Tyne Estuary Field Trip

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Tyne Estuary Field Trip
Notes for Field Leaders and Demonstrators
• What are the intended learning outcomes?
1) An understanding of the dominant processes that determine the sedimentary
characteristics of the primary depositional environments (beach, barrier sands,
estuary in lower tidal and upper tidal environments, and channel settings).
2) An understanding of the principal physical, mineralogical and biological
components that characterise each of the environments.
2) An appreciation of the present-day boundaries between these environments
(gradual versus abrupt)
3) A basic understanding of how these environments would appear if
superimposed in the stratigraphic record.
John Muir Country Park,
Dunbar, East Lothian
This part of East Lothianand and its coastline was covered in ice at the last glacial maximum (approx. 18,000 yrs ago). Since then,
the glacial sediment has been reworked by fluvial and tidal currents to form the coastline sediments seen today. This trip aims to study
the variety of sedimentary environments, what determines their sedimentary chracteristics, and how this might be preserved in the
stratigraphic record.
Logistics
•
•
•
The buses will be departing one hour after each other at 9.00, 10.00 and 11.00 o’clock. It will take
approximately 45 minutes to drive there, and the buses should park in the John Muir country park,
the car park being next to the East Links Family Park (see maps below).
The field component of the day will take 4 hours maximum, and the conditions will be windy and
cold. The low tide is at 11.34, rising to high tide at 18.06.
The trip involves walking straight out from the car park across the lagoonal region, and over the
dunes onto the beach (see map below). Having left the car park, turn left along the footpath by the
toilet block. Follow this footpath for about 100m until a bridge, then turn right soon after to cross the
marshy lagoon area (it’s drier up here). Having crossed the lagoon and the dunes, turn left along the
beach (see map). From there, walk along the beach until the end of the spit is in sight. From here, the
aim is to carry out a traverse of the main environments based around four main stops (shown below).
They should construct a topographic cross-section in their notebooks as
they go along. It will be about 1km long from location 1 to 4, and will
need to have some vertical exaggeration. Onto this, they should
annotate the important points concerning the environments, and the
the boundaries between environments.
Planned route for virtual excursion and location numbers
spit
1
2
estuary
3
45
Coaches park here
STOP 1
•
From this stop, the questions to pose are:
1.
What are the dominant processes that transport sediment on the beach environment? Ans – wave and tide energy.
Tidal range is 2.5 (neap) to 4.0 m (spring) – Discuss spring and neap tides.
2.
What implications does that have for the surface form? Ans – Planar, dipping offshore, usually too rough to
preserve ripples in winter, but common in calm summer conditions as tide recedes.
3.
Where does the sediment come from?Is it along-shore drift, from the rivers or from wave erosion of modern
rocky shorelines? Ans. Mainly from wave and tide reworking of vast piles of glacially derived sediment
generated during the last glaciation finishing around 18 Ka.
4.
What implications does it have for sedimentary texture? Look at sediment and consider grain sizes, sorting,
shape. Is there much bioclastic material? How is it preserved? Ans - lot of broken fragments. COLLECT A
SEDIMENT SAMPLE TO EXAMINE IN THE PRACTICAL
5.
Are there any living organisms that might be preserved in the ancient as fossils? Not many, except lugworms and
some razor shells at very low tide. SO both should generate vertical burrows. The vertical burrows reflect the
necessity for organisms to be able to move vertically in beach environments in order to respond to rapidly shifting
levels of sediment.
6.
What defines the boundaries to this environment and is this a gradual or abrupt change? Ans – High tide and low
tide marks, but as this varies depending on weather and neap/spring cycles, the boundary is gradual over several
to tens of metres. The most recent high tide mark is characterised by scattered weed and debris, but usually there
is evidence for storm high tides.
7.
