Marine sedimentary media

Apart from the sedimentary media of transition between the continent and the sea, the purely marine media are made up of the continental shelf on the one hand and the precontinental edge and the abyssal plain on the other. A large amount of detrital materials transported by rivers and sedimented in the sea end up on the continental platform, giving rise to deltaic forms. Of them, the finest are distributed on the platform. In addition, it is here where organogenic sedimentation reaches greater development (for example, coral reefs). In the precontinental edge and abyssal plain there are two types of sedimentation. An autochthonous or pelagic sedimentation product of the accumulation of shells of planktonic organisms, either calcareous or siliceous. And on the other, allochthonous, or detrital type, based on the materials that from the continent and passing through the continental platform, end up at the foot of the slope. This transport of detrital materials is carried out either by gravitational landslides from the platform, or by turbidity currents located in the submarine canyons that, upon reaching their mouth, are scattered over the abyssal plain, building fans or "deltas" of sedimentation.

Types of marine sedimentation

Sedimentation on continental shelves

The transition zone between the outer limit of the beach and the continental shelf itself (offshore) participates in the sedimentological characteristics of both. It is an area dominated by silt and shale sedimentation, although there may be sandy interbedded layers originated during large storms (storm sand layers). Due to the great dominance of life (in species and individuals) the sediment is frequently bioturbid and, furthermore, it is not uncommon to find layers formed by the accumulation of shells. On the continental shelf itself there is a domain of sedimentation of marls, silts or clays. Most of the silt and shale materials have been transported in suspension from the mainland. In the most proximal part there may still be layers caused by large storms, although less frequently than in the transition zone to the beaches. The fauna can be varied according to the areas. Thus, local accumulations of shells can occur. The bioturbation of the materials is locally very strong, giving rise to burrows that sometimes have well-defined shapes. It is also common to find accumulations of fecal pellets. In warm seas, much of the sediment is the product of shell erosion by drilling organisms. Emery (1952-1968) classifies the sediments of the current continental shelves as relict and modern. The relicts, which would represent, according to this author, 70% of the total, would have been deposited there when the area in question was part of another sedimentary environment, generally more proximal because the sea level was at lower levels than today. This would have occurred during the Quaternary era, when, as a consequence of the ice ages, there were rapid transgressions and regressions. These sediments, at present, are not in equilibrium with the environment where they are found. They are, therefore, inherited and largely reworked by organisms (relict sediments). The modern ones divide them into detrital material (transported in suspension, either by water, wind or ice); organogenic material (product of the accumulation of shells and their fragments), and autogenous minerals (or minerals formed in the environment itself, such as phosphorite and glauconite). Relict sediments can be reworked by marine currents and give rise to layers of different geometry. Among the most important are giant ripples and sand ribbons. In ancient sediments, the most frequent platform materials are marls and clays, sometimes siltstones, with parallel stratification, sometimes nodulous due to diagenesis, and with fauna characteristic of this environment.

Carbonate sedimentation on platforms

Geological section of a carbonate platform, showing the different depositional sub-ambients and the distribution of the corresponding sedimentary facies.

Irwin (1965), studying the "Mississippian" deposits of the Williston Basin in North America, devised a theoretical model for carbonate sedimentation on shelves. These deposits are characterized by presenting three different types of facies, which represent each other lateral changes. These are: a) cyclical evaporitic; b) bioclastic or oolitic limestone and dolomites, and c) finely stratified argillaceous limestone.

Facies a) Consists mainly of dolomites and anhydrite and minor amounts of halite, clay and sandstone. These materials are rhythmically distributed in the following sequence: it begins with pel- and biomicrites, which move upwards to microcrystalline dolomites with scattered shell fragments (these dolomites contain veinlets of anhydrite and, towards the top, nodules) and the rhythm culminates with anhydrites with dolomite veinlets.

Facies b) It is composed of mud-free calcarenites, well classified, sometimes dolomitized or cemented by sparite, but often retaining primary intergranular porosity. These rocks are frequently oolitic, and sometimes sandy skeletal, composed mostly of crinoid remains. Upwards they pass to pelesparites which, with increased calcareous mud, pass to the pelmicrites of facies a). As fossil fragments they include crinoids, brachiopods, bryozoans, corals, foraminifera and algae.

Facies c) They are dark gray clayey limestones, laminated or finely stratified; locally they are siliceous and interbedded with cherts. The fauna is similar to that of facies b) but less abundant and better preserved, with few corals or algae. Fossils are sometimes silicified.

