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Geologic maps graphically communicate vast amounts of geologic information. A geologic map representents the projection on a flat piece of paper of the intersection between geological 3D features with the surface topography with the added benefit of depicting the relative age, composition, and relationships among rocks and sediments at and near the earth’s surface.
A detailed geologic map shows what it is you are standing on; where similar rocks or sediments may be found; how old they are; what they are composed of; how they formed; how they have been affected by faulting, folding, or other geologic processes; and what existing or potential mineral resources and geologic hazards are nearby.
Geologic information shown on maps is necessary for countless reasons, from finding natural resources (water, minerals, oil and gas) to evaluating potential hazards (earthquakes, landslides, floods, volcanic eruptions) to describing a fundamental part of the environment that controls distribution of plants and animals. General purpose geologic maps address all of these themes, however, your task is to create a geological map showing lithology and structure.
Our field mapping project will introduce basic field techniques including: use of geological compass, and recording of field notes and field data. You will start by learning the local stratigraphy and how to identify the units you will encounter throughout your mapping exercises.
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How will you make your Geologic Map?
You will roam the landscape (namely up and down numerous huge mountains, but commonly along section perpendicular to the strike of major structures), plotting the location of geologic contacts, faults, shear zones, folds, and other features on aerial photographs, that you use as a base map. You will make structural measurements using a geological compass and take copious field notes and drawing numerous sketches (2D and 3D) as you go. In the evenings, you will tidy your maps and plan your route for the next day.
Geologic maps normally include cross sections or block diagrams that reveal the structure or arrangement of rocks below the Earth’s surface. Such diagrams give map users a glimpse below the ground surface and a better understanding of the three-dimensional arrangement of the rocks. Understanding this third dimension is particularly important for the discovery and assessment of mineral and energy resources. You are required to make a geological cross-section as part of your assessment. We suggest that you make rough cross-sections each evening to help you grasp the structure as you map.
In a lithologic key, geologic units are placed in stratigraphic (age) order--youngest at top (left) and oldest at bottom (right). The composition of each unit is summarized in the legend captions. Closely related geologic units have similar colors--yellow and gold for unconsolidated Quaternary and Tertiary sediments, purple for mafic igneous rocks and pinks/reds for felsic igneous rocks, blues for limestone, and brighter colours for metamorphic rocks.
Structural symbols for folds and faults are especially important elements of surficial geologic maps.
Sedimentation & Stratigraphy: We study sedimentary rocks in Proterozoic strata, all of them moderately to strongly metamorphosed. We instruct you in the methods of facies analysis, and you apply these methods by recording bed shapes, sedimentary structures, textures, and mineralogy of the formations to identify younging directions and to interpret environments of deposition.
Structural Geology: Our exercises in structural geology are done in at least two different areas that are gradational in their complexity of folding, faulting and tectonic history. These exercises are designed to give you the background you need to develop three-dimensional perspectives. For more information, see the section on Field Exercises.
Igneous Geology: Both mafic and felsic igneous and meta-igneous rocks may be found throughout the mapping area in the form of dykes and plutons. We will examine their mineralogy and timing with respect to deformation and metamorphism.
Metamorphic Geology: Most of the mapping area is metamorphosed up to amphibolite grade. We will help you identify key metamorphic minerals and relate these back to the protolith.
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The ant, the geologist and the concept of tectono-lithostraticgraphic unit.
At the scale of the ant all what is to be seen of a Claude Monet painting is what appears to be randonly distributed colored dots. To get an overall understanding of the impressionist work the ant would have to map the distribution of colors. To make this work possible the ant would have to use a much reduced palet of color than the one used by the artist. A few uniform blue will be use to map the sky, a few uniform greens for grassy area, etc. At the end of this process the ant will have a sketchy but accurate representation of the chef-d'oeuvre it is roaming on.
Indeed the real thing is much nicer...
