Richard Douglas Elmore

A paleomagnetic and petrographic study of host carbonate rocks and impact breccias at the Kentland impact structure was conducted to better constrain the timing of the impact and to test for alteration by hydrothermal fluids. The Ordovician‐Silurian target rocks sampled are fossiliferous wackestone/packstones with minor dolomite. Polymict impact breccias, also sampled, occur as dikes/sills and contain clasts of dolomite, host carbonate, sandstone, sphalerite, and rare coated grains which contain clays, dolomite, calcite, and hexagonal silica resembling tridymite. The host carbonates contain brecciated zones near the polymict breccias that display flow textures of aligned and elongated clasts and minerals. Authigenic minerals present include sylvite, apatite, gypsum, magnetite, and hematite. These observations suggest alteration by hydrothermal fluids, which probably had an estimated duration of ~7500 yrs after impact. Alternating field (AF) and thermal demagnetization of impact breccia and host carbonate specimens removed two post‐tilting magnetic components: one with southerly declinations and moderate negative inclinations and the other with northerly declinations and positive inclinations. Demagnetization results suggest the magnetizations primarily reside in magnetite as well as hematite and possibly pyrrhotite. Petrographic and rock magnetic results are consistent with this interpretation. These magnetizations are interpreted as chemical remanent magnetizations acquired through a reversal, which formed from alteration by hydrothermal fluids generated after the impact. The paleomagnetic poles (mean pole, 75.7° N, 98.4° W) fall near the Early Jurassic part of the apparent polar wander path, which suggests the alteration in the breccias, and likely the impact, occurred in the Early Jurassic (175–185 Ma).

Editorial: Reviews in geomagnetism and paleomagnetism

Editorial: Remagnetization and Diagenesis

This paper presents an integrated paleomagnetic, rock magnetic and geochemical study of the Devonian Antrim shale in the Michigan Basin. Specimens were obtained from two unoriented vertical cores (named St. Chester and Krocker) and sampled relative to a master specimen orientation line. Alternating field (AF) demagnetization experiments isolated a lower coercivity component (LC) from 0 to ∼60 mT in the Krocker core and corresponds to a Jurassic (170 Ma ± 25) magnetization. Higher coercivity components (HC) in the Krocker core are unblocked from ∼60 to 120 mT and unlike LC components, occasionally exhibit stable unblocking temperature ranges (e.g., 150–450°C) and higher AF demagnetization ranges (∼60–120 mT). HC components in the Krocker core are unique to certain members within the Antrim shale with a poorly resolved Middle Permian/Late Triassic magnetization in the Paxton member and Late Pennsylvanian (305 Ma ± 10) component in the Norwood. The St. Chester well exhibited a LC component among all specimens with a slightly older (205 Ma ± 10) than the Krocker core. Rock magnetic parameters indicate the magnetization resides in PSD/SD magnetite in both HC and LC components. Overall, the magnetizations are interpreted to be chemical remanent magnetizations (CRMs) with the HC component residing in SD magnetite which formed during hydrothermal activity in both cores. The LC component likely resides in PSD/MD magnetite and is interpreted to have formed in response to fluid flow associated with tectonic uplift and/or hydrocarbon migration. Petrographic observations indicate mineral assemblages consistent with hydrothermal mineralization (e.g., baroque dolomite). Depleted δ18O data from carbonate fractures are consistent with a hydrothermal fluid magnetization mechanism. Consistent anisotropy of magnetic susceptibility (AMS) lineations indicates either a long-lived paleocurrent direction or far-field tectonic shortening originating from the neighboring Acadian orogeny. Inorganic geochemistry indicate that paramagnetic Fe-rich clays carry the AMS signal. Lastly, an evaluation of the origin of chalcopyrite and monazite is performed through provenance analysis. These minerals are attributed to hydrothermal mineralization however, strong evidence suggests that some fraction of these grains were sourced from the Michigan Peninsula.

An integrated paleomagnetic, magnetic fabric, and petrographic study of two cores (EC, oriented; DB, unoriented) in the Devonian Marcellus Formation from the Plateau Province (PA) indicates the presence of chemical remanent magnetizations (CRMs) and extensive diagenetic alteration. Anisotropy of magnetic susceptibility results indicate a predominately oblate fabric. Some specimens contain a prolate fabric that is interpreted as diagenetically altered. A well-developed viscous magnetization, possibly contaminated by a drilling induced component, is not present in either core, and cannot be used for orientation. An intermediate temperature component (ITN) with steep down inclinations is removed in both cores below ∼300°C. Specimens in DB also contain a component with steep up inclinations (ITR) that is removed between 300 and 440°C. The ITN is interpreted to reside in magnetite and/or possibly pyrrhotite, and the ITR resides in magnetite. The ITN could be a reversal of ITR, acquired in the Cretaceous to Cenozoic based on a comparison with the expected inclinations. Alternatively, the ITN could be a contaminated by a Brunhes viscous magnetization. Specimens from both cores contain a characteristic remanent magnetization (ChRM) with shallow inclinations that is removed by 500°C in DB and 460°C in EC, and is interpreted to reside in magnetite. The DB ChRM has an inclination of −9.0°, which corresponds to a range of ages between 310 and 225 Ma based on the expected inclinations. The ChRM in EC has streaked declinations, which may have been caused by core barrel rotation. Shallow inclinations (−3.7°) in this core suggest a range of acquisition from 305 to 230 Ma. Rock magnetic data are consistent with the presence of magnetite but provide conflicting evidence for pyrrhotite. The ChRM in DB is interpreted as a CRM whereas the ChRM in EC could be thermoviscous in origin. Diagenetic studies indicate a complex paragenesis with authigenic minerals such as barite, sphalerite, baroque dolomite, and sylvite suggestive of alteration by external fluids. The presence of the CRM in highly altered zones suggests it is related to alteration by external fluids, although its presence in other facies suggests that burial diagenetic processes could also be a cause of remagnetization.

