Correlating paleomagnetic, geochemical and petrographic evidence to date diagenetic and fluid flow events in the Mississippian Turner Valley Formation, Moose Field, Alberta, Canada (original) (raw)
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Canadian Journal of Earth Sciences, 2000
The Rainbow Field is in reefal carbonates of the Middle Devonian Keg River Formation in the Western Canada Sedimentary Basin. Alternating field and thermal step demagnetization was done on specimens from unoriented core from a vertical, an inclined, and a horizontal well core in the dolomite reservoir. Although they had no viscous remanent magnetization component to use for orientation, most specimens had a well-defined characteristic remanent magnetization (ChRM) that resides in single to pseudosingle domain magnetite. By rotating the mean ChRM direction around its core axis, a small circle can be generated for each core and the small circles intersect in the true ChRM direction of D = 168°, I = -73.5 o (α 95 = 5.8°, k = 32.2). Its paleopole of 164°E, 83°N (A 95 = 10°) defines a Tertiary age with one sigma limits of Middle Eocene to Middle Miocene age. Petrographic examination defines four generations of dolomite. Matrix dolomite has 60-100 µm diameter crystals that were later recrystallized to 200-400 µm. Dolomite cements are represented by vug-filling coarse dolomite (100-200 µm) and saddle dolomite (1000 µm). All four generations of dolomite give similar δ 18 O values of -10.7 to -16.5‰ (Peedee Belemnite, PDB), δ 13 C values of +0.7 to +3.2‰ (PDB), and Sr isotopic ratios of 0.70826 to 0.70846 that do not match the expected Middle Devonian carbonate or seawater values. We interpret these data to indicate that mixed pre-Laramide basinal fluids, heated by burial during the Laramide Orogeny, were present during late Laramide time when the dolomites were recrystallized and (or) precipitated prior to petroleum migration and accumulation in the Rainbow "A" reservoir. Thus the combined use of paleomagnetism, geochemistry, and petrography has been proven to be a useful technique to date and identify dolomitization events and pathways for the migration of hydrocarbons.
Three classic sections of Middle and Late Triassic fossiliferous limestones cropping out around Williston Lake in British Columbia, Canada, were sampled for paleomagnetic study. The objective was to test the suitability of these units for detailed magnetobiostratigraphic study with the aim of improving the reference Triassic geomagnetic polarity time scale. The Williston Lake characteristic magnetizations differ, however, from any Triassic North America cratonic reference directions. A satisfactory agreement is found instead with Cretaceous -early Cenozoic North America cratonic reference directions. The exclusive occurrence of normal polarity suggests that remagnetization likely occurred during the Cretaceous long normal superchron. Remagnetizations may have been triggered by connate brines, which moved along aquifers of porous sandstones and carbonates in the early stages of Laramide folding.
Canadian Journal of Earth Sciences, 2001
Three classic sections of Middle and Late Triassic fossiliferous limestones cropping out around Williston Lake in British Columbia, Canada, were sampled for paleomagnetic study. The objective was to test the suitability of these units for detailed magnetobiostratigraphic study with the aim of improving the reference Triassic geomagnetic polarity time scale. The Williston Lake characteristic magnetizations differ, however, from any Triassic North America cratonic reference directions. A satisfactory agreement is found instead with Cretaceous -early Cenozoic North America cratonic reference directions. The exclusive occurrence of normal polarity suggests that remagnetization likely occurred during the Cretaceous long normal superchron. Remagnetizations may have been triggered by connate brines, which moved along aquifers of porous sandstones and carbonates in the early stages of Laramide folding.
