Chalcedony veins occur as local stratabound arrays at multiple levels within the finer-grained sediments of the White River Group, making up to 2%-3% of the outcrop volume. The veins are commonly deformed by small folds, faults with well-developed striae, and various fold-fault combinations, and they also exhibit striae and slickenslides on vein walls. These indicate significant vertical shortening of the veins. The combination of a stratabound distribution and vertical shortening is consistent with an origin by diagenetically driven deformation, where changes in clay and/or silica phases drive syneresis and associated dewatering and compaction. In this way, the chalcedony veins bear similarities in origin to stratabound polygonal normal fault systems seen in fine-grained marine strata. Smectite clays, silica phases, and clinoptolite in the White River Group are associated with diagenetic reactions that could produce syneresis. At different localities, vein strike distributions vary from being statistically random to highly organized. These distributions are also consistent with a syneresis origin, with local stress fields organizing the distribution into multiple coeval directions in some cases. Chalcedony veins locally occur inside and parallel to clastic dikes, clearly indicating that the veins were emplaced at the same time as or after the dikes. Thin-section textures from dike-vein composites indicate that vein formation occurred while the clastic fill was unlithified and still mobile. These relationships, along with common orientations when in proximity, link clastic dike and chalcedony vein formation. Dikes also show complex strike orientation distributions that differ by locality. Internal dike features indicate multiple fill events with intervening lithification. Evidence for vertical dike shortening suggests synchronous or later compaction. The clastic dikes are also postulated to result from syneresis. We suggest that chalcedony vein formation, silica mobilization, local uranium mineralization, and clastic dike formation are part of diagenetically driven fracture development that produced a fluid flow network, initiating feedback relationships among diagenesis, dewatering, fluid migration, and associated compaction. Given that the clastic dikes occur within the Sharps Formation, the event was Miocene or later.

Fragments of ancient continental lithosphere, entrained in the shallow oceanic mantle, have been found in a number of locations in the Southern Hemisphere. Isotopic analyses of material from Quaternary volcanic centres in Hispaniola indicate that the lavas are derived from an ancient lithospheric fragment with affinities to the supercontinent Gondwana.

The integration of student research into a general chemistry laboratory and an environmental geology course has been evaluated for its effectiveness to improve (i) student attitudes about science and chemistry, (ii) student understanding of the nature of experimental science and the scientific method, and (iii) student perceptions of the application of science and the interdisciplinary nature of science. Students in introductory science courses frequently devote all or most of their time and effort to learning basic laboratory techniques by following predefined procedures that have intentionally predictable outcomes. Though this longstanding model of laboratory structure is an important means of effectively preparing students for future chemical study, it often neglects items i−iii. Students were guided through a research experience by an interdisciplinary team of faculty so that students could begin the research and bring it to an appropriate conclusion, including formal presentations, within a single semester. Evaluation of this instructional strategy indicated that students believed they were doing work similar to a research scientist, that they appreciated this opportunity to do research, that it increased how much they like science in general, and that they were more likely to consider majoring in chemistry. The advanced instrumentation, personal relevance of the research, and collaborations (within and outside the department) contributed significantly to the overall success of the project.

Iron ooid beds are unusual deposits that have been linked to greenhouse conditions and the transgressive flooding of shallow shelves, and which were globally prevalent during certain periods. Within the marine, Aptian–Albian, Carolinefjellet Formation of Spitsbergen, chamosite ooids have been found within distinctive sandstone beds at six localities, and at a consistent stratigraphic position within the basal Dalkjegla Member. Distinctive characteristics include the iron ooids themselves, a coarser grain size, intercalation with silty siderites, grading, cross-beds indicating offshore or longshore transport, and a lack of burrowing. The enclosing sands display planar and hummocky crossstratification and abundant oscillation ripple marks, and are interpreted as lagoon-attached bar complexes. The stratigraphic position and traits of the iron ooid sands are consistent with seaward storm transport and preservation within interbar swales. Ooids vary in shape considerably, and display evidence for multiple growth events. Nuclei of quartz, opaques, carbonate clasts and laminated crusts are typically encircled by finer grained tangential chamosite and opaque laminae, sometimes with outer overgrowths of calcite and/or radial chamosite. The Dalkjegla Member is the marine portion of a large-scale transgressive tract, attached to underlying fluvio-estuarine Helvetiafjellet Formation strata. A lagoonal environment associated with the basal shales of the Dalkjegla Member represents a logical setting, where riverine iron concentration and iron silicate growth could occur. The Spitsbergen iron ooid beds extend the known occurrence of Cretaceous examples, representing a less common High-Latitude example, and one not directly associated with a transgressive flooding surface.

