Deep Water Clastic Systems

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Introduction

Processes and facies in deep-water sedimentary systems. (A) Downslope, Modified from [1]; (B) Along slope and vertical settling [2]
Processes and facies in deep-water sedimentary systems. (A) Downslope, Modified from [3]; (B) Along slope and vertical settling [2]

A deep water clastic system involves sediment erosion, transport, and deposition on the seafloor. Prior to modern techniques and studies, the previous assumption was that sediment processes in deep water settings were predominantly pelagic in nature [4]. Pelagic sediment consists of fine-grained sediment that accumulates on the seafloor from settling processes. Some other processes include gravity-driven and current-driven processes [2]. Three distinct sediment facies exist in deep water systems: turbidites, contourites, and hemopelagites. Though these facies are distinct, there arises issues with each process and there is a broad spectrum of processes and deposits [2]. Figure 1 illustrates all the subtle differences in downslope and alongslope processes and facies within a deep water system.

What is the importance of Deep Water Clastic Systems?

Deep water systems are often used for oil and gas exploration with one example coming from NW Europe which was created by a series of sand rich turbidites which acted as reservoirs. These deep water systems can produce important hydrocarbon play fairways which include notable areas such as the North Sea and Atlantic margins [5]. The ideal source rock within these plays is produced by low rates of fine-grained sediment deposition over long periods of time. Oftentimes, there are deep-water sandstones which become interbedded within the source rock which act as reservoirs. Seals consist of thick beds of fine-grained sediment which are formed from sea level rise. These seals are produced similar to the source, however they do not become mature enough to become a viable source rock. [5].

Geomorphology & Modern Analog

The geomorphology of a deep water clastic system is best visualized as a terrestrial to basin floor model. There is the terrestrial environment which grades into a shallow marine environment throughout the continental slope and further into deep water settings. The overall driving forces or gravity which is why there are many slope driven deposits in a deep-water system. Deep channels and gorges can exist within a deep-water system which can be remnants of when ancient rivers used to flow all the way out into the deep ocean. Ridges can also be present especially in a tectonically active area. Some modern examples of deep water clastic systems can be seen off the coast of California which represent deep channels and gorges. These modern examples are heavily dominated by deep water turbidites.

Depositional Processes and Depositional Facies

Schematic view (partial 3D) of a turbidity current, identifying the head, body and tail regions. High-concentration flows commonly develop a distinctive dense basal layer towards the flow front [2]

Turbidity Currents

Turbidity currents are powerful ways to transport large amounts of sediment from the continental shelf to the deep sea and can include fine, medium, and coarse-grained material. These types of currents are usually high density and play a very important role in burying future oil and gas source rock due to rapid mass sediment deposition [6]. Turbidity currents form when there are slides or slumps of material on the continental slope and as they evolve downstream, they become debris flows which in turn become turbidity currents which can be seen in Figure 2. Figure 2 represents a high-concentration turbidity current which differs from a low-concentration turbidity current. In a low-concentration turbidity current the distal portion carries the finest material which ultimately becomes upwardly mixed. The upward mixing of the finest sediment allows for a slow settling of suspended sediment transported with the current [2]. Turbidity currents can occur as short-lived surge events which travel a few kilometers downslope, or they can occur over longer periods of time and are usually more uniform, traveling significantly farther than short-lived events [7].

Schematic view (3D) of a bottom (contour) current, identifying the current core, eddies and strands within a deep-water mass. Typical physical parameters as shown [2]

