Geological mapping method

A geological map is a horizontal projection of the boundaries of various geological bodies on the earth's surface. It uses line segments, text symbols and pattern legends to represent the nature, shape, spatial geometric relationships and relative timing of geological bodies in the survey area. It is a reflection of the geological map author's understanding of the geological structural characteristics and evolutionary history of the study area. The process of representing geological bodies on a map is called geological mapping or geological cartography.

3.2.2.1 The division and comparison of strata and the basic types of geological maps

(1) The division and comparison of strata is an important part of geological mapping work and is an important step in reshaping the history of geological development. , an important basis for studying structures and mineral distribution patterns. Stratigraphic division is based on the characteristics of rocks, biological fossils, geophysics, geochemistry and other characteristics of the strata, dividing the strata into different types and different levels of stratigraphic units to express the relative sequence or relative age relationship of the strata. Modern stratigraphy advocates the multiplicity of stratigraphic divisions, believing that there are as many ways to divide strata as there are rock formations that can be used as the basis for dividing strata, and changes in one feature are not necessarily consistent with changes in another feature.

For a specific research object, it is neither possible nor necessary to use all types of stratigraphic divisions. Instead, the corresponding division system should be adopted according to practical possibilities or for a certain application purpose. There are currently three most commonly used stratigraphic classification systems: ① Lithostratigraphy is divided into four-level units of groups, groups, sections, and layers based on the rock characteristics of the rock layers; ② Rock layers containing fossil parts are classified according to the fossils or fossil combinations contained in the rock layers. Biostratigraphy that is divided into various biological zones; ③Dividing into universes, realms, systems, systems, stages, and time zones based on the inferred or interpreted geological ages (epochs, generations, epochs, epochs, periods, times) of rock formations Chronostratigraphy. Only chronostratigraphic units have a fixed and consistent time meaning; most other stratigraphic units are diachronic, that is, they have an oblique relationship with the isochronic plane. Chronological stratigraphic units are divided according to attributes, which belong to the cognitive category and are variable. The classification of the first two categories is based on the objective existing characteristics of rock formations, which do not change according to changes in people's understanding. However, the content of fossils requires a process of accumulation and is also subject to chance or opportunity, and detailed research on fossils is not something that ordinary geologists can do, especially in the wild. Therefore, only lithostratigraphic division is the first procedure in stratigraphic research. On the other hand, as the first objective geological entity, it is permanent and cannot be limited or modified by other concepts.

In large and medium-scale regional geological surveys, the division of groups and the selection of their boundaries play an important role in the quality of mapping. You should correctly understand the meaning of a group and master the conditions for establishing a group. The stratigraphic specification stipulates: "The important meaning of a group lies in the unity of lithology, lithofacies and metamorphic degree. A group may be composed of one type of rock, or include one main type of rock with repeated interlayers, or be composed of two, It is composed of three types of rocks that overlap repeatedly, and may also be characterized by very complex rock compositions, which are different from other simple formations. "The formation of marine strata is often a simple lithology combination of one phase, continental phase." The lithology of transitional phases between sea and land is relatively complex and is often formed by the merger of several adjacent phases. The group must have a certain lateral stability and a certain thickness. Under general conditions, the distribution range of the group should not be less than the range of the third-level stratigraphic division, and the thickness should not be less than 50M. However, the formation of rock formations with special structural and lithofacies significance is not subject to this restriction. The boundaries of groups are generally the boundaries of lithology, lithofacies, sedimentary cycles or erosional discontinuities, and should have obvious identification marks. Fossils are not a necessary condition for building a group, but the Phanerozoic strata have their own fossil assemblages and even established fossil belts. If there are only fossil boundaries and no obvious difference in lithology, there is no need to build a group. The group is the largest local unit, usually equivalent to a large sedimentary cycle including different phases, with complex lithological combinations and large thickness. A segment is a lithostratigraphic unit at a lower level than a group. It can be a separation of a single lithology or single lithofacies within the group, or it can be a subdivision of differences in lithology combinations within the group. It does not necessarily require considerable lateral stability. sex, does not require a certain fossil content.

