Geology Lab: The Sedimentary Record

Lithification and Diagenesis

After these sediments have been deposited at the Earth's surface, they may later be buried and cemented at low temperatures and pressures to form solid rock. The process of changing the soft sediment into rock is known lithification. It may involve a number of changes resulting from heat and pressure associated with burial, biological activity, and the reaction of sediment and rock with circulating groundwaters carrying other materials in solution. The sum of all changes that occur to sediment and rock after burial is known as diagenesis. Depending on the diagenetic environment, sedimentary rocks can display various degrees of lithification.

Transitional Depositional Environments

As rivers transport weathered sediments from the continental environment to the ocean and interior lakes, deltas form at the mouths of rivers where large volumes of clastic sediments are deposited. Thick accumulations of sand, silt, and mud form in several environments, including stream channels, flood plain, coastlines (beaches), tidal bars and tidal flats. From the beach outward, well sorted, clean, sandstones accumulate in the wave-dominated near shore environment. Behind the dunes along the beachfront, lagoons form as coastal bodies of brackish water where clastic sediments, organic matter, and evaporites can accumulate. Sahbkas environments form in coastal regions where shallow marine platforms have limited circulation and arid conditions persist, representing hyper-saline, low elevation coastal environments that are inhospitable to most organisms. Salinas are coastal lagoons and inland seas that have limited open marine circulation with seasonal influx of meteoric water drastically altering the salinity of the water. This decrease in salinity generally results in an increase in planktonic growth during the wet season and associated carbonate precipitation.

Bioherm Structure and Turbidity

Bioherm structures are associated with moderate turbidity.

Bioherms

Bioherms are sedimentary depositional structures similar to reefs, but exhibit much lower biodiversity. Bioherms are positive relief structures that are generally associated with a more stressed depositional environment that may not have stable temperatures, salinity or turbidity. These positive relief structures also focus marine nutrients into a thinner vertical water column, usually as a result of colonial growth, but also because of the less stable environment, as such bioherms are dominated by just a few individual species. Bioherms often form grainstone to boundstone rocks, depending on the turbidity and extent of terrigenous sediment that is incorporated. Bioherms may occur in regions where there is an overabundance of a dominant nutrient source, such as the chemotropic ecosystems associated with methane seeps. While total population density of bioherms may rival those of reef complexes, the diversity of organisms is significantly limited in these environments. However, like reefs, the organisms within bioherms provide a structural framework for the positive relief structures they create.

Carbonate Depositional Systems

Carbonate Depositional Systems represent some of the most diverse ecological environments throughout the geologic time, which host significant quantities of fossils, while clastic depositional systems more commonly host significant quantities of trace fossils. Associated with these carbonate depositional systems, there is significant variability in faunal diversity and abundance that reflects the environmental setting of the depositional environment and provides clues to water depth and nutrient supply. Environments with abundant nutrients, stable salinities and generally low environmental stresses exhibit high biodiversity. Similarly, marine environments associated with arid regions and exhibit very low biodiversity, or no biodiversity, because of highly fluctuating salinities that promoted seasonal evaporite (gypsum, halite) precipitation.

Sparite and Micrite

Carbonate rocks can be further described based on the presence and relationship of these allochems as described by Dunham. In carbonate rocks, the allochems are held together by either cement or matrix. The most common carbonate cements are calcite and dolomite which are formed by the recrystallization of carbonate clasts and precipitated as sparite (carbonate cement) between allochem grains. Matrix in most carbonate rocks is a fine-grained calcium carbonate mud known as micrite. Classifying carbonate rocks requires two key observations: 1.) Is the material between the allochemical grain matrix (micrite) or cement (sparite)? 2.) Do the allochems form a framework that is self-supporting without the presence of matrix or cement? If there are sufficient numbers of allochemical grains and they support themselves without the intervening matrix or cement, they are called grain-supported. However, if the allochemical grains cannot support themselves without the intervening micrite, they are called mud-supported.

Carbonate Depositional Environments

Carbonate rocks, those that contain the minerals calcite and dolomite, are the most common chemical precipitate sedimentary rocks and are classified based on the individual grains, cement and matrix that make up the rock, similar to the classification of detrital rocks. Limestones and dolomites can have a crystalline texture or can be composed of grains referred to as allochems with a clastic texture. Allochems are classified as fossils (broken or whole skeletal remains of organisms), oolites (spherical grains formed by the precipitation of carbonate sediment around a nucleus), fecal pellets, and intraclasts (sand or gravel size pieces of limestone or dolomite).