Bottom Currents

In a deep water setting, there are bottom currents which are analogous to terrestrial rivers as seen in Figure 3. These features are less volatile than the turbidity currents explained above and are more of a semi-permanent feature that exist over geologic time [2]. Bottom currents generate deposits that are associated with wind-driven bottom currents, tidal forces, and deep thermohaline circulation (Juan et al., 2018). Thermohaline circulation is best understood as what moves the deep water currents which are the fluxes of heat and freshwater across the surface of the sea. These changes in water temperature and salinity produce deep currents which increase and decrease in intensity as heat and salinity change [8]. Both wind-driven bottom currents and thermohaline currents behave somewhat similarly in terms of how they erode the seafloor and affect contourite deposition. After the sediment is eroded from the source and introduced to the ocean, the sediment and current proceeds to follow the downslope until they arrive at a specific density interval that matches their salinity-temperature property. At this interval, the sediment and currents no longer flow downslope and instead begin following the gradient alongslope, beginning a contourite [2]. Both thermohaline and wind-driven bottom currents have different velocities which change with the differences in slope gradient and topographic irregularities. Bottom currents can be wide and slow over low gradient slopes and ocean basins, intermediate and constricted over slopes that are steeper with the introduction of topographic irregularities, and finally the highest velocity is found at the steepest gradient which produces narrow channels [2]. Deep-water tidal currents are continuous and can alter the velocity of bottom currents based on what angle they meet contourites. When tidal currents run parallel to contourites, the velocity at which the bottom-current travels can be severely increased or decreased based on the intensity of the tidal forces [2].

Schematic view (3D) of the hemipelagic and pelagic processes, identifying sediment supply from terrigenous and biological sources as well as its dispersion and settling through the water column. Typical physical parameters as shown [2].

Pelagic and Hemipelagic Settling

Pelagic settling involves gravity driven forces in which biogenic material and very-fine grained sediment fall slowly to the sea floor (Figure 4). The factors that drive the rate of sediment accumulation are flocculation amount and organic material. Sediment accumulation is typically continuous and slow in these settings and periods of high zooplankton presence can cause higher periods of accumulation. The blooming of producers can produce a large amount of fecal pellets which eventually settle to the sea floor [2]. Hemipelagic settling occurs closer to the shelf than pelagic settling and involves both vertical settling and slow lateral advection [2]). With this settling occurring so clause to the shelf, the driving factors that affect the lateral advection involve rivers, glacial melting, diffusion, internal tides, and waves. Hemipelgic settling that occurs around 1000-2000 m of water is responsible for allowing large quantities of organic carbon to accumulate which ultimately produce black shales which are important for oil/gas generation.

Turbidites

Turbidites are created by all sediments deposited by turbidity currents. They typically have well defined bedding and have distinct bases but as you move toward the top of a turbidite, there is usually normally graded bedding present. Structures can range from parallel and cross-lamination bedding to large-scale cross-bedding or dunes. Structureless beds are also common especially in thicker beds and at the base of some turbidites there can be scours. Bioturbation can occur in turbidites but only if turbidites have an extended period of geological time between each deposition. The alteration occurs near the tops of the structures and certain turbidite settings contain certain ichnofacies [2].Some examples are that in thick-bedded sandstone sequences, there tends to be Ophiomorpha and Thalassinoides ichnofacies found [9]. Grain sizes and sorting within turbidites can range from silt/clay to coarse grained with fine and medium grained turbidites containing the better sorting. The composition of a turbidite is usually siliciclastic but can be bioclastic or volcaniclastic. The overall characterization of the composition is based on what material moves from shallow water or continental source which can increase the variability [2].

Contourites

Bottom currents are the main ways that contourites are developed and since currents are the driving factors, they can greatly alter how contourites are preserved in the rock record. In areas where there are high velocity currents, the bedding is well preserved due to increased sand and silt and interbedded mudstone. In low velocity currents, the bedding is poorly preserved and thick mud rich successions are observed [2]. Structures within contourites are absent in mud rich systems but there can be some cross-lamination seen in sand/silt rich systems. Bioturbation is pervasive in contourites and can be seen in all beds. Based on the strength of the currents, there can be different ichnofacies that can be found. Glossifungites, for example, are present where long periods of non-deposition are overlain by sand [10]. Contourites can vary in composition but most commonly are displayed as mixed terrigenous-biogenic composition.

