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

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)

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

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.

Stratigraphy and Time

Stratigraphy is the study of sedimentary rocks in space and time. It is the basis of interpreting what happened in the past. We use facies to interpret depositional environments from the rocks. Changes in facies both vertically and horizontally allow us to interpret changes in ancient landscapes and processes.

Example: Beach Facies. Beach environments grade laterally into each other. The offshore areas grade into the swash zone of the foreshore. The foreshore grades into the berm (the highest point of the beach) and backshore (if present). Eolian (wind) dunes, marshes or erosional cliffs can be present landward of the beach. Rock facies similarly grade into each other because they were deposited in different depositional environments. If the depositional environments stay in exactly the same place through time, a stratigraphic column in each place would consist of a uniform facies, but each stratigraphic column would have a different style of rock (facies). However, depositional environments tend to migrate back and forth as sea level rises or falls, basins fill in with sediment, etc. Thus, facies in stratigraphic columns tend to change upward. They also vary laterally. See figure 19.8 on pg. 308 of Nichols or this figure: http://www.ocean.odu.edu/~spars001/geology_112/laboratory/session_04/walthers_law.jpg from this page discussing stratigraphic correlations: http://www.ocean.odu.edu/~spars001/geology_112/laboratory/session_04/handout.html

Changes in sea level and depositional environment lead to variations in stratigraphic columns both laterally and vertically. If you compare different stratigraphic columns, there are several ways you might "correlate" them. If you correlate different rock types, e.g. lithostratigraphy, you are marking regions with similar characteristics, but the sediments in each unit were not necessarily deposited at the same time. In contrast, if you correlate rocks that were deposited at the same time, e.g. chronostratigraphy, each unit often consists of more than one facies. This is obvious when you look at the distribution of depositional environments now. Different areas are accumulating different types of sediment at the same time.

Lithostratigraphic correlations are easy because you can directly observe rock type. These correlations are very useful for studies of reservoir properties, where one might want to identify a porous sand that acts as a water or hydrocarbon reservoir. However, these correlations do not help you interpret ancient depositional environments because they do not represent an ancient landscape. Chronostratigraphic correlations tell you the most about depositional environments and their distribution through time, but they can be VERY difficult because you have to have a time marker that tells you what deposits were synchronous. Sometimes volcanic ash beds or other depositional events allow you to directly observe which rocks were deposited at the same time, but these events are rare. Often, chronostratigraphic correlations require detailed facies analyses and an understanding of how depositional environments change through time.

Walther’s Law is key for understanding the differences between lithostratigraphy and chronostratigraphy. Walther’s Law states that environments that are adjacent to each other are represented as vertical successions of facies in the rock record if there is no break in sedimentation (no unconformity). If sea level is rising relative to the shore line, the different depositional environments are migrating inland. This leads to different facies accumulating progressively inland as well. The most landward deposits are river deposits and alluvial plain deposits, followed by marsh and then marine deposits. Vertically, you see the facies representing those depositional environments in the same order. At any given time, rocks are being deposited in all of the different environments.

Chronostratigraphy - Chronostratigraphy enhances the interpretation of the stratigraphic record in terms of Earth history. Even when one has a detailed map of the distribution of depositional environments, it is difficult to say exactly how to correlate section in terms of time. In real rocks, there are a number of tools that you can use to get correlations of various accuracy. These include: fossils (biostratigraphy); magnetic properties (magnetostratigraphy); absolute ages of interbedded volcanic ash beds and basalt flows; some chemical properties such as elemental isotope ratios in carbonates; geological instantaneous depositional events such as huge storms, meteorite impacts, etc.; and unconformities due to sea level falls and the geometry of sedimentary deposits (sequence stratigraphy). We will get back to all of these in more detail throughout the quarter, particularly near the end.

Distribution of Rock and Time - One might think that sections can be correlated based on assuming that the same amount of sediment gets deposited in all places in the same amount of time. This is a BAD assumption, although many researchers are forced to use it. It is important to understand that the preserved rock does not represent all of time. What I mean is that time is not evenly represented by rock thickness. For example, with turbidites, the sandstones may have been deposited in a couple hours to a day at most, whereas the shales (Bouma E) represent 100’s to 1000’s of years of fine grains settling out. Thus, most of the "time" is represented in the much thinner shales. In addition, there is erosion at the base of some of the turbidites. Thus, there is a significant amount of time that is only represented by an erosional surface which produces a gap in the rock record. Generally, sedimentation is thought of as a continuous processes. This is NOT true. Sedimentation is episodic and there are unconformities in the stratigraphic record spanning all time ranges from minutes to millions of years. Gaps of minutes might occur in a river if there is a burst of strong flow that is erosive rather than depositional. Gaps of hours occur at low tides when the intertidal zone is exposed. Gaps of years to thousands of years can occur in land environments where there is no source of sediment or the topography is too high to collect sediment. Gaps of millions of years also occur in terrestrial environments, especially if there is erosion. The longer time gaps usually represent regional changes in deposition and can be very useful for correlating rocks chronostratigraphically. Also, different depositional environments accumulate sediment at different rates: thickness does not equal time!

