficient for a range of major and minor element, trace element and isotopic analyses, except for very coarse grained or heterogeneous rocks, where about 1 kg may be required. • Particular care needs to be taken to ensure that the sample is fresh. This might mean that weathered material needs to be removed first. If possible it is better to remove the weathered material in the field to ensure that what you take back is a fresh sample, and also because it is easier to allow for the extent of the weathering in the field. • Colour changes are a good indication of weathering, but also the fracture pattern and hardness of the rock tend to change. • In addition some rocks contain minerals that are the product of weathering (e.g. weathered mudstones often develop gypsum crystals). Some caution may also need to be exercised if the sample is being analysed for metal isotopes in order to avoid contamination from metal chisels and hammers. Sampling ► Samples for mineral extraction • The amount of sample that needs to be collected for mineral analysis depends on the minerals to be extracted and the composition of the rock. • For heavy minerals such as zircon that are present only in low abundance, 1 - 2 kg of rock may be required. • For volcanic glass and feldspars for ,,Ar - ,,Ar and K- Ar dating, generally at least 1 kg of rock is required. • For pyroclastic and sedimentary rocks it is particularly important to collect from the base of the bed as the coarser - grained minerals that are more easily picked out will be most abundant there. Sampling ► Samples for fossils • Samples for megafossil analysis: • Megafossil samples and their supporting rock material tend to be large. They are best wrapped in paper. If they are fragile or the rock needs to dry out slowly, wrap them in cling fi Im/plastic food wrap and then paper. They commonly have commercial value and therefore special permission may be needed to remove and/or ship them out of the locality or country where they were collected. Important specimens should be donated to a museum after analysis and their sample (or museum acquisition) number referred to in any subsequent publication. Sampling ► Samples for fossils • Samples for microfossil analysis: • The size of samples for microfossil analysis depends on the likely abundance of fossils within the sample. • For an average abundance of foraminifers 200 g should be sufficient; for nanofossils and diatoms a small 10 g sample is ample. For palynology samples of 0.5-1 kg are required. • Samples for molecular fossils: • Very small samples are required for molecular fossils but the sample needs to be large enough (2 - 5 g) to avoid contamination. • The samples should be packaged in metal foil or glass or polythene bags of known composition. • In addition care needs to be taken not to cross - contaminate samples by carefully cleaning the sampling instruments between each sample. Sampling ► Sampling for regional studies • Regional and low - resolution studies require samples that are representative of the whole area. In addition samples should be collected from units that need laboratory analysis because their composition is enigmatic. It is difficult to generalize here though, as the sampling strategy will need to be adapted according to the hypothesis being tested. Collecting a sample from each of the main lithostratigraphic units is probably a good place to start. If the region is fairly monolithic (for instance a large igneous province) then take regularly spaced samples that are spatially separated both horizontally and vertically, in addition to other key features such as potential feeder dykes. Other samples that are worth considering are a set to test for compositional variation across an igneous body, representative fossils, tectonic contacts, representative samples from metamorphic zones and representative samples from a set of dykes. Sampling ► Labeling • On the sample bag or outside of the packing material itself it is useful to record the following as appropriate: • • • • • • sample number; cross reference to field notebook page and/or date collected; location; whether or not the sample is orientated; rock type; associated samples. Area-Date-Sample Number MNL-02032018-00l(A) References ► Coe, A. (2010). Geological Field Techniques. 9600 Garsington Road, Oxford: Wiley-Blackwell A John Wiley & Sons, Ltd., Publication.