GLY 155 Introduction to Physical Geology, W. Altermann. Press & Siever, compressive forces. Compressive forces cause folding and faulting.

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1 Press & Siever, 1995 compressive forces Compressive forces cause folding and faulting. faults 1

2 Uplift is followed by erosion, which creates new horizontal surface. lava flows Volcanic eruptions cover the surface with lava flows. 2

3 Tensional forces cause normal faults, creating down-dropped dropped blocks and breaking up earlier features. tensional forces normal faults Fish River Canyon, Namibia 3

4 Bedding and unconformities: All sedimentary rocks are characterized through bedding: The bedding originates from alternating conditions of deposition in time. The layers of sediment are deposited horizontally The law of layer superposition: Bedding and unconformities: Missing bed B - deposited and eroded or not deposited? 4

5 Bedding and unconformities: Bed D is pinching out to the right and has not been deposited in the right profile Bedding and unconformities: Reconstruction of relative time events from bedding and conformity relationships 5

6 Bedding and unconformities: Bedding and unconformities: Two erosional unconformities in the Grand Canyon 6

7 Bedding and unconformities: Reconstruction of relative time events from bedding and conformity relationships Bedding and unconformities: Paraconformity Disconformity Nonconformity Angular unconformity Unconformities: Time gaps in sedimentary record Paraconformity: Time of non deposition without noticeable erosion Disconformity: Time of erosion within a depositional sequence Nonconformity: Time of exhumation and erosion of a much older igneous rock Angular unconformity: Time of exhumation and erosion of a folded or tilted rock sequence 7

8 Old land surface -- Erosional surface Transgression conglomerate Transgression conglomerate on the Sardic unconformity 8

9 Bedding and unconformities: Reconstruction of relative time events from bedding and conformity relationships Bedding and unconformities: Sequence of events: 1. Sedimentation of the sequence of A to G 2. Folding and faulting of A to G 3. Intrusion of a granite into A to G 4. Exhumation and erosion of A to G and of the granit 5. Deposition of X to Z as horizontal layers 6. Tilting of the entire system to the E (right) 7. Intrusion of a vertical dyke into the granite and A to G 9

10 Chapter 7: 7 Deformation- Modification of Rocks by Folding and Fracturing Plate Tectonic Forces Deformation tensional forces compressive forces shearing forces Outcrop basic source of geologic information in the field 10

11 Mapping Geologic Structure Mapping Geologic Structure 11

12 Mapping Geologic Structure S W Dip direction 45 Mapping Geologic Structure Strike direction Dip angle N E S W 45 Dip angle N E Water trickles down slope parallel to dip. 12

13 Outcrop basic source of geologic information in the field Dinosaur Ridge, located west of Denver, Colorado. Mapping Geologic Structure Geologic maps geologic maps represent the rock formations exposed at Earth s s surface A common scale for geologic maps is 1:25000 Geologic cross sections geologic cross sections diagrams showing the features that would be visible if vertical slices were made through part of the crust 13

14 Basic Deformation Structures 14

15 Example of a strike- slip fault Basic Deformation Structures 15

16 Basic Deformation Structures 16

17 joints 17

18 Styles of Continental Deformation Styles of Continental Deformation 18

19 Styles of Continental Deformation 19

20 Example of tensional tectonics Example of compressive tectonics 20

21 21

22 Example of shearing tectonics Thought questions for this chapter In what sense is a geologic map a scientific model of the surface e geology? Is it fair to say that geologic cross sections in combination with a geologic map describe a scientific model of a three-dimensional geologic structure? Why is it correct to say that large-scale geologic structures should be represented on small-scale scale geologic maps?? How large a piece of paper would be required to make a map of the entire U.S. Rocky Mountains at 1;24,000 scale? Can you explain the geologic story in Exercise 6 (Understanding Earth, p. 188, ) in terms of plate tectonics? 22

23 Thought questions for this chapter The submerged margin of a continent has a thick layer of sediments overlying metamorphic basement rocks. That continental margin collides with another continental mass, and the compressive forces deform it into a fold and thrust belt. During the deformation, which of the following geologic formations would be likely to behave as brittle b materials and which as ductile materials? (a) sedimentary formations in the upper few km (b) metamorphic basement rocks at depths of km (c) lower crustal rocks below 20 km In which of these layers would you expect earthquakes? 23