Paleomagnetic and petrographic studies of a quartz sandstone bed from the Silurian Rose Hill Formation of the Central Appalachians show that hematite carrying a Late Paleozoic remagnetization is authigenic. Comparison of paleopole positions from the Rose Hill Formation with Late Paleozoic poles for North America suggests a synfolding remagnetization. The sandstone cement is characterized by pore-lining hematite, pore-filling berthierine and intergrowths of hematite, berthierine and quartz. These cements postdate pressure solution of detrital quartz grains. Late Paleozoic cementation is a consequence of diagenesis associated with Alleghanian deformation. Detrital FeMg silicate grains are believed to have been a local source of Fe for some authigenic hematite and berthierine, although mass balance calculations suggest that an external source is also required. Some berthierine aggregates are pseudomorphic after detrital Fe-bearing silicate grains. Trace amounts of detrital tourmaline are the only remnants of the detrital FeMg silicates.

We have undertaken a comparative study of ARM and IRM anisotropies in Paleozoic carbonate rocks from the Nashville and Jessamine Domes in the Southern Appalachian Basin. The ARMA ellipsoids differ markedly from the IRMA ellipsoids. ARMA appears to reflect a pre-deformation magnetic fabric due to deposition and/or compaction with minimum axes near vertical and a weak lineation. Conversely, IRMA has minimum axes near horizontal and oriented NE-SW, which is compatible with a tectonic fabric due to Alleghanian deformation. Percent ARMA is consistently greater than IRMA. ARM was imparted at low alternating fields (30 mT), and thus ARMA may reflect the fabric residing in low and intermediate coercivity (coarse grained) magnetite. However, the IRM was imparted in saturating fields (300 mT) and appears to be dominated by the single domain fraction, which is believed to carry the observed Kiaman Superchron remagnetization.

A characteristic magnetization has been isolated from 18 sites in Middle Ordovician igneous and sedimentary rocks from the Builth inlier, Wales. Two conglomerate tests demonstrate that the characteristic magnetization in two of the volcanic units was acquired early, perhaps during initial emplacement. Six sites from the northern and central parts of the inlier give a mean direction of D=153.8-degrees, I=69.8-degrees (k=45.8; alpha-95=10-degrees) that passes a regional fold test. However, 12 sites from the southern end of the inlier, in and near the Llanelwedd quarry, yield a significantly different direction after correction for local tilt: D=176.4-degrees, I=44.5-degrees (k=42.4; alpha-95=6.7-degrees). The Llanelwedd quarry data are anomalous with respect to both results from the northern parts of the inlier, and those obtained from approximately coeval rocks from the Shelve inlier. We have searched for an explanation for the discrepancy between the magnetization directions of the two areas and have now ruled out some possibilities, including superimposed magnetization components at one of the two areas. The Llanelwedd quarry hes within a wedge of dominantly andesitic volcanic rocks that appear only locally, and which are overlain by two unconformities that am also not present elsewhere in the inlier. One possible explanation for the anomalous nature of the Llanelwedd quarry magnetization is that the strata were not horizontal at the time of magnetization, either due to early volcano-tectonic tilting or primary dips on the flanks of a volcano. Another possibility is that the Llanelwedd quarry rocks were extruded rapidly enough that the geocentric coaxial dipole field hypothesis cannot be applied. Results from Cenozoic volcanic rocks from the western United States suggest that even highly anomalous fields can be preserved virtually unchanged over a number of separate flow events. We currently regard this to be the most viable explanation for the anomalous nature of the Llanelwedd quarry magnetization. We conclude that the data from the 6 northern sites represent the best estimate of the Middle Ordovician axial dipole field for the inlier.

Paleomagnetic and rock magnetic studies of sedimentary carbonates in the northern Appalachian Basin demonstrate that the late Paleozoic remagnetization in this area is probably related to authigenesis of single-domain magnetite. Fifteen sites within Middle Devonian carbonates were sampled along a transect extending from Albany, New York, to the Algonquin Arch in southeastern Ontario. Paleomagnetic studies show that all sites except the two westernmost sites on the Algonquin Arch were remagnetized during Alleghenian time. The very weakly magnetic unremagnetized sites retain a dual-polarity magnetization of probable Devonian age. In the remagnetized sites, both the size of the Kiaman components and magnetite concentration, estimated from low-field susceptibility (X), are highest in the central part of the transect and lower on either side. X ray powder diffraction analyses demonstrate that K-feldspar is also most abundant in the middle of the transect, with lower concentrations to the west and east. We suggest that formation of authigenic magnetite and K-feldspar may be related in this setting. Sites with the largest Kiaman components have the highest unblocking temperatures and the largest ratios of anhysteretic remanent magnetization (ARM) to bulk susceptibility (ARM/X), indicating that these sites have the greatest amounts of single-domain magnetite. Several remagnetized sites exhibit northerly, shallow magnetic components which may be Kiaman "normal" directions. We conclude that remagnetization along the New York-Ontario transect is a chemical remanent magnetization resulting primarily from authigenesis of single-domain magnetite. We present a model in which pulses of heated fluids triggered magnetite formation. Scanning electron microscope observation of magnetic extracts from 14 sites show that most of the resolvable magnetite grains are multidomain grains of irregular shape; many sites also contain a small percentage of magnetic spheres and/or euhedral magnetite particles. A majority of the magnetite grains contain Fe as the only cation, but many also contain minor amounts of other cations including various combinations of Cr, Ni, Ti, Zn, Mn, Si, Al, Ca, and Mg. Single-domain magnetite is not resolvable with our observational techniques, but its presence is inferred from rock magnetic tests. Much of this magentite may be of authigenic origin.

