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16-1121: Dorrite was first reported in 1982 by A. Havette in a basalt-limestone contact on Réunion Island off of the coast of Africa. The second report of dorrite was made by Franklin Foit and his associates while examining a paralava from the Powder River Basin , Wyoming in 1987. Analyses determined that this newly found mineral was surprisingly similar to the mineral rhönite, lacking Ti but presenting dominant Fe in its octahedral sites. Other minerals that coexist with this phase are plagioclase , gehlenite - akermanite , magnetite -magnesioferrite-spinel solid solutions, esseneite , nepheline , wollastonite , Ba-rich feldspar , apatite , ulvöspinel , ferroan sahamalite , and secondary barite , and calcite . Dorrite can be found in mineral reactions that relate dorrite + magnetite + clinopyroxene , rhönite + magnetite + olivine + clinopyroxene , and aenigmatite + pyroxene + olivine assemblages in nature. These assemblages favor low pressures and high temperatures. Dorrite

32-453: A change in depositional environment, but with only a minor hiatus in deposition with no significant erosion. A brief hiatus without erosion is known as a diastem . Gradational contacts occur where the change in depositional environment takes place over a longer period of time. They are further divided into progressive gradual contacts where the change in lithology is more or less continuous, or intercalated contacts where thin beds of

48-477: A form of secondary structure in the rock beds. Conformable contacts represent no time gap in the geologic record. They are usually planar, though they may have slightly irregular topography. These contacts represent continual, uninterrupted deposition and accumulation of sedimentary rocks, or represent lava flows. A conformable contact can be abrupt , where the contact separates beds of distinct lithology. Abrupt contacts coincide with bedding planes and represent

64-416: A new lithology appear at the base of the contact zone, become thicker upwards, and completely replaced the old lithology at the top of the contact zone. Closely related to intercalated contacts is interfingering (also known as intertonguing , interdigitating , or interlocking ) of laterally adjacent sedimentary rock bodies. Here the contact breaks down into a series of wedges or tongues that penetrate

80-421: A physical break in stratigraphy. Fault surface contacts show discrete breaks and have an attitude and position which describes the contact between two formations. These fault surfaces can be polished into slickensided surfaces which depict striations in the direction of the fault movement. Shear zones are different as there is no physical break displayed, but there is displacement. Identifying and understanding

96-461: Is a boundary which separates one rock body from another. A contact can be formed during deposition , by the intrusion of magma , or through faulting or other deformation of rock beds that brings distinct rock bodies into contact. The geologic subdiscipline of stratigraphy is primarily concerned with depositional contacts, while faults and shear zones are of particular interest in structural geology . Faults and shear zones can be regarded as

112-411: Is intruded by an igneous body of rock which formed when magma forced upward through fractures , or melted through overlying rock . Magma then cooled into solid rock, different from the surrounding country rock. Sometimes, a fragment of country rock will break off and become incorporated into the intrusion, and is called a xenolith , from Greek , ξένος , xenos , "strange,", and λίθος , lithos ,

128-673: Is stable in strongly oxidizing , high-temperature, low-pressure environments. It occurs in paralava, pyrometamorphic melt rock, formed from the burning of coal beds. Researchers conclusively determined that dorrite is triclinic-pseudomonoclinic and twinned by a twofold rotation about the pseudomonoclinic b axis. The parameters for dorrite are a=10.505, b=10.897, c=9.019 Å, α=106.26°, β=95.16°, γ=124.75°. Calcium 8.97% Magnesium 5.44% Aluminum 6.04% Iron 37.48% Silicon 6.28% Oxygen 35.79% CaO 12.55% MgO 9.02% Al 2 O 3 11.41% Fe 2 O 3 53.59% SiO 2 13.44% Contact (geology) A geological contact

144-417: Is typically less dense than the rock it intrudes, widening and filling existing cracks, sometimes melting the already-existing country rock. The term "country rock" is similar to, and in many cases interchangeable with, the terms basement and wall rocks . Country rock can denote the widespread lithology of a region in relation to the rock which is being discussed or observed. Settings in geology when

160-455: The adjacent rock body and pinch out individually. These record fluctuations in the location of the boundary between different depositional environments where sediments were simultaneously being laid down. Unconformities are gaps in the geologic record within a stratigraphic unit. These gaps can be caused by periods of non-deposition or by erosion. As a result, two adjacent rock units may have significantly different ages. Intrusive contacts are

176-779: The beginning of the Paleocene Series. Located in Tunisia, the contact is described as a reddish layer at the base of a dark clay layer. This reddish layer is the Iridium Anomaly , representative of the fallout of the major impact that resulted in the mass extinction event that ended the Cretaceous. Country rock (geology) In geology , country rock is the rock native to an area, in contrast to any intrusion of viscous geologic material, commonly magma , or perhaps rock salt (in salt domes ) or unconsolidated sediments . Magma

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192-620: The immense heat from the intruding body in numerous ways. Chill margins are created if the magma is cooled too quickly to fully crystallise. The result is a distinct boundary of very fine grain igneous rock along the border of the country rock. The surrounding rock may be "baked" through contact metamorphism , resulting in non-foliated metamorphic rocks. Rocks that were originally limestone, quartz sandstone, and shale become marble , quartzite , and hornfels , respectively. Fault and shear zone contacts can be represented by either discrete breaks and discontinuities, or ductile deformation without

208-408: The relationship between contacts is important in determining relative ages of rocks and formations. Contacts are a key feature used to create geological maps. Cross-cutting relationships of these contacts can be used to determine the relative geological history of an area or of an outcrop. The golden spikes on geologic timescales represent internationally agreed upon references for the boundaries of

224-534: The stages in the geologic timescale. These contacts have been identified by the International Commission on Stratigraphy and they are known as GSSPs , or global boundary stratotype section and points. Some of these boundary points are at physical locations, while others are in ice drill core sections, or have been defined chronometrically. The GSSP for the Danian Stage marks the end of the Cretaceous and

240-462: The surfaces between host (or country ) rock and an intrusive magmatic body. The older country rock is crosscut by a younger magmatic body. The nature of the intruding body depends on its composition and depth. Common examples are igneous dikes, sills, plutons , and batholiths . Depending on the composition of the magma, the intrusive body may have a complex internal structure which can provide insight into its emplacement. The country rock responds to

256-495: The term country rock is used include: When describing a pluton or dike , the igneous rock can be described as intruding the surrounding country rock , the rock into which the pluton has intruded. When country rock is intruded by a dike, perpendicular to the bedding plane , it is called discordant intrusion, while a parallel intrusion by a sill indicates a sub-parallel or concordant intrusion . Most intrusions into country rock are via magma . Usually, country rock

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