Origin of the sedimentary magnetic record at Ocean Drilling Program Sites on the Owen Ridge
1993
The 3.2-m.y. whole core magnetic susceptibility record obtained during Ocean Drilling Program leg 117 from the Owen Ridge in the western Arabian Sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information. The salient features of this record are that (1) it correlates strongly with variations in the concentration and flux of eolian dust; (2) it is driven strongly at Earth orbital periodicities; and (3) there is a significant change in the spectral character at 2.4 m.y., which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas. In view of its potential paleoclimatic significance, we have examined the origin of the rock magnetic signal from the Owen Ridge in more detail. We find that desDite the strong relationship throughout the record between magnetic susceptibility and percent terngenous content, there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis. We ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process. The first occurs at about 1.5 m depth and is characterized by a loss of fine-grained ferrimagnetic material. The second stepshift in magnetic properties occurs at a depth of 6-8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity (magnetite) and high coercivity (hematite and/or goethite) magnetic mineral components. Scanning electron microscope and energy dispersive Xray analyses show that the bulk of the coarse-grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the Fe:Ti ratio of these grains across the lower diagenetic front. This suggests that titanomagnetite grains with higher Ti contents may be more resistant to the process of reductive diagenesis, and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diagenetic front. 1. INTRODUCTION Recent magnetic studies of deep-sea sediment cores from several ocean basins demonstrate correlations between rock magnetic parameters such as magnetic susceptibility and paleoclimatic indices such as oxygen isotopes, carbonate content, microfossil assemblages, and terrigenous fraction concentration, grain size and mineralogy [e.g., Kent, 1982; Robinson, 1986; Bloemendal et al., 1988; Doh et al., 1988; Mienert and Bloemendal, 1989; Hall et al., 1989a; Hall et al., 1989b; Bloemendal and DeMenocal, 1989; Robinson, 1990]. The rock magnetic properties reflect variations in the concentration, grainsize distribution, and mineralogy of the magnetic minerals which are present as trace components of the sediments. In most cases the rock magnetic-1Department of Geography, University of Liverpool, England. •3raduate School paleoceanographic linkage results from the climatic control of one or a combination of the following: (1) variations in carbonate productivity and preservation, which have the effect of alternately diluting and concentrating the flux of detrital magnetic minerals, (2) the transport mechanisms and pathways by which detrital magnetic minerals reach the ocean basins, e.g., via the activity of processes such as bottom current, fluvial, eolian activity, and ice rafting, and/or (3) cycles of continental aridity and humidity which control the quantity of terrigenous material (and associated lithogenic and pedogenic magnetic minerals) available for eolian transport to the oceans. Variations in the character of sedimentary magnetic minerals effected by these processes can be monitored very sensitively and rapidly by rock magnetic methods (some of which can be used on whole, unsplit sediment cores) and their use in paleoceanographic reconstructions is likely to increase. An implicit assumption of these studies is that the detrital magnetic minerals which dominate the magnetic properties of many deep-sea sediments are geochemically stable on the time scales of interest.
The rock magnetic fingerprint of chemical remagnetization in midcontinental Paleozoic carbonates
Geophysical Research Letters, 1992
Results of a paleomagnetic and rock magnetic survey of Paleozoic carbonates from 39 sites in the midcontinental U.S. show that many of these sites carry a stable remanence of apparent Permian age. Many of these remagnetized sites also have relatively high anhysteretic susceptibilities, and higher saturation remanence than most of the sites where the late Paleozoic remanence is absent. However the correlation between late Paleozoic remanence and high anhysteretic susceptibility or high saturation remanence is imperfect. The most diagnostic rock magnetic parameter for recognizing remagnetized sites is a ratio of anhysteretic remanence/saturation remanence exceeding 10%. We have found high ratios in almost all remagnetized sites, but in very few sites where the Late Paleozoic remanence is absent. The high ratios reflect the presence of a significant fraction of extremely fine-grained magnetite (a few tens of nanometers), spanning the superparamagnetic-single domain threshhold. Introduction Abundant evidence has accumulated in recent years indicating that the Paleozoic carbonate strata of cratonic North America were extensively remagnetized during the late Paleozoic Kiaman reverse polarity superchron [e.g., McCabe and Elmore, 1989; Van der Voo, 1989]. At least three principal lines of evidence favor a chemical mechanism for the remagnetization, involving growth of new magnetite. First, petrographic studies have shown that Ti-free magnetites with apparent diagenetic or authigenic morphologies (e.g., spheroidal or botryoidal) occur in most remagnetized carbonates [e.g., McCabe et al., 1983; Suk et al., 1990 a, b; Lu et al., 1990]. Second, many remagnetized strata occur in areas for which there is little or no evidence of any significant burial or heating; this by default favors a chemical mechanism. Finally, recent detailed rock magnetic studies or .,everal well-documented examples of rernagnetized strata (Trenton limestone [McCabe et al., 1984], Knox dolomite [Bachtadse et al, 1987], and Onondaga limestone [Kent, 1979, 1985]) have shown that these units share a set of quite unusual magnetic properties, apparently due to 1) an absence of uniaxial (shape or stress) anisotropy in the magnetic grains, and 2) the presence of a significant fraction of ultrafine-grained magnetite, near the boundary between single-domain (SD) and superparamagnetic (SP) sizes [Jackson, 1990; Jackson et al, 1992].
