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Geology LibreTexts

1. Sediments and Strata: An Introduction

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.  Sedimentological interpretations are placed into a stratigraphic context for detailed interpretations of changes in environments over time.

Principle of Uniformitarianism 

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.  The processes that formed ancient deposits are the same as those that form modern deposits.

Here are some videos and images that connect these ideas:

Current Ripples:

Wave Ripples:

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.  The team is looking at grains and rocks, trying to understand what the ancient environments were like.  The only way they 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

Original Horizontality and Younger-Over-Older

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 <b>principle of original horizontality</b>.  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:

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).  

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, Mars.  One of the reasons they 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, the team is analyzing 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:  

Walther's Law

 

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).

 

(*<i>Conformably</i> 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.)

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