Cambrian (geology)

	Main Article	Discussion	Related Articles  [?]	Bibliography  [?]	External Links  [?]	Citable Version  [?]
	This editable Main Article is under development and subject to a disclaimer. [edit intro]

The term Cambrian is applied to both a system of strata and a period of time. These are related but there are important differences. In the first instance, the name refers to a chronostatrigraphic system and in the latter a geochronological unit of geological measure. The Cambrian System encompasses four chronostatrigraphic series and ten stages. Correspondingly, the Cambrian Period in the Paleozoic Era of the Phanerozoic Eon, encompasses four geochronological epochs and ten ages.

The name is derived from Cambria, the Roman word for Wales, the area where the rocks of the Cambrian was first studied.^[1]

(See main article Geologic ages of earth history)

Boundaries[edit]

Geochronologically, the Cambrian Period is recognized by the ICS as the period from 542 million years ago (with an error factor plus or minus 1.0 mya) to 488.3 (plus or minus 1.7 mya).^[2]

Stratigraphically, the Cambrian System (collective groups of layers or strata) is distinctive for a large increase in the fossil record indicating a significant increase in plant and animal life during that time (also referred to as the "Cambrian Explosion") ^[3]The lower boundary^[4] is characterised by trace fossils, the lowest occurrence of Treptichnus (Phycodes) pedum and a near base of negative carbon-isotope excursion. The end or lower boundary is characterised by Conodont fossils, and the lowest occurrence of Iapetognathus fluctivagus; just above the base of Cordylodus lindstromi conodont Zone, and just below lowest occurrence of planktonic graptolites.

Historically it has been subdivided into three epochs (not recognized by the ICS)

• Early Cambrian (542 - 513 mya)
• Middle Cambrian (513 - 501 mya)
• Late Cambrian (501 - 488.3 mya)

Regional variations are also in use. An example of the Laurentian Timescale from the Geological Society of America: Laurential Regional Designations^[5]

• D Epoch
  • Sunwaptan (495 – 490 mya)
  • Steptoean (500 – 545 mya)
• C Epoch
  • Marjuman (506 – 500 mya)
  • Delamaran (512 – 516 mya)
• B Epoch
  • Dyeran (516 – 512 mya)
  • Montezuman (520 – 516 mya)
• A Epoch (543 – 520 mya)

Currently the Furongian Series (or Epoch) and the Paibian and Drumian Stages (or Ages) are defined and designated by name. ^[6] The earliest undefined series (unamed) encompasses the as yet unamed Stage 1 and Stage 2 (earliest to latest). Series 2 extends from 521 mya to 510 mya with two stages. Series 3 (from 510 to 501 mya) is composed of Stage 5, the Drumian Stage (or Age). and Stage 7. The final and last series to be created in the Cambrian, the Furongian, encompasses the Paibian Stage, Stage 9 and Stage 10 ^{[7][8][9][10]}

GSSP

In accordance with ICS rules, a period is defined by an event recorded in rock formations termed the global stratotype section and point (GSSP). The GSSP is the reference point that defines the international “standard” for recognition of the base, or lower boundary, of the period. The initial GSSP of the beginning of the Cambrian Period is in Newfoundland, Canada.^[11]

Defining the Cambrian Period[edit]

Developing the parameters of the Cambrian

Originally the Cambrian was an undefined period noted for the apparently sudden and unexplained appearance of complex animals. Charles Darwin referred to this as the stratigraphic product of massive record failure. ^[12][13]

In 1914, Charles Doolittle Walcott later expanded on Darwin’s view, defining a Lipalian Interval as “the unrecorded period of time reflected in the unconformity between lowermost Cambrian strata and the (commonly deformed) rocks that lay beneath them.” In other words, the beginning of the Cambrian period was not clearly marked. However, there were other stratigraphers at that time who knew there were successive Cambrian layers in various regions that lay atop distinctive, that is to say, discernable sedimentary layers up to thousands of meters in thickness. Wolcott and Darwin did not have a complete picture from which to work.

Establishing the beginning of the Cambrian strata.

