Depositional environment/tectonic setting tip sheet

This page was written by Steven Holland (stratum@uga.edu).

Get help with Depositional environments or Tectonic settings, or see the Glossary of commonly used terms, if you need a term you can't find on the pulldown menus.


Depositional environments

Facies interpretations are best made from a combination of lithologic features and geologic context. The lithologic features (rock type, sedimentary structures, etc.) of any given facies may commonly be very similar to those of another facies. In cases like these, geologic context becomes important. Attention should be paid to overlying and underlying facies, but only where the contacts with those facies are gradational. Upward shallowing is the most common stratigraphic motif, so underlying facies are generally deeper and overlying facies are generally shallower. Where contacts between facies are sharp, such as at flooding surfaces and sequence boundaries, context is much less informative and should not be relied upon. Paleogeographic context can also be important where the geographic position of a given facies belt can be evaluated relative to surrounding facies belts. Again, in these conditions, a given facies interpretation can be checked against its regional context.

All depositional environments listed on the pull-down menu are shown here in bold green.

Ask first, Is the overall environmental setting marine & eolian carbonate, coastal & marine siliciclastic, or terrestrial & fresh-water?

Carbonate settings

In cases where the environment cannot be determined more precisely than just marine and carbonate, choose carbonate indet.

Some eolian environments are composed of carbonate grains and these use the same environmental terms as for siliciclastics. Eolian settings are divided into dune environments, characterized by large-scale cross-stratification and interdune environments, characterized by, depending on climate, wind ripples, adhesion ripples, evaporites, and bioturbation from plant roots. Where the particular eolian setting cannot be determined, use eolian indet.

Shallow marine carbonate systems are dominated by peritidal, shallow subtidal, sand shoal, and reef settings. The peritidal environment includes supratidal and intertidal settings and may be characterized in arid climates by evaporite minerals, dolomite, and desiccation cracks, but in humid climates by bioturbated mudstones with fenestral pores. The shallow subtidal environment includes settings of up to a few meters of water depth, characterized by high rates of carbonate production and bioturbation, and is consequently composed of thick bioturbated beds of typically micrite-rich carbonates. Shallow subtidal settings may have normal marine salinities, in which case they are considered open shallow subtidal, or they may have either hypersaline or hyposaline conditions, in which case they are called lagoonal / restricted shallow subtidal. Where salinity cannot be determined, use shallow subtidal indet. The sand shoal environment is characterized by strong wave or tidal currents and consequently displays large-scale trough cross-stratification of well-sorted grainstone, but may also display seaward-inclined laminae where it is developed in a foreshore setting.

Reef, buildup, and bioherm settings encompass a diverse group of features and are classified based on their location on a carbonate platform. Those on the shallow-water portion of a platform or shelf, but not near its margin are labeled as intrashelf / intraplatform reef. Those at a platform or shelf margin are termedplatform-margin / shelf-margin reef. Reefs and buildups on the slope of a carbonate platform are identified as slope / ramp reef, and those developed in deep-water settings are called basin reef. Where its geographic position is uncertain, it is termed simply reef, buildup or bioherm.

Carbonate platforms also contain environments near and below storm wave base and these are classified based on their position relative to typical storm wave base and whether they are developed on seaward-dipping ramp or on a relatively flat-topped platform or shelf. Deep subtidal environments include those deposited between normal wave base and typical storm wave base. They commonly consist of a mix of carbonate mud and carbonate grains, segregated into storm beds. Such deposits found a ramp are termed deep subtidal ramp, while those deposited on a relatively flat-topped shelf or platform are called deep subtidal shelf. Where platform morphology is unknown, use deep subtidal indet. Offshore environments are those that lie below typical storm wave base, although they may be affected by exceptionally strong storms. Their facies are typically muddy, which may be either carbonate or siliciclastic, but they may contain distal storm beds. In ramp settings, these are called offshore ramp and in shelf or platform settings, these are called offshore shelf. As for the deep subtidal, where platform geometry is unknown, use offshore indet.

Deep-water carbonate systems include slope environments characterized by slumps, slides, debrites, and turbidites. They also include basinal oozes that are predominantly carbonate, called basinal (carbonate). In some settings, these deep-water deposits may be cherty and are called basinal (siliceous) or composed of terrigenous mud and called basinal (siliciclastic).