An oriented Woodford Shale core from the eastern Ardmore Basin was sampled to test if the shale was an open or closed system to hydrothermal fluids, and to determine the timing of alteration. Mineralized fractures are ubiquitous in the core, and the shale exhibits a complex paragenesis with multiple hydrothermal minerals, including biotite, magnesite, norsethite, gorceixite and potassium feldspar present in and around the fractures. These minerals suggest that the Woodford Shale was an open system during part of its diagenetic history. Vitrinite reflectance (R_o) measurements indicate values of ∼1.81 % (∼230 °C). Palaeomagnetic analysis reveals a characteristic remanent magnetization (ChRM) with south-southeasterly declinations and shallow inclinations that resides in magnetite. This ChRM is interpreted to be either a chemical remanent magnetization (CRM) or a thermochemical remanent magnetization (TCRM) that was acquired at 245 ± 10 Ma during Late Permian time based on the pole position (51.0° N, 115.6° E). Because the palaeomagnetic specimens show evidence of extensive hydrothermal alteration, the CRM/TCRM is interpreted to date the migration of hydrothermal fluids through the shale. The agreement in timing with other studies that report hydrothermal alteration in southern Oklahoma and the Ouachita Mountains in Late Permian time, suggest that there were post-collisional fluid-flow events which accessed reservoirs of warm fluids.

The Mississippian limestone is a petroleum exploration target in northern Oklahoma, and diagenetic events are significant factors in controlling porosity. In this study, paleomagnetic data, supported by petrographic results, were used to determine the origin and timing of diagenetic events in five unoriented cores from northern Oklahoma. Petrographic analysis indicates a complex paragenetic sequence, which includes precipitation of sphalerite and baroque dolomite. Thermal demagnetization removes a low-temperature viscous remanent magnetization (VRM) and a chemical remanent magnetization (CRM) in magnetite. An attempt was made to orient the cores using the VRM but this resulted in a streaked distribution of declinations. The inclinations of the CRM in the specimens in the five cores are similar (mean = −2.6°) and the age of the CRM was determined by comparing the inclinations with the expected inclinations for the study area. This indicates remanence acquisition in the Permian (~310–290 Ma). This is consistent with dates for mineralization in the nearby Tri-State MVT deposit and for a hypothesized Permian hydrothermal alteration event in the study area. The age of the CRM and the presence of sphalerite and baroque dolomite suggest that the CRM was acquired via hydrothermal fluids in the Permian.

The objective of this study is to test if external fluids (e.g., hydrothermal) altered the Woodford Shale in the southeast Anadarko Basin, and how the diagenesis caused by such fluids, particularly in mineralized fractures, has affected the reservoir quality and mechanical behavior of the Woodford Shale. Two Woodford Shale cores from the Anadarko Basin were sampled in order to identify diagenetic events, interpret their origin, and determine the shale’s diagenetic history. Both cores were analyzed using petrographic, cathodoluminescence, and scanning electron microscopy in order to identify minerals and cross-cutting and textural relationships. X-ray computed tomography was conducted to further characterize fracture networks. Early diagenesis is dominated by events in the matrix and allochems, with later diagenesis dominated by events associated within fracturing and brecciation. The mineralized fractures and brecciated intervals contain complex mineralogies. Multiple minerals, interpreted to be hydrothermal in origin, are present, including magnesite, norsethite, witherite, gorceixite, potassium feldspar, sphalerite, chalcopyrite, and saddle dolomite. Cathodoluminescence in minerals within fractures reveals evidence of evolving fluids and multiple fluid-flow events. While porosity is present within pyrite framboids, between clay sheets, and as dissolution vugs within fractures and allochems, fracture porosity was destroyed by mineralization which filled fractures and permeated into the matrix. Vitrinite reflectance data from both cores shows a correlation between thermal maturity and level of hydrothermal alteration, with Core A (0.80% VRo (~125°C)) displaying a lower amount of alteration and Core C (~1.5% VR_o (~210°C)) displaying a higher amount of alteration. The extensive faulting present in the southeastern Anadarko Basin likely facilitated the movement of the hydrothermal fluids through the unit. The presence of hydrothermal minerals not only has implications for the thermal maturity and reservoir quality of the Woodford Shale, but also contributes to the discussion of whether or not shales behave as open or closed systems.