SEM/STEM Observation of Magnetic Minerals in Presumably Unremagnetized Paleozoic Carbonates from Indiana and Alabama
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Tecto~ophysics, 215 (1992) 255-272 2.55 Elsevier Science Publishers B.V., Amsterdam SEM/STEM observation of magnetic minerals in presumably unremagnetized Paleozoic carbonates from Indiana and Alabama Dongwoo Suk, Rob Van der Voo and Donald R. Peacor Department of Geological Sciences, Uniile~si~ of Michigan, Ann Arbor, MI, 481~-1~3, USA (Received March 51992; revised version accepted August 24, 1992) ABSTRACT Suk, D. Van der Voo, R. and Peacor, D.R., 1992. SEM/STEM observation of magnetic minerals in presumably unremagnetized Paleozoic carbonates from Indiana and Alabama. Tecfonop!tysics, 215: 255-272. The Silurian Wabash Formation in Indiana and the Mississippian Pride Mountain Formation in Alabama appear not to have been affected by a late Paleozoic remagnetization event. In an attempt to characterize the magnetic mineralogy in these (presumably) unremagnetized carbonates and in order to compare their magnetic mineralogy to that of remagnetized carbonates, scanning and scanning transmission microscope tSEM/STEM) observations and rock magnetic investigations were carried out. It is possible to recognize differences in magnetic mineralo~ in the unremagnetized carbonate from that in remagne- tized carbonates: (1) iron oxides associated with iron sulfides are hematite (in this study) as a result of replacement of pyrite (instead of magnetite as was found elsewhere); (2) occurrences of large euhedral pure-iron oxides of secondary origin are common in the unremagnetized carbonates; and (3) a rare occurrence of fine-grained single-crystal magnetite capable of carrying a remanence in the unremagnetized carbonates is noticeable as compared to the abundance of such grains in the remagnetized carbonates. Although the abundance of the fine-grained magnetite grains in remagnetized carbonates is inferred to be a diagnostic factor to distinguish the remagnetized from the unremagnetized carbonates, this clarifies only the carriers in the remagnetized rocks and leaves the question of the carriers in unremagnetized limestones unresolved to a large extent. The lack of remagnetization is commonly attributed to a restricted amount of fluid influx into the rocks. For the Wabash and the Pride Mountain Formations this may also be true; early cementation has significantly reduced the porosity and permeability in the Wabash Formation in Indiana, whereas the presence of the impermeable Chattanooga Shale may have ‘protected’ the Mississippian Pfide Mountain Formation in Alabama. Introduction to find reliable primary remanence for apparent polar wander path (APWP) construction, the A late Paleozoic remagnetization has been causes and possible carriers of the remagnetiza- widely recognized in many of the early to middle tion rapidly became interesting topics in and of Paleozoic carbonates in eastern North America themselves. The ancient remagnetization has been as having been acquired during the Kiaman re- found not only in areas affected by major tectonic versed Polarity Superchron of the late Paleozoic activity but also in the stable mid-continent where (Kent, 1979; Scotese et al., 1982. McCabe et al., the effects of tectonic activity are less prominent 1983, 1984; Bachtadse et al., 1987; Jackson et al., (Lu et al., 1990). There have been two different 1988). Although the remagnetization was an ini- explanations for the remagnetization, a CRM tial disappointment to paleomagnetists who hoped (chemical remanent magnetization) and a VRM (viscous remanent magnetization). A CRM mech- Correspondence to: D. Suk, Department of Geology and Geo- anism, in which the magnetization was acquired physics, University of Utah, 717 Browning Bldg., Salt Lake during the formation of new magnetic minerals in City, UT 84112, USA. limestones (McCabe et al., 1983, 1984; Bachtadse 004%1951/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved 256 et al, 1987; Suk et al., 1990a,b) has generally eral localities in the northeastern U.S.A. in our been favored over the VRM model (Kent, 1985). previous scanning and scanning transmission Moreover, it is now generally accepted that fluid electron microscope (SEM/STEM) investigations is a necessary ingredient in authigenesis of mag- (Suk et al., 1990a,b, 1992), we have extended our netic minerals in the remagnetized carbonates. studies to the unremagnetized carbonates in the Despite the pervasive and widespread remag- hope that they can provide a representative pic- netization in early to middle Paleozoic carbon- ture of the magnetic mineralogy in pristine car- ates, several carbonates have nevertheless been bonates. If so, such studies could in turn lead to a found not to have been affected by the late better understanding of