1. Primary Structures
Geologic Structure A definable shape or fabric in a rock
Primary Structure: A structure formed during or shortly after deposition (sedimentary) or formation (igneous) of rocks
Secondary Structure: A structure formed after its host rock is formed
Tectonic Structure: A structure formed as a result of strain due to tectonic deformation

2. Primary Sedimentary Structures
Bedding: The primary surface in a sedimentary rock, separating beds with different composition, texture, color, cement (make sure you recognize beds based on these criteria!)
Different beds represent different source, sedimentary processes, and environments of deposition
Emphasized in outcrop by parting and differential weathering and erosion
A plane of separation, along which the rock has a tendency to split or fracture parallel to bedding (don’t confuse with fracture!)
Commonly due to the weak bonds between different beds, or preferred orientation of clays
Commonly, there is a bedding-parallel fracture which forms due to unloading or rocks
Closely-spaced parting is called fissility (e.g., in shale).

3. Bedding between inter-bedded sandstone and conglomerate

4. Reasons Why Clays or Clasts are Preferrably-Oriented?
Sedimentary settling of elongate or planar flakes in the gravity field (syn-depositional)
Rotation and reorientation of flakes in a flowing fluid (syn-depositional). Flakes reorient so that the traction is minimized
This may lead to imbrication (grains overlap like roof singles) which may be used for paleo-current analysis (finding past flow direction and regime) e.g.,
Pebble Imbrication where shingled flat pebbles indicate current direction
Reorientation (rotation) due to post-depositional compaction (squeezing of unlithified sediment due to the weight of the overlying rocks).

5. Bedding is Important in Structural Analysis
Bedding is used as a paleo-horizontal, or nearly horizontal reference frame (recall the principle of original horizontality)
Bedding as a primary structure (S0, or original surface) is the first object that becomes deformed. The subsequent deformation surfaces created (S1, S2, S3) are compared relative to the S0
Structures, textures, fossils, etc, in beds provide clues as to the:
Depositional environment
Stratigraphic facing (younging direction) to identify right-side-up or overturned beds
Current direction
Beds help us to better map stratigraphic contacts, and identify large structures such as folds, faults, and unconformities.

6. Bedding between sandstone & conglomerate

7. Graded Beds
Graded beds: Progressive fining of clast grain size, from the base to the top of a bed; form as a consequence of deposition by turbidity currents (e.g., in turbidite)
Can indicate which way is up provided the bed is not inversely graded
Provide information for stratigraphic facing and possibly current direction, e.g., if cross-beds are present
Must know what kind of depositional environment deposited the bed – example:
debris flows - deposit inverse graded beds,
storm deposits (tempestites) & turbidites are typically graded beds

8. Graded Bedding

9. Cross Beds provide information for facing and possibly current direction
Cross beds: Are surfaces within a thicker, master bed that are oblique to the bedding in the master bed
Defined by subtle parting or concentration of grains
Form when grains move from the windward or upstream side of a dune ripple, toward the leeward or downstream side
Topset: thin, usually concave upward, laminations parallel to the upper master bedding.
Foreset: inclined, curved, laminations or beds deposited parallel to the slip face. These merge with the topset and bottomset beds. Foresets define the cross beds. Current direction is perpendicular to the strike of the foreset
Bottomset: thin laminations parallel to the bottom master bedding

10. Cross Bedding

11. Cross Beds
Erosion truncates the topset and upper part of the foreset, juxtaposing younger bottomsets on the older foreset; this forms higher foreset angles at the upper bedding compared to the tangential angles below (used for facing).
The foreset beds are inclined at an angle to the main planes of stratification.
- Truncated at top
- Tangential at bottom .
- Dip direction indicates transport direction

12. Ripple Marks
Ridges and valleys on the surface of a bed, formed due to current flow. Cross stratification with wave amplitude < 6“
(1)     Oscillation or Symmetric Ripple Marks
Oscillation wave produced ripples (current moving in two opposite directions)
Crests are pointed and troughs are curved
Symmetrical concave up small scale (amplitude < 6") cross stratification.
Good facing indicator
(2)     Current or Asymmetric Ripple Marks
Asymmetric cross stratification produced by current moving in one direction; i.e., uniformly flowing current
Good current direction indicator

13. Ripple Marks

14. Mud Cracks Polygon shape in map view.
Result from desiccation into an array of polygons separated by mud cracks.
Thin (typically sand filled) fractures that taper down in cross section because each polygon curls upwards along its margin.
Good facing indicator (individual cracks taper downward.