Detrital Sedimentary Rocks

Conglomerate: coarse; gravel; >2mm, Clastic, Rounded rock fragments. Breccia: coarse; gravel; >2mm, clastic, angular rock fragments. Quartz Sandstone: medium; sand; 1/16-2mm, clastic, >75% quartz grains. Arkose: medium; sand; 1/16-2mm, clastic, Quartz sand with at least 25% feldspar minerals. Graywacke: Medium; sand; 1/16-2mm, clastic, Quartz sand with silt and clay sediments within the pore space. Siltstone: fine; silt; 1/16-1/256mm, clastic, Quartz and clay minerals. Shale: very fine; clay; <1/256mm, clastic, Quartz and clay minerals.

Abbreviated classification of basic sedimentary environments and the sedimentary rocks and structures produced in each

Continental Environment: Alluvial fans - arkose, conglomerate, sandstone, cross beds, graded bedding River beds (fluvial) - sandstone, conglomerate, cross beds, ripple marks River floodplains - siltstone, mud cracks Lakes (lacustrine) - shale, siltstone, marl, ripple marks, mud cracks. Swamps - coal, shale, bedding Desert (aeolian) - sandstone, evaporites, cross beds, ripple marks, bedding Glacial - sandstone, conglomerate, graded bedding in outwash plain Shoreline/Transitional: Deltaic - sandstone, siltstone, shale, cross beds Coastline (beaches) - sandstone, coquina, conglomerate, clastic limestone, shale, ripple marks, cross beds Tidal bar - sandstone, clastic limestone, cross beds Tidal flats - sandstone, siltstone, shale, mud cracks Lagoon - coal, siltstone, shale, evaporites, bedding Sabkha/Salina - evaporites, micrite, bedding Marine: Shallow (<10m) - clastic limestone, sandstone, shale, cross beds, graded bedding Deep (>10m) - micrite, chalk, shale, bedding Abyssal plain (>4000m) - diatomite, shale, chert, bedding

Geez

Depositional environments can help scientists interpret geologic environments; sediments have been deposited in the past in environments that do not exist in the present, such as an atmosphere with no free oxygen, or catastrophic environments disturbed by a gigantic meteorite impact. Therefore, by examining sedimentary rocks as windows into these environments, we can learn about earth processes that we would otherwise know little about, and deduce details about them such as the chemistry of the air or water with which the sediments were in contact and the physical processes that were occurring in that environment. Interpreting depositional environments of the past provides knowledge of earth processes through time, including catastrophic environments associated with earthquakes, tsunamis, landslides, and volcanic eruptions. Geologists also use analyses of depositional environments to help locate natural resources such as fossil fuels, economic minerals, and construction materials, and water resources.

Marine Depositional Environments

Farther offshore, at the edge of the continental shelf, is the continental slope and rise, down which gravity flows or turbidities move poorly-sorted sands and muds down into the deep ocean basins. On the deep abyssal plains, far from the influence of turbidite transported continental materials, organic muds or marine oozes are the result of a fine rain of the shells of microorganisms filtering down from near the surface. These become gradually finer from fine sand through silt and into quiet water offshore mud environments, including parallel layered shales. In mid and high latitudes, these muds continue out onto the continental shelf. In tropical latitudes coral reefs or carbonate platforms often form offshore from the interlinked skeletons of carbonate secreting corals, molluscs, etc. Coral requires sunlight and warm clear water. Therefore limestone reefs do not form in the deep oceans (too dark) or in siliclastic, wave-dominated, turbid, near-shore environments. In marine depositional environments with large sediment fluxes associated with rapid weathering of continental environments, many ecosystems can be inundated by turbid waters that prevent the large-scale growth of filter-feeding communities. Evidence of life in these environments is widespread in the form of trace fossils. In areas that are not as severely impacted by terrigenous influx, reefs, bioherms, and typical platform deposits can develop.

Trace Fossils

In all depositional environments, relatively few of the actual organisms that lived in that environment are preserved as fossils (preservation bias); however, the presence of these organisms is often preserved as trace fossils that represent the tracks, trails, borings and burrows of these organisms. These are called ichnofossils, which record in the sedimentary rock record the activity of organisms, which bioturbation (degree of burrowing of organism in sediments) providing clues as to the relative abundance of organisms that reworked seafloor sediments, thus destroying bedding features in depositional environments.

