Showing posts with label basin. Show all posts

7 Jun 2020

June 07, 2020 Comment

Sedimentary Basins

The sedimentary veneer on the Earth?S surface varies substantially in thickness. If you stand in central Siberia or south-vital Canada, you'll ?Nd your self on igneous and metamorphic basement rocks which can be over one billion years vintage sedimentary rocks are nowhere in sight. Yet in case you stand alongside the southern coast of Texas, you'll ought to drill thru over 15 km of sedimentary beds before achieving igneous and metamorphic basement. Thick accumulations of sediment form best in special regions wherein the surface of the Earth?S lithosphere sinks, presenting area wherein sediment collects. Geologists use the term subsidence to consult the procedure through which the floor of the lithosphere sinks, and the term sedimentary basin for the sediment-?Lled melancholy. In what geologic settings do sedimentary basins shape? An information of plate tectonics idea gives the solutions.

Categories of Basins in the Context of Plate Tectonics Theory

The geologic setting of sedimentary basins.

Geologists distinguish amongst exceptional types of sedimentary basins in the context of plate tectonics idea. Let?S keep in mind some examples (determine above).

Rift basins: These form in continental rifts, regions where the lithosphere is stretching horizontally, and therefore thins vertically. As the rift grows, slip on faults drops blocks of crust down, producing low areas bordered by narrow mountain ridges. These troughs fill with sediment.
Passive-margin basins: These form along the edges of continents that are not plate boundaries. They are underlain by stretched lithosphere, the remnants of a rift whose evolution successfully led to the formation of a mid-ocean ridge and subsequent growth of a new ocean basin. Passive-margin basins form because subsidence of stretched lithosphere continues long after rifting ceases. They fill with sediment carried to the sea by rivers and with carbonate rocks formed in coastal reefs.
Intracontinental basins: These develop in the interiors of continents, initially because of subsidence over a rift. They may continue to subside in pulses even hundreds of millions of years after they formed, for reasons that are not well understood.
Foreland basins: These form on the continent side of a mountain belt because the forces produced during convergence or collision push large slices of rock up faults and onto the surface of the continent. The weight of these slices pushes down on the surface of the lithosphere, producing a wedge-shaped depression adjacent to the mountain range that fills with sediment eroded from the range. Fluvial and deltaic strata accumulate in foreland basins.

Transgression and Regression

Sea-level changes, relative to the land floor, manage the succession of sediments that we see in a sedimentary basin. At times throughout Earth history, sea degree has risen with the aid of as plenty as a couple of hundred meters, creating shallow seas that submerge the interiors of continents. At different times, sea stage has fallen through more than one hundred meters, exposing the continental cabinets to air. Global sea-stage adjustments can be because of various of things, such as weather change, which controls the amount of ice stored in polar ice caps and reasons modifications inside the quantity of ocean basins. Sea level at a location can also be due to the local uplift or sinking of the land surface.

The concept of transgression and regression, throughout deposition of sedimentary collection.

When relative sea level rises, the coastline migrates inland. We name this process transgression. When relative sea level falls, the coast migrates seaward. We name this system regression (discern above). The process of transgression and regression results in the formation of broad blankets of sediment.

Diagenesis

Earlier we discussed the procedure of lithi?Cation, by means of which sediment hardens into rock. Lithi?Cation is an issue of a broader phenomenon called diagenesis. Geologists use the time period diagenesis for all of the bodily, chemical, and biological techniques that remodel sediment into sedimentary rock and that alter traits of sedimentary rock after the rock has formed.

In sedimentary basins, sedimentary rocks may additionally come to be very deeply buried. As a result, the rocks bear higher pressures and temperatures and are available in contact with heat groundwater. Diagenesis, below such situations, can cause chemical reactions in the rock that produce new minerals and also can purpose cement to dissolve or precipitate.

As temperature and pressure boom nevertheless deeper in the subsurface, the adjustments that take location in rocks end up greater profound. At suf?Ciently excessive temperature and strain, metamorphism starts, in that a new assemblage of minerals forms, and/or mineral grains emerge as aligned parallel to each different. The transition between diagenesis and metamorphism in sedimentary rocks is gradational and takes place among temperatures of 150C and 300C. In the subsequent bankruptcy, we input the area of real metamorphism.