15. Mud Cracks

16. Other Casts
Erosion or scraping, filling, subsequent erosion produce positive relief casts. Good indicators of current direction
Groove casts - Elongate nearly straight ridges
Bounce, Brush, Skip marks
All are discontinuous type of groove cast
Flute Cast – Asymmetric troughs formed by fluid vortices or eddies (mini-tornadoes) that dig into unconsolidated sediment
Stronger vortex at the upstream end cuts deeper and narrower than the downstream part which is shallower and wider. Thus, flute casts taper down-stream!

17. Sole Marks - Load Casts
Bulbous protrusions of denser sand into less dense mud layers
Forms due to density instability when sediment is still soft (i.e., still unlithified)
The sinking is triggered by the disturbance during earthquake, storm, or slump
At greater depths, partially consolidated mud breaks into pieces and sink into underlying sand, forming disrupted bedding

18. Contacts Contact: Boundary between two geologic units of any kind.
Depositional contact: a sedimentary unit is deposited on top of another.
Fault contact: two units are juxtaposed by a fault.
Intrusive contact: an igneous cuts across another rock body.

19. Unconformities
Conformable contact: The boundary between adjacent beds or units does not represent substantial gap in time
A succession of beds of nearly the same age that represent nearly continuous deposition
Diastem
Erosion surfaces within a conformable succession of strata
Unconformable contact (unconformity):
Represents an interruption in sedimentation, such that there is a substantial gap in time (called hiatus), few years to billions of years, across the contact
Contact represents erosion or non-deposition of strata

20. Unconformity

21. Four Types of Unconformity
Angular unconformity - Beds below and above the unconformity have different attitudes.
Beds below are truncated by the unconformity.
Buttress (onlap) unconformity – New beds lie on areas with significant pre-depositional topography.
The younger layers are truncated by the rugged unconformity (difference with angular unconformity).
Beds above and below the unconformity may or may not parallel the unconformity.
There is an angular discordance between the beds above and below the unconformity

22. Types of Unconformity
Disconformities – Beds above and below the unconformity are parallel, but there is a hiatus, created by non-deposition or erosion.
A disconformity is hard to recognize in the field Fossils, paleosols, or scour features help!
Nonconformities – Strata deposited on older, crystalline (igneous or metamorphic) basement rocks

23. Identifying Unconformities
Basal conglomerates, rest on unconformable surface and contain fragments (clasts) of underlying rocks
Topographic relief
Paleosols - Ancient soils, weathered zone just below the unconformity
Recognized by color change, and soil structures

24. Soft Sediment Penecontemporaneous Structures
Sediments may be deposited with a gentle initial dip. In this case, gravity may pull them down during storm or earthquake. The downslope movement is helped by fluid pressure
If sediments that move down the slope are soft, they may produce a slurry of clasts suspended in a matrix called debris flow. When the debris flow comes to rest, it forms a poorly-sorted conglomerate
If the sediments are compacted sufficiently before they are dislodged by gravity, they maintain their cohesion, and produce what is called slumping

25. Penecontemporaneous structures
The folds and faults formed during slumping are called penecontemporaneous
Penecontemporaneous means that they formed almost (hence “pene”) at the same time as the original deposition of the layers
Penecontemporaneous folds and faults are characteristically chaotic
They are intra-formational, i.e., bounded above and below by relatively undeformed strata

26. Growth Faults
 Synsedimentary faulting - fault displacement continues as sediment is deposited on top of the fault blocks
Thickness of sedimentary units varies across the fault

27. Volcanic Structures Flow Layering
Layers of volcanic flows defined by color, texture and weathering.
Flow structures
Pahoehoe; Ropy lava - Good flow direction indicator
Pillow Structures
Flat bottomed, curved top basalt encased in thin obsidian cover
Good facing indicator

28. Volcanic Structures, cont’d
Vesicles
Voids formed by gas bubbles typically more numerous at the top of the flow
Good facing indicator
Columnar Jointing
Fractures formed in basaltic lava due cooling and shrinkage
Polygonal columns
Product of slow cooling, top of flow does not have as well defined columnar joints as base of flow. Good facing indicator

29. Intrusive - Plutonic Structures
Flow Foliation
Aligned minerals in intrusive igneous rocks occurs while rocks are still melted or partially melted and flowing.
Indicates flow direction