Hull Geological Society
SEDIMENTOLOGY – PROCESS AND PRODUCT.
Held 5th to 7th October 2012 at
Hull University.
A
joint meeting of the Hull Geological Society, University of Hull Department of
Geography and the Yorkshire Geological Society.
Abstracts
Cross-Bedding Indicates Variable Preservation Potential Within River Channels.
The assumption that all boundary conditions are
internally homogenous in well-defined environments is not necessarily true. For
example, our models of preservation of fluvial bedforms suggest that only the
bases of the biggest dunes are preserved. However, fully-preserved dunes are
frequently observed in Ground Penetrating Radar images from the Rio Paraná,
Argentina. Their presence can be related to a combination of flow unsteadiness
(that causes abandonment) and
non-uniformity (i.e. a bar trough, where
recurrence of erosion is minimal or absent). Theory states that most variables
that control bedform preservation change in non-uniform conditions, such as in
the lee of bars. Indeed, measurements from cores, trenches and flume experiments
indicate that bedforms migrating down low-angle bar slopes are on average
thicker. Complete and increased preservation of bedforms in natural (non-uniform
and unsteady) flows constrains the validity of our bedform preservation models
to the uniform flows for which they were developed. Further research at the
University of Hull addresses a range of fundamental aspects needed to understand
the processes and boundary conditions that cause variable preservation potential
within individual river channels.
The
Digital Approach To Understanding the Quaternary Evolution of the Vale Of York,
UK.
Anthony Cooper, Jon Ford and Simon Price, British Geological Survey.
Detailed geological surveying in the Vale of York, north-east England has
utilised GIS, to bring together digital map data, digital terrain models, and
field mapping. The use of extensive borehole databases (about 8000 boreholes)
and borehole modelling has allowed the understanding to be taken into 3
dimensions. The uncovering of this 3D model permits time slices to be
constructed for the Devensian glaciation showing how it advanced and waned
extending the modelling into 4D. As the ice advanced, a large pro-glacial lake
was impounded by ice in the North Sea. Thick laminated clays with marginal sands
and gravels formed in this lake. These were overridden by the advancing ice,
which extended south to form the Escrick and then York moraines. The Vale of
York ice blocked the rivers courses around its margins and formed numerous
overflow channels that, with the drainage from the ice-sheet, fed the lake in
front of the moraines. As the ice front retreated more glacial lakes with
laminated clay deposits formed to the north of the moraines. Eventually the
eastward drainage to the North Sea was re-established and the present topography
developed.
Origin
of Cretaceous Flints.
Nodular cherts (“flint”) in the Upper Cretaceous Chalk of western Europe are a
product of early diagenetic silica precipitation that occurred 5-10 m below the
sediment surface. Trace element and stable isotope data suggest that
silicification occurred in a sediment of 70-80% porosity and was caused by
widespread carbonate dissolution. Silicification began with a phase of skeletal
replacement, followed by en-mass precipitation of opal-CT lepispheres, and
growth of interstitial chalcedony cement. The opal-CT later recrystallised to
the present day quartz mineralogy.
Paramoudras (round vertical columnar flints containing a cemented chalk core
around a central burrow) provide a vital clue to understanding such localised
carbonate dissolution. Anaerobic bacterial sulphate reduction released excess
sulphide (H2S or HS-) which diffused outwards to become oxidised at the
oxic-anoxic boundary. The resulting hydrogen ions from sulphide oxidation caused
dissolution of the host chalk sediment, seeding dissolved biogenic silica to
form a flint.
The
more common “bedded” flint bands result from more widespread sulphate reduction
and re-oxidation of resultant sulphide at the base of the oxic zone. Local
porosity and permeability variations resulted in heterogeneous mixing of the
sulphide and oxygen to produce the characteristic digitate-nodular form of most
flints. Less marked permeability variations resulted in more regular “tabular”
flints, and early stage compaction joints became the sites of “sheet” flint
formation.
The
Palaeo-Valley Fill Sediments Of Flamborough Head Rediscovered: Their
Significance For Dating The Advance Of The North Sea Lobe Of The Last
British-Irish Ice Sheet Into Holderness, Eastern England.
Ian
Heppenstall, Colin Clark, Rodger Connell, Derek Gobbett, Dennis Haughey, Mike
Horne, Stuart Jones, Brian Kneller, Chris Leach, Paul Richards, & Rod Towse of
the Flamborough Quaternary Research Group, Hull Geological Societ.