In the field, a geologist is in a situation similar that the one experienced by our little ant. As she walks across mountains and valleys the geologist experience first hand the contrast between the human scale and that of the region she tries to understand. Only in mapping the distribution of rock formation would the 3D geological structure be revealed to be cartographer.
For convenience, and to make the mapping duable, rock formations are assembled into tectono-lithostratigraphic units. A tectono-lithostratigraphic unit is a package of rock that share the same origin and have the same tectono-metamorphic history. Hence, the boundary between different units can be an unconformity, a fault or shear zone, or a bedding plane across which the sedimentary caracteristics or the composition changes significantly. Without the the concept of lithostratigraphic units mapping would be an extremely teneous and time consuming work as lithologies can change over the centimeteric scale.
In each unit the cartographer must decide which, if any, rock formations will be mapped. A balance must be achieve between the time allocated for the mapping, the scale of the map and the map resolution. A color will be assigned to each tectono-lithostratigraphic unit according to standard geological color scheme. Various shade of the same color can be used to decipher particular rock formations inside each unit.
The cartographer must focuss her attention in mapping the boundaries between units and boundaries between formations. One methodology consists in identifying the main boundaries and to follow them in the field and plotting their location and other characteristics on a base-map.
The Concepts of Form-Lines and Form-Surface Mapping.
In the field, you may locally be able to spot the intersection of prominent beds with the surface topography. This intersection appears to draw lines that run more or less parallel to each others. These lines are called form-lines and can also be mapped, a technique called form-surface mapping. A good trick is to use fine darker lines drawn on top of a more pale color that represents the lithostratigraphic unit. Form-lines help the geologist to visualize the structure at the scale of the landscape and help them to identify key area such as large scale fold hinges, unconformities etc. Form lines can be seen on aerial photographs, therefore form lines seen in the field can be extrapolated relatively easily to a large area. NB: in folded terranes, form-lines usually run parallel to each other in fold limbs. As one enter into the hinge of the fold the from-lines are no longer well organized and the outcrop has a patchy look.
|Form lines can be seen in the distance. This lines represent the intersection of the surface topography with bedded rock formations. Differential erosion makes the hard beds stand above the ground surface.|
|Line drawing of the form lines. In the top right corner fo the photograph is seems that the form lines rotated. A good geologist will record on her map the form lines and will go the check whether or not this rock formation is folded.|
|On this outcrop the form lines are parallel to each other. In a folded terrane this pattern would suggest that the geologist is walking across a fold limb. |
On her base map the geologist would use a pale color to map the area covering the entire outrcop and a few lines of the same color but darker to underlines the orientation of the form lines.
Measurement of the strike-dip-dip direction of the beds and other foliations along with the record of way-up criteria would complete the data collection on this spot.
Geological Map from Sequenced Cross-Sections.
A good starting point for a mapping exercise is to construct a cross-section along a profile perpendicular to the regional structural trend and to report on your base-map geological information as you go. The cross-section will help you to define your lithostratigraphic column as well as helping you to understand the overall regional structure of the area you have to map. In fact, a geological map can be constructed from a set of parallel cross-sections and aerial photographs to extrapolate the geology lying in between two adjacent cross-sections. Remember however that, on cross-sectins, the vertical and horizontal scale must be the same. Should you fail to apply that rule dips and thicknesses would have no meaning.
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Beddings, Cleavages and the their Relationships
Geological mapping is more than putting dots of color on a base map (aerial photograph or topographic maps). The orientation of structural elements such as bedding planes, fold axes and folds axial plane, cleavages, lineations etc have to be measured and reported on the map as well as on a field book. Certainly one of the most important skill a field geologist have to develop is to be able to decipher the original bedding from other planar features such as cleavages and metamorphic foliations. Bedding surfaces (S0) separate beds of different composition. No matter how small this change in composition is it can be picked-up by the geologist either because there is a slight change in color, or there is a change in grain size, or a change in hardness than makes some beds standing out.
|Bedding-cleavage relationships 1: |
The rock layer at the top and the two layers at the bottom of the pictures stand out relatively to the layer in the middle. The boundaries between these layers are clearly bedding planes. In this example differential erosion emphasizes minute change in composition.