Introduction to Sedimentology and Stratigraphy

Sediments and Strata
Sediments and sedimentary rocks cover most of earth (and large parts of mars), and weathering is occurring on the rest of it. The reshaping of the surface of the earth has had a huge influence on the planet, affecting everything from the evolution of life to the tectonics of mountain ranges. Sediments and sedimentary rocks record the events and processes that shaped the surface of earth – and other rocky planets. They provide the temporal framework that connects processes within the earth to those at the surface. They are important for:

1. Earth (and mars) history. Sedimentary rocks contain features that allow us to interpret ancient depositional environments, including the evolution of organisms and the environments they lived in, how climate has changed throughout earth history, where and when faults were active, etc.

2. Economic resources. Petroleum reservoirs have organic-rich, sedimentary source rocks that produced the petroleum when heated, most oil and gas migrates through sedimentary rocks, and most of the reservoirs are hosted in sedimentary rocks. Water aquifers are dominantly found in sedimentary rocks (although some are in fractured metamorphic and igneous rocks). The composition of the rocks strongly influences water quality due to water-rock interactions. (Why does Davis water taste bad?) Sedimentary rocks also host economic minerals such as gold and diamonds, which are eroded from other rocks and concentrated to specific areas during sediment transport.

3. Environmental geology. Sediments cover 2/3 of the continents and essentially all of the ocean floor, which totals 89% of the surface of earth. They host the biosphere, and they are most of the rocks we interact with directly and indirectly. Our actions as humans have an extremely strong effect on sedimentation and erosion. Understanding our impact on the environment - and the environment’s impact on us - must include deep appreciation for sediments and sediment transport.

Sedimentology and Stratigraphy
Sedimentology is the study of sediment transport processes and sedimentary rocks, and it is the focus of most of this course. If covers scales ranging from a single grain to entire planets, and focuses on processes. Stratigraphy is the study of the distribution of sediments and sedimentary rocks in space and time. It is essential for understanding earth history, reservoir properties, etc. We will place our sedimentological interpretations into a stratigraphic context with examples in lecture, the homework, and tests.

Sedimentology and stratigraphy are about as old as mineralogy as a field of study. Leonardo da Vinci provided one of the first environmental interpretations from sediments; he interpreted fossils in the Italian Apennines as evidence of an ancient ocean. He used the logic behind what we now call the Principle of Uniformitarianism: Similar organisms produce similar shells. The logic is, “If you see shells on the tops of mountains that look like those from organisms that live in the ocean today, the shells on the mountain tops were probably once in the ocean, too.” (Did the mountains go up or did the ocean go down? That is a question that was not thoroughly answered until we understood plate tectonics!) Here is a more formal statement of the Principle of Uniformitarianism:

Key Concept: The characteristics of sedimentary rocks can be used to determine the environmental conditions under which they were deposited, and the environmental conditions allow you to predict the characteristics of sediments that are likely to be deposited. This is the Principle of Uniformitarianism (formulated by James Hutton in the mid 1700’s) – the processes that formed ancient deposits are the same as those that form modern deposits.

Current ripples forming in a flume:  http://youtu.be/rSzGOCo4JEk
Wave ripples forming in a flume:  http://youtu.be/MBDadRqSHOc
Modern Current Ripple Image: http://tinyurl.com/yjlw7gq
Ancient Current Ripple Image:  http://tinyurl.com/yh46jlm
Modern Wave Ripple Image: http://tinyurl.com/7dbwr82
Ancient Wave Ripple Image: http://tinyurl.com/7t8p2bz

In the much more recent past, for example in the 1970’s, the Principle of Uniformitarianism was interpreted by some as requiring continuous, incremental processes and as excluding dramatic, rapid events. For example, a meteorite impact was seen as a non-uniformitarian event. However, my view of uniformitarianism can encompass rare events. The basic idea is that catastrophic events also produce characteristic features. For example, a meteorite impact produces similar deposits no matter when it occurs in time. We can recognize impact spherules from Archean sedimentary rocks that formed and were deposited in essentially the same way as those from the Cretaceous impact that killed the dinosaurs. Or an impact on mars will produce features similar to those produced by an impact on earth.