The sandstone petrography of sample suites from four sites spanning the Rurikfjellet (Hauterivian) to Carolinefjellet (Aptian–Albian) formations in central Spitsbergen was investigated. The sandstones show a distinct stepwise shift in composition from quartz arenites to sublitharenites and lithic arenites, typically within the upper part of the Helvetiafjellet Formation. This shift is related to the introduction of 10 - 25 % (grain %) plagioclase grains and volcanic lithics, and a notable increase in basement and sedimentary lithics. Quartz grain character also changes, and grain shapes become more varied. The shift is also associated with the transgressive arrival of marine sediments in the area, and the introduction of sands from the east-northeast by shore-parallel transport. Regional regression and subsequent transgression, and the change in sandstone composition is attributed to the development of the High Arctic Large Igneous Province in the region. The relative constancy of sand composition and volume of volcanic detritus within the Carolinefjellet Formation suggests long term (≈ 20 M) stability of the sediment system and a large volcanic source area, consistent with LIP (Large Igneous Province) derivation, along with significant exposure of basement rocks. Sample spacing and sediment recycling and mixing do not allow detection of events that would have changed sandstone composition that were less than ≈ 1 M duration. Preservation of significant amounts of plagioclase in a sediment-starved shelf can be explained by relatively cold climatic conditions.

The northeastern portion of the Idaho batholith (NIB) intruded Proterozoic rocks of the Belt-Purcell supergroup between 50 and 90 Ma. Whole-rock Sm-Nd isotopic analyses of batholithic rocks yield depleted mantle model ages ($$T_{DM}$$) between 1.72 and 1.93 Ga and values of $$\epsilon_{Nd}$$ between -17.7 and -21.2, similar to associated metamorphic rocks and within the range for Belt-Purcell sedimentary rocks. Premagmatic zircons from one sample of the NIB were analyzed individually using the SHRIMP ion microprobe and yielded a single age population at 1.74 Ga. This apparently single-aged source contrasts with the range of ages reported for zircons from sedimentary rocks of the Belt-Purcell supergroup and suggests that the batholith was not the product of melting Belt-Purcell sediments, nor was it significantly contaminated with these sediments. The source of the batholith, however, appears to be of appropriate age and composition to be a major contributor of sediment to the Belt basin. In addition, the near coincidence of $$T_{DM}$$ and the age derived from premagmatic zircons in one sample suggests the source of at least part of the batholith was extracted largely from 1.74 Ga depleted mantle, with little or no input from older rocks. If so, this crust may represent a possible continuation of crust of similar age and character exposed to the north in the Canadian cordillera and to the south in Nevada, Arizona, and southeastern California.

The vertebrate fauna of the Leisey Shell Pit near Tampa Bay, Florida, represents one of the more significant Irvingtonian mammalian faunas of North America. The fossil vertebrates occur in thin bone beds bounded above and below by massive shell beds containing a rich invertebrate (chiefly molluscan) fauna. Debate has arisen concerning the precise age of the faunas at Leisey. Although generally agreed to be Pleistocene, estimates based upon vertebrate biostratigraphy suggest a somewhat older age than do estimates based upon molluscan biostratigraphy. To help resolve this controversy, 87Sr/86Sr ratios were determined on molluscan shells throughout the section. These ratios were then correlated to the global sea water 87Sr/86Sr curve for age detemination. The Sr isotopes support an early Pleistocene age for the vertebrate fauna and suggest a complex history for the shell accumulations.