Holderness, East Yorkshire, contains the typesite of
the Dimlington Stadial of the Late Devensian glaciation of the UK (a time from
ca. 28,000 to 14,700 cal years BP). At Dimlington the base of the Skipsea Till
(the older of two Late Devensian tills) can be seen above the radiocarbon dated
Dimlington Silts (22.0 ± 0.5 and 21.8 ± 0.3 cal. ka BP. (Hughes
et al., 2011)). The base of the
Skipsea Till can also be seen at the classic site of Sewerby, near Bridlington,
where it overlies periglacial sediments and an important sequence of last,
Ipswichian, interglacial (MIS 5e) deposits. At Eppleworth, west of Hull, close
to the ice sheet limit weathered Skipsea Till overlies a silt rich gelifluction
deposit TL dated to 17.5 ± 1.6 ka BP. See Catt (2007) for the most recent review
of all three important sites.
The two Late Devensian, till units of Holderness, the
Skipsea and Withernsea Tills, were deposited by multiple advances of the North
Sea Lobe of the last British-Irish Ice Sheet advancing southward. See Evans and
Thomson (2010) and Bateman et al.
(2011) for a review of the sedimentology of the deposits and new dating
information from the Dimlington typesite respectively. However, the chronology
of the North Sea Lobe’s advances is dependent on these few sites in Holderness,
and sites in the Vale of York where a large proglacial lake was impounded as the
lobe blocked the Humber gap. See Murton
et al. (2009) for a review of recent dating evidence for Glacial Lake Humber
sediments. As the base of the Skipsea/Withernsea till assemblage is only rarely
visible in coastal Holderness it is critical to search for further sites to test
and elaborate the evolving chronology and potentially enhance its resolution.
Particularly so as it remains unclear if the advance of the North Sea Lobe was
driven by climate effects, intrinsic ice sheet behaviour, or both.
Two
sites which appear to have the potential to add important detail to the
chronology of the North Sea Lobe are present in the southern cliffs of
Flamborough Head, just a few kilometres east of the Sewerby site. Here sediments
infilling what appear to be pre-Devensian chalk dry valleys are seen in section
in the modern cliffs (Figures 1 and 2 below). These sites were recorded by both
Daykns (1880) and Lamplugh (1890, 1891) but the full significance and potential
of the earliest deposits has not been appreciated by more recent workers.
However, one of us, Ian Heppenstall, visited these sites in 2002 as part of a
University of Hull, Centre for Lifelong Learning course. Shortly afterwards the
Flamborough Quaternary Research Group of the Hull Geological Society was formed
to carry out research on the sites and others on Flamborough Head, and this work
continues. Brief descriptions of the sedimentology and stratigraphy of the
deposits identified in the palaeo-valleys are given in the figure captions below
(Figures 1 and 2) but this short talk will concentrate on the earliest deposits
present resting on bedrock. A number of samples have been collected at the two
sites for optically stimulated luminescence dating (OSL) at the University of
Sheffield and results are eagerly awaited. More details of the deposits will be
presented in the talk together with a review of their likely significance for
the chronology of the dynamic North Sea Lobe in eastern England during the
Dimlington Stadial.
References
Bateman, M.D., Buckland, P.C., Whyte, M.A., Ashurst, R.A., Boulter, C. and
Panagiotakopulu, E. 2011. Re-evaluation of the Last Glacial Maximum typesite at
Dimlington, UK. Boreas 40, 573 – 584.
Catt,
J.A. 2007. The Pleistocene glaciations of eastern Yorkshire: a review.
Proceedings of the Yorkshire Geological Society 56, 177 – 207.
Dakyns, J.R. 1880. Glacial deposits north of Bridlington. Proceedings of the
Yorkshire Geological and Polytechnic Society 7, 246 – 252.
Evans.
D.J.A. and Thomson, S.A. 2010. Glacial sediments and landforms of Holderness,
eastern England: A glacial depositional model for the North Sea Lobe of the
British-Irish Ice Sheet. Earth-Science Review 101, 147 – 189.
Hughes, A.L.C., Greenwood, S.L. and Clark, C.D. 2011. Dating constraints on the
last British-Irish Ice Sheet: a map and database. Journal of Maps 2011, 156 –
184.