Coastal and marine siliciclastic settings

Coastal and marine siliciclastic settings include all settings in which siliciclastic sediments accumulate under marine influence.

If you are working with a coastal or marine siliciclastic setting, is it a paralic setting (estuaries, bays, lagoons, etc.), a deltaic coastal setting, a non-deltaic coastal setting, or a deep-sea setting? If you cannot make any finer determination than that you are working with a siliciclastic coastal or marine system, choose marine indet. If you know that it is a siliciclastic marine setting, but you can rule out the deep-sea alternative, choose marginal marine indet. If you can also rule out that it is a paralic setting, that is, you know it is a fully marine non-deep sea setting, but cannot tell if it is a deltaic or non-deltaic coast, choose coastal indet.

Siliciclastic paralic

Paralic environments include coastal settings characterized by both marine and freshwater influence, specifically estuary/bay environments and lagoonal environments. Recent sedimentological classifications have underscored the overall similarity of bays and lagoons, with lagoons reflecting semi-enclosed bodies of water that are elongate parallel to shore and estuaries being elongate broadly perpendicular to shore. This distinction is often difficult in the rock record without adequate outcrop coverage and where this distinction cannot be made, you should choose paralic indet. Both estuary/bay and lagoonal systems are tripartite, with an outer zone dominated by strong wave and tidal currents, a middle muddy zone characterized by weak currents and extensive mud deposition, and an inner zone dominated by fluvial currents within bayhead deltas. These distinctions are currently not used, but could be noted in the comments field. Both estuary/bay and lagoon systems can range in salinity from hypersaline, to normal marine, to hyposaline. Again, these distinctions are not currently used, but could be noted in the comments field.

Siliciclastic deltaic coastal

Deltas include a variety of settings that range from marine to terrestrial/aquatic. Included here are only those settings that show some marine influence; for deltaic settings that bear evidence of only terrestrial to aquatic settings, see alluvial or fluvial settings above. Seaward portions of the delta plain, including interdistributary bay environments and other environments essentially right at sea level may contain marine to brackish water faunas. The delta front is typically more open marine, but may be brackish owing to significant fresh-water input. Delta front facies are characterized by gently inclined beds of sand and mud deposited by turbidity currents shed from the distributary mouth bars. These pass seaward into progressively muddier deposits of the prodelta, which can be difficult to distinguish from offshore deposits (see below, under non-deltaic coastal), except based on overall context. In cases where a specific marine-influenced deltaic environment cannot be determined, choose deltaic indet.

Siliciclastic non-deltaic coastal

Non-deltaic coastal settings are characterized by four principal depositional environments. Foreshore facies are composed of sandstone with gently seaward dipping planar lamination with minimal bioturbation. Shoreface facies from most margins are characterized by sandstone with large-scale trough and tabular cross-bedding and a Skolithos ichnofacies suite. On coasts with a weak wave climate, the shoreface may be highly bioturbated with thin intervals of planar and wave-ripple laminae. Transition zone / lower shoreface deposits bear a mix of mudstone and hummocky cross-stratified sandstone, with a mixed Skolithos-Cruziana ichnofacies. Offshore facies are composed primarily of shale, but may contain thin beds of sandstone or siltstone deposited by storms. Bioturbation is commonly pervasive in offshore facies, with a Cruziana ichnofacies. Where a specific non-deltaic coastal environment cannot be determined, select coastal indet.

Siliciclastic deep-water

The database currently distinguishes among a few types of deep-sea environment. All submarine fan environments are lumped together and these are collectively characterized by a variety of slumps, slides, debrites, and turbidites. Three deep-sea mud and ooze and environments are distinguished, based on composition. Siliciclastic muds dominated by clays are handled by basinal (siliciclastic), Siliceous oozes commonly composed of chert derived from diatom and radiolarian tests are called basinal (siliceous), and calcareous oozes are grouped under basinal (carbonate). Where a specific determination of deep-sea deposit cannot be made, use deep-water indet.


Terrestrial and fresh-water settings

Terrestrial and aquatic environments are limited to those without marine influence.

