# Shallow water marine environment

Shallow water marine environment refers to the area between the shore and deeper water, such as a reef wall or a shelf break.This environment is characterized by oceanic, geological and biological conditions, as described below. The water in this environment is shallow and clear, allowing the formation of different sedimentary structures, carbonate rocks, coral reefs, and allowing certain organisms to survive and become fossils.

## Sediment

The sediment itself is often composed of limestone, which forms readily in shallow, warm, calm waters. While siliciclastic and carbonaceous sediments can coexist, shallow marine environments can also contain only one or the other. Shallow water marine sediment primarily features larger grain sizes because smaller grains have washed out to deeper water. Within carbonaceous sedimentary rock, evaporite minerals such as gypsum, anhydrite, and halite may be present. The most common evaporite minerals found within modern and ancient deposits are gypsum, anhydrite, and halite. These minerals can occur as crystalline layers, isolated crystals, or clusters of crystals.
Approximately 75% of surface sediments are in shallow marine environments, holding most Phanerozoic and Precambrian sedimentary rock. This is visible in the North American and Caribbean regions. However, shallow marine sediment quantity varies significantly over geologic time due to supercontinent breakup and shifting tectonic plate processes.

## Sedimentary Structures
### Shallow Marine Environments

Shallow marine environments are characterized by various types of sedimentary structures, including:
* Graded Bedding: characterized by a vertical gradation in grain size, with the smallest grains at the top
* Ripples: the smallest bedform type
* Dunes: larger and similar in shape to ripples

### Carbonaceous Rocks in SMEs

Carbonaceous sedimentary rock in SMEs contain significant amounts of non-skeletal matter along with siliciclastic or chemical constituents and can exhibit a range of sedimentary structures, including:
* Cross stratification: a layering structure found in gravel, sand, and coarse silt-sized sediment, characterized by distinct layers that are steeply inclined to the underlying surface.
* Desiccation cracks: cracks formed by the drying of newly deposited mud in sub-aerial climates.
* Flame structures: mud shaped like flames that intrude into the overlying layer of rock.
* Convolute Folds: sediment folded into irregular anticlines and/or synclines.
* Flutes: outstretched ridges that are rounded at one end and flared at the other.
* Groove casts: outstretched, almost straight grooves in sediment caused by the dragging of an object.
* Chevron structures: a v-shaped type of groove cast, resulting from stress applied in two or more directions, found on the bottoms of beds deposited in shallow water environments.
* Syneresis cracks: cracks formed by other than by sub-aerial climate exposure, including contraction by clay clumping, contraction from settling during faulting, smectitic clay compaction from dewatering due to a salinity change in surrounding water, compaction dewatering under sediment causing injection from below or collapsing from above, and tensional opening due to down-sloping of a surface mud layer.
* Fenestrae: open or partially filled spaces occupied by different sediment or cement

## Water composition
### Characteristics

Shallow marine environments are typically characterized by clear and shallow water. The distributional patterns of marine organisms in these environments can be used to define different types of shallow marine environments based on temperature, which can also provide insights into past patterns in Paleolithic zones.
The boundaries between different shallow marine environment types in terms of climatic zones are not always agreed upon. However, three major criteria are used to define SME types: faunal provinces, faunal elements, and latitude.

#### Carbonate factory zones

Carbon dioxide is removed from the atmosphere when it dissolves in seawater and turns into carbonic acid. The acid then weathers rocks, creating bicarbonate and other ions. Calcium carbonate is a precipitate from calcium and the bicarbonate ions, while the carbon is precipitates as limestone. Many shallow marine environments are associated with carbonate factory zones, where processes that remove CO $_2$ from the water change bicarbonate ions into carbonate ions, supporting lime precipitation. These processes include increasing temperature, intense evaporation, and mixing water that is high in CO $_3$ and low in calcium cations with seawater.

#### Changes in limestone composition over geologic time

Over geologic time, the composition of limestone has changed from calcite-rich to aragonite-rich. This change is influenced by the presence of magnesium ions, which can inhibit calcite precipitation. Aragonite and calcite have the same chemical formula but a different crystal system, although aragonite is less prone to magnesium inhibition. Changes in the Mg/Ca ratio over geologic time, influenced by seafloor spreading and tectonic plate movement, have also increased aragonite abundance.

## Organisms

Shallow marine environments, particularly the intertidal zone, are home to a diverse range of organisms, including starfish, sea anemones, sponges, marine worms, clams, mussels, predatory crustaceans, barnacles and small fish.

### Hydrozoa and microinvertebrates

Hydrozoa, also known as hydroids, inhabit SMEs and feed on surrounding algae and zooplankton. Species of isopods and amphipods are found in intertidal zones, creating complex burrows and surface tracks in the sediment. Brittle stars may bury themselves in sediment with their arms exposed.

### Carbonate reefs

SMEs are characterized by carbonate reefs, which support many species. Estimates suggest that coral reefs alone may host between 1-9 million species. The three main types of reef formations are:
* Fringing Reefs: attached to the shore
* Barrier Reefs: separated from the mainland by a lagoon
* Atoll Reefs: ring-shaped reefs surrounding a lagoon

### Reef Organisms

Organisms that inhabit these reefs include red algae, green algae, bivalves and echinoderms. Many of these organisms contribute to reef formation. Furthermore, unicellular dinoflagellates live in coral tissues, engaging in a mutualistic relationship where they provide corals with essential organic molecules.

## Fossils

The majority of the fossil record has been discovered in lithified SMEs, which were once home to diverse organisms. Many of these fossils date to periods when much of the Earth was covered in shallow seas.
Several types of fossils can be found in these environments, including:
* Skolithos Ichnofacies: vertical, cylindrical, or U-shaped burrows created by organisms for protection. These are classified as trace fossils.
* Glossifungites Ichnofacies: vertical, cylindrical, U-shaped, or tear-shaped borings or burrows created by organisms such as shrimp, crabs, worms, and bivalves. These are also classified as trace fossils.
* Stromatolites: laminated sedimentary structures formed when cyanobacteria create microbial mats, trapping clay, silt sediment, and organic materials. These mats eventually lithify, embedding fossils.