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Detecting storm and tsunami deposits in coastal lagoons. Preliminary results from Lagoa de Óbidos (W Portugal).
by
Costa, Pedro
Department of Geography and Earth Sciences, Brunel University, Uxbridge, London, UK
Coauthors: Leroy, Suzanne, Kershaw, Stephen, Dinis, Jorge
A series of lakes and coastal lagoons are being studied along the Portuguese and Mauritanian coasts with the aim of: - Discovering and studying the 1755 tsunami sediment layer - Detecting storm deposits and - Detecting major environmental changes in the study areas over the last five centuries. Identifying storm and tsunami deposits is still a matter under intense debate. The hydrodynamic of this type of events contributes to the complex deposition. To clearly distinguish storm and tsunami deposits from the under and overlying layers a multi proxy analysis is required. We conducted a group of palaeolimnological and palaeoseismological proxies in order to establish the full extent of the catastrophic event, but we also looked at other non-catastrophic changes in the study area. The techniques used include magnetic susceptibility, x-ray imaging, sediment visual description, laser granulometry, geochemistry (Atomic Absorption Spectrometry), Pb210 and OSL (Optically Stimulated Luminescence) dating, palaeontological studies and a range of sedimentological and palaeoecological proxies with the focus of obtaining well-dated tsunami/storm indicators such as salinity changes, grain size increase, erosive and compaction microstructures. We also used historical data to collect complementary information about the effects of the tsunami and to reinforce the age-depth model of the sedimentary sequences. Just over the last 20 years a significant number of articles have been published about the sedimentary signature of tsunamis. Possible tsunami deposits can be identified using a group of diagnostic criteria (presented below). 1) Stratigraphic characteristics of tsunami deposits are: The event unit fines upward and inland (Foster, 1991; Dawson 1994). The lower contact is unconformable or erosional (Dawson, 1988; Moore, 1988). The deposit can contain intraclasts of reworked material (Dawson, 1994; Moore, 1994). The stratigraphic criterion reflects the high energy of such events. 2) The granulometric criteria to identify tsunami or storm deposit are: An anomalous sand unit in the lagoon sequence (Ota, 1985; Moore, 1988; Minoura 1994; Dawson 1994). However, the size of the particles depends on the material available on the coastline. 3) The palaeontological criteria are: An increase in abundance of marine to brackish water diatoms (Dawson, 1988; Minoura, 1994; Hemphill-Halley, 1996; Clague, 1999). Marked changes in foraminifera and other microfossils assemblages (Patterson, 1996; Shennan, 1996;Clague, 1999). Shell-richness of the event unit. 4) The geochemical criteria are: Increases in sodium, calcium, magnesium and chlorine relative to under and overlying layers (Minoura, 1994; Goff, 1998). 5) Dating tsunami and storm deposits is complicated due to older debris that are incorporated in the event deposit. OSL is the method that has achieved better results. The criterion presented reflects both the high energy of the events, as well, as a marine invasion. However, both tsunamis and storm surges can cause such impacts in the coastal stratigraphy. The uniqueness of a tsunami event, when compared with the more frequent storm surges, is still one of the main, if not, the decisive, criteria, although not a scientifically sound one. Obviously this can only be applied in regions not subject to frequent tsunamis. It is undoubtedly important to be able to distinguish such events in order to establish patterns of coastal flooding.
Fig. 1: Lagoa de Óbidos (39o23’ N 9o14 W’), Portugal.
Our research is focusing on the well-known Lisbon 1755 tsunami and its consequences. We focused on areas that were strongly affected by the tsunami. Based on historical records we choose to collect cores from Lagoa de Óbidos (Figure 1).