Ichnofabric Index Levels

Index 1: Undisturbed sediment indicating no burrowing organisms Index 2: Discrete, isolated trace fossils with <10% disturbance indicating minimal burrowing organisms. Index 3: Isolated, overlapping burrows with 10% to 40% disturbance indicating moderate activity of burrowing organisms. Index 4: Minimal bedding structure visible with overlapping burrows that are visible but not well defined including 40% to 60% disturbance; activity by burrowing organisms. Index 5: No original bedding structures visible but burrows remain discrete in a few places indicating abundant activity by burrowing organisms. Index 6: Completely homogenized sediment with no distinct burrows indicating extensive activity by burrowing organisms. The level of bioturbation determines the Ichnofabric Index.

Chemical Precipitate/Organic/Other Sedimentary Rocks

Micrite: calcite, crystalline, fine-grained lime mud. Travertine: calcite, crystalline, banded limestone. Fossiliferous Limestone: calcite, clastic, macro-fossils in lime mud matrix. Oolitic Limestone: calcite, clastic, cemented oolites. Chalk: calcite, clastic, microscopic shelled organisms cemented together. Coquina: Calcite, clastic, cemented fossil hash. Dolomite: dolomite, crystalline, light to tan, reacts with acid when powdered. Gypsum: gypsum, crystalline, light colored, soft, can be banded. Rock Salt: Halite, crystalline, white to light gray with salty taste. Chert: fine-grained silica, crystalline, occurs in variety of colors, conchoidal fracture. Coal: organic matter, clastic, carbonized plant fragments.

Continental Depositional Environments

On the continents, sedimentation might be thought to begin with clastic materials shed from the flanks of mountain ranges. These alluvial fans are characterized by poorly sorted, boulder and gravel dominated, debris flow conglomerates. Fluvial (river) facies include cross-bedded and rippled river sandstones and parallel or cross-bedded floodplain mudstones (siltstones and clay shales). Lacustrine (lake) facies include sands deposited at the mouths of rivers which empty into the lake and along the shoreline as well as muddy facies on the deep lake bottom. Swamps often form in low-lying areas (for example, the area near sea level behind the shore environment) in which parallel layered, organic-rich black shales and coal form. In arid regions with little vegetation and few rivers, aeolian (wind-deposited - sand dunes) environments may dominate. Glacial deposits are formed as meltwaters transport sand and silt into lower elevations in braided stream environments.

Dunham classification of carbonate rocks

Original components not bound at deposition: contains mud (particles of clay and fine silt size): mud-supported: less than 10% grains (MUDSTONE) vs. more than 10% grains (WACKESTONE). All of that, but grain-supported = PACKSTONE. Lacks Mud: grain-supported = GRAINSTONE. Original components bound together at deposition. Intergrown skeletal material, lamination contrary to gravity, or cavities floored by sediment, roofed over by organic material but too large to be interstices = BOUNDSTONE.

Platforms

Platform environments may have local positive relief structures that include specific ecological niches for bioherms and reefs; however, the open platform environments consist of loosely consolidated sediment over vast areas with minimal relief. Platforms are horizontal to gently-sloping along continental margins where the lack of bathymetric relief prevents the focusing of nutrients into specific regions. These ecosystems generally exhibit moderate biodiversity but low population density, with carbonate rocks ranging from mudstone to packstone, based on the amount of wave energy in the depositional environment. Organisms within the platform environments tend to be largely mobile, enabling them to cover larger areas in search of nutrients, unlike the dominance of filter-feeding organisms that dominate reef and bioherms communities. Many platform deposits appear largely devoid of macroscopic fossil material because of the low population density, but are often extremely rich in microfossils that have settled on the ocean floor from overlying water column. Today, platform environments vary greatly in population density where vast regions appear to be devoid of life and localized areas have high populations where a large nutrient source has been added, such as dead and decaying fish or a larger vertebrate that has settled to the ocean floor.

Reefs

Reefs are sedimentary depositional structures where the organisms grow in-situ usually forming a boundstone. Reefs are high biodiversity, positive relief structures that generally are associated with the photic zone in stable environments. These positive relief structures generally occur at bathymetric changes when ocean currents focus marine nutrients into a vertically limited water column, thus increasing nutrient supply to a competitive ecosystem. High biodiversity is maintained by stable temperatures, salinity, and turbidity. Therefore, mot reefs are associated with tropical waters that do not have a significant component of terrestrial, clastic sediment influx. In the late Paleozoic, large reef complexes formed in what is now the Southwest of the United States (west Texas, New Mexico), in what has been termed the Permian Basin by the petroleum industry. These were massive, biologically-diverse structures several hundred meters tall with the main framework organisms being colonial sponges with crinoids, brachiopods, bryzoa, and trilobites mixed throughout.

