LECTURE 04

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Evolution of the Earth Chapter MAIN TOPICS: ROCK FACIES RELATIVE TIME GEOLOGIC TIME SCALE RELATIVE GEOLOGIC TIME • As geologic thought progressed in the early 1800’s it became necessary to classify and organize material (fossils) and concepts (maps)) in a more orderly and manageable form • In 1835 Adam Sedgwick and Roderick Murchison proposed formal names for the entire European stratigraphic succession • Eras (Paleozoic, Mesozoic, Cenozoic) • Periods (Vendian, Cambrian, Ordovician, etc.) • Based solely on fossils (e.g fossil life spans) • Based on fossils and Steno’s Laws GEOLOGIC TIME SCALE ORIGINATED IN ENGLAND, FRANCE, GERMANY GEOLOGIC TIME SCALE • PALEOZOIC ERA (383 My) – LOWER PALEOZOIC • • • – CAMBRIAN (Cambria) ORDOVICIAN SILURIAN (Silures) UPPER PALEOZOIC • • DEVONIAN (Devonshire) CARBONIFEROUS (coal-bearing) – – • • PERMIAN MESOZOIC ERA (183 My) – – – • MISSISSIPPIAN (N America) PENNSYLVANIAN (N America) TRIASSIC (Trias, Germany) JURASSIC (Jura Mtns.) CRETACEOUS CENOZOIC ERA (66 My) (cont.) Fig 4.2 Geology of Northwestern Europe where much of the geologic time scale was developed Note the unconformities and lateral extend of major rock units FACIES – DEFINITION & RELATIONSHIPS • Sedimentary Facies Overall lithology (rock-type) reflecting or diagnostic of depositional environment Examples: – – – – Sandstone facies Mud facies Carbonate facies Salt facies • General rule: adjective describing depositional environment + “facies” • Note: metamorphic petrologists use facies concept in a similar fashion, e.g kyanite facies Relations between Old Red Sandstone in Wales and marine facies in Devonshire Intertonguing relationships established Devonian age of the Old Red Sandstone Fig 4.4 Source of nomenclature feud between Sedgwick and Murchinson As their field areas converged it became apparent that each has included the same rocks in his own classification Sedgwicks’ top of Cambrian overlapped Murchinson’s lower Silurian After their deaths, the dispute was resolved by naming a new system, the Ordovician Fig 4.5 Relative Geologic Time Cross-section across Scotland showing superposition, cross-cutting and included-fragment relationships What is the sequence of events here? Included fragments: Any rock represented by frag ments in another rock must be older than the host rock Cross-cutting relationships: Any igneous rock or any fault must be younger that the rocks it cross cuts Fig 4.5 Relative Geologic Time Sequence of events here: the primitive and transition rocks were (1) folded, intruded by granites, uplifted and deeply eroded before deposition of Old Red Sandstone on unconformity surface This was followed by injection of dikes and sills Fig 4.6 ROCK UNITS (LEFT) AND TIME CHART (RIGHT) Observed rock unit (left) and interpreted time chart on right Note hiatus corresponding to unconformities (Hiatus is a time of nondeposition and/or erosion.) Fig 4.7 FORMATION – a mappable unit either in the field or by well logs A GROUP consists of or more FORMATIONS A formation is the basic rock unit in geology IT IS NOT A TIME UNIT It is defined by its properties: type (sandstone, limestone, etc e.g (Bell Shale), color (Brown Niagrian), texture, geometry The choice is fairly obvious in A, but more difficult in B In B and C the choice of subdivisions is somewhat arbitrary Correlation using three different index fossils A single fossil zone is shown in blue Note that range and maximum development (indicated by pattern width) vary from place to place Fig 4.17 Significance of different rates of evolution and changes in environment (due to transgression) The brachiopod evolved slowly and stayed in/on sand facies It is a poor index fossil The cephalopods evolved rapidly and are free swimmers They were changing and widely distributed and thus excellent index fossils Fig 4.18 Volcanic ash layers (bentonites) make excellent time markers and permit correlations between facies (provided the ash layer is preserved) Fig 4.19 Conodont A conodont is a preserved bony part of an extinct eel It evolved rapidly and is widely distributed across many facies types They are excellent index fossils and can be used to determine maximum burial temperatures as well Conodont specialists were once highly sought after by oil companies Fig 4.20 Block diagram showing relationships between formations and index fossils Fossil zone C shows that Formation is synchronous everywhere, but zones and vary in age How can you tell? Fig 4.21 Fig 4.22 SLOSS SEQUENCES (6) Six unconformity bounded sequences from which a world-wide sealevel fluctuation curve has been inferred (the “Vail” curve) Note two maxima (highs) at about 500 and 75 million years and three minima at 600, 200 and the present Max +350 to -150 m above and below present Fig 4.23 Effects of sea level change on sediment accumulation and unconformities at a continental margin Additional Relative Time Scales • In addition to index fossils, there are several other ways to determine relative time: • The sequence unconformities just discussed • Magnetic reversals • Isotope geochemistry e.g strontium isotopes work pretty well back to end ot Miocene Fig 4.24 Magnetic reversals over past 80 million years These reversals are recorded in sediments and can be used for relative time dating Fig 4.25 Graph of depositional rates of a hypothetical sequence of strata The continuous average rate of deposition is computed by dividing the strata thickness by the time interval The actual rate accounts for changes in rate of deposition as well as for erosional events PALEO-RECONSTRUCTION EARLY DEVONIAN 400 MY • global hemisphere views one centered on North America and the other centered on the Tethys-Indian Ocean region A global mollewide projection with labels and 1storder tectonic elements shows the whole Earth for the Early Devonian http://jan.ucc.nau.edu/~rcb7/paleogeographic.html http://jan.ucc.nau.edu/~rcb7/paleogeographic.html PALEO-RECONSTRUCTION • global hemisphere views one centered on North America and the other centered on the Tethys-Indian Ocean region A global mollewide projection with labels and 1storder tectonic elements shows the whole Earth for the Late Devonian LATE DEVONIAN 370 MY http://jan.ucc.nau.edu/~rcb7/paleogeographic.html http://jan.ucc.nau.edu/~rcb7/paleogeographic.html
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