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 event 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

Cretaceous-Tertiary Boundary Example

The end of Cretaceous time is marked by a major extinction, a meteorite impact, and reversals in the Earth's magnetic field.  The following are some figures we'll use in class.  I'll update the notes and reference the figures after class (my computer just crashed and I lost what I'd written - save often!)

Regional Strat Column Correlations

Guest lecture by Cara Harwood on regional correlations of stratigraphic columns across the Precambrian-Cambrian boundary in the western US

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Geology 109: Sediments and Strata

March 12, 2012

Cara Harwood

Integrated Stratigraphic Analysis:  Correlating multiple columns using lithology, biostratigraphy, and chemostratigraphy

Recap:  You have been looking at interpreting stratigraphic columns based in lithology, distinctive structures, sequences of structures.  We use Walther’s law to predict how rocks will be stacked vertically.  

Remind yourself what Walter’s law is.  (http://dawnssedstrat.blogspot.com/2012/01/uniformitanianism-original.html)

Today:  Looking at multiple stratigraphic columns from the same time period that correlate across a region.  Focusing on the Precambrian-Cambrian boundary and how multiple columns are used along with biostratigraphy and chemostratigraphy to understand and recognize this key time period. 

The Precambrian-Cambrian transition records one of the most important intervals in the history of life.  It encompasses appearance and diversification of metazoans, a change in style of bioturbation (organisms are moving to different types of environments within the sediment), and mineralization of skeletons.  

Sedimentologists, stratigraphers, paleontologists, have studied this interval to try to learn about: 

• the time of the boundary, and correlating the boundary globally
• timing of evolutionary events
• what environments were like where these events were happening
• how environments were distributed

They studied stratigraphic columns and looked at lithologic, paleontologic, and chemostratigraphic data.  We will talk about each of these types of data and how they are correlated in the context of the pC-C boundary in southern California. 

Stratigraphy Types

Biostratigraphy: characterization and correlation of rocks based on their fossil content, based on the principle that organisms have undergone successive changes through geologic time

Chemostratigraphy:  characterization and correlation of rocks based on their chemical composition, based on the principle that certain chemical signatures occur globally through geologic time

Why is studying the pC-C boundary in California interesting? 

• The boundary is defined by a trace fossil that is present in siliciclastic rocks
• It is also defined by a specific chemical signature in carbonate rocks
• Many regions that preserve this boundary do not have both siliciclastics and carboantes together, but they are both present in this region so we can see how they correlate

Paleogeographic map of North America 

550 Ma:

http://www2.nau.edu/rcb7/namPC550.jpg

510 Ma:

http://www2.nau.edu/rcb7/namC510.jpg

Regional map showing positions of the craton margin relative to the Death Valley region and White-Inyos

From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf

 

Look at White-Inyo stratigraphic column (on left of the following figure) and talk through the whole thing - lithology, fossils including trace fossils...

From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf

Formations around the Precambrian-Cambrian boundary:

• Wyman Formation: interbedded mudrock, siltstone, quartzite; interbedded carbonate layers that increase in number upsection --> shallow marine deposition
• Reed Dolomite: cross bedded oolitic grainstone, stromatolites --> shallow marine, subtidal to intertidal; then hummocky cross bedded sandstone --> deposition below normal wave based, but above storm wave base. 
• Deep Spring Formation: siliciclastic carbonate couplets, with ripple laminated quartzites with hummocky cross stratification, cross bedded oolites, intraclastic grainstone --> high energy shallow water depositional environment.  

Each of these is a formation: a mappable unit that can be defined by its lithology and stratigraphic position; has some degree of homogeneity in rock type, mineralogical composition, sedimentary structures, fossil content.  

Treptichnus pedum - pC-C boundary index fossil globally; it is one of the first (and once thought to be the first) complex trace fossils, indicating more complex organisms that made the trace.   Show examples of this trace fossil.

Once we get into the Cambrian trilobites are good index fossils that are useful for biostratigraphy; specific time zones with unique assemblages of trilobite species.