the remagnetization pro- Paleozoic remagnetization. The Silurian Wabash cesses by allowing us to discriminate between the Formation in Indiana (McCabe et al., 19851, the remagnetized and unremagnetized carbonates on Lower Ordovician Oneota dolomite from the up- the basis of their oxide textures, morphologies, per Mississippi River Valley in Iowa, Wisconsin modes of occurrence, chemistry, or mineralogy. and Minnesota (Jackson and Van der Voo, 19851, and the Mississippian Pride Mountain Formation in Alabama (McCabe et al., 1989) have been Sample localities and methods inferred to have paleomagnetic directions of de- positional ages; at the very least, no evidence for The Silurian Wabash carbonates in Indiana Iater Paleozoic remagnetization has been re- and the Mississippian Pride Mountain carbonate vealed by the paleomagnetic investigations. in AIabama were sampled for this study. Samples Whereas the remagnetized Paleozoic carbonates of the Silurian Wabash Formation were collected in eastern North America contain authigenic from two localities in north-central Indiana: one magnetite grains as a possible remanence carrier is a small outcrop in a railroad cut near the town (Suk et al., 1990a,b), the unremagnetized carbon- of Wabash, and the other is in the Pipe Creek Jr. ates were not expected to contain such secondary Quarry near Gas City. These two localities are iron oxides; if that were the case, one could then the same as those for the paleomagnetic study of conclude that the latter rocks did not experience McCabe et al. (1985). The Wabash Formation is the authigenesis that caused the introduction of divided into three principal lithofacies in the Pipe new magnetic minerals long after the formation Creek Jr. Quarry; grainstone, grainstone-mud- of the rocks. Instead, the unremagnetized carbon- stone, and mudstone lithofacies (Devaney et al., ates have hitherto been inferred to contain mag- 1986). The Wabash Formation in the quarry is netic minerals of detrital or near-primal (early dominantly limestone with minor doiomitic mud- diagenetic) origin. This speculation was sup- stone, while it is completely dolomite in the rail- ported by the presence of titaniferous iron oxide road outcrop. About 20 kg of carbonate samples grains found in magnetic extracts from Mississip- of grainstone-mudstone lithofacies and dolomite pian carbonates in Alabama (McCabe et al., 1989). from the quarry and dolomite from the outcrop in Those titaniferous magnetite grains, which dis- the railroad cut were collected for SEM/STEM played high-temperature oxidation exsolution tex- observations of thin sections and magnetic ex- tures, constitute the only direct evidence for a tracts, and for rock magnetic experiments (Fig. 1). possible magnetic carrier of the primary magneti- The Mississippian carbonate from a quarry in zation in the unremagnetized carbonates. Mc- Margerum, Alabama, was provided by C. Mc- Cabe and colleagues (1989) attributed the lack of Cabe (see McCabe et al., 1989). remagnetization to at most a limited amount of Representative samples were prepared for magnetite-forming fluid restricted by an imper- SEM/STEM observations as polished thin sec- meable shale horizon between older carbonates tions using “sticky wax” as an adhesive so that and the Mississippian carbonates in Alabama. selected areas could be detached for STEM sam- Since our successful characterization of iron ple preparation. Areas containing iron oxides, as oxides in the remagnetized carbonates from sev- identified by SEM, were prepared for STEM SEM/S’I‘EM OBSERVATION OF MAGNETIC MINERALS 257 observations using an ion mill to produce thin Michigan to identify magnetic minerals, size and edges [see Suk et al. (1990a) for detailed descrip- domain state, and magnetic interaction between tion]. In addition, magnetic extracts were recov- magnetic minerals. Saturation isothermal rema- ered from insoluble residues produced by dissolv- nent magnetization (SIRM) acquisition curve was ing limestones with buffered acetic acid @lcCabe obtained by imparting an isothermal remanent et al., 1983). For the Mississippian limestone from magnetization (IRM) in a stepwise manner at 12 Alabama, dichloromethane was used to remove steps to a maximum availabIe field of 1.4 T, heavy bitumen in the rock prior to the acetic acid Modified Lowrie-Fuller tests (Lowrie and Fuller, treatment. The magnetic separates were then 1971; Johnson et al., 1975) were carried out spread on slides or on Cu grids in preparation for through acquisition and alternating field (AF) SEM and STEM observations, respectively. Sev- demagnetization of IRM and anhysteretic rema- eral grains of selected separates were mounted nent magnetization (ARM). The ARM was in- on glass fibers to obtain Gandolfi powder X-ray duced using a 100~mT AF field with a O.l-mT DC diffraction patterns for mineral