Fine-grained sedimentation on the Chenier Plain Coast and inner continental shelf, northern Gulf of Mexico

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2003This thesis examines the evolution of a mud-dominated coastal sedimentary system on multiple time scales. Fine-grained systems exhibit different properties and behavior from sandy coasts, and have received relatively little research attention to date. Evidence is presented for shoreline accretion under energetic conditions associated with storms and winter cold fronts. The identification of energetic events as agents of coastal accretion stands in contrast to the traditional assumption that low-energy conditions are required for deposition of fine-grained sediment. Mudflat accretion is proposed to depend upon the presence of an unconsolidated mud sea floor immediately offshore, proximity to a fluvial sediment source, onshore winds, which generate waves that resuspend sediment and advect it shoreward, and a low tidal range. This study constrains the present influence of the Atchafalaya River on stratigraphic evolution of the inner continental shelf in western Louisiana. Sedimentary and acoustic data are used to identify the western limit of the distal Atchafalaya prodelta and to estimate the proportion of Atchafalaya River sediment that accumulates on the inner shelf seaward of Louisiana's chenier plain coast. The results demonstrate a link between sedimentary facies distribution on the inner shelf and patterns of accretion and shoreline retreat on the chenier plain coast.Among my funding sources was a two-year fellowship from the Clare Booth Luce Foundation. I have received research grants from the Geological Society of America Foundation (Grant 6873-01) and the American Association of Petroleum Geologists (Kenneth H. Crandall Memorial grant)

Climate stability during the Pliocene warm period

We present a high-resolution climate record from a sediment core spanning an 80-kyr interval of time during the mid-Pliocene epoch, when warmer conditions and lower global ice volume prevailed worldwide. Oxygen and carbon isotope analyses were made on benthic and planktonic foraminifera from ODP Site 981 in the North Atlantic. The amplitude and approximate recurrence interval of suborbital variations in these records are comparable to those of Holocene and marine isotope stage 11 (MIS 11) records from the North Atlantic. We conclude that the mid-Pliocene warm interval was a time of relative climatic stability. These results suggest that warmer climatic conditions alone may not necessarily enhance variability in the climate system, a finding that may facilitate predictions of 21st century climatic response to anthropogenic warming

Arc–continent collision and the formation of continental crust : a new geochemical and isotopic record from the Ordovician Tyrone Igneous Complex, Ireland

Author Posting. © Geological Society of London, 2009. This is the author's version of the work. It is posted here by permission of Geological Society of London for personal use, not for redistribution. The definitive version was published in Journal of the Geological Society 166 (2009): 485-500, doi:10.1144/0016-76492008-102.Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth’s history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian-Taconic Orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U–Pb zircon dating yields ages of 493 ± 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 ± 10 Ma from a light-rare-earth-element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc-continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material which would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian-Taconic Orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc-continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust.This work was supported by the University of Aberdeen. LA-MC-ICPMS dating was conducted at the Arizona LaserChron Center with the assistance of George Gehrels and Victor Valencia and was supported by NSF-EAR 0443387

Effects of river regulation on aeolian landscapes, Colorado River, southwestern USA

Connectivity between fluvial and aeolian sedimentary systems plays an important role in the physical and biological environment of dryland regions. This study examines the coupling between fluvial sand deposits and aeolian dune fields in bedrock canyons of the arid to semiarid Colorado River corridor, southwestern USA. By quantifying significant differences between aeolian landscapes with and without modern fluvial sediment sources, this work demonstrates for the first time that the flow- and sediment-limiting effects of dam operations affect sedimentary processes and ecosystems in aeolian landscapes above the fluvial high water line. Dune fields decoupled from fluvial sand supply have more ground cover (biologic crust and vegetation) and less aeolian sand transport than do dune fields that remain coupled to modern fluvial sand supply. The proportion of active aeolian sand area also is substantially lower in a heavily regulated river reach (Marble–Grand Canyon, Arizona) than in a much less regulated reach with otherwise similar environmental conditions (Cataract Canyon, Utah). The interconnections shown here among river flow and sediment, aeolian sand transport, and biologic communities in aeolian dunes demonstrate a newly recognized means by which anthropogenic influence alters dryland environments. Because fluvial–aeolian coupling is common globally, it is likely that similar sediment-transport connectivity and interaction with upland ecosystems are important in other dryland regions to a greater degree than has been recognized previously