Figure

Sediment deposited within a specific depositional environment will develop characteristics based on the mode of transportation and the environment in which they are deposited. Main sedimentary depositional environments: alluvial, glacial, aeolian, evaporite, fluvial, lacustrine, lagoonal, beach, deltaic, tidal, shallow water marine, deep water marine, and reef.

Sedimentary Depositional Environments

Sedimentary rocks also provide a number of clues as to their mode of transportation and depositional environment. They form in a wide variety of environments; any place that sediment can accumulate is a potential environment for future sedimentary rock. At the Earth's surface, sediment accumulates in three major environments: continental; shoreline or transitional; or marine. These sedimentary environments, as well as the energy level of transportation and diagenetic processes, will dictate many of the characteristic features found in sedimentary rocks.

Minerals

Sedimentary rocks can be composed of a variety of minerals, but the most common minerals that are found in sedimentary rocks are quartz, calcite, and clay minerals. Quartz is chemically stable at the Earth's surface and is commonly found as grains, cement, or chemical precipitates in all types of sedimentary rocks. Calcite is the primary mineral component of limestone and can function as cement in various detrital sedimentary rocks. Clay minerals are formed from weathered feldspars (orthoclase and plagioclase) and micas (biotite and muscovite). These very fine-grained minerals compact to form shale, the most abundant sedimentary rock.

Figure 2

Sedimentary structures help geologists determine past depositional environments and interpret Earth history. Ripple marks are indicators of current activity, in a fluvial environment or along a shoreline. Mud cracks are indicators of alternating wet and dry conditions. Cross bedding is an indicator of current activity also, either wind or water, causing sediment to accumulate on sub-horizontal planes. Graded bedding is an indicator of high energy environments such as storm surges or major floods.

Sedimentary Structures

Sedimentary structures such as bedding, cross-bedding, mud cracks, ripple marks, and graded bedding. Provides important information about paleoclimates and the environment in which the rocks formed. The lateral continuity of these layers reflects the areal extent and uniformity of the environment in which they were deposited. When the environment of deposition changes in a particular region, the nature of the sediment accumulating there also changes. A bed of sandstone on top of a bed of shale reflects a change in sedimentary conditions from one in which clay was deposited to one in which sand was deposited. Often times, the past environment of deposition can be vastly different than the environment in the present; and this is one of the ways in which geologists interpret changes in the rock record.

Evaporites

Some sedimentary rocks form as a result of evaporation of sea water and are collectively called evaporites. The rocks form in arid environments where evaporation exceeds precipitation such as deserts, sabkhas (arid coastlines), and restricted basins with limited circulation. The most common rocks that form in evaporite conditions are rock salt and gypsum. These climate sensitive sediments and rocks are important archives of Earth's history and depositional environments. They are also sources of economic minerals and can form traps or seals for sub-surface hydrocarbons. The Jurassic-aged salt domes of the Gulf Coastal area provide structural traps for oil and gas and an economic source of salts and sulfur for industrial processes. Paleozoic salt deposits across Michigan, Ohio, and New York have been mined extensively, and Michigan leads the nation in salt production. The thick sequences of Permian-aged gypsum and anhydrite in West Texas provides gympsum for construction and low permeability traps for lucrative hydrocarbon extraction in the Permian Basin. The Salado Formation, a thick sequence of Permian-aged salt near Carlsbad, New Mexico is used to store transuranic waste from nuclear facilities.

Texture

Texture refers to the size, shape and arrangement of minerals in a rock and is related to the mode of transportation and the depositional environment. Clastic texture refers to sedimentary rocks that are composed of broken bits and pieces of minerals, other rocks and fossils, all detrital sedimentary rocks and some chemical precipitate rocks have a clastic texture. Crystalline texture refers to sedimentary rocks that are formed from an interlocking network of crystals; many chemical precipitate rocks have a crystalline texture.

Ichnofabric

The extent of disturbance of sedimentary layers resulting from animal burrows is termed ichnofabric. If bioturbation is minimal or non-existent, then there will be no disturbance of sedimentary layers within a depositional system. If bioturbation is extensive, then burrowing may completely destroy any evidence of horizontal layering expected in a normal depositional system. Therefore, ichnofabric is characterized by the level of bioturbation which indicates the level of organism activity within the depositional environment. Ichnofabrics are subdivided into six Ichnofabric Index Levels.

