Geologic interpretation is one of the critical work in geologic field. To get the results of geological interpretation that closer to the truth are required adequate data, important geologic data packaging also affect results of interpretation that obtained. Geologic maps is one of geologic data presentation that most competent and is used for various purposes either for science or exploration of natural resources. Geologic maps is maps that consist of geologic information of outer layer of earth crust, they are variation of litology, distribution of geologic structure, stratigraphy and geomorphology. All of that information read by symbols and colors. Detail of geologic maps depend on scale that be choosen, the density of observations in the field, and the accuracy of the observations in the field. The final result from all geologic methods is geologic maps (its basic for all geologic analysis).
To make a good geologic maps, a geologist must have good skill in geologic mapping and know well about how to read geologic maps. Basic step of geologic mapping consist of observation and measurement in field, take samples, and analysis sample in laboratory. Geologic mapping is comprehensive of all geologic science such as petrology, structure geology, geomorphology, paleontology, stratigraphy, sedimentology, etc. Geologic mapping consist of two kind based on selection trajectory, they are:
- Systematic mapping
- Smart mapping
Systematic mapping is used as technic only, without consider geomorphology, distribution of geomorphology and structure geology pattern. While smart mapping is done consider by the results of image or topographical maps analysis, so it would be more efficient in geologic mapping accordance with the desired scale. In this paper we more focus on method of smart mapping because this method is most often used by geologists.
Smart mapping used for several reasons, such as mapping area is too large, a short time mapping and lithological variations are relatively homogeneous. While systematic mapping is used when variations of litology are very complex so observation of some point isn’t enough to represent all litology and structures in the mapped area and the availability of sufficient time. Therefore it is very important to know the purpose of geologic mapping before mapping to desired area.
Key point of smart mapping is :
- Smart mapper will start by looking at aerial photos to determine where the available outcrop is and then make 2-3 traverses across strike to groundtruth the photo interpretation
- Search patterns will focus in on key areas of interest eg shear zones
- Less time is spent in areas where the rocks are uniform and a lower density of observations will serve
- Most useful observations are the predictable geometrical relationships between bedding, cleavage, lineations and folds as well as movement indicators for brittle and ductile shear zones
Fig. 1. Ilustration of systematic mapping (upper) and smart mapping (bottom)(Derived from Pramumijoyo, 2014)
Geologic mapping is process of problem solving. It is known as the system of multiple working hypotheses. (It is armed with a number of ideas about the geology developed from looking at published maps, literature, satellite image, air photos, topographic map and/or gravity map). Prediction / interpretation done to an area and field observations is to prove the results of the interpretation. Don’t falling in love with just one model.
Geological mapping is a scientific process and corresponds to the classic scientific method: theorizing, making predictions from the theories and designing experiments (field observations or geochemistry) to test the predictions.
Geologic mapping consist of three point steps, they are :
Pre Mapping Phase
Pre-mapping step is performed before data collecting in the field. This stage is important because data is retrieved and the process that must be done when the mapping depends on the results of the pre mapping analysis. This process aims to obtain early predictions about the area to be mapped in the form of a tentative interpretation of geological maps, tentative interpretation of geomorphology map, making trajectory plan, lineament analysis, and making a plan or schedule for geological mapping.
Analysis of Images and Topographic Maps
The initial phase that should be done is secondary data collection. This data will be the basis for interpretation of pre mapping. Secondary data required are:
- Topographic Maps
- Satelits Images
- Regional Geologic Maps
- Landuses Maps
- Paper or Journal about Geologic Research in Area that will be mapped
- Geologic Maps of mapping area that have been made (if available)
After all secondary data collected, analysis process can start. Aims of analysis process to determine the following points:
- Landscape and Strike/dip of bedding
- Distribution of rock units/Formation
- Structure Geology pattern
- Tentative map
How to interpret strike and dip rocks bed from Images or Topographic Maps
Determining the direction strike and dip can be done by looking at the secondary data. the result of this determination is still tentative and should be checked in the field. Step determination strike / dip rock layers through a topographic map is as follows:
- Choose a hill or high topography that extends
- Pull the longitudinal direction of the hill or high topography as the direction of strike
- Directions of dip is determined by looking at the density contours on both sides of the line of strike that has been withdrawn
- Directions of dip paralel to the most tenuous density contour of both sides of the strike
Make Tentative Geomorphology Map
Maps used as a basis to draw the boundary unit is geomorphology is a topography map. Topographic maps depict clearly the difference reliefs appear on the surface of the earth. Difference reliefs is caused by differences in the type of lithology, geological structure, and exogenic processes that control the morphology. Satellite imagery and Digital Elevation Model (DEM) can be used as supporting data to clarify the morphology of the mapping area.