(2) Stratigraphic comparison is based on stratigraphic division and comparison with domestic and foreign standard sections to determine the position of strata in the geological time scale; on the other hand, it is the comparison of corresponding layers in the survey area. To determine the geological boundaries of mapping units, the isochronism of boundaries between adjacent map sheets, and the development rules of strata. The comparison of biostratigraphic units can be carried out with the help of standard fossils, biota or fossil combinations. The phylogenetic evolution or direction of biological evolution of organisms can also be used, and paleoecological data can be used to compare simultaneous heterogeneous strata. To this end, systematic fossil collection and ecological observation must be carried out in the wild. Lithostratigraphic units can use data such as lithological characteristics, marker layers, sedimentary rhythms, heavy minerals, trace elements, paleomagnetic and geophysical detection wells. Therefore, in the actual measurement of stratigraphic profiles, a large number of specimens and sample collection work need to be carried out.

(3) Under the guidance of modern stratigraphic theory, there are two types of geological maps in the world, namely group maps and series maps. The group map uses the group of lithostratigraphic units as the basic cartographic unit (such as Huangnigang Formation, Yanwashan Formation, etc.). It is a direct reflection of the surface rock components and their geometric relationships, and a true record of the geological history and the evolution of the tectonic environment. It can promote the combination of remote sensing data, geophysical data and geological research, and is suitable for large-scale (>1:100,000) geological mapping. The group map can be used as a lithology distribution map and has a wider service area.

The series map is filled in based on the "series" of chronostratigraphic units established based on the relative sequence of biological evolution. It is suitable for small-scale (such as 1:1 million) geological surveys and can be used for theoretical analysis of large-scale geological structures.

The map is an objective and direct reflection of the actual geological body. It can be measured in the field, and its boundaries are relatively stable. It can also be mapped directly using remote sensing data. The geological age represented by the series of maps is deduced from indoor analysis and identification based on paleontological and other aspects of data. Due to the limitations of data accumulation in different periods and differences in personal understanding, geological time boundaries often change significantly. The family map is not suitable for direct mapping in the field. It takes a lot of effort to find the chronological and stratigraphic boundaries in areas without intuitive signs, and it may even be impossible to figure out. It can be seen that the group map should be the basic map of geological survey. On the basis of group diagrams, family diagrams of different eras can be compiled.

3.2.2.2 Principles and methods for the arrangement of observation lines and observation points

Continuous geological observation based on certain intervals of routes and certain intervals of control points is the basis of geological mapping method. Its function is to use different line and point densities to reflect the accuracy of regional geological surveys at different scales, and it also helps to systematically catalog field observation materials.

(1) Principles and methods of laying out geological observation routes

There are two basic forms of geological observation routes, namely crossing routes and pursuit routes. The crossing route is a vertical or basically vertical arrangement of stratigraphic or regional structural lines that traverses the entire survey area at a certain distance. Geologists collect geological and mineral data along the observation route, plot geological boundaries, and collect necessary specimens and samples. The geological boundaries between the lines are connected using the "V" rule and a small amount of tracing. The advantage of this route is that it can quickly grasp the basic characteristics of the geological structure of the survey area, the spatial changes of stratigraphic sequences, phase changes and contact relationships. The disadvantage is that there will be errors and omissions in the geological details between the lines. If aerial photos are used, this defect will be greatly improved. The tracing route is arranged along the stratigraphic boundary, the boundary of the geological body or the structural trend, and is used to study the lateral changes of the geological body (such as stratigraphic phase changes, contact relationships, ore-bearing layers, faults, etc.). The mapping accuracy is high, but the efficiency is low. Traverse routes and retrieval routes should be used together on geological maps of different scales. In medium- and small-scale geological mapping, the crossing method is the main method, and in large-scale mapping, the number of retrieval routes should be significantly increased. As for the 1:1000 to 1:5000 mapping of the mining area, tracing and delineation are the main focus.