Why do storms induce flooding? Combination of high waves, and storm set-up that is generated by low pressure
systems. For each millibar of pressure change, sea-level responds by approximately 1cm, hence typical low
pressure systems raise sea-level by approximately 0.5m. Consider the importance of low frequency, large
magnitude events.
Location 1 - the seaward beach
1
2
3
45
How is the sediment transported?
Ebb & flood
tidal currents
River flow
Q. Do you think the majority of the coastal sediments
come from along-shore drift or from river discharge?
A. Most sediment will come from along-shore drift and
wave reworking of glacial sediments deposited at the edge
of the ice sheet which was near the coastline around here.
Where does the sediment come from?
In-situ bioclastic
production (e.g. shells)
Alongshore
tidal transport
River transport
Q. What proportion of the beach sediment do you think
is made up of bioclastic material?
Location 1 - Beach with high wave energy and tidal fluctuations.
Beach environment is laterally
homogeneous and continuous
High tide mark with
coarsest debris
Q. How might you expect the
sediment in the dunes to differ from
that on the beach? Clues - consider
the different ability of wind and water
to carry different grainsizes?
A.
The dune sediments will be better sorted, and
finer grained, as wind can
B.
only transport grains up to ~0.5 mm in diameter
and grainsize
Boundary between beach and
dune environments is transitional
over a few metres.
Wind-blown sand preserved
in dunes with Marrum grass
colonisation
What orientation would you expect the boundary between the sediments
that record these
environments to be when projected beneath the ground? Clue- consider what would happen to this boundary during a sea-level
rise?
Ans. During sea-level rise it should dip beneath the beach, and during
sea-level fall, it would dip inland.
Typical Beach sands
Location 2
2
3
45
1
STOP 2
•
1.
2.
3.
4.
5.
Walk over the dunes, through the Marrum grass to the estuary side and consider the
contrast between the processes and deposits on the beach at location 1, and in the
dunes, and then at the margins of the estuary. Pose a similar set of questions to each
of these environments as at location 1. Having observed the processes and
sedimentary characteristics, make sure they understand the following points:
That the dune environment requires wind transport, and that therefore, the grain sizes
are smaller (usually less than 0.25mm), and well sorted, and lack a lot of the
bioclastic component. The surface forms are irregular, and therefore, cross-bedding
or irregular bedding will be the normal internal structure. Rootlets are also likely to
be preserved. COLLECT A SAMPLE
That the estuary environment is very different, and that there is an abrupt boundary
between it and the landward side of the spit.
That the estuary environment contains more mud, and that if you dig down, there are
alternations of mud and sand – these are likely related to storm wash over the dunes.
The estuary also contains a high diversity of shelly material (oysters, cockles,
mussels, and other large bivalves), plus more evidence of worm burrowings – you
should be able to see worms if you dig fast enough.
That the estuary is protected from high wave energy, and strong tidal currents, hence
this increases the mud content, the nutrient levels, and the diversity and number of
organisms. COLLECT A SAMPLE
Think about what these environments are doing in response to the sea-level rise that
is now taking place along this shoreline (see figure for rates of sea-level change).
Topography of dune environment
Extensive, fine network of rootlets in the dune sands
Location 2 - Behind the dunes and onto the estuary.
Back of dune environment
Abrupt boundary between the dunes
and the protected estuarine setting
Q. How will this boundary be preserved
in the stratigraphic record?
A. It depends on how fast these
environments are getting buried
by sediment versus how fast the
environments are moving - the practical
will investigate this in more detail.
Colour change between the two settings
indicating different sedimentological characteristics.
Coarse, bioclastic (shelly),poorly sorted sands. This material
is part of a spit that is encroaching over the estuary. During
each incoming high tide the spit is remobilised by both wave
and tidal currents resulting in its progressive migration.
Q. Where are you more likely to
find well-preserved shells in their
life position?
A. In the muds as they record less
reworking and abrasion by waves
and tides
Muds with
shell fragments
Abrupt boundary
between spit and estuary
Q. Which of these environments do you think is has a greater volume of
organisms living in it, and why?
