Misplaced Pages

#72927

23-581: The Snake Indian Formation is a stratigraphic unit of Middle Cambrian age that is present on the western edge of the Western Canada Sedimentary Basin in the northern Canadian Rockies of Alberta and British Columbia . It was named for Snake Indian River in Jasper National Park by E.W. Montjoy and J.D. Aitken in 1978. The type locality was established on Chetamon Mountain . The Snake Indian Formation

46-399: A hiatus because deposition was on hold for a period of time. A physical gap may represent both a period of non-deposition and a period of erosion. A geologic fault may cause the appearance of a hiatus. Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout

69-461: A 1669 work on the fossilization of organic remains in layers of sediment. The first practical large-scale application of stratigraphy was by William Smith in the 1790s and early 19th century. Known as the "Father of English geology", Smith recognized the significance of strata or rock layering and the importance of fossil markers for correlating strata; he created the first geologic map of England. Other influential applications of stratigraphy in

92-454: A geological region, and then to every region, and by extension to provide an entire geologic record of the Earth. A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively, the gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity. It is called

115-402: A lithostratigraphy or lithologic stratigraphy of the rock unit. Key concepts in stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries imply about their original depositional environment. The basic concept in stratigraphy, called the law of superposition , states: in an undeformed stratigraphic sequence, the oldest strata occur at

138-557: A relative scale until the development of radiometric dating , which was based on an absolute time framework, leading to the development of chronostratigraphy. One important development is the Vail curve , which attempts to define a global historical sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon -bearing reservoir rocks, seals, and traps of petroleum geology . Chronostratigraphy

161-436: A section. The samples are analyzed to determine their detrital remanent magnetism (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. For sedimentary rocks this is possible because, as they fall through the water column, very fine-grained magnetic minerals (< 17  μm ) behave like tiny compasses , orienting themselves with Earth's magnetic field . Upon burial, that orientation

184-413: Is fossiliferous and includes the remains of several genera of Middle Cambrian trilobites , as well echinoderms , other marine invertebrates , and stromatolites . Stratigraphy Catholic priest Nicholas Steno established the theoretical basis for stratigraphy when he introduced the law of superposition , the principle of original horizontality and the principle of lateral continuity in

207-417: Is affected. These insolation variations have influence on Earth's climate and on the deposition of sedimentary rocks. The main orbital cycles are precession with main periods of 19 and 23 kyr, obliquity with main periods of 41 kyr, and 1.2 Myr, and eccentricity with main periods of around 100 kyr, 405 kyr, and 2.4 Myr. Precession influences how much insolation each hemisphere receives. Obliquity controls

230-411: Is preserved. For volcanic rocks, magnetic minerals, which form in the melt, orient themselves with the ambient magnetic field, and are fixed in place upon crystallization of the lava. Oriented paleomagnetic core samples are collected in the field; mudstones , siltstones , and very fine-grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with

253-413: Is the branch of stratigraphy that places an absolute age, rather than a relative age on rock strata . The branch is concerned with deriving geochronological data for rock units, both directly and inferentially, so that a sequence of time-relative events that created the rocks formation can be derived. The ultimate aim of chronostratigraphy is to place dates on the sequence of deposition of all rocks within

SECTION 10

#1732786811073

276-564: The natural remanent magnetization (NRM) to reveal the DRM. Following statistical analysis, the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale. This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for

299-615: The ambient field during deposition. If the ancient magnetic field were oriented similar to today's field ( North Magnetic Pole near the North Rotational Pole ), the strata would retain a normal polarity. If the data indicate that the North Magnetic Pole were near the South Rotational Pole , the strata would exhibit reversed polarity. Results of the individual samples are analyzed by removing

322-407: The base of the sequence. Chemostratigraphy studies the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Carbon and oxygen isotope ratios vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. This has led to the specialized field of isotopic stratigraphy. Cyclostratigraphy documents

345-454: The behaviour of sedimentary systems. Cyclostratigraphy also aids the study of planetary physics, because it provides information on astronomical cycles that extends beyond 50 Ma (astronomical models are not accurate beyond this). Assigning time ranges to these astronomical cycles can be used to calibrate Ar/ Ar dating . Uncertainties also arise when using cyclostratigraphy. Using radioisotope dating to set parameters for time scales introduces

368-436: The early 19th century were by Georges Cuvier and Alexandre Brongniart , who studied the geology of the region around Paris. Variation in rock units, most obviously displayed as visible layering, is due to physical contrasts in rock type ( lithology ). This variation can occur vertically as layering (bedding), or laterally, and reflects changes in environments of deposition (known as facies change). These variations provide

391-631: The estimation of sediment-accumulation rates. Cyclostratigraphy Cyclostratigraphy is a subdiscipline of stratigraphy that studies astronomically forced climate cycles within sedimentary successions. Astronomical cycles (also known as Milankovitch cycles ) are variations of the Earth's orbit around the Sun due to the gravitational interaction with other masses within the Solar System . Due to this cyclicity, solar irradiation differs through time on different hemispheres and seasonality

414-561: The intensity of the seasons. Eccentricity influences how much insolation the Earth receives altogether. Varied insolation directly influences Earth's climate, and changes in precipitation and weathering are revealed in the sedimentary record. The 405 kyr eccentricity cycle helps correct chronologies in rocks or sediment cores when variable sedimentation makes them difficult to assign. Indicators of these cycles in sediments include rock magnetism, geochemistry, biological composition, and physical features like color and facies changes. To determine

437-673: The northern Canadian Rockies of Alberta and British Columbia where it reaches thicknesses of up to about 610 metres (2000 ft). It unconformably overlies the Gog Group , and is conformably overlain by the Titkana Formation in the north and the Eldon Formation in the south. It is equivalent to the Mount Whyte , Cathedral , and Stephen Formations of the southern Canadian Rockies. The Snake Indian Formation

460-478: The often cyclic changes in the relative proportions of minerals (particularly carbonates ), grain size, thickness of sediment layers ( varves ) and fossil diversity with time, related to seasonal or longer term changes in palaeoclimates . Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in the rock layers. Strata from widespread locations containing the same fossil fauna and flora are said to be correlatable in time. Biologic stratigraphy

483-580: The time range of a cyclostratigraphic study, rocks are dated using radiometric dating and other stratigraphic methods. Once the time range is calibrated, the rocks are examined for Milankovitch signals. From there, ages can be assigned to the sediment layers based on the astronomical signals they contain. Cyclostratigraphic studies of rock records can lead to accurate dating of events in the geological past, to increase understanding of cause and consequences of Earth's (climate) history , and to better understand controls on depositional mechanisms of sediments and

SECTION 20

#1732786811073

506-419: Was based on William Smith's principle of faunal succession , which predated, and was one of the first and most powerful lines of evidence for, biological evolution . It provides strong evidence for the formation ( speciation ) and extinction of species . The geologic time scale was developed during the 19th century, based on the evidence of biologic stratigraphy and faunal succession. This timescale remained

529-478: Was deposited in shallow marine environments along the western shoreline of the North American Craton during Middle Cambrian time. It is a thick sequence of shale and calcareous shale with interbeds of limestone and siltstone . Mudcracks in the basal shales indicate that there were periods of subaerial exposure during the early stages of deposition. The Snake Indian Formation is present in

#72927