The layout of the crossing route should consider: the direction of the main structural lines, crossing conditions, outcrop distribution, base station design and field work organization, etc. The pursuit routes are mainly arranged in key special research areas. Different types of natural geographical areas (such as plain river network areas, alpine mountainous areas, forest covered areas, deserts, etc.) must be arranged flexibly according to local conditions. The average density of routes must comply with regulatory requirements, but the density distribution of routes in different parts of the survey area should vary based on the complexity of geological structures, mineralization prospects, and the degree of interpretation of aerial photographs.

(2) Arrangement principles and positioning methods of geological observation points

Observation points can be divided into geological boundary points, structural points, mineral points, hydrological points, geomorphological points, etc. according to their properties. The role of observation points is to accurately control the spatial position of geological bodies; to organize and systematize the cataloging of original data; to control the connections between various geological data and the correspondence between textual and graphic data and field locations; to facilitate the sorting and review of original data and check the quality of work. The arrangement of points is based on the principle of effectively controlling geological boundaries and various geological elements. Generally placed at the boundaries of mapping units, landmark layers, fossil points, obvious changes in lithology and lithofacies, rock mass contact zones, facies zones, ore bodies or mineralization, faults, fold hubs, wells, springs, and wells with important hydrogeological significance. Landforms, etc. The error of equidistant mechanical point distribution is obvious, but control points in a large-scale single geological body are also necessary. It is a measure to avoid the omission of important geological phenomena and mineral clues.

The observation point positioning should be accurate, and the drawing error should not exceed 1mm. Fixed-point method: ①Visual observation method: direct positioning based on terrain and features; ②Rear intersection method: use a compass to perform rear intersection based on three known terrain and feature points. The angle between the azimuths of each point shall not be less than 45°. If the three lines intersect to form a parallax triangle, take the center of gravity as the point, or calibrate it based on the detailed features of the terrain; ③ Use aerial photos to fix the point and transfer it to the topographic map. ④GPS method: It uses a remote sensing satellite positioning instrument to directly quantitatively measure the longitude, latitude or Gaussian coordinates of a certain point. In special geographical conditions such as forest coverage areas and alpine canyons, compass positioning and polar coordinate positioning by walking distance can be appropriately used. An empty box barometer can also be used to measure relative elevation for indirect correction. To ensure accuracy, climb to nearby highlands as much as possible. Establish some control points to correct the fixed points.

(3) Density quotas for observation routes and observation points

The density quotas for observation routes and observation points are the quality standards for geological surveying.

The "Interim Requirements for 1:50,000 Regional Geological and Mineral Survey" (Trial Draft) stipulates that the line distance in bedrock areas is generally 400~800M, and the point distance is generally 300~500M. In areas with a high degree of aerial photo interpretation, strata with single lithology or areas with wide exposures, the lines and point spacing can be appropriately thinned. In the large Quaternary distribution area, the line distance can be relaxed to 1000~1500M.

1:50,000 geological map, only calibrating closed geological bodies with a diameter greater than 100M; linear geological bodies with a width greater than 50M and a length greater than 50M; faults and fold structures with a length greater than 250M. Direct and indirect prospecting signs and geological bodies of special significance that are smaller than the above scale should be appropriately enlarged or combined for representation. In the bedrock area, the Quaternary system with an area less than 0.5km2 and a valley width less than 100M is still mapped as bedrock on the map. Large areas covered by the Fourth Series can be exposed through projects as appropriate based on geophysical and geochemical exploration work. The calibration error of layer boundaries, contact zones, fossil layers, sign layers and mineralization marks shall not be greater than 50M.

Taking into account the specific conditions of the Jiangshan internship area, the standards adopted in this internship area are: line spacing and point spacing are 300M and 100~150M respectively, and the density of observation points is effective points per square kilometer. About 27.

