Chapter 3 Sedimentation of clay minerals

Transcription

1 Chapter 3 Sedimentation of clay minerals 3.1 Clay sedimentation on land 3.2 From land to sea 3.3 Clay sedimentation in the sea Clay sedimentation on land Deserts Glaciers Rivers Lacustrine 2 University 1

2 3 Clay mineralogy of sediments (large circles) and air-borne dusts (small circles) in the northwestern Africa. 4 University 2

3 Glaciers Glacier ice often incorporates and preserves colluvial and aeolian particles. Glacierderived clays are basically detrital. The weathering under glacial conditions is very weak; transportation by running water does not allow identifiable mineralogical changes, especially if temperature is low. Paleoenvironmental applications: stratigraphic correlation, glaciation history, source analysis, and climate. 5 Rivers There is a good correspondence between the clay composition of fluvial deposits and of the formations submitted to erosion. Essentially, the character of fluvial clay suites is detrital, and allows precising the origins. The mineral distribution in suspended matters depends on the average size of different species submitted to hydrodynamical forces. 6 University 3

4 Clay minerals constitute by far the major component of all river suspensions, and chiefly reflect the composition of soils in the drainage basins. 7 Environmental applications: reflecting geological formations and recent river alluvium, physical and/or chemical weathering. 8 University 4

5 Environmental applications: minimizing the contribution of soils, and therefore the importance of climatic control on river sedimentation. 9 Environmental applications: The clay associations change strongly and irregularly form the source area downstream, and reflect the geologic formations successively eroded and drained. 10 University 5

6 Lacustrine Most freshwater and saline lakes contain clay suites that result exclusively form the erosion of drainage basins. Saline lakes and evaporative processes do not automatically favor clay authigenesis. Clay genesis is reported in a few recent lakes and corresponds to specific conditions, namely to highly ferriferous, magnesian or siliceous environments. The lacustrine formation of fibrous clays and associate Mgsmectite does not obviously occur in Quaternary time, but was extensive in several geologic periods marked by a hot climate, strong hydrolysis and evaporation, and high carbonate and silica accumulation. 11 Detrital supply: A general correspondence exists between the mineral composition of most freshwater lakes and the average clay mineralogy of rocks and soils in the surrounding drainage basins. 12 University 6

7 Authigenic clay in saline lakes: Surficial sediments of Lake Chad offer a succession of mineralogic zones, characterized by different types of smectites, from the slightly acid fluvial environment to the alkaline northernmost part of the lake. 13 Volcanic environment: Illite is considered to be the result of smectite transformation in a closed basin submitted to both saline and alkaline conditions, during a stage of very low level due to a dry climate. Smectite is thought to be illitized by the incorporation of potassium released by evolution of the K/Nazeolites to Na-zeolites. 14 University 7

8 3.2 From land to sea Estuary and delta Clay sorting and settling Aeolian input 15 Estuary and delta Sedimentary particles transported from the continent to the sea experience changes in salinity, ph, and other physicochemical characters. Various data indicate the existence of a general correspondence between the composition of estuarine and deltaic clay assemblages and the mineral composition of rocks and soils in the hinterland. The geochemical break occurring between land and sea was not strong enough to permit significant crystallochemical changes of clay minerals. 16 University 8

9 Clays in estuaries: Smectite-rich assemblages exist in both river and open sea sediments, which was first attributed to in-situ transformations and later to a differential deposition process. 17 Differential settling and flocculation: Differential settling depends on various parameters, like water density, temperature, turbulence, ph, organic matter content, dissolved ions, etc. 1.3 m/day for smectite, 11.8 m/day for kaolinite, 15.8 m/day for illite. Smectite statistically appears to settle less rapidly than other clay minerals. 18 University 9

10 19 Progressive dilution of land-derived materials in marine waters. 20 University 10

