Misplaced Pages

#918081

25-564: The Porcupine Hills Formation is a stratigraphic unit of middle to late Paleocene age in the Western Canada Sedimentary Basin . It takes its name from the Porcupine Hills of southwestern Alberta , and was first described in outcrop by George Mercer Dawson in 1883. The Porcupine Hills Formation is composed primarily of mudstones , siltstones and sandstones . The sediments were derived from

50-399: 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

75-454: 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

100-402: 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

125-557: 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

150-436: 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

175-411: 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

200-467: Is slightly different as the processes involved in laying down archaeological strata are somewhat different from geological processes. Human-made intrusions and activity in the archaeological record need not form chronologically from top to bottom or be deformed from the horizontal as natural strata are by equivalent processes. Some archaeological strata (often termed as contexts or layers) are created by undercutting previous strata. An example would be that

225-413: 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

250-472: Is the present day erosional surface , so its original thickness can not be determined; however, the maximum recorded thickness is 1,200 metres (3,940 ft) in the Porcupine Hills. The Porcupine Hills Formation is exposed at surface or is overlain by Quaternary sediments. Its base is a thick sandstone that rests disconformably on a paleosol horizon of the underlying Willow Creek Formation . In

275-643: The Canadian Cordillera during tectonic uplift and erosion in the late stages of the Laramide Orogeny , and were transported eastward by river systems and deposited in fluvial and floodplain environments. The mudstones are characterized by well-developed paleosols and caliche nodules, and the sandstones are cross-bedded and cemented with calcite . In contrast to the Paskapoo Formation , an equivalent formation farther to

SECTION 10

#1732801443919

300-421: The law of superposition , the principle of original horizontality and the principle of lateral continuity in 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

325-564: 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

350-615: 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

375-407: 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

400-417: The estimation of sediment-accumulation rates. Law of superposition The law of superposition is an axiom that forms one of the bases of the sciences of geology , archaeology , and other fields pertaining to geological stratigraphy . In its plainest form, it states that in undeformed stratigraphic sequences, the oldest strata will lie at the bottom of the sequence, while newer material stacks upon

425-591: The importance of fossil markers for correlating strata; he created the first geologic map of England. Other influential applications of stratigraphy in 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

450-719: The north near the Bow River, the Porcupine Hills Formation grades into the equivalent Paskapoo Formation. It is correlated with upper part of the Ravenscrag Formation of southern Saskatchewan and the Fort Union Formation of Montana and North Dakota, which are not contiguous with it but are of similar age. Stratigraphy Catholic priest Nicholas Steno established the theoretical basis for stratigraphy when he introduced

475-579: The north, there are no coaly beds. This has given rise to the hypothesis that during the deposition of the Porcupine Hills Formation the climate was semi-arid , while the climate farther north was more humid. The Porcupine Hills Formation is present in southwestern Alberta, from the Waterton River near the Canada–US border in the south to the Bow River near Calgary in the north. The upper limit

500-478: 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

525-498: The oldest layers on the bottom, thus enabling paleontologists and paleobotanists to identify the relative ages of any fossils found within the strata, with the remains of the most archaic lifeforms confined to the lowest. These findings can inform the community on the fossil record covering the relevant strata, to determine which species coexisted temporally and which species existed successively in perhaps an evolutionarily or phylogenetically relevant way. The law of superposition

SECTION 20

#1732801443919

550-412: The silt back-fill of an underground drain would form some time after the ground immediately above it. Other examples of non vertical superposition would be modifications to standing structures such as the creation of new doors and windows in a wall. Superposition in archaeology requires a degree of interpretation to correctly identify chronological sequences and in this sense superposition in archaeology

575-410: The surface to form new deposits over time. This is paramount to stratigraphic dating , which requires a set of assumptions, including that the law of superposition holds true and that an object cannot be older than the materials of which it is composed. To illustrate the practical applications of superposition in scientific inquiry, sedimentary rock that has not been deformed by more than 90° will exhibit

600-419: 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

625-721: Was first proposed in 1669 by the Danish scientist Nicolas Steno , and is present as one of his major theses in the groundbreaking seminal work Dissertationis prodromus (1669). In the English-language literature, the law was popularized by William "Strata" Smith , who used it to produce the first geologic map of Britain. It is the first of Smith's laws , which were formally published in Strata Identified by Fossils (1816–1819). Superposition in archaeology and especially in stratification use during excavation

#918081