In addition to the beddings (in red on the picture below) there is another planar features (in green) that cuts accross the bedding planes. This planar feature is a flattening plane called cleavage plane.
On this outcrop strike-dip-direction of both the bedding and the cleavage must be measured and reported on the map and on the field book along with an oriented sketch showing the angular relationship between both planar features. In addition it is a good idea to measure the plunge-plunge direction of the intersection between the two planes. Indeed such a lineation is parallel to the fold axis with admitt the cleavage plane as axial plane.
NB: To an inexperienced geologist this cleavage-bedding association could be misinterpreted in terms of SC mylonite with the red line representing the shear planes and the green lines the foliation. A close look would confirm that there is no deflection of the flattening plane across the red planes and no stretching lineation.
|Bedding-cleavage relationships 2: |
Here a strong change in color emphasize the successive beds. These beds are cut by a strong steeply dipping cleavage plane.
A close observation reveals that the dark thick bed running in the middle of the picture show is affected by a successon of short wavelenght folds whose axial plane is parallel to the cleavage.
|The high angle between S0 (in red) and S1 (in green) indicates that this outcrop is located at or close to a fold hinge.|
|Cleavage-bedding relationship 3: |
I spy with my little eyes..., not one, not two but three planar features... you know that you are in trouble.
Bounded by two red lines (on the picture below) is a darker quartzo-feldspathic beds in between two more pelitic layers. The bedding is emphasized by a slight change in color and a change in the average grain size.
|In green is a cleavage plane cutting through a more pervasive planar fabric in blue. This blue fabric is an older metamorphic foliation. Assuming that we called Sn the metamorphic foliation the cleavage must then be called Sn+1. If no other fabric is found then Sn and Sn+1 will be replaced by S1 and S2. |
The strike-dip and dip direction of each of the three planar features must be recorded in the field book as well as plotted on the map, indeed on must use different symbols.
The plunge and plunge direction of the intersection between Sn and Sn+1 will give the orientation of the F2 fold axis.
Finally the fold vergence must be recorded in the field book with a sketch illustrating the relationship between S0, Sn and Sn+1.
|Cleavage-bedding relationship 4: |
Here the change is composition is revealed by the grow ofth metamorphic minerals (andalusites) in the pelitic beds. In succession with pelitic layers there are a few quartzo-feldspatic layers. These layers are more fine grained, a bit stronger and devoid of andalusites.
|The pelitic layers are weaker that the surrounding quartzo-feldspatic layer. Therefore the foliation (in green) is better expressed in the pelitic layers. |
The vergence of the fold (direction of the next antiformal closure) is toward the right. Remember that fold vergence must be determined by looking toward the same direction, indeed for an obervation looiking from an opposition direction the vergence of that fold would be toward the left.
|Detail of the precedent picture.|
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Folds, Cleavages and Foliations
There is no cleavage or foliation without folding. The geometry of folding says a lot about the depth at which these folds developed and about the tectonic regime responsible for the deformation. Interestingly the geometry and orientation of folds is scale independent which means that the characteristics of a small scale parasitic fold are applicable to larger scale folds.
