About This Item
- Full TextFull Text(subscription required)
- Pay-Per-View PurchasePay-Per-View
Purchase Options Explain
Share This Item
The sedimentary environment is investigated by study of the lake's origin, topography and hydrography of the region draining into the lake, chemistry of the lake waters and tributary streams, and plant and animal life in the lake. The lake is a colloidal suspension in which a concentrated electrolyte is the dispersing medium and clay the main constituent of the disperse phase. The electrolyte is a complicated solution of salts of different solubilities. Calcium and magnesium carbonate and a hydrous magnesium silicate have been precipitating out for a considerable time, and at low lake levels sodium chloride has crystallized on the bottom. Cold weather results in temporary sodium sulphate precipitation. Three types of permanent sediments are most common, namely: the clays, oolites, and calcareous algal deposits. All come from the same source materials but varying mechanical conditions or the activity of the lake plants and animals have caused different physical and chemical characteristics.
The clays are exceptionally smooth and plastic, which property is due chiefly to the base ions, particularly Na+, absorbed by the clay particles. The average origin of the plastic sediments is approximately as follows: clastic and colloidal material transported by lake currents, 20 per cent; clastic material (possibly some colloidal) transported by wind and deposited in water, 44 per cent; and material precipitated from solution, 36 per cent.
The identifiable minerals are the common detrital ones with quartz most common. They reflect the nature of the rocks of the nearest shore, but probably contain some admixture from more distant regions by wind transportation. The chemical precipitates are aragonite, CaCO3; dolomite, 2MgCO3·CaCO3; parasepiolite (?), 2MgO·3SiO2·4H2O; and absorbed bases, Ca, Mg, Na, and K. A fine white colloidal clay complex is believed to be made up chiefly of colloidally transported montmorillonite (?); chemically precipitated parasepiolite (?); colloidal size particles of quartz, feldspar and other detrital minerals; and absorbed ions of Ca, Mg, Na, and K.
The average moisture content of the clays is 39 per cent on a total weight basis. Compaction and dehydration appear to be occurring even at shallow depths. A crude hexagonal fissure system is an interesting consequence at two places, as are also the effects of a recent earthquake in liberating cold salt water in fissures around an almost buried bedrock hill. The organic content of the clays averages about 1 per cent. Bacteria are very active and emit sulphurous fumes. The diagenic effect of the bacteria on the carbonate and organic content could not be determined. Careful bacteriologic work is necessary. The lake waters are reductive to iron compounds and prevent the appearance of red coloring in the sediments.
The oolites are found only along the shores exposed to vigorous wave activity. The
oolitic sand is everywhere rippled. The oolites consist of nucleus (mineral particles and faecal pellets of the brine shrimp) and concentric layers (about 84 per cent CaCO3, 5.5 per cent 2MgCO3·CaCO3 and 5.6 per cent of very fine clay). The oolites grow by accretion from solution of carbonate molecules which build acicular, submicroscopic crystals of aragonite with some dolomite crystals interspersed, all probably oriented normal to the surface of attachment. The Ca++ and Mg++ ions of the carbonate crystals attract and precipitate clay particles less than 350 microns in diameter from suspension by base exchange. The clay becomes entrapped along the sides of the growing fibrous crystals. A small amount of organic matter become occluded in the same way. Constant agitation by the waves in the troughs of ripple marks exposes all parts of the oolite surface equally to accretion of the carbonate and clay and removes any excess clay that might settle. Radial structure is formed by inversion of aragonite to calcite with merging of crystals until microscopically visible. The enmeshed clay is pushed to the side of the growing bundles of calcite crystals or to the lamination contacts, and results in white, opaque inter-ray areas.
Extensive calcareous bioherms are present along the shores exposed to wave activity. They are principally the deposits of the alga, Aphanothece Packardii but bacterial precipitation of carbonate may also be important.
The CaO:MgO ratio of the soluble contents of the river waters that empty into the lake is about 2.6:1. The weighted ratio of the same oxides, soluble in weak, hot HCl, from all of the lake sediments is 7.3:1. This difference can not be accounted for by continued concentration of magnesium in the lake waters, but is understood, at least in part, by base ion absorption of Mg++ ions in greater proportion than Ca++ ions in and around the various colloidal particles of the clays.
About one-third of the lake sediments are the rod-shaped faecal pellets of the brine shrimp, Artemia gracilis. The pellets differ in composition from the bottom clays only in a higher carbonate content indicating that the shrimps consume a great deal of inorganic matter but that some carbonate is, perhaps, precipitated in the digestive tract.
The lateral sedimentary variations and the burial of islands are described. Future structures for oil accumulation are probably in the making.
Pay-Per-View Purchase Options
The article is available through a document delivery service. Explain these Purchase Options.
|Watermarked PDF Document: $14
|Open PDF Document: $24
Members of AAPG receive access to the full AAPG Bulletin Archives as part of their membership. For more information, contact the AAPG Membership Department at [email protected].