Lamplugh, G.W. 1890.On a new locality for the arctic fauna of the “Basement”
boulder clay in Yorkshire. Geological Magazine 7, 61 – 70.
Lamplugh, G.W. 1891. On the drifts of Flamborough Head. Quarterly Journal of the
Geological Society of London 47, 384 – 431.
Murton, D.K., Pawley, S.M. and Murton, J.B. 2009. Sedimentology and luminescence
ages of Glacial Lake Humber deposits in the central Vale of York. Proceedings of
the Geologists’ Association 120, 209 – 222.
The
Pleistocene Geology of Flamborough Head: A Display of Photographs And Diagrams.
Members of the Flamborough Quaternary Research Group, Hull Geological Society.
Deciphering Ancient Fluvial Successions From Modern Channels: Some Insights From
Small And Large Modern Multiple-Channel Rivers
Jim
Best, University of Illinois, U.S.A.
Interpreting the ancient using the modern has lain at the heart of process
sedimentology, and provides many insights into the dynamics and deposits of
ancient sediments, allowing us to make predictions regarding the size, shape and
architecture of sedimentary successions. Advances in technology over the past
two decades have permitted us to take a new and higher resolution look into the
stratigraphy of modern alluvial successions, and examine the controls upon their
preserved deposits. This talk will illustrate some of this progress from recent
studies of both small and large multi-thread rivers, using the South
Saskatchewan River, Canada, and the Río Paraná, Argentina, as case studies.
These studies have adopted an approach that couples monitoring recent migration
and channel change, with an examination of the deposits left behind. The work
has centered on the use of ground-penetrating radar in imaging and quantifying
the subsurface, and has linked this to remote sensing of channel change. This
talk will illustrate the detailed sedimentary architecture of these rivers,
assess how similar or different the sedimentary facies of small and large rivers
may be, and discuss the influence of event magnitude on the sedimentary record
locked up within their deposits. This latter work suggests the signature of
large, low-frequency floods, may be difficult, if not impossible, to recognize
in the ancient rock record.
Abandonment Overprints On Channel Belt Architecture in the Upper and Central
Rhine Delta, The Netherlands.
K.M.
Cohen, W.H.J. Toonen, M.G. Kleinhans, E. Stouthamer, Utrecht University.
The
mapping and dating of channel belts in the Rhine delta is a traditional subject
at our department at Utrecht University. The network of channels in the delta
has multiple bifurcation nodes and forms and changes by successive avulsions.
From Middle to Late Holocene times a set of primary bifurcations developed
upstream in the delta (apex region), where the undivided Rhine splits in
long-lived channel belts of considerable width and functioning multiple
millennia. A second set of bifurcations persists in the central delta, also
feeding split channel belts of considerable size and prolonged lifespan (~1000
yrs). Further smaller channel belts also exist in the central delta, some of
them thought to have functioned 1000 yrs, others much shorter. The present
network of Rhine branches did essentially begin 2500 years ago, when the current
channels branched off from former routes as partial avulsion creating a new
bifurcation. The larger channels seem to have slowly evolved and matured in
size. The new insight is that this is sedimentologically recorded in abandoned
channel belts over multiple meander lengths distance downstream of their former
feeding bifurcation. Over the time it took new avulsed channels to mature and
take over the full discharge, the slowly abandoned bifur kept receiving and
storing bed load and fines – in the case of larger distributaries over periods
over to 500 years. This causes an abandonment overprint not only on the
architecture of the residual channel fills (the final phase of channel filling,
when only flood waters bring sediment), but also the channel belt architecture.
It makes the internal build up of meandering delta channels differ from the
classic meander belt models. This is demonstrated by a study of the Rhine apex
bifurcation, that preserved a section of slowly abandoned bifur meander belt
(Kleinhans et al. 2011; Toonen et al. 2012). The new insight has made that
bigger former branches are ‘separation anxious’, i.e. that their abandonment was
a slow process, now propagates to our mapping of the main channel belts in the
central delta, where we recognise the same slow abandonment features in the
sedimentary build-up and avulsion-event dating. It makes that us rethink our
delta network evolution as relatively more ‘bifurcation lifetime’, than
‘avulsion frequency’ determined.
Metripol Birefringence Imaging Of Unconsolidated Glaciotectonised And Ice Keel
Scoured Sediments: Identification Of Unistrial Plasmic Fabric.
Lorna.
