Chronostratigraphy

The idea behind chronostratigraphy is to correlate rocks that formed at the same time. This is useful for reconstructing events and depositional environments in earth history as well as finding resources like oil. There are several techniques that can be used for chronostratigraphy, including: event stratigraphy, magnetostratigraphy, chemostratigraphy, biostratigraphy, and sequence stratigraphy. Here, I will address event stratigraphy, magnetostratigraphy, and biostratigraphy. Sequence stratigraphy is very powerful, and lots of resources on it can be found at: http://sepmstrata.org/

Event Stratigraphy
Event stratigraphy involves identifying the sedimentary effects of an unusual even in multiple stratigraphic columns. If one can demonstrate the the effects were all produced by the same even, one can reasonably interpret the effects to have happened at the same time in the different columns. For example, if a volcano erupts and deposits ash over a broad region, that ash is preserved in the stratigraphy, and a geologist can demonstrate that the ash in multiple sections came from the same eruption, then the geologist can create a chronostratigraphic correlation among the sections. Other events that are useful for event stratigraphy can include impact debris layers (for example at the Cretaceous-Tertiary boundary), tsunami deposits, and sometimes large storms.

There are some shortcomings of event stratigraphy. First, there has to be an event that affects the stratigraphy. For it to be useful, it needs to be something that affects multiple depositional environments in a way that produces a distinctive set of features that can be distinguished from the normal depositional processes. Second, for a specific event to be useful for correlations, it has to have affected the stratigraphy at the sites of interest. For example, a tsunami that affected the west coast of North America might help one correlate Pleistocene coastal deposits in Oregon and Washington. However, it would not be helpful for correlating Pleistocene rocks in Florida because it did not influence them. Third, if there are multiple events, the geologist has to sort out which correlate with each other. For example, if there are multiple volcanic eruptions at different times, the geologist needs to evaluate which eruptions the ash beds might represent. It can become complicated to correlate many events. Sometimes correlations are more reliable if there are fewer events, but then there are not as many potential temporal ties between the stratigraphic columns. Even with these complications, event stratigraphy is a very valuable tool. When the events are volcanic, the ash beds can often be dated, providing a precise age for a segment of the stratigraphic column.

Magnetostratigraphy
Magnetostratigraphy uses preserved magnetization of rocks for correlation. The magnetization comes from the alignment of magnetic minerals in sedimentary rocks (and other types of rocks) with the earth’s magnetic field. Small magnetic minerals, especially clay-sized minerals, align like little magnets, and when the sediment is lithified, that magnetization can be preserved. Under the right conditions, samples can be collected and the direction of magnetization measured. Data can be used to reconstruct the direction of the earth’s magnetic field. This magnetic field can reverse directions due to the dynamics of circulation in the core. In other words, sometimes the magnetic field is aligned such that magnets point north (as they do now, and called “normal” in the scientific literature) and sometimes it is aligned such that magnets point south (called “reversed”). The earth’s magnetic field changes at close to the same time globally, so the effects are seen everywhere. Paleomagnetists have studied well dated sedimentary and volcanic rocks and have mapped out the times in earth history where the magnetic field was normal and reversed (see: http://www.geosociety.org/science/timescale/). This provides a reference that can help correlate other stratigraphic sections.

To correlate a suite of stratigraphic sections using magnetostratigraphy, one would collect samples, measure their magnetic properties using a variety of techniques, and evaluate whether or not they have been remagnetized. If they have not been remagnetized, changes in the direction of earth’s magnetic field can often be interpreted from the results. If a geologist has multiple sections that were deposited at the same time, they can interpret the changes in the direction of earth’s magnetic field to have happened at the same time. Unfortunately, however, one can not necessarily independently tell the many normal intervals apart from each other nor the many reversed intervals apart. Thus, the geologist needs additional information to make reliable chronostratigraphic correlations.

Biostratigraphy
Biostratigraphy is an extremely powerful tool for chronostratigraphic correlation. Life evolves through time, with new species emerging and other species going extinct. For time intervals and species that are well studied, the process of evolution provides a detailed temporal framework for correlating stratigraphic columns. The basic idea is for the geologist to identify fossils in the stratigraphic columns, compare them to the ranges of those organisms know from previous studies, and then interpret the age of the rocks from documented extinction and species origination events. This is an extremely powerful approach to correlating stratigraphic columns because each species is unique and changes through time. However, not all organisms are useful for biostratigraphy.

