Abstract In Scotland and Ireland, a Laurentian passive margin sequence, the Dalradian Supergroup, was deformed during the c. 470–460 Ma Grampian orogeny, resulting in the formation of crustal-scale recumbent nappes. In Ireland, this passive margin sequence is in general bounded to the SE by the Fair Head-Clew Bay Line (FHCBL), a segment of a major lineament within the Caledonides. Adjacent to the FHCBL, Dalradian metasediments in two separate inliers have undergone post-Grampian strike-slip movement, with the initially flat-lying Grampian nappe fabric acting as a décollement-like slip surface in both cases. As the orientation of these foliation slip surfaces was oblique to the local shear plane in both inliers, displacement along these pre-existing foliation surfaces was also accompanied by crenulation slip. However, the crenulation-slip morphologies produced imply the opposite sense of movement in the two inliers. 40 Ar- 39 Ar dating of muscovite defining the crenulation-slip surfaces indicates that post-Grampian dextral displacement took place along the FHCBL at 448 ± 3 Ma. A subsequent phase of sinistral movement along the FHCBL took place at c. 400 Ma, based on previously published Rb-Sr muscovite ages for synkinematic pegmatites. The kinematic information obtained from crenulationslip morphologies combined with geochronology can thus be used to constrain the reactivation history of a major crustal-scale shear zone.
The majority of carbon sequestration at the Earth’s surface occurs in marine continental margin settings within fine-grained sediments whose mineral properties are a function of continental climatic conditions. We report very high mineral surface area (MSA) values of 300 and 570 m 2 g in Late Cretaceous black shales from Ocean Drilling Program site 959 of the Deep Ivorian Basin that vary on subcentennial time scales corresponding with abrupt increases from approximately 3 to approximately 18% total organic carbon (TOC). The observed MSA changes with TOC across multiple scales of variability and on a sample-by-sample basis (centimeter scale), provides a rigorous test of a hypothesized influence on organic carbon burial by detrital clay mineral controlled MSA. Changes in TOC also correspond with geochemical and sedimentological evidence for water column anoxia. Bioturbated intervals show a lower organic carbon loading on mineral surface area of 0.1 mg-OC m -2 when compared to 0.4 mg-OC m -2 for laminated and sulfidic sediments. Although either anoxia or mineral surface protection may be capable of producing TOC of < 5%, when brought together they produced the very high TOC (10–18%) apparent in these sediments. This nonlinear response in carbon burial resulted from minor precession-driven changes of continental climate influencing clay mineral properties and runoff from the African continent. This study identifies a previously unrecognized land–sea connection among continental weathering, clay mineral production, and anoxia and a nonlinear effect on marine carbon sequestration during the Coniacian-Santonian Oceanic Anoxic Event 3 in the tropical eastern Atlantic.
Summary Three zones can be recognised in the Canadian Appalachian-British Caledonian belt. The NW Marginal Zone extends from NW Scotland and Ireland through N Newfoundland to Quebec. It consists of late Precambrian to early Palaeozoic clastic sequences and an extensive Cambro-Ordovician carbonate platform. Orthotectonic deformation and metamorphism occurred in the early Ordovician, and middle Ordovician allochthons and obducted ophiolite characterise this zone in N America. The SE Marginal Zone extends from Wales and the English Midlands through SE Ireland to E Newfoundland, Nova Scotia and S New Brunswick. It is characterised by extensive late Precambrian volcanicity, late Precambrian orthotectonic deformation and metamorphism and the development of lower Palaeozoic basins in Wales and Nova Scotia. Late Precambrian orogenic activity locally involved ophiolite. The Axial Zone can be subdivided into a number of subzones, some of which do not extend for great distances along strike. It contains local ophiolites, trench complexes and calc-alkaline volcanics ranging in age from Cambrian to Silurian. Volcanicity was most pronounced in the Ordovician on both sides of this zone and extended into the marginal zones. Middle Ordovician deformation and metamorphism is locally intense but elsewhere Ordovician and Silurian sequences are apparently conformable. The Axial Zone and adjacent parts of the marginal zones have all suffered Siluro-Devonian, generally paratectonic, deformation and metamorphism. The history of the whole belt indicates that subduction was active on the SE in the late Precambrian, on the NW and in the Axial Zone in the lower and middle Ordovician and extended into the Silurian. Final closure of the Iapetus ocean was delayed until the upper Silurian-Devonian. Plate-tectonic models must involve multiple subduction zones.
Kula et al. (2012) reanalyze our sequence stratigraphic interpretation and carbon isotope data for the younger Neoproterozoic cap carbonate interval in the northeastern Amadeus Basin of central Australia (Kennedy and Christie-Blick, 2011), and conclude that they are incompatible.This conclusion is unwarranted.Our paper focused on the interval between the upper part of the late Cryogenian Olympic Formation (glacial marine) and the lower part of the early Ediacaran Pertatataka Formation (a basin-wide marine siltstone representing post-glacial transgression).In relatively deep-water successions, such as the type section of the Olympic Formation (MS-11 in fi gure 3 of Kennedy and Christie-Blick, 2011), glacial deposits are sharply overlain by a several-meter-thick cap carbonate that is typical of Neoproterozoic successions worldwide, and that passes upward in turn to siltstone.To-
Research Article| June 01, 2011 Early Cambrian metazoans in fluvial environments, evidence of the non-marine Cambrian radiation Martin J. Kennedy; Martin J. Kennedy * Department of Earth Science, University of California−Riverside, Riverside, California 92557, USA *Current address: School of Earth and Environmental Science, University of Adelaide, Adelaide, SA 5005, Australia. Search for other works by this author on: GSW Google Scholar Mary L. Droser Mary L. Droser Department of Earth Science, University of California−Riverside, Riverside, California 92557, USA Search for other works by this author on: GSW Google Scholar Geology (2011) 39 (6): 583–586. https://doi.org/10.1130/G32002.1 Article history received: 15 Dec 2010 rev-recd: 04 Feb 2011 accepted: 09 Feb 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Martin J. Kennedy, Mary L. Droser; Early Cambrian metazoans in fluvial environments, evidence of the non-marine Cambrian radiation. Geology 2011;; 39 (6): 583–586. doi: https://doi.org/10.1130/G32002.1 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 Here we describe the oldest evidence of non-marine animals from the early Cambrian Wood Canyon Formation, California, evidence created by metazoans of a variety of sizes and behaviors. Millimeter-sized vertical trace fossils, including the U-shaped burrow Arenicolites and the vertical burrow Skolithos, as well as a centimeter-scale horizontal trace fossil, occur in conglomerate and gritty arkosic sandstone bed tops within fluvial channels. These fossils demonstrate that animals were dwelling in this habitat coincident with, or possibly predating, the first trilobites, and extend the freshwater record of animals back at least 80 m.y. The development of a functioning terrestrial ecosystem was concurrent with the early Cambrian marine radiation and suggests that freshwater environments were populated early by metazoans and that ecological opportunity likely played a determining role in metazoan exploitation of non-marine habitats versus commonly assumed influences from physiological or nonbiological barriers. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
We show that 85% of variation in total organic carbon can be explained by mineral surface area in a black shale deposit from two locations in the late Cretaceous Western Interior Seaway, United States. This relation suggests that, as in modern marine sediments, adsorption of carbon compounds onto clay mineral surfaces played a fundamental role in the burial and preservation of organic carbon. Our data also provide evidence for organic matter within the smectite interlayer. This association implies that organic carbon sequestration in a representative oil-prone black shale facies may be more closely related to patterns of continental weathering and clay mineralogy than to ocean water chemistry or marine productivity.
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