Sedimentology and Authigenesis of the Lower Devonian Torbrook Formation Ironstone, Torbrook, Nova Scotia, Canada
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Sedimentology and Authigenesis of the Lower Devonian Torbrook Formation Ironstone, Torbrook, Nova Scotia, Canada By Luke A. Marshall Thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of Science with Honours in Geology Department of Earth and Environmental Science Acadia University Wolfville, Nova Scotia © Luke Marshall, 2011 This thesis by Luke A. Marshall is accepted in its present form by the Department of Earth and Environmental Science as satisfying the thesis requirements for the degree of Bachelor of Science with Honours Approved by the Thesis Supervisor ___________________________ ________________ (Dr. Peir K. Pufahl) Date Approved by the Head of the Department ___________________________ ________________ (Dr. Robert Raeside) Date Approved by the Honours Committee ___________________________ ________________ (Dr. Sonia Hewitt) Date ii I, Luke A. Marshall, grant permission to the University Librarian at Acadia University to reproduce, loan, or distribute copies of my thesis in microform, paper, or electronic formats on a non-profit basis. I however retain the copyright in my thesis. ________________________________________ Luke A. Marshall ________________________________________ Date iii Acknowledgements I would like to thank my supervisor, Dr. Peir Pufahl, for his guidance and assistance. Don Osburn is gratefully acknowledged for preparing thin and polished sections. I would also like to thank Drs. Chris White and Sandra Barr for providing information related to the Torbrook Formation (Rockville Notch Group). Haixin Xu assisted with the scanning electron microscopy and Sara Akin helped while in the field. Ivan Trimper is acknowledged for the historical background on mining in Torbrook. I would also like to thank Acadia University and the Department of Earth and Environmental Science for use of their laboratory equipment. Funding was provided by a NSERC Discovery Grant to Dr. Peir Pufahl. iv Table of Contents Approval Page…………………………………………………………………………….ii Permission for Duplication page…………………………………………………………iii Acknowledgements………………………………………………………………………iv Table of Contents…………………………………………………………………………v List of Figures……………………………………………………………………………vii Abstract……………………………………………………………………………...…..viii 1. Introduction……………………………………………………………………………1 2. Background…………………...……………………………………………………….2 2.1 Regional Geology and Stratigraphy…...……………………………………………...2 2.2 Paleoenvironments of the Torbrook Formation ..…………………………….............5 2.3 Mining……………………………………………………………………….………...5 2.4 Phanerozoic Ironstones ………………………………………...………………….....6 3. Methods………………………………………………………………………………...7 4. Sedimentologic attributes and Paleoevnironments…….…………...……………….8 4.1.1 Lithofacies 1 – Interbedded laminated mudstone and ripple cross laminated sandstone .………………………………………...……………………………………...10 4.1.2 Lithofacies 2 – Dark-grey to black, laminated, pyritic, mudstone…………..……..11 4.1.3 Lithofacies 3 – Brachiopod-rich, medium-grained sandstone…………….……….12 4.1.4 Lithofacies 4 – Nodular ironstone hummocky cross-stratified, fine-grained sandstone……………...………………………………………………………………….13 4.1.5 Lithofacies 5 – Iron-rich, hummocky cross-stratified, fine grained sandstone…....15 4.1.6 Lithofacies 6 – Bioturbated mudstone……………………………………..………18 4.2 Paraseqence descriptions ……………………………………....……………..........19 5. Discussion………………………….………………………………………...……….20 5.1 Authigenesis in the Torbrook Formation…………………………...………………..20 5.2 Coated iron grains and seafloor processes………………………………….……....24 v 6. Conclusions…………..…………………………………………………...………28 7. References……………………….…………………………………………..........30 Appendix I (Sample Descriptions)..……………………………...…………………..33 Appendix II (Thin Section Microscopy Descriptions) ……………………………....35 Appendix III (Stratigraphic Field Logs)……………………………...…….………..37 vi List of Figures 1. Stratigraphic map of Spinney Brook study area……………………………..…………4 2. Torbrook Formation stratigraphic column showing parasequences, lithofacies and stratigraphic sample locations.………………………………...……..…………….……...9 3. Transmitted light photomicrograph, crossed polars (XP) of stylotitic mudstone and burrow filled with lithified coarser sediments…………………………………………...10 4. Transmitted light photomicrograph (XP), pyrite crystal with crinoid grain…………..11 5. Transmitted light photomicrograph (XP), brachiopod shell fragment………………...12 6. Transmitted light photomicrograph (XP) of an iron nodule…………………..………13 7. SEM backscatter X-ray image of an iron nodule……………………………...………14 8. An Iron-rich, hummocky cross-stratified, fine-grained sandstone with scour surfaces, and corresponding transmitted light photomicrograph (XP) with quartz grains cemented with hematite and francolite…………………………………..