Brief summary of how geologists identified the K-T impact: https://www.e-education.psu.edu/earth501/content/p3_p5.html

The key point is that similar processes produce similar products. All processes are not active at all times (large meteorites are not continuously bombarding earth!), and some, like burrowing by worms for example, did not occur at specific times, e.g. before the evolution of worms. However, if a feature is present that is characteristic of a specific process, e.g. a thin tube-shaped area in a siltstone with a slightly different color and a specific geometry, it is reasonable to interpret that process, e.g. burrowing by a worm, as having produced the feature. This is how we extract earth history from rocks, e.g. the absence of worm burrows before 540 Ma allows us to state with confidence that worms did not exist before 540 Ma. However, it is often challenging to identify which processes produce which features. There is rarely the nice, exact correlation between features and processes that one would wish for. For example, a specific color variation in a rock could reflect a burrow or a water flow path or both if the burrow influenced water flow. One needs to understand the uncertainties in geological interpretations.

Right now, I am using the Principle of Uniformitarianism as a scientist on the NASA Mars Science Laboratory (http://mars.jpl.nasa.gov/msl) - a mission that is using the rover Curiosity to investigate ancient environment in Gale Crater on mars. We are looking at grains and rocks, trying to understand what the ancient environments were like. The only way we can do this is to assume that sedimentary processes on mars produce the same features they would produce if they occurred on earth. There may be some processes on mars that are very rare or do not occur on earth, and these might produce features we do not recognize. However, if we see a current ripple, we can infer that there was flowing water on mars. The details of the flow speed, water depth, etc. might have been different (and we could calculate that), but the basic process was similar.

Summary of the Principle of Uniformitarianism: http://www.youtube.com/watch?v=ifdlx_dFzPU

There are two other really important concepts that were first articulated in the 1660’s that we use on both earth and mars: The Principle of Original Horizontality and the fact that younger sediments overlie older sediments. Nicolas Steno was the first to write down the idea that strata (or sedimentary rock layers) are deposited in a nearly horizontal position, an idea called the principle of original horizontality. Some layers are deposited exactly flat, but most layers follow the tilt of the depositional surface, which is not exactly horizontal. However, most sedimentary layers are close to horizontal for our purposes here. If layers are no longer horizontal, later deformation must have changed their orientation.

Images "horizontal" and tilted strata:
Grand Canyon "horizontal" strata:  http://www.flickr.com/photos/grand_canyon_nps/sets/72157626136162880/
Rainbow Basin tilted and folded strata:  http://en.wikipedia.org/wiki/File:Rainbow_Basin.JPG

This idea is intimately associated with the idea of time in rocks.

Key Concept: Younger sediments overlie older sediments (if they are still approximately horizontal)– obviously, at least to us now.

Steno first wrote this down in 1667. The relative ages of sedimentary rocks gives us time. We can interpret changes in processes through time using the principle of uniformitarianism combined with the relative age (or stratigraphic succession) of rock layers. Steno’s work provided the intellectual framework for understanding relative time in a local areas. It was not until much later that the idea of “faunal succession” (articulated by William Smith, early 1880’s) provided a global time scale. Smith (and other geologists at the time) recognized that fossil organisms succeed one another in the stratigraphic record in an orderly, recognizable fashion. They were learning about a key component of evolution, although they did not yet have the intellectual framework of evolution by natural selection. They formulated the basic ideas that if organisms evolve through time, rocks containing similar organisms are approximately the same age.

Discussion of the distribution of time in sedimentary rocks: http://www.youtube.com/watch?v=fBjR_1vK9ug

The MSL team is using these ideas to interpret the time history of events in Gale Crater. One of the reasons we chose to go to this landing site was that Mt. Sharp, which is a 5 km-high mountain in the crater, contains layers of rock. These layers record a history of processes and events that occurred in Gale Crater, they contain clues to what mars was like about 3 billion years ago. We do not yet know very much about the rocks. However, we do know that the earliest history is in the rocks at the bottom, and changes in the rocks going upward will reflect changes through time. In other words, the history book starts at the bottom.

Strata in Mt. Sharp from the Curiosity landing site: http://mygeologypage.ucdavis.edu/sumner/gel109/Mars/MtSharp_M100.jpg

On earth, geologists have studied the time succession of rocks to create the Geological Time Scale. This time scale is largely based on fossils (for rocks younger than mid-Neoproterozoic time), with radiometric ages providing absolute time and refinements on our understanding of evolution. Continual refinements in the time scale provide insights into the process and history of earth that are core to geology. In many ways, the geological time scale documents our substantial understanding of earth. In contrast, the Martian Geological Time Scale is just being built. We know very little about the history of mars, and right now, we can only use how old rocks look - the number of craters or how many other rocks are on top of them - to get relative ages. Some planetary scientists have noted a correlation between apparent age and mineralogy, with the oldest rocks having more clay minerals, medium aged rocks having more sulfate minerals, and the youngest rocks being mostly wind deposits. On average, this may be true, and this model provides a testable hypothesis, that we can test in part with Curiosity. Rocks in Mt. Sharp show spectral signatures of both clay and sulfate minerals. Using Curiosity, we will analyze the mineralogy of these rocks, in the context of their sedimentary structures, grain size, etc. to interpret the environments they formed in and their relative ages.

