An interval of time in earth history (Cambrian period) & its rock record (Cambrian system). The Cambrian period spanned about 70,000,000 years & began with the first appearance of marine animals with calcareous shells. The Cambrian system includes many different kinds of marine sandstones, shales, limestones, dolomites & volcanic. There is very little provable records of nonmarine Cambrian environments.
The concept that great systems of rocks recorded successive periods of earth history was developed in England in the early 19th century. The Cambrian, which was one of the first systems to be formally named, was proposed by the Revered Adam Sedgwick in 1835 for a series of sedimentary rocks in Wales that seemed to constitute the oldest sediments in the British isles. At that time, there was no real idea of the antiquity of Cambrian rocks. They were recognized by distinctive fossils & by their geologic relations to other systems. In the early part of the 20th century, radiometric techniques for obtaining the ages of igneous & metamorphic rocks evolved. Because of the difficulty of finding rocks that can be dated radiometrically in association with rocks that can be dated empirically by fossils, the age in years of most Cambrian deposits is only approximate. The best present estimates suggest that Cambrian time began about 570,000,000 years ago & ended about 475,000,000 years ago. It is one of the longest Phanerozoic periods.
For most practical purposes, rocks of Cambrian age are recognized by their content of distinctive fossils. On the basis of the successive changes in the evolutionary record of Cambrian life that have been worked out during the past century, the Cambrian system has been divided globally into Lower, Middle & Upper series, each of which has been further divided on each continent into stages, each stage consisting of several zones. The divisions of the Cambrian system presently recognized in North America are shown in Fig.1. Despite the amount of work already done, precise intercontinental correlation of series & stage boundaries & of zones is still difficult & refinement of intercontinental correlation of these ancient rocks is a topic of research.
LIFE: the record preserved in rocks indicates that essentially all Cambrian plants & animals lived in the sea. The few places where terrestrial sediments have been preserved suggest that the land was barren of major plant life & there are no known records of Cambrian insects or of terrestrial vertebrate animals of any kind
Plants: the plant record consists entirely of algae, preserved either as carbonized impressions in marine black shales or as filamentous or blotchy microstructures within marine buildups of calcium carbonate, called stromatolites, produced by the actions of these organisms. Cambrian algal stromatolites were generally low domal structures, rarely more than a few meters high or wide, which were built up by the trapping or precipitation of calcium carbonate by one or more species of algae. Such structures, often composed of upwardly arched laminae, were common in regions of carbonate sedimentation in the shallow Cambrain seas. SEE STROMATOLITE
Animals: the animal record is composed almost entirely of invertebrates that had either calcareous or phosphatic shells (Fig-2). A few rare occurrences of impressions or of carbonized remains of soft-bodied organisms indicate that the fossil record is incomplete & biased in favor of shell-bearing organisms. The fossils of shell-bearing organisms include representatives of several different classes of arthropods, mollusks, echinoderms, brachiopods & poriferans. Coelenterates (other than jellyfish impressions), bryozoans, radiolarians & foraminiferans are unknown from Cambrian rocks. Some fossil groups of widespread occurrence, such as Archaeocyatha, are known only from Cambrian rocks & several extinct groups of Paleozoic organisms such as hyolithids & conodonts have their first appearance in Cambrian rocks.
[19sepr12R vbm] Pd umumx tanaman & hewan pd jaman Kambrium hidup di laut. Walaupun terdpt sedimen2 darat ttp tdk dijumpai fosil2 tumbuhan & hewan darat pd jaman Kambrium.
Pd umumx tanaman yg berasal dr masa Kambrium adl golongan alga yg mana fosilnya terawetkan pd btlp hitam & jg membentuk STROMATOLIT
Hewanx umumx adl gol invertebrata yg mempunyai cangkang dr kapur atau fosfat (Gambar-2).
Hewan2 tsb adl golonga arthropoda, moluska, echinodermata, brachiopoda & porifera. SEbaliknya gol coelenterata, bryozoa, radiolaria & foraminifera jarang atau tdk ditemukan didlm batuan Kambrium. Selain itu dijumpai jg Archaeocyatha, hyolithids & conodonts
[21sept12J vbm] Diversity: although the record of marine life in the Cambrian seems rich, one of the dramatic differences between Cambrian marine rocks & those of younger periods is the low phyletic diversity of most fossiliferous localities. The most diverse faunas of Cambrian age have been found along the ocean-facing margins of the shallow seas that covered large areas of the Cambrian continents. Because these margins were often involved in later geologic upheavals [pergolakan, turbulence], their rich record of Cambrian life has been largely destroyed. Only a few localities in the world remain to provide a more accurate picture of the diversity of organisms living in Cambrian time. In North America, the richest localities are in the Kinzers Formation of southern Pennsylvania, the Spence Shale of northern Utah & the Burgess Shale of British Columbia.