Good biostratigraphic species: 1) have short geological ranges, e.g. they did not live for millions of years, and evolved quickly; 2) were distributed over a large region of the earth; 3) were easily preserved; and 4) were abundant. They also need to be well studied.

Zones of well documented species with distinct origination and extinction times can be defined a number of ways. A zone could consist of the total time of existence of a fossil, it could consist of the time where two or more fossils coexist, it could be defined as the time between the origination of one fossil and the extinction of a different fossil, etc. An example of a biostratigraphic zone chart, combined with magnetostratigraphic reversals can be found at: http://www-odp.tamu.edu/publications/189_IR/chap_02/c2_f6.htm

Interpreting Stratigraphic Columns

Here is an outline of how to approach interpreting the depositional environment represented by a stratigraphic column.

Step 1: Look for sedimentary structures that are characteristic of a specific environment or process
Examples:

HCS

wave ripples (vs current ripples)

herringbone cross stratification

reactivation surfaces

mud drapes in sandstone

bouma sequence

mud cracks

root casts

ripple cross lamination with reverse grading

meter-high dunes in fine sand

diamictites with facetted clasts and striations

lone (or drop) stones in laminated shale


Step 2: Evaluate how these distinctive structures relate to each other in the stratigraphic column to develop a tentative environmental interpretation

Are there several indicators of waves or storms?

Are there several indicators of tides?

Are there several indicators of wind-deposited sediment?

Are there several indicators of glacial activity?


Step 3: Compare the tentative interpretation to flow implied by other sedimentary structures in the column and evaluate whether they are consistent with your tentative environmental interpretation.
Examples of other sedimentary structures:

Trough cross stratification

Planar cross stratification

Current ripple cross lamination

Planar lamination

Step 4: Evaluate how the vertical sequence of sedimentary structures changes to refine or correct your environmental interpretations.
Do structures occur in a distinctive pattern that suggests a depositional environment?

Is there an erosion surface followed by dune stratification followed by ripple lamination followed by a rooted horizon? (Then it might be migrating river channels or tidal channels if there are indicators of tidal currents.)

Do the structures suggest an environment that shallows upward into a river system? (Then it might be a delta building out into standing water.)


Step 5: Use Walther's Law to refine your environmental interpretations and to test whether or not they are reasonable.
Try to sketch neighboring environments and interpret how they shifted through time. Are your interpreted vertical changes in environments consistent with neighboring environments horizontally? Does you interpretation require any jumps in environments or imply an unconformity? Revise your interpretation until it is consistent with your data.

Often, there is some ambiguity about the depositional environment(s) represented in real rocks. By going through this process, you can reach a reasonable interpretation that is well supported by the data. You will also understand where the ambiguities are. This is particularly helpful if it is your own data and you can make more observations by doing more field work.

Alluvial Fans (in brief)

Alluvial fans are cone-shaped accumulations of coarse sediment deposited at the transition from confined flow in a canyon to unconfined flow in a basin. This also corresponds to a break in slope. As the slope shallows and the flows spread out, the flows slow down and deposit much of the sediment that they were able to transport in the canyon. (Think about the Hjulstrom diagram.) Fan geometry is determined by the rate of deposition. At the canyon mouth, it is steeps (up to 15°) due to rapid deposition of coarse sediment. It shallows to about 5° over the main part of the fan and shallows even more to 1-2° at the toe. Only the suspended sediments are transported beyond the toe, along with dissolved ions. If the water can pond, the fine grains settle out and the water evaporates forming minerals like gypsum and halite, and creating playa lake deposits. Deposition on a given alluvial fan is very rare - one event occurs about every 300 years on most fans in the southwestern US.

Flow types - Two types of flows are common: 1) debris flows and 2) sheet flows. Debris flows are slurries of mud, rock debris, and just enough water to make the sediment into a viscous flow. Due to the high viscosity, the flow is laminar, like a glacier, and like a glacier, there is no significant sorting of grain sizes. Debris flows can transport very large blocks. Debris flows continue to move until the internal friction of the flow due to viscosity exceeds the flow’s momentum when it freezes into place. This can occur due to either the loss of water or lower slope. The resulting deposits show little sorting and would be classified as a mud supported breccia or a diamictite. In most cases, debris flow deposits are unsorted and lack any form of stratification. They are laterally restricted because they don’t spread out too much, and they are commonly an even thickness throughout, with steep edges to the flows.