………………………….16 9. Transmitted light photomicrographs of coated grains………………………..……….17 10. Hematite cementing coarser silt-sized grains of in-filled burrow….…………..….....18 11. Formation of iron oxide, iron silicate and francolite………………..…………….....21 12. Redox aggraded and Uncnformity bounded grain formation through redox pumping……………………………………………………………………………….…25 13. SEM backscatter X-ray image of an unconformity-bounded grain.………..………..27 14. SEM backscatter X-ray images of several unconformity-bounded grains……..……28 vii Abstract Ironstone in the lower Devonian Torbrook Formation of Nova Scotia accumulated in an array of storm-dominated, middle and distal shelf environments. Lithofacies stacking patterns indicate deposition occurred through the development of seven parasequences. Paraseqeunces range in thickness from 40 to 250 m thick and become progressively thicker through the Torbrook Formation. Individual parasequences coarsen from parallel laminated mudstones and ripple cross-laminated sandstones to iron cemented hummocky cross-stratified sandstones that are capped by marine flooding surfaces. Ironstone firmgrounds that mark flooding surfaces contain in situ and transported brachiopods and crinoids, suggesting mesotrophic nutrient levels predominated across the shelf. Ironstone development is interpreted to have been restricted to flooding surfaces because low or net negative rates of sedimentation stabilized the iron redox interface within the sediment, which allowed pore waters to become saturated with iron. Saturation and precipitation of iron minerals across this redox interface is interpreted to have occurred through iron redox pumping. Iron redox pumping is a cyclic mechanism that concentrates iron in pore water through the dissolution of iron (oxyhydr)oxide below the iron redox interface. Precipitation occurs when ferrous iron diffuses upward through the sediment to combine with oxygen in suboxic pore water. This process links the cycling of iron to phosphorus and is likely responsible for the high levels of bioavailable phosphorus necessary to sustain firmground populations of brachiopods and crinoids. Important authigenic phases in these ironstone firmgrounds include hematite, chamosite, brethierine, and francolite. viii 1. Introduction Ironstone is a marine chemical sedimentary rock composed of more than 15 weight % Fe (Young and Taylor 1989). Ironstones are commonly associated with black shale that accumulated during the Devonian-Ordovician and Jurassic-Paleogene (Young and Taylor 1989). Rising sea level increased accommodation space and starved submerging shelves of diluting terrigenous clastics (Young 1989), preconditioning shelves for ironstone deposition (Taylor et al. 2002). Ironstone precipitates as peloids and cements within condensed sediments when iron is concentrated in pore water (Pufahl 2010). An oxygenated seafloor is a prerequisite for concentration because such conditions permit pumping of iron in pore water. Iron redox pumping is a cyclic mechanism that concentrates iron in pore water through the dissolution of iron (oxyhydr)oxide below the iron redox interface. In general, ironstones are Phanerozoic in age and contain more silica than Precambrian iron formations (Pufahl 2010). Iron formations also precipitated from an anoxic water column or at the seafloor from hydrothermally derived Fe and Si (Simonson and Hassler 2003). The purpose of this thesis is to investigate the sedimentology and stratigraphy of ironstone in the lower Devonian Torbrook Formation, Torbrook Mines Nova Scotia (Fig. 1). This 950 m thick succession provides an excellent opportunity to examine the depositional and authigenic processes of ironstone formation. An important aspect of this research is to understand the relationship between the benthic phosphorus cycle and iron precipitation, which is a common association in many Phanerozoic ironstones. Such information promises to yield new insights into the importance of firmground 1 development, nutrient cycling, and benthic productivity in middle and distal shelf environments. 2. Background 2.1 Regional Geology and Stratigraphy Sedimentary rocks in Nova Scotia form two major Gondwana Terranes with different Early Paleozoic depositional histories. The Avalon Terrane extends from northeastern mainland Nova Scotia to Cape Breton. The Meguma Terrane encompasses nearly all of southwestern mainland Nova Scotia. They were structurally juxtaposed during the Devonian Appalachian orogeny along the Cobequid–Chedabucto fault (Force and Barr 2005). The original relationship between the Avalon and Meguma terranes in the Paleozoic Appalachian orogen is controversial. Two major hypotheses exist: (1) these terranes developed along different parts of the Gondwanan margin in the late Neoproterozoic and were accreted to Laurentia as