These three types of facies are distributed areally, being a) the most proximal and c) the most distal. The most proximal facies, that is, a), has been deposited in a restricted marine environment separated from the open sea by bars. Pelmicrites are typical of current lagoons, while dolomites and evaporites may be from primary precipitation on lagoon bottoms or by diagenesis in intratidal or supratidal deposits, similar to current sebkhas. In the intermediate facies, that is, b), the faunal fragments, the presence of oolites and the absence of mud, indicate a high-energy sedimentation medium with strong movement of skeletal sand, building bars. In the most distal facies, c), the fine grain size and, above all, the fauna, indicate purely marine sedimentation with low energy, as would correspond to an area of open sea, below the action of the waves and far from the waves. bottom currents. By evolving this model with the time imposed by transgressions and regressions, it allows predicting the appearance of a certain lithology by applying Walther's law, whereby any vertical lithological change in a section results from a lateral migration of different media. In the case of cyclical series, they will be interpreted as transgressive-regressive sequences, the regressive stage being similar to the transgressive one, but the facies migrating in the opposite direction to the transgressive one. In other words, an ideal transgressive series would entail the superposition of facies a, b and c in this order, and in regressive it would be: c, above it b and facies a crowning the cycle. This theoretical model can also be applied to areas of current carbonate sedimentation, such as the Persian Gulf and the Caribbean Sea. The extension of each one of the zones is logically different, imposed by the topography. It can also be identified in areas of terrigenous sedimentation.

Reef sedimentation

A reef (Lovenstan, 1950) is a calcareous deposit of remains of organisms that possessed sufficient ecological potential to maintain a life position, in a rigid structure and resistant to waves, and that originate accumulations of characteristic geometry. There are many terms to designate the different types of deposits, of which we will only cite: bioherm, characterized by growth structures with a tendency to dome shape, surrounded by other lithologies, and biostroma, corresponding to geometry with a stratified tendency. The organisms that originate reefs are very diverse and have had varied importance throughout the stratigraphic column, highlighting corals, calcareous algae, stromatoporids, rudistas, oysters, bryozoans, and even some carbonate-secreting worms, since the necessary ecological potential to give a construction, it is a value relative to the energy of the medium capable of destroying the construction. Due to their geometry and facies relationships, marginal reefs are usually distinguished, attached to the coast and with a linear trend; the atolls, with a circular geometry enclosing a protected lagoon inside; and the barrier reef, with a linear tendency, but which, due to its protective role, creates a lagoon in its rear area, and is the most widespread form. Lateral changes between different types are frequent. A reef gives rise to three fundamental types of facies:

• (a). The construction facies formed by the calcareous skeletons of the organisms growing interconnected and giving a very porous structure that is filled with detritus originated by the partial destruction of the skeletons and calcareous mud of diverse origin. Coral algae, or laminar stromatoporides, often act as cement or construction ligants.
• (b). The tense, elastic front facies, which passes laterally to the platform marine sediments. If the growth of the reef is very fast can appear slides and structures that remember the means of turbidites, being the main components large broken fragments of the reef embedded in bio-clinical sediments of fine sizes.
• (c). Post-arreciferous phasesback-reef), characterized by a very weak energy environment, by the mechanical protection of the construction that individualizes a lake sometimes without defined limits, characterized by bioclinical sands and calcareous fangs with fecal pellets that indicate strong biological activity. In cases of rapid-growth reefs, clastic facies similar to those of the tense front can be developed, but smaller. Within the lake, independent tense-buildings of the main reef can be developed.

Slope and continental edge sedimentation and deep sedimentation

As previously noted, at the foot of the continental slope are accumulated the materials deposited in the outer part of the continental shelf and that have slipped down the slope. The sedimentation in this area will be predominantly clayey without intercalations of olistostromic levels. If there is a mouth of a submarine canyon, it will build its typical depositional fan, formed by predominantly turbiditic series. Both the continental edge materials, as well as those of submarine fans, pass laterally to the deeper sediments. These are formed by thin layers of material transported by turbidity currents and by autochthonous sediment, constituted, to a large extent, by pelagic marls in which the shells of foraminifera abound. In the areas where detrital material does not reach, very fine materials are deposited that are in suspension in the waters and shells of pelagic foraminifera, or else, the accumulation of radiolarian shells, originating, in this case, a siliceous rock (radiolarite).