Genetic relationships between beach ridge plain and superimposed dunes (Maule region, Chile)

Jose ́ F. Araya Vergara* and Cristian Araya Cornejo Departamento de Geografía, Universidad de Chile, Av. Portugal 84, Santiago, Chile

*Email: [email protected]

Abstract The relationships between beach ridge plain and granulometry, morphoscopy and mineralogy of samples superimposed coastal dunes are studied in La Trinchera were carried out in laboratory. erg, Maule region, by means of surface geomorphology, drilling and sedimentary analysis. The underlying strand The landforms were observed in satellite images and plain is composed by a set of multiple beach ridges, which directly in the field. The strand plain of multiple beach represents a prograded shore plain. The superimposed dunes have been formed in three different pulses: old, ridges and its structure were defined following the works middle and modern. The allostratigraphy and regional of Otvos (2000) and Orford et al. (2003). The sets of dunes correlations support the model of alternate sediment were classified as systems organized and superimposed in decoupling for the formation of the old dune system and the different formative pulses, taking the experiences of model of sequential sediment sourcing for the construction Araya-Vergara, (2001) and Araya (2010). The concept of of the middle and modern pulses. allostratigraphy is used following the principles exposed by Walker (1990) and Lønne (1995). Keywords: strand plain, beach ridge, superimposed dunes, models of decoupling.

1 Introduction Barchanoid Rotated distal belt transverse dunes

Some former works show dune fields overlaying multiple Aeolian deflational and gravel-dominated beach ridges (Orford et al. , 2003; Carter, aspersional surfaces Beach ridges

1982). Orford and Carter (1988) examined the problem of the decoupling of gravel beach ridges into dune sand. The A exact mechanism for this superimposed succession is studied and discussed by Orford et al. (2003), reviewing two models of possible decoupling: a) Sequential sediment sourcing (SSS), assuming that gravel ridges are emplaced as a complete set under a reflective dynamic regime. When the sediment source is Barchanoid Aeolian deflational and exhausted, the superimposition of the dune cover is belt aspersional surfaces produced under a dissipative domain, because of energy Beach ridges and sediment sourcing changes. Parabolic

B b) Alternate sediment decoupling (ASD), assuming synchronous deposition of ridges and dunes. This mechanism reflects multiple switches in the beach sediment. Sand and gravels are decoupled rhythmically. Figure 1. Types of cross section in the La Trinchera system: A = So, in the present work a dune field overlaying multiple northern part; B =central part. gravel-dominated beach ridges is analyzed in order to test Beach Narrow the cited models or propose another mechanism. ridge swale Swale Shoreline 1

m 2 Method, Samples, Results

2.1 Methods and samples

180 m The stratigraphy of the system was known by means of cores (type sand auger) and casing drilling. The grain size Figure 2. Simplified sketch, showing the structure of the strand and form of the sands of different cores were described in plain. the field using the table Sand Gauge. The more precise

680 2.2 Results Vergara (1989) observed three principal dune generations in Central Chile. The old dunes are foredunes, parabolic or The figures 1 and 3 show the following groups of upsiloidal and they contain soils. The middle dunes are landforms, which are mentioned sequentially according to parabolics, without soils. The modern ones are principally their relative age: a) the strand plain of multiple beach barchans, barchanoids and transverse. Therefore, the old ridges; b) the old parabolic and complex dunes; c) the systems were stabilized as deflational dunes in a period of middle parabolic; d) modern group, which contains aeolian meagre sand supply. The time elapsed between this deflational and aspersional sand sheets, rotated distal stabilization and the birth of the middle dunes was long. transverse dunes, barchanoid belt and foredunes with So, periods of both decrepitude and sand recharge have blow-outs (Table 1). characterized the morphogenetic sequences of these ergs. These characteristics are observed also in La Trinchera erg, Table1. Predominant sedimentological characteristics in where only the old system show direct relationship with groups of landforms the underlying beach ridge strand plain, starting likely Group of Textural from foredunes. Its observable sand mass is low. In landform composition exchange, the superimposed mass of the middle and modern dunes is evidently higher, represented principally Strandplain Gravels, medium sand and soils by a thick set of barchanoids and transverse dunes. So, the sources of its sands are not in the beach ridges of the local Old Fine sand and strand plain, but in outer systems. incipient soils Consequently, the two models studied by Orford et al. Middle Fine sand (2003) are not antithetic in order to explain the decoupling of the strand plain into the dune field in La Trinchera. The Modern Medium and fine sand allostratigraphy and regional correlation support the model ASD for the formation of the old dune system and SSS for The strand plain, as oldest unity, presents a structure in the construction of the middle and modern systems. which each beach ridge re-covers partially the shoreface of the immediately former, and so successively toward the References shoreline (Figure 2). The old dunes overlay directly the strand plain; they are masked, morphologically Araya-Vergara, J.F. 1989. Remnant coastal dunes and their transformed and difficultly visible. The middle dunes are significance in Chilean ergs. In Second International Conference on Geomorphology Geoöko-Plus 1:15, Darmstadt principally visible through their outcropped summits, because they are almost totally covered by a modern Araya-Vergara, J.F. 2001. Los ergs del desierto marginal de aeolian deflational and aspersional sand sheet. Their Atacama, Chile. Investigaciones Geográficas, Chile 35: 27-66. parabolic form only can be observed directly in isolated sites. The modern dunes, including the deflational- Araya, C.I. 2010. Morfogénesis evolutiva y sedimentología del aspersional sheet, cover the bigger part of the landscape. sistema de dunas costeras de La Trinchera, región del Maule. Memoria de Título (Unpublished), Universidad de Chile, The whole sand sea shows that the increasing of the sand Escuela de Geografía: 86 p. mass is inversely proportional to the relative age of the dune groups. That is to say, the sand supply has been Carter, R.W.G. 1982. Recent variations in sea level on the north and clearly bigger during the formation of the modern pulses. east coasts of Ireland and associated shoreline response. In Proceedings Royal Irish Academy, 82B: 177-187, Ireland.

The analysis of cores (Figure 3) shows a recurrent Lønne, I. 1995. Sedimentary facies and depositional architecture of presence of gravels in de base of the stratigraphic columns, ice-contact glaciomarine systems. Sedimentary Geology 98: coincident with the presence of beach ridges in the strand 13-43. plain. Well differentiated bodies of medium and fine sands and complementary morphoscopical observations suggest Orford, J.D.; Carter, R.W.G. 1988. Shelf-coast evolution; an Irish that the cores represent different pulses in the formation of perspective. In Shelf Coast Interaction (van de Plasche; editor). Vrije Universiteit: 88-91, Amsterdam. the sand sea. The bigger sediment thicknesses correspond to the fine sands of the modern dunes. This fact agrees Orford, J.D.; Murdy, J.M.; Wintle, A.G. 2003. Prograded Holocene with the bigger mass that these dunes show in the beach ridges with superimposed dunes in north-east Ireland: landscape and the inverse relationship between the age and mechanisms and time scales of fine and coarse sediment the sand mass of the pulses in the sand sea. decoupling and deposition. Marine Geology 194: 47-64.

Otvos, E.G. 2000. Beach ridges—definitions and significance. 3 Discussion and Comments Geomorphology 32: 83-108.

The results of former researches can be used in order to Walker, R.G. 1990. Facies modeling and sequence stratigraphy. apply models for the evolution of this sand sea. Araya- Journal of Sedimentary Petrology 60: 777-786.

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Figure 3. Geomorphological map and cores showing stratigraphy

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