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ET Sedimentation Info

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Pages 32-34
[b]Subduction[/b] of between 5,000 to 15,000 lineal kilometres of pre-existing East Pacific seafloor crust beneath the American continent is also not required. In Figure 6.6 the Pacific Ocean is instead shown to originate during early-Jurassic times as two separate marine sedimentary basins. A North Pacific basin was located between northwest Australia, Canada, and China, and a South Pacific basin was located between east Australia, South America, New Zealand and Antarctica.

Both of these marine basins progressively opened to the south and north, along the west coasts of North and South America respectively. These basins then merged to form a single Pacific Ocean basin during the mid- to late-Jurassic Period. Remnants of this early basin history are now preserved as continental margin and marine plateaux sediments within the South East Asian and Coral Sea regions—shown as white areas in Figures 6.6 and 6.7.

... Throughout the Mesozoic Era the North Pacific Ocean underwent a very rapid enlargement, with an asymmetric spreading axis extending southeast into the South Pacific region. This spreading and mid-oceanridge development curved along the west coasts of North and South America and ultimately extended west into the Coral Sea region during the Cretaceous.

... Development of the Pacific Ocean on an increasing radius Earth during the Cenozoic Era is characterised by the initiation and rapid development of symmetric-style seafloor spreading. This commenced within the Tasman Sea region, located southeast of Australia, during the Paleocene and it continued to extend east towards South America during the Eocene. From there, symmetric spreading continued north, forming the present East Pacific spreading ridge, and then extended along the west coast of North America to its present location adjacent to
California.

Page 34
[b]Tethys Sea.[/b] The evolution of the Mediterranean to Middle East regions on an increasing radius Earth (Figure 6.8) represents the remnants of a more extensive continental Tethys Sea—as distinct from a conventional Tethys Ocean. The Tethys Sea will be shown later to have had an extensive crustal and sedimentary basin history, extending back to the early Precambrian times.

Page 38
[b]Fluids Origin.[/b] In addition to returning the seafloor volcanic lava and marginal sediments to their places of origin it is also conceivable to consider that the bulk of the ocean waters and much of the atmospheric gases must be returned to the mantle. Black smokers discharging hot water and gases from along the seafloor mid-ocean ridges as well as volcanic eruptions are modern-day examples of this new water and gas discharge process in action and by reversing these, and any other discharge processes, back in time the water and gases must be returned to the mantle where they came from.

Page 38
[b]Continental Composition.[/b] In contrast to the relatively simple seafloor crusts, continental crust is made up of a diverse range of present-day to ancient rocks dating back to the earliest Archaean times. These rocks include ancient granite and volcanic rocks, deformed and physically altered sediments eroded from the more ancient lands, intrusive and extrusive magmatic rocks, as well as multiple layers of overlying younger sediments deposited in past low-lying regions. These younger rocks, in particular, often cover vast areas of older rocks. Studies elsewhere also show that the average composition of the continental crust is that of granite. That is, rocks rich in silica and aluminium in the form of quartz and feldspar minerals. This contrasts with the seafloor crust which has an average composition of basalt—a lava rich in iron and magnesium.

... The continental geology shown in this figure is further complicated by subsequent deposition of many layers of young sedimentary and volcanic rocks, which generally cover and overprint the older crustal rocks lying below them. These are, in turn, complicated still further by many periods of metamorphism, folding, faulting, weathering, and erosion that may have occurred intermittently throughout Earth history.

Page 39
[b]Craton.[/b] A craton is defined as a part of the Earth’s crust that has attained relative crustal stability and the rocks have been little deformed for a prolonged period of time. By definition, cratons must have reached crustal stability by about 2,400 million years ago (the end of the Archaean Eon) and since then have undergone little deformation compared to adjacent parts of the crust.

[b]Orogen.[/b] An orogen refers to a belt of rocks characterised by regional folding, metamorphism, and intrusion of magmatic rocks. The rocks of an orogen can include deformed, eroded, and reworked parts of older, early-formed cratons, as well as volcanic and sedimentary rocks. A distinct tectonic phase of Earth movement, over a relatively short period of time, first establishes an orogen. It is also possible
for an orogen to become re-activated during subsequent tectonic events and the belt normally remains as a permanent zone of relative weakness within the Earth’s crust.