Tentative geomorphological map drawn up early because morphology is the appearance of the most easily seen and identified. Morphological changes reflect differences in lithology, geological structure, exogenic process, and time.
Withdrawal process of geomorphology unit boundary can be done with following step:
- Create a boundary line unit by taking into differences in density contour (dense and loose)
- Make an geomorphology section in some direction to be certain limits morphology most appropriate unit
- Create the pattern of morphological lineament (ridge) and geological structures (valley)
- Based on the characteristics of stream patterns, lineaments, and density contour, morphology units are named, better use morphogenesis name because it represents landscape and processes that influence.
Make Tentative Geoogic Map
Tentative geological map is a map that is subsequently created, the boundary of geomorphology units that made in geomorphological map reused in making a tentative geologic map. Geomorphology unit boundary also represent different types of litology.
To determine the litology that may compose each litological unit can be used several indicators, such as :
- Different kind of vegetations, vegetation sometimes represents the difference of litology. For example teak trees usually grow above carbonate rocks. Although not usually true, but the data is quite useful as a first approximation.
- roughness and fineness of the contour pattern, contour pattern may also indicate differences in litology. Dense contour pattern indicate solid and hard rock such as metamorphic rocks, igneous rocks and sedimentary rocks that are resistant while the smooth contour pattern indicate sedimentary rocks that are not resistant (mudstone).
In the tentative geological map we can also draw a lineament that estimate geological structures such as faults. Fault marked with change of contour patterns drastically or shifted in a narrow area wide. Estimated geological structure will facilitate observations in the field to look for evidence of the structure.
Tentative geological map is conjectural and therefore should be investigated the truth on the field. But the geological map is very useful as a guide geological mapping plan, because the main object of mapping is to find the boundary of geological units and structural geology, so that the field observations can be focused on areas that indicate differences in litology from tentative map.
Make Traverse Plan
Determination of track plan is a step that should have been done before go to field. Geological mapping that suitable for areas that have tropical climate, with the possible of limiting data is large use smart mapping.
Trajectory mapping is mapping done by combining several observation points. Two things to be noted are: efficient of trajectory direction and location selection of the observation point. Efficient track direction is the direction perpendicular to the common dip or perpendicular to the longitudinal axis of the fold or spreading of rock or lengthening morphology element. For more efficient, before go to field create a map of the distribution of the main road to the footpath and distribution of streams and valleys Find the intersection direction of the road and the river is approximately perpendicular / angular blunt with unit geomorphic boundary (relief: are allowed so synonymous with boundary of litology units). Plan a cutoff point as stop side for observation. While in the field, there are some basic criteria that can be used to make the observation location :
(1) a common contact / litology boundary (don’t forget that the geological maps essentially is a map that illustrates the spreading direction contacts!)
(2) The point where the common boundary morphology is sharp (perhaps also the contact between rocks).
(3) The place where the common structure (fault, stocky, folds, etc.)
(4) The point where (1) to (3) don’t exist but there are good outcrops / complete
(5) The point where there is found positive geological potential and / or negative potential such as landslide, etc
(6) Point somewhere where possible morphological descriptions in the other direction (from a height in the direction of lower or of the plain towards the hills around
(7) Points on a map that distance more than 2 cm from another point.
Make Scheedule on the Field
First thing to note is the availability of time. Whether to spend one full month? 3 weeks? Decide first. Note glance topography maps, to see accesibility. Does it take a lot of time or medium?
Second, determine the time design. Is in field for a week, two days off? Or four days field, one day off? Why should there be a day off? There are some function, they are :
- With the time off (not go to field / only in basecamp), there will be time to check the fieldnotes. Is it well organized? cluttered of fieldnote will only make a mess interpretation. Sample management that not good, just be a real headache. Big problem is working twice in the same stop site could happen.
- The time off, update the field strategy. Plan alternative / new trajectories. Certainly, the initial trajectory plan needs to be corrected, at the time off is the one you do that revision.
- Takes your body to rest for a moment. Rest for a moment of physical activity and mind tiring.
