Stratigraphic Record 1-5

What is stratigraphy?

he study of rock layers (strata) and their relationships to determine the geological history of an area

What types of events can be identified through stratigraphy?

Climate, tectonic, biological events, and depositional records

Give examples of climate events identified through stratigraphy.

occurrence of coral reefs indicating climate warming, and dropstones indicating a glacial period

What are some examples of tectonic events identified through stratigraphy?

tilting of strata, folding and metamorphism indicating orogenesis and change from sub-rift to post-rift strata indicating rifting and continental breakup

How can biological events be identified through stratigraphy?

Via the appearance or disappreance of certain species

What can the depositional record in stratigraphy reveal?

types of sediments deposited in a specific environment

relative geological ages

The sequence of rock layers and events in relation to each other, without specifying an absolute number of years

numerical (absolute) geological ages

the absolute number of years since a particular event occurred

principle of superposition

states that each bed is younger than the one below it

principle of cross-cutting relationships

a bed or unit is younger than the one it cuts across

principle of original horizontality

consolidated strata that are tilted now were originally deposited horizontally

principle of continuity

That in a basin, the same bed is of identical age in every part of the basin. Beds terminate where deposition was not possible or when they are eroded or deformed

principle of inclusion

that if an element A is included in an element B, then A is older than B

faunal succession

fossil assemblages can be used to correlate and determine the relative ages of rock formations

How can faunal succession be applied in stratigraphy?

By comparing the vertical succession of fossils in different rock formations, relative ages are determined

What is the concept of time in stratigraphy?

Relative dating and faunal succession establish the chronological order of events recorded in the rock layers.

What is lithostratigraphy?

subdivision of stratigraphic units based on their lithological character and stratigraphic position

What are the hierarchical levels in lithostratigraphy?

supergroup, group, formation, member, and bed

What is a key or marker bed?

thin and distinctive bed that is widely distributed and can be used as a reference horizon

How are boundaries placed in lithostratigraphy?

At lithologic changes, which can be either sharp or gradational.

What is biostratigraphy?

study of the distribution of fossils in rocks to determine the relative ages of rock formations

How can species be used in biostratigraphy?

to indicate the presence, absence, coexistence, or abundance of certain species during specific periods of geological time

biozones

intervals of strata characterized by the presence of particular fossil species or assemblages

index fossils

fossils of organisms that lived for a relatively short time but had a wide geographic distribution

what are index fossils used for?

Correlating and dating rock formations.

radiometric dating

uses the decay of radioactive isotopes to determine the age of rocks or geological events

How does radiometric clock measure ?

the time of crystallization

How does the radiometric clock work in radiometric dating?

measures the ratio of radioactive nuclides to their original nuclides, using the decay constant and the known half-life of the radioactive isotop

What are the limitations of radiometric dating?

requires a fresh sample with a closed system, no original daughter nuclides present, and a variety of minerals

isotope stratigraphy

study of variations in isotope ratios of elements over time, which can provide insights into environmental, crustal, and biological changes.

How can isotope ratios be used in paleoclimate reconstruction?

by analyzing the variations in isotopes in geological samples, such as oxygen isotopes in ice cores or carbon isotopes in sediment records.

Why are isotope ratios compared to standards?

To determine their deviation from the expected natural composition, for highly unequal natural concentrations it reduces uncertainty

magnetostratigraphy

study of Earth's magnetic field preserved in rocks to determine the chronology of events & changes in the Earth's magnetic polarity

rock magnetization

atural orientation of magnetic minerals in the Earth's magnetic field, which creates a remnant magnetization in rocks below the Curie point

Curie point

temperature at which certain magnetic materials undergo a sharp change in their magnetic properties

What causes the flip in the orientation of Earth's magnetic dipole?

geological processes, resulting in changes in the magnetic field's polarity

What does the standard geological timescale provide?

framework for events, including tectonic events, changing sedimentary environments, biological evolution, and climate change

facies

a body of sediment or rock that has distinct lithological, physical, and biological characteristics, setting it apart from surrounding units

What are descriptive characteristics of facies?

