출처: 앨런 레스터 연구소 – 콜로라도 볼더 대학교
지질지도는18세기 중반에서 후반유럽에서 처음 만들어졌으며 활용되었습니다. 그 이후로, 그들은 지구 표면, 지하, 그리고 시간을 통해 수정에 바위 분포를 이해하기 위해 노력하는 전 세계 지질 학적 조사의 중요한 부분이었다. 현대 지질지도는 2차원 계획 보기에서 바위와 바위 구조물을 데이터로 풍부한 표현입니다. 대부분의 지질지도의 기본은 지형맵으로, 특정 암석 단위를 나타내기 위해 어떤 색상 변형이 배치되었는지에 대한 것입니다. 바위 단위 사이의 경계를 접점이라고 합니다. 접촉선 외에도 지질지도에는 암석 유닛의 딥 앤 스트라이크, anticlines 및 동기화선 및 결함 표면의 흔적과 같은 주요 기능을 나타내는 기호가 포함되어 있습니다.
2차원 맵 뷰가 유용하지만 지질학자의 주요 작업 중 하나는 지하에서 바위의 유형과 방향을 추론하는 것입니다. 이것은 지질 학적 규칙, 추론 및 표면에서 아래쪽으로 투영을 사용하여 수행됩니다. 그 결과 협곡 벽이나 도로가 보이는 것처럼 본질적으로 컷어웨이 이미지를 제공하는 지질학적 단면이 생성됩니다.
지구로 이 가상의 슬라이스는 3차원(깊이)을 제공하는 지질학적 응용 분야의 핵심입니다. 단면은 시간이 지남에 따라 암석 형성의 시간적 모델을 평가하는 데 사용됩니다. 즉, 바위와 구조물이 먼저, 마지막으로, 그리고 그 사이에 온 단계별 시퀀스를 재현하는 것이 목표입니다. 또한 바위가 압축, 확장 또는 기타 스트레스를 받았는지 여부에 관계없이 특정 변형 모드를 결정하는 데에도 사용됩니다.
지질 학적 단면은 지하수 이동 의 영역을 식별하고, 경제 광물 퇴적물에 대한 잠재적 인 사이트를 평가하고, 석유 및 가스 저수지를 찾는 데 도움이됩니다.
For this demonstration, a portion of the Carter Lake, Colorado, USGS 7.5-minute Quadrangle Map was used. This notation means that 7.5 minutes of longitude and 7.5 minutes of latitude define the E-W and N-S boundaries on the map. On the east side of the cross section line A-A’, the rock layers dip to the west; in contrast, on the west side, the layers dip to the east. It can be inferred that these layers meet in the subsurface to form a bowl-shaped fold-structure, known as syncline. Ultimately, all folds (whether down-warps, such as synclines, or up-warps, such as anticlines) are a product of compression-style deformation. When rocks have been squeezed, they show plastic deformation features (folding), especially if deformation has occurred relatively rapidly, with high-confining pressures and elevated temperatures in Earth’s upper crust. In contrast, rapid application of stress, low-confining pressure, and low temperatures are more likely to produce brittle deformation, known as faulting.
Cross-sections provide a means to analyze and assess the subsurface orientation of rock units. Geologists use the relative dating rules of cross-cutting and superposition to determine the timing of deposition and deformation. For example, when one layer sits above another, it can be inferred that the top layer is most likely younger than the layer below. Furthermore, if a fault cuts across a particular rock unit, then the fault is most likely younger than the rock unit it offsets.
Some specific applications include the determination of geologic history, groundwater flow analysis, mineral deposits, and oil and gas reservoirs. Relative dating techniques permit an assessment of a sequence of geologic events, including deposition, intrusion, and deformation (folds and faults). Geologists seek to understand the earth in not only the three spatial dimensions, but also within the context of a temporal dimension- the idea being to reconstruct geological change through time.
Cross sections are a key to evaluating fluid flow in the subsurface. Understanding the orientation of flow-enhancing layers (aquifers) versus flow-preventing layers (aquicludes) is the key to evaluating the motion of groundwater. This also provides an application for determining where wells are best to be drilled. It allows for analysis of aqueous pollutant movement and possible mitigation strategies. In general, rock types that contain considerable pore space (e.g. sandstone or highly fractured igneous/metamorphic rocks) will be aquifers. In contrast, rock types that contain limited pore space (or pores that lack inter-connectivity) will more likely be aquicludes.
Most economic mineral deposits (e.g., Au, Ag, Cu, Mo, etc.) are associated with igneous rocks. If igneous rocks outcrop on the surface, and their surface contacts can be assessed, then one can determine where possible ores can be found in the subsurface. Most oil and gas reservoirs are associated with sedimentary rocks, because these are the rock types that contain hydrocarbon sources (decayed organics, both terrestrial and marine). Here, cross section analysis is absolutely critical to determining where fold or fault traps are likely to exist, and if they contain petroleum resources. For example, up-warps (anticlines) are a classic location for oil and gas drilling. This is because mobile hydrocarbons tend to flow upward, within permeable layers, until they reach the peak (or axis) of an anticline. If the permeable layer is capped by an impermeable layer, then a hydrocarbon reservoir accumulates and pools at the apex of the fold.