A Gondwanan origin for the pre-Cretaceous basement of Florida is suggested by U-Pb ages of 515 to 2860 Ma for single zircons separated from subsurface samples of lower Paleozoic sandstone of the Suwannee basin (Alachua County, Florida) and Neoproterozoic Osceola granite (Osceola County, Florida). Forty individual grains analyzed by SHRIMP ion microprobe yielded ages from 515 to 2860 Ma; ages >1000 Ma were predominantly concordant. Two principal populations are evident: (1) 515 to 637 Ma (avg. = 574 Ma; Pb-206/U-238 ages) and (2) 1967 to 2282 Ma (avg. = 2130 Ma; Pb-207/Pb-206 ages). Only six zircons were recovered from the granite; four are Pan-African and two are Archean. For the sandstone, the similarity between the Sm-Nd model age (1245 Ma) and the average of the single zircon ages (1326 Ma) suggests that these zircons are chronologically representative of the aggregate provenance of the sandstone. The two dominant zircon age groupings correspond chronologically to the Pan-African and Birimian or Eburnian (Africa) and to the Brasiliano and Trans-Amazonian (South America) orogenic cycles. The presence of detritus from rocks of these two orogenic cycles clearly places the basement of Florida in Gondwanaland proximal to the West African and/or Trans-Amazonian-San Luis cratons in the early Paleozoic, a location it probably shared with other circum-Atlantic exotic terranes (Avalonian, Cadomian, and/or Carolina).

The southern Madison Range of southwestern Montana contains two distinct Precambrian lithologic assemblages: (1) a complex of tonalitic to granitic gneisses that has been thrust over (2) a medium-grade metasupracrustal sequence dominated by pelitic schist. Crystallization ages for the protolith of a granodioritic gneiss that intruded the meta-supracrustal sequence (approximately 2.6 Ga) along with an intercalated meta-andesite (approximately 2.7 Ga) confirm the sequence as Archean. Chemical (major and trace element), isotopic (Rb-Sr, Sm-Nd, Pb-Pb), and geochronologic (U-Pb zircon) data for selected components of the gneiss complex indicate two groups of gneisses: an older, tonalitic to trondhjemitic group (approximately 3.3 Ga) and a younger, mostly granitic group (approximately 2.7 Ga).Both groups of gneisses exhibit the radiogenic Pb and nonradiogenic Nd isotopic signature characteristic of Middle and Late Archean rocks from throughout the Wyoming province. The older gneisses, in particular, appear to be compositionally, isotopically, and chronologically comparable to other Middle Archean gneisses from the northern part of the province (for example, Beartooth Mountains). The Late Archean gneisses, however, exhibit some distinct differences relative to their temporal counterparts, including (1) trace-element patterns that are more suggestive of crustal melts than subduction activity and (2) higher initial Sr isotopic ratios that suggest more involvement of older crust in their petrogenesis. These comparisons suggest that the juxta-position of Late Archean terranes in the northern Wyoming province was the result, at least in part, of intracratonic processes.

Lavas from Akutan Island, located in the eastern Aleutian arc at the transition between continental and oceanic crust, show a gradual change in their petrologic and chemical characteristics over the last 4 million years. The oldest lavas exposed on the island, the Hot Springs Bay Volcanics (HSBV), range from magnesian basalt to dacite (45%–62% SiO2). The most mafic basalts contain salitic clinopyroxene, Cr- and Al-rich spinel, and pargasitic amphibole suggesting that they were derived from relatively hydrous magmas at greater pressures than lavas from the younger Akutan Volcanics (AKV) and the modern volcano (MOD). AKV lavas also range between basalt and dacite (46%–63% SiO2), but contain no hydrous phenocrysts and seem to have fractionated within a shallow level magma chamber. Lavas from the modern volcano are andesitic (52%–57% SiO2) and have a mineral assemblage similar to that of AKV lavas of similar composition. With the exception of clinopyroxene and spinel in the most mafic lavas, the compositions of plagioclase (An92−45), olivine (Fo88−51), orthopyroxene (En69−56), and titanomagnetite (15%–21% TiO2) phenocrysts found in these lavas are within the range observed in lavas from other Aleutian volcanoes. Variations in the major element chemistry of the older lavas can be reproduced by fractional crystallization of the observed mineral assemblages, however closed system crystal fractionation models are inadequate to explain the trace element variations. During the last 4 million years, La/Yb ratios have decreased (6.5–3.3 for HSBV lavas and 2.9–1.9 for MOD lavas) whereas Ba/La ratios appear to have increased slightly (37–43 for HSBV and AKV, and 41–45 of MOD). The lower La/Yb ratios of MOD lavas correspond with lower total abundances of the REE and slightly higher Sr and Pb isotopic ratios. The increased87Sr/86Sr ratios and Pb isotopic ratios in the MOD lavas, the less enriched LREE, and the higher Ba/La ratios may result from partial melting of an arc source which has experienced previous melting events but has continued to be contaminated by a component from the subducting slab. It may also indicate a change in the degree of partial melting of the underlying mantle, which corresponds to a different percentage of a slab derived component being incorporated into the overlying mantle.

