Chad McCabe

Diagenetic magnetite carries ancient yet secondary remanence in some Paleozoic sedimentary carbonates

Many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite. When such magnetizations are observed, it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition. However, paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the Helderberg and Bonneterre carbonates (United States) indicate that the magnetite present in these rocks is most likely of diagenetic (i.e., postdepositional) origin.

Comparison of magnetic hysteresis parameters of unremagnetized and remagnetized limestones

For white magnetite-bearing Mesozoic pelagic limestones from Italy which carry a “primary” magnetization, the values of Mrs/Ms and Hcr/Hc generally lie in the pseudo-single domain (PSD) field of the Day et al. (1977) plot. The logarithmic plot of Mrs/Ms against Hcr/Hc gives a straight line (R = 0.814) with slope and intercept close to the empirical mixing line of Parry (1982) for single domain (SD) and multidomain (MD) magnetite. For one of the white pelagic limestone formations (Maiolica Formation), samples with hysteresis ratios closer to the MD field display increased paramagnetic susceptibility and are from the upper part of the formation characterized by increased detrital clay. We therefore associate the increased MD magnetite with increased detrital influx. For pinkish and reddish varieties of the Italian pelagic limestones, the presence of hematite is manifest by high saturation fields, a wide range of Hcr/Hc, and “wasp-waisted” hysteresis loops attributed to the mixing of magnetite and high-coercivity authigenic hematite. The hysteresis ratios for a collection of Paleozoic and Mesozoic remagnetized magnetite-bearing limestones from Britain, Nevada, Alaska and the Appalachians lie mainly outside the PSD field and appear to follow a power law trend. Following Jackson et al. (1993), the high values of Hcr/Hc and the characteristically “wasp-waisted” hysteresis loops can be interpreted in terms of a fine-grained subspherical high-coercivity SD magnetite mixed with a high proportion of superparamagnetic magnetite. The slope and intercept of the power law relationship for Mrs/Ms and Hcr/Hc in the remagnetized limestones are distinct from those observed for the Italian limestones, and may provide a means of fingerprinting magnetite of “primary” as opposed to diagenetic origin.

Occurrence of secondary magnetite within biodegraded oil

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.

Magnetite authigenesis and diagenetic paleotemperatures across the northern Appalachian basin

The concentration of magnetite in the Lower and Middle Devonian Helderberg and Onondaga carbonate formations varies in a distinct pattern along an east-west profile across New York State. Magnetite concentrations are uniformly low in the western half of the profile, increase sharply eastward to a peak value near Syracuse, and decrease farther east. The pattern strongly resembles previously reported variations in the extent of clay mineral diagenesis that reflect differences in paleotemperatures along the profile. Previous paleomagnetic studies have documented that the magnetite carries a well- defined magnetization of Pennsylvanian-Permian age, and magnetite authigenesis is therefore no younger than late Paleozoic. We attribute the correlation between high magnetite concentration and high illite content to temperature-dependent diagenesis triggered by orogenic fluids. The large proportion of secondary magnetite indicates that over most of the area, the mechanism of late Paleozoic remagnetization was principally chemical. Thermal effects played a less direct role in the remagnetization of the strata by controlling the extent of the chemical processes that resulted in overprinting of the original remanence.

Late Paleozoic remagnetization in limestones of the Craven Basin (northern England) and the rock magnetic fingerprint of remagnetized sedimentary carbonates

Samples of Lower Carboniferous Chatburn and Pendleside limestones collected from 27 sites in the Craven Basin (northern England) carry a syn-Hercynian remagnetization residing in magnetite. The paleomagnetic directions are all of reversed polarity consistent with a Kiaman Superchron remagnetization. The partially (62%) tilt corrected mean direction has the minimum correlation statistic implying a synfolding magnetization. The resulting pole appears to be Late Carboniferous in age, and there is no indication of any magnetization component that predates the remagnetization event. The magnetization appears to be of the same type as the synfolding remagnetizations documented in several Appalachian carbonate units. Jackson (1990) reported that remagnetized Appalachian carbonates have distinctive hysteresis properties, and we have obtained similar hysteresis loops from remagnetized Paleozoic carbonates from the Craven Basin (England), the Great Basin (Nevada), Alaska, and New York State. The hysteresis loops are all “wasp-waisted” and yield highly anomalous hysteresis ratios, compared to data from synthetic or “typical” magnetite-bearing samples. In contrast, unremagnetized limestones from Italy show hysteresis behavior which is more typical for magnetite-bearing rocks (Channell and McCabe, this issue). These results suggest that remagnetization in magnetite-bearing carbonates can be readily detected on the basis of hysteresis properties, prior to a full-scale paleomagnetic investigation.

