63 research outputs found
Landscape evolution and sediment routing across a strike-slip plate boundary
No full textThe North Sakhalin Basin is a polyphase Neogene basin situated on an active strike-slip plate boundary between the Amur and Okhotsk microplates. This basin contains a sedimentary record of the Amur River, as well as the tectonic processes which have resulted in the formation and deformation of the basin. I use a multi-disciplinary approach, involving heavy mineral analysis, seismic interpretation and geomorphic observations and analyses, to constrain the evolution of the landscape, sedimentary basins and fluvial systems at this active continental margin. Detrital mineralogy and integrated sediment accumulation rates indicate that the drainage basin of the Amur River has been relatively stable since the Early Miocene, with no evidence for continental-scale drainage capture during this time. Instead, sediment delivery to the basin has responded to a number of tectonic and climatic processes, most notably the onset of Northern Hemisphere glaciation in the Pliocene and uplift and erosion of the North Sakhalin Basin in the Late Neogene. By contrast, the Colorado River, which also has a delta on an active strike-slip margin, has been profoundly affected by tectonic processes at the continental margin, resulting in major drainage re-organisation in the Late Neogene. Sediment delivery to the North Sakhalin Basin has been strongly affected by tectonic processes along the Sakhalin-Hokkaido Shear Zone. The basin underwent a phase of transtension during the Early Miocene (>15 Ma), followed by continued strike-slip offset during the Middle-Late Miocene (15-6.3 Ma), and finally transpression during the Pliocene which is still ongoing today. The diachronous, northeastward-propagating deformation and uplift of the North Sakhalin Basin (initiated between 6.3-3.6 Ma) is preserved in the geomorphic characteristics of fluvial networks, the first-appearance of recycled deltaic sediments and by onlapping reflector terminations on offshore anticlines. The landscape of Sakhalin is transient, and continuing to deform in the present day.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Erosion and deposition beneath the Subantarctic Front since the Early Oligocene
Get PDFThe Antarctic Circumpolar Current (ACC) spills across the Falkland Plateau into the South Atlantic as a series of high-velocity jets. These currents are a driving force for global overturning circulation, and affect climate by modulating CO2 exchange between the atmosphere and ocean, but their timing of onset remains controversial. We present new evidence of strong currents associated with the Subantarctic Front (SAF) jet since the earliest Oligocene (~34 Ma) based on a widespread erosional surface on the Falkland Plateau, preserved below a 30,000 km2 contourite sand deposit. This is the largest such feature ever to be recognized, and provides the most robust constraint of the initiation of the SAF to date. By contrast, the South Falkland Slope Drift is dominated by contourite mud of Pleistocene-Recent age, substantially younger than previous estimates, indicating a significant decrease in long-term current strength at that time. As ACC strength is primarily a function of the position of the South-Westerly Winds, our data indicates that associated currents are likely to increase substantially in a warming world. Likely implications include increased upwelling and associated carbon flux from the deep ocean to the atmosphere, a positive feedback loop not included in most future projections of atmospheric CO2.</p
Early Cretaceous deep-water bedforms west of the Guinea Plateau revise the opening history of the Equatorial Atlantic Gateway
No full textThe Equatorial Atlantic Gateway (EAG) was critical to Earth's climatic and oceanographic evolution during the Mesozoic, yet its early opening history remains enigmatic. Here, we present new 2D seismic reflection data and biostratigraphic ages from DSDP Site 367, integrated with tectonic reconstruction models, to constrain the sedimentary response to the evolution of the gateway. Seismic analysis reveals five stratigraphic units (U1 to U5) documenting tectonic and oceanographic changes in the Guinea Plateau margin. Morphosedimentary features identified in units U2 to U4, including sediment waves and contourite drifts, document changing current dynamics during EAG opening. We propose a two-stage model for the initial gateway opening: i) middle-late Aptian (∼117 to ∼113 Ma) formation of a marginal sea in the western EAG and overflow of Equatorial waters into the Central Atlantic, producing large sediment waves northwest of the gateway, and ii) latest Aptian-late Albian (from ∼113 Ma onwards) widening and deepening of the gateway, establishing more continuous water exchange and leading to the transition into contourite deposition. This direct sedimentary evidence shows the establishment of a marine connection started at around 117 Ma, significantly earlier than previous estimates, and coinciding with the onset of global climate cooling. These findings show the dynamic interplay between gateway opening, ocean circulation, and climate change during the middle Cretaceous, highlighting the pivotal role of ocean gateways in Earth's climate system
The Subantarctic Front as a sedimentary conveyor belt for tsunamigenic submarine landslides