Hemipelagites

With sediment settling being the driving force for hemipelagites, the primary grain size of sediment is fine-grained. Muds and sandy muds are pervasive and the expanse of these deposits are vast. Hemipelagites don’t have very distinct beds but are instead defined by alternating layers of color which directly reflect the presence or absence of carbonate material. Light colored bands indicate more carbonate/biogenic-rich sediment while darker bands indicate more carbon-rich material [2]. Since there are no currents driving deposition, there are limited structures present in hemipelagites. One of the only structures seen is parallel lamination; however, they can only occur during periods of high sedimentation and low oxygen rates. Bioturbation is wide-spread in these deposits with Zoophycos and Nereites being some examples [11].

Controls on Depositional System Evolution

Turbidites

Tectonics play an important role in the formation of turbidites. Areas where there is an active tectonic margin such as the Cascadia Subduction Zone, there can be varying turbidite systems that are aided by tectonically controlled deep-sea channels [12]. One example of volcanic influence is that of the eruption of Mount Mazama in the southern cascades. This eruption produced large amounts of volcanic debris and caused much of the forest around the area to become a large woody debris flow. All of this material followed river drainages to the ocean and eventually aided in the formation of turbidites which were characterized by their woody beds [12]. Turbidites are also affected by sea level change. Periods of low sea level produced thick sand bedded turbidites while periods of high sea level coupled with warmer climate produced thin sand bedded turbidites [12].

Contourites

Contourites are easily most affected by weather patterns. Large storms can produce sediment that ultimately gets caught in the suspended load of a contourtie. Once the storm subsides and the sediment can settle, the sediment accumulates in the bedload of the controurite. Storms can also produce strong currents which can increase the velocity of a contourite [2]. This is also similar to how sea level changes affect contourites. Tectonic settings which create deep troughs or ridges that create a deep water levee system have the most effect on contourites because they create a deep water channel that is more stable [13].

Hemipelagites

Since hemipelagites are composed of both biogenic and terrigenous inputs, seasonality is an important factor in their formation. During times of increased sun exposure, there are periods where blooming of primary producers can increase the amount of biogenic material being deposited. The same principles can be applied to the terrigenous material. In times of increased physical erosion produced by either wind or rivers, the river plumes or wind can facilitate the transportation of sediment to the deep ocean. Sea level change can affect the development of hemipelagites especially if sea level drops substantially. With decreased water depth, there can be increased energy that can reach the sea floor which would reduce the sediment settling [2].

Facies Models

The medium-grained turbidite family for sand and sand-mud turbidites. The ideal Bouma facies models showing the complete sequence of divisions A-E and typical partial sequences found commonly in nature, is given. F is now commonly used for pelagites above a turbidite [2]
The fine-grained turbidite family for silt and mud turbidites. The ideal Stow facies model showing the complete sequence of divisions T0-T8, and typical partial sequences found commonly in nature, is given [2]
Medium-grained turbidite family: photographic examples. (A) Sandstone turbidite succession (thin-, medium- and thick-bedded) interbedded with fine-grained turbidites (mud-rich), Annot Basin, SE France. (B) Sandstone–mudstone turbidite succession, thin- and medium-bedded, Zumaia Formation, N Spain. Scale stick 1.5 m. (C) Sandstone–mudstone turbidite succession, thin-bedded, Aberystwyth Grit Formation, Wales. (D) Sandstone–mudstone turbidite succession, thin- and medium-bedded, Apennines, Italy. (E) Example from Aberystwyth Grit Formation, Wales, showing Bouma sequence A–E turbidite. (F) Example from Misaki Formation, Japan, showing Bouma sequence A–E turbidite [2]

