Lab 10: Geologic Structures, Maps and Block Diagrams

Objectives:

- Geologic maps and block diagrams

- 3 types of unconformities

- Strike and dip

- Brittle v. ductile deformation

- Interpretation of geologic maps

Introduction:

Structural geology: the study of how geologic units are arranged when first formed and

how they are deformed afterward, understanding the relationship between stress and strain

Stress: directed pressure applied to a rock or sediment

Strain: response to stress, generally distortion (translation- moving from one spot to

another, rotation- spin about an axis, or dilation- volume change)

Outcrop: exposures of rock at the earth’s surface

Geologic maps:

-2D representation of outcrop data, the location and distribution of units of rocks

-can be used to infer the three-dimensional arrangement of units

Structural Geology:

Formations: mappable units of rock, grouped on the basis of color, texture or composition

Members: subdivision of formations, many members make up a formation

Contacts: boundaries between geologic units

Geologic maps show geologic units and topography

Geologic cross-section: drawing of a vertical slice through Earth

Block diagram: combination of the geologic map and cross section, 3D representation

Measuring the Attitude of Rocks Units: Figure 10.1

Attitude: the orientation of a rock formation or surface

Dip direction: the direction that water would flow down the bedding plane of a rock

Dip angle: the angle between horizontal and the surface of a bedding plane

Strike: the compass bearing of a line perpendicular to dip

Figure 10.3 - Geologic map symbols

Figure 10.4 – Variations of outcrop width of a formation

Unconformities: Figure 10.4

Disconformity: an unconformity between relatively parallel strata

Angular unconformity: an unconformity between nonparallel strata

Nonconformity: an unconformity between sedimentary rock and non-sedimentary

Brittle deformation - faults: Figure 10.5

Review of stress types: tension, compression and shear

Headwall: top surface of the fault, sits on top of the footwall

Footwall: lower surface of the faults, sits below the headwall

Normal faults: tensional stress, gravity pulls the headwall down relative to the footwall

Reverse faults: compression, pushes headwall up relative to the footwall at a steep angle

Thrust faults: compression, pushes headwall up relative to the footwall at a low angle

Strike slip faults: shear, horizontal motion of rocks

-Right lateral: rocks opposite your side will move to your right

-Left lateral: rocks opposite your side will move to your left

Ductile deformation - folds: Figure 10.6

Folds: upward or downward bends of rock layers

Antiforms: upfolds

Anticlines: an antiform where the oldest rocks are in the middle

Synform: downfolds

Synclines: a synform where the youngest rocks are in the middle

Figure 10.8

Fold axis (hinge line): an imaginary axis around which stratum is folded

Axial plane: the plane made by connecting all axial lines in a section of rock

Figure 10.9

Plunge: the angle between the fold axis and horizontal

Trend: bearing in the direction of plunge

Limbs: each side of a fold on either side of an axial plane

Monoclines: two axial planes that separate two nearly horizontal limbs from a steep limb

Figure 10.10

Domes and basins: large circular structures that are warped upwards or downward

Geologic maps and block diagrams:

Rules for interpreting geologic maps – Figure 10.11

Contacts between horizontal beds are parallel to topographic contours

Anticlines – oldest beds in center

Synclines – youngest beds in center

Anticlines plunge towards the nose of the structure

Synclines plunge towards the open end of the structure

True dip angles can only be seen in cross section if the section is perpendicular to

the fault or to the strike of the beds