It was this process on earth that led to the definition of Cambrian, Ordovician, etc. Comparisons of the spatial distribution of similar features leads to many important insights. Da Vinci recognized that fossils can be used to interpret ancient rocks; he interpreted similar environments because the shells he saw were essentially identical to those in the modern oceans. To Smith and other English geologists, similar fossils suggested the rocks were similar ages.

These two ideas reflect the two key components of stratigraphy: rock types vary in both space and time, but the same type of rock can be deposited in different places at different times. These changes can be organized based on how different environments are distributed:

Key Concept: (Walther’s Law) Depositional environments vary in space and time such that “The facies [rock types] that occur conformably* next to one another in a vertical section of rock will be the same as those found in laterally adjacent depositional environments” (Johannes Walther, 1894).

(*Conformably means that there is neither a break in sedimentation nor erosion between the two environments, e.g. there is no unconformity between them. Jumps in depositional environment can occur if the rocks do not provide a complete record of the environmental changes that occurred; rock types in a vertical succession separated by the unconformity do not necessarily represent neighboring environments.)

Images of environments and a Google Earth tour: Google Earth kmz files (You must have the program Google Earth installed on your computer. Download and open this file.):
Environments from Davis to San Francisco: http://mygeologypage.ucdavis.edu/sumner/gel109/GoogleEarth/DavisSFOEnvironments.kmz
Tidal Environments near Derby, Western Australia: http://mygeologypage.ucdavis.edu/sumner/gel109/GoogleEarth/TidalEnvironmentsDerby.kmz

One of the most important implications of Walther’s Law is that rocks of the same type are not necessarily deposited at the same time. There is a BIG difference between correlating rocks based on having the same lithology and rock being deposited at the same time. This is a critical conceptual idea that we will focus on throughout the class. And what does this suggest for the martian time scale, which is based on mineral compositions?

Summary of Walther’s Law: http://www.youtube.com/watch?v=ZSsULiPouTo

Next time - Sediment transport
Reading:
Chapter 4 is the most important for Wednesday’s lecture.
Chapters 2 and 3 cover classification of sediments and sedimentary rocks. We are not going to cover them explicitly in class, but knowing the terminology will be critical. Thus, read these chapters soon! Chapter 2 will be very useful for those of you in lab.

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.

Fluvial Review, Deltas and Marine Processes Part 1

Review: General Characteristics of Fluvial Sediments:
1) On a large scale, river deposits consist of sheets and lenses of sand deposited in channels associated with flat laminated shales and silts with rare rippled sand beds deposited on floodplains.
2) Fining upward sequences of beds in the sands with decreasing flow sedimentary structures
3) Abundant cross stratification in well sorted sands, particularly trough cross stratification
4) Cut banks at the edges of channels - these are good indicators of a migrating river channel, but can be hard to see in outcrop
5) Soil development in associated shales deposited in the floodplain environment.

Look at pictures of fluvial rocks at http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html

http://gigapan.org/gigapans/67910
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/FluvialCycles.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezDebrisFlow.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezGravelChannel.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezMudCracks.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHRiverXStrat.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHDuneChannelXStrat.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHUpBar2.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughXStrat3.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughMap.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/FluvialX1.jpg

Deltas and Estuaries - Introduction

When rivers reach standing water such as a lake or the ocean, the flow speed slows down dramatically. And when flows slow down, sediment is deposited. Almost all of the sediment transported in a river is thus deposited close to the river mouth, with the exception of grains that are fine enough to remain in suspension. Lacustrine and marine processes can rework the deposited sediment to distribute it along shorelines.

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 end in 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.

I will draw cross section and map views of a delta 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.

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. We will come back to these delta types after discussing the marine processes.

Marine Depositional Processes

Most of Earth is covered with oceans, there is abundant life in the oceans, most sediments eventually get transported into the oceans, and shallow marine deposits are the most abundant in the in sedimentary record due to their large volume and the low erosion rates in shallow marine environments. You need tectonics to uplift them above sea level to get significant erosion. This happens commonly, so that we can also see them exposed.