Trilobites: the most abundance remains of organisms in Cambrian rocks are trilobites (Fig-2a-c). They are present in almost every fossiliferous Cambrian deposit & are the principal tools used to describe divisions of Cambrian time & to correlate Cambrian rocks. These marine arthropods ranged from a few millimeters to 50cm in length, but most were less than 10cm long. Although some groups of trilobites such as the Agnostida (Fig-2a) were predominantly planctonic in habitat, most trilobites seem to have been benthic or nekto-benthic & show a reasonably close correlation with bottom environments. For this reason, there are distinct regional differences in the Cambrian trilobites faunas of the shallow seas of different parts of the Cambrian world.
[22sept12St vbm wisuda naim] Batuan umur Kambrium kaya akan fosil2 yg habitatnya adl di laut. Walaupun dmk dari segi keragaman “phyletic” fosil2 Kambrium adl kurang beragam dibandingkan fosil2 lain dari jaman yg lebih muda. Fosil2 Kambrium yg lebih beragam banyak ditemukan di sedimen2 laut dangkal. Fosil2 jaman Kambrium banyak ditemukan di Fm Kinzers di selatan Pennsylvania & di Burgess Shale di British Columbia (Canada). Fosil yg banyak ditemukan pd batuan Kambrium adl Trilobit (Fig 2a-c). Oleh krn itu fosil ini sering dipakai unt membagi jaman Kambrium menjd bbrp sub-perioda & jg unt keperluan korelasi. Trilobit adl golongan Arthropoda laut yg ukuran panjangnya berkisar dari bbrp milimeter hingga 50cm, tetapi rata2 mempunyai ukuran panjang kl 10cm. Gol Trilobit umumnya hidup secara plantonik spt Agnostida (2a), sedangkan yg lainx hidup secara bentonik atau nektonik. Oleh krn itu karakter fauna Trilobit yg berasal dr lokasi geografis yg berbeda adl tdk sama.
[23sept12M vbm] Brachiopods: the next abundant Cambrian fossils are brachiopods (Fig 2d-f). These bivalved animals were often gregarious & lived on the sediment surface or on the surfaces of other organisms. Brachiopods with phosphatic shells, referred to the Acrotretida (Fig 2f), are particularly abundant in many limestones & can be recovered in nearly prefect condition by dissolving these limestones in acetic or formic acids. Upper Cambrian limestones from Texas, Oklahoma & the Rocky mountains yield excellent silicified shells of formerly calcareous brachiopods when they are dissolved in dilute hydrochloric acid.
Archhaeocyathids: limestones of early Cambrian age may have large stromatolites formed by an associations of algae & an extinct phylum of invertebrates called Archaeocyatha (Fig 2m-n). Typical archeocyathids grew conical or cylindrical shells with two
walls separated by elaborate radial partitions. The walls often have
characteristic patterns of perforations.
[23sept12M vbm] Selain Trilobit fosil yg umum terdpt pd batuan Kambrium adl Brachiopoda (Fig 2d-f). Fosil ini bentukx bivalved hidup dipermukaan atau menempel pd organisma lain. Brachiopoda yg cangkangx terbuat dr fosfat dinamakan Acrotretida (Fig 2f), sangat berlimpah di batugamping. Unt melarutkan btgp (spy mendptkan foislx yg utuh) dipakai asam asetat atau asam format. Fosil2 jaman Kambrium ini banyak terdpt di Btgp di Texas, Oklahoma & peg Rocky dmn cangkang2x sdh mengalami silisifikasi (seblmx terbuat dr zat kapur). Unt menghilangkan pengotorx btgp bisa dilatutkan dg memakai asam klorida encer.Btgp dari perioda Kambrium Awal kadang mengandung fosil stromatolit berukuran yg dibentuk oleh kumpulan algae & jenis invertebrata yg sdh punah ini namax Archaeocyatha (Fig 2m-n). Bnt cangkang Archeocyathids ini adl spt konus atau silendris. Dinding cangkang sering memiliki karakteristik pola perforasi.