11 Estuarine mineral assemblages result simply from the mixing of river and marine sources. 21 Source mixing appears to be the best understood mechanism for explaining the distribution of clay and associate mineral suites in estuaries. Flocculation and differential settling processes are still imperfectly understood, because of the diversity and complexity of factors involved and because of the difficulty in applying experiments to natural situations. 22 University 11

12 Clay sorting and settling The scarcity of the clay fraction in shallow water sediments, due to winnowing through the action of waves, tide, and currents, contrasts with the huge accumulation of argillaceous deposits in the ocean. The differential settling of smectite: late flocculation of smectite relatively to kaolinite University 12

13 Mechanisms: chemical transformation; differential flocculation; physical size segregation. 25 Particle aggregation and advection: Suspended minerals are convected with the rapid sinking of organic matter in the water column. 26 University 13

14 Smectite is suggested to be supplied laterally at mid-water depths, not through surface current or bottom nepheloid layers. 27 Clay mineral segregation by differential settling represents a common phenomenon on certain continental margins characterized by a simple influx from river to ocean. The mechanisms responsible for clay changes recorded an continental margins appear to be dominated by grain size sorting. The aggregation of clay and other lithogenic particles by marine amorphous organic matter appears to represent a widespread phenomenon, responsible for the rapid vertical sinking of land-derived particles. 28 University 14

15 Aeolian input Various methods are used to identify and to quantify aeolian transport over the world ocean. Geochemical analysis of aerosols often provides a good estimation of the abundance of dust constituents. 29 The major sources of dust emissions are situated in the subtropical desert belt extending from West Africa to Central Asia. The Sahara is probably the world s most important source of dust. 30 University 15

16 31 32 University 16

17 The most reliable and convincing information on the importance of aeolian input within the oceans arises from the direct comparison of minerals contained in wind-borne dust and in underlying marine sediments. 33 Distribution of aeolian sediments 34 University 17

18 35 36 University 18

19 Clay sedimentation in the sea Surface distribution on seafloor Transport of clay minerals 38 University 19

20 Kaolinite: low-latitude minerals Resulted from strong hydrolysis, presenting warm and humid climate conditions. Windom (1976) 39 Chlorite: high-latitude minerals Resulted from weak hydrolysis, presenting cold and dry climate conditions. Windom (1976) 40 University 20

21 Illite Resulted from weak hydrolysis, presenting cold climate or direct erosion of rocks Windom (1976) 41 Smectite Resulted from warm/humid environments, related to weathering of volcanic rocks Windom (1976) 42 University 21

22 43 Polar regions: As cold land masses are almost exclusively submitted to physical weathering, all mineral species contained in outcropping rocks can theoretically be removed by erosion and transported within the polar oceans. 44 University 22

23 Volcanic sources: Volcanic rocks weather preferentially into smectite, regardless of climate conditions if there is sufficient water to allow hydrolytic processes. 45 Mixed geologic and pedologic sources: Clay detrital assemblages reflect the combined influences of land petrography and continental climate. This is systematically the case in temperate regions, where chemical weathering is weak enough to allow the recognition of geologic substrates, and strong enough to permit the mineralogic expression of pedologic processes. 46 University 23

24 47 Offshore hydrodynamic: The transportation by nearshore surface or density currents can modify the original clay zonation induced by climate. 48 University 24

25 Combination of terrigenous supply and transport agents: Marine current, submarine volcanism, land geology and climate University 25

26 51 Terrigenous supply represents the dominant agent responsible for the constitution of clay suites in most recent sediments of the world ocean. Illite, chlorite, associated quartz, feldspars and various dense minerals, commonly called primary minerals, have long been considered typically terrigenous species. The basic zonation of terrigenous clays in the ocean is controlled by the climate. The average petrography of source regions becomes essential as soon as chemical weathering is very low. The transportation by nearshore surface or density currents also modify the original clay zonation induced by climate. 52 University 26

27 Homeworks-1 53 Homeworks-2 University 27