D. Linch* and jaap. J. M. Van der meer, Queen Mary University of London.
In unconsolidated sediments subject to strain, clays
and silts are realigned into particular optical birefringent arrangements
(plasmic fabrics), which provide information on style and intensity of
sediment deformation. A relatively new,
non-destructive, optical microscopy technique for automatically recording and
quantifying birefringence (previously commercialised under the name ‘Metripol’)
is pioneered in this study as a valuable and innovative micromorphological tool
to examine deformation in unconsolidated sediments. Metripol is applied to
unistrial plasmic fabric in glaciotectonised and ice keel scoured sediment from
the Netherlands and former Glacial Lake Agassiz (Manitoba, Canada) respectively.
Colour coded images are produced where colour represents relative optical
retardation and thus optical anisotropy through the quantity of │sinδ│ and
optical orientation of anisotropy through the angle Ø (also indicated by azimuth
lines). In this study Metripol shows that generally the better developed the
unistrial plasmic fabric the higher the │sinδ│ values, the larger the areas of
high │sinδ│ values, and the longer and more densely populated the azimuths. In
addition, some unistrial plasmic fabrics under Metripol demonstrate lower │sinδ│
than the surrounding sediment despite being ‘perceived’ as demonstrating higher
birefringence than the surrounding sediment under a standard petrographic
microscope. This is particularly true in clay-rich sediments and holds
implications for the way we currently describe and interpret unistrial plasmic
fabrics in unconsolidated sediment. Finally, identification and quantification
of additional structures that would otherwise have gone undetected using a
standard petrographic microscope (e.g. linear structures that are likely to
represent shears), highlights the potential for Metripol to gather additional
information on the stress history of unconsolidated sediments that standard
techniques cannot.
The
Morphology and Function of Thrombolitic Carbonate Biofilms: A Universal Model
Derived From Freshwater Mesocosm Experiments.
Martyn
Pedley, University of Hull.
Microbialites with laminar (stromatolite) and thrombolitic (thrombolite) fabrics
are widespread limestone producers within the freshwater sedimentary record
though the biology and physiology of the living prokaryote–microphyte biofilms
which produced them is only now becoming understood. The present contribution
describes a flowing water experimental mesocosm study spanning over 2.5 years
and run under near–natural conditions. This work focussed on the development
processes in microbial biofilms which produce thrombolitic fabrics capable of
preservation in the geological record. In particular, the roles of microbial
guilds and carbonate precipitation processes were examined and recorded at all
stages of thrombolite development. The mesocosm experiments convincingly
demonstrated that the biofilm community actively encourages the precipitation of
calcium ions derived from flowing waters. This precipitation took the form of
amorphous calcium carbonate nanosphere clusters. These were not randomly
distributed within the biofilm Extracellular Polymeric Substances (EPS) but were
focussed in the close vicinity of living filament and coccoid bacterial clusters
within individual living biofilm layers. The precipitates never replaced
microbial cell walls and never buried the living microbes. During nanosphere
precipitation EPS was progressively occluded from between the developing
nanosphere clusters. However, EPS was never totally occluded until the
aggregates neomorphosed into microspar crystals.The orientation of microspar
crystals within the biofilm appeared to be controlled by the host EPS fabric
(cf. typical crystal growth from solid substrates). During nanosphere
development living EPS was maintained centimetres into the underlying fabric.
Precipitates were organised around the margins of a cancellate microfabric
developed by a range of microbial guilds within each biofilm layer. This lead to
the development of a distinct thrombolitic fabric to the carbonate precipitates
which is quite distinct from the laminar fabrics typical of stromatolites.
It is
concluded that the mesocosm grown freshwater biofilms and their associated
microbialite calcite fabrics provide a universally applicable model which
explains the process of thrombolitic microbialite development throughout the
geological record.
Limestones - Orbital Forcing, Climate And Tectonic Controls On Deposition: Are
Beds In Shelf Carbonates Millennial-Scale Climate Cycles?
Maurice Tucker, Durham University.