Overtime, research uncovered a growing number of immediately sub-Cambrian strata. In 1952, Boris Sokolov proposed the name Vendian to label a discrete system of siliciclastic rocks.^[14] The Vendian underlay the Lower Cambrian strata in a geological formation known as the Russian Platform in the Ural Mountains. Sokolov later expanded the Vendian System to encompass the Laplandian glacial level exposed in the Ural Mountains.^[15]

In 1960, Termier and Termier proposed that there was an unrecorded global Ediacarien interval that immediately preceded the Cambrian strata (i.e. indicated immediate pre-Cambrian time).^[16] In addition, as suggested by Darwin, the interval contained fossils of simple animals.

By the 1960s, two significant factors were known to be manifest on a global scale: There was strong evidence of continental glaciation ^[17] and Ediacaran fossils.^[18] These both had a major impact on attempts to correlated research findings regionally and globally.

In 1991, the International Commission on Stratigraphy did ratify a series of Proterozoic periods but the time interval immediately prior to the Cambrian (the terminal Proterozoic period) was left undefined. The ICS Subcommission on the Terminal Proterozoic Period eventually voted to recommend a definition of the initial GSSP for the Ediacaran Period, the terminal Proterozoic period, originally proposed by P. E. Cloud and M. F. Glaessner in 1982.^[19][20]

Characteristics of the Cambrian[edit]

The time before the Cambrian Period[edit]

The Ediacaran Period immediately before the Cambrian Period began with the termination of the last great global glaciation when continental glaciers reached sea level in tropical latitudes (appropriately, the name of the preceding period, the Cryogenian, basically means “genesis of ice”). The end of the period (marked by the initial GSSP of the Cambrian Period) is characterized by the fossils of animals who had a bilateral anatomic structure—a symmetrical left and right side as is prevalent today.^[20]

Stratigraphy[edit]

Biotic Characteristics[edit]

Bilaterian animals[edit]

Bilaterian animals are symmetrical, divided into equivalent left and right sides, on either side of a central axis, and with dissimilar ends, that is to say, an anterior-posterior polarity. They may have appeared about 555 million years ago, before the end of the Ediacarean Period and became profusely abundant on a global scale at the dawn of the Cambrian Period.

Initially they were macroscopic—larger than microscopic life forms—and soft bodied. Their lack of hard tissue also means that during the process of fossilisation, they left, at best, trace fossils (impressions or casts of the animal) rather than fossilised tissue.

Few animal body fossils are found in strata deposited during the Proterozoic Era (2.5 billion to 544 million years ago). Trace fossils from this era are typically only a few millimetres wide and are found at the interface between water and sediment.

The trace fossils found in the Cambrian strata (deposited 544 to 490 million years ago) show much more diverse animal body fossils such as trilobites. The Cambrian trace fossils are more diverse in size, shape and depth of penetration into the sediment.^[21]

Conodonts[edit]

Conodonts are "tooth-like" microfossils (usually ~0.1 to 5mm) that may have been part the feeding structure of an extinct animal resembling a hagfish-like vertebrate (Phylum Chordata). Conodonts are found in layers of carbonate strata deposited in marine environments. Composed of calcium carbonate fluorapatite and other organic matter, they are found in commonly in black shales associated with graptolites, radiolarians, fish remains, brachiopods, cephalopods, trilobites and palaeocopid ostracods.

Ubiquitous, they are disseminated globally. They occur commonly in the Precambrian and are prolific in the Cambrian Strata, eventually becoming extinct in the late Triassic Period. They provide a valuable source of stratigraphic information in that they are believed to have undergone rapid evolutionary development from the Precambrian to the Triassic.^[22][23]

Sources[edit]

• Proposed Global Standard Stratotype-Section and Point for the Paibian Stage and Furongian Series (Upper Cambrian) International Subcommission on Cambrian Stratigraphy, International Commission on Stratigraphy (ICS), International Union of Geological Sciences (IUGS).
• Subcommission on Neoproterozoic Stratigraphy International Commission on Stratigraphy (ICS), International Union of Geological Sciences (IUGS).
• EarthTime

References[edit]