If you are dealing with a terrestrial or aquatic setting, is it more specifically an alluvial or fluvial setting, a lacustrine setting, an eolian setting, a karst setting, or some other setting that does not fit neatly into any of these categories (such as glacial, swamp, spring, or tar)? If you are not sure, you should choose Terrestrial indet.

Alluvial and fluvial environments

If you know that you are dealing with an alluvial or fluvial environment, but cannot specify the environment in more detail, choose fluvial indet.

The alluvial fan environment is characterized by steep gradients with both debris flow and braided stream deposition. Braided-stream settings without evidence of debris flows should be placed into one of the fluvial environments, below.

Fluvial settings can be broadly separated into channel-associated settings and floodplain-associated settings. For channel deposits, it may be possible recognize the channel lag, characterized by pebble to boulder-sized material, consisting of large siliciclastic grains, mud rip-up clasts, bone, and wood, overlying the basal erosional surface of the channel. One may also distinguish channel fill deposits based on grain size and thereby delineate coarse channel fill and fine channel fill environments. In some cases, it may not be possible to distinguish any of these channel environments, in which case you should select "Channel" as the environment.

Several environments are present in floodplain settings, which are generally characterized by muds and silts, compared to the sands and gravels of channel deposits. It may be possible to recognized poorly-drained wet floodplain, based on gray to black colors of mudstones and an abundance of plant material, or well-drained dry floodplain, characterized by striking red and brown colors of mudstones. Where it is not possible to distinguish between these two, choose "floodplain". Associated with floodplain settings are crevasse splay deposits, marked by thin beds of rippled to bioturbated sandstone encased by floodplain mudstones. Levee deposits consist of fine sands and silts at the transition from channel deposits to floodplain deposits, and may also be rippled or bioturbated, depending on the extent and type of vegetation that was present. Bogs, swamps, and marshes may be present on a floodplain and are characterized by organic-rich mudstone, peat, and coal; all would be classified as mire/swamp facies. If you are dealing with a lake on a floodplain, such as an ox-bow, use one of the lacustrine environments. If you know that the environment is associated both with rivers and lakes, but cannot determine the environment more precisely, choose fluvial-lacustrine indet.

Near marine coastlines, fluvial systems grade imperceptibly into deltaic settings. Within the delta plain, fluvial systems begin to branch into distributary channels and begin to feel the effect of marine influence, such as tides. In cases where it is not possible to determine whether you are in a fluvial system or the deltaic system, choose fluvial-deltaic indet.

Lacustrine

For the database, lacustrine environments are classified primarily on the basis of size, with categories for lacustrine - large, lacustrine - small, and pond. A special case of lake is reserved for those developed within craters or volcanic calderas and these are termed crater lake. A variety of terms are also used for deltas associated with lakes. The lacustrine delta plain term is used for the topset beds of a delta entering a lake and are characterized by a variety of distributary channel and interdistributary settings. In some cases, it may be possible to identify lacustrine interdistributary bay as such. The lacustrine delta front is characterized by inclined strata deposited from turbidity currents shed from distributary mouth bars. These pass towards the center of the lake into muddy deposits of the lacustrine prodelta. In some cases, it may be clear that you are dealing with a delta associated with a lake, but it may not be possible to determine the environment more precisely, in which case you should select lacustrine deltaic indet. Finally, in cases where all that you can determine is that the environment is a lake, but you cannot make any finer interpretation, select lacustrine indet. Note that carbonate lake deposits should use one of these lake environments, not one of the carbonate environments, which are reserved for eolian and marine deposits.

Eolian

The eolian environments include both carbonate and siliciclastic settings under the same terms. Distinguishing these two can be accomplished through the lithology menus. Eolian settings are broadly divided into dune environments, characterized by large-scale cross-stratification and interdune environments, characterized by, depending on climate, wind ripples, adhesion ripples, evaporites, and bioturbation from plant roots. The loess environment is used for deposits of wind-blown dust (usually silt-sized particles). Where the particular eolian setting cannot be determined, use eolian indet.

Karst

Three specific karst-related environments can be selected. The cave environment reflects the accumulation of sediment within caves, that is, within a semi-enclosed solution-formed feature with an open connection to the surface. The fissure fill represents the accumulation of sediment within a solution-widened joint. The sinkhole environment is reserved for semicircular pits formed by solution and collapse of strata. The term karst indet. should be used when it is not possible to choose among these three.

