R. Douglas Elmore

Associations between burial diagenesis of smectite, chemical remagnetization, and magnetite authigenesis in the Vocontian trough, SE France

Results of a paleomagnetic, rock magnetic, geochemical, and petrographic study on Jurassic and Cretaceous carbonates in the Vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization (CRM) carried by magnetite. Where smectite has altered to other clay minerals, limestones are characterized by a prefolding, secondary, normal polarity magnetization throughout the basin. The magnetization is interpreted to be a CRM based on low burial depths which cannot cause thermoviscous resetting. Where significant smectite is still present, the CRM is absent/weakly developed, and where the clays show no evidence for burial alteration, the units are characterized by a primary magnetization. CRM intensity also varies with the amount of smectite and burial. Isothermal, anhysteretic, and natural remanent magnetization intensities increase where smectite has altered, both stratigraphically and geographically. This is interpreted to indicate magnetite authigenesis associated with clay diagenesis. Superparamagnetic magnetite is more dominant in highly altered units based on the results of low-temperature experiments. All sections away from the Alps have 87Sr/86Sr values that are similar to coeval seawater, and stable isotopes of carbon and oxygen show no sign of alteration. Orogenic-type fluids therefore are not a likely agent of remagnetization. Near the Alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the CRM through a reversal. A postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic-type fluids. We conclude that burial diagenesis of smectite is the likely cause for the development of the widespread CRM in the Vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere.

Remagnetization by basinal fluids: Testing the hypothesis in the Viola limestone, southern Oklahoma

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 in the unit. The Viola Limestone 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 sulfides) precipitated from basinal fluids which were warm, saline, and radiogenic. The radiogenic 87Sr/86Sr values of the limestones in the alteration zones, and the fact that there is more significant alteration closer to the veins suggests that the basinal fluids were also responsible for alteration in the limestones. The coincidence of the geochemical and remagnetization trends suggests that the Permian CRM dates the migration of basinal fluids through the veins. Geochemical results from the Viola Limestone containing the pervasive CRM indicate that it is relatively unaltered with no evidence for basinal fluids. The lack of evidence for basinal fluids suggests that other mechanisms for the origin of the pervasive CRM need to be tested. The results of this study indicate that flow of basinal fluids was focused in veins and only locally altered the host limestone.

Chemical remagnetization and burial diagenesis: Testing the hypothesis in the Pennsylvanian Belden Formation, Colorado

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-250°C) remanence that is interpreted to be a modern, viscous magnetization. An intermediate unblocking temperature (250–400°C) 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 (400–570°C) remanence component which is interpreted to be a CRM. Fold tests from different parts 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 87Sr/86Sr 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 parts 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.

Sedimentologic-magnetic record of western Pangean climate in upper Paleozoic loessite (lower Cutler beds, Utah)

Sedimentologic, pedologic, and magnetic data within the upper Paleozoic lower Cutler beds of the southwestern Paradox basin (Utah) record high- and low-frequency climate changes that operated at equatorial latitudes of western Pangea. The lower Cutler beds consist of ∼250 m of lithified eolian silt (loessite) and marine-reworked and fluvially reworked loessite, with abundant intercalated paleosols comprising Protosols, Argillisols, and Calcisols. The evolution from loessite and marine-reworked loessite with abundant Calcisols in the lower section to loessite and fluvially reworked loessite with abundant Argillisols in the upper section records a long-term transition from semiarid conditions in western equatorial Pangea in latest Pennsylvanian time to seasonally wet conditions in earliest Permian time. This shift could record intensification of the Pangean megamonsoon and associated seasonal incursions of moisture-laden westerlies. Paleosols record relatively high-frequency fluctuations between drier, dustier glacials and wetter interglacials of the late Paleozoic. Bulk magnetic-susceptibility values in paleosols exhibit variations that track paleosol type and are significantly elevated relative to parent loessite, attributable to the occurrence of both ultrafine-grained (superparamagnetic) and coarser-grained (remanence-carrying) magnetite. This signature reflects in situ pedogenic production of ferrimagnetic phases and a subordinate component of allochthonous, magnetic dust influx during pedogenesis, analogous to processes inferred for the magnetic signature in the Pliocene–Pleistocene loess-paleosol sequences of, e.g., the Chinese Loess Plateau. Integration of sedimentologic, geochemical, and magnetic data further suggests that enhancement of magnetic susceptibility in loessitic paleosols of this section relates primarily to climatic conditions and secondarily to durations of pedogenesis. Whereas peak susceptibility values in mature paleosols (Argillisols and Calcisols) do not vary significantly through the study section, peak values for Protosols track facies evidence for wetter conditions through time. Accordingly, relative changes in paleosol susceptibility values can provide paleoclimatic information, but should be integrated with other data to fully assess the origin of the signature. Overall, our data document the applicability of analytical approaches used on recent loess to very ancient loessite; this result is significant, because loess commonly records high- resolution evidence of terrestrial climate and climate change.