3.2.2.3 Observation procedures and cataloging requirements for route geology

The general procedures for route geological observation are: ① fixed point; ② observe and describe the geological and mineral phenomena around the point; ③ measure occurrence; ④ tracing and mapping geological boundaries; ⑤ collecting specimens and samples. After the work on the points is completed, continuous geological observations and descriptions will be carried out along the forward direction of the route, and continuous Xinshou section maps will be compiled at the same time.

The description of the geological observation point is as follows:

(1) Date and weather conditions.

(2)Routes and tasks.

(3)Personnel composition.

(4) Point number: that is, the number of the observation point. It is marked with the unified number of the survey area, and the name of the map where the point is located is written.

(5) Point location and elevation: The geographical location, coordinate network and structural location of the observation point as well as the rear intersection direction must be stated. The elevation is determined based on the barometer or actual intersection point, etc., and should be clearly stated when recording so that people can understand its reliability. For GPS fixed point, just record the latitude and longitude or Gaussian coordinates and elevation.

(6) Point or purpose: The purpose refers to what problem needs to be solved. For example, it mainly describes the marker layer and its changes, stratigraphic boundaries and contact relationships, or observes folds or fault structures, etc.

(7) Outcrop situation: describe the quality of the outcrop near the observation point, which strata are exposed, the nature of the outcrop (natural outcrop or artificial quarry), the size of the outcrop area, extension, weathering degree and vegetation Coverage etc.

(8) Geomorphic features: describe the terrain features near the observation point. Such as hillsides, ridges, cliffs or gullies, etc., the lithology, origin and relationship with geological structures.

(9) Content description: The general order of description is from oldest to newest, but it can also be described in reverse. First, the contact relationship and age of the stratigraphic units on both sides of the interface should be briefly explained, and then their lithology and other characteristics should be described respectively.

(10) Description and route summary along the way: After an observation point is described, continuous observations and descriptions should be made to the next observation point; when a route observation is completed, a route summary should be carefully written. In this way, field data can be systematized in a timely manner, making the original record an organic whole, rather than the description of some isolated geological points.

The cataloging format and description of route observations are as follows:

The weather was sunny on Friday, April 5, 2013

Location: At the bottom of Ou Pond, Jiangshan

Route I: 150M in the direction of 310° on the 272.2 highland - by the west pond of Ooutangdi Village

Task: geological survey of the route, the main task is to determine the boundary between O1n and O1y

Personnel : Liu Lushui (record), Zhang Qingshan (specimen collector),...

Working map: 1:10,000 topographic map (Jiangshan area topographic map)

No.001

Point location: 150M in the 310° direction of highland 272.2 (GPS coordinates can also be used such as: : Artificial (good)

Micro-landform: roadside

Point: boundary point (O1n and O1y)

Content description:

Diandong: Yinzhubu Formation (O1y) yellow-green and purple-red shale interspersed with a small amount of pressure-soluble nodular limestone. The nodular limestone is purple-red and has developed nodular structures. The rock is composed of two parts: the tumor body and the matrix. The tumor body is ellipsoid, lentil-shaped, ginger-shaped, etc., ranging in size from 2 to 5cm, and is composed of microcrystalline calcite. The long axes of the tumors are arranged roughly parallel to the layers, accounting for 60% to 70% of the whole rock, and have clear and smooth boundaries with the matrix. The matrix is ??composed of calcium and mud, which foams slightly when exposed to acid. The weathered surface of rock forms a honeycomb-like appearance due to the dissolution or peeling of nodules. The nodular limestone is thin-medium-thick layered, unstable in trend extension, and has a phase change relationship with the mudstone.

Occurrence: 320°∠42°

Point West: Ningguo Formation (O1n) dark gray thin to medium-layered microcrystalline limestone.

Occurrence: 318°∠45°

O1n has an integrated contact relationship with the underlying O1y.

......

(Brief description of other phenomena such as structure, landform, and hydrological phenomena.)

No.001-No.002 (description between points)

Description along the way: 0-10cm dark gray thin-medium microcrystalline limestone.

10~50M gray-black shale intercalated with thinly layered black chert, with well-developed horizontal bedding and scattered pyrite nodules. Rich in graptolite fossils.

