Geophysical characterization of the Precambrian basement in northwest Pennsylvania, U. S.
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Summary Few integrated studies combining different geophysical data sets have concentrated on the Precambrian basement in the northwestern part of the Pennsylvanian Appalachian basin. In this study, we present an indirect method for characterizing the structure and composition of the Precambrian basement utilizing newly acquired magnetic and gravity data. Depth-to-basement estimates made by using the radially averaged power spectrum analysis, Standard Euler calculations and 3-D model inversions over the gravity and magnetic data indicate general agreement with a sedimentary basin having a base at approximately −9,000 to up to −15,000 ft subsea. A gently dipping Precambrian basement with laterally varied density and magnetic susceptibility with a significant high susceptibility area found within the basement likely due to lithological variations within this unit. These results may have implications for areas of enhanced oil and gas production found within the region.Keywords:
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Summary Few integrated studies combining different geophysical data sets have concentrated on the Precambrian basement in the northwestern part of the Pennsylvanian Appalachian basin. In this study, we present an indirect method for characterizing the structure and composition of the Precambrian basement utilizing newly acquired magnetic and gravity data. Depth-to-basement estimates made by using the radially averaged power spectrum analysis, Standard Euler calculations and 3-D model inversions over the gravity and magnetic data indicate general agreement with a sedimentary basin having a base at approximately −9,000 to up to −15,000 ft subsea. A gently dipping Precambrian basement with laterally varied density and magnetic susceptibility with a significant high susceptibility area found within the basement likely due to lithological variations within this unit. These results may have implications for areas of enhanced oil and gas production found within the region.
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Recent compilation of data by the Kansas Geological Society's Basement Rocks Committee regarding the Precambrian in part of the Mid-Continent provides a framework in which to analyze further this rock complex. In Kansas alone, more than 2,100 wells are known to have penetrated the Precambrian, and approximately 50-60 tests a year are drilled into the basement. Studies are now in progress to attempt to determine detailed spatial relationships of these rocks and their intricate geologic history. By using only preliminary information, it is possible to differentiate general categories of rock types at the Precambrian surface, which in Kansas is buried beneath Paleozoic rocks at depths from 500 to 9,000 feet. Rocdk types recognized include granite, granodiorite, syenite, diabase, rhyolite, and metasediments; the interrelations of these are exceedingly complex. Sediments, chiefly alternating silicate-cemented sandstone and indurated shale, have been described from Missouri and may be abundant elsewhere. Outliers of schist capped by resistant quartzite form buried hills in central Kansas. Diabase and related types of mafic rocks are found; syenite may be in the form of intrusive plugs. Extrusive rocks are represented by the rhyolite and associated suites. Granite of various kinds is by far the most extensively recorded rock type in the Mid-Continent; granodiorite has very limited distribution. In many areas, the deeply weathered and perhaps even reworked pre-Reagan (or Lamotte) basement rock constitutes granite wash assumed here to be Precambrian. Geographic distribution of different rock types is suggestive of pre-Paleozoic structure. A wide band of metasediments through central Missouri, northeastern Kansas, northeastern Nebraska, and southeastern South Dakota forms a large arc convex southwest perhaps outlining the southwestern flank of the old Wisconsin Highlands. Present dip of the metasediments in west-central Missouri is known to be southwestward. Outside the belt of metasediments are igneous rocks and some metasediment outliers. Potassium-argon ages determined by J. L. Kulp on five samples from Barton, Rush, and Morris Counties, Kansas, yielded dates of 1,165 to 1,460 million years, comparable with ages elsewhere in the central United States. End_of_Article - Last_Page 272------------
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Abstract Abstract The central Lockne area is located in the Caledonian thrust front in Jämtland. Though most of the Caledonian orogenesis is thought to have influenced only the Palaeozoic sedimentary rocks that were deformed in a décollement-style, there is clear evidence that at least some Palaeozoic deformation of the Precambrian basement has taken place. In the described area the basement is brecciated within a NW-SE trending zone. This brecciation has occurred during at least two episodes, one occurring in pre-Aseri time; another is not older than of Dalby age. They are perhaps connected with Palaeozoic reactivation of Precambrian (transform?) faults. In a new quarry in the vicinity of Lockne church, crushed Precambrian crystalline rocks overlie Cambrian alum shales. This suggests that Caledonian thrusting may influence the basement in this part of Jämtland. Key Words: Lockne areaJämtlandSwedenbrecciaPrecambrian basementsedimentary dykesalum shaleasphaltiteoverthrust granite-gneiss
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Divergent views exist as to whether the Precambrian basement under the Songliao basin exists or not. Older zircons were discovered in a number of samples during the SHRIMP zircon dating of Early Cretaceous volcanic rocks in the northern Songliao basin, and their ages indicate that they mainly formed in the Precambrian. It is suggested that they were probably captured from the basement rocks during Early Cretaceous volcanic eruption. Three age groups have been broadly distinguished for the captured zircons:(1848±34) Ma, (1600±29) Ma and(1293±41) Ma, which implies that the basement of the northern Songliao basin experienced multiple tectono-magmatic events during the Paleo- and Mesoproterozoic. Therefore, it is believed that the Precambrian basement exists under the northern Songliao basin.
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In 1968, the Missouri Geological Survey (MGS) established the Operation Basement program to address three objectives: a) to obtain drill hole and underground mining data relative to the structure and composition of the buried Precambrian basement; b) to expand mapping in the Precambrian outcrop area and conduct research related to Precambrian geology and mineral resources; and c) to publish the results of the first two objectives in the Contribution to Precambrian Geology series (Kisvarsanyi, 1976). The database presented here represents the first of those objectives. It was originally compiled in close cooperation with exploration and mining companies operating in Missouri, who provided drillhole data, core and rock samples to MGS. These data enabled geologists to study otherwise unexposed basement rocks from a large area of the state for the first time, allowing better classification and understanding of the Precambrian basement across the state. MGS is continuing data collection and database compilation today as information becomes available, furthering our knowledge of the Missouri Precambrian basement. This effort was supported through a cooperative agreement with the Mineral Resource Program of the U.S. Geological Survey.