Studia Geophysica et Geodaetica, 2012
sections, Belgium. Both sections are divided into a lower unit, dominated by biostromal and external ramp facies (biostromal unit) and an upper unit, only consisting of lagoonal facies (lagoonal unit). The variations in χ in signal are mainly driven by magnetite variation, mostly pseudosingle-domain (PSD) magnetite. Clay minerals, pyrite, hematite and obviously calcite and dolomite are also present but their contribution to the χ in pattern is not significant. There is a correlation between detrital proxies (Zr, Rb, Al 2 O 3 , TiO 2 ) and χ in for the Tailfer biostromal unit and the entire Villers section. The pervasive presence of fine-grained magnetite is interpreted as related to remagnetization. In absence of external fluids, the iron released during the smectite to illite transition remains in situ. In those situations χ in may reflect an inherited primary synsedimentary signal. In the lagoonal unit of the Tailfer section, remagnetization appears to have obscured the original detrital information prompting the need for an evaluation of the composition of the susceptibility signal for individual case studies.
Geology, 1983
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.
Lower Cretaceous magnetic stratigraphy in Umbrian pelagic carbonate rocks
Geophysical Journal International, 1980
A record of geomagnetic field polarity for the Barremian, Aptian and Albian stages of the Early Cretaceous has been derived in three overlapping sections of pelagic carbonate rocks in the Umbrian Apennines of northern Italy. The remanence carrier in the greyish-white Majolica limestone and Fucoid Marls is magnetite, with haematite also an important constituent in a zone of 'couches rouges' within the Fucoid Marls. The weak remanent magnetizations were measured with a cryogenic magnetometer. Alternating field or thermal demagnetization was used to isolate the characteristic remanent magnetization (ChRM) in 655 specimens from 248 stratigraphic levels. The samples respond positively to a tectonic fold test, indicating that the ChRM predates the Late Tertiary folding of the Umbrian sequence. The magnetic stratigraphy derived from variations of virtual geomagnetic pole latitude clearly defines the recognizable reversal pattern associated with Mesozoic marine magnetic anomalies MO to M4. The sections have been zones palaeontologically on the basis of planktonic foraminifera and calcareous nannofossil assemblages. The ages of magnetic anomalies MO to M4 determined in this way are somewhat older than those in the reversal time scale of Larson & Hilde (1975). Anomaly MO is located in the Early Aptian, close to the Aptian/ Barremian boundary. A long period of normal polarity in the Aptian and Albian corresponds to the early part of the Cretaceous magnetic quiet zone. It is interrupted in the Late Aptian by a reversal which we fmd in only one of the Fucoid Marl sections, and which has not been reported in oceanic magnetic anomaly investigations. The record of geomagnetic polarity during the Mesozoic and Cenozoic is known best from interpretation of the magnetic anomaly lineations recorded in oceanic crust formed during this time, since the break-up of Pangaea in the Late Triassic. The largest part of this polarity record has not yet been confirmed by independent palaeomagnetic investigations. The youngest portion of the magnetic time scale of correlates excellently with the polarity sequence determined in radiometrically dated lavas younger than about 3.5 Myr . The older Cenozoic and Late Cretaceous anomalies were dated by simple linear extrapolation from this short baseline. Although McDougalletaZ. (1977) have extended recently the baseline length to about 6.5 Myr in stratigraphically sequential lavas, the geomagnetic polarity record for the remainder of the Cenozoic has not yet been verified. For most of this period the errors inherent in radiometric dating prohibit development of a magnetic reversal sequence from stratigraphically unrelated samples. On the other hand recent major improvements in magnetometer technology now permit rapid and accurate investigation of sedimentary sequences whose remanent magnetizations were previously measurable only with great difficulty or not at all. Particularly well suited for magnetic stratigraphy studies are sequences of pelagic carbonate rocks.