50~80M gray silty shale and siltstone intercalated with thin layers of siliceous rock.

Occurrence: 308°∠39°

……

(Xinshou cross-sectional view, the scale is the same as the plan view, drawn on the graph paper on the left. )

No.002

Point location: West pond of Ooutangdi Village

Elevation: 158M

Outcrop condition: natural and good

Micro-landform: pond edge

Point: structural point

Content description:

This point is a fault observation point. The fault strikes 320°, tilts to the southwest, and the dip angle is nearly upright. The fault extends to both ends into adjacent observation routes. The east wall of the fault is O3c yellow-green shale with an occurrence of 168°∠57°. The west side of the fault is C1y gray-brown medium-thick layered pebbly coarse sandstone and conglomerate, with an occurrence of 182°∠72°. The fault fracture width is 40-60cm, and it is composed of mudstone and sandy conglomerate fragments without cementation. The nature of the fault needs to be further clarified.

No.002-No.003

(Continuous description, the method is the same as before.)

Today’s route ends here.

Route Summary

1.......

2.......

......

Route Geological Observation In the process, we must diligently pursue and strike, observe and think frequently, record and outline frequently, and maintain a strong spirit of exploration. In principle, any geological phenomena on and between points should be fully observed and recorded. Make sure that the terminology is accurate, the concepts are clear, the text is concise and hierarchical, and the spatial location is clear. We should be diligent in thinking, pay attention to analyzing the connections between geological phenomena, and constantly improve the predictability of route observation. Maintain an objective attitude towards actual phenomena and cannot make arbitrary choices or even exaggerate or falsify. After the data of each route is sorted in the room, a route summary is written on the same day, summarizing the data on major geological issues, pointing out existing problems, and serving as a reference for adjacent routes.

In addition to the above methods for recording route geological observations, there are also various methods such as form cards suitable for computer processing and recording pens for recording in the field and then sorting it out indoors.

3.2.2.4 Determination of occurrence elements and delineation of geological boundaries

Occurrence elements are important data for determining the spatial geometric relationship of geological bodies. Pay attention to the reliability, representativeness and systematicness of the occurrence. To judge the reliability of the occurrence of rock strata, we must first identify whether it is a bedrock outcrop or a turnstone; whether it is a layer, a joint surface or other structural surface. Pay attention to the structural location of the occurrence, and identify whether there are local changes in the occurrence of the rock formation caused by secondary structures or gravity on the slope. Pay attention to selecting representative occurrences, which is very important for correctly understanding the regional tectonic framework. Occurrence elements must be measured systematically and distributed evenly on the map. They should be located in key structural parts (such as the wings of folds, turning ends, and dip ends, the two rock layers of faults, fault planes, strata above and below unconformity, and intrusions). Body contact surfaces, primary flow structures, etc.) must have sufficient occurrence notes. The shape is written in the format of 290°∠36°, where the former represents the tendency and the latter represents the inclination. The determination of occurrence factors mainly relies on the compass. Since the compass uses a magnetic needle for positioning, in order to read the geographical position directly on the compass, magnetic declination correction is required. The magnetic declination values ??for each region can be found on topographic maps. The magnetic declination angle in the practice area is 2°58′ westward. The dial should be turned so that true north falls on the 357°02′ scale line. In order to facilitate the projection of occurrence symbols on the map, the ordinate of the kilometer grid (Gauss-Krüger coordinates) is often used as 0° of the plane azimuth angle, so the meridian convergence angle needs to be corrected for the occurrence. Meridian convergence angles can be found on topographic maps. If the coordinate line is east of the meridian, the corrected azimuth angle = true azimuth angle - meridian convergence angle; if the coordinate line is to the west of the meridian, the corrected azimuth angle = true azimuth angle + meridian convergence angle. The coordinate line of the internship area is 0°41′ on the west side.