Clast Size

The first key to identification of sedimentary environments is the relationship between energy and clast size. Generally speaking, the higher the energy of the transportation mechanism, the larger the grains that can be kept in suspension. Therefore, sedimentary deposits reflect the energy of the depositional environment and contain sediments too large to remain in suspension. As an example, a swift mountain stream might keep even gravel sized objects in suspension, so its deposits would only consist of pebbles and boulders; or a large river might be flowing so slowly that it is unable to keep sand in suspension, so its deposits would contain sand sized grains and larger. Thus the smaller the dominant clast found in a sedimentary environment, the lower the energy level of the transportation mechanism and depositional environment.

Why are Depositional Environments Important?

The sedimentary record and knowledge of depositional environments is important for reconstructing earth history, understanding earth processes, and helping humans exploit and utilize natural resources produced as the result of geologic processes through time. By analyzing sedimentary rocks and interpreting depositional environments, geologists can deduce what was happening on earth at the place and time the sediment was originally being deposited. Geologists utilize all the processes we have learned about today to interpret past environments, including rock and sediment identification, interpretation of sedimentary structures, the use of allochems to interpret carbonate environments, and the use of trace fossils to determine biological activity.

Type

The three main criteria used to classify sedimentary rocks are type, texture, and mineral composition. Type refers to the main categories and one sub-category of sedimentary rocks: Detrital - derived from the weathering and erosion of other rocks; Chemical Precipitate - formed by chemical precipitation; or Other/Organic - considered a sub-category of chemical precipitate sedimentary rocks, chert can form as irregular masses in other rocks or as bedded layers between sequences of carbonate rocks; coals form as a result of an accumulation of organic matter in low oxygen environments.

Transportation

These sediments are transported by a variety of means to new environments where they are eventually deposited. The mechanism of transportation varies, with water being the most common mode of transportation; other mechanisms include wind, gravity, glaciers, and biologic activity. Gravity assists in all forms of sediment transport as material at higher elevations is transported to lower elevations due to gravitational forces.

Salina-like Deposits

Towards the end of the Permian, large-scale, salina-like deposits formed throughout the Permian Basin when growth of the great Permian reef structures completely enclosed the inland basins. As a result, several hundred meters of interbedded limestone, gypsum and halite deposits filled the sub-basins (Delaware, Midland, and Val Verde) of the Permian Basin at the end of the Paleozoic. Similar occurrences were deposited in Europe at this time, all of which are generally known as Zechstein Evaporites named for deposits in Germany. While these are not classic salina deposits because of the vast area they cover, they are effectively formed by the same process and very similar in occurrence.

Trace Fossils Part 2

Trace fossils preserve part of the depositional environment and are often found in locations where body fossils are not. Dinosaur tracks are one of the most publicly visible forms of trace fossils, but the tracks left by a trilobite crawling along the ocean floor are other forms of trace fossils. Trace fossils can indicate where an organism settled or crawled across the sea floor, leaving behind a track or imprint of the organism. Trace fossils may also be three-dimensional and represent the burrowing activity of an organism where it created a hole to live in or where it crawled through the sediment as a detritivore. Either way, trace fossils represent the movement of an organism either on the surface of the seafloor or through the sediments that compose the seafloor.

Weathering

Weathering processes, both mechanical and chemical, produce sediments from preexisting rocks, including: Detrital sediment - the solid particles derived by predominantly mechanical weathering, which retain the same composition as the original source rock. Chemical sediment - the products of chemical weathering which includes mineral alterations and the resulting ions that are placed in solution from dissolution.

More

What we learn about geologic history of a region comes mostly from examining the layers of sedimentary rock from the area and determining their depositional environments. Because sedimentary rocks are stratified in age sequence, layers of sedimentary rock act as a record of how that area was changing, physically and biologically, over the extent of geologic time spanned by the sedimentary rock layers. Reconstructing depositional environments enables geologists to observe climates of the past, life forms of the past, and geography of the past such as the location of mountains, basins, large rivers, and bays of the ocean. Changes over time in climate, life forms and geographic location constitute the geologic history of a region. Ultimately, regional geologic histories are compiled into a history of the Earth over the whole course of its existence, including the formation, growth, and movements of continents and ocean basins, the growth and erosion of major mountain ranges, and the history of life on earth.