Processes - Several processes are unique to shallow marine deposition (and some large lakes): Waves, storms, and tides

Waves - Waves have oscillating current directions every few seconds. The flow in both directions is equal in deep water, but not necessarily near shore. Draw a picture of wave water motion. (Water at the top of the wave moves in the direction the waves move.)

Wave Ripples - Wave ripples are like current ripples, except that they experience transport in both directions. Draw a picture with the laminar boundary layer, etc. At low flow, the boundary layer doesn’t have enough speed or momentum to remove the crest of the ripple and deposition of the grains that are moved are deposited right on the upper part of the lee slope. Thus, crests are sharp. At higher flow, the crests erode due to the higher speeds and momentum and deposition occurs farther down the lee slope. Thus, high flow ripples have rounded crests. Wave ripples can be recognized in rocks by their symmetric shape (if flow in each direction is the same speed) and most importantly, the presence of x-laminations dipping in two directions. This is the truly distinctive feature and can be present even if the ripples are not very symmetric.

In shallow water, currents along the bottom from the waves are strong enough to flatten out the ripples, but they are not consistent enough in one direction to form dunes. Thus, the sedimentary surface tends to be planar or broadly scalloped as the waves are focused into certain areas. This produces a flat lamination (not upper planar lamination) where waves are in very shallow water relative to their height, e.g. from the breaker zone towards the shore.

KEY POINT FOR WAVES: Bi-directional flow every few seconds

Storms - Storms produce both large waves and strong, irregular currents. The combination and interference of these produces some unique deposits which can be used to recognize the importance of storms in a given marine sequence. Storms generally start far from shore and can approach through time. Then they either die out or move on. Thus, deposits that storms affect, i.e. those on continental shelves, tend to start out with low energy flows, increase to erosional (if strong enough) and then decrease back to lower energy flows. For example, sharp crested wave ripples might transition into round crested wave ripples, followed by cross stratification due to large waves and strong currents, followed by erosion, deposition of the coarsest sediment, and a reverse of the sedimentary structures. However, because there is usually little sediment being deposited at the beginning of a storm because there is not much sediment in motion and because flow speeds are increasing, there is usually no record of the first half of this sequence in the rock record. It is only the second half that gets preserved.

HCS - The cross stratification that is deposited as a combination of strong currents and large waves is unique to storms (and is found only in medium to fine sands). It is called hummocky cross stratification (HCS) and swaley cross stratification. When currents are washing eroded sand into an area with strong oscillatory flow, rounded mounds or hummocks of sand develop on the sea floor separated by lows (swales). These mounds are a few to 10 cm high and 10’s of cm across. See Figures 14.3 and 4 in Nichols. Variations in current strength cause erosion locally, and the locations of the hummocks and swales change through time. This produces erosional surfaces which truncate the older laminae (note that Fig 14.2 has the wrong laminae truncated). HCS is characterized by low angle laminae truncated by low angle surfaces. There are abundant concave and convex up laminae and many fewer flat laminae.

Storm Sequence - A sample stratigraphic column consists of: Mud, scoured surface, sole marks, (gravel at base), normally graded, HCS, flat laminae or wave rippled top, return to suspension settling. Contrast this to a turbidite - I will ask you to do this!

KEY POINT FOR STORMS: Multi-directional flows over seconds, low to high to low energy in deep water

Tides - Two key characteristics that are unique to tides: 1) flow changes direction 1 or 2 times per day; and 2) The speed of flow is cyclical with flow going onshore, stopping at hight tide, then flowing offshore, and stopping at low tide. There is lots of variability in tides depending on geography. Flow speeds vary, producing different sedimentary structures. In the Bay of Fundy, which has the highest tides recorded in the world (up to 16m - a 5 story building), the water moves up to 15 km/hr (417 cm/sec) which is fast enough to transport boulders and is well above the upper flat lamination zone for smaller grain sizes. At the low end, tidal currents are essentially non-existent. Also, there are times of slack tides when the water is essentially still or wave-dominated. Thus, the range of sedimentary structures is wide, including dunes (often called tidal bars when very large) and ripples. The main characteristic to look for, though, is variations in flow speed and DIRECTION.

Tidal sedimentary structures - Due to changing flow directions, two sediment transport directions are common, one for onshore flow and one for offshore flow. Often the onshore and offshore flows are not in the same location, but they shift around. This gives rise to current ripples showing transport in two directions and dune migration in two directions producing herringbone cross stratification. See figures 11.6 and 11.7 in Nichols. If the dunes are small and sedimentation rates are very high, you can get herringbone cross stratification in one tidal cycle in a modern environment. It is usually not preserved in the geological record because it is eroded prior to lithification. It is almost always the longer term changes in current locations that gives rise to preserved herringbone cross stratification. Dunes migrate in one direction for a while, and then currents patterns change and they migrate in the other direction. Herringbone cross stratification is almost always due to tidal processes, although it is not all that common in the sedimentary record. Commonly, one tidal current is much stronger than the others or the flow locations aren’t systematically shifting, so tabular cross stratification is more common. It is not unique to tidal environments, however.

