Regional Late Paleozoic Remagnetization of Shallow-Water Carbonate Rocks in the Great Basin: The Usefulness of “Micro”-Fold Tests from Late Compaction Fabrics
Published:January 01, 1993
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Stephen L. Gillett, John W. Geissman, 1993. "Regional Late Paleozoic Remagnetization of Shallow-Water Carbonate Rocks in the Great Basin: The Usefulness of “Micro”-Fold Tests from Late Compaction Fabrics", Applications of Paleomagnetism to Sedimentary Geology, Djafar M. Aïssaoui, Donald F. McNeill, Neil F. Hurley
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Paleomagnetic data have been obtained from heterogeneous, shallow-water, miogeoclinal carbonate rocks of the Pogonip Group (Early Ordovician) in the Desert Range of southern Nevada, the Egan Range of east-central Nevada, and the southern House Range of western Utah. These rocks locally contain abundant replacive chert that preserves relict textures from the host limestones as well as clearly detrital grains (e.g., blue-luminescing feldspars). Stylolites are abundant and are interpreted as late diagenetic features, as they cut late cements and truncate bedding lamination. Differential compaction along stylolites wrapping around the chert masses has resulted in macroscopic deformation, as evidenced by tilting of bedding of over 25° about chert masses in some cases. We have used the differential compaction fabrics in these rocks to test for the age of acquisition of a generally well-grouped and well-defined characteristic magnetization.
All three carbonate sections give a low-inclination, southerly to southeasterly magnetization residing in magnetite (e.g., Decl. = 152°, Incl. = -21°, α95 = 3°, kl = -61, k2 = -21, N = 48 independent samples, site 12; Pogonip Group, Sawmill Canyon, Egan Range). The magnetization is interpreted to be secondary and acquired after local compaction because directions of magnetizations from different samples are not dispersed by the compaction deformation. The uniform reversed polarity in addition to the direction of the magnetization, moreover, is interpreted to suggest a late Paleozoic age of remagnetization. In the Desert Range, the remagnetization had been previously attributed to a viscous partial thermoremanent magnetization (VPTRM) from deep burial. Based on several observations, we now argue for a chemical origin from late diagenetic magnetite, such as is now well-documented in the Appalachians and mid-continent. The cherts are almost nonmagnetic, as would be expected from their impermeability if the magnetite were precipitated from late fluids. Abundant authigenic alkali feldspar in the Desert Range is also consistent with late metasomatism. Finally, in the Egan Range, the remagnetization extends through a section exceeding 3 km in thickness, into rocks as young as Mississippian, which were never buried as deeply and thus not heated to the same degree as lower Paleozoic strata.
These results underscore the utility of integrating observations based on paleomagnetic data with carbonate textures. “Micro”-field tests can constrain both the timing of magnetization acquisition and of diagenetic events. The micro-fold tests discussed apply to features that are not formed by tectonic deformation. The availability of field tests from early formed textures in sedimentary rocks is especially important given the recent recognition of widespread remagnetization in ancient rocks.
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Applications of Paleomagnetism to Sedimentary Geology
Applications of Paleomagnetism to Sedimentary Geology - Based on the 1991 SEPM Research Symposium, the results are directed towards bringing the disciplines of Paleomagnetism and sedimentary geology closer together. Advances in the field of sedimentary geology will likely result from continued development of new ideas, questioning of old dogma, and, most importantly, providing means for testing these new hypotheses. It is hoped that the union of these two disciplines will help address many fundamental geological questions, such as the perennial problems of precise age-dating, stratigraphic correlation and geometries, understanding the timing and nature of post-depositional diagenetic fabrics, and the intriguing relationship between hydrocarbons and magnetization. The reader will find an unusual diversity of research topics presented in this volume. This diversity serves as a testimony to the potential applications awaiting the sedimentary geologist willing to explore these new paleomagnetic tools.