[25sept12Sl vbm] Mollusks & echinoderms: the Cambrian records mollusks & echinoderms is chracterized by many strange-looking forms that lived for only short periods of time & left no clear descendants (Fig 2g-l). Representatives of these phyla, such as cephalopods, clams & true crinoids, which are abundant in younger rocks are absent from Cambrian rocks or are found only in the very latest Cambrian deposits. Snails, however, are found throughout the Cambrian. Discoveries of primitive clams have been made in Early Cambrian beds, but they are apparently absent from the later record of life for tens of millions of years until post-Cambrian time
Extinction: the stratigraphic record of Cambrian life in North America shows perhaps five major extinctions of most of the organisms living in the shallow seas. These extinction events form the boundaries of evolutionary units called biomeres (Fig 1). Their cause & their presence in the Cambrian records of other continents is under investigation. However,perhaps it was these periodic disasters that prevented clear continuity in the evolutionary records of many groups & which led, particularly, to the records of the echinoderms & mollusks. See ANIMAL EVOLUTION; EXTINCTION (biology)
Faunal origin: another major unsolved problem is the origin of the entire Cambrian fauna. Animal life was already quite diverse before Cambrian time. The earliest Cambrian beds contain representatives of more than 20 distinctly different invertebrate
groups.All of these have calcified shells, but none of the Precambrian organisms have any evidence of shells. There is still no clear evidence to determine whether shells evolved in response to predation or to environmental stress, or as the result of some change in oceanic or atmospheric chemistry. See PRECAMBRIAN
Geography: knowledge of Cambrian geography & of the dynamic aspects of evolution & history in Cambrian time is derived from rocks of this age that have been exposed by present-day erosion or penetrated by borings into the Earth’s surface. Despite the antiquity of Cambrian time, a surprisingly good record of marine rocks of Cambrian age has been preserved at many localities throughout the world.
[27sept12K vbm] Informasi ttg geografi jaman Kambrium seta pola evolusi pd jaman ini diperoleh dr batuan2 sedimen laut yg tersingkap dibanyak tempat di dunia ini.
[27sept12K vbm]Each of the differnt rock types contains clues about its environment of deposition that have been derived from analogy with modern marine environments. From this information, together with knowledge gained from fossils of about the same age within the Cambrian & information about the present geographic distribution of each Cambrian locality, a general picture of world geography & its changes through Cambrian time is available.
The developing theory of plate tectonics has provided criteria whereby ancient continental margins can be identified. By using these criteria & the saptial information about marine environments derived from study of the rocks, the Cambrian world can be resolved into at least 5 major continents. These were (1) North America, minus a narrow belt along the eastern coast from eastern New-foundland to nortehr Georgia which is most closely related to Cambrian Gondwana; (2) Baltica, consisting of present-day northern Europe north of France & west of Ural mountains, but excluding most of Scotland & northern Ireland, which are remains of marginal Cambrian North America; (3) Gondwana, a giant continent whose present-day fragments are Africa, South America, India, Australia & Antartica, parts of southern Europe, the Middle East & SE.ASia; (4) Siberia, including much of the northeastern quarter of Asia; (5) Mongolia, including the present-day Mongolia & northern half of China. Unfortunately, there is not enough reliable information to accurately locate these continents relative to one another on the Cambrian globe. See CONTINENT FORMATION; PLATE TECTONICS
History: At the beginning of Cambrian time, the continents of the world were largely exposed, much as they are now. Following some still unexplained event, the seas were suddently populated by a rich fauna of shell bearing invertebrates after 4,000,000,000 years of nearly barren existence. Belts of volcanics islands comparable to those of the western Pacific ocean today fringed eastern North America, the Australian & western Antarctic margins of Gondwana & southern Siberia.