A. The muds as they will be much richer in organic nutrients
on which worms, mussels, cockles and shrimps will feed.
Coarser intervals in the succession at the boundary
between the dune and estuarine environments
STOP 3
This stop involves a brief pause while traversing the edge
of the estuary to look at the nature of the sediments as you
progress inland. Note that they become finer grained (very
muddy), and that they preserve fully articulated bivalve
shells. These fine grained sediments are deposited at slack
high water. In these settings, grain size actually increases
as you go further offshore due to the higher flow velocities
during rising and falling tides compare to the slack high
water. The well preserved shells also reflect the quiet water
environment. Also note that the mussel banks are in the
lower intertidal regions. Again, get them to COLLECT A
SAMPLE
Mid- to upper Estuarine setting dominated by muds with current
ripples (ebb tidal flow) and fully articulate bivalve shells.
Mussel banks in the lower estuary setting
STOP 4
•
1.
2.
At this location, it is possible to view the characteristics of a small river
channel that feeds into the edge of the estuary. Ask the students to make a
sketch and label the following:
The meandering character of the channel, and consider how the gradient of
the channel banks vary with respect to the meanders. Is there evidence of
erosion on any of the banks? (see photo showing erosion on outer bend of
meander). Note where sediment accumulation will take place on the lower
gradient inner bend and introduce the term ‘point bar’ at this stage. Note
that if the point bar is the point of sediment accumulation, then the
depositional character of the resultant sediments should incorporate low
angle cross-bedding. These features can be seen upstream of the bridge
where the floodplain is vegetated (see photo).
The evidence of coarse pebbles and boulders in the base of the stream.
Consider where these might have come from – Ans. They’re presumably
reworked glacial sediments from higher up that are very occasionally
remobilised during high discharge.
Point bar
Erosion of outer bank
Coarse debris in channel
Reworked glacial debris?
Location 5
Point Bar
Erosion of outer bank
STOP 5
•
This stop attempts to bring together some of the environmental observations in terms of a
consideration of how they might get preserved in the stratigraphic record. The students should make
a drawing of the small cliff section, and observe the primary boundary within it, and incorporating
the vegetation at the top. There should be cross-bedded, grey shelly sands in the lower part of the
succession, and then finer grained, reddened cleaner sands at the top.
•
A probable interpretation for this is that it represents the seaward motion of the coastline during the
~6000 year stillstand in sea-level in this region. During this time, dune environments migrated out
over planar and cross-bedded sands of the beach environment. Since then, sea-levels continued to fall
in response to the post-LGM isostatic rebound. However, the recent tidal records suggest that the sealevels are now rising along this coastline, and so the recent estuarine deposits will accumualte over
the unconformity that represents the post LGM lowstand. Get the students to schematically project
this beneath the ground on their annotated cross section.
•
Clearly, the above is an oversimplification as the tidal regime was probably different 6Ka, but the
important point to convey is that there has been a seaward superposition of environments, and that
this is the basis of Walther’s Law (I doubt anyone has mentioned this phrase to them before, John
brings it in at the beginning of second year).
•
Having finished, follow the footpath between the woods and the fields of the East Links family park
back to the car park (see map).
Location 2
2
3
45
1
Modern soil
Finer grained red sandstones
Abrupt boundary
Lower, cross-bedded shelly
sands
Seaward superposition
of environments
Reddened rootleted fine sandstones – dune facies?
Planar bedded sands – typical beach facies
Cross-bedded shelly sands
For a modern analgue, it is possible to see large dune
fields in the lower intertidal zone of the Tyne estuary/beach.
Exact interpretation of this succession is not simple. The
Tyne estuary was much larger, flooding all the way to East Linton,
Therefore, it would have had greater tidal amplification, hence, the modern
System is not an exact analogue cf. 6Ka.
Modern Soil with roots penetrating underlying environments
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