Get PDFThe Subantarctic Front (SAF), one of the three main jets of the Antarctic Circumpolar Current (ACC), flows through a narrow gap in the North Scotia Ridge and then north-westward across the continental slope of Burdwood Bank, ~150 km south of the Falkland Islands. There, the SAF flows across a fold-and-thrust belt caused by oblique convergence at the active plate boundary between the Scotia Plate and South American Plate. We here use regional 2D and 3D seismic reflection data to show the interaction of the associated bottom currents with the active margin, particularly to understand the causes and consequences of a number of large submarine landslides located in the adjacent foredeep. Kinematic indicators from the landslide deposits show that they are derived from a single point source located in an embayment on the northern slope of Burdwood Bank, where we identify a large contourite drift deposit. This drift forms the depositional sink for an along-slope sediment routing system driven by currents associated with the SAF, with sediment being eroded from the Burdwood Terrace, transported ~200 km westward, and plastered against the middle-upper continental slope. The contourite drift is undercut by the core of the current, making the slope inherently unstable in this area. Numerical modelling of the landslides and resultant waves indicates the tsunamigenic potential of these events. Modelled peak wave elevations of up to 40 m inundate the southern coast of the Falklands for a ~100 km3 volume landslide, with a recurrence interval of 1 Ma or less. This research highlights preconditioning mechanisms for submarine failure on continental slopes dominated by strong ocean currents, and specifically, oceanographic controls on the frequency, magnitude and location of submarine landslides associated with contourite systems
Late glacial to holocene sedimentary facies of the Eirik Drift, southern Greenland margin: Spatial and temporal variability and paleoceanographic implications
No full textThe Eirik Drift, off southern Greenland, is one of a series of contourite deposits in the northern North Atlantic that record changes in the strength and location of western boundary currents in the region. To date however, the sedimentary facies, and particularly the variation in facies across this drift, have received relatively little investigation. Here, we present an analysis of the sedimentary facies observed within a transect of cores from the crest to toe of the Eirik Drift from late Pleistocene to Holocene. The Holocene sequence consists of muddy contourites with high sedimentation rates at the drift toe, and a condensed sequence of sandy contourites on the upper drift flanks, consistent with winnowing under strong bottom currents on the upper drift and deposition under a low velocity, sediment-laden current at the drift toe. We interpret this to be a combined result of episodic, high-energy benthic storm events associated with the East Greenland Current (EGC) on the upper drift and more continuous, lower velocity Deep Western Boundary Current (DWBC) on the drift flanks. The deglacial interval is represented by muddy contourites across the drift, with evidence for decreasing current activity (both EGC and DWBC) and more widespread ice-rafted deposition from the Bolling-Allerod into the Younger Dryas. Palaeocurrent data from this interval show two separate current directions at the crest of the drift, suggesting temporary, local detachment of the DWBC or EGC, linked to temporal variation in current strength. The late glacial interval consists of glaciomarine hemipelagites and muddy contourites, with evidence for a higher degree of current influence at shallower depths, consistent with a moderate EGC and weak DWBC. This is the first time that the EGC is recognised as having a significant role in sedimentation on the Eirik Drift
Defining regional and local sediment sources in the ancestral Colorado River system: A heavy mineral study of a mixed provenance unit in the Fish creek-vallecito basin, Southern California
No full textThe Colorado River has flowed across the dextral strike-slip plate boundary between the
North American and Pacific plates since the latest Miocene or earliest Pliocene. The Fish Creek-
Vallecito Basin (FCVB) lies on the Pacific Plate in southern California, dextrally offset from the
point where the modern Colorado river enters the Salton Trough; it contains a record of ancestral
Colorado River sedimentation from 5.3–2.5 Ma. The basin stratigraphy exhibits a changing balance
between locally derived (L-Suite) and Colorado River (C-Suite) sediments. This paper focuses on the
Palm Springs Group (PSG), a thick fluvial and alluvial sequence deposited on the upper delta plain
(between 4.2–2.5 Ma) when the Colorado was active in the area, allowing the detailed examination
of the processes of sediment mixing from two distinct provenance areas. The PSG consists of three
coeval formations: 1) Canebrake Conglomerate, a basin margin that has coarse alluvial fan deposits
derived from surrounding igneous basement; 2) Olla Formation, fan-fringe sandstones containing
L-Suite, C-Suite, and mixed units; and 3) Arroyo Diablo Formation, mineralogically mature C-Suite
sandstones. Stratigraphic analysis demonstrates that the river flowed through a landscape with relief
up to 2000 m
Zircon and apatite thermochronology of the Nankai Trough accretionary prism and trench, Japan: sediment transport in an active and collisional margin setting