Geological boundaries and rock formation occurrences are the most basic raw data for geological maps to reflect the spatial distribution patterns and interrelationships of geological bodies, and must be mapped in the field. Geological boundaries in bedrock areas can be determined directly based on the markings and contact relationships of mapping units. However, in areas with large vegetation and soil coverage, you can refer to natural signs such as the distribution of rock fragments in the residue, landform characteristics, soil color and structure, vegetation type and development level, and you can also use excavations of animal caves and roadbeds. , telephone poles, ditches and other artificial exposures. Geological boundaries must be drawn strictly in accordance with the "V" shape rule on large-scale maps, while small-scale maps must be drawn according to stratigraphic occurrences and with reference to topography and features.

3.2.2.5 Geological Sketching and Photography

Sketching and photography are more intuitive and vivid forms of recording field geological phenomena. Geological sketch is a combination of flat and cross-sectional drawings commonly used in geological work and sketches in paintings. It includes: ① plane sketch using pattern legends (Fig. 3.1a); ② combination of sketch and geological patterns (Fig. 3.1b); ③ complete sketch (Fig. 3.1c). In sketches representing regional geological structural landscapes, three-dimensional topographic line drawings plus geological symbols are often used (Figure 3.2), or joint section sketches are used (Figure 3.3). Geological sketches must have a prominent theme, appropriate selections, respect for reality, and concise lines.

Figure 3.1 Different types of geological sketches

Figure 3.2 Landscape geological sketch

Figure 3.3 Joint section sketch

Geological photography in geology The contrast between body tones is obvious, and the effect is better when the contrast between landforms is strong. In most cases, it needs to be supplemented by sketches. When shooting, the number, shooting location, shooting direction and shooting subject should be noted in the record book, and the photography technical parameters should be recorded.

3.2.2.6 Collection of specimens and samples

There are many types of specimens and samples that need to be collected during regional geological surveys. Including:

(1) Specimens used for rock ore identification must fully reflect the main types and combination characteristics of rocks in the survey area. The specimen size for display is 9cm×6cm×3cm, and the specimen for identification and sectioning is 6cm×4cm×3cm. Rock specimens should be collected from fresh, unweathered rocks as much as possible.

(2) Rock formation analysis samples.

(3) Paleontological fossil specimens. When measuring stratigraphic profiles, they are collected layer by layer and cataloged layer by layer. For strata where no macrofossils are found, samples for micropaleoanalysis (such as conodonts, ostracods, spore pollen, etc.) should be collected.

(4) Bedrock spectrum and metal content measurement sample, sample weight 50g. Used to study regional geochemical characteristics.

(5) Natural heavy sand samples and artificial heavy sand samples, the sample weight is 10~20kg.

(6) Silicate analysis and carbonate analysis samples.

(7) Ore chemical analysis and ore technical and physical properties measurement samples.

(8) The size of isotope age samples, paleomagnetic samples, and paleomagnetic orientation specimens should be larger than 10cm×10cm×10cm.

Sample collection must have a clear purpose, be fully representative, and must meet the technical requirements for sample processing and experimental analysis. Attention should be paid to the compilation and cataloging of samples and their analysis and identification results.

3.2.2.7 Regional geological survey content in sedimentary rock areas

Sedimentary rocks are the most widely distributed rocks on the earth’s surface and are the main objects of regional geological survey work. Because of its obvious layered characteristics, the sedimentary rock area is different from the magmatic rock area and metamorphic rock area in its regular belt-like geological structural landscape. Regional geological surveys in sedimentary rock areas have accumulated rich experience and formed a very complete set of working methods. Its main work procedures are to measure profiles to establish stratigraphic sequences, study the structural characteristics of the survey area through geological mapping, and search for mineral-bearing layers with the help of sedimentary facies and paleogeography research. The main work includes: ① dividing and comparing strata and establishing stratigraphic sequences; ② sedimentary petrology research; ③ collection of biological fossils; ④ sedimentary facies and paleogeographic investigation; ⑤ research on structures (folds, faults) and stratigraphic contact relationships; ⑥ sedimentation Mineral survey; ⑦ Exploration of geological development history, etc.