<h1 class="pgc-h-arrow-right" >9</h1>
By looking at the strata, can we characterize the sedimentary environment? To determine the sedimentary environment, geologists, like crime scene investigators, look for clues: detectives look for fingerprints and blood stains to identify the culprit, while geologists examine particle size, composition, classification, sedimentary structure, and fossils (remnants of organisms buried in sediment) to determine the sedimentary environment. Each environment leaves a different fingerprint in the formation:
< h1 class = "pgc-h-arrow-right" >9.1 terrestrial sediments</h1>
When we explore sedimentary environments, we first consider those that develop inland from the coast so that they are not affected by ocean tides and waves. These are non-marine environments, or terrestrial sedimentary environments, including drylands, lakes, streams, and swamps.
Usually, oxygen in surface water or groundwater reacts with iron in land sediments, producing rust-like iron oxide minerals that give the sediment an overall red color. Geologists informally refer to this hue of formation as the red layer.
< h1 class="pgc-h-arrow-right" > 9.1.1 glacial deposits</h1>
If we travel in the mountains, or go to the polar regions, where the weather is very cold and more snow accumulates in winter than melts, we will find that glaciers are growing and flowing slowly. Glaciers can carry a variety of sediments: from clay sizes to boulder sizes, falling from neighboring cliffs onto glaciers, or peeling from the ground at the bottom or side.
When glaciers melt, sediments in or above them form glacial moraine. Since ice does not classify sediments by size, it usually contains larger detritus, which are randomly distributed in mudstone or siltstone. This type of rock is called mixed land source sedimentary rock.
<h1 class="pgc-h-arrow-right" >9.1.2 mountain stream sedimentation</h1>
Crossing the end of the glacier, we enter a field of rapids of a river that runs down from the valley. Turbulent currents carry huge debris that are ground into circles. When the water flow slows down, pebbles and large rocks settle down, while the river carries sediment further downstream. When lithification occurs, the stream gravel becomes conglomerate.
< h1 class="pgc-h-arrow-right" > 9.1.3 alluvial fan deposition</h1>
When we reached a mountain front, a turbulent stream flowed into the plain, slowing down the water flow, causing sediment to accumulate. In arid regions, this accumulation forms a wedge-shaped pile called an alluvial fan, made up of grit. The special sand of the alluvial fan may contain feldspar particles that have not yet weathered into clay, so the sediment of the alluvial fan becomes feldspar sandstone and conglomerate after lithification.
< h1 class= "pgc-h-arrow-right" > 9.1.4 dune deposition</h1>
In very dry climatic conditions, few plants grow, so the ground is exposed to wind. Flowing air forms sand dunes, which are made up of well-sorted, round sand. If buried, these dunes turn into thick, interlaced layered sandstones.
< h1 class = "pgc-h-arrow-right" > 9.1.5 river sedimentation</h1>
Rivers transport gravel, sand, silt and mud. Thicker sediments tumble along riverbeds, gathering in staggered layers, while finer sediments drift in the water. These fine sediments settle along river banks or on floodplains. River sediments form sandstone, siltstone, and shale (or mudstone).
In general, coarser sediments congregate in river channels, surrounded by layers of fine-grained floodplain sediments. On the transverse profile, the channel sediment is lens-shaped. The shale in these sediments may contain mud cracks. Geologists often refer to river sediments as alluvials (derived from the Latin flovuius, meaning river).
< h1 class = "pgc-h-arrow-right" > 9.1.6 lake sedimentation</h1>
The tranquil lake inshore water cannot carry coarse sediment, so only fine clay reaches the center of the lake, where it is deposited on the lake bed to form silt. As a result, the lake sediments become fine layered shales. In lake deltas, gravel usually gathers on a horizontal layer close to the shore, sand gathers on an offshore inclined bed, and silt gathers on the lakeside level.
Glacial sedimentation; mountain stream sedimentation; alluvial fan sedimentation in Death Valley, California; sand dunes in Brazil; river channel sedimentation; laminated mud accumulated on the lake bed
The cross-section of the delta, formed where the river enters the lake. Different types of sediments accumulate in different parts of the delta
< h1 class= "pgc-h-arrow-right" >9.2 marine sediments</h1>
There are a variety of sedimentary environments along the coast, shallow sea and deep sea. All of these environments are types of marine sedimentary environments. Sediments in a particular marine environment depend on water depth, wave energy and water temperature, as well as the presence or absence of debris from quartz and clay.
< h1 class="pgc-h-arrow-right" > 9.2.1 marine delta sediments</h1>
Where rivers flow into the sea, a huge ocean delta can be formed. Such deltas are more complex than the lake deltas we mentioned earlier because the ocean deltas have a variety of sedimentary environments, including swamps, waterways, floodplains, and ocean floor slopes. Changes in sea level over time can cause the location of these environments to change. As a result, marine deltas produce multiple types of sedimentary rocks.
< h1 class="pgc-h-arrow-right" > 9.2.2 coastal clastic deposition</h1>
Ocean currents and waves carry sand along the coastline, and the sand washes back and forth in the waves, forming a good classification and smoothness. The slopes of the coast are gentle, forming wide tidal flats where rippled silt beds accumulate and silt settles in the waves-protected lagoons.
Offshore, the sea is deep, the wave energy has little effect on the seabed, and silt and mud will accumulate. There may be a large number of organisms on or in the sediment, but because the water is stationary, there are no ripples on the surface. So, if you find well-sorted sandstones, as well as siltstones and shales that contain marine fossils, you see clastic sediments along the coast and in shallow seas.