Turbidites

Turbidites are divided into three main categories: coarse-grained (Lowe), medium-grained (Bouma), and fine-grained (Stow). Lowe turbidites are caused by high density currents which consist of the base being turbulent flow and the top being a more dilute layer of suspended sediment. Between the two zones, there can be a gradual or sharp boundary. The general facies model is gravel sized material at the base of the turbidite with evidence of scouring present. Above this base layer are usually alternating coarse-grained layers that show periods of traction and suspension. Ultimately the top of the turbidite is derived from settling material and is usually composed of smaller grain sizes. Bouma turbidites (Figure 5) are caused by low density turbidity currents which consist of majority suspended particles in turbulence. There is a small layer of sediment acting as traction at the base, but the majority of the body is sediment in suspension. Bouma turbidites have massive graded bedding present at the base with sand size particles which are poorly sorted. As you move up the turbidite the structures evolve from planar laminae to ripples/contorted laminae and back to planar laminae due to changing flow regime. Mud sized particles are present at the top of the turbidites with increased bioturbation present. The Bouma model represents an overall fining upward sequence [2]. Stow turbidites are relatively low energy depositional systems. They tend to have smaller grain sizes and have alternating layers of sand sized lenses within mud/clay sized beds (Figure 6). There can be fading ripples present at the base of Stow turbidites and convolute laminations as well. Bioturbation is much more present in Stow turbidites than the other variations. Some real life examples of turbidites are present in Figure 7.

The bi-gradational contourite family for fine-grained mud-sand contourites. The ideal bi-gradational facies model showing the complete sequence of divisions C1-C5 and typical partial sequences found commonly in nature is given [2]
The sandy contourite family for muddy sands, fine-medium sands and medium-to-coarse sand [2]
Contourite family: photographic examples of ancient contourites. (A) Miocene siliciclastic contourite succession, Rifean Gateway, Morocco. The view shows large-scale alternation of more mud-rich (grey) and more sand-rich (brown) contourites, as well as small-scale contourite cyclicity. (B) Detail from (a) above of sandy contourite, poorly sorted and structureless. (C) Detail from (a) above of muddy/silty contourite, poorly sorted and structureless. Note that surface mud wash has been partly scraped away to show a mud (grey) to silt (brown) sequence. (D,E) Sandy contourite succession, siliciclastic, Rifean Gateway, Morocco. The view shows large-scale cross-stratification and parallel stratification. It is interpreted as deep tidal bottom currents enhancing flow speed of thermohaline bottom currents [2]

Contourites

There are two major contourite facies: bi-gradational and sandy. In a bi-gradational contourite facies, there is a symmetrical shape that reflects similar lithology as seen in Figure 8. At the base and top there can be bioturbated mud layers which are immediately followed by mottled silt and mud which is also bioturbated. At the center of this facies, there is the most sand rich sediment which is also bioturbated [2]. In a sandy contourite facies, there is also mud at the base and top of the model with the sand being isolated in the middle (Figure 9). The sand in the middle (if muddy-sands) can be bioturbated with some structures or no structures at all. If the sand in the middle is more fine-medium sand, there can be sparse bioturbation with indistinct laminae or cross-laminations occurring within. If the sand in the middle is more medium-coarse, there is limited bioturbation and more distinct structures such as cross-bedding or parallel laminations [2]. Figure 10 shows real life examples of contourites.

Hemipelagite facies models. The standard model shows simple compositional cyclicity between more clay-rich and more biogenic-rich parts. Variations depend on the input of different components [2]
(C) Bioturbated hemipelagites–pelagites (whitish) interbedded with graded mud turbidites (dark brown), Plio-Pleistocene, DSDP Site 530, SE Angola Basin, S Atlantic. (D) Detail from succession as above, hemipelagite over turbidite with intense bioturbation. (E) Pelagite (micritic limestone), Eocene, Petra tou Romiou, southern Cyprus. Some evidence for interbedding with fine calcareous contourites, i.e., small bi-gradational sequence from calcilutite to calcisiltite and back to calcilutite (marked with a black line). (F) Pelagite, white micritic limestone (chalk) and black chert nodule (from siliceous pelagite), Durdle Door, Dorset, southern England. (G) Hemipelagite (pale) interbedded and inter-bioturbated with volcaniclastic ash layers (dark). Some evidence of possible transport by bottom currents. Miocene Misaki Formation, Miura, Japan. (H) Pelagites: interbedded limestone (white) and organic-rich chert (black) beds, Cretaceous, central Umbria, Italy [2]

Hemipelagites

Hemipelagites are extremely wide-spread over a deep-water system and are almost always fine-grained. With the grain size being more uniform than turbidites or contourites, the facies model is less complicated. The facies model for hemipelagites show indistinct bedding which is defined by compositional differences. The alteration colors of light and dark illustrate changes in biogenic and terrestrial sediment input. Hemipelagite facies lack depositional structures but there can be a zonation of different fossils in the presence of bioturbation [2]. An overall facie model can be seen in Figure 11. Real life examples can be seen in Figure 12.