Sheet Flows - Sheet floods are turbulent flows with significantly more water and less mud than debris flows. Since the flows are turbulent, there is significant grain sorting and normally graded, fining upward deposits are common. Once a flow reaches the mouth of the canyon, the flow spreads out and the coarsest rocks are deposited first. Finer grains are deposited later and farther down the fan and later in time. This produces normally graded beds, but deposition is very rapid and the grading is commonly poor. The suspended load may make it to the toe of the fan if the water doesn’t filter into the fan first. Sheet flood deposits produce broad deposits that are clast supported, with some imbrication of clasts. Unlike a debris flow, sheet flows commonly cover 1/3 to 1/2 of a fan surface.

Other types of flows - A number of other flow types are also common on fans. For example, if there isn’t sufficient rain to produce a sheet flow, ephemeral rivers can flow down the surface of the fan - which is more common. This produces braided river type deposits. There is also a significant gradation between debris flows and sheet floods. They represent two end members, and there are lots of variations in mud content and water content which variously affect the viscosity of the flow and thus the resulting sedimentary deposits.

Laminar & Transitional Flows

Re= l*u*ρ/µ

Ice
Ice flows are laminar because they have very high viscosity. This can be seen in the ice cliffs along the edges of glaciers in Taylor Valley, Antarctica. The ice is particularly cold and is so viscous that it does not flatten out on the time scale of at least dozens of years. Because the flow is laminar, when the ice melts or sublimates, it dumps all grain sizes into one deposit, called a diamictite. (Diamictites are defined as very poorly sorted sedimentary rocks with no grain size sorting within them. Thus, debris flows also deposit.) If one knows that the diamictite was deposited by ice, it is then called till or tillite.

If the glacier melts on land, it leaves piles of till in moraines. Water from melting of the ice is turbulent (unless it contains lots of mud-sized sediment). Thus, the turbulent water from melting ice can sort sediments, usually associated with braided river facies due to a high proportion of bed load sediment.

If glacial ice melts over water, the debris is deposited into the water, commonly forming a till sheet. If only a few large clasts are deposited in the water, they are called “drop stones”. This is one way to get large grains in very deep, standing water.

Mudflows & Debris Flows
High concentrations of mud-sized grains, particularly if they are clay minerals, increase the viscosity of the flow and decrease the flow speed. Both of the reduce the Reynolds number, leading to laminar or, more commonly, transitional flows. These flows can transport boulders at very low flow speeds compared to a water flow with little mud. Also, the grains are not well sorted by size. All grain sizes are transported together and when the flow stops, they are deposited all at once. This creates a diamictite, like glacier ice melting does. (However, it does NOT produce a tillite, which has to have a glacial origin.)

Mudflows (few large clasts) and debris flows (many large clasts) commonly form when there is a very high supply of mud-sized grains relative to the amount of water. The ratio of mud to water affects viscosity and thus the level of turbulence in the flows. If sediment is deposited or water is added, they can become more turbulent through time. In contrast, if water drains out or more sediment is added to the flow through erosion, they become more viscous. If they become too viscous, they can “freeze” in place, leaving a thick layer of diamictite.

Flash floods and land slides are processes that can initiate mudflows and debris flows.

Here is a play list of laminar and transitional debris flows:
http://www.youtube.com/user/sumnerd#grid/user/93B0D9433B8C4275

Carbonates (in brief)

Carbonates
Carbonate rocks form from ions in seawater. Thus, their deposition and accumulation is somewhat different than it is for siliciclastic sediments. They do not require that sediment is transported into the environment. Rather, they require specific chemical, temperature, and biological conditions in the environment where they form.

