Photos of Pacific Coast, Cascades, Columbia Plateau
Geology of the Pacific Northwest

Basics -- Depositional Environments

Introduction

A depositional environment is a specific environment in which sediments are deposited. They are sometimes called sedimentary environments. The layers of sediment that accumulate in each type of depositional environment have distinctive characteristics that provide important information regarding the geologic history of an area.

Why are depositional environments important?

To put together the geologic history of a region, the depositional environments of its sedimentary rocks must be analyzed. By reconstructing depositional environments geologists are able to reconstruct the climates of the past, life forms of the past, and geography of the past-where the mountains, basins, large rivers, and bays of the ocean were.

The knowledge that is gained this way has practical applications. To give one example, by reconstructing depositional environments of certain sediments on the coast of the Pacific Northwest, geologists concluded that great subduction earthquakes and tsunamis (giant waves) created by the earthquakes were the driving forces of the depositional environment. This has led to re-evaluation of the earthquake hazards in western Washington and Oregon and rewriting of some of the plans for roads and bridges in the area.

Other examples include looking for energy deposits in the form of coal or oil, each of which originates in a certain type of depositional environment. if you know what to look for in the rock layers to lead you to that type of depositional environment, you may be able to uncover major sources of fossil fuel (should you be so included).

There are many depositional environments. We will focus on depositional environments that are important in the Pacific Northwest, including the deep ocean floor, the continental shelf, and river floodplains.

Examples of sediments and their particular depositional environments

Turbidites

The oceans receive most of the clastic sediments that erode from the continents. On the edges of the continental shelves, where the submarine slope tilts down into much deeper water, accumulations of mud and sand deposited by rivers build up. Eventually so much sediment builds up on the edge of the steepening slope that it is likely to give way into an underwater landslide. The submarine landslide will flow down the slope into deeper water, mixing with seawater as it goes to form what is called a turbidity current. As the sediments gradually settle out of the turbidity current onto the deeper ocean floor, the coarser-grained sediments (those sediment grains with larger diameters) will settle to the bottom first, followed gradually by finer and finer sediments. This creates a graded sequence of sediments-it grades upward from a bed of sand through a layer of silt to a top layer of fine mud. This graded deposit becomes a rock known as a turbidite. Over the years one turbidite is likely to be deposited on top of another, over and over again thousands of times. This creates repeated beds of coarse sand to fine mud, which may total thousands of feet thick. If parts of the ocean floor end up becoming part of a continent, turbidites are likely to be a major component the accreted terrane.

Marine Limestone

Limestone, rock made of the calcium carbonate mineral known as calcite, can form in a variety of depositional environments, from hot spring deposits in lakes to coral reefs in the tropical oceans. Most limestone originates in shallow waters of tropical oceans, and may carry fossils of plants and animals that lived in those environments. However, limestones made of buried coral reefs are not as common as limestones made simply from lime mud. Lime mud originates from disintegrated organisms that have hard parts made of calcium carbonate. As a result, limestone is commonly massive, fine-grained, and fossil-poor.

Transgressive and Regressive Sequences

The gradual rise and fall of sea level relative to the land creates distinctive sequences of depositional environments. In a low area near the sea, as sea level gradually rises (transgresses), the environment will start as a beach, then become an offshore tide flat, and then a deeper bay with a muddy bottom. Eventually, in warm water, it may become a place where lime mud accumulates.

Thus, from bottom to top, as the sea transgresses across an area it will leave a sedimentary sequence of sand overlain by mud overlain by lime. If these layers are buried deeply enough to be lithified into sedimentary rock, they will become sandstone, shale and limestone. This is a transgressive sedimentary sequence, from the bottom up, sandstone-shale-limestone.

transgressive sequence diagram

A regressive sedimentary sequence represents the lowering of sea level and the retreat of the sea from the low-lying land. It is the reverse of a transgressive sequence. A regressive sequence in the stratigraphic record would be, from the bottom up, limestone-shale-sandstone.

Tsunami Deposits

When devastating subduction zone earthquakes occur along a coast, extremely large water waves called tsunamis are generated. At the same time, sea level changes relative to land level along the local shore. The combination of a sudden drop in land level and a tsunami washing over coastal lowlands creates several distinctive markers in the sediment layers that remain. These include muddy coastal marsh deposits overlain by gravel or sand deposits that have indications of high-energy wave flow. Where the coast is nearly flat rather than steep, these tsunami deposits can extend miles inland. Groves of cedar trees or other evergreen trees that grow adjacent to marshy areas, barely above sea level, may drop down and have their tree roots subjected to salty water. This will kill the trees, though they may stand in place for several hundred years as "ghost forests," silent testimonials to great earthquakes of the recent past.

Coal

Coal is a chemical sedimentary rock made mostly of carbon. It forms from the remains of plants that lived in moist environments rich in trees, shrubs, water, and mud. In such swampy settings, the dead plant debris is quickly buried and thus escapes rotting away at the earth's surface. Upon being buried, heated and compressed within the earth's crust, the dead plants will become coal if the right conditions of heat and pressure are achieved.

Meandering Rivers

Sequences of beds of sandstone, conglomerate, siltstone, shale, and plant fossils indicate sediment deposition by a system of meandering rivers. If there were thick woods and swampy areas, there may also be coal. Details in the sedimentary structures, characteristic signatures of particular depositional processes, will confirm if there were meandering river channels, sandbars, stream bank erosion, and occasional floods.

Glossary terms that appear on this page: subduction earthquake; tsunami; floodplain; silt; turbidite; limestone; calcium carbonate; sand; mud; sandstone; shale; gravel; chemical sedimentary rock; conglomerate; siltstone; meandering river; sedimentary structures


Geology of the Pacific Northwest
Basics--Depositional Environments
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 7/2/13