Geologic cross-sections can assess temporal models of rock formation through time.
Using geologic maps, cross-sections can be generated which predict the strata of the rocks sub-surface, and estimate the rock shape above ground prior to erosion.
The resulting cross-section is a cutaway image much like those seen in canyon walls or road cuts. While geologists may be able to infer such features from a plan-view geologic map, the addition of a cross-section provides a third dimension of information that can greatly enhance the ability to evaluate folds and faults.
This video will illustrate the process of creating a geologic cross section, and highlight some of the extensive uses of this geological tool.
The first step in creating a geologic map is to take a topographic map and onto this color-code the regions containing different rock types. In the field, geologists observe mineralogic and textural features, which are then used to identify distinct rock types and rock units. The lines between each rock unit section are the contacts. Within each rock type, strike and dip data will be added to illustrate the surface outcrop orientation of the rock strata.
These strike and dip data indicate fold-type deformations that generate up-warped strata, analogous to an upside down bowl, which are referred to as anticlines. The folds that involve down-warped strata are synclines. In contrast, faults are a result of brittle deformation, whereby rocks break instead of bending along a distinct surface-of-rupture. This surface is the “fault-plane.”
Taken together, rock type, position, and orientation, are used to create a geologic cross-section. The first step is to create a topographic profile, which shows the elevation and contour of the target region. The geologic data is then added to this profile. This cross-section can now be used to infer the subterranean structure. For example, beds dipping away from a central axis are indicative of anticlines, whereas beds that dip towards would indicate synclines.
Further, geologic cross sections are used to reconstruct folds and faults that may be cryptic, due to the effects of erosion on the surface features. This is achieved by extrapolating the existing surface and subsurface data upwards above the existing plane.
Now that we are familiar with the principles behind the construction of a geologic cross section, let’s take a look at how this is carried out on an example map.
To construct a geologic cross-section, first take a geologic map of the target survey area. Begin by choosing two points that define a cross section profile of interest. Label these points as A and A’. These should be selected so that a line between them will be approximately perpendicular to the strike directions of the intervening rock units. Connect these points, and create a topographical profile, without vertical exaggeration, based on the contours that intersect the line. Next, take a strip of paper and align it along the A-A’ line, and carefully mark the contacts between the different rock units.
At each contact, the dip information of the adjoining layers is used to project the boundary into the subsurface. Note that in the projection to the subsurface, we use an average dip across the fold. This maintains constant bed thickness in the projection.
Using a protractor, measure the angle of the dip according to the original map, and extend the rock layers in straight lines below the surface. Projecting this information at each contact point will give a rough predicted cross-sectional view of the rock strata beneath the surface. Next, look for patterns in the rock projections that may indicate folds of the same type of rock strata. If these predicted strata lines appear to meet, this indicates folding of the same substrate, and they should be joined in a smooth projection based on the dip magnitudes given at the surface.
Finally, extend the rocks layers into the above ground region. This shows the inferred presence of rocks and geologic structure prior to erosion.
The map used for this demonstration shows a portion of the MASONVILLE, COLORADO, 7.5 minute quadrangle, USGS geologic map. The rock layers and contacts have been transferred to the geologic profile, and projections made into the subsurface and surface. In the case of one of the units, the Dakota group, labeled KD and highlighted in green, we see the layers dipping on one side of what is referred to as the anticline, to the east, and to the west on the opposite side. Overall, the projections suggest an anticline-syncline combination, and the crest of the anticline is recorded on the original map itself as a dashed line, with the trough (pronounce “trof”) of the syncline indicated to the west by a different dashed line. This combination results in a bowed down set of rock formations, and a bowed up formation, produced by past compressional stresses on the rock strata. The Dakota group, which follows this anticline-syncline pattern, is a unit of importance as it represents a sandstone, which will contain water or oil, which may be of interest for mining.
Geologic cross-sections are useful tools for a number of types of geological investigation. Some of these applications are explored here.
Analyzing sequences of deposition, intrusion, deformation, or erosion over time can inform not only the spatial dimensions of the rock, but also the temporal dimension. Using this information, it is also possible to simulate and anticipate future changes in the Earth’s structure, such as the erosion of softer substances, leaving harder rock exposed.
Most economically important mineral deposits; including gold, silver, copper, and molybdenum; are associated with igneous rocks. If such rocks are found on the surface during a geological survey, and their surface contacts can be assessed, it is possible to use a geologic cross section to extrapolate where possible ores can be found in the subsurface.
Geologic cross-sections are key to evaluating fluid flow in the subsurface. Understanding the orientation of flow-enhancing layers, or aquifers, versus flow preventing layers, or aquicludes, allows geologists to predict the motion of groundwater, and potentially determine suitable areas for drilling of wells. In general, rock types containing considerable pore space, like sandstone, will be aquifers, and those with denser structure and little pore space, like slate, will act as aquicludes. Crucially, this information also allows for analysis of aqueous pollutant movement, and development of possible mitigation strategies in such events.
You’ve just watched JoVE’s introduction to geologic cross-sections. You should now understand how to create a geologic profile from a geologic map, and the uses and applications of these geologic cross-sections.
Thanks for watching!
