High-Resolution Architecture of Late Holocene Highstand Prodeltaic Deposits from Southern Spain: the Imprint of High-Frequency Climatic and Relative Sea-Level Changes
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ARTICLE IN PRESS Continental Shelf Research ] (]]]]) ]]]–]]] High-resolution architecture of late Holocene highstand prodeltaic deposits from southern Spain: the imprint of high-frequency climatic and relative sea-level changes L.M. Fernandez-Salas! a,*, F.J. Lobob, F.J. Hernandez-Molina! c, L. Somozad, J. Roderoe,V.D!ıaz del R!ıoa, A. Maldonadoe a Instituto Espan˜ ol de Oceanograf!ıa, Centro Oceanografico! de Malaga,! Puerto Pesquero, s/n, Apartado 285, Fuengirola 29640, Spain b CIACOMAR-Universidade do Algarve, Avenida 16 de Junho, s/n, Olhao* 8700-311, Portugal c Facultad de Ciencias, Departamento de Geociencias Marinas, Universidad de Vigo, Vigo 36200, Spain d Instituto Geologico! y Minero de Espan˜ a (IGME), Servicio Geolog!ıa Marina, R!ıo Rosas 23, Madrid 28003, Spain e Instituto Andaluz de Ciencias de la Tierra, CSIC-Univ. Granada, Fuentenueva s/n, Granada 18071, Spain Received 31 January 2002; received in revised form 15 April 2003; accepted 2 June 2003 Abstract The stratigraphic architecture and recent evolution of two late Holocene prodeltas (Guadalquivir and Guadalhorce rivers) from the south of Spain are compared on the basis of a high-resolution seismic stratigraphic analysis. These sedimentary bodies display a similar internal architecture, being characterised by a thin transgressive systems tract at their bases and an overlying thick, wedge-shaped highstand systems tract (HST). The internal structure of the HST deposits shows the repetition of two progradational/aggradational cycles, whose generation seems to be related to the influence of high-frequency climatic changes during the last 6–7 ka. Those changes probably involved small amplitude (o3–4 m) sea-level oscillations and associated changes of sediment supply, conditioning the development of delta switching processes. Small differences in the distribution and characteristics of the seismic units in each area are attributed to local factors, such as physiography of the coastal plains, the wave climate and increased sediment supplies conditioned by anthropic actions. r 2003 Elsevier Ltd. All rights reserved. Keywords: Holocene; Sea-level changes; Climatic changes; Gulf of Cadiz;! Alboran Sea; Geographic bounding coordinates: Guadalquivir area: (36150N,7150W) (36500N,5350W); Guadalhorce area: (36300N,4400W)(36450N,4230W) 1. Introduction The application of sequence stratigraphy con- cepts to late Pleistocene and Holocene shelf and coastal deposits has shown that the large-scale *Corresponding author. Tel.: +34-952471907; fax: +34-952- 463808. architecture of deltas, defined by the geometry of E-mail address: [email protected] progradation, aggradation and retrogradation, is (L.M. Fernandez-Salas).! mainly governed by sediment yield and relative 0278-4343/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0278-4343(03)00120-1 ARTICLE IN PRESS 2 L.M. Fernandez-Salas! et al. / Continental Shelf Research ] (]]]]) ]]]–]]] sea-level change (Thorne and Swift, 1991; Postma, 1987). Those processes were thought to be 1995). These deltaic sequences can be subdivided generated by rather random controlling factors, generally into three stratigraphic units (SU I, II such as changes in river mouth domains, neotec- and III) (Warne and Stanley, 1995). SU III is the tonic activity and fluctuations in sediment supply most recent unit and represents the Holocene (D!ıaz et al., 1990; Goodbred and Kuehl, 2000). deltaic deposits. It is composed of transgressive However, several authors have recently suggested systems tract (TST) to highstand systems tract that the avulsion of deltaic lobes can be attributed (HST), which are differentiated by the stacking to superimposed processes, such as climate change pattern of the parasequences. It is believed that and sea-level fluctuations (Swift et al., 1991). In eustatic changes played a major role in determin- this sense, fifth-order sea-level cycles (1–2 Â 103 ing the structure of those sequences. However, years), associated with major climatic fluctuations their evolution may also be controlled by the rate during the middle to late Holocene, would of sediment supply and by tectonics, but those have favoured delta lobe switching (Lowrie and factors can only modify the relative growth Hamiter, 1995). patterns of such systems tracts. For example, huge Some authors (Hernandez-Molina! et al., 1994; sediment loads would favour the formation of Fernandez-Salas,! 1996; Somoza et al., 1998) have aggrading TST, and early initiation of HST suggested that Holocene highstand deposits do not progradation (Goodbred and Kuehl, 2000). constitute a unique sedimentary body, but are The stratigraphic architecture of the HST made up of an assemblage of minor sedimentary component of the Holocene prodeltaic deposits bodies. The internal structure of HST deltaic has been studied by a number of workers. deposits has been linked to the role of fifth- and Traditionally, most of them considered a unique sixth-order sea-level oscillations. Progradational clinoform structure with a wedge-shaped external events are caused by the fifth-order cycles, whereas form and a characteristic topset–foreset–bottomset the modification of deltaic lobes has been attri- internal configuration (Nittrouer et al., 1986; D!ıaz buted to the influence of sixth-order cycles. These et al., 1996). This wedge-shaped structure inter- cycles have been used to explain the depositional nally shows lateral transition of seismic facies, architecture of many coastal and shelf deposits in from proximal stratified facies to distal transpar- different areas, as the Mississippi delta (Boyd et al., ent facies, which is related to different sedimentary 1989; Lowrie and Fairbridge, 1991; Lowrie and environments (Nittrouer et al., 1986; Trincardi Hamiter