Fundamental Structures - Introduction Part 2 PDF

Fundamental Structures ▪ Primary Structures: These are sedimentary structures that may be in strata prior to deformation. They may be quite useful as strain markers (giving us an initial state) and as way-up indicators, etc. ▪ They must not be mistaken for secondary structures,...

Fundamental Structures ▪ Primary Structures: These are sedimentary structures that may be in strata prior to deformation. They may be quite useful as strain markers (giving us an initial state) and as way-up indicators, etc. ▪ They must not be mistaken for secondary structures, which are the result of deformation. ▪ Primary structures are either sedimentary or igneous only. ▪ Structures which form during metamorphism are secondary. ▪ Layering (bedding and lava flows) is the most common and important primary structure, and most other primary structures can be related to their position in the layers. ▪What do primary structures tell us: The environmental conditions at the time of sedimentations etc. Geometry of the basin of deposition The younging vector points from the The original orientation of the rocks (horizontal) base of the bed The stratigraphic sequence (“younging”). towards the top of the bed ▪ An example of the use of “younging” in structural geology: If A and B are younging in the same This is quite direction then A is a younger bed than B common A B A B and we probably have a normal stratigraphic sequence. If A and B are younging in opposite directions,. it is likely that A and B are the same bed Primary structures formed at the tops of beds repeated by tight folding and the sequence is not a normal stratigraphic one.. ripple marks trace fossils rain prints ropy lava top Raindrop impressions can appear as: small craters on the top of the layer on which the rain was fallen, Delicate raindrop imprints formed by rain Or as: small raised bumps on the bottom of the upper layer Primary structures formed at the bottoms of beds flute casts bottom of the bed load casts scour-and-fill top of the bed palaeocurrent low density silt injects bottom of direction erosional truncation upwards into denser sand top of the bed of beds the bed groove casts Primary structures formed within the beds (internal structures) Convolute bedding palaeocurrent graded cross direction bedding water escapes bedding upwards  Angle of Repose Cross-beds caused by desert sand dunes Convolute Bedding- Bahariya Oasis Gebel El Dist  Some more Primary volcanic structures which are useful for younging disc-shaped vesicles vesicle shapes pillow lavas near top of the flow spherical vesicles chilled lava base tubular vesicles the base of a pillow conforms to at base of the flow the shape of the pillows below baked contact Some other primary igneous structures flow lineations flow foliations These form planar and linear fabrics schlieren are complex flow bands defined by concentrations of dark minerals, e.g. biotite Columnar Joints Columnar Joints There is an internal structure called a SLUMP which forms by collapse of wet sediments in response to tilting or agitation or loading: palaeoslope They are formed by sliding downslope so they are good palaeoslope indicators Slumping is particularly common in cross-bedded strata because the cross-beds are already tilted Slumps are folds, often tight but confined to a single bed Since they form after deposition they cannot be truly primary structures Since they form after deposition they Structures formed after cannot be truly primary structures lithification are SECONDARY Sediment deposition Sediment lithification time Structures formed after deposition but before any lithification processes are called PENECONTEMPORANEOUS e.g. slumps, mud cracks Penecontemporaneous Structure  Mud Cracks Mud cracks - cracks produced by desiccation (drying) of clays/silts during subaerial exposure  Unconformities ▪ Boundaries separating one rock unit from another are of two types: ▪ 1. Normal conformable Conformable contact ▪ 2. Unconformable ▪ Horizontal contact between rock units with no break in deposition or erosional gaps is a normal conformable contact Unconformable contact Unconformities ▪ Erosion surfaces representing a significant break in deposition Angular unconformity (and geologic time) ▪ Angular unconformity ▪ Disconformity ▪ Non-conformity Angular Unconformity, with conglomerate erosional surface Bahariya Oasis large-scale cross-bedding with prominent, V-shaped erosive scours (unconformity surface) infilled by transgressive lag deposit at the top, Fayoum Desert, Egypt Disconformity ▪ Erosional gap between rock units without angular discordance Angular unconformity ▪ example: fluvial channel cutting into underlying sequence of horizontally bedded deposits  Nonconformity Sedimentary strata overlying igneous or metamorphic rocks across a sharp contact. example: Precambrian-Paleozoic contact in Jordon. Angular Unconformity, with conglomerate erosional surface Bahariya Oasis  Structural Relations ▪ The structural relations between bed contacts are important in determining: ▪ presence of tectonic deformation/uplift ▪ sequence of events ▪ relative ages of rock units ▪ principle of original horizontality ▪ principle of original lateral continuity ▪ principle of cross- cutting ▪ principle of inclusion Dykes and Faults are Discordant Structures - Wadi Feiran Sinai (Discordant relationship). Principle of Inclusion 1 2 Inclusions within a host rock are always older than the host (J. Hutton) Fluorite in Quartz Principle of Original Horizontality ▪ Sedimentary rocks are deposited as essentially horizontal layers (Steno, 1600’s) ▪ Exception is cross- bedding (e.g. delta foresets) ▪ Dipping sedimentary strata implies tectonic tilting and/or folding of strata ▪ Sedimentary strata extend as laterally continuous layers in all directions within a basin Horizontal Layers until they thin and pinch out and terminate against the basin edges. Deformation  Deformation ▪ Deformation is a general term that refers to all changes in the original form and/or size of a rock body ▪ Most crustal deformation occurs along plate margins ▪ Deformation involves oForce – that which tends to put stationary objects in motion or changes the motions of moving objects ▪ Deformation involves oStress - force applied to a given area oTypes of stress ▪ Compressional stress – shortens a rock body ▪ Tensional stress – tends to elongate or pull apart a rock unit ▪ Shear stress – produces a motion similar to slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom  Differential Uniform Stress Stress Stress ▪ Uniform o compressional ▪ Differential o Compressional o Tensional o shear Differential Stress Compressional Stress Tensional Stress Shear Stress  You Than k