Primary Structures

Geologic StructureA 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

Primary Sedimentary StructuresBedding: 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 depositionEmphasized in outcrop by parting and differential weathering and erosionA 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 claysCommonly, there is a bedding-parallel fracture which forms due to unloading or rocksClosely-spaced parting is called fissility (e.g., in shale).

Bedding between inter-bedded sandstone and conglomerate

Reasons Why Clays or Clasts are Preferrably-Oriented?

1. Sedimentary settling of elongate or planar flakes in the gravity field (syn-depositional)

2. 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

3. Reorientation (rotation) due to post-depositional compaction (squeezing of unlithified sediment due to the weight of the overlying rocks).

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 environmentStratigraphic facing (younging direction) to identify right-side-up or overturned bedsCurrent direction

Beds help us to better map stratigraphic contacts, and identify large structures such as folds, faults, and unconformities.

Bedding between sandstone & conglomerate

Graded BedsGraded 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 gradedProvide 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

Graded Bedding

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 bedDefined by subtle parting or concentration of grainsForm when grains move from the windward or upstream side of a dune ripple, toward the leeward or downstream sideTopset: 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 foresetBottomset: thin laminations parallel to the bottom master bedding

Cross Bedding

Cross BedsErosion 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

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 MarksOscillation wave produced ripples (current moving in two opposite directions)

• Crests are pointed and troughs are curvedSymmetrical 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

Ripple Marks

Mud CracksPolygon 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.

Mud Cracks

Other CastsErosion 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!

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

ContactsContact: Boundary between two geologic units of any kind.

1. Depositional contact: a sedimentary unit is deposited on top of another.

2. Fault contact: two units are juxtaposed by a fault.

3. Intrusive contact: an igneous cuts across another rock body.

UnconformitiesConformable 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

1. DiastemErosion 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 contactContact represents erosion or non-deposition of strata

Unconformity

Four Types of UnconformityAngular 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

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

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

Soft SedimentPenecontemporaneous 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

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

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

Volcanic StructuresFlow Layering

Layers of volcanic flows defined by color, texture and weathering.

Flow structures Pahoehoe; Ropy lava - Good flow direction indicator

Pillow StructuresFlat bottomed, curved top basalt encased in thin obsidian coverGood facing indicator

Volcanic Structures, cont’dVesicles

Voids formed by gas bubbles typically more numerous at the top of the flowGood facing indicator

Columnar JointingFractures formed in basaltic lava due cooling and shrinkagePolygonal columnsProduct of slow cooling, top of flow does not have as well defined columnar joints as base of flow. Good facing indicator

Intrusive - Plutonic Structures

Flow FoliationAligned minerals in intrusive igneous rocks occurs while rocks are still melted or partially melted and flowing.

Indicates flow direction