Most carbonates (during Phanerozoic time) are created by living organisms as shells and skeletons. (During Precambrian time, microbial communities strongly influenced carbonate mineral precipitation.) Corals, snails, clams, etc. are good examples. The reaction to form the carbonate minerals calcite or aragonite (which have the same mineral formula) is:

Ca²⁺ + CO₃^2- = CaCO₃

Because HCO₃^- is more abundant in seawater than CO₃^2-, the actual reaction that takes place is:

Ca²⁺ + 2HCO₃^- = CaCO₃ + H₂O + CO₂

The = means the reaction can go either way. If it goes from left to right, calcium carbonate minerals - calcite or aragonite - form. If it goes to the left, calcium carbonate minerals dissolve.) The chemical reaction to form the minerals is most likely in warm water, particularly in warm agitated waters. Breaking waves help get rid of the CO₂ produced by the reaction, which makes the reaction proceed even faster. Also, some corals contain photosynthetic organisms within their tissues, and those organisms consume CO₂, which also helps with the mineralization process. Thus, carbonate minerals form in warm, shallow seawater. The accumulation of carbonates creates “carbonate platforms” around many tropical islands (e.g. Caribbean islands, Bahamas, Hawaii, etc.) and along tropical shorelines (Florida, Great Barrier Reef of Australia, etc.).

Channels south of Highbourne Cay, Bahamas:

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This atol NW of Hawaii consists entirely of a reef that built up to sea level and the extinct volcano that underlies it subsided:

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One major source of carbonate sediments is from calcifying algae. These organisms are very abundant. They produce the minerals as sand or mud grains (depending on the species) within their tissues. When the algae die, the carbonate grains are released into the environment to be transported by waves and currents. At this point, the grains behave more or less the same as siliciclastic grains, with coarser sediment requiring high flow speeds to be transported, and mud-sized grains requiring very low flow speeds to settle from suspension. In shallow environments, muds accumulate in deeper areas of lagoons or get transported off shore into deeper waters. The grains get concentrated into shoals where water speeds slow down, for example, where water is channeled through a reef into a lagoon. These grains can also grow through carbonate mineral precipitation forming coated grains, or ooids. Thus, they get coarser with time.

A second distinctive feature of carbonates is the growth of reefs. Corals and other skeletal organisms grow well in high energy zones with breaking waves. Their skeletons make them resistant to erosion, and the breaking waves enhance carbonate mineral formation. Also, the precipitation of more carbonate as cements makes the structures hard and very resistant to erosion even though they are in high energy zones with breaking waves. These reef ecosystems can grow very quickly, creating a topographic high located off shore. This high induces more breaking waves, changing the energy distribution across the carbonate platform. The distribution of grain sizes around a reef depend on the flow speeds, similar to the dependence for siliciclastic grains, but the reef itself is cemented in place and provides a unique environment. Grains that are broken off tend to be transported to the inside or outside of the reef where water depths increase and flow speeds slow.

As reefs grow upward, they create very steep slopes, sometimes almost vertical slopes. These slopes can be unstable long-term, and they can fail, creating breccia in deep water and inducing turbidites as in siliciclastic sediments.

See figure 15.12 in Nichols, Edition 2 for the distribution of environments across a reef.

Marine Shorelines

Shorelines are the interface between the land and the oceans. Their characteristics vary depending on the balance of sediment supply and transport processes. When the sediment supply from rivers is large compared to the rate at which transport processes redistribute the sediment, deltas form, building out into the ocean. If sediment supply is low compared to the rate of sediment transport seaward of the shoreline, the shoreline erodes back. When sea level rises, river valleys can become flooded with marine water, creating estuaries. When sea level falls, rivers tend to erode downward into the previously coastal sediments.

The balance between tides and waves also affects the geometry of shorelines. Wave-dominated shorelines tend to have beaches, whereas tide-dominated shorelines tend to have broad marshy flats. Either can be erosional if the offshore transport of sediment is higher than the sediment supply or constructional if offshore transport is lower. They can shift back and forth through time if sediment supply or transport processes change. Thus, most shorelines are dynamic environments that vary significantly on human time scales.

Wave Influenced Shorelines
Waves have very specific sediment transport characteristics, with the highest energy flows near the breaker zone and lower flows both onshore and offshore. The onshore flows transport sediments to form beaches. The swash zone is the area that forms the primary beach. During storms, the waves are commonly higher, and, if sufficient sediment is available, they carry sediment farther up the beach, creating a berm. This gives the beach a characteristic slope up away from the shore, a crest, and then a slope downward. In some cases, the beach can extend off the coastline, creating a barrier bar or barrier island. A lagoon then forms between the beach and the main coastline. When there is a large sand supply, these barrier bars and islands can grow to be quite large. However, waves also transport sand off shore, going from the high energy breaker zone to the lower energy deep water. If the sand supply is low, more sand can get transported offshore than is delivered to the beaches. This causes beaches, barrier bars, and barrier islands to erode.

