6.6: Introduction to Strata and Basins

Overview

Plates move and collide, mountains rise, sea level rises and falls. Chemical and physical weathering processes, buoyed by important interactions between the Earth’s spheres, break rock down in place. After it breaks down, erosional forces powered by gravity and aided by wind, water, and ice provide the energy and work necessary to move weathered materials downhill, downstream, into a new place of rest. Many variables determine the outcome for these sediments: latitude and climate, the composition of parent rock that weathered, the degree of physical and chemical weathering, life forms that interacted with the grains, etc., ultimately determine the story written in the layers of sediment. As these layers accumulate, so do the chapters in the history book of the Earth.

Strata

Sedimentary rocks come in strata, or layers, much like a nice birthday cake or pages in a storybook, and stratigraphy is the study of the record of these layers. These rocks are formed from sediments originally deposited in a basin. A basin is an area of depressed elevation generally surrounded by land at higher elevations. Basins can be on dry land or be filled with freshwater or saltwater. Over time, the land surrounding a basin erodes and the resulting sediment begins to fill the basin. From the point of origin in the highlands to the point of deposition in the basin, a great deal can happen to this sediment. Weathering, in various physical and chemical forms, takes its toll. By the time of their final moment of deposition, these sediments record a memory of their former existence as rocks, of their transport journey, and of processes that occur in the basin. Much of this sediment is solid and arrives as particles. Some of it arrives in aqueous solution, becoming the salts we experience in the seas around us. Some of it forms in the bodies of organisms, such as carbon-rich plant tissues or the skeletal remains of animals.

Figure \(\PageIndex{1}\): Brown, Green, white, and red sedimentary strata, indicating different levels of oxidation of disseminated iron in the rock. Proterozoic Spokane Formation, Glacier National Park, Montana. (With permission for educational purposes from Marli Miller Photography.)

Depending on the distance traveled, some sediments will remain the size of massive boulders and some will be reduced to the size of tiny flakes of clay. The range of grain sizes, or sorting, is the result of weathering and subsequent changes in energy as the particles move downslope. Some are well-rounded, others retain their rough edges. Some sediment, found far from its source, will be composed primarily of quartz, with less stable minerals having rotted away. Other sedimentary deposits will still contain unweathered minerals like micas and feldspars. Once deposited, sediment and rock will begin to undergo physical and chemical processes associated with the local environment. Soil formation will begin on land. In marine settings, chemical processes associated with water chemistry and biogenic materials will occur, adding onto the earlier story of transport. These layers of sediment, accumulating over time, are the subject of stratigraphy.

Figure \(\PageIndex{2}\): Sedimentary facies commonly found in a variety of basins.

The journey a sediment grain takes ultimately ends with deposition in a basin. Throughout that journey, sediments will take up residence for a time, waiting for a sufficiently energetic event to move them further along toward this end. The collective experience of sedimentation along these myriad pathways creates situations, as illustrated above, where different depositional environments are created. Geologists refer to the unique sedimentary characteristics of these varying environments with the term facies as discussed previously. Real-time facies migrate, or change, at a location as conditions change. For example, when sea level rises and an area is submerged, the type of sediment that deposits changes. All of the environments in the figure above are dynamic in nature. We can also assume, using uniformitarianism, that similar environments have existed in the past, producing rocks in the geologic record that record similar characteristics we see in their modern counterparts. As sediments deposit and facies migrate, strata are generated. Geologists call such strata, beds. As beds representing similar environments accumulate and lithify, they will come down to us as mappable units called formations.

All of this action ultimately occurs in sedimentary basins.

What is a Basin?

Sedimentary basins are geographic depressions where sediment accumulates. Some basins are epicontinental; they reside on top of the continental platform. Other basins are tectonic, located around the margins of the continents, after the continental shelf break. The power of stratigraphy lies in its ability to help scientists understand the history of basins in general. Basins sometimes contain water and sometimes do not. If the basin is dry, as with Death Valley, California, then the primary control on the creation of stratigraphic sequences would be changes in tectonics, such as the uplift or subsidence of the land itself. If the basin is connected to a body of water, then the cyclical rise and fall of this water will be recorded in the material deposited in the basin. Climatic and astronomical factors can cause changes regionally or locally that can affect the level of the water. The melting of ice or the thermal expansion of water are among the variables that can cause sea level to rise in a basin. Such changes typically do not occur within the lifespan of a single human. The record left behind is the pattern of sedimentary deposits. This pattern is preserved when the sediments become sedimentary rocks, and reading it is how we understand the history contained in this deep time.

The long-stable igneous and metamorphic cores of the original continents are referred to as cratons. Some portions of these cratons are exposed at the surface and some other portions are covered with blankets of sedimentary rock. The exposed portions are called shields. The portion of the craton that is buried is referred to as the platform. It is on these platforms where basins exist and where sedimentary strata accumulate.

Figure \(\PageIndex{3}\): USGS image depicting shields, platforms, basins, recent orogens (mountain belts), and other features. The cratons are made up of the shield and platform areas. The basins, in blue, are just some of the many sedimentary basins that exist globally. In modern as in ancient times, the oceans themselves are basins.