Measurements of wind, aeolian sand transport, and precipitation in the Colorado River corridor, Grand Canyon, Arizona--November 2003 to December 2004

70 pages (PDF version). 23961.018 KB

Origin and significance of the Delaney Dome Formation, Connemara, Ireland

Author Posting. © Geological Society of London, 2002. This article is posted here by permission of Geological Society of London for personal use, not for redistribution. The definitive version was published in Journal of the Geological Society 159 (2002): 95-103, doi: 10.1144/0016-764901034Dalradian meta-sediments of the Laurentian margin and mafic intrusions thereof in SW Connemara, Ireland, tectonically overlie meta-rhyolites of the Delaney Dome Formation. The two units are separated by the Mannin Thrust. A new U–Pb age of 474.6 ± 5.5 Ma shows that the Delaney Dome Formation is a temporal equivalent of arc volcanic rocks preserved in the adjacent South Mayo Trough: the Tourmakeady Volcanic Group, erupted during the collision of an oceanic island arc with the Laurentian margin in the Grampian Orogeny. New rare earth and high field strength element data show that the Delaney Dome Formation and Tourmakeady Volcanic Group are chemically similar and arc-like in character. This suggests that the Delaney Dome Formation is an along-strike equivalent of the Tourmakeady Group, strike-slip faulted south of the South Mayo Trough during or after the Grampian Orogeny. Further correlation of these units with northern Appalachian rhyolites is also possible. The Delaney Dome Formation is an extrusive temporal equivalent of intrusions that penetrate the Connemara Dalradian. Thus, movement along the Mannin Thrust brought mid-crustal plutons and Dalradian country rocks tectonically above the extrusive volcanic sequence. The Mannin Thrust is identified as a major imbricating structure within a continental arc, but not a terrane boundary

Vegetation and substrate on aeolian landscapes in the Colorado River corridor, Cataract Canyon, Utah

Vegetation and substrate data presented in this report characterize ground cover on aeolian landscapes of the Colorado River corridor through Cataract Canyon, Utah, in Canyonlands National Park. The 27-km-long Cataract Canyon reach has undergone less anthropogenic alteration than other reaches of the mainstem Colorado River. Characterizing ecosystem parameters there provides a basis against which to evaluate future changes, such as those that could result from the further spread of nonnative plant species or increased visitor use. Upstream dams have less effect on the hydrology and sediment supply in Cataract Canyon compared with downstream reaches in Grand Canyon National Park. For this reason, comparison of these vegetation and substrate measurements with similar data from aeolian landscapes of Grand Canyon will help to resolve the effects of Glen Canyon Dam operations on the Colorado River corridor ecosystem

Geochemical evolution of arc magmatism during arc-continent collision, South Mayo, Ireland

The Lough Nafooey arc, in the western Irish Caledonides, collided with Laurentia during the Early Ordovician. This event is recorded in the stratigraphy of the South Mayo trough, the preserved forearc basin of this system. Lavas at the base of the oldest Lough Nafooey Group show intraoceanic arc composition. εNd(t) decreases and light rare earth elements (REEs) become more enriched upsection in the Lough Nafooey Group (ca. 495-481 Ma), reflecting early collision with Laurentia. The subsequent Tourmakeady Group (ca. 481-470 Ma) is rhyolitic, light REE enriched, and has strongly negative εNd(t). These rocks were erupted during the Grampian orogeny. The Rosroe and Mweelrea Formations (\u3c470 Ma) show wide scatter of La/Sm and Nb/Zr values, suggesting mixed mantle sources. This correlates with rapid exhumation of the adjacent Connemara metamorphic terrane. The chemical evolution of the arc supports models of collision, orogeny, and tectonic unroofing within ∼15 m.y., and shows that genesis of magmas more enriched than continental crust can occur during arc-continent collision, clarifying the potential role of arc volcanism in continental crust formation