Paleomagnetism of the Upper Silurian and Lower Devonian carbonates of New York State: evidence for secondary magnetizations residing in magnetite

Abstract Paleomagnetic directions for the Upper Silurian and Lower Devonian carbonates of the Helderberg escarpment (New York State) differ from expected Late Silurian and Early Devonian directions for cratonic North America. The mean direction (D = 165°, I = −10°; paleopole at 50°N 129°E) is similar to Late Carboniferous and Early Permian results. Negative fold tests, and a lack of reversals, suggest that the magnetization is secondary. However, low coercivities, low blocking temperatures, the thermomagnetic curves (TC near 570°C) and the acquisition of isothermal remanent magnetizations all suggest that the remanence is carried by magnetite. If a detrital origin of these magnetites is assumed, the secondary nature of the remanence would argue for thermal resetting as a result of deep burial of the rocks. However, no evidence for such thermal resetting is seen in the alteration of conodonts. More likely perhaps is a chemical or thermochemical origin of the remanence; this would require the magnetites to be authigenic.

Paleomagnetic results from volcanic rocks of the Shelve Inlier, Wales: evidence for a wide Late Ordovician Iapetus Ocean in Britain

Abstract In order to shed light on the early Paleozoic paleogeography of Iapetus-bordering continents, samples for paleomagnetic study were collected from middle Ordovician (Llanvirn) volcanic rocks of the Shelve Inlier, Welsh Borderland. The folding within the inlier is constrained to late Ordovician (Ashgill) time, thus offering a fold test with tight age brackets. After stepwise thermal demagnetization, a pre-folding characteristic magnetization component was isolated from 95 specimens. The mean of 11 site mean directions for the characteristic magnetization gives D/I=116.4/+ 67.9(k = 89.4; α 95 = 4.9° ), and the paleomagnetic pole lies at 26.6°N, 36.1°E ( δp = 6.9°, δm = 8.2° ). The fold test is positive at the 99% confidence level, indicating that the magnetization was acquired during the middle or late Ordovician. We infer from geological considerations that this magnetization is not a primary thermoremanence, but was acquired during late Ordovician time as a result of a thermochemical disturbance of that age. Our results indicate that southern Britain was near 51° south latitude during the later Ordovician. Since middle to late Ordovician paleolatitudes for Gondwana and Laurentia are well-determined, our result can be used to test various scenarios for the closure of Iapetus. We conclude that: (1) the Iapetus Ocean in Britain still had a latitudinal width of some 30° at this time, thus rendering a postulated Ordovician closure unlikely; (2) southern Britain, Avalon, and Hercynian Europe all occupied high latitudes during the later Ordovician, consistent with a paleoposition near the northern margin of Gondwana; and (3) the Acadian orogeny in Britain and North America could have been the result of the collision of Gondwana and Laurentia during early Devonian time.

A genetic link between remagnetization and potassic metasomatism in the Devonian Onondaga Formation, Northern Appalachian Basin

Whole-rock chemical analyses indicate that the Onondaga Formation along a Rochester-Syracuse (New York) transect has a marked eastward increasing trend in K/Al ratio, which correlates with an increase in authigenic magnetite content and degree of diagenetic illitization. An eastward increase in the content of some authigenic minerals including K-feldspar, quartz, and Fe-rich chlorite in the rocks is indicated by petrographic and X-ray diffraction studies. These observations suggest a link between remagnetization and potassic alteration of Fe-bearing silicates in the Onondaga. Two hypothetical reactions are proposed in which magnetite authigenesis is related to the breakdown of Fe-bearing smectite in the presence of potassium. Products of these reactions include authigenic K-feldspar, illite, quartz and chlorite, as well as magnetite. Our results suggest that remagnetization was triggered by a basin-wide migration of K-rich fluids during the Alleghenian Orogeny.

Anhysteretic remanent magnetic anisotropy and calcite strains in Devonian carbonates from the Appalachian Plateau, New York

Jackson, M., Craddock, J.P., Ballard, M., Van der Voo, R. and McCabe, C., 1989. Anhysteretic remanent magnetic anisotropy and calcite strains in Devonian carbonates from the Appalachian Plateau, New York. Tectonophysrcs. 161: 43-53. Anisotropy of anhysteretic susceptibility (AAS) is a recently developed high-resolution method of measuring the magnetic fabric of rocks. In order to test the applicability and limitations of AAS for estimation of strain orientations in weakly-deformed and weakly magnetic rocks, we have used the method to examine the magnetic fabric of samples from a series of sites in limestones of the Helderberg and Onondaga formations along a 500-km E-W transect across New York State. Two distinct shortening directions have been previously identified and interpreted in terms of two separate phases of Alleghenian deformation. Over most of the transect, minimum anhysteretic susceptibility axes within the plane of bedding closely parallel the compression direction of the earlier (“Lackawanna”) phase. A few sites show minimum anhysteretic susceptibility parallel to the later (“Main”) phase. The threshold for resolution of the tectonic signal by AAS is at anhysteretic susceptibilities of about 2 X lo-’ (SI) and strain magnitudes of 0.5 to 1% as recorded by twinning in calcite. The central part of the transect exhibits minimum horizontal anhysteretic susceptibility perpendicular to the inferred tectonic compression, rather than parallel to it. We attribute this to either: (a) anisotropic transmission of stresses from the larger calcite matrix grains to the smaller magnetite grains during twinning; or (b) preferential recording of a late-stage non-coaxial stress direction in this area by late diagenetic magnetite.

Deposition of magnetic pyrrhotite during alteration of crude oil and reduction of sulfate

Abstract 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.