Reactivation Surfaces - Reactivation surfaces form when flow in one direction is stronger than the other, but the other flow is strong enough to modify the bedform shape. See figures 11.6 and 11.9 in Nichols. Reactivation surfaces are erosion surfaces within the sets of cross stratification. They look like irregular surfaces that are similarly oriented to the foresets, but usually do not dip quite as steeply. Also, the foresets above and below the reactivation surface commonly have a slightly different orientation. Reactivation surfaces indicate varying flow directions, which is very common in tidal environments.

Mud Drapes - Flow speeds are also cyclical. During slack tides (low or especially high), fine grained sediment can fall out of suspension draping tidal bedforms with mud. Because mud is cohesive, it does not necessarily erode during the next tidal flow, particularly in the separation zone where flow is slow, e.g. at the bases of ripples and dunes. Thus, sand foresets coated with mud are very common in tidal environments as well. See figures 11.6 and 11.8 in Nichols.

KEY POINT FOR TIDAL PROCESSES: Bi-directional flow with varying speeds over hours

Facies, Turbidites, and Stratigraphy

The Midterm is next Wednesday, Feb. 8. Use the study guide I sent out earlier today and posted on SmartSite.

Facies
Facies are groupings of rocks based on similar features. Rocks (and sediments) can be grouped by a suite of different characteristics, for example grain size, sedimentary structures, grain composition, grain rounding, etc. The goal of assigning rocks or sediments to facies is to provide a useful categorization scheme.  See http://youtu.be/ydpJOgZYNBM.

Turbidites
Turbidites provide a good summary of the ideas we have been talking about, e.g. facies and sedimentary structures related to flows. Turbidites are deposited from slurries of sediment and water in any standing body of water (lakes, oceans).

1) Turbidity flows start with slope failure in soft sediment. Slopes become oversteepened where sedimentation rates are very high, such at the mouths of rivers. Because flow speeds are very low in standing water, the sediment does not get washed downslope. Rather, it builds up until there is a subaqueous slope failure. Earthquakes can trigger these slides, too.

2) Sediment and water mix creating a “fluid” that is denser than the surrounding water because of the entrained sediment. Thus, it flows downhill even if the slope is very low (1°).

3) The base of the flow is commonly erosional on steep slopes, so more sediment is entrained in the flow.

4) Enough sediment is entrained that erosion stops. Deposition begins as the slope gets shallower or the flow starts to slow down. Initially, the coarsest grains are deposited (remember the Hjulstrom diagram) and then finer grains, so the sediment is “graded”. However, the sediment is usually poorly sorted because the flow is a slurry of water and sediment so hydraulic sorting is reduced. (Facies = Bouma a)

5) Sediment concentration decreases with deposition, so one gets more hydraulic sorting. The flow is very fast so the sediment has upper plane bedding. (Facies = Bouma b)

6) As the flow slows more, grain size decreases and ripples start to form. Dunes do not usually form for two reasons: a) often only fine sand and finer grains are left in the flow by this point; and b) dunes do not have time to develop. (Facies = Bouma c)

7) Eventually, the flow slows to the point that bedload transport stops and deposition is mostly settling of silt and then clay. The progressive settling of coarser and then finer grains produces a faint lamination, but it is not as strong as the planar laminations in Bouma b. (Facies = Bouma d)

8) Mud settles out producing shale. This can look identical to background settling of clays brought into the lake/ocean as suspended sediment. (Facies = Bouma e)

Bouma divisions a-d can take hours or a day or so to be deposited. However, division e, which is usually the thinnest, commonly accumulates over months or longer (e.g. hundreds of years) depending on how frequent turbidites are in the area.

Watch these movies of turbidites in flumes:
http://faculty.gg.uwyo.edu/heller/SedMovs/middletonturb.htm
http://faculty.gg.uwyo.edu/heller/SedMovs/Turbidity%20ignition.html

Changes in Character Downslope - The parts of turbidites that are deposited change downslope and usually only a few of the subdivisions are preserved. In the most proximal (upslope) environments, divisions a and b are most common. In the more distal areas, all of the coarser sediment has already been deposited upstream, so divisions d and e are most common. Generally, there are also channels which fan out producing variations in rock types that change in space and through time.

For HW #3, you will draw a stratigraphic column of a turbidite and then describe the facies characteristic of different environments.