What has Earth's geography been like during past geological eras? Movements of the continents since the breakup of the supercontinent Pangea about 180 million years ago are very well known from studies of the world's ocean basins. However, Pangea existed for a relatively short interval of geologic time, having assembled from precursor continents late in the Paleozoic era about 250 million years ago. Paleozoic ocean crust has been destroyed by its descent into the Earth's mantle as the Paleozoic continents converged to form Pangea. Thus, there is no ocean floor record to constrain Paleozoic continental movements. The configuration of the continents during the Paleozoic must be inferred from land‐based observations. This work is fraught with difficulties and our knowledge has, therefore, increased slowly.

Reviews of GeophysicsVolume 29, Issue S1 p. 377-383 Geomagnetism and Paleomagnetism The Occurrence and Origin of Remagnetization in the Sedimentary Rocks of North America R. DOUGLAS ELMORE, School of Geology and Geophysics University of Oklahoma, Norman, OklahomaSearch for more papers by this authorCHAD MCCABE, Department of Geology and Geophysics Louisiana State University, Baton Rouge, LouisianaSearch for more papers by this author R. DOUGLAS ELMORE, School of Geology and Geophysics University of Oklahoma, Norman, OklahomaSearch for more papers by this authorCHAD MCCABE, Department of Geology and Geophysics Louisiana State University, Baton Rouge, LouisianaSearch for more papers by this author First published: 1991 https://doi.org/10.1002/rog.1991.29.s1.377Citations: 25Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Citing Literature Volume29, IssueS11991Pages 377-383 RelatedInformation

Crude oil seepage is widespread in carbonate cap rock and in overlying sediments of the Damon Mound salt dome, Brazoria County, Texas. Scanning electron microscopy of calcite-lined cavities provides insight to the latest stage of mineral deposition in this salt dome environment. Solid crude oil residues and microbes occur in association with surface minerals such as gypsum and barite. Deposition of pyrrhotite, pyrite, marcasite, sphalerite, and elemental sulfur is attributed to microbial sulfate reduction, and deposition of aragonite and calcite is related to microbial oxidation of crude oil hydrocarbons. The C1—C4 hydrocarbons in pyrrhotite-bearing cap rock samples are a biodegraded residue derived from crude oil. Higher molecular weight crude oil components in cap rock are altered by water washing and to a lesser extent by biodegradation. Hydrocarbon compositions could be explained by limited aerobic biodegradation, but a component of anaerobic biodegradation cannot be excluded. Some Damon Mound samples are highly magnetic because of abundant ferrimagnetic pyrrhotite. This study provides new evidence of a link between crude oil migration, microbial activity, and diagenetic magnetic anomalies in rocks.

Although it has been known for 25 years that some Paleozoic sedimentary rock units in Europe and North America were remagnetized during Pennsylvanian or Permian time, it is only very recently that the complex and widespread nature of the late Paleozoic remagnetization phenomenon has been generally acknowledged. It is now recognized that many Paleozoic paleomagnetic poles for North America that were once considered reliable are in fact the result of remagnetization, and as a consequence the paleomagnetic data base for the Paleozoic is undergoing rapid and drastic revision. The causes of late Paleozoic remagnetization in North America are currently the focus of much interest and active research. The remagnetization can reside in either hematite or magnetite, and different remagnetization mechanisms have been important in different settings. Chemical remagnetization processes, some related to specific diagenetic events, are dominant in hematite‐bearing sandstones and carbonates. In magnetite‐bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important, but it is difficult to determine which process is the dominant one in some settings. Some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain‐building events that affected much of the continent during the late Paleozoic. Paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids.

