Complete Geologic Time Scale Hadean to Recent Phanerozoic – “visible life”
Geologic Time Scale for 1 st 3.8 Billion Years of Earth Existence Proterozoic - “hidden life” Archean – life first appears (?) and remains viable Hadean – meteorite bombardment, life started and restarted?
Chap. 8 - Origin of Continental Crust Main Topics –Earth cooled sufficiently to permit formation of early continental (granitic) material –Isotopic age dates within continents “cluster” suggesting several periods of “orogeny” –Early continents seem to represent “partial melts” of andesitic volcanics or early sediments. –Most of the present-day volume of continental material had formed by ~2.5 billion yrs. ago.
Chap. 8 - Origin of Continental Crust Main Topics (cont.) –Archean (3800 – 2500 Bya) rocks characterized by “greenstone” belts and texturally immature sediments (graywackes), largely form oceanic arcs. Suggesting plate tectonics may have started? –Proterozoic (2500 – 540 Bya) rocks are texturally and compositionally mature, include chemical sediments (carbonates and evaporites). Stromatolites are present showing life had evolved while evaporites suggest that sea water had also evolved to its present composition
Fig. 8.1 Atrists conception of what surface of earth looked like during its first 500 million years. Surface was largely molten, with a few of the original microcontinents beginning to form. Intense meteorite bombardment heated surface to melting. Moon was twice as close, exerting a very strong gravitational pull. Early atmosphere had no O 2, but probably consisted of N 2, CH 4, NH 3, CO 2 and H 2 O. Note no oceans.
Evidence of Crustal Development from Igneous and Metamorphic Rocks Importance of Granite Rock-types surviving from early Cryptozic are mainly granitic in composition and they are arrangemed in highly deformed orogenic belts. This has led to hypothesis of continential accretion of early granitic masses into protocontinents and then continents.
Evidence of Crustal Development from Igneous and Metamorphic Rocks However field evidence suggests that granitic continental crust was not original and must have increased in volume through time. Original crust was thin and mainly basalt. Weathering, erosion and igneous activity converted some of the original crust to granite to form embryonic continents. Embryonic continents persisted on surface of earth and accreted slowly to form larger continents.
Fig Archean granite (light) intruding metavolcanic (metamorphosed volcanic ash, etc.) sediments. Nestor Falls. Ontario. Granite is about 2.5 By (Algoman orogeny).
Fig. 8.2 High-grade metamorphic rock (gneiss) typical of ancient “shield” regions. Sondre Stromfjord, SW Greenland. Age of rocks in this picture are ~3.8 By. Cryptozoic (“hidden life”) Eon
Fig. 8.6 Cross-section from N. Shore of L. Superior to northern Michigan. Numbers refer to relative age (1 = oldest).
Development of a Cryptozoic Chronology Age dating of ancient rocks showed patterns of old rocks bounded by younger rocks in patterns that suggested accretion of younger material onto a core of older, mostly granitic, rock. Thus the modern continents have a history of growth by addition of smaller granitic masses, which persisted through time because of their greater buoyancy.
Fig. 8.3 Map showing locations of all Cryptozoic and early Paleozoic rocks in the world. Numbers refer to age in By.
Fig These geologic provinces form the core of the North American craton. The older rocks probably accreted about Bya. The Grenville Province was sutured about 1.0 Bya. (craton = stable nucleus of a continent) Isotopic age dates show great discordance when mapped over the entire N. American craton.
Greenstone Belts “Greenstone Belts” are basically metamorphosed basalts and graywacke (discussed below) sandstones deposited as pillow lavas and turbidity flows on the floors of ancient seas. When protocontinents collided and accreted, the ocean floors filled with these basalts and graywackes collapsed, forming greenstone belts that also accreted to the growing protocontinent. Thus some of the early seafloor survived destruction (by subduction) and became part of the stable craton.
Fig Evolution of greenstone belts. A. Basins between protocontinents fill with basalts, B. when protocontinents collide, they “collapse” the oceans filled with basalts and graywackes, forming greenstone belts.
Fig Hypothetical scenario for assembly of N. American craton during Proterozoic. Based on dates and tectonic patterns in previous figure.
Interpretation of Crustal Development from Sediments Terrigenous vs. nonterrigenous sediments Composition of sedimentary rock reflects source –Clastic sediments – primarily silicates, derived from erosion of older rocks in land areas –Chemical sediments – evaporites (salt – NaCl, gypsum – CaSO4) and carbonates. Precipitates or bio-precipitates in warm, shallow seas
Fig Stages in the development of textural maturity in a sand through abrasion and sorting of grains. Size tends to decrease with time and transport distance. Clay minerals form, from from chemically unstable minerals such as feldspars and amphiboles and quartz is concentrated in residue. Final stage is a pure quartz sandstone, but often only after several tectonic (erosion, burial, uplift) cycles.
Fig Steps in the evolution of a mature sand from initial weathering of a granite. Texturally mature sand is mono-minerallic (quartz), well-rounded and of a uniform grain size. This indicates a long time spent in transport or washing around on a beach. It may also be 2nd or even 3rd cycle. Graywacke suggests rapid transport and burial (why?) while arkosic sands suggest longer transport or more intense weathering in place, since most unstable minerals (amphiboles) are missing. graywacke arkose quartzite
Fig. 8.16a Photomicrograph of a graywacke sandstone showing lack of textural maturity (angular grains, many unstable minerals and poor sorting (a wide range of grain sizes. This rock is 1 st cycle, deposited rapidly, perhaps as a turbidite and spent little or no time in a high-energy environment such as a beach. This type of rock would be expected to be common on the early (Archean) earth.
Fig. 8.8a Graded bedding (grain size decreases upward in the gray beds) in Archean graywacke from Ely, Mn.