grain type and composition, texture (grain size, sorting, roundness), sedimentary structures, color, and biological structures and content

What are the purposes of studying facies?

describing rocks, creating maps, interpreting paleoenvironments (past environments), and identifying hydrocarbon reservoirs or extracting groundwater

lithofacies

a specific facies name that reflects observable characteristics of rocks

How does the concept of lithofacies help in rapid logging or mapping?

convenient classification system that allows similar observations to be grouped together for quick logging, mapping, or detailed section measurements

How can the subdivision of facies be useful in analyzing facies distribution?

allows vertical and lateral trends in the distribution of facies can be analyzed, providing insights into the geological processes and environments

How is the scale of facies analysis determined?

objectives of the study, considering factors such as time, effort, budget, and the intended purpose

Issues with facies

Observation based, & single facies is characteristic of a depositional environment but often non-unique

facies association

collection of multiple facies that are genetically related and formed within a single depositional system

What types of analogues can be used for making the leap to environmental interpretation?

modern environments, ancient environments, theoretical models, and experimental studies

What are some characteristics of a deep-marine lobe facies association?

typically exhibits no major erosion, has widespread near-tabular beds, indicates decelerating turbidity currents, shows deposits that pinch out like a wedge, and represents a terminal part of the system

What are some features of a typical point-bar facies association?

includes a cut bank and point bar formed by migrating rivers in meandering systems, occasional formation of oxbow lakes, presence of lines representing previous river belt beds, and a decrease in grain size indicating lower energy conditions.

What are deep-water depositional systems?

sedimentary systems that lie predominantly below the storm wave base and serve as the ultimate resting place for sediment, organic matter, and pollutants

How do sediment gravity flows enter deep marine or lacustrine basins?

confined or unconfined, or a combination of the two

What factors influence the large-scale architecture of sand bodies in deep-water systems?

The interaction of sediment gravity flows with the slope over which they flow and the influence of local topography

What are some examples of topographic features that can affect sediment gravity flows?

Gravity tectonics, slump-induced topography, folding, and faulting (both pre-existing and syn-depositionally active features)

What is the range of sizes for deep-water sediments?

micron-scale pelagic suspension deposits to megaclasts and kilometer-scale submarine landslides

What were the consequences of the Grand Banks Earthquake in 1929?

submarine landslides that broke seafloor cables and provided the first direct evidence of turbidity currents in the wild

What are the classes of sediment transport in deep water?

Six classes that range from creep of consolidated material to full of completely unconsolidated sediment.

What are the two types of sediment gravity flows?

fluidal flows (turbidity currents) and laminar flows (debris flows)

turbidity current

flow driven by gravity that carries sediment-laden fluid, with sediment maintained in suspension by turbulent flow cells

turbidites

deposits formed by turbidity currents

How are turbidity currents initiated?

submarine landslides triggered by earthquakes or over-steepening, as well as through homopycnal and hyperpycnal flow

What are the different types of gravity flow deposits?

Turbidite (turbidity current), Debrite (debris flow), Hyperpycnite (hyperpycnal flow), Pelagite (pelagic sediment), Hemipelagite (hemipelagic sediment), Channel fill.

What are low-density turbidites also known as?

Bouma Sequence

hat are high-density turbidites associated with?

higher flow velocities and larger grain sizes

How are high- and low-density turbidites distributed?

High-density turbidites are typically found closer to the source of sediment supply, while low-density turbidites are more widely distributed.

What are some characteristics of thick-bedded turbidites?

channel deposition, bed amalgamation, clasts (intraformational mudflakes & extraformational grains), mix of facies including thin- and thick-bedded turbidites, bypass parts, debris flow deposits, and coarse grains

What are the characteristics of thin-bedded turbidites?