On shorter time scales, sediments accumulate as individual beds. Eventually, groups of beds that are widespread enough to be geologically long-lasting will be referred to as formations. Platforms make up the continental areas in which formations accumulate. The stacking of formations is like adding pages to a book. Some platform environments are dominated by carbonate sedimentation while others are dominated by siliciclastic sedimentation.

All basins have limited space. The interplay between tectonic changes, global sea level, local water level, and sediment accumulation all affect changes in space within any given basin. This “accommodation space” is the available volume within a basin to accommodate influxes of sediment. Accommodation space is not static. It adjusts as changes occur in tectonics, sedimentation, water level, and global (eustatic) sea level. As Earth’s systems interact over time, the ability of a basin to accumulate sediment or even the amount of sediment being produced fluctuates. This is true of all basins.

Stratigraphy in Basins

Basins can be thought of as a library filled with books, which are the layers of rock that accumulate. These layers tell the story of global geologic changes, regional changes, and changes within the basin itself. There is great variety among sediments and other materials deposited in basins and in the layers, or strata, that they create. This variety and layering provides the equivalent of text on the pages of a novel or even in this textbook. Individual pages are the strata, or beds, and their characteristic portion of the story on that page are the facies. These pages are laid down horizontally, building up vertically over time, reflecting the order in which they occurred (superposition). Eventually, these pages (beds) accumulate into books (formations).

Processes that occur after deposition, such as lithification, deformation, and further weathering and erosion can then make changes to this record that are akin to the deterioration seen in ancient Egyptian papyrus, or in a book whose pages and cover have been subject to the ravages of the elements. Strata can be torn (faulting), wrinkled (folding), or younger material may be injected (igneous intrusions). Depending on the context, these modifications may provide some confusion or clarification to the overall story. Over time, some of the material that makes up the strata and rock of this “book” can be lost or altered (erosion, metamorphism, secondary chemical alteration, etc.).

Sometimes, even entire pages, or beds, are removed. Entire books, or formations, can be truncated, leaving gaps in the storyline (unconformities). Stratigraphers grapple with a record that was incomplete to begin with, and is further reduced over geologic time. Still, the remaining story these layers of sediment have to tell is powerful and important. It is of critical value in providing a more complete understanding of the history of our planet, including your own home landscape. Geologists who study this record are called stratigraphers. They are the historians within the geosciences.

Consider the office bookcase below as a stratigraphic metaphor:

Facies = The words, notations, etc. on an individual page of a book;

Beds = Pages within periodicals or books;

Formations = Books full of pages, some complete, some perhaps incomplete;

Groups of Formations = Shelves of books with similar topics (assuming they are organized as in a library!);

Bookcase = Stratigraphic record of the entire basin.

(Note: the limitation of this metaphor is that most of the “stratigraphy” you see here is vertical, implying a great deal of geologic alteration. Remembering the “Principle of Original Horizontality”, we know that if these were rocks, they would be stacked differently.)

The gigapan below depicts the Hampshire Formation. It was taken along Corridor H (US 48) in West Virginia. The image depicts a single formation. If you observe this formation, akin to a book, you may first see the varying color. These are beds. Some of these beds, or pages, could be broken down further into smaller beds. Ultimately, these beds are all related because they were deposited in similar depositional environments, as illustrated by their facies.

Figure \(\PageIndex{4}\): A geologic model of the Grand Canyon. Formations are indicated with numbers and letters (ie 6a through 6d are all formations, or books of rock). Formations are then assembled into Groups, where appropriate (ie 1 through 6). Groups are like shelves of similar books. (CC BY-SA 3.0; The original uploader was Maveric149 at English Wikipedia. – Transferred from en.Wikipedia to Commons, https://commons.wikimedia.org/w/index.php?curid=2257589.)

Layers of sediment are not static recipients of the story only. They are also participants. As we will learn in the next section, they are recording their story and being acted upon by outside forces such as tectonic change, sea level change, and lithification. But, they are also having their own impact on tectonics and sea level. Layers of sediment also play an important role in the record of geologic history that is important to understand also. As these layers accumulate in basins, they record the history of an area. Stratigraphy is the study of that history.

Key Terms

• bed – a small recognizable layer of rock sedimentary rock
• craton – a large, stable block of the Earth’s crust that forms the old, tectonically inactive core of a continent
• formation – a distinct, mappable body of rock with unique physical characteristics, such as color, composition, or particle size, that differentiates it from the surrounding rock layers
• platform – a portion of a craton that is covered by relatively undeformed, horizontally lying or gently tilted sedimentary rocks, overlying older crystalline basement
• sedimentary basin – a low area of the Earth’s crust where sediment accumulates over time to form thick, laterally extensive sequences
• shield – a large, exposed area of a craton composed of old crystalline igneous and metamorphic rocks, typically tectonically stable and deeply eroded
• strata – distinct layers of sedimentary rock or volcanic deposits that can be distinguished by composition, texture, or other features
• stratigraphy – the branch of geology concerned with the description, correlation, and interpretation of layered rocks, including their age relationships and depositional environments