Credits: Stephen Marshak (Essentials of Geology)

11 May 2020

May 11, 2020 Comment

Banded-iron formations (BIFs) - Evidence of Oxygen in Early Atmosphere

Our knowledge about the rise of oxygen gas in Earth’s atmosphere comes from multiple lines of evidence in the rock record, including the age and distribution of banded iron formations, the presence of microfossils in oceanic rocks, and the isotopes of sulfur.

However, this article is just focus on Banded Iron Formation.

BIF (polished) from Hamersley Iron Formation, West Australia, Australia

Summary:Banded-iron formations (BIFs) are sedimentary mineral deposits consisting of alternating beds of iron-rich minerals (mostly hematite) and silica-rich layers (chert or quartz) formed about 3.0 to 1.8 billion years ago. Theory suggests BIFs are associated with the capture of oxygen released by photosynthetic processes by iron dissolved in ancient ocean water. Once nearly all the free iron was consumed in seawater, oxygen could gradually accumulate in the atmosphere, allowing an ozone layer to form. BIF deposits are extensive in many locations, occurring as deposits, hundreds to thousands of feet thick. During Precambrian time, BIF deposits probably extensively covered large parts of the global ocean basins. The BIFs we see today are only remnants of what were probably every extensive deposits. BIFs are the major source of the world's iron ore and are found preserved on all major continental shield regions.

Banded-iron formation (BIF) isconsists of layers of iron oxides (typically eithermagnetite orhematite) separated by layers ofchert (silica-rich sedimentary rock). Each layer is usually narrow (millimeters to few centimeters). The rock has a distinctively banded appearance because of differently colored lighter silica- and darker iron-rich layers. In some cases BIFs may containsiderite (carbonate iron-bearing mineral) or pyrite (sulfide) in place of iron oxides and instead of chert the rock may contain carbonaceous (rich in organic matter)shale.

It is a chemogenic sedimentary rock (material is believed to be chemically precipitated on the seafloor). Because of old age BIFs generally have been metamorphosed to a various degrees (especially older types), but the rock has largely retained its original appearance because its constituent minerals are fairly stable at higher temperatures and pressures. These rocks can be described as metasedimentary chemogenic rocks.

Jaspilite banded iron formation (Soudan Iron-Formation, Soudan, Minnesota, USA

Image Credits: James St. John

In the 1960s, Preston Cloud, a geology professor at the University of California, Santa Barbara, became interested in a particular kind of rock known as a Banded Iron Formation (or BIF). They provide an important source of iron for making automobiles, and provide evidence for the lack of oxygen gas on the early Earth.

Cloud realized that the widespread occurrence of BIFs meant that the conditions needed to form them must have been common on the ancient Earth, and not common after 1.8 billion years ago. Shale and chert often form in oceanenvironments today, wheresediments and silica-shelled microorganisms accumulate gradually on theseafloor and eventually turn into rock. But iron is less common in younger oceanicsedimentary rocks. This is partly because there are only a few sources of iron available to the ocean: isolated volcanic vents in the deep ocean and material weathered from continental rocks and carried to sea by rivers.

Banded iron-formation (10 cm), Northern Cape, South Africa.

Specimen and photograph: A. Fraser

Most importantly, it is difficult to transport iron very far from these sources today because when iron reacts with oxygengas, it becomesinsoluble (it cannot be dissolved in water) and forms asolid particle. Cloud understood that for large deposits of iron to exist all over the world’s oceans, the iron must have existed in a dissolved form. This way, it could be transported long distances in seawater from its sources to the locations where BIFs formed. This would be possible only if there were little or no oxygen gas in the atmosphere and ocean at the time the BIFs were being deposited. Cloud recognized that since BIFs could not form in the presence of oxygen, the end of BIF deposition probably marked the first occurrence of abundant oxygen gas on Earth (Cloud, 1968).

Cloud further reasoned that, for dissolved iron to finallyprecipitate and be deposited, the iron would have had to react with small amounts of oxygen near the deposits. Small amounts of oxygen could have been produced by the first photosynthetic bacteria living in the open ocean. When the dissolved iron encountered the oxygen produced by the photosynthesizing bacteria, the iron would have precipitated out of seawater in the form ofminerals that make up the iron-rich layers of BIFs: hematite (Fe2O3) and magnetite (Fe3O4), according to the following reactions:

4Fe3 + 2O2 → 2Fe2O3

6Fe2 + 4O2 → 2Fe3O4

The picture that emerged from Cloud’s studies of BIFs was that small amounts of oxygengas, produced by photosynthesis, allowed BIFs to begin forming more than 3 billion years ago. The abrupt disappearance of BIFs around 1.8 billion years ago probably marked the time when oxygen gas became too abundant to allow dissolved iron to be transported in the oceans.