Basins in ARC-Continent Collisions

Arc-continent collisions occur commonly in the plate-tectonic cycle and result in rapidly formed and rapidly collapsing orogens, often spanning just 5-15 My. Growth of continental masses through arc-continent collision is widely thought to be a major process governing the structural and geochemical evolution of the continental crust over geologic time. Collisions of intra-oceanic arcs with passive continental margins (a situation in which the arc, on the upper plate, faces the continent) involve a substantially different geometry than collisions of intra-oceanic arcs with active continental margins (a situation requiring more than one convergence zone and in which the arc, on the lower plate, backs into the continent), with variable preservation potential for basins in each case. Substantial differences also occur between trench and forearc evolution in tectonically erosive versus tectonically accreting margins, both before and after collision. We examine the evolution of trenches, trench-slope basins, forearc basins, intra-arc basins, and backarc basins during arc-continent collision. The preservation potential of trench-slope basins is low; in collision they are rapidly uplifted and eroded, and at erosive margins they are progressively destroyed by subduction erosion. Post-collisional preservation of trench sediment and trench-slope basins is biased toward margins that were tectonically accreting for a substantial length of time before collision. Forearc basins in erosive margins are usually floored by strong lithosphere and may survive collision with a passive margin, sometimes continuing sedimentation throughout collision and orogeny. The low flexural rigidity of intra-arc basins makes them deep and, if preserved, potentially long records of arc and collisional tectonism. Backarc basins, in contrast, are typically subducted and their sediment either lost or preserved only as fragments in melange sequences. A substantial proportion of the sediment derived from collisional orogenesis ends up in the foreland basin that forms as a result of collision, and may be preserved largely undeformed. Compared to continent-continent collisional foreland basins, arc-continent collisional foreland basins are short-lived and may undergo partial inversion after collision as a new, active continental margin forms outboard of the collision zone and the orogen whose load forms the basin collapses in extension. © 2012 Blackwell Publishing Ltd

Sedimentary processes in modern and ancient oceanic arc settings: Evidence from the Jurassic Talkeetna formation of Alaska and the Mariana and Tonga Arcs, Western Pacific

Sediment deposited around oceanic volcanic arcs potentially provides the most complete record of the tectonic and geochemical evolution of active margins. The use of such tectonic and geochemical records requires an accurate understanding of sedimentary dynamics in an arc setting: processes of deposition and reworking that affect the degree to which sediments represent the contemporaneous volcanism at the time of their deposition. We review evidence from the modern Mariana and Tonga arcs and the ancient arc crustal section in the Lower Jurassic Talkeetna Formation of south-central Alaska, and introduce new data from the Mariana Arc, to produce a conceptual model of volcaniclastic sedimentation processes in oceanic arc settings. All three arcs are interpreted to have formed in tectonically erosive margin settings, resulting in long-term extension and subsidence. Debris aprons composed of turbidites and debris flow deposits occur in the immediate vicinity of arc volcanoes, forming relatively continuous mass-wasted volcaniclastic records in abundant accommodation space. There is little erosion or reworking of old volcanic materials near the arc volcanic front. Tectonically generated topography in the forearc effectively blocks sediment flow from the volcanic front to the trench; although some canyons deliver sediment to the trench slope, most volcaniclastic sedimentation is limited to the area immediately around volcanic centers. Arc sedimentary sections in erosive plate margins can provide comprehensive records of volcanism and tectonism spanning \u3c 10 My. The chemical evolution of a limited section of an oceanic arc may be best reconstructed from sediments of the debris aprons for intervals up to 20 My but no longer, because subduction erosion causes migration of the forearc basin crust and its sedimentary cover toward the trench, where there is little volcaniclastic sedimentation and where older sediments are dissected and reworked along the trench slop. Copyright © 2006, SEPM (Society for Sedimentary Geology)