Turbidite Facies Models - Over the decades, sedimentologists have described and interpreted sedimentary rocks and defined generalized facies and facies associations that are characteristic of different depositional environments. These generalized facies and associations are called Facies Models. Each depositional environment or system has its own facies model. This is a VERY powerful tool for interpreting ancient environments.

Extra on Turbidites - Turbidite facies analysis and the resulting facies model led to the discovery of a new process. Sedimentologists had characterized turbidites all over the world. They all had the same flow characteristics consisting of a very strong erosive flow, deposition of a normally graded bed which was massive, followed by upper plane bedding, rippled finer sands, coarsely laminated silts, then shales. Comparisons with known flows showed that this sequence of deposits must come from a strong initial flow that slowed through time to still water. And this repeated again and again. The associated facies and the succession of different facies in these sequences suggested that the deposits had to be in deep water. For example, the fossils were all characteristic of deep water, shales were abundant and only settle from still water (shallow or deep), and they were sometimes associated with deep water storm deposits. Thus, the sedimentologists proposed slope failure and turbid currents flowing downslope and called them turbidity currents. A process like this had not been observed in modern depositional environments, so the idea was controversial. Many geologists did not believe that you could generate strong enough currents underwater to get those flow characteristics. Eventually in 1964, two geologists Heezen and Drake realized that an event in 1929 provided strong evidence for turbidity currents. In 1929, without satellites, under water telegraph cables were strung from Newfoundland to Europe. In November, about 30 cables broke in order from farthest north and shallowest to farther south and deeper water. At the time, people did not know why they broke, but Heezen and Drake suggested that a turbidity current was triggered by an earthquake and the cables broke as the turbidity current passed over them (they are strong flows!). Because they were continuously used for communication, the time each cable broke was very well known. Heezen and Drake calculated that the front of the flow traveled at 250 km/h (36,000 cm/s) when it first formed and then slowed to around 20 km/h (7000 cm/s) by the time the last cables broke 500 km from the source. This was a fast, strong flow and may be typical of turbidites. These speeds are above the upper end of the Hjulstrom diagram and are very erosive. It is only after the turbidite slows down even more that you get deposition. The characteristics of the flow seen by the breaking cables fit the flow characteristics proposed by the sedimentologists, and now turbidity currents and the facies model developed for turbidites are widely accepted and often treated as an ideal example of rocks that closely reflect flow characteristics. Turbidites and their interpretation are almost an ideal example of a good Facies Model.

Extra on Dense Sediment Flows
Sometimes with slope failures on land or under water, much more sediment can be put into motion than the flow would normally erode. Depending on the amount of water mixed with the sediment, the flow characteristics are different. When abundant water is present, the sediment can form a thick slurry with a higher density than sediment-free water, commonly leading to a higher Re and more turbulent flow (Re=u*l*r/µ). Also, collisions between grains become extremely important. Both of these tend to keep the sediment moving. Grain-to-grain collisions also have an important effect on grain sorting. The collisions tend to make sorting much less efficient and the sediment that gets deposited tends to consist of whichever grains make it to the base of the flow and are not kicked back up again. Usually, the largest grains are part of this first deposit because they weigh more, but small grains are also present. As the amount of sediment decreases, the flow becomes more like typical water flows. Turbidites are subaqueous flows that start out with a very high sediment load and decrease in time to more normal flows. They have characteristic sedimentary structures associated with them that reflect these changes.

If there is very little water associated with a clay-rich sediment flow, the flow can be very viscous due to the charge attraction among clay particles. The high viscosity makes the flow laminar (Re=u*l*r/µ). Debris flows with lots of cohesive mud are like this. In laminar flows, there is no mixing of the water or grains (or ice) and there is no sorting of grain sizes. Thus, the sediment remains mixed up with large grains, sometimes boulders, “floating” in mud. They flow down hill pulled by gravity until the flow seizes up and stops. This can be due to too low a slope or loss of water. Underwater debris flows can also be diluted by water that gets incorporated at the edges of the flow and become less viscous and more turbulent.

There also are dry sediment flows in which air is present between grains. For example, rock avalanches and some pyroclastic flows from volcanoes lack water. For these to move significant distances, large amounts of energy from either gravity or explosions are necessary to keep the sediment in motion.

A nice, hour long lecture on turbidites in the Monterrey Bay canyon, CA, can be found at: http://online.wr.usgs.gov/calendar/2010/jun10.html.  The actual lecture starts about 5 minutes into the video.  My summary can be found at: http://youtu.be/G05juwK2OTI

Stratigraphy and Time:
Stratigraphy is the study of sedimentary rocks in space and time.

Stratigraphy is the basis of interpreting what happened in the past. We use facies to interpret depositional environments from the rocks. Changes in facies both vertically and horizontally allow us to interpret changes in ancient landscapes and processes.