As of September 13, 2017, the Microsoft Access database OperationBasement.accdb has information from 915 drill holes that penetrated the Precambrian basement tabulated by county. When available, core logs and petrographic descriptions are included. Location information provided is as specific as possible with a field denoting location impediments.
References:
Kisvarsanyi, Eva B., 1974, Operation Basement: buried Precambrian rocks of Missouri their petrography and structure: American association of Petroleum Geologists Bulletin, v. 58, p. 674-684
Kisvarsanyi, Eva B., 1975, Data on Precambrian in drillholes of Missouri including rock type and surface configuration (Contribution to Precambrian Geology No. 5): Mo. Dept. of Natural Resources, Geol. Survey, Rept. 56, 24 p.
Kisvarsanyi, Eva B., 1976, Missouri Precambrian Revisited: Progress in Studies of Precambrian Geology, 1961-1976: Mo. Dept. of Natural Resources, Geol. Survey, Rept. 61, p. 1-11
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Skaly prekambryjskie tworzą podloze calego kraju, lecz zostaly szczegolniej wyroznione i zbadane w trzech regionach: na prekambryjskiej platformie wschodnioeuropejskiej i w Karpatach, gdzie tworzą fundament krystaliczny ukryty pod skalami pieter osadowych, oraz na Dolnym Śląsku, gdzie są wlączone w system faldow waryscyjskich, wypietrzone i odsloniete. Skaly prekambryjskie roznią sie nie tylko polozeniem, ale takze budową tektoniczną i strukturalną, metamorfizmem regionalnym, wiekiem i stylem przebudowy. Odmiennie ksztaltowaly sie dzieje ich badan i niejednolity jest stopien rozpoznania. THE PRESENT STATE AND DIRECTIONS OF STUDIES ON THE PRECAMBRIAN ROCKS OF POLAND Although the whole basement of Poland is built of Precambrian rocks, these rocks were recorded and recognized in three regions only. They form crystalline basement overlain by rocks of sedimentary stages in the Precambrian Platform and the Carpathians, being incorporated in the Variscan fold system, uplifted and exposed in the Lower Silesia. Precambrian rocks of these three regions differ in location as well as in tectonic and structural units, regional metamorphism, age and style of reactivation. The history of studies of these rocks is also different, similarly as the degree of knowledge. In Poland, these rocks built crystalline basement on which rocks of Paleozoic and Meso-Cenozoic sedimentary stages were originating and undergoing structural rebuilding. The development of the latter was determined by movements of crystalline basement blocks, responsible for regional and tectonic differentiation of the sedimentary covers. The studies of crystalline basement of the East-European Platform were initiated in 1954 by making three drillings preceded by geophysical surveys. In the next years, there took place intense development in there studies, determined by search for metal ore deposits and bitumens in the sedimentary cover of the Platform. The geological studies on the crystalline basement are concentrated in the Geological Institute which makes possible appropriate planning of the works and synchronization of geophysical, drilling and mapping programs. On the basis of results of petrographic, mineralogical, geochemical, geochronological and deposit studies of core material from almost 200 boreholes, and rich geophysical data (including those from magnetic and gravimetric semi-detailed and detailed mapping), it was possible to prepare petrological map of the top of crystalline basement in the scale 1:500 000 and, subsequently, the map in the scale 1:200 000 in 34 sheets. The tectonic model and stratigraphic scheme of rocks of the crystalline basement were also prepared. The studies have been crowned with discovery of vanadium-bearing titanomagnetite deposit in anorthosites in the Suwalki area and demonstration of its resources. Moreover, chemogenic hematite taconites were found at Łochow near Warsaw at 2400 m depth, and concentrations of rare earth elements – in nepheline syenites and carbonatites in the areas of Elk and Tajno. The history of studies on the Precambrian basement of the Carpathians and southern Poland began when crystalline rocks were found by a drilling at Rzeszotary in 1909. However, at that time it was not certain whether the drilling entered an exotic or autochtoneous crystalline massif. This question has not been solved before the end of the sixties, when deep drillings penetrated the Carpathian Flysch and entered crystalline basement. The knowledge of crystalline basement in this part of Poland is unsatisfactory as petrological maps, tectonic model and stratigraphic scheme are still missing. The available data show that the basement is strongly eroded ill the west, where the Carpathian Flysch rests on amphibolites fades metamorphic rocks, whereas in the east the Flysch rests on strongly diagenesed or weakly metamorphic rocks of the uppermost Precambrian. After the world war II, the Geological Institute began studies on crystalline rocks of the Lower Silesia, with detailed geological mapping of areas for which maps were still missing. Up to the present, 100 sheets of geological map in the scale 1: 25000 are completed and the remaining ones are in preparation. To carry out programs of stratigraphic, lithological, petrographic and tectonic studies and search for deposits, there has been called into existence the Lower Silesian Branch of the Geological Institute in Wroclaw. Along with time, the Lower Silesian Branch of the Geological Institute started introducing modern research techniques, as e.g. mesoscopic analysis in studies on tectonics, petrotectonic studies, geochemical analysis, analysis of radiogenic age, physical studies of rocks, geophysical studies, and biostratigraphic studies of “silent rock series”. The developments in the studies were accompanied by widening their range by deep drillings, especially in the forefield of the Sudety Mts. The results of these studies markedly changed views on stratigraphy, lithological development and metamophic alterations of the Precambrian.
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