There
is an ever-growing literature on sub-Milankovitch, millennial-scale cyclicity
recorded from Quaternary strata with mechanisms invoked including rapid warming
/ cooling for Dansgaard-Oeschger (D-O) events and Bond cycles, ice-sheet
dynamics for ice-rafted debris in Heinrich events, and changes in climate and /
or solar irradiance for millennial-scale periodicities (~1500 yrs) in δ18O in
Greenland ice cores. Ancient shelf and slope limestones can also be expected to
have been affected by millennial-scale fluctuations in solar output/climate
change and/or oceanic–atmospheric interaction, and these would have caused
changes in depositional conditions, one way or another. However, there have been
few descriptions of such high-frequency cycles; or is it that we have simply
overlooked them? Could it be that the beds, which are such a distinctive feature
of shallow-marine limestones generally, are the product of high-frequency
climate/environmental changes, driven by fluctuations in solar output?
Platform limestones deposited in 5-30 metres water depth are widespread
throughout the geological record and are well represented in the Carboniferous,
Permian and Jurassic of Yorkshire. They commonly have a well-developed
stratification, 10-100 cm thick, defined by shale partings, which is laterally
extensive over 10s to 1000s of sq km. The duration of bed deposition is on the
millennial scale. In some cases there appear to vertical patterns of bed
thickness variation, defining thinning-up and thickening-up units. There may be
corresponding variations in δ18O, TOC and trace elements too, within individual
beds and within bed-sets.
All
these features indicate an allocyclic control on bed and bed-set deposition. The
most likely explanation for the beds involves variations in the quantities of
wind-blown dust or river-borne clay and subtle salinity changes, resulting from
arid to humid climate changes; these would generate shale partings and affect
carbonate productivity. Fluctuations in temperature and nutrient supply could
also have contributed. Fluctuations in solar output may have been the climate
driver. Millennial-scale climate rhythms will have operated throughout
geological time. Their record in stratigraphy is there in the beds.
The
Brick-Clays Of North-East England And Their Products. A Display of Bricks.
Ron
Harrison, Hull Geological Society.
Water
Level Changes In Lake Anstazewo And Lake Skulskie (Central Poland) During The
Last 20 Years And Its Stable C Isotope Record: Implications For Paleolimnology.
Reconstruction of temporal changes in lake water levels is one of the greatest
challenges of paleolimnology. There are many tools to estimate past lake levels
and the ratio of stable C isotopes in lacustrine carbonates and/or organic
matter is one of the most promising paleohydrological indicators. However, the
interpretation of δ13C signatures is often ambiguous and requires calibration of
δ13C data with the instrumental records of lake water levels.
In our
study we aimed to show and discuss δ13CTOC record of water level changes in Lake
Anastazewo and Lake Skulskie (SE part of the Gniezno Lakeland, central Poland)
within the last 20 years. During that time both lakes displayed considerable
lowering in the water level which was explained by brown coal open-cast mining
and temporal changes in the precipitation pattern in the vicinity of these
lakes.
Despite the fact that the lakes differ significantly with respect to the surface
area and the water depth the amplitudes of lake water level drop is similar
(c.a. 0.5-2.0 m). Consequently, we are able to compare geochemical imprints of
the same process in a small and shallow as well as large and deep water body. We
analyzed vertical/temporal changes in the stable C isotope composition of bulk
organic matter (δ13CTOC), and bulk geochemical composition (TOC/N, TOC, SiO2ter,
SiO2biog, carbonates) in the sediment cores from Lake Anastazewo and Lake
Skulskie. Geochemical data were supplemented with the analysis of subfossil
diatoms and plant macroremains. The age model was established on the basis of
the 210Pb dating. Lake water levels were measured since mid-1960s.
Preliminary results are shown in Fig. 1 and Fig. 2.
In
both cores stable C isotopes clearly reflect the fluctuations of lake water
level, however in a shallow lake, Lake Anastazewo, δ13CTOC varies parallel to
the lake water level, and in a deeper lake, Lake Skulskie, the relationship
between δ13CTOC and lake level is the reverse, i.e. the higher lake water level,
the lower δ13CTOC.
The
obtained relationships can be explained by the changes in the source of C used
by phytoplankton to produce organic matter in the studied lakes. In Lake
Anastazewo enrichment in 12C occurs owing to increased contribution of C derived
from decomposing organic matter. This isotopically lighter C is released to the
water when organic-rich deposits in the nearshore section of the lake bottom are
exposed to the atmosphere during lowstand levels of the lake.
