Tectonic Control of Sediments Prepared By ARSHAD AYUB

Introduction Sedimentation is controlled by three extrinsic variables: Tectonics, Climate and Sea level. Climate and sea level depends upon each other and they both depends upon tectonics. Tectonics affects sedimentation in two different but often related ways: 1) Uplift of the land 2) Subsidence of the crust

Introduction Subsidence may be formed by vertical upset of fault. Uplifting expose various rocks to weathering and erosion and is then transported. Subsidence provides the accommodation space for the accumulation and burial of these sediments to be deposited in the sedimentary basin. Although sediments may be deposited in tectonically in-active and topographically low areas of the crust.

Introduction Among these variables, which effect sedimentation, Tectonics has the most important control on sedimentation. The important effect of tectonics on sedimentation, wither direct or indirect include: 1. Nature of sediments 2. Rate of sediment supply 3. Rate of deposition

Introduction 4. Depositional environments 5. Nature of source rock 6. Nature of vertical succession Sedimentation also affect tectonics, although to a much lesser extent, mainly by increasing lithospheric load on the basin.

Why to study Sediments and Tectonics? We study sediments and sedimentary rocks in order to know the effect of extrinsic and intrinsic processes on the depositional history of a sedimentary basin. Sedimentary record in sedimentary basin tell us about paleotectonics in the best way. The deposition of sediment types, sediment thickness and paleocurrent in a basin gives us evidence of the existence and location of elevated areas of the crust created by tectonics.

Example Siwaliks of Pakistan: Study of the Siwaliks of Pakistan show us that they are formed from the sediments weathered and eroded from Himalayas. These Himalayas were uplifted by Tectonics since Miocene.

Tectonics and Erosion There are three processes that can increase rate of erosion at high elevation and provides abundant sediments to be deposited: 1) Active faulting that exposes fresh unweathered rocks 2) High altitude mechanical weathering due to steep slope, lack of vegetation and glacial weathering 3) Orographic rainfall on windward slope of mountain ranges

Tectonics and Erosion

Tectonics and Sedimentation As sedimentation depends upon Tectonics, so first we must know about Tectonics. Tectonics is the study of origin and arrangement of the structural features of the earth`s surface, including mountain belts, continents and earthquake belts. The concept of Plate Tectonics was born in the late 1960s by combining two pre- existing ideas; 1) Continental Drift and 2) Sea Floor Spreading. Basically Tectonics means that the earth surface is divided into few large and many small plates that move slowly and change in size.

Tectonics and Sedimentation Plate is a large mobile slab of a rock that is part of the earth`s surface. These plates move along plate boundaries and intense geologic activity occurs at these boundaries. These plates either move away from each others (Divergent Boundary), past one another (Transform Boundary) or towards one another (Convergent Boundary). More tectonic activities will lead to more sedimentation on the earth`s surface mostly along basins.

Divergent Margins If two plates are separating, they form a divergent margin. It is characterized by extensional features like seafloor spreading, normal faulting etc. In the first stage continental crust begin to rift apart and hot mantle plumes move upward. This upward movement produces domed uplifts that shed coarse, immature alluvial and fluvial deposits onto their flanks.

Divergent Margins In the second stage, uplift and extension breaks apart the crust, and a crustal block drops down to form a fault graben called a rift valley. This stage is occurring today in the East Africa Rift Valley. These rift valley form basins for the sediment that erodes from the up-thrown areas. Most of the sediments are coarse, immature alluvial debris and lesser fluvial deposits. In the center of the basin, small lakes may form lacustrine shales or limestone and even evaporates.

Convergent Margins If two plates are moving towards each other, they form a convergent margin. Such margins are characterized by folding and thrusting. Due to plate convergence several types of mountain belts can be produced. Convergence of the two plates may be: i. Continent-Continent convergence ii. Ocean-Continent convergence iii. Ocean-Ocean convergence

Continent-Continent Convergence In this case non of the plate subducts beneath the other, so they become uplifted and deformed and finally suture along the line of collision. The suture belt itself is an area of erosion. It sheds coarse clastic debris and fluvial deposits off its flank into adjoining plains. Himalayas is one of such sutures. The molasse sequence of the Siwalik hills of Pakistan and India are the product of the Himalaya uplift since the Miocene.

Himalayas Himalayas have been formed by the collision of Indian and Eurasian Plate. This collision began 50 million years ago and still continue today. The Himalayas are still rising by a rate of 1 cm per year as India continue to move Northwards into Asia. But at the same time weathering and erosion are lowering the Himalayas at about the same rate. Himalayas contain sedimentary rocks of marine origin that were originally deposited below sea.

Himalayas

Ocean-Continent Convergence In this type of convergence the oceanic plate subducts beneath the continental plate. Due to this type of convergence continental-margin arc forms. As this region is epicontinental and sub-aerial, so the sediments filling the basin are mostly immature alluvial and fluvial sandstones and shales. Most of the pelagic sediments of the oceanic plate is scraped onto the continental plate and accommodate along the arc to form accretionary wedge.

Sedimentation along Subduction Zone

Transform Margins If two plates are sliding past each others, neither separating nor converging they form a Transform margin. They are characterized by horizontal shear and strike-slip faulting. No such basins are associated with transform margins. Because of the irregularities in the sliding, local compression and extension occurs, forming two types of fault bounding troughs. 1) Transtensional or pull-apart basin 2) Transpressional basin

Transform Margins 1) Transtensional or pull-apart basin is formed by extensional gap. Salton Trough along the Southern Sand Andreas Fault is its common example. 2) Transpressional basin is formed by the compressional down warping. Mio- Pliocene Ridge Basin of California They accumulate thick pile of sediments. Thick, narrow and immature accumulation of sediments can only occurs in basins on transform margin.

Transform Margins Because of rapid rate of deposition and burial of organic materials, fault bounded basin are good places to explore for petroleum. Many of the petroleum occurrences in California are related to basins that are formed as a result of the Sand Andreas transform fault.

San Andreas Transform Fault

Tectonics and Type of sediments The windward side of a mountain receive more rainfall than the Lee side. Due to more rainfall on the windward side most of the sediments will lack feldspar because of its alteration to other minerals. So, in such case the rocks formed will have no or very less feldspar. Less rainfall leads to increased aridity on the Lee side. The sediments and sedimentary rocks so formed will have more feldspar.

Clues and Evidences There are various clues to show that sedimentation is strongly controlled by Tectonics. Siwaliks Fiambala Basin, Which includes: i. Punaschotter Formation ii. Tamberia Formation iii. Guanchin Formation

Siwaliks Sediments study of the Siwalik group show that the detritus or sediments deposited here, came from the weathering and erosion of the Himalayas.

Fiambala Basin Fiambala basin bounding ranges have been uplifted by the late Miocene (~7-6 Ma). The Punaschotter Formation, Tamberia Formation and Guanchin Formation of the Fiambala Basin record exhumation and erosion of the surrounding ranges. Regional tectonics were the most probable cause of deposition of these sediments/conglomerates. These conglomerates may record global climate changes, tectonic activity or both, and can serve as a proxy for tectonic activity worldwide.

Fiambala Basin

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