Geologic mapping (UGM)

Wiki Write-Off Entry
Student Chapter	Universitas Gadjah Mada
Competition	June 2015

Geological interpretation is one of the critical works in geological field. To get the result of geological interpretation that’s closer to the truth, we need required adequate data and important geological data packaging that will affect the result of interpretation that we got. Geological map is one of geological data presentation that’s most competent and is used for various purposes either for science or exploration of natural resources. Geological map is a map that’s consist of geological 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 geological map depends on the scale that’s choosen, the density of observations in the field, and the accuracy of the observations in the field. The final result from all geological methods is geological map (it’s basic for all geological analysis).

Geological mapping

To make a good geological map, a geologist must have good skill in geological mapping and know well about how to read geological map. Basic step of geological mapping consists of observation and measurement in field, taking samples, and analyze samples in laboratory. Geological mapping is a multidisciplinary method that combines all geological disciplines such as petrology, structure geology, geomorphology, paleontology, stratigraphy, sedimentology, etc. Geological mapping consists of two kind based on selection trajectory, they are:

1. Systematic mapping
2. Smart mapping

Systematic mapping is done by mapping technically only, without considering geomorphology, distribution of geomorphology and geological structure pattern. While smart mapping is done by considering the results of satellite image or topographical maps analysis, so it would be more efficient in geological mapping according to the desired scale. In this paper we are more focus on the method of smart mapping because this method is most often used by geologists.

Smart mapping is 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 litologies and structures in the mapped area and the availability of sufficient time. Therefore it is very important to know the purpose of geological mapping before doing the mapping to desired area.

The key points of smart mapping are :

• 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

• Searching patterns will focus in on key areas of interest e.g. 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, lineaments and folds as well as movement indicators for brittle and ductile shear zones

FIG 1

FIG 2

Geological mapping is a 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, aerial photographs, topographic map and/or gravity map). Prediction / interpretation is done to an area and field observations is to prove the results of the interpretation. Don’t be too focused to 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 analysis) to test the predictions.

Geologic mapping consists of three point stages, they are :

DIAGRAM

Pre-mapping stage

Pre-mapping stage is performed before collecting data 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 is aimed to obtain early predictions about the area that will be mapped in the form of a tentative interpretation of geological maps, tentative interpretation of geomorphological map, making trajectory plan, lineament analysis, and making a plan or schedule for geological mapping.

Analysis of images and topographic maps

The initial step that should be done is secondary data collection. This data will be the basis for interpretation of pre-mapping. Required secondary data are: Topographic Map

• Satellite Images
• Regional Geological Map
• Landuse Map
• Paper or Journal about Geological Research in Area that will be mapped
• Geological Maps of mapping area that have been made (if available)

After all secondary data collected, analysis process can start. Aim of analysis process is to determine the following points:

• Landscape and Strike/dip of bedding
• Distribution of rock units/Formation
• Geological Structure pattern
• Tentative maps

How to interpret strike and dip of bedding rocks from images or topographic maps

Determining the direction of 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. Steps for determination of strike / dip of rock layers through a topographic map are 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 of contour lines on both sides of the line of strike that has been withdrawn
• Directions of dip is parallel to the most tenuous density contour of both sides of the strike

MAPS HERE

Make tentative geomorphological map

Map that’s used as a basis to draw the boundary unit of geomorphology is a topography map. Topographic maps depict clearly the different reliefs that appear on the surface of the earth. Different reliefs are caused by differences in the type of lithology, geological structure, and exogenic processes that control the morphology. Satellite image and Digital Elevation Model (DEM) can be used as supporting data to clarify the morphology of the mapping area.

Tentative geomorphological map is drawn up early because morphology is the the most easily seen and identified appearance. Morphological changes reflect differences in lithology, geological structure, exogenic process, and time.

Determining process of geomorphology unit boundary can be done with following step:

• Create a boundary line unit by taking into differences in density of contour lines (dense and loose)
• Make a geomorphological section in certain direction to be certain of the limits of morphological unit
• Create the pattern of morphological lineament (ridge) and geological structures (valley)
• Based on the characteristics of stream patterns, lineaments, and density contour, morphological units are named, better use morphogenesis name because it represents landscape and processes that influence.

FIG 3

Make tentative geological map

Tentative geological map is a map that is subsequently created, the boundary of geomorphological units that are made in geomorphological map will be reused in making a tentative geological map. Geomorphological unit boundary also represents 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 line patterns, contour line pattern may also indicate differences in litology. Dense contour line pattern indicates solid and hard rock such as metamorphic rocks, igneous rocks and sedimentary rocks that are resistant while the smooth contour line pattern indicates sedimentary rocks that are not resistant (e.g. mudstone).

In the tentative geological map we can also draw a lineament that estimate the geological structures such as faults. Fault is marked by the change of contour line patterns drastically or shifted in a narrow area. Estimated geological structure will facilitate observations in the field to look for evidence of the structure.

Tentative geological map is conjectural and therefore we should investigate the truth in the field. But the geological map is very useful as a guide in geological mapping plan, because the main object of mapping is to find the boundary of geological units and geological structure, 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 for areas that have tropical climate, with the possibility of limited data mostly use smart mapping.

Trajectory mapping is mapping done by combining several observation points. Two things to be noted are: efficiency 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 cut off point as stop site for observation. While in the field, there are some basic criteria that can be used to make the observation location:

1. a common contact / lithology 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 schedule on the field

First thing to note is the availability of time. Whether to spend one full month? 3 weeks? Decide first. Note to 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 functions, they are:

1. With the time off (not go to field / only in basecamp), there will be time to check the fieldnotes. Is it well organized? cluttered fieldnote will only make a mess interpretation. Sample management that’s not good, just be a real headache. Big problem is working twice in the same stop site could happen.
2. The time off, is also to 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.
3. Takes your body to rest for a moment. Rest for a moment of physical activity and mind tiring.

Third, then refer to the tentative litology / geological map. Certainly on the tentative litology / geological map, will appear relationships of stratigraphic units and the possibility of geological structure.

1. Notice, where it is possible contacts. Do continue, discontinue, or interfingering? Strategic locations where outcrop of stratigraphy is found (river valleys, escarpment erosion, mining areas, etc.)
2. Where is the estimates locations that have interpretive structures on the map? Locations to be visited : the possibility of a fault, peak / fold axis, the wings of fold, etc;
3. 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 and 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.