Banded Iron Formation

Source is unknown

It is interesting to note that BIFs reappeared briefly in a few places around 700 millionyears ago,during a period of extremeglaciation when evidence suggests that Earth’s oceans were entirely covered with sea ice. This would have essentially prevented the oceans from interacting with theatmosphere, limiting the supply of oxygengas in the water and again allowing dissolved iron to be transported throughout the oceans. When the sea ice melted, the presence of oxygen would have again allowed the iron toprecipitate.

References:

1. Misra, K. (1999). Understanding Mineral DepositsSpringer.

2. Cloud, P. E. (1968). Atmospheric and hydrospheric evolution on the primitive Earth both secular accretion and biological and geochemical processes have affected Earth’s volatile envelope.Science, 160(3829), 729–736.

3. James,H.L. (1983). Distribution of banded iron-formation in space and time.Developments in Precambrian Geology, 6, 471–490.

4 May 2020

May 04, 2020 Comment

Basics of Basin Analysis

· A sedimentary basin is an area in which sediments have accumulated during a particular time period at a significantly greater rate and to a significantly greater thickness than surrounding areas.

· A low area on the Earth’s surface relative to surroundings e.g. deep ocean basin (5-10 km deep), intramontane basin (2-3 km a.s.l.)

· Basins may be small (kms2) or large (106+ km2)

· Basins may be simple or composite (sub-basins)

· Basins may change in size & shape due to:

1. erosion

2. sedimentation

3. tectonic activity

4. eustatic sea-level changes

· Basins may overlap each other in time

· Controls on Basin Formation

1. Accommodation Space,

a. Space available for the accumulation of sediment

b. T + E = S + W T=tectonic subsidence E= Eustatic sea level rise S=Rate of sedimentation W=increase in water depth

2. Source of Sediment

a. Topographic Controls

b. Climate/Vegetation Controls

c. Oceanographic Controls (Chemical/Biochemical Conditions)

· The evolution of sedimentary basins may include:

1. tectonic activity (initiation, termination)

2. magmatic activity

3. metamorphism

4. as well as sedimentation

· Axial elements of sedimentary basins:

1. Basin axis is the lowest point on the basement surface

2. Topographic axis is the lowest point on the depositional surface

3. Depocentre is the point of thickest sediment accumulation

· The driving mechanisms of subsidence are ultimately related to processes within the relatively rigid, cooled thermal boundary layer of the Earth known as the lithosphere. The lithosphere is composed of a number of tectonic plates that are in relative motion with one another. The relative motion produces deformation concentrated along plate boundaries which are of three basic types:

1. Divergent boundaries form where new oceanic lithosphere is formed and plates diverge. These occurat the mid-ocean ridges.

2. Convergent boundaries form where plates converge. One plate is usually subducted beneath theother at a convergent plate boundary. Convergent boundaries may be of different types, depending on the types of lithosphere involved. This result in a wide diversity of basin types formed at convergent boundaries.

3. Transform boundaries form where plates move laterally past one another. These can be complex andare associated with a variety of basin types.

· Many basins form at continental margins.

Using the plate tectonics paradigm, sedimentary basins have been classified principally in terms of the type of lithospheric substratum (continental, oceanic, transitional), the position with respect to a plate boundary (interplate, intraplate) and the type of plate margin (divergent, convergent, transform) closest to the basin.

· Plate Tectonic Setting for Basin Formation

1. Size and Shape of basin deposits, including the nature of the floor and flanks of the basin

2. Type of Sedimentary infill

· Rate of Subsidence/Infill

· Depositional Systems

· Provenance

· Texture/Mineralogy maturity of strata

3. Contemporaneous Structure and Syndepositional deformation

4. Heat Flow, Subsidence History and Diagenesis

· Interrelationship Between Tectonics - Paleoclimates - and Eustacy

1. Anorogenic Areas------>

· Climate and Eustacy Dominate