Maria T. Cioppa

Crossover plots: A useful method for plotting SIRM data in paleomagnetism

The effective grain-size of magnetic minerals, and therefore domain type, controls the saturation isothermal remanent magnetization acquisition and subsequent demagnetization response and can be used to create “crossover” plots for paleomagnetic specimens, where %SIRM is plotted as a function of the field applied, using a logarithmic scale. This paper presents templates of such plots for magnetite, pyrrhotite, hematite and goethite, onto which sample data can be plotted for conventional paleomagnetic specimens to characterize their magnetic mineralogy and effective grain-size.

Correlating paleomagnetic, geochemical and petrographic evidence to date diagenetic and fluid flow events in the Mississippian Turner Valley Formation, Moose Field, Alberta, Canada

Abstract Petrographic, geochemical and paleomagnetic analyses of the Mississippian Turner Valley Formation provide constraints on diagenesis and fluid flow events in the Western Canada Sedimentary Basin. Paleomagnetic plugs and companion geochemical samples were taken from two drillcores, with Fullbore MicroImage log orientations. Dolomite from both wells yielded two magnetization directions. The low-temperature, low-coercivity direction is a drilling-induced remanence rather than a viscous remanent magnetization. The high-temperature, high-coercivity remanence direction is Cretaceous, and there is no sign of a primary Mississippian direction. Geochemical analyses of matrix dolomite yield δ18O values ranging from 0.65 to −3.34‰ (VPDB standard) and δ13C values ranging from 1.77 to 3.05‰ VPDB. The least depleted samples have stable isotope values consistent with, or only slightly depleted from, postulated Mississippian dolomite values. The remaining sample values exhibit a negative covariant trend consistent with either mixing with another diagenetic fluid or recrystallization during burial. Petrographic analysis reveals the presence of a recrystallization event that caused zoning and a gradual increase size of the dolomite crystals. This event is thought to have caused both the Cretaceous paleomagnetic remanence and the altered geochemical values. The minor enrichment in Sr radiogenic isotopes, relative to coeval seawater values, suggests that both an extrabasinal source for any fluid and large-scale fluid flow are unlikely. The results also indicate that magnetic remanences are very sensitive to visually minor changes in carbonate recrystallization from heat or pressure, so that great care must be taken in correlating paleomagnetic and geochemical data.

Beach sediment magnetism and sources: Lake Erie, Ontario, Canada

ABSTRACT Measurements were made along the northwestern shore of Lake Erie, Canada to determine whether grain magnetic properties can be used to identify and distinguish sources of beach sediment. Although surface magnetic susceptibilities were highly variable, ranging from 56 to 9867 × 10-5 SI (Bartington MS2D), there was generally a gradual increase from the low beach (near the waterline) towards the high beach; there were also narrow, shore-parallel bands with high susceptibility at various points on the beach surface. Magnetic mineralogy on the beaches was dominated by low-Ti magnetite (570°<Tc<580 °C), and the effective grain-size varied from pseudosingle domain in the low beach to multidomain on the high beach. Sandy bluff sediments in the eastern part of the study area had magnetic properties (e.g. S-ratios, hysteresis loops, thermomagnetic curves) that were similar to those on the beaches, whereas the magnetic properties of the extensive till bluffs and river basin sediments were quite different. The data suggested that, whereas the beaches in the western part of the study area are supplied with sediment from bluffs several tens of kilometres to the east, the source of the high magnetic concentrations on the eroding beaches of eastern Point Pelee remains to be determined.

Magnetic evidence for the nature and timing of fluid migration in the Watrous Formation, Williston Basin, Canada: a preliminary study

Abstract Paleomagnetic analysis of red siliclastics from the Watrous Formation in the Williston Basin of Saskatchewan defines aprimary or early diagenetic remanence of Pennsylvanian age held in hematite. Similar analysis of associated green-grey reduced zones (‘green beds’) defines a latest Cretaceous or earliest Tertiary characteristic remanence held in pyrrhotite and magnetite. The ‘green beds’ may have formed as an alteration product of fluid flow occurring in reducing conditions, which would therefore be constrained to have occurred late in the tectonic history of the Williston Basin. Such a reducing fluid could potentially carry hydrocarbons.

