-~aleomagnetism of frecambrian Rhyolites from Southeastern Missouri A Thesis Presented in Partial Fulfillment of the Requirements for the Degree, Bachelor of Science by Mark David Gilliat The Department of Geology and Mineralogy Ohio State University 1984 Approved by Hallan C. Noltimier, Advisor Department of Geology and Mineralogy ABSTRACT Twenty four samples of rhyolite from six sites in the Taum Sauk Mountain region were sampled for paleomagnetic investigation. Paleomagnetic techniques were applied to the rocks in an attempt to identify a component of rota­ tional movement along a mapped fault. All samples indicate multi-component behavior, with remanence beinq carried by magnetite and/or hematite. Within core specimen magnetic dispersion was low, however between sample dispersion was significantly high. This high between sample dispersion made any attempt to identify a rotational component of movement along the fault impossible. Sources of the dis­ persion of magnetic results include: 1) variation in the primary magnetic combnent due to inhomogeneous aquisition of remanence within a cooling unit.; 2) differential wea­ thering within a unit. ; 3) lightning strikes. ACKNOWLEDGMENTS I would like to thank Dr. Hal Noltimier, my advisor, for the "long distance" assistance. His critical reviews were very helpful. Eric Cherry contributed knowledge and support throgh­ out the project. Thanks Eric, it couldn't have been com­ pleted without your help. Thanks to Nick Schear for help with the word process­ ing program SCRIBE. Financial support for the field work was provided by a grant from Sigma Xi. Financial support for the laboratory work was provided by a grant from the Friends of Orton Hall. 1 Introduction The St. Francois Mountains of southeast Missouri lie near the crest of the Ozark dome (fig 1). The terrain consists of Precambrian granites and rhyolites, dated at 1.5 b.y. (Bickford and Mose, 1975), cropping out as structural and topographic highs (fig 2). The St Francois Mountains are signifigant in that they are one of the few mid-continent exposures of Precambrian rocks, and these rocks offer an opportunity to study the structural and chemical evolution of the mid-continent, basement complex. The purpose of this investigation is to apply paleomagnetism to the structural relationships between two crustal blocks separated by a fault that outcrops on Taum Sauk Mountain. Taum Sauk Mountain (elevation 1772 ft.) is located in the western St. Francois Mountains, and has been mapped in detail by Berry (1970, 1976). Berrys' geologic map shows a major northeast trending fault in the saddle between Taum Sauk and Russell Mountains ( elevation 1726 ft.) (fig 3). The repeated section of rock across the fault offers an opportunity to observe the structural relationships between the fault blocks. In the present investigation these rocks are studied by paleomagnetic techniques in an attempt to identify a component of rotational movement across the fault. The rocks were subjected to standard laboratory alternating field and thermal demagnetization techniques to establish a stable virtual geomagnetic pole (VG P) for each unit. Reflected light studies have been used to better understand magnetic properties of the individual units. In addition, information obtained on the general igneous petrology provided an independent test of the previous petrologic correlation of the units across the fault. Identification of any rotational component of movement along the fault is made possible by comparing the established VGP's for each unit, with its counterpart across the fault. Assuming the relative movement along the fault is simply translational, paleomagnetic results should show no statistically signifigant difference between the VG P's on either side of the fault. On the other hand, if rotational movement has occured, the rotation between the blocks can be estimated both in terms of local angle and sense of relative rotation. EXPLANATIC" m Rocks of Ordo•,c-c,. -S• ~ Rocks of Co111b,,c,. •;• E~ Pr1ca111llricn rocas lll1noi1 Basin Modifiod from Stoto (1932) Figure 1 Location of the St. Francios Mountains in relation to the Ozark uplift. 0 IOO 200 MILES ...._~~~~~~~~........ ~~~~~--~~-___J Figure 2 Generalized cross-section of the St. Francios Mountains. T 33 N ...· -,... t --. ·. ~ ... ,. ::/' J.· ···.', 4 . ,"- _....;. "- ,. ·, .. -- _9 1000 0 1000 2000 3000 4000 5000 FEET D CAMBRIAN SEDIMENTS D RUSSELL MOUNTAIN RHYOLITE D TAUM SAUK RHYOLITE w LINDSEY MOUNTAIN RHYOUTE D ROYAL GORGE RHYOLITE CJ IRONTON RHYOLITE BUCK MOUNTAIN SHUT -INS D BELL MOUNTAIN RHYOLITE D FORMATION D WILDCAT MOUNTAIN RHYOLITE D POND RIDGE RHYOLITE FAULT; BAR ON DOWNTHROWN · SIDE Figure 3 Geologic map of the Taum Sauk Mountain region. 