Example: Beach Facies. Beach environments grade laterally into each other. The off shore areas grade into the swash zone of the foreshore. The foreshore grades into the berm and backshore (if present). Eolian (wind) dunes or erosional cliffs can be present landward of the beach. Rock facies similarly grade into each other because they were deposited in different depositional environments. If the depositional environments stay in exactly the same place through time, a stratigraphic column in each place would consist of a uniform facies, but each stratigraphic column would have a different style of rock (facies). However, depositional environments tend to migrate back and forth as sea level rises or falls, basins fill in with sediment, etc. Thus, facies in stratigraphic columns tend to change upward. They also vary laterally. See figure 19.8 on pg. 308 of Nichols.

Changes in sea level and depositional environment lead to variations in stratigraphic columns both laterally and vertically. If you compare different stratigraphic columns, there are several ways you might "correlate" them. If you correlate different rock types, e.g. lithostratigraphy, you are marking regions with similar characteristics, but the sediments in each unit were not necessarily deposited at the same time. In contrast, if you correlate rocks that were deposited at the same time, e.g. chronostratigraphy, each unit often consists of more than one facies. This is obvious when you look at the distribution of depositional environments now. Different areas are accumulating different types of sediment at the same time.

Lithostratigraphic correlations are easy because you can directly observe rock type. These correlations are very useful for studies of reservoir properties, where one might want to identify a porous sand that acts as a water or hydrocarbon reservoir. However, these correlations do not help you interpret ancient depositional environments because they do not represent an ancient landscape. Chronostratigraphic correlations tell you the most about depositional environments and their distribution through time, but they can be VERY difficult because you have to have a time marker that tells you what deposits were synchronous. Sometimes volcanic ash beds or other depositional events allow you to directly observe which rocks were deposited at the same time, but these events are rare. Often, chronostratigraphic correlations require detailed facies analyses and an understanding of how depositional environments change through time.

Walther’s Law is key for understanding the differences between lithostratigraphy and chronostratigraphy. Walther’s Law states that environments that are adjacent to each other are represented as vertical successions of facies in the rock record if there is no break in sedimentation (no unconformity). If sea level is rising relative to the shore line, the different depositional environments are migrating inland. This leads to different facies accumulating progressively inland as well. The most landward deposits are river deposits and alluvial plain deposits, followed by marsh and then marine deposits. Vertically, you see the facies representing those depositional environments in the same order. At any given time, rocks are being deposited in all of the different environments.

Chronostratigraphy - Chronostratigraphy enhances the interpretation of the stratigraphic record in terms of Earth history. Even when one has a detailed map of the distribution of depositional environments, it is difficult to say exactly how to correlate section in terms of time. In real rocks, there are a number of tools that you can use to get correlations of various accuracy. These include: fossils (biostratigraphy); magnetic properties (magnetostratigraphy); absolute ages of interbedded volcanic ash beds and basalt flows; some chemical properties such as elemental isotope ratios in carbonates; geological instantaneous depositional events such as huge storms, meteorite impacts, etc.; and unconformities due to sea level falls and the geometry of sedimentary deposits (sequence stratigraphy). We will get back to all of these in more detail throughout the quarter, particularly near the end.

Distribution of Rock and Time - One might think that sections can be correlated based on assuming that the same amount of sediment gets deposited in all places in the same amount of time. This is a BAD assumption, although many researchers are forced to use it. It is important to understand that the preserved rock does not represent all of time. What I mean is that time is not evenly represented by rock thickness. For example, with turbidites, the sandstones may have been deposited in a couple hours to a day at most, whereas the shales (Bouma E) represent 100’s to 1000’s of years of fine grains settling out. Thus, most of the "time" is represented in the much thinner shales. In addition, there is erosion at the base of some of the turbidites. Thus, there is a significant amount of time that is only represented by an erosional surface which produces a gap in the rock record. Generally, sedimentation is thought of as a continuous processes. This is NOT true. Sedimentation is episodic and there are unconformities in the stratigraphic record spanning all time ranges from minutes to millions of years. Gaps of minutes might occur in a river if there is a burst of strong flow that is erosive rather than depositional. Gaps of hours occur at low tides when the intertidal zone is exposed. Gaps of years to thousands of years can occur in land environments where there is no source of sediment or the topography is too high to collect sediment. Gaps of millions of years also occur in terrestrial environments, especially if there is erosion. The longer time gaps usually represent regional changes in deposition and can be very useful for correlating rocks chronostratigraphically. Also, different depositional environments accumulate sediment at different rates: thickness does not equal time!

A discussion of time in sedimentary rocks can be found at: http://youtu.be/9ch-6HiOAW4