What little is known has been discovered largely in the past four or five decades.
The oldest rocks in the world occur in the Canadian Shield. Their ages have been calculated from precisely measured ratios of the radioactive decay of trace amounts of certain isotopes in the rock sample. The ratio of… The Precambrian environment Several rock types yield information on the range of environments that may have existed during Precambrian time.
Evolution of the atmosphere is recorded by banded-iron formations BIFspaleosols buried soil horizonsand red beds, whereas tillites sedimentary rocks formed by the lithification of glacial till provide clues to the climatic patterns that occurred during Precambrian glaciations.
Paleogeography One of the most important factors controlling the nature of sediments deposited today is continental drift. This follows from the fact that the continents are distributed at different Precambrian to cambrian, and latitudinal position affects the temperature of oceanic waters along continental margins the combined area of the continental shelf and continental slope ; in short, sedimentary deposition is climatically sensitive.
At present, most carbonates and oxidized red soils are being deposited within 30 degrees of the Equator, phosphorites within 45 degrees, and evaporites within 50 degrees. Most fossil carbonates, evaporites, phosphorites, and red beds of Phanerozoic age dating back to the Cambrian have a similar bimodal distribution with respect to their paleoequators.
If the uniformitarian principle that the present is the key to the past is valid meaning the same geologic processes occurring today occurred in the pastthen sediments laid down during the Precambrian Precambrian to cambrian have likewise been controlled by the movement and geographic position of the continents.
Thus, it can be inferred that the extensive evaporites dating to 3. It can also be inferred that stromatolite-bearing dolomites of Riphean rock, a sedimentary sequence spanning the period from 1.
Riphean rock is primarily located in the East European cratonwhich extends from Denmark to the Ural Mountains, and in the Siberian craton in Russia. Today, phosphate sediments are deposited primarily along the western side of continents.
This is the result of high biological productivity in nearby surface waters due to the upwelling of nutrient-rich currents that are moving toward the Equator. The major phosphorite deposits of the Aravalli mountain belt of Rajasthan in northwestern India, which date from the Proterozoic Eonare associated with stromatolite-rich dolomites.
They were most likely deposited on the western side of a continental landmass that resided in the tropics. Paleoclimate Evolution of the atmosphere and ocean During the long course of Precambrian time, the climatic conditions of the Earth changed considerably.
Evidence of this can be seen in the sedimentary record, which documents appreciable changes in the composition of the atmosphere and oceans over time. Oxygenation of the atmosphere Earth almost certainly possessed a reducing atmosphere before 2.
The minerals uraninite UO2 and pyrite FeS2 are easily destroyed in an oxidizing atmosphere; confirmation of a reducing atmosphere is provided by unoxidized grains of these minerals in 3. However, the presence of many types of filamentous microfossils dated to 3.
The presence of fossil molecules in the cell walls of 2. Oceans of the Archean Eon 4.
The oxygen that combined the ferrous iron was provided as a waste product of cyanobacterial metabolism. A major burst in the deposition of BIFs from 3. This enabled the atmospheric oxygen level to increase appreciably.
By the time of the widespread appearance of eukaryotes at 1. These relatively high concentrations were sufficient for oxidative weathering to take place, as evidenced by hematite-rich fossil soils paleosols and red beds sandstones with hematite-coated quartz grains.
A second major peak, which raised atmospheric oxygen levels to 50 percent PAL, was reached by million years ago. It was denoted by the first appearance of animal life metazoans requiring sufficient oxygen for the production of collagen and the subsequent formation of skeletons.
These sediments, which include abraded detrital zircon grains that indicate water transport, are interbedded with basaltic lavas with pillow structures that form when lavas are extruded under water. The stability of liquid water that is, its continuous presence on Earth implies that surface seawater temperatures were similar to those of the present.
Differences in the chemical composition of Archean and Proterozoic sedimentary rocks point to two different mechanisms for controlling seawater composition between the two Precambrian eons.
During the Archean, seawater composition was primarily influenced by the pumping of water through basaltic oceanic crustsuch as occurs today at oceanic spreading centres.
In contrast, during the Proterozoic, the controlling factor was river discharge off stable continental margins, which first developed after 2.
The present-day oceans maintain their salinity levels by a balance between salts delivered by freshwater runoff from the continents and the deposition of minerals from seawater. Climatic conditions A major factor controlling the climate during the Precambrian was the tectonic arrangement of continents.
At times of supercontinent formation at 2.In Cambrian Period: Fossil record of the Precambrian-Cambrian transition. The preservation of the record of the Precambrian-Cambrian transition was significantly affected by global changes in sea level.
During latest Precambrian time, the sea level was relatively low, resulting in spatially restricted oceans and expanded continents. The super eon Precambrian (Pre-Cambrian) is an informal name for the three eons of the geologic timescale that came before the current Phanerozoic eon.
It spans from the formation of Earth around Ma (million years ago) to the evolution of abundant macroscopic hard-shelled animals, which marked.
The Precambrian ( to million years ago) is a vast period of time, nearly 4, million years long, that began with the formation of the Earth and culminated with the Cambrian Explosion.
THE PRECAMBRIAN ERA The Precambrian Era.
The name means: "before the Cambrian period." This old, but still common term was originally used to refer to the whole period of earth's history before the formation of the oldest rocks with recognizable fossils in them.
THE PRECAMBRIAN ERA The Precambrian Era. The name means: "before the Cambrian period." This old, but still common term was originally used to refer to the whole period of earth's history before the formation of the . Precambrian definition, noting or pertaining to the earliest era of earth history, ending million years ago, during which the earth's crust formed and life first appeared in the seas.