[12nov12Sn lab mikropal] p.666
The term used to designate rocks & time older than the Cambrian, which is the oldest geologic period from which abundant fossils have been recovered. Age determinations by means of radioactive elements place the upper boundary of the Precambrian at about 600,000,000 years ago. The oldest dated rocks appear to be 3,800,000,000-400,000,000 years old. All evidence indicates that these rocks are not remnant of the original earth crust, & it follows that the Precambrian interval of earth history lasted well over 3,500,000,000 years. See CAMBRIAN; RADIOACTIVE MINERALS; ROCKS, AGE DETERMINATION OF
[13nov12Sl vbm] Istilah yang digunakan untuk menunjuk batu & waktu yang lebih tua daripada Kambrium, yang merupakan periode geologi tertua yg banyak mengandung fosil. Berdasarkan unsur radioaktif umur Prakambrium adl 600 juta th yl, sedangkan umur batuan yg paling tua adl ant 3.8 milyar-400 jt th yl. Batuan tsb bukan merupakan bagian dr kerak benua/samudra. Rentang waktu Prakambrium adl 3,5 milyar th.
Subdivision: formerly, the Precambrian was divided into the Archean (or Archeozoic) & the Proterozoic (or Algonkian). Other names were also used, especially in Europe & elsewhere, but the names of Archean & Proterozoic have been most widely used. Students of the Precambrian, however, are discarding this terminology. Some geologists still retain the terms, but generally restrict the meaning of the Archean & expand the Proterozoic into Lower, Middle & Upper. The majority of Precambrian investigators prefer to discard the former terms entirely & to make use of Early, Middle & Late Precambrian, or of some numerical subdivision, until a more precise & better-documented time scale can be worked out.
[13nov12Sl vbm] Sebelumnya, Prakambrium dibagi kedlm 2 jaman yi Arkean (atau Archeozoic) & yang Proterozoikum (atau Algonkian). Pembagian tsb lazim dipakai di Eropa. Perkembangan selanjutnya para ahli membagi lebih detail lagi Proterozoicum menjd 3 yi: Bwh, Tengah & Atas. Sedangkan ahli yg lainx membg jaman Prakambrium menjd 3 yi:Bwh, Tengah & Atas.
The present state of knowledge is such that a twofold subdivision appears highly inadequate & construction of a general time scale to represent the sequence of Precambrian events for the entire world is yet impossible. Lack of continuous outcrops, lack of fossils & destruction of primary characteristics of rocks by subsequent metamorphism make correlation of Precambrian rocks & events much more difficult than correlation of later strata & crustal disturbances. Data from radioactive determinations may solve the problem, but they are yet too few. Data are accumulating in increasing numbers, however, & have begun to show patterns of possible major events in Precambrian history. The world picture is still developing & is not yet clearly delineated. Continental subdivisions are emerging but cannot yet be considered more than tentative, & world-wide correlation is further in the future. The fact that certain dates, or comparatively narrow ranges of dates, are being obtained much more frequently than others, appearing to reflect more widespread & significant events, has led the US Geological Survey to adopt an interim scheme for subdividing Precambrian time. The scheme is based strictly on age dating (without regard to formational boundaries, tectonic episodes, erosional intervals), designed solely for the US, & meant to serve as a convenient temporary vehicle for classification until a satisfactory subdivision is developed. The Precambrian is divided into 4 units, somewhat as other geologists have suggested, but here called Precambrian W (oldest), Precambrian X, Precambrian Y & Precambrian Z (youngest), with dividing lines set at 2,500,000,000, 1,600,000,000 & 800,000,000 years. If older subdivisions are desired as information is acquired, they may be added as Precambrian V, Precambrian U & so forth, for progressively older intervals.
The variety of rocks found in Precambrian terranes is great, but one continent differs little from another in this regard. The Precambrian rocks of Australia, Canada, Scandinavia & other parts of the world show the same range in variety & abundance. They are represented by all kinds of sedimentary rocks (conglomerates, sandstones, shales & limestones) & extensive lave flows.
[13nov12Sl vbm] Sedikitnya singkapan batuan & tdk adax fosil Prakambrium serta kebanyakan batuanx telah mengalami metamorfosa menyebabkan kesulitan dlm penelitian jaman ini
Ahli geol US membg jaman Prakambrium berdaskan kpd adting menjd 4 unit yi (tua ke muda) Prakambrium W,X,Y&Z. Umur ant W-X= 2.5 milyar; X-Y= 1.6 milyar & Y-Z= 800 jt th yl.
Batuan Prakambrium dari Australia, Kanada, Skandinavia & bagian lain dunia diwakili oleh semua jenis batuan sedimen (konglomerat, batupasir, serpih & batugamping) & aliran lava.