Lagoa de Óbidos is one of the largest coastal lagoons in Portugal. Located just north of Cabo Carvoeiro, in the western Portuguese coast, Lagoa de Óbidos is a shallow lagoon surrounded by abrupt cliffs. This lagoon was an estuary but due to sedimentary accumulation, especially after the Little Ice Age, became a lagoon with a small channel (Aberta) to communicate with the sea. The lagoon has 2 major areas; one closest to Aberta, marine dominated and another area, to the east of the small overwash fan, marking the landward limit of storm surges in the area, where the marine influence is smaller and the polluted rivers are the main sedimentary contributors. The lagoon with an approximate area of 6 km2 is in a slow process of infilling. In the village of Óbidos the 1755 earthquake caused serious destruction, churches (S. Tiago and S. Pedro), the City Hall, the City Wall and many houses collapsed. According to Sousa Moreira (1993) in Porto Novo, 25 km south of Óbidos, the run-up of the 1755 tsunami was 20 meters and the penetration in land was 2.5 km. Porto Novo is a geomorphological area very similar to Óbidos. We collected 10 cores (app. 1 m long each) from Lagoa de Óbidos using a Livingstone (piston) corer. A transect with an orientation NNW-SSE (Fig. 1) was followed. The cores were stored in PVC pipes after extrusion. Magnetic susceptibility measurements on the unopened cores were then conducted using a Bartington MS2C magnetometer. The results help us anticipate major depositional changes in the cores. The cores were further sectioned in two halves. The standard core description, according with the ODP visual core description form, included granulometry, macroscopic sedimentary structures, the nature of contact between different stratigraphical units and colour (according to the Munsell chart). We identified four different stratigraphic Units (Fig.2). Unit A, the deepest unit, consists of coarse clay, darker in the eastern stations and present in all. In places this unit contains a discontinuous millimetric scale laminae of dark brow clay interlayed with light brown clay. The contact between Unit A and the overlying Unit B is erosional. Unit B, or event unit, consists of olive green sandy material, which fines inland. Moreover, it is not present on Station 2, the most eastern station. The shell richness of this unit was also evident. A flame structure was also recognised on the base of this unit. Unit C, overlaying Unit B, consists of very fine clays, without any visible sub-units, is present in all stations. The contacts between Unit C and the over and underlying units are transitional.
Fig. 2: Stratigraphic description of Lagoa de Óbidos cores.
The top unit, Unit D, only present on the westernmost two stations, consists of clay material that becomes coarser towards the top. In places it was also possible to identify some laminae, alternating coarser, light millimetric sandy material with finer, darker clays. The stratigraphy of the cores represents the complexity of the depositional system in Lagoa de Óbidos. The marine influence is clearly noticed on stations 1 and 4, and less relevant on stations 5 and 2. Each unit represents different depositional conditions, related with the balance between the marine and fluvial deposits. The abruptness of the contact between Unit A and B suggests a sudden and short-lived depositional event. X-ray photographs were made of selected intervals to reveal sedimentary structures. We used x-rays to highlight features that are not visible by visual description. The results of both digital and x-ray photos are presented in Fig.3. The strong contrast between Unit A and B was exposed on both analyses. The shell (many of them broken) richness of Unit B is better seen in the x-ray photos.
Fig. 3: Digital (left) and x-ray photograph (right) of core OB 4.2
Small samples (c. 5 g) were collected every 2 cm and sequential loss-on-ignition was used to calculate organic and carbonate content. The organic matter content was determinated after combustion at 550 °C for 2 hours. The carbonate content was calculated after combustion at 925 °C for 24 hr. An increase both in organic matter and in carbonates was detected at c. 1.20 and 0.56 m. The base of Unit B is marked by a very slight increase in carbonates and a very small decrease in organic matter. Grain size analysis was conducted using a Malvern 2000 Series Laser Granulometer on sub-samples from all cores The results indicated a silt-dominated material in the cores. Furthermore, the results showed three major events, with sharp increases in grain-size, at 1.20 and 0.56 and also at the base of Unit B. The grain-size analysis also demonstrated that the event unit fines upwards. A sequence of 3 fining upward sub-units is clear (Fig. 4). Moreover, the average grain size of Unit B decreases towards the east demonstrating the loss of energy of the abrupt depositional event responsible for its deposition. The fining of the average grain size towards the east demonstrates the decrease of marine influence.