2 Regional Geology The St. Francois Mountains of southeast Missouri are located at the crest of the Ozark dome. The St. Francois terrain is characterized by Precambrian igneous rocks exposed through a cover of lower Paleozoic sedimentary rocks. The principle rock types exposed in the St. Francois terrain include silicic volcanics, and associated granitic plutons. The silicic volcanics, chiefly alkali rhyolite ash-flow tuffs, form about two-thirds of the outcrop area {Pratt et. al, 1979). Three types of granite have been identified using bulk chemical methods; 1) Amphibole granites representing ring fracture and porphyry intrusives 2) Biotite granites representing sub-volcanic massifs 3) Tin granites representing the main batholiths (E. Kisvarsanyi, 1980). Hypabyssal basic dikes and sills of the Skrianka diabase cut across all volcanic and plutonic rocks of the region and represent the youngest period of igneous activity. The St. Francois terrain is interpreted as representing several volcanic-plutonic ring complexes, identified on the basis of petrographic: geochemical, structural subsurface, and aeromagnetic data (E. Kisvarsanyi, 1980). The regional terrain has been dated at approximately 1.5 b.y. using U-Pb ages of separated zircons (Bickford and Mose, 1975). This time interval corresponds to a time when anorogenic volcanic-plutonic complexes were emplaced in the mid­ continent basement (Muehlberger et. al, 1967; Silver et. al, 1977). The hills in the St. Francois terrain, composed of igneous rocks, rise as much as 250 m. above the valleys. Upper Cambrian sedimen_try rocks fill the valleys, lying unconformably on the Precambrian igneous rocks. Stratigraphic relationships indicate that there was an extensive period of erosion, prior to the deposition of the Cambrian sediments, that produced considerable relief throughout the igneous terrain (Sides et. al, 1981). The present landscape is primarily the exhumed pre-late Cambrian terrain, where the topographic highs are supported by resistant volcanic rocks, and the topographic lows are easily weathered granites, or areas of low paleorelief filled with Cambrian or younger sedimentary rocks. 3 Volcanic Terrian The Precambrian volcanic terrain of the St.Francois Mountains consists primarily of ash-flow tuffs, lava flows, and minor bedded tuffs, chiefly of rhyolitic composition (R. Anderson, 1962, 1970). These volcanics are preserved only locally on the tops of buried basement knobs and along the flanks of the volcanic terrain (fig 4) (E. Kisvarsanyi, 1981). These sites of preservation correspond to former structural depresions, such as cauldron subsidence stuctures and extra cauldra depresions, where the volcanics were protected while the surrounding highlands were eroded. Several structural complexes in the St. Francois terrain have been suggested to represent cauldron subsidence structures. This interpretation is supported by several lines of evidence. 1) The presence of sub-volcanic massif granites surrounding granite-porphyry ring intrusives 2) The ring intrusives of granite porphyry enclosing remnants of the volcanic terrain, suggesting the structural control involved in preserving the rhyolites. The granite porphyry is assumed to have intruded along the Precambrian ring faults that bound the cauldron subsidence structure 3) Random stuctural attitudes in the volcanics, suggesting large scale brecciation often associated with cauldron subsidence and cauldron collapse stuctures. 4) Downfaulting of the volcanics allowing preservation while the surrounding region was eroded (E. Kisvarsanyi, 1981). It should be noted that observed contacts between rhyolites and biotite granites (sub-volcanic massifs) generally dip at low angles, and lack evidence of forceful intrusion (Tolman and Robertson, 1969). This relationship further suggests that the volcanic pile was supported by its own subvolcanic massif and lacked a solid floor. The sub-volcanic massif granites are essentially the intrusive equivilants of the rhyolites. These rising plutons charged with volatiles exploded through the surface, producing the volcaniclastic deposits and lava flows, and subsequently consolidated below the volcanic pile (E. Kisvarsanyi, 1981). The Eminence region of the St. Francios terrain exemplifies these structural relationships (fig 5). The Graniteville pluton northwest of Ironton coritains structural evidence suggesting a resurgent cauldron structure. A resurgent cauldron (Smith and Baily, 1962, 1964) is defined as a cauldron within which the cauldron block, after initial subsidence, has been uplifted, usually in the form of a structural dome. Crosscutting relationships show that the tin granites (those forming the central plutons) of the Graniteville pluton are younger than the amphibole and biotite granites of the subvolcanic