Sites with beaches and barrier bars include (I strongly recommend exploring the larger maps):
North Carolina Coastal Plane:

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Niger Delta:

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An estuary feeding into the Black Sea:

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Tide Influenced Shorelines
Tidal currents flow on and off shore every day or twice a day. When tidal ranges are high, tidal currents can be strong, redistributing sediment either onshore or offshore. These tidal currents often become channelized, and they begin to act like rivers, with meanders, etc.

Sites with tidal channels include:
An erosional tidal shoreline north of Derby, Western Australia:

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An estuary in Madagascar:

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The Ganges-Brahmaputra Delta:

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Constructional Shorelines: Deltas
Deltas form at the mouths of rivers that transport enough sediment to build outward. (Building outward is a key component of the definition of a delta. Rivers where the ocean or lake floods the river valley flow into estuaries.) Deltas require substantial accumulation of sediment, in contrast to estuaries which do not build outward. Sedimentary facies are similar to other depositional environments, but the association of subenvironments are recognizable as deltas. Some of the sub environments include: river facies with all the associated sub environments; shore line deposits including beaches, marshes/swamps, etc.; submarine shelf and slope facies, including storm deposits and turbidites; etc.

In lecture, I drew cross section and map views of deltas showing the delta plane, delta slopes, and prodelta. Rivers flow through delta planes and slow when reaching water, producing a mouth bar. Grain size decreases with distance away from the river mouth into the ocean/sea/lake.

Progradation - Because deltas are sites of sediment building outward from the coast, they are progradational; the landward depositional environments move seaward over more marine/lacustrine deposits. Thus, delta sequences in the rock record start with deep water, marine, fine grained sediments and grade upward into shallower water, possible more freshwater, coarser grained sediments. This is one of the distinguishing aspects of deltas that let you define them in the sedimentary record. These changes in grain size and environment typically occur over 1’s to 100’s of meters in the rock record and include many beds.

Sediment Transport Type - All deltas (by definition) have their sediment transported to the delta by rivers. Thus, riverine deposits are always associated with them. In addition, depending on marine (or lacustrine) conditions, waves and tides can redistribute the riverine sediment changing the morphology and facies of deltas. There are three main end member categories of deltas when characterized by processes: 1) River dominated; 2) Wave influenced; and 3) Tide influenced.

See my video summary of deltas at http://www.youtube.com/watch?v=_XOb07MDjAs

River Dominated Deltas - River dominated deltas have very low wave energy and a very small tidal range. Delta top deposits are well developed and are very similar to meandering river deposits, including channel, levees and overbank deposits. Overbank areas are commonly heavily vegetated and result in peat and coal deposition. Channels build out into the ocean (or lake) on top of their mouth bars. This leads to a coarsening upwards of grain sizes within the mouth bars as well as a change from some marine processes to unidirectional river flow. Avulsion of the rivers is common due to low gradients on the delta plain. Lobes of the delta become abandoned creating a “bird’s foot delta”. Sheltered bays are common between the lobes, and are filled with overbank deposits from floods as well as marshy deposits. The Mississippi River Delta is a classic river dominated delta.

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Wave Influenced Deltas - Waves redistribute the sediment deposited by the rivers. Progradation of channels is limited because mouth bars are reworked by waves into shore parallel sand bars and beaches. Spits of sand are also common. The waves sort the sediment better than rivers and, if the grains are not already well rounded, the waves will round them. The big differences for wave influenced deltas are that beach facies are abundant and channel fill and overbank facies are less common. The Niger River Delta is a wave influenced delta.

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Tide Influenced Deltas - Tides rework sands into elongate bars perpendicular to shore (vs. waves). These bars are analogous to mouth bars, but they contain tidal sedimentary characteristics including bi-directional flow indicators and slack tide mud drapes. Overbank areas can include tidal flats. The Ganges-Bramhaputra delta in Bangladesh is a tide dominated delta.

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Constructional Shorelines: Coastal Planes
Coastal planes are broad areas where there is sufficient sediment for the land to build seaward, but it is not localized at a single delta mouth. Examples of coastal planes include the Everglades area of Florida and the coast of the Carolinas.