Get PDF[1] The Nankai accretionary complex is the most recent addition to the accretionary complexes of southwest Japan and has preserved a record of sediment flux to the trench during its construction. In this study, we use U-Pb zircon and fission track analysis of both zircons and apatites from sediments taken from the forearc and trench of the Nankai Trough, as well as rivers from southwest Japan to examine the exhumation history of the margin since the Middle Miocene. Modern rivers show a flux dominated by erosion of the Mesozoic-Eocene Shimanto and Sanbagawa accretionary complexes. Only the Fuji River, draining the collision zone between the Izu and Honshu arcs, is unique in showing much faster exhumation. Sediment from the Izu-Honshu collision is not found 350–500 km along the margin offshore Kyushu indicating limited along-strike sediment transport. Sediment deposited since 2 Ma on the midtrench slope offshore the Muroto Peninsula of Shikoku (ODP Site 1176) and on the lower slope trenchward of the Kumano Basin (IODP Sites C0006E and C00007E) shares the dominant source in the Shimanto and Sanbagawa complexes seen in the modern rivers. Prior to 5 Ma, additional sediment was being sourced from further north in more slowly exhumed terrains, ~350 km from the trench axis. Around 9.4 Ma, U-Pb zircon ages of ~1800 Ma indicate enhanced erosion from the North China Craton, exposed in northern Honshu. In the middle Miocene, at ~15.4 Ma, the sediment was being derived from a much wider area including the Yangtze Craton (U-Pb ages ~800 Ma). We suggest that this enhanced catchment may have reflected the influence of the Yangtze River in supplying into the Shikoku Basin prior to rifting of the Okinawa Trough at 10 Ma and migration of the Palau-Kyushu Ridge to form a barrier to transport. The restriction of Nankai Trough provenance to Mesozoic source partly reflects continued uplift of the Shimanto and Sanbagawa complexes since the Middle Miocene
Defining Regional and Local Sediment Sources in the Ancestral Colorado River System: A Heavy Mineral Study of a Mixed Provenance Unit in the Fish Creek-Vallecito Basin, Southern California
No full textThe Colorado River has flowed across the dextral strike-slip plate boundary between the North American and Pacific plates since the latest Miocene or earliest Pliocene. The Fish Creek-Vallecito Basin (FCVB) lies on the Pacific Plate in southern California, dextrally offset from the point where the modern Colorado river enters the Salton Trough; it contains a record of ancestral Colorado River sedimentation from 5.3–2.5 Ma. The basin stratigraphy exhibits a changing balance between locally derived (L-Suite) and Colorado River (C-Suite) sediments. This paper focuses on the Palm Springs Group (PSG), a thick fluvial and alluvial sequence deposited on the upper delta plain (between 4.2–2.5 Ma) when the Colorado was active in the area, allowing the detailed examination of the processes of sediment mixing from two distinct provenance areas. The PSG consists of three coeval formations: 1) Canebrake Conglomerate, a basin margin that has coarse alluvial fan deposits derived from surrounding igneous basement; 2) Olla Formation, fan-fringe sandstones containing L-Suite, C-Suite, and mixed units; and 3) Arroyo Diablo Formation, mineralogically mature C-Suite sandstones. Stratigraphic analysis demonstrates that the river flowed through a landscape with relief up to 2000 m. Satellite mapping and detailed logging reveal a variable balance between the two suites in the Olla Formation with an apparent upward increase in L-Suite units before abrupt cessation of Colorado sedimentation in the basin. Stable heavy mineral indices differentiate L-Suite (high rutile:zircon index: RZi 40–95) from C-Suite (RZi: 0–20). Both suites have garnet:zircon index (GZi) and apatite:tourmaline index (ATi) mostly above 50, although many L-suite and mixed Olla samples have much lower ATi (20–50), suggesting that the distal floodplain was wet and the local sediment had a longer residence time there, or went through several cycles of erosion and redeposition. Heavy mineral analysis, garnet geochemical analysis, and detrital zircon U-Pb age spectra allow us to quantify the amount of mixing from different sediment sources. These data show that about 30% of the mixed units are derived from the Colorado River and that up to 20% of the L-Suite is also derived from the Colorado River, suggesting that there was mutual cannibalisation of older deposits by fluvial channels in a transitional area at the basin margin. Although this study is local in scope, it provides an insight into the extent and nature of sediment mixing in a two-source system. We conclude that most ‘mixing’ is actually interbedding from separate sources; true mixing is facilitated by low subsidence rates and the rapid migration of fluvial channels
Tying catchment to basin in a giant sediment routing system: a source-to-sink study of the Neogene–Recent Amur River and its delta in the North Sakhalin Basin
No full textThis paper uses an extensive dataset from more than 200 samples to provide a comprehensive source-to-sink analysis of the Amur River and its delta in the Russian Far East. The majority of sand-sized sediment in the Amur River and its former delta comes from upstream of the Lesser Khingan Ridge, shown by uniformity of sediment composition in the lower 1700 km of the river. Stable mineral ratios, U–Pb age spectra and garnet geochemistry show little stratigraphic provenance-specific variation in the Neogene delta. This renders Miocene–Pliocene drainage capture models unlikely. The onset of uplift in the delta is marked by a decrease in the apatite–tourmaline index (ATi) in Upper Pliocene offshore well samples, caused by dissolution of apatite as sediments were uplifted and eroded onshore Sakhalin. These wells also show variable ATi and garnet–zircon index (GZi) values in Lower Miocene samples, which could potentially be used for stratigraphic correlation. A positive correlation between GZi values and distance from the river mouth is attributed to hydrodynamic sorting across the delta system. This has negative implications for the use of this stable mineral index and others of a similar hydraulic equivalence as regional correlation tools on a basin scale (>100 km)
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