INTRODUCTION
Structural Geology
Detailed Structural Analysis
Introduction
• The architecture of the Earth
– Need to accurately describe the structures that are present
– Tools for viewing the surface and interpreting what is occurring in the third dimension
• The Geologic Map
– Shows the geology on the surface
– Cross sections to interpret the third dimension
– Formations—Smallest unit that can be traced across a map area
• Sometimes divided into members
• Sometimes lumped into groups
Types of Structures
• Primary Structures
– Originated at the time the rock was formed
– Reflect local conditions at the time the rock formed
– Examples
•
•
•
•
Bedding
Cross bedding
Ripple marks
Flow structure
Bedding
• The most important primary structure
– Allows application of laws of original horizontality, superposition, and lateral
continuity
– Can determine which way is up
• Younger rocks on top of older rocks
• Facing direction
–
In complexly deformed rocks, called S0
Sedimentary Structures
• Bedding
• Cross-bedding
Sedimentary Structures
• Mudcracks
• Ripple marks
– Oscillation
– Current
Sedimentary Structures
• Sole marks
Soft-sediment structures
• Penecontemporaneous folding and faulting
Types of Structures
• Contacts—Boundaries that separate one rock body from another
–
–
–
–
–
–
Most often they separate formations
Normal depositional contacts
Unconformities
Intrusive contacts
Fault contacts
Shear zone contacts
Contacts
• Conformable contacts
– Age differences between adjacent layers are negligible
• Unconformable contacts
– A depositional contact between two rocks of measurably different ages
– Represent times when there was no deposition or during which rock
material was removed by erosion
Unconformities
• Angular unconformity—layers above the unconformity are not parallel to layers
below the unconformity
• Disconformity—unconformity separating strata that are parallel
• Nonconformity—unconformity separating distinctly younger volcanic or
sedimentary rocks from distinctly older igneous or metamorphic rocks
• Buttress unconformity—sediments deposited in a topographic low, against a
steep slope
Angular unconformity at Siccar Point
Three kinds of unconformities
Recognizing unconformities
•
•
•
•
Fossil evidence
Paleosols
Basal conglomerate
Channels
Contacts
• Intrusive (igneous)—magma invades and crystallizes against
country rock
• Diagnostic Features
– Chilled margin or chill zone
– Apophyses
– Xenoliths
– Contact metamorphic auerole
Pillow Lavas
• Formed under water
• Rounded part up, point down
Igneous Intrusions
“liths”
• Cactolith—A quasi-horizontal chonolith composed of anastomosing
ductoliths, whose distal ends curl like a harpolith, thin like a
sphenolith, or bulge discordantly like an akmolith or ethmolith.
(from “Geology and Geography of the Henry Mountain Region,
Utah” by C.B. Hunt, P. Averitt, and R.L. Miller, 1953, USGS, p.150.
Contacts
• Intrusive, but not igneous
– Clastic dikes
• May move up or down
• Material may be older or younger than the rocks they invade
– Soft-sediment intrusions
– Salt diapirs—density inversion of denser sediment on top of salt
– Mud diapirs—unconsolidated, low density mud forced upward into denser
sediments
Clastic Dike
Types of Structures
• Secondary structures—Form in sedimentary and igneous rocks
after the rock formed or during the formation of metamorphic rocks
• Stresses that created these structures are commonly related to
regional deformation
• Examples
–
–
–
–
Joints and shear fractures
Faults and shear zones
Folds
Cleavage, foliation, and lineation
Brittle Structures
• Joints—tensional features
• Shear fractures—faults with minimal displacement
• Transitional tensile fractures—part joint and part shear fracture;
oblique motion that combines both tension and shear
• Faults—discrete fracture surfaces or zones of fractures along
which rocks have been offset by movements parallel to the fracture
surface
Fault contacts
• Fault surface
• Fault zone
– Gouge
and breccia
– Slickensides
– Mylonite
• Shear zone
Brittle Structures
• Slickensides—striated surface produced by faulting
• Slickenlines—linear striations produced by faulting
• Slickenfibers—linear mineral grains which grew in the direction of
fault motion
Ductile Structures
• Folds
• Foliation—planar alignments of features like micas, ribbons of
quartz, aligned phenocrysts, shear surfaces, or flattened objects
• Cleavage—foliation along which the rock tends to split
– May be due to the alignment of minerals
– May be caused by pressure-solution
– Most commonly associated with folding and nearly parallel to the axial
plane of the fold
Ductile Structures
• Lineation—linear alignment of objects
• Shear Zones
– Not really always ductile, but where offset is distributed across a zone
– Exhibit ductile characteristics when they form under conditions of elevated
temperature
Deformed Structures
• Fossils
– Crinoids
– Trilobites
• Sand volcanoes
Detailed Structural Analysis
• Descriptive Analysis—completely describe what is present,
including measures of size and orientation
• Kinematic Analysis—what movements are responsible for forming
the structure
–
–
–
–
Translation
Rotation
Dilation
Distortion
Detailed Structural Analysis
• Dynamic Analysis—interpret deformational movements in terms of
forces and stresses which caused them
Descriptive Analysis
• Foundation is geologic mapping
• “Geologic cross sections…represent the ‘best’ interpretation of the
geology as it projects into the subsurface or, up into the sky.”
• Mining, oil, and hydrology require even more detailed subsurface
information obtained by drilling and geophysical techniques
Descriptive Analysis
• Scale of observation
– We see different things at different scales
– “Big dogs have little fleas”
• Read excerpt of Price and Mountjoy (1970) in D&R, p. 21-22.
Descriptive Analysis
• Structural Elements
– Physical elements—Those that are real and tangible
– Geometric elements—Imaginary elements
• The gate
– Physical elements—2 x 4’s, slats, hinges, latch
– Geometric element—rectangular cracks between slats, or discontinuities
• The fold
– Physical elements—folded layers, hinge
– Geometric elements—fold axis, axial plane, and discontinuities
The Gate
• Physical elements—2 x 4’s, slats, hinges, latch
• Geometric element—rectangular cracks between slats, or
discontinuities
The Fold
• Physical elements—folded layers, hinge
• Geometric elements—fold axis, axial plane, and discontinuities
Pattern Recognition
• Joints in aerial photograph of Entrada sandstone
• 3 sets of joints
• Define a joint system
Kinematic Analysis
• Motions
–
–
–
–
Translation
Rotation
Dilation
Distortion
• Strain analysis
– Circular structure deformed to ellipse
Aellipse =  ab Acircle =  r2
• If the areas are equal,
r2 = ab and
r can be determined
•
Kinematic Analysis
• Penetrative deformation
– Spaced so closely at the scale being studied that they appear to be
everywhere
– Partly a question of scale
• Slip versus Flow
– Also a question of scale
– Flow is continuous, slip has discontinuities
Dynamic Analysis
•
•
•
•
•
Interprets forces, stresses, and mechanics
Requires the greatest inference
Physical models
Mathematical models
Timing
– Relative timing
– Absolute timing
– Timing that varies from place to place
Mathematical Model
•
L=2t (/61)1/3
• The wavelength (L) of a fold in a layer of viscosity  and thickness t is equal to
the product of 2 pi times the thickness times the cube root of the ratio of the
viscosity to 6 times the viscosity of the rock in which the layer is contained
Analysis of a Pizza
Analysis of the San Manuel Fault