Fig. 4: Grain size distribution in Station 1
Statistical grain size parameters, e.g. standard deviation, skewness and kurtosis, were calculated, using formulas based on MacManus (1988). The granulometric data confirmed the uniqueness of Unit B in the lithostratigraphy of Lagoa de Óbidos. The geochemistry of the sediments was analysed to detect changes in the following elements: Al, Ca, Fe, K, Mg, Mn, Na, Pb, Zn. Flame Atomic Absorption Spectroscopy with a Thermo Jarrell Ash S11 and a Perkin-Elmer Model 2380 Spectrophotometer measured the elements. The bottom of Unit B presents an increase in Mn (anoxic conditions), probably due to sudden deposition of large amount of sediments. The results showed clearly a sharp change linked to the event. After a more marine influenced environment (high values of Ca, Na, Mg and K) underlying Unit B, a totally different environment has developed after the top of Unit B. The sharp change can be explained by one of two reasons; the closure of the sand barrier after an abrupt event or, a strong period of storminess below Unit B with frequent marine intrusions in the lagoon. It was also noted that also the top 25 cm present a slight increase in Ca, Na, Mg and K. The 0.56 m event is also registered geochemically with increases in Ca, Na, Mg and K. Studying Unit B, at higher resolution, we can detect three sharp increases in K, Mg, and Na in lower 10 cm of Unit B. These results can be correlated with the three fining upwards grain size sequences. The geochemical data proved, beyond doubt, the unique character and consequences of Unit B. Unit B was dated using OSL at the Geochronology Laboratories, University of Gloucestershire. The Luminescence age was obtained by dividing the mean equivalent dose value by the mean total dose rate value. The error on luminescence age estimates represents the combined systematic and experimental error associated with both the equivalent dose acquisition and dose rates values. Figure 5 shows the composite probability mass function obtained for the sample, incorporating the equivalent dose acquisition and dose rate uncertainties for each aliquot, along with 1
Fig. 5: Age estimates generated for Unit B (in core OB1.2)
A palaeontological study is being carried out. The preliminary results showed that the samples are rich in Molluscs and in plant debris. The event layer is richer in shell fragments and in plant debris, indicating a strong energy event. Moreover, the presence of large quantities of quartz in Unit B samples is also a strong indicator of the marine character of the deposit.
This research is still in its early stages, but we can already conclude that a strong depositional event occurred in Lagoa de Óbidos, at the base of Unit B. Moreover, this unique event presents some of the tsunami sedimentological diagnostic criteria. It cannot be concluded that it was deposited by one tsunami. The Unit also marks a strong alteration in the marine influence in the lagoon (mode shift). This work highlights the problem of recognising the extent of ancient tsunamis. The key challenge in the recognition of tsunamis in past sedimentary records is establishing a distinctive signature of tsunami that will clearly separate it from other sedimentary processes. It is important to analyse the effects of the 1755 tsunami in the western coast of Portugal, on the shores of Morocco and Mauritania, to have a broader picture of the causes, the behaviour and the vast devastation caused by the event. That is the objective of our ongoing research.
Clague J.J., Hutchinson I., Mathewes R.W. and Patterson R.T., 1999. Evidence for late Holocene tsunamis at Catala Lake, British Columbia. Journal of Coastal Research 15, no.1. 45-60 Dawson A.G., Long D. and Smith D.E., 1988. The Storegga Slides: evidence from eastern Scotland for a possible tsunami. Marine Geology 82. Pages 271 to 276. Dawson A. G., 1994. Geomorphological effects of the tsunami run-up and backwash. Geomorphology. Volume 10, Pages 83-94. Foster I.D.L. , Albon A. J., Bardell K. M., Fletcher J. L., Jardine T. C., Mothers R. J., Pritchard M.A. and Turner S.E., 1991. High energy coastal sedimentary deposits; an evaluation of depositional processes in Southwest England. Earth Surface Processes
Date received: November 25, 2003
Copyright © 2003 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # camu-09.