|When a strong cleavage exists folding may not be easy to spot. Here a quartzo-feldspatic bed bounded by more pelitic layers is folded into an open fold. The cleavage (in green) strongly disrupts the continuity of the bedding plane (in red). The geologist needs to focus of the envelop of the bedding surface to visualize the folding.|
|A tight fold. The blue arrow shows the plunge of the fold B axis. On such outcrop one can verify that the intersection between S0 and the cleavage, which can be seen on limb on the photograph, is indeed parallel to the fold axis.|
|Isoclinal fold. When a rock formation has been isoclinaly folded the beds and the cleavage show a consistent orientation (ie same strike, dip and dip direction) and both planar structures are parallel to each other excepted around fold closures. Unfortunately, fold hinge are narrow and can be easily missed.|
|Folding in a micaschist. The bedding is emphasized by a strong variation in color. The dark beds are pelitic layers (rich in micas), the quartzo-feldspatic layers are more white with less micas. Note that only in the fold hinge the foliation is perpendicular to the bedding plane. Micas growth paralell to the fabric which means that we are dealing with a metamorphic foliation.|
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Folding has sometimes the unfortunate consequence to put things on their head, and not only puzzled geologists. During folding entire sections of rock formation can be overturned into a position where younger layers ly underneath older ones (i.e younding direction down). In the field this translates into a succession of fold limbs with alternating younging direction. Younging direction can be inferred from syn-depositional, gravity-controled, sedimentary structures called way-up criterion. The change in younging direction helps geologists to locate fold closures. In addition, way-up criteria combined with S0-S1 relationships help to define the fold vergence.
|Way-up Criteria 1: Cross-stratifications |
Cross-beds develop during the formation of dunes under the action of water current or wind. Particles from the face opposing the direction of flow are transported toward the face in pressure shadow position. The pressure shadow face develop a spoon-shape surface in which the lower part tangentially merges with the underlying bed whereas the upper part of the dune is eroded.
|The lines in red underline the eroded top surface of sand dunes. Across the red lines there is a clear truncation of the dune internal layering (cross-beds). The yellow lines underline the base of the dunes where the dunes internal layering merge tangentially with the bed underneath. |
The younging direction of this rock formation is top-up.
|Way-up criteria 2: Dewatering structures |
This way-up criteria develops when a high-porosity water-rich layer such as a silstone is overlain by a layer with lower porosity such as a sandstone. Compaction of the silstone layer under gravity leads to fluid-overpressure. Fluids (muds) escape upwards forming pillar structures that represent fluid-path ways. In silica-rich fluid quartz (white area on that picture) can precipitate in the pillar.
|The layering in the silstone (yellow lines) is deformed during dewatering. This deformation is said to be hydroplastic as it occurs when the sediment was water-rich and poorly consolidated. The red line marks the boundary between silstone and sandstone. The cuspate shape of the bedding in the silstone points toward the younging direction which is again upward on that outcrop.|
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Now that we have a better idea of that we have to look for it is important to keep a proper record of these observation. Sketches are indeed extremely important as they help the cartographer to put together various information related to a particular feature. For instance, a 3D block diagram of a fold seen in the field will summarize the relationship between the geometry of that fold, the orientation of its axis, axial planar cleavage, bedding plane and S0-S1 intersecting lineation. A good field geologist does not fall in the trap that consists in replacing sketches by photographs. Sketching involves more detailed analyses and give more time to the geologist to think about what she is sketching. Digital cameras will never replace a field book, a pencil, a rubber, and a brain.
Sketches do not have to be fancy. They purpose is to provide a schematic illustration of the relationship between the prinicipal elements of a particular feature or outcrop.
Every sketch must be properly referenced with its GPS coordinates, it must have a scale and an orientation (arrow showing the North for instance). It is on the basis on these sketches that your tectonic models will be designed.
NB: The more you draw the better your sketches will become.
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Geological maps uses standard color and patterns. As much as possible try to use the following standard colors, patterns and symbols.
Tectonic geopoesie... Your poster should tell the geological story of your mapping area in the form of a sucession of 3D block diagrams summarizing the main geological events. This story should be backed-up by sketches of key field observations accurately located on a map including GPS coordinates.