On the other hand, lowered δ13CTOC in Lake Skulskie during higher water level can be explained by the input of 12C from the catchment’ soils. As the increase in the lake water level is coincident with enhanced atmospheric precipitation it seems likely that lighter C is delivered to the lake by percolating meteoric waters. This mechanism results in the enrichment of lake water DIC in 12C and consequently in a shift towards more negative values of δ13C in autochthonous organic matter.
Glacial sedimentary environments at Barmston Beach
Barmston beach, 8km south of Bridlington, East Yorkshire, has recently found
fame starring in a TV commercial for holidaying in the UK during2012! The cliff
sections are no less interesting! The site provides excellent exposures of
sediments relating to both the advance and retreat of the North Sea Lobe of the
last British-Irish Ice Sheet into Holderness during the Dimlington Stadial.
Depending on the state of the sections we will be able to examine diamictons
deposited during this advance together with sediments formed in a range of
environments as the ice sheet withdrew to the north east. The sediments contain
a range of deformation structures which are presently the focus of research at
the site by members of the Flamborough Quaternary Research Group of the Hull
Geological Society.
Glacial sedimentary environments at Flamborough
During
the last two decades of the 19th century two workers recorded valley forms cut
into the chalk cliffs of Flamborough Head and filled with a range of sediment
types. They were J.R. Dakyns (during the geological survey of the district) and
G.W. Lamplugh (then a local amateur geologist). Research on these valley-fill
sediments has been neglected until recently and their significance has gone
unappreciated. Ongoing research by members of the Flamborough Quaternary
Research Group (Hull Geological Society) will be demonstrated at either Danes’
Dyke or South Landing (both on the south shore of the headland) to the north
east of Bridlington, East Yorkshire. Detailed analysis of the sediments and
their palaeoenvironmental interpretation, together with preliminary OSL dating
of selected units by Professor Mark Bateman, has revealed the potential
significance of the sites for the chronology of the advance of the North Sea
Lobe during the last glaciation of eastern England.
Foraminifera Are Sorted By Size And Taxon During Post-Mortem Transport
Palaeoenvironmental reconstruction using foraminifera
relies on the assumption that assemblages reflect the ecological conditions at
the time of deposition. However, the distribution of taxa can be greatly
affected by transport and reworking of tests, and although this issue is generic
to all environments with active submarine currents this issue is particularly
pronounced in the canyon and fan systems in which much of the worlds hydrocarbon
reserves are found. Here, we provide critically important new empirical
constraints on 1) the behaviour of empty foraminiferal tests compared to
quartzose sediments and 2) the likelihood that different taxa will behave
differently and thus may become sorted during transport. Measured settling
velocities range from 0.01 to 0.06 ms-1 (with foraminifera 200-500 although
larger specimens fall faster than smaller ones, differences between taxa are
also significant. Of the five taxa investigated,
Elphidium crispum exhibits the
fastest average settling velocity of 0.03 ms-1 while
Planorbulina mediterranensis falls
with the lowest average settling velocity of 0.01 ms-1. As predicted by this
result, we also report significant spatial separation of taxa within a single
flow using a simple waning turbidity current experiment within a laboratory
flume. These results are highly compatible with the results of settling
experiments, indicating that slowly settling tests such.
P. mediterranensis and Cibicides
lobatulus remain suspended in the current for longer, and are thus
transported further, than more rapidly settling taxa such as
E. cripsum and
Ammonia spp. Consequently,
assemblages of foraminifera transported within a turbid current should be
assumed to be sorted by shape (i.e. by taxon) in addition to being sorted by
size, and that hydraulic rather than ecological factors are likely to control
spatial distributions within moderate to high energy environments.
Submarine Landslides And Turbidity Currents: New Insights Into The Largest Mass
Flows On Our Planet.
By far
the largest sediment flows and landslides occur underwater. The Storegga
landslide offshore Norway is larger than Scotland, occurred 8,200 years ago, and
produced a tsunami that ran up for 3-20 m around the northern UK coastline.
Perhaps the most remarkable fact about the Storegga Slide (and many other
submarine mega-landslides worldwide) is that they occur on sea floor gradients
of just 1-3°. For comparison, a premiership soccer pitch has a 1° gradient to
help water drain. This talk will start by exploring current ideas on why huge
submarine landslides might occur on such low gradient continental margins.