Conodont CAI and magnetic mineral unblocking temperatures: implications for the Western Canada Sedimentary Basin

Abstract The laboratory unblocking temperature of a magnetic component ( T UB ) is dependent on the time duration and temperature of its acquisition and thus is similar in concept to the conodont color alteration index (CAI). The CAI provides an independent determination of the past thermal history that can be related to the observed T UB . We present simple CAI versus T UB plots that can be used to determine the possibility of a thermal remagnetization in specific magnetic minerals, given the measured T UB , or alternatively, to estimate the minimum T UB necessary for a magnetization component to have been unaffected, given a specific CAI value. Failure to correctly correlate CAI values in the Cordillera on the western side of the Western Canada Sedimentary Basin may have led to secondary uplift remagnetizations being interpreted as primary remanent magnetizations in earlier paleomagnetic studies.

Timing of hydrocarbon generation and migration: paleomagnetic and rock magnetic analysis of the Devonian Duvernay Formation, Alberta, Canada

Abstract Paleomagnetic and rock magnetic analysis of the black anoxic shales, brown shales and carbonates of the Devonian Duvernay Formation in the Western Canada Sedimentary Basin indicates that the organic-rich black shales carry a different magnetization from the other shales and carbonates. Thermal maturation of the black shales has affected the rock magnetic properties, effecting a change from magnetite and hematite magnetization carriers in the immature sediments to magnetite in the mature sediments and indicating a more reducing environment.

Initial analysis of the Jurassic “Nordegg Member”, Fernie Formation: using paleomagnetism to date source rock thermal maturation

Paleomagnetic analysis demonstrates that the mean inclination, and therefore the age of the magnetization, in the organic-rich shales of the “Nordegg Member” of the Jurassic Fernie Formation correlates with thermal maturity indices. In the immature wells, the magnetization is probably Early to Middle Jurassic (I=60°±5°), in the mature wells, Late Jurassic (I=70°±3°) and in the overmature wells, Late Cretaceous (I=77°+7/−5°). The change in inclination with time, which reflects varying ages of magnetization, also allows the possibility that the inclination can be used as a proxy for the maturity level in the “Nordegg Member” and thereby aid in petroleum exploration.

Dating penecontemporaneous dolomitization in carbonate reservoirs: Paleomagnetic, petrographic, and geochemical constraints

The predominant dolomitization in the Mississippian Debolt Formation, Western Canada sedimentary basin was hypothesized to be early in three gas fields, based on stable oxygen and carbon isotopic values and on strontium isotope ratios that are similar to postulated Mississippian dolomite values and seawater ratios, respectively. As the absolute age could not be determined by this method, paleomagnetism was used to place constraints on the age of the dolomitization. The magnetic analyses on Debolt specimens from the same wells revealed three magnetization components that could be tied to geologic/diagenetic events: (1) a low-temperature ( 350 degreesC) and coercivity (>80 mT) C component of primary or early diagenetic origin that is found mostly in the fine-grained limestone and dolomitic muds. The preservation of a primary or early diagenetic magnetization, combined with the preservation of primary isotopic values, indicates that little or no extrabasinal fluid flow is likely to have occurred. Thus, in this particular area, orogenically induced fluid flow cannot explain the presence of the Cretaceous B magnetization. Consideration of the potential methods for forming this B component suggests that it is likely an in-situ chemical remanence resulting from either hydrocarbon migration or dissolution and reprecipitation of Fe-rich minerals in the original pore fluids.

Multiple magnetizations in Ordovician–Devonian carbonates in the Williston Basin (Manitoba, Canada)

Abstract Palaeomagnetic and rock magnetic data collected from the Upper Ordovician Red River, Silurian Interlake and Devonian Winnipegosis, Souris River and Birdbear carbonates in one well from southwestern Manitoba (Canada) reveal a complex magnetization history for the north-eastern Williston Basin. Rock magnetic analysis (thermal demagnetization, anhysteretic remanent magnetization (ARM)/saturation isothermal remanent magnetization (SIRM), S-ratios, partial ARM (pARM), SIRM crossover curves and points and coercivity) show three magnetic carriers for different magnetizations seen in this well. An Early–Mid-Jurassic remagnetization observed in the lower Red River and Souris River formations is carried by single-domain to pseudo-single-domain (SD–PSD) magnetite and was probably produced by basement fluids circulating along fractures and faults created by the Hartney impact/volcanic structure and/or tectonic movements along the Superior Boundary Zone. In the Winnipegosis Formation a possible primary depositional or early depositional magnetization (Devonian age) is carried by PSD pyrrhotite. In the Birdbear Formation two different magnetizations of uncertain age are present: one carried in hematite in the upper strata, possibly originating from the younger Amaranth Formation, and magnetite dominates in the lower strata. The upper Red River and Interlake formations contain both magnetite and pyrrhotite; however, the weaker palaeomagnetic data reveal little in terms of a magnetization age.