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The Willyama Complex (Mawson, 1912; Vernon 1969) straddles the New South Wales and South Australian border and has been informally divided into two blocks (Fig. 1-1) arbitrarily along the border (Thompson, 1976), or along a geophysical boundary representing the probable southwestern extension of the Pliocene Mundi Mundi Fault (Stevens, 1986). To the east, the Broken Hill Block has been the focus of considerable work with incentive provided by the stratiform Pb-Zn-Ag orebodies at Broken Hill. Detailed systematic mapping by the Geological Survey of New South Wales generated a broad stratigraphic sequence (Stevens et al., 1980) with subsequent work deriving a formal stratigraphy with detailed subdivisions (Willis et al., 1983). Structural studies (Hobbs, 1966; Rutland & Etheridge, 1975; Marjoribanks et al., 1980) have recognised a succession of deformation events but have failed to accurately constrain strain patterns associated with the early recumbent to reclined fold structures. Contrasting with the extensive data available for the Broken Hill Block is modest mapping of the Willyama Complex within the Olary Block by the South Australian Department of Mines and Energy (Pitt 1977, Forbes & Pitt, 1980) and structural work of a fragmentary nature with limited regional context (Talbot, 1967; Berry et al., 1978).
The gneisses and schists of the Olary Block occur as semi-isolated blocks (Fig. 1) which are in fault contact with the late Proterozoic 'Adelaidean' sedimentary succession (Mawson & Sprigg, 1950; Sprigg, 1952) to the west and southwest, but are onlapped by 'Adelaidean' rocks to the north and northeast (Forbes & Pitt, 1980). Meta-sedimentary rocks in the Olary Block have been divided into five conformable rocks suites (Clarke et al., 1986), which can be correlated with those of the Early Proterozoic Willyama Supergroup of the Broken Hill Block (Willis et al., 1983). The resolution of a stratigraphy for the Olary Block has allowed analysis of the regional structure, with the outcrop geometry explained in terms of a refolded recumbent terrain (Clarke et al., 1986). Evidence for the earliest foliations, S1 and S2, is preserved by oriented quartz and mica inclusions in garnet, staurolite and andalusite porphyroblasts that predate a recumbent S3 foliation. An L3 mineral and stretching lineation, indicating a regional D3 transport direction, occurs at a high angle to recumbent F3 fold hinges. Inverted limbs that are several kilometres long suggest F3 folds of a similar scale. S3 cleavage-bedding relationships, combined with stratigraphic younging directions, indicate a southeast-directed shear during F3 folding. S3 and the recumbent F3 folds are refolded about upright F4 folds, which have a vertical or steeply dipping axial-surface foliation (S4 ); the orientation of S4 is consistent with it having developed during continued compression in the same sense and direction as inferred for D3 (Clarke et al., 1986). The D1-4 events together form the 'Olarian' Orogeny. At least two phases of granitoids intrude the metasediments. Post-D3, pre-D4, albite-rich and less abundant microcline-rich granitoids are prevalent in the southern part of the Olary Block and are accompanied by numerous and widely distributed sodic pegmatites (Forbes & Pitt, 1980, 1987; Clarke et al., 1986; Ashley, 1984). One of these has been dated at 1579±2 Ma by conventional U-Pb analyses of zircon grains (Ludwig & Cooper, 1984). A later phase of granitic to granodioritic intrusions post-dates D3 (Forbes & Pitt, 1980; 1987).
The Olary Block has been divided into three zones on the basis of assemblages in metapelitic rocks, with higher grade conditions in the southeast (Clarke et al., 1987). Mineral assemblages developed during peak metamorphism, which accompanied D3, include andalusite in Zones I and II and sillimanite in Zone III. Upright folding and overprinting of mineral assemblages occurred during D4, the new (retrograde) mineral assemblages including fibrous sillimanite and kyanite in Zone II and prismatic sillimanite and kyanite in Zone III. The timing relationships of the aluminosilicate polymorphs, together with the peak metamorphic and over-printing parageneses, imply an anti-clockwise P-T path for the "Olarian" Orogeny, pressure increasing with cooling from the metamorphic peak (Clarke et al., 1987). The Olary Block is cut by retrograde shear zones that post-date both the 'Olarian' Orogeny and the latest granitoids, but began prior to the deposition of the Late Proterozoic 'Adelaidean' sediments (Vernon & Ransom, 1971; Etheridge & Cooper, 1981). The Cambrian Delamerian Orogeny caused the folding of the 'Adelaidean' sequence and resulted in the development of discrete zones of high strain within the Early Proterozoic gneisses (Clarke et al., 1986). Movement on these zones resulted in the outcrop pattern of the Olary Block (Clarke & Powell, 1988).
