Seds - Sequence Stratigraphy Flashcards
Definition of stratigraphy?
The study of the vertical sequence of the layered (strata) rocks and the lateral variations of individual beds on a regional basin
Layer cake and correlation principle
• Core of all geology
• Simple way of age dating and correlation
• Principles hold for any sequence of layered rock
• Sedimentary beds or strata (singular stratum) are deposited as layers
• Bounding surface separating layers (beds) are called bedding planes
• Bedding planes reflect changing conditions of sedimentation – very important for facies interpretation
– Gradational - gradual change in grain size
– Sharp - a period of non-deposition
Lithostratigraphy
Study of the physical relationship among rock units. There is no time connotation other than superposition
Geologists need to know the correct way-up of strata e.g. sedimentary structures, stratigraphic relationships etc.
Units are defined based on lithology
Rock units vary in space and time
Boundaries are subjective, and not physical since lateral facies changes are gradational
Biostratigraphy
trace index fossils
• They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil. If the species concerned were short-lived (in geological terms, lasting a few hundred thousand years), then it is certain that the sediments in question were deposited within that narrow time period. The shorter the lifespan of a species, the more precisely different sediments can be correlated, and so rapidly evolving types of fossils are particularly valuable. The best index fossils are common, easy-to-identify at species level, and have a broad distribution. Graptolites make excellent zonal index fossils in the Lower Palaeozoic.
• Best = common, broadly distributed and lived in a short time period
Chemostrigraphy
- Inorganic whole rock geochemical data used to define stratigraphic correlations in the petroleum industry for over a decade
- The stratigraphic technique of chemostratigraphy relies upon recognizing changes in element concentrations through time and using those to model changes in geological features, such as paleoclimate and provenance
- Gamma ray
- background radiation – using thorium mostly
- Trend with gamma ray content correlates with rock type – high gamma ray = sandstone, low gamma ray = mudrock
Sedimentation terms:
- Normal and Catastrophic sedimentation qualify processes and the sediments formed by these processes.
- Abundant and rare refer to the proportion of the facies in a sequence.
- Exceptional may be used for an event, a process or unique deposit
- Preservation potential – facies preservation can vary – magnitude and frequency of ‘energy’ levels
Unconformities in stratigraphy:
- A surface of erosion or non-deposition separating younger strata from older rocks, along which there is evidence of subaerial erosional truncation (and, in some areas, correlative submarine erosion) or subaerial exposure, with a significant hiatus.
- Unconformities mean a break in time - Unconformity shows change from one rock to another = erosional surface – angular
- Angular unconformity -erosion surface on tilted or eroded strata
- Disconformity - erosion surface or nondeposition between parallel beds
- Paraconformity – erosion surface between parallel beds – can be difficult to identify
- Nonconformity - erosion surface cut into Ig or Met Rx’s and covered by sedimentary Rx’s
Allostratigraphy
- Involves stratigraphic interpretation, correlation, and mapping which uses discontinuities and surfaces to subdivide the sedimentary section. These discontinuities and surfaces are assumed to have time-stratigraphic significance and may include unconformities, omission surfaces, ravinement surfaces, and flooding surfaces.
- Bounding surfaces – use of allostratigraphy
Sequence stratigraphy
The study of the relationship between sea level change and sedimentation
• Sequence stratigraphy is governed by sea level and works best with paralic sequences in shallow marine – e.g. deltaic systems, beach and shoreface systems and shallow water carbonates.
• Deltas – best examples of applying sequence stratigraphy
• Baselevel or sea level rise and fall is a fundamental control on sedimentation and also known as a allocyclic control on sedimentation.
• Sea level controls nic points of rivers so that river needs to equilibriate as sea level changes
• Non-marine (fluvial) areas are not influenced by sea level so sequence stratigraphy cant be applied
Concept of facies
- Facies: a distinctive packet of sedimentary rock with specific sedimentary features. The characteristics of the rock unit come from the depositional environment
- Lithofacies: facies type based on grain size and mineralogy e.g. shale facies; oolitic limestone facies. Can also have biofacies.
- Facies Relationships
- Walther’s Law: Vertical succession of facies reflects lateral changes in environment. Fundamental for understand transgressions and regressions
Accommodation Space:
• The space available for potential sediment accumulation”.
• This space is the combined product of movement of:
o The sea surface (eustasy: global sea level measured from a datum such as the center of earth)
o The sea floor (tectonics)
o Changes in rates of sediment accumulation.
• Accommodation space is a primary control upon sedimentation and ultimately facies relationships
• The space created for the storage of sediment is controlled by several external factors – one of which is Sea level and vital for correlation.
Parasequences:
• Relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces and their correlative surfaces.
In addition to these defining characteristics, most parasequences are asymmetical shallowing-upward sedimentary cycles.
Deep Sea Sedimentation:
• Deep-sea Sedimentation has two main sources of sediment: external material from the land and internal-biogenic and authigenic from the sea.
• Red Clays: Kaolinite Chlorite Quartz Feldspar
o The Late Cretaceous chalks of NW Europe are made from predominantly calcareous microfossils called coccolithophores, which rained down on the sea floor from the sunlit waters above. Each miniscule individual has a spherical skeleton called a coccosphere, formed from a number of interlocking calcareous discs called coccoliths. After death, most coccospheres and coccoliths collapse into their constituent parts and accumulate to form chalk a type of fine-grained limestone
• Turbidites – deep marine sands and silts
• Pelagic chalk
• Pelagic - Pelagic sediment or pelagite is a fine-grained sediment that accumulates as the result of the settling of particles to the floor of the open ocean, far from land
Growth faults – Extentional faulting
- High sedimentation rates with commonly fine-grained sediments
- Occur at same time as sedimentation – syn-tectonic
- Commonly associated with deltaic environments
Strike-slip tectonics – pull-apart basins
- Thick, locally confined sediments (up to ~6 km)
- Often locally sourced sedimentation
- Strong tectonic influence on sedimentation