The Precambrian nonesuch formation of the North American mid-continent rift, sedimentology and organic geochemical aspects of lacustrine deposition

Abstract The Precambrian Keweenawan Trough in the Lake Superior region of North America is part of a failed intractonic rift that is filled with a thick sequence of volcanic and sedimentary rocks. One unit in this sequence, the None-such Formation, is composed of black-to-green siltstones and shales and is conformable with the underlying alluvial Copper Harbor Conglomerate and the overlying fluvial Freda Sandstone. The formation is exposed as a lens in northern Michigan and Wisconsin and attains a maximum thickness of ∼ 200 m. Based on the integration of outcrop and core data, nine facies have been recognized in the formation; each has been assigned to one of three genetic facies assemblages. A marginal lacustrine assemblage, characterized by interbedded lithic sandstones, siltstones, mudstones and sandstone-shale couplets, is interpreted as a record of deposition on a sandflat-mudflat complex. A lacustrine assemblage is characterized by massive to well-laminated dark shaly siltstones, carbonate laminites, shales, siltstones and mudstones. These sediments were probably deposited in a progressively shallowing perennial lake. Bottom conditions were anoxic and became increasingly more oxic as lake regression progressed. A gradual transition from a lacustrine environment to a fluvial environment is represented by the red to brown, massive, horizontally laminated and rippled fine sandstones, siltstones and mudstones of the fluvial-lacustrine assemblage. Interactions among rates of subsidence, rates of sedimentation, fluctuations in lake level, possible changes in climate and differences in tectonic setting controlled sedimentation in the Nonesuch Lake and resulted in variable vertical facies sequences. Total organic carbon (TOC) analyses show a strong correlation between organic richness and the shale facies (lacustrine assemblage) and mudstone facies (marginal lacustrine assemblage). Detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two prevalent organic petrographies which could be distinguished using combined pyrolysis-gas chromatography-mass spectrometry (PY-GC-MS) and, to a limited extent, by stable carbon isotope mass spectrometry. The geographic and stratigraphic distribution of rocks containing the two organic petrographies suggests that differential preservation (e.g., microbial degradation) accounts for the difference rather than maturity or variable source input.

Stable isotope (C, S, N) and molecular studies on the Precambrian nonesuch Shale (Wisconsin-Michigan, U.S.A.): Evidence for differential preservation rates, depositional environment and hydrothermal influence

Abstract The organic-rich, Middle Proterozoic Nonesuch Formation is part of a thick volcaniclastic rift-fill sequence (Mid-Continent Rift System, northern Wisconsin and Upper Peninsula Michigan) with a mild thermal history. Despite stratigraphic/sedimentologic similarities between the east (Michigan) and west (Wisconsin) study areas, distinctions in organic petrologic, elemental and stable isotopic parameters are noted. Geologic arguments and predictable relationships among organic carbon content, bulk and molecular pyrolysate composition and petrologic parameters are sufficient to invoke differential preservation of the same (or similar) primary producers as the principal cause of variability. Isotopic signals indicating precipitation of calcite via intense organic production (planktonic blooms) is evident among carbonate laminites. Incorporation or concentration of organic degradation by-products is evident, particularly in the west, from carbon and nitrogen stable isotope data. It is proposed that whereas preservation of organic productivity of a particular organism may be nearly complete in one organic facies in the east and west, organic remains in other intervals in the west were subject to extensive degradation and reconstitution to form protokerogens. Implications of the study include establishing parameters for recognizing pre-Devonian lacustrine vs. marine euxinic systems, clues to the complexity of Middle Proterozoic aquatic systems and in recognizing the input of hydrothermal fluids to water bodies and early sediments.

Paleoclimatic inferences from paleopedology and magnetism of the Permian Maroon Formation loessite, Colorado, USA

The Maroon Formation in the eastern Eagle basin (Colorado) consists of >700 m of lithified loess with >200 paleosols interpreted as Protosols and Argillisols on the basis of field, petrographic and geochemical data. Additionally, magnetic susceptibility aids assessment of the intensity of pedogenesis. Bulk magnetic susceptibility (χ b ) through the section repeatedly fluctuates between low values (average 3.51 × 10 −8 ± 1.59 × 10 −8 m 3 /kg) in parent loessite and higher values (average 5.70 × 10 −8 ± 2.70 × 10 −8 m 3 /kg) in paleosols. Moreover, magnetic susceptibility positively correlates with abundance of clay-sized material as well as Al 2 O 3 and K 2 O and effectively distinguishes Protosols and Argillisols. Low- temperature demagnetization indicates the presence of ultra-fine-grained magnetite. The integration of geochemical, petrographic, and rock magnetic data suggest that changes in magnetic susceptibility reflect pedogenesis and relate primarily to climate- and time-dependent pedogenic production and concentration of ultra-fine- grained magnetite. The Maroon Formation loess and associated soils accumulated in an overall arid system, documented in part by formation of incipiently formed paleosols developed into Argillisols by eolian clay and carbonate additions rather than by in situ clay formation. However, the paleosols showing bulk magnetic susceptibility values of greater than 200 χ b document a high- frequency (10 4 −10 5 yr) fluctuation between arid times of loess accumulation and slightly wetter times of reduced silt influx and resultant pedogenesis. This fluctuation likely reflects glacial-interglacial climate shifts that operated in low-latitude Pangea during icehouse conditions. These results suggest that climate-related magnetic susceptibility variations within loess successions can be preserved and useful in very ancient (pre– Pliocene–Pleistocene) sequences.