Third, then refer to tentative litology / geological map. Certainly on the tentative litology / geological map, will appear relationships of stratigraphic units and the possibility of geological structure.
- Notice, where it is possible contacts. Do continu, discontinu, or interfingering? Strategic locations to see usually outcrop of stratigraphy were (river valleys, escarpment erosion, mining areas, etc.)
- Where is the estimates locations interpretive structures on the map? Locations to be visited : the possibility of a fault, peak / fold axis, the wings of fold, etc;
- If you already understand this passage, mark it on the new topographic map (clear) as a sign founded stop site. Create as much as possible in the confines of the unit and structure.
Fourth, the time plan and the distribution of stop site points, create relationships between nearby stop site. Tracks should be sequential, don’t jump around. Create a closed path (circular) so that there are trajectory that leave basecamp and back to basecamp. Its function is to crosscheck and facilitate the achievement of the coverage area of study.
What should prepared?
- Mapping Tools
Geological field is about tools. What tools are appropiate to use in the field. Geologist should knows about the tools, it will make easier if we easier in the field. The tools are:
– Geological hammer
Its use to sampling rocks or many physical data. There are two kinds of hammer :
– Pick point (for crystalline rocks or hard-rocks)
– Chisel point (for sedimentary rocks or soft-rocks)
– Geological compass
Its use to knows the direction, strike-dip measurement, slope measurement and structural geology data recording. There are many type of compass, such as Brunton compass, Topochaix-Universal compass, and many more, but we usually using Brunton compass.
– Geological loupe
Its use to detail-observed of the rocks. The rocks compose many macro until micro minerals and materials. Thats why we need it.
– Hydro-chloric acid solution 0,1 M
If we works in carbonate fields, we need it to distinguish the carbonate rocks and non-carbonate rocks. If there is carbonate, it will be fizzing. In Indonesia, absolutely Java island, carbonate fields abundant in Southern Mountain zone, Kendeng zone, Rembang zone.
– GPS (Global Positioning System)
It used to determine our location in the field. GPS working with coordinate system. Many coordinate system are UTM (Universal Transvere Mercator) and Longitude-Latitude system. We usually using UTM cause its easier to check in the field.
– Sample bag
We need it to carrying our sample. Should be strong, water and tear resistant, and enough for our sample, hand-specimen minimum.
Used to write all of data and make a sketch. Keep the fieldnotes from wet, fire, and anything which make your data loss.
The map can be topographic map or your tentative map. It used to make traverse for our track, make a boundaries of lithology, and to write all of field data.
– Grain comparator
Its used to distinguish the grain size. Its very useful for sedimentary rocks. We can knows the grain size more detail.
– Jacob stuff
Its used to measure the stratigraphic on the outcrop.
To write and make the data recording easier.
To collect samples, so easier to carrry anywhere.
– Stratigraphic measurements sheet
To record the stratigraphic data based on our measurement at the field.
– Digital camera
To take a documentation in the field. Every sketch will be better if we take the documentation to. Its used to knows the other non-geological data at the field.
- Safety First
Before we go to the field, we should prepare all of mapping tools, include safety equipment. It is very important, cause its concern our safety. We dont know what will happen at the outside. Many dangerous things like reptiles or insect bites, hit a rocks, the rebound of rock-particles while we samping, or hitting by hammer, it probably happening. So we should well-prepared. The standard suit to use are :
– Safety shoes
– Safety glasses
– A long-sleeves suit
– Raincoat and Jacket
– Cap/field cap
– Safety helmet (Important if we down to hole or cave)
– First-aid medicine
– Knowledge about first-aid healing
– Knowledge about emergency code
- Etiquette is important
As geologist, we should know and respect to the local culture. We should knows the ethic codes as a scienstist. Every civilization have different culture, and if we involved we should obey the culture and rule of life. Begin for a simple things such as :
– Ask for permission to government and civil organization before we start to mapping.
– Do not littering
– Do not kill anything except time
– Do not make a violences
– Respect all of local residents, examples if we meet with the residents, give him/her a warm greetings and trying to speak about another topics.
– Do not take anything except somethings you need, such as samples. Take samples appropiately.
– Obey the local rules and respect the folk story. It is real, and don’t underestimate.