Fig. 8.8b Archean graywacke showing multiple graded beds and interstratified limestones. East of Great Slave Lake, Northwest Territories, Canada.
Fig. 8.16b Photomicrograph of a pure quartz sandstone characterized by good sorting (mono- minerallic, one dominant grain size) well-rounded grains and absence of clay and unstable minerals. This type of rock would be expected to be found on a stable craton where it could spend a lot of time (millions (?) of years ) washing around as loose grains on a beach. This rock could be 2 nd or 3 rd cycle from pre-existing sediments as they were buried, consolidated and then uplifted and eroded.
One example of a classification chart for sedimentary rocks Sediment composition triangle The diagram shows the range of sedimentary rock types represented as mixtures of three components: calcium (plus magnesium) carbonates, clay minerals (represented by the hypothetical hydrated aluminum and iron oxides as the end member), and silica (silicon dioxide). Sediments and sedimentary rocks have the same ranges of composition. Iron-rich laterites and aluminum-rich bauxites are the products of intense weathering. Sandstones are primarily composed of indurated sandy sediments, in many cases dominantly quartz. Argillaceous rocks are formed by lithification of clay-rich muds. Sediments or sedimentary rocks rarely, if ever, have compositions represented by the white area of the triangle. Cherts are the sedimentary rock equivalent of biologically deposited siliceous deposits. During the transformation into rock, the amorphous silica, originally deposited by diatoms and radiolarians, is transformed into very hard microcrystalline quartz-rich rock.
A simple model showing how different tectonic regimes lead to different types of sandstone deposition. QFL triangular diagrams are usual method of depicting sandstone composition and hence provenance (source) and history. QFL = Quartz, Feldspar, Lithic fragments
SEDIMENTARY DEPOSITIONAL ENVIRONMENTS “Long” vs “short” system models for sedimentary deposition environments. Note both systems eventually result in submarine fans but long reach has more and varied environments.
Fig. 8.9 Cross-bedded 1.75 By sandstones from the Big Bear Formation, Coppermine River, NW Territories, Canada. Cross-beds are produced when coarse sand is deposited by water (fluvial) or wind (aeolian). These are probably aeolian._
Fig Ripple marks in early Proterozoic (Huronian) quartzite. 30 miles east of Sault Ste. Marie, Ontario. Ripple marks contain information on direction of sediment transport as well as being “tops” indicators.
Block diagram showing origin of cross-stratification by migration of ripples. Cross-bedding reveals top and bottom as well as current direction.
Fig Comparison of relative sorting of sand grain sizes by different sedimentary processes. Sorting can help determine the origin of a sandstone.
Origin of Life - Stromatolites A special type of rock exists throughout the geologic record, called stromatolites, which record the very first visible evidence of life, as early as billion years ago. These rocks are actually comples colonies of different types of bacteria, each type occuping a special niche in the colony. The most important are the photosynthetic cyanobacteria (formerly blue green algae) common pond scum. These amazing life forms are highly adaptable and form the base of the first food chain. Oh yes, they also are responsible for all the oxygen in the air. O2 is a waste product of their photosynthesis. Plants later likely simply incorporated a version of cyanobaterial to carry out their photosynthesis. Nature rarely reinvents a wheel.
Fig Outcrop of a stromatolite “reef” from 1.6-billion year old Proterozoic carbonate in the Wopmay orogen. These reefs were formed by colonies of photosynthetic “blue-green” algae, cyanobacteria and represent some of the first life forms on earth.
Fig Modern algae from Shark Bay Australia. They survive in the hypersaline lagoons because predators cannot tolerate the high salt content. Shark Bay – A Glimpse into the Archean
Fig Model showing schematically how cyanobacteria changed the world. Note the iron minerals (BIFs) in A and the oxygen segregation in the oceans (B).
Fig. 8.7 Banded Iron Formation (“BIF”) near Jasper Nob, Ishpeming MI. Chert (red) iron (gray).
Fig Oolites in Banded Iron Formation (BIF), N. Michigan. Oolites are now chert (SiO2) but were most likely originally deposited as carbonate (CaCO3). Jolter’s Key in the Bahamas may be a modern analog for the original depositional environment.
Modern habitat of ooids Jolter’s Cay in Bahamas (Island in center of picture). Modern ooids form in the warm, shallow waters in the lee of the island
Fig SEM photographic of chert showing the sponge spicules that make up the bulk of the rock. Magnification 160x.
Fig Continental growth by accretion of small plates (“strange terrains”). Note the “suture” zone between the two colliding granitic masses. The following slides of E. Africa show a modern “aulacogen” in the process of developing.
Fig Another product of a failed rift, the mid- continent gravity high thought to be a result of a failed arm back in the Keweenawan (1Bya). The floor of the high is largely dense basalts that poured out of the upper mantle before the arm failed, again similar to what is happening in E. Africa today.
Fig Global distribution of late Proterozoic (Varangian) glacial deposits (triangles) showing their occurrence in equatorial regions. The glacial deposits are interbedded with limestones which further suggest a low latitude origin. The Earth may have narrowly escaped freezing over completely in the Varangian.
Fig Mud cracks in red shales in the Chuar Group of the Grand Canyon. 1.8 Bya. Rocks like these indicate hot, dry conditions (mudcracks) while the red color indicates that there was not enough oxygen in the atmosphere to turn the rocks rusty red.
Fig Laminated mudstone with scattered pebbles and sand grains dropped from above. Gowganda Formation, Blind River Ontario. This textures suggests the stones dropped from a drifting iceberg.
Fig. 8.5 Pillow basalts in Archean “greenstones” 15 km west of Marquette, MI. “Protusions” on lower side of several of the pillows indicate (point to) bottom.
Fig. 8.4 Early field geologists working on Lake Mistassini, Quebec, 1885.