The origin, distribution and hydrocarbon generation potential of organic-rich facies in the Nonesuch formation, central North American Rift System: A regional study

Abstract Precambrian, unmetamorphosed, organic-rich clastic rocks, whilst uncommon in the geologic record, are of interest with respect to their potential for hydrocarbon generation. The ∼ 1.1 billion year old Nonesuch Formation is primarily a lacustrine deposit which is part of a thick sequence of volcanic and clastic sedimentary rocks that filled an aborted rift in central North America. Previous organic geochemical investigations of the Nonesuch Formation have been limited to isolated samples from the basal portion of the Nonesuch Formation at a copper mine (White Pine) in northern Michigan. In the present study, 183 outcrop and core samples of the Nonesuch Formation from northern Wisconsin and Michigan (Upper Peninsula) were collected for sedimentologic and organic geochemical analyses. Total organic carbon (TOC) values for the samples ranged from 0.0 to 2.5% and showed a strong correlation between organic richness and depositional environment. Detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two major organic petrographies (1 and 2) which could be further distinguished using combined pyrolysis-gas chromatography/mass spectrometry (PY/GC/MS). Whereas the composition of Organic Petrography 1 kerogens is primarily aliphatic, the composition of Organic Petrography 2 kerogens is dominated by aromatic and/or phenolic constituents. The geographic and stratigraphic distribution of rocks bearing the two organic petrographies indicate that variable source-input or preservation, superimposed over maturity differences, is responsible for both organic petrographic and kerogen pyrolysate compositions.

Absolute dating of dedolomitization by means of paleomagnetic techniques

Paleomagnetic analysis, in conjunction with petrographic studies, is an approach whereby absolute dates can be placed on diagenetic events. Hematite is associated with dedolomite in the Lower Ordovician Kindblade Formation in south-central Oklahoma and is interpreted as a by-product of the dedolomitization of ferroan dolomite. Paleomagnetic results date the time of dedolomitization as Late Pennsylvanian-Early Permian. Dedolomitization was probably caused by oxidizing fluids with high calcium contents that migrated through the Kindblade Formation following uplift of the Arbuckle Mountains.

Remagnetization of the folded Belden Formation, northwest Colorado

Paleomagnetic and geochemical results are used to evaluate the origin of an apparent synfolding magnetization in Belden Formation (Pennsylvanian) limestones on an asymmetric fold in Colorado. The natural remanent magnetizations contain three components: a modern viscous magnetization removed at low temperatures, a reversed Tertiary magnetization (declination (D) = 186.7°, inclination (I) = −54.6°) of thermoviscous origin removed at intermediate temperatures, and an apparently synfolding Late Cretaceous/early Tertiary characteristic magnetization (D = 338.7°E, I = 62.5°, precision parameter k = 54.9, radius of 95% probability cone ∝95 = 5.9°, 65% unfolding) removed by 580°C. Hysteresis properties suggest that the characteristic magnetization resides in single-domain magnetite. Low burial temperatures and the presence of authigenic magnetite are consistent with a chemical origin for this magnetization. The 87Sr/86Sr values (0.7088–0.7100) show the limestones to be radiogenic, with 87Sr enriched halos around calcite-cemented tectonic fractures. There is no control on the distribution and intensity of the magnetization by the veins or structural position (e.g., highly mineralized versus less mineralized rock). Lithology, rather than fluid alteration, controls the magnetization. Fecal pellets contain pyrite, with alteration rims identified as magnetite by scanning transmission electron microscope X ray diffraction patterns. Selective demagnetization has established that the fecal pellets carry the characteristic magnetization. It is not possible to unequivocally establish whether the characteristic magnetization is synfolding or prefolding. A synfolding magnetization caused by the syndeformational mineralizing fluids, however, can be ruled out. Structural modification of a prefolding component and a synfolding piezoremanent magnetization cannot be ruled out but are not considered likely mechanisms of remanence acquisition. Overprinting of a prefolding chemical magnetization by a postfolding viscous magnetization residing in multidomain grains is a likely explanation for the synfolding test result. The replacement of pyrite by magnetite is interpreted as prefolding in origin, perhaps triggered by an oxidizing fluid or by diagenesis of hydrocarbons.