Collapse of volcanic islands can also generate extremely large submarine
landslides, and the magnitude of the tsunamis they generate is also the subject
of vigorous debate. The talk will continue by outlining recently collected field
data sets that show how volcanic landslides are emplaced, which is the major
unknown factor for predicting tsunami magnitude. This includes the first
drilling and 3-D seismic data for volcanic island landslide collected in 2012
during IODP Leg 340 offshore Montserrat and Martinique in the Lesser Antilles.
The
talk will conclude by describing submarine turbidity currents that can sometimes
run out for thousands of kilometres into the deep ocean. A single turbidity
current can transport more sediment than the annual sediment flux from all of
the world’s rivers combined. Turbidity currents are the longest run out sediment
flows yet recognised on Earth, and they produce the most extensive sediment
accumulations (submarine fans). The most remarkable fact about turbidity current
is how few direct measurements we have from monitoring active events. The
sediment concentration of turbidity currents that reach submarine fans has never
been measured, ever, in any location. The talk will conclude by outlining
ongoing efforts to monitor active flow events, and summarise recent outcrop and
experimental work that suggests turbidity currents are not like our textbook
models.
Reconstructing a regional, mid-Holocene palaeoprecipitation signal of the Asian
Summer Monsoon from organic geochemical analysis of Pearl River sediments,
China.
Rachel
Flecker,
University of Bristol
Palaeoprecipitation is reconstructed from proxies such
as cave speleothems and lake levels which have small catchment areas. Their
regional signal is therefore vulnerable to local overprinting. Sediments
deposited at the mouth of large rivers may capture a more regional signal if it
is integrated over the drainage basin. The D18O record, which lies in the Pearl
River catchment (China), has been interpreted as recording a regional decline in
Asian Summer Monsoon precipitation over the last 6.5 kyrs. However, it differs
from the bulk sedimentary organic δ1313Corg) record from a core in the Pearl
River Estuary, and the regional nature of the Dongge palaeoprecipitation signal
has therefore been questioned. Our study re-evaluates both records by
constructing, for the same estuarine core, biomarker and compound-specific δ13C
records, which have substantially better constrained terrestrial and marine end
13Corg. The core’s BIT indices, which reflect the ratio of soil versus marine
organic matter, covary with the Dongge Cave 18O record suggesting a common,
regional control. By contrast, sterols, n-alcohols
and n-fatty acid ratios parallel the
13Corg record between 6.5-2 kyrs documenting a decline in terrigenous organic
matter input to the estuary. The leaf wax δ13C record reflects an abrupt change
in vegetation at 2 kyr, probably resulting from local, anthroprogenic
cultivation. The catchment scale of these estuarine records equates to up to 15
grid cells in typical Earth System Models permitting spatially equivalent
comparison of PMIP2 model simulations of the mid Holocene with our new data for
the first time.
Photographs Of The Sewerby Buried Cliff November 2007.
Stuart
Jones, Hull Geological Society.
After
severe south-easterly storms this classic exposure was exceptionally clean.
Reconstructing Fluvial Morphology From Set Thickness Statistics.
Preservation is the link between fluvial surface morphodynamics and what is
recorded in the fluvial sedimentary. Reconstruction of the original channel
morphology from stratification can provide important information about paleoflow
conditions. To infer the original dimensions of paleomorphological features such
as river channels from the fluvial record, a detailed understanding of the
relation between morphodynamics and preservation is needed. So far, theories to
reconstruct the original morphology from preserved stratification have not been
tested for meandering river channels for lack of detailed bathymetry.
We
report on a series of controlled flume experiments and Delft3D physics-based
numerical model runs with the objectives to i) test the prediction of set
thickness as a function of the morphology formed by a meandering river channel,
and ii) explore and explain spatial and temporal set thickness variations in the
resulting channel belt. High-resolution measurements of time-dependent surface
elevation were used to quantitatively relate the preserved stratification to the
river morphology. Experimental design corresponds to the predicted hydraulic
geometry for a non-cohesive gravel-bed river, and the width-depth ratio is
chosen such that alternate bars form.
We
find that the mean set thickness agrees well with the theoretical prediction
from channel morphology. The mean preserved set thickness is 30% of the mean
channel depth. Finally, there is much systematic spatial variation in set
thickness related to repetitive point bar growth and chute cutoff. We find
undisturbed and thick sets close to channel belt margins and more irregular
stratification with stacked thinner sets in the channel belt center. We conclude
that set thickness statistics can be used to provide quantitative error bounds
for the reconstruction of paleochannel dimensions.
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