Journal of Geophysical Research, 1997
Lower Pennsylvanian Belden Formation carbonate rocks from Colorado were subjected to paleomagnetic, rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical remanent magnetization (CRM), carried by authigenic magnetite, and burial diagenesis. Thermal demagnetization results indicate the presence of two components of natural remanent magnetization (NRM) after removal of a low unblocking temperature (NRM-250øC) remanence that is interpreted to be a modem, viscous magnetization. An intermediate unblocking temperature (250-400øC) remanence component with normal and reversed polarity Tertiary directions is interpreted to be a thermoviscous remanent magnetization. Many limestones also contain a high unblocking temperature (400-570øC) remanence component which is interpreted to be a CRM. Fold tests from different pans of the basin indicate that the CRM was acquired either before or during Laramide folding. This CRM is interpreted to be carried by authigenic magnetite that formed by replacement of pyrite. Hysteresis ratios are consistent with those reported for other remagnetized carbonates and indicate that the CRM is carried by single-domain/pseudo single-domain magnetite. Although elevated S7Sr/S6Sr values indicate passage of radiogenic fluids through the limestones, the results of contact vein tests do not support the hypothesis that these fluids were responsible for the CRM. The time of CRM acquisition, which varies from late Paleozoic to Cretaceous, coincides with the modeled time of organic matter maturation in different pans of the basin. This suggests that diagenetic reactions, that were triggered by low to moderate burial temperatures, may have caused the magnetite authigenesis and probably gave rise to the CRM. 1. Introduction Pervasive, basin-wide, secondary magnetizations have been recognized from many Paleozoic carbonate units within the North American craton [e.g., Van der Voo, 1989; Elmore and McCabe, 1991; Lu et al., 1991 ]. Abundant evidence indicates that many of these magnetizations are chemical in origin and that they reside in authigenic magnetite that formed by alteration of preexisting pyrite framboids [e.g., Suk et al., 1990a, 1990b; Jackson et al., 1993]. A mechanism that is commonly invoked to explain such chemical remanent magnetizations (CRMs) in carbonate rocks is based on large-scale migration of orogenic fluids [Oliver, 1992]. Evidence for this model includes a spatial and temporal association of the CRMs with orogenic belts and events. The orogenic fluids are believed to be responsible for causing widespread mineralization and other diagenetic changes in rocks, including magnetite authigenesis [e.g., Oliver, 1992]. This model can explain some ofthe CRMs in Paleozoic carbonates [e.g.,
Remagnetization by basinal fluids: Testing the hypothesis in the Viola limestone, southern Oklahoma
Journal of Geophysical Research: Solid Earth, 1993
Migration of orogenic or basinal fluids is a recently invoked mechanism to explain the widespread presence of late Paleozoic secondary magnetizations in the rocks of North America. Paleomagnetic and geochemical results from the Ordovician Viola Limestone in southern Oklahoma are evaluated to assess the role of basinal fluids in leading to secondary magnetizations (as recently summarized by Elmore and McCabe [1991]). Many secondary magnetizations are interpreted to be chemical in origin and are evidence of one or more diagenetic events. If a direct connection between a secondary magnetization and the diagenetic event can be established, then paleomagnetic data can provide important magnetizations in the unit. The Viola Limestone information on the timing and nature of the contains what we interpret to be a pervasive Pennsylvanian synfolding magnetization residing in magnetite and a localized Permian magnetization which resides in hematite and occurs in alteration zones around mineralized veins. Both secondary magnetizations are interpreted as chemical remanent magnetizations (CRM) based on low burial temperatures and the presence of authigenic magnetic phases. The relative proportion of the Permian CRM in hematite gradually decreases whereas the magnetite CRM increases with distance from the veins. Fluid inclusion and Sr isotope studies indicate that the vein mineralization (calcite with Mississippi-Valley-type oxides and event. The role of fluids in general, and particularly orogenic fluids expelled as a result of tectonism, has recently received considerable attention as a possible agent of remagnetization [