Characteristics of the Oaxaca Terrane of southern Mexico suggest that the record of a complete Wilson cycle is present. The local basement is composed of high-grade rocks of the Oaxaca Complex, which is considered to have North American affinities based on Grenvillian lithologies and ages. In contrast, early Paleozoic sedimentary rocks in depositional contact with the Oaxaca Complex have very close faunal affinities with the Olenid-Ceratopygid trilobite province of Argentina. Late Paleozoic faunas indicate that by this time the Oaxaca Terrane was once again associated with North America. In an attempt to gain further insight into the drift history of this area, we have undertaken a paleomagnetic study of the Paleozoic sedimentary rocks of Oaxaca. The results of our study indicate that the entire Paleozoic section was remagnetized in the same paleomagnetic field at some time following late Paleozoic sedimentation, emplacement of an igneous complex, and the earlier of two folding events. Fold and conglomerate tests show that the remagnetization occurred prior to deposition and folding of overlying early Cretaceous sediments. The Oaxaca Paleozoic rocks were therefore remagnetized sometime between late Permian and early Cretaceous. Since the exact age of remagnetization is not known, we compare our result with data from cratonic North America for the bracketed range of magnetization ages. This analysis indicates that as much as 28° of net counterclockwise rotation could have occurred between Oaxaca and cratonic North America subsequent to the Oaxaca remagnetization.

Research Article| January 01, 1987 Occurrence of secondary magnetite within biodegraded oil Chad McCabe; Chad McCabe 1Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803 Search for other works by this author on: GSW Google Scholar Roger Sassen; Roger Sassen 1Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803 Search for other works by this author on: GSW Google Scholar Barbara Saffer Barbara Saffer 1Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803 Search for other works by this author on: GSW Google Scholar Author and Article Information Chad McCabe 1Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803 Roger Sassen 1Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803 Barbara Saffer 1Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (1): 7–10. https://doi.org/10.1130/0091-7613(1987)15<7:OOSMWB>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Chad McCabe, Roger Sassen, Barbara Saffer; Occurrence of secondary magnetite within biodegraded oil. Geology 1987;; 15 (1): 7–10. doi: https://doi.org/10.1130/0091-7613(1987)15<7:OOSMWB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Samples of solid bitumen from the Thornton Quarry (Illinois) and the Cynthia Quarry (Mississippi) were found to be strongly magnetic and to have rock magnetic properties suggesting that the magnetizable grains present are magnetite. Studies of magnetic isolates revealed that magnetite is present primarily as spherical crystal aggregates that appear identical to magnetite spherules isolated from re-magnetized Paleozoic carbonate units from other localities. Organic geochemical analyses of the solid bitumen suggest an origin by microbial attack on what once was liquid crude oil. The occurrence of secondary magnetite as inclusions within solid bitumen suggests a relationship between crude oil biodegradation and development of that mineral in our samples. We infer that secondary magnetite in other geologic environments may be related to the presence of hydrocarbons. The discovery of a natural association of secondary magnetite and hydrocarbons has important implications for paleomagnetism and for petroleum exploration. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A Middle/Late Silurian paleomagnetic pole has been obtained from reefal limestones in north central Indiana. Eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization. One additional site gave the same characteristic direction after great circle analysis. These 12 site‐level directions were used to calculate the mean paleomagnetic pole. Reef flank beds dip steeply (about 38°) away from reef centers. Field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds. Present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12°. A 12° tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement, indicating that the magnetization was acquired early. The presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence. The mean of the site paleomagnetic poles (with sites from steeply inclined flank beds corrected for 12° of tilt) is 16.8°N, 124.9°E (K = 74, A 95 = 5.1°), in good agreement with the Middle Silurian Rose Hill pole for North America.

One hundred eighty-eight oriented samples of the Late Cambrian Bonneterre carbonates of southeastern Missouri were taken from Viburnum Trend mines and local surface outcrops. Initial natural remanent magnetization directions have south- southeasterly declinations and downward inclinations. Progressive demagnetizations using thermal and AF techniques give two directions: one is a viscous present-day field direction, and the other is of reversed polarity, with south-southeasterly declinations and very shallow inclinations. The resulting paleopole, at 43øN, 126øE, is located near the Pennsylvanian to Early Permian segment of the North American apparent polar wander path. A negative conglomerate test supports this assignment for the magnetization age. Magnetic extracts from the mineralized as well as unmineralized zones of the Bonneterre contain spherical crystal aggregates of pure magnetite of late diagenetic origin. The secondary magnetization resides in the magnetite spheroids, and we infer that the spheroids formed prior to sulfide mineralization.

One hundred eighty‐eight oriented samples of the Late Cambrian Bonneterre carbonates of southeastern Missouri were taken from Viburnum Trend mines and local surface outcrops. Initial natural remanent magnetization directions have south‐southeasterly declinations and downward inclinations. Progressive demagnetizations using thermal and AF techniques give two directions: one is a viscous present‐day field direction, and the other is of reversed polarity, with south–southeasterly declinations and very shallow inclinations. The resulting paleopole, at 43°N, 126°E, is located near the Pennsylvanian to Early Permian segment of the North American apparent polar wander path. A negative conglomerate test supports this assignment for the magnetization age. Magnetic extracts from the mineralized as well as unmineralized zones of the Bonneterre contain spherical crystal aggregates of pure magnetite of late diagenetic origin. The secondary magnetization resides in the magnetite spheroids, and we infer that the spheroids formed prior to sulfide mineralization.