Other

Several environments do not fit neatly into any classification. The tar environment is characterized by tar or asphalt, such as the La Brea tar pits of southern California. The Mire/swamp environment can be associated with fluvial, lacustrine, and deltaic settings and include all coal and peat-forming settings. The spring environment may also occur in a variety of terrestrial settings. Finally, the glacial environment should be selected for settings immediately associated with a glacier.


Tectonic setting

All tectonic settings listed on the pull-down menu are shown here in bold green.

Rift - Basins characterized by normal faults and stretching-dominated subsidence. Includes not only relatively long basins that evolve into passive margins, but also shorter spurs that cease to subside and become failed rifts or aulacogens. Modern examples include the Red Sea and the Gulf of California.

Passive Margin - Basins with adjacent oceanic lithosphere and characterized by thermal subsidence following an initial rifting stage. Basins are asymmetrical, with greatest amount of accommodation occurring on the oceanic side, far from the sediment source, as opposed to foreland basins. Mesozoic and Cenozoic examples include the Atlantic margins of North America, South America, Europe, and Africa.

Back-arc Basin - Similar to passive margins in origin and subsidence mechanism, but generally smaller in scale. Back-arc basins occur behind a volcanic arc, away from a plate margin. A modern example is the Sea of Japan.

Cratonic Basin - Basins characterized by slow thermal subsidence, occurring in the interior of continents, and tending to form a circular, bulls-eye pattern of outcrop. Examples include the Michigan, Illinois, Williston, and Paris basins. Also included here are mid-continental arches separating cratonic basins, such as the Cincinnati, Findlay, Kankakee, and Transcontinental Arches of North America.

Deep Ocean Basin - Basins floored by oceanic lithosphere. Any basin floored by continental lithosphere should be classified as another type of basin.

Forearc Basin - Large basins characterized by a complicated array of subsidence mechanisms that differ from basin to basin, include cooling, stretching, and flexure from both horizontal and vertical loads. Forearc basins lie between a volcanic arc and its associated accretionary wedge. Puget Sound is a modern example, and the central valley of California is a Cenozoic example.

Foreland Basin - Large basins characterized by flexural subsidence driven by the emplacement of loads in the form of thrust sheets and accretionary wedges. Basins are asymmetrical, with thickest portions adjacent to thrust loads, which also serve as the predominant sediment source. Subsidence histories show characteristic stair-step pattern of episodic thrust loading followed by long (10's m.y.) periods of tectonic quiescence. Includes peripheral foreland basins, in which the edge of continental lithosphere is loaded, typically by an accretionary wedge, island arc, etc., and retroarc foreland basins, in which crustal shortening occurs on the continent itself, behind a volcanic arc. Examples include the Appalachian Basin and the Sevier Basin and associated Western Interior Seaway of North America.

Intermontane Basin - A typically relatively small basin developed as a local low between adjacent uplifting mountains.

Intramontane Basin - A typically small basin developed within an uplifting mountain range.

Piggyback Basin - Basins formed on top of one or more thrust sheets and carried along by those thrust sheets. The Peshawar Basin of Pakistan is a modern example.

Pull-Apart Basin - Generally small (<50 km) basins characterized by extremely rapid stretching and thermal subsidence. Tend to have very short histories (~10 m.y.) of rapid subsidence followed by rapid filling with sediment. Best examples include Los Angeles Basin and related basins along releasing bends of the San Andreas fault system.

Volcanic Basin - A basin produced by subsidence on a volcano, such as in a caldera (e.g., Yellowstone Lake, Crater Lake, Ngorongoro crater) or on the flank of a volcano. This term should not be used for larger-scale basins associated with volcanic arcs; for them and island-arc settings in general, use Forearc Basin.

Impact Basin - A typically circular basin produced by the impact of an extraterrestrial object.

Non-subsiding Area - Should be seldom used, since non-subsiding areas generally do not accumulate appreciable stratigraphic records. Some non-subsiding areas may contain fluvial deposits temporally stored in a state of dynamic bypassing. The post-Jurassic Russian Platform is perhaps one of the very few examples of a large region with a stratigraphic record that apparently has undergone no tectonic subsidence.


Glossary of commonly used terms

Depositional environments

Tectonic settings