The large-flow delta formed along the coast is a synthesis of multiple sedimentary environments
< h1 class= "pgc-h-arrow-right" > 9.2.3 coastal carbonate deposits</h1>
In shallow sea environments relatively free of sand and clay, warm, clear, nitrogen-rich water can accommodate large numbers of organisms that produce carbonate shells. Nearby beaches gather sand composed of small shell fragments, lagoons accumulate carbonate mud, and on reefs, organisms, such as corals, create fixed carbonate mineral mounds. The farther away from the reef, the reef debris will form a circle. Products of shallow carbonate environments are buried to form fossil-bearing limestones and mudstones.
Carbonate reefs form along the coastline with clear and warm waters
A tropical island surrounded by reefs
< h1 class= "pgc-h-arrow-right" > 9.2.4 deep sea sedimentation</h1>
On deep-sea plains far from land, only fine clay and plankton provide a source of sediment. The clay eventually settles deep into the ocean floor, forming fine-grained laminated mudstones. Plankton shells are deposited and lithified to form flint or chalk. Thus, the deposition of mudstones, chalks or layered flints indicates a deep-sea sedimentary environment.
Tiny plankton shells are made up of carbonate minerals; the white chalk cliffs in southeast England consist of carbonate plankton shells from 80 Ma ago
< h1 class = "pgc-h-arrow-right" >10. Origin and evolution of sedimentary basins</h1>
The thickness of sedimentary layers on the Earth's surface varies greatly. If you stand in south-central Canada, you'll find yourself on the foundations of igneous and metamorphic rocks that formed 1 billion years ago, and sedimentary rocks will be nowhere to be found. But if you're standing on the south coast of Texas, you'll have to drill through 15 kilometers of sediment to get to the igneous and metamorphic substrates.
Sediment accumulation only forms in certain areas, i.e. the Earth's surface sinks or undergoes sinking, forming depressions where sediments are collected. Geologists call sediment-filled depressions sedimentary basins. Under what geological conditions did sedimentary basins form? Why do the characteristics of sediments deposited at a location in a basin change over time? We must consider plate tectonic theory and changes in sea level to find answers.
<h1 class="pgc-h-arrow-right" >10.1 Why do sedimentary basins form? </h1>
Geologists distinguish between different types of sedimentary basins in the context of plate tectonic theory:
The environment of the sedimentary basin
<h1 class="pgc-h-arrow-right" > 10.1.1 Rift Valley Basin</h1>
In continental rift valleys, lithospheres stretch horizontally and thinner vertically, and the earth's crust slides down along faults, forming low troughs with narrow ridges as the edge. These sediment-filled troughs are called rift valley basins.
<h1 class="pgc-h-arrow-right" > 10.1.2 Passive Marginal Basin</h1>
Passive continental margins, which are continental margins rather than plate boundaries. They were formed after continental rift valley action was completed and a new mid-ocean ridge was formed. The stretched and heated continental crust slowly cools and sinks during the Rift Valley event, forming passive marginal basins. These basins range from 150km to 400km wide and are filled with various types of marine sediments. The apex of the passive margin basin is the continental shelf.
< h1 class= "pgc-h-arrow-right" > 10.1.3 inland basins</h1>
Bowl-like depressions in the interior of the continent are called inland basins. They may have formed where the late Precambrian continent experienced unsuccessful rift valley action, as the rift valley and surrounding continents cooled and settled when rift valley action stopped.
<h1 class="pgc-h-arrow-right" >10.1.4 foreland basins</h1>
In continental collisions or plate aggregation boundary interactions along the edge of the orogenic belt, large chunks of rock push faults up the continental surface. The weight of these rock masses is pushed down to the surface of the lithosphere, forming a wedge-shaped depression close to the mountain range, which is filled with sediment from the erosion of the mountain range. Rivers and delta strata are deposited in foreland basins.
<h1 class = "pgc-h-arrow-right" > 10.2 sea advance and sea retreat</h1>
When relative sea level rises, the coastline moves inland, a process called sea advance; When relative sea level drops, the coastline moves toward the ocean, a process called sea retreat. Sea advances and retreats affect the sedimentary environment of an area. The process of sea advance and retreat can form a large sediment, which is not deposited at the same time. Historically, land has been widely submerged, after which sea levels have fallen and these areas have become drylands. Therefore, we can find records of sea advances and retreats in the formations of the interior of the continent.
Effects of sea advance and retreat on sedimentary sequences
< h1 class = "pgc-h-arrow-right" > 10.3 Underground Change: Diagenesis</h1>
We discussed the process of lithification, through which sediments harden into rock. Lithification is an aspect of a broader process that geologists call diagenesis. The term includes not only all the physical, chemical, and biological processes that transform sediments into sedimentary rocks, but also processes that change their characteristics after sedimentary rocks have formed.
In sedimentary basins, strata can be buried very deeply. As a result, these rocks are subjected to high pressures and temperatures and come into contact with warm groundwater. Diagenesis under such conditions can cause chemical reactions that produce new minerals in the rock, which can also lead to the dissolution of existing cements and the precipitation of new cements.
When geologists use the term diagenesis, they refer to processes that occur in sedimentary rocks and do not produce structures and minerals found in metamorphic rocks. The gradient process from diagenesis to metamorphism occurs at temperatures between 200 and 300 °C. In the next part, we will enter the realm of real deterioration.