Start to mapping
- Knows the direction and record it
Before we start, we should determine the direction and decide the track. It will be simpler if we make Traverse Map, contain our tracking plan. There are two kinds of mapping track, i.e. open polygon and close polygon. Open polygon means that we doing mapping from the first point > second point > third point and so on, until the finish point is not the first point in one day. It’s like a rally track, the start point it’s not the finish point.
Close polygon means that the start point same as finish point. So we make a full-polygon in one day mapping. We can choice which one better, look at the topographic, track condition, mapping method, and times.
Dont forget to traverse our track every time we tracking. We can plotting our track on topographic map or secondary map.
- Finding an outcrop
To take a data, we need outcrop. It is very important things that we need in mapping project. Highly recommended to find a fresh outcrop, cause the data are more accurate and well-recorded. The condition of topographic can affect the abundant of outcrop. In exposed area, we easier to find outcrop than in unexposed area. It happen in Indonesia, cause have tropical climate than the erosional very effective. It easier to find a soil than a fresh rocks. So to overcome, we need to looking for the indirect evidence such as :
– Soil colors, signing what is the original rocks, taking from man-made holes, animal burrows, or cliff.
– Vegetation, can showing the bedrocks. Example that limestones signing with the juniper or beech
– Go to the river, and look at the river banks. It more effective cause every river bank certainly eroded and the bedrock can exposed.
– Go to the structural geology area, it can be indicate the uplifted or unresistant rocks. You can know it from secondary mapping by interpretating it.
- Finding a boundaries and contacts
To find a lithology boundaries, you should know the dipping. Law of superposition said that “the oldest rocks is sedimented under the youngest rocks, except structural was controled it”. To find a youngest rocks, we should walking cross and perpendicular the dip. You can find a lithology contact. Before you find it, literature study is the main.
- At the Stop Site
Notes there are should doing while data recording is record with detail and correctly, don’t manipulating the data. What should recorded?
– (gambar BCL scan)
For drawing sketch, preffered to drawing the geological data. Other geological data such as vegetation, civil objects are additional.
For taking sample, we should know the procedure so that we can do it safely. First, use our safety suit (safety glasses, gloves, mask, and helmet) to protect from the rebound of rock particles. We should know that sample is hard rock (igneous or metamorphic rocks), soft rock, or unlithified sediment. If hard rock, we use pick point hammer. Notes, there is very dangerous cause its more dense than sedimentary rocks or unlithified sediment, that can produce many sharp particles and can wound our body. For, soft rocks using a chisel point hammer. Dont forget to put our sample in the sample bag and give note about the information of stop site like a number of stop site, date, sample number, and observator name.
- Stratigraphic Measurements
It’s important things to do, because we can know more about the rocks strata and corelate them to predict a geologic history. Many stratigraphic science can use to interpretating the history, that are :
– Biostratigraphy, using fossils (biological data) to determine the rock groups in strata
– Lithostratigraphy, that’s the same kinds of rocks is the same group in strata
– Magnetostratigraphy, it’s depends on magnetic reversals
– Chemostratigraphy, depends on temporal exursions and sequencial changs in chemical characteristics
– Sequence stratigraphy, depends on progression of sedimentary rocks in changes sea level
To make a stratigraphy successions, we should needs a Jacob stuff, it’s a 150 meters stuff with a sign delimiter every 10 centimeters and clinometer in the top part and Stratigraphic measurements sheet.
The steps to doing measurements are :
- Determine the outcrops or ways to measured. It should be :
– Showing the well bedding
– Avoid the geological structure like a fault
- Before we use the Jacob stuff, we should make sketch of bedding. Its help us to make the measurement efficiently.
- Place the Jacob stuff perpendicular with bedding. Use the clinometer to measure dip, so that Jacob stuff can make perpendicular with the bedding.
The Jacob stuff should be perpendicular with the bedding
(Source : Stratigraphy Principal mapping archive)
- Every Jacob stuff has 10 cm interval every red-white line. Its used to measuring the width of bedding. Keep place perpendicular with the bedding
(Source : Stratigraphy Principal mapping archive)
Post-Mapping Phase (Read Geologic Maps)
After collecting geological data (such as lithology, geological structure, and geomorphology) from field observation, the next step that should be done is the poster which consists of points of observation’s map, geological map, geomorphological map, stratigraphic column, geological history, and photos and diagrams from petrography, paleontology and structural geology analysis. Those data and maps should represent your field geologically.