[b]Basin.[/b] A basin refers to an area that is underlain by a substantial thickness of sedimentary rocks. These rocks possess unifying characteristics of both sediment type and deformation history. Within a basin, sediments are deposited during a regionally restricted period of time, often extending for tens to hundreds of millions of years, during crustal depression or a related sequence of such events. The term basin is usually synonymous with the term sedimentary basin and it represents a regional topographical down-warp of the Earth’s surface, generally filled with water.

Page 47
8.1 [b]Assumptions[/b]
Sediments deposited in continental sedimentary basins, as well as magmatic intrusions and volcanic eruptions, represent new rocks or crusts that accumulate primarily within areas of extensional continental crust.

Page 49
[b]Flatlands.[/b] Deposition of sediments eroded from the exposed lands was confined to a global network of continental sedimentary basins which coincided with relatively shallow continental seas. Breakup and subtle jostling of each of the ancient cratons during changes in surface curvature was first initiated during this phase, in particular within the established network of sedimentary basins, giving rise to long linear zones of crustal weakness. Because of the prolonged period of time involved in this phase, the continental crust may have had a subdued featureless topography for much of the time and continental seas were relatively shallow.

Page 50
[b]Phase 2[/b]: A mid-Proterozoic to Carboniferous—from 1,600 to 360 million years ago —phase ... Crustal extension was again mainly confined within a coincident network of crustal weakness, sedimentary platform basins, and shallow seas. Over time, the network of sedimentary basins and seas continued to increase their surface areas throughout the early- to mid-Palaeozoic Era — 540 to 360 million years ago.

[b]Phase 3[/b]: A Carboniferous through to late-Permian — 360 to 250 million years ago —
transitional phase, where continental crustal extension was exceeded and extension changed to crustal rupture, rifting, and initiation of continental breakup and opening of the modern oceans. Continental seas commenced draining and deposition of sediments progressively shifted away from the established continental sedimentary basins into newly opening marine basins. Evidence for this early marine basin phase is now preserved seawards of the continental shelf margins and as remnant ocean plateaux within many of the modern oceans, such as the Lord Howe Rise in the Tasman Sea. During this phase reptiles, dinosaurs, plants and mammals evolved to eventually dominate the lands.

Page 58
[b]Spreading.[/b] During this Pangaean time, rupture and breakup of the continental crusts had initiated draining of the continental seas which was in turn accompanied by a shift in where eroded sediments were being deposited. This shift changed from sediments being deposited within an existing network of continental sedimentary basins, to being deposited within newly opening marine basins and along the continental shelf margins of the newly formed modern continents. This influx of sediment, along with intrusion of new volcanic and magmatic rocks, is now commonly preserved within submerged marine plateaux as well as continental shelf settings surrounding most of the modern continents.

Page 64
[b]Margins.[/b] Deposition of sediments within the ancient Tethys Sea region was then disrupted and deposition of eroded sediments shifted into the newly formed marine basins, now located around the margins of many of the modern continents.

Page 100
[b]Rigidity.[/b] It is now known that the Earth’s continental crust is primarily made up of stabilised ancient cratons, complexly folded and metamorphosed orogens, sedimentary basins, and volcanic seafloor crusts. The ancient cratons comprise mainly granite and volcanic rocks with lesser sediments and can, by definition, be considered as rigid crusts. The orogens comprise mainly metamorphosed sedimentary rocks—rocks that have been folded and recrystallized as a result of Earth pressures and temperatures—and when originally formed these rocks were essentially flexible crusts. These orogenic rocks may have since increased their rigidity during metamorphism. The orogenic rocks can then be considered as having an intermediary strength between the ancient cratonic rocks and the younger basin sediments. The rocks forming the sedimentary basins and seafloor crusts are, by comparison, truly flexible rocks. Over the extended period of geological time available the basin sediments in particular will readily flex, fold, fault, shear, stretch, and distort, as highlighted in many rock exposures throughout the world today.

Pages 103-104
[b]Surface.[/b] Relief of surface curvature to the new Earth radius is then balanced by the strength of the crust and downward acting weight of the sediments deposited within the trough. During a radial increase in radius of the Earth from A to B, sediments within the geosynclinal trough are then shown to undergo compression and orogenesis during ongoing changes in surface curvature. This may have also been accompanied by intruded granite magma and volcanism.

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