Research Article| October 01, 1987 Florida as an exotic terrane: Paleomagnetic and geochronologic investigation of lower Paleozoic rocks from the subsurface of Florida Neil D. Opdyke; Neil D. Opdyke 1Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Douglas S. Jones; Douglas S. Jones 2Florida State Museum, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Bruce J. MacFadden; Bruce J. MacFadden 2Florida State Museum, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Douglas L. Smith; Douglas L. Smith 3Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Paul A. Mueller; Paul A. Mueller 3Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Robert D. Shuster Robert D. Shuster 3Department of Geology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: GSW Google Scholar Geology (1987) 15 (10): 900–903. https://doi.org/10.1130/0091-7613(1987)15<900:FAAETP>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Neil D. Opdyke, Douglas S. Jones, Bruce J. MacFadden, Douglas L. Smith, Paul A. Mueller, Robert D. Shuster; Florida as an exotic terrane: Paleomagnetic and geochronologic investigation of lower Paleozoic rocks from the subsurface of Florida. Geology 1987;; 15 (10): 900–903. doi: https://doi.org/10.1130/0091-7613(1987)15<900:FAAETP>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract New geophysical and geochronological data are presented for a lower Paleozoic subsurface (794–827 m depth) sedimentary core recovered from Alachua County, northern Florida. Paleomagnetic data from samples taken at various depths along the core indicate that a high unblocking temperature component is isolated during thermal demagnetization between 400 and 600 °C. The mean inclination of this component is 65°, yielding a paleolatitude of 49°. Previous paleogeographic reconstructions for the early Paleozoic of North America place Florida at a significantly lower paleolatitude (∼28°), whereas our data are consistent with a Gondwanian fit (∼50°–60°). U/Pb isotopic systematics of zircons separated from core samples yield five dates ranging from ca. 1650 to 1800 Ma. Because there are no known source rocks of this antiquity in southeastern North America, the most probable provenance for these detrital zircons is either Africa or South America. These two new lines of geologic data provide strong evidence confirming previous suggestions that Florida was part of Gondwana during the early Paleozoic and that its current configuration is that of an exotic terrane sutured to North America during the fragmentation of Pangea. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late Archean rocks of the eastern Beartooth Mountains range in composition from basaltic andesite to granite and were emplaced 2.73-2.80 Ga ago in a middle to early Archean terrane as indicated by U-Pb zircon studies. Although trace element abundances are extremely variable for this group of rocks, their initial Pb, Sr, and Nd isotopic compositions are remarkably homogenous. A composite Rb-Sr isochron (>30 samples) yield an age of 2.79/plus minus/0.04 Ga with an initial ratio of 0.7022/plus minus/2 while /epsilon/Nd 2.78 Ga ago ranges from -1.5 to -3.1 (av. -2.2). Whole-rock Pb data for these rocks scatter about a 2.75 Ga isochron and feldspar Pb data suggest initial 206/204 = 13.88, 207/204 = 14.96, and 208/204 = 34.3. These values lie well above values for average crustal leads 2.78 Ga ago as modeled by Stacey and Kramer (1975) and would require development in a reservior with /mu/= 12 from 3.7-2.8 Ga (/mu/= 7.2, 4.5-3.7 Ga). The marked differences between these values and those of the late Archean mantle require that an early to middle Archean crust played a role in the genesis of these rocks. The compositional variety and isotopic homogeneity may have developed as the result of crust-mantle mixingmore » similar to that observed in modern volcanic-plutonic arcs along continental margins where crustal materials can be subducted, and fluids derived from these materials added to the overlying mantle wedge and lower crust. During this period, contaminated mantle may have been generated on a regional scale as evidenced by the isotopic systematics of young mafic volcanics from the northwestern U.S. (e.g. Snake River Plain, Yellowstone, Columbia River).« less