Index fossils generally - can be trace fossils or body fossils; they mark specific time periods.  Good index fossils are taxa that appear and disappear (evolve rapidly), that are widespread - global distribution, that are distinctive and abundant, and that are facies independent.   **T. pedum is only found in siliciclastic rocks, so not found everywhere at the boundary.    

Look at how the White-Inyo section is correlated with other sections in the region - Death Valley and craton margin sections.  

Note that the scale of these columns is different than others that we have been looking at - the total thickness of the White-Inyo column is 2900 m, so we’re not looking at each individual bed, but rather looking at facies and dominant lithologies.  


Look at the 510 Ma paleogeographic map again to see the broad environments where each of these columns are from.  

Marine deposits in the west correlate to the east with non-marine facies: glacial deposits, non-marine sandstone and conglomerate, and there are more unconformities.  Also notice how the overall thickness of the columns is becoming thinner to the east. 

What general observations can we make about correlating stratigraphic columns across a region? 

• sediment packages thicken as we move from the craton (land) out into the basin
• more unconformities (can happen when exposed, above sea level) towards land
• facies shift from being a mix of non-marine and marine to mostly marine and deeper water facies
• correlating long distances - can’t do lithostratigraphy (i.e. correlating sands to sands)
• correlations based on fossil/trace fossil occurrence are robust

Look at chemostratigraphic signature (C isotopes) from ‘offshore’ (White-Inyos) to ‘onshore’ (Death Valley and craton sections):

From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf

Correlations across a region (and globally!) can also be made based on chemostratigraphy.   Even though the lithology is different, carbonate rocks track the concentration of ions in seawater, so the marine rocks everywhere can have the same signature. 

We’re not going to talk about what these chemical signatures mean, but notice how in the Lower Deep Spring Formation there is a positive 13 C value, and then around the pC-C boundary there is a negative value.  This is a global trend during this time period. 

Other elements also have global trends and can be used for correlating based on chemostratigraphy.  Show examples of Sr and O isotope curves.  

Using this integrated approach (chemostratigraphy and biostratigraphy) showed that the index fossil and the distinct chemical signature occur at the same time.  With this time constrained, we can apply either one of these (chemical signature or fossil) to correlation in other sections, and use this to better understand this time period.

Wrap up - this is an example of using stratigraphy.  Key points: 

• Looking at mixed carbonate-siliciclastic facies allows us to combine data types that occur in just one
• We get a complete picture of the time period by looking at a combination of data sets (lithostratigraphy/facies, biostratigraphy, and chemostratigraphy)...
• ...and how they are distributed across the region - looking at multiple stratigraphic columns

Interpreting Stratigraphic Columns

Interpreting Stratigraphic Columns

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

Examples:

• Hummocky Cross Stratification - waves plus currents (storms)
• Wave Ripples (vs current ripples) - waves (standing water)
• Herringbone Cross Stratification - bidirectional flow over hours or longer (tides)
• Reactivation Surfaces - reshaping of bedforms due to changes in flow (tides)
• Mud Drapes in sandstone - flow stops (tides)
• Bouma Sequence - rapid flow slowing down (turbidity current)
• Mud Cracks - mud contracts (exposed to air)
• Root Casts - from plants, usually land plants (land)
• Faint Ripple Cross Lamination with reverse grading - (eolian ripples)
• Meter-high Dunes in fine sand - (eolian dunes)
• Diamictites - laminar flow - (debris flows, mud flows, melting ice)
• Diamictites with Facetted Clasts and Striations - (glacial)
• Lone (or drop) Stones in laminated shale - large grains rafted over quiet environment (icebergs; trees possible)

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 or Stratification

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 your 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.


Here are some example stratigraphic columns to think about:

Marine Shorelines and Interpreting Stratigrahic Columns

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.

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.

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.

Deltas consist of 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.

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.

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.

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.

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.

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.

Each of these processes creates distinctive features in stratigraphic columns.