References

[14]
[15]

[16]

  1. Stow, Dorrik AV. "Fine-grained sediments in deep water: An overview of processes and facies models." Geo-Marine Letters 5.1 (1985): 17-23.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 2.24 2.25 2.26 2.27 2.28 2.29 2.30 2.31 Stow, Dorrik, and Zeinab Smillie. "Distinguishing between deep-water sediment facies: Turbidites, contourites and hemipelagites." Geosciences 10.2 (2020): 68.
  3. Stow, Dorrik AV. "Fine-grained sediments in deep water: An overview of processes and facies models." Geo-Marine Letters 5.1 (1985): 17-23.
  4. Jenkyns, Hugh C., and Christopher J. Clayton. "Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic." Sedimentology 33.1 (1986): 87-106
  5. 5.0 5.1 Hurst, A., et al. "Deep-water clastic reservoirs: A leading global play in terms of reserve replacement and technological challenges." Geological Society, London, Petroleum Geology Conference series. Vol. 6. No. 1. Geological Society of London, 2005.
  6. Heerema, Catharina J., et al. "What determines the downstream evolution of turbidity currents?." Earth and Planetary Science Letters 532 (2020): 116023.
  7. Middleton, Gerard V. "Sediment deposition from turbidity currents." Annual review of earth and planetary sciences 21.1 (1993): 89-114.
  8. Rahmstorf, Stefan. "Thermohaline circulation: The current climate." Nature 421.6924 (2003): 699-699.
  9. Uchman, Alfred, and Andreas Wetzel. "Deep-sea ichnology: the relationships between depositional environment and endobenthic organisms." Developments in Sedimentology. Vol. 63. Elsevier, 2011. 517-556.
  10. Wetzel, A., F. Werner, and D. A. V. Stow. "Bioturbation and biogenic sedimentary structures in contourites." Developments in sedimentology 60 (2008): 183-202.
  11. Uchman, Alfred, and Andreas Wetzel. "Deep-sea ichnology: the relationships between depositional environment and endobenthic organisms." Developments in Sedimentology. Vol. 63. Elsevier, 2011. 517-556.
  12. 12.0 12.1 12.2 Nelson, C. HANS, et al. "External controls on modern clastic turbidite systems: three case studies." External Controls on Deep-Water Depositional Systems: SEPM, Special Publication 92 (2009): 57-76.
  13. Owen, Matthew J., et al. "Control of sedimentation by active tectonics, glaciation and contourite-depositing currents in Endurance Basin, South Georgia." Global and Planetary Change 123 (2014): 323-343.
  14. Whaley, J., 2017, Oil in the Heart of South America, https://www.geoexpro.com/articles/2017/10/oil-in-the-heart-of-south-america], accessed November 15, 2021.
  15. Wiens, F., 1995, Phanerozoic Tectonics and Sedimentation of The Chaco Basin, Paraguay. Its Hydrocarbon Potential: Geoconsultores, 2-27, accessed November 15, 2021; https://www.researchgate.net/publication/281348744_Phanerozoic_tectonics_and_sedimentation_in_the_Chaco_Basin_of_Paraguay_with_comments_on_hydrocarbon_potential
  16. Alfredo, Carlos, and Clebsch Kuhn. “The Geological Evolution of the Paraguayan Chaco.” TTU DSpace Home. Texas Tech University, August 1, 1991. https://ttu-ir.tdl.org/handle/2346/9214?show=full.