Upper Proterozoic and overlying Cambrian metasediments in South Australia (Sprigg, 1952; Thompson, 1969b; Rutland et al., 1981) form a sigmoidally shaped fold belt extending northeast from Kangaroo Island to the lower Proterozoic basement comprising the Willyama Complex (Mawson, 1912; Vernon, 1969). An additional northward extension of metasediment forms the northern Flinders Ranges (Fig. 1). These areas comprise the Adelaide Geosyncline, which will be referred to hereafter as the Adelaide Foldbelt, after Rutland and Murrell (1975). The Torrens Hinge Zone separates folded basinal sedimentary rocks to the east from thin, essentially undeformed shelf sedimentary rocks to the west (Preiss et al., 1981); not all sedimentary sequences in the Adelaide Foldbelt are present to the west. The undeformed sediments, which comprise the Stuart Shelf (Sprigg, 1952), unconformably overlie the Archaean to mid-Proterozoic crystalline basement comprising the Gawler Craton (Webb et al., 1986; Fig.1). Since strata in both the Stuart Shelf and Adelaide Foldbelt are correlated with extensive upper Proterozoic to early Palaeozoic basins throughout Australia (Daily, 1956; Preiss and Forbes, 1981), the presently exposed margins to the sequence do not necessarily represent the limit of sedimentation (Daily et al., 1973; c.f. von der Borch, 1980). However, both the Willyama Complex and Gawler Craton have acted as source regions for at least some of the sedimentary cover rocks (Rutland et al., 1981). The present topography of much of the Foldbelt is largely the result of Tertiary block faulting and uplift (Sprigg, 1936)
Mawson and Sprigg (1950) and Sprigg (1952) originally defined the chronostratigraphic terminology for the upper Proterozoic "Adelaidean" rocks which Thompson (1969a) grouped into four main lithostratigraphic units. In order of decreasing age, these are known as the the Callana Beds, the Burra Group, the Umberatana Group, and Wilpena Group.The Callana Beds rest unconformably upon another crystalline basement inlier, the Mt. Painter Block (Coates and Blisset, 1971; Fig.1), and are divided into two sequences; (1) the Lower Callana Beds which include basaltic and andesitic volcanics, conglomerate, altered limestone, and sandstone; (2) the Upper Callana Beds which represent a thick sequence of phyllite, conglomerate, sandstone, siltstone and dolomite. Sediments of the Burra Group overlie these rocks with local unconformity, and are characterised by a basal sandstone-conglomerate unit passing up into an argillite-carbonate succession. The Umberatana Group contains three parts. It commenced with a glacial sequence which contains massive tillite, arkose and quartzite. These are overlain by a thick pile of laminated siltstone and shale with some stromatolitic carbonate, and then an upper unit consisting of a marine glacial sequence overlain by non-marine red sandstone and siltstone. The Wilpena Group contains a sequence of shale, siltstone, limestone and quartzite with red-bed affinities.
In the Northern Flinders Ranges, and southwest of Adelaide (Fig.1), the Wilpena Group passes with apparent conformity into limestone and shale containing Cambrian fauna (Daily, 1956, 1963). These rocks form the Hawker Group (Dalgarno, 1964) and equivalents (Thompson, 1969b). Southwest of Adelaide they are unconformably overlain by a thin sequence of greywackes which are correlated with the Kanmantoo Group (Sprigg and Campana, 1953), a non-fossiliferous sequence of metamorphosed arkose, greywacke, pelitic and calcareous rocks. In most localities the Kanmantoo Group is in fault contact with underlying Adelaidean and Cambrian rocks (Offler and Fleming, 1968), but a mid-Cambrian age is assigned to the Group from lithostratigraphic correlations.
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Field Mapping Area
Satelite images coupled with geological maps provide excellent information on the nature of surface geology. Here is your field mapping area:
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