FF14 - ALL CHOKED UP EarthCache
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The shoreline is
where the land meets the sea and it is continually changing. Coastal
lagoons are but one example that occur at this interphase sea-land and
provide not only dynamic geological environments as well as local
ecological safe havens for myriad species of animals, plants and birds.
A lagoon is a body of comparatively shallow salt or brackish water
separated from the deeper sea by a shallow or exposed sandbank, coral
reef, or similar feature.
The application of the name lagoon in English dates from 1769. It
adapted and extended the sense of the Venetian laguna (cf Latin lacuna,
‘empty space’), which specifically referred to Venice’s shallow,
island-studded stretch of saltwater, protected from the Adriatic by the
barrier beaches of the Lido refers to both coastal lagoons formed by
the build-up of sandbanks or reefs along shallow coastal waters, and
the lagoons in atolls, formed by the growth of coral reefs on slowly
sinking central islands. Lagoons that are fed by freshwater streams are
also called estuaries.
The Albufeira Lagoon
Approximately 25 km south of central Lisbon, in the Sesimbra county
lies the Albufeira Lagoon.
This lagoon occupies an average area of 1.3 km2 with a long axis
oblique to the coastline with a NE-SW orientation. It’s maximum length
and width is 3.5 km and 625 m, respectively. This lagoon is fed by the
Apostiça River and is made up of two water bodies: the Lagoa Grande
(Large Lagoon) and the Lagoa Pequena (Small Lagoon). Although detailed
bathymetry is unknown, it is estimated that the lagoon does not exceed
15 m depth.
This lagoon is separated from the sea by a continuous sand bank that ii
artificially opened once a year to allow replenishing of the nutrients
and sediments within the lagoon.
This sand bank is approximately 1200 m long, orientated in a N-S
direction, and varies in width from 608 m in the north to 432 m in the
south. It is “anchored” to land via cliff faces of approximately 20 m
high that are made up of Miocene-Pliocene (11 - 2 M.a.) formations.
The sand bank is maintained in place and effectively cuts the lagoon
off from the sea by active longshore drift that moves sediment around
from both north and south depending on whether the wave direction is
from the S/SW or N/NW. Rarely; it hasn’t happened naturally in this or
the previous century, is this sand bank breached by high water levels
within the lagoon. Preferential sediment movement is to the south and
dunes are frequently developed within the sand bank.
Genesis model
Approximately 10000 years ago the mean sea level was 20 m below todys
mean sea level. At that time this lagoon did not exist and in its place
would have been only a deep valley. Between 10000 and 6000 there is an
abrupt sea level rise close to what it is today. The invading ocean
would have created a system of tidal inlets. At about 5000 years before
present the rate of sea level uplift was drastically reduced that
allowed the sand bank to start forming and block the access of the
fluvial waters and sediments to the sea. Since that time the valley has
been progressively being filled with river sediments causing a decrease
in the extension of the valley and causing it to attain a flat bottom.
This EarthCache will take to the sand bank of the Albufeira Lagoon in
order to get to know it better from a sedimentological and geological
point of view. You will be able to see in loco many of the features
described in this page and from your observations be able to carry out
a few tasks so you may log a “found”.
Your tasks are:
1- What size are the sand grains that make up the sand bank?
2- Would you class this sediment as fine-grained, Medium-grained or
coarse?
3- What type of dunes are present in the sand bank? ?(barchan or seif)
?
4- On what day of the year is the lagoon artificially opened to the
ocean? This will require some sleuthing amongst the local population.
Send these answers via e-mail .
Additional Hints
(No hints available.)