The first thing that you need to make is the poster. The poster is consist of :
- Cover Page (with title and your name)
- Points of Observation’s Map
- Geological Map with Profile and Legends
- Geomorphological Map with Profile
- Stratigraphic Column and Its Information
- Geological History that’s arranged based on the stratigraphy and structure of the area
- Photos and Diagrams from Laboratory Observations or Analysis
- Validity Sheet
- Attachments (photos, sketches and diagrams that are optional)
Poster must be arranged with a logical line of reasoning and neatly structured. Below is the explanation about each components of the poster :
- Cover Page
The cover page is consist of the title of poster (includes the village, town, city and country of the mapping area), name of the writer (full name and student id number), symbol of university/college or institutions, and date of published.
- Points of Observation’s Map
The procedure about how to make this map have been mentioned in the pre-mapping phase. This map explains about the route that you’ve observed in mapping. Each points symbolize an area that you’ve visited to do some field work such as lithology descriptions, measured sections and strike/dip measurements. There will be some tracks which connect points that are nearby. The component of this map is almost the same as geological map, but with addition of the number of points and tracks in the legend and also the body of the map.
- Geological Map with Profile and Legends
From this map, you will know the information about distribution of rocks, the relationship between the rock units, the geological structure and the order of the rocks. The map is consist of title, scale, legend, coordinate, lithological symbol, contour, geological structure symbol, strike/dip, river and administrative location. And also a geological profile by making section in the body of the map.
- Geomorphological Map with Profile
This map explains about the distribution of geomorphology. It’s consist of title, scale, legend, geomorphological symbol, contour, coordinate and also the used classification of geomorphology. And also a geomorphological profile by making section in the body of the map.
- Stratigraphic Column and Its Information
This column explains about the order of lithology based on the geological age and also the relationship between the rocks unit. From this column, we can know the depositional environment of the rocks.
- Geological History
From all geological data that you’ve got, you will make interpretation of the geological processes and depositional environment of the mapping area. From the interpretation, you will make a narration that’s full of facts that you’ve got from your work in field and laboratory. The narration will explain about the geological processes and the change of depositional environments from the oldest age to now.
- Photos and Diagrams
Photos from petrography and paleontology observation in laboratory may be put in the poster to support the map and interpretation that you’ve made. Diagrams from geological structure analysis also may be put on a show on the map.
All of the literature and books, papers, and other publications that support your work during mapping should be written in Bibliography page.
- Validity Sheet
Before the poster is shown to public, it must be signed by the advisor or the lecturer to inform that this poster is able and good enough to be published.
This page is optional. If there’s some data such as sketches, or other photos and diagrams that you want to add. You can add it in attachment page.
To make geological map and geomorphological map correctly and based on the data that we got from field observation, there are some rules that we need to apply in making the map. Below is the rules and the right procedure about how to make map :
- The accuracy and precision of geological map is mainly based on the scale of the map that you want to make. The greater scale means the more accurate the map that you make. For example, 1 : 25000 scale is more accurate than 1 : 50000 scale. Also the greater scale will make you get more geological information than the smaller scale.
- To make a boundary between two different rock units, we need to measure strike and dip in field observation. And also we need to pay attention closely to any contact between two different rock units that we found in the field. More value of strike and dip means the more accurate the geological structure construction and more rocks variance.
- To decide the boundary, we can apply the “V Rule”, which has these principle points below :
- If the dip of bedding is 0⁰ or < 5⁰, the boundary will be drawn by following the contour line.
- If the dip of bedding is > 5⁰, if the direction is the same as the dip of slope from topography, then the boundary will be drawn by cutting off the contour line but a bit inclined to downward. And if the direction is the opposite of the dip of slope from topography, then the boundary will be drawn by cutting of the contour line but a bit inclined to upward. Both appearances will be seen if the boundary is cutting of the river line.
- This rule is not applicable in a field where there’s fault.
- This rule is also not applicable in a field that has igneous and metamorphic rocks.
- To make cross sections, we need to make a line for a cross section which will show the appearance of the distribution of all rock units in the field and also the geological structure. Usually, the direction of the cross section is from north to south with a low or steep dip.
Sukanndarumidi. 2011. Pemetaan Geologi. Yogyakarta : Gadjah Mada Press
Pramumijoyo, Subagyo. 2014. Metode Geologi Lapangan. Yogyakarta : Unpublished