External levee sands or basin plain turbidites. With climbing ripples, no erosional structures, & lateral continuity from channel to pinch out. Internal levees may display multiple ripple sets, palaeocurrent complexity, and a less organized structure.

hybrid beds

Exhibit characteristics of both turbidity currents and debris flows. They are often well-developed in off-axis and distal positions.

pelagic sediment deposits in deep water

Pelagites

debris flow

high concentration mixture of fluid and sediment that flows downslope due to gravity

How are particles supported in a debris flow?

flow matrix strength and pore fluid pressure, which creates a buoyancy force

deposits of debris flows

debrites

What are the characteristics of debris flow deposits (debrites)?

chaotic mixtures of different grain types and sizes, poorly sorted, clasts are disorganized, and they occur frequently within canyons or channels in deep-water settings

What can make it difficult to discern debrites in core samples?

looks similar to bioturbated textures and local remobilization and/or injection. Large clasts in debrites may also be difficult to distinguish from laterally persistent mudstones

Debris flows

mixture of fluid and sediment

slides/slump

dominated by intact but rotated bedding that occur mainly on slopes but can also occur on the front- and back-sides of levees due to oversteepening or flow undercutting.

slides/slumps in subsurface and seafloor settings

mass transport deposits

thermohaline currents

flows driven by temperature and salinity differences in the water column.

shallow water

typically above the storm wave-base and influenced by wave and tidal activity. In the oceans, it is typically above 50m water depth

three modes of sediment transport in shallow sea/rivers

dissolved load/wash load, suspended load, and bedload

What drives sediment transport in shallow seas?

flow strength, wave action, and tidal activity.

Why is sediment transport in shallow seas important?

Shallow seas are highly populated and sensitive to sea-level change. They also play a crucial role in food production, infrastructure development, and sediment deposition.

What is the significance of regressive and transgressive shorelines?

Sensitive to sea-level change, and understanding their past changes is important for predicting future changes

shallowing upward

characteristic sedimentary deposits in shallow marine environments, where sediments coarsen-up, thicken-up, and shallow as they accumulate, separated by flooding surfaces

What is the effect of subsidence on transgressive and regressive deposits?

Subsidence accelerates transgression and slows down regression, leading to the fast deposition of transgressive deposits and slower building-out of regressive deposits. Transgressive deposits are often thin or absent in the rock record.

What are deltas and how do they form?

land formations that extend into the sea, formed when rivers deposit sediment faster than it can be transported away by waves and tides

main mechanism driving flow in deltas

pre-existing inertia of the river as it enters the sea drives the flow in deltas

How does the density of the river jet affect its behavior in deltas?

When the density of the river jet is higher than the ambient water, the jet hugs the bed. When the density is lower, the jet detaches from the bed.

What are the three types of river jets in deltas based on density?

hyperpycnal (density higher than ambient water), hypopycnal (density lower than ambient water), and homopycnal (density similar to ambient water).

How do mouth bars and mouth bar formation contribute to delta evolution?

cause choking of the river mouth and lead to the rapid switching and migration of the distributary channels, resulting in the formation of radial "fan" deltas

What are the classic characteristics of delta deposits?

exhibit seaward-dipping surfaces called clinoforms and coarsening-upward successions, with prodelta muds, delta front (mouth bar) heterolith and sandstone, and channel-based sandstones.

What determines the long-term fate of a delta?

analyzing stacking patterns and the vertical and lateral cycles of shallowing-up.

What are the different continental clastic environments?

rivers (fluvial, alluvial), lakes (lacustrine), glaciers (moraine, outwash), deserts (aeolian), volcanic areas (pyroclastics, debris flows), soils, and urban areas.

What is the concept of source-to-sink?

A complete sediment routing system and involves studying the link between sources, transport, & deposition.

Why is the source-to-sink approach useful?

improves our understanding of landscape and seascape evolution, encourages interdisciplinary thinking, and enhances predictability in ancient systems

Why are fluvial systems important?

They deliver sediment, organic carbon, & pollutants to the coast, provide water resources, cause flooding, generate power, & fishing

river

large conduit for water and sediment flow

fluvial

processes and bedforms related to rivers

alluvial

processes and deposits occurring outside the river channel (such as floodplains and deltas)

What are the two main kinds of rivers?

bedrock rivers, where part of the bed is bare rock, and alluvial rivers, where the bed consists of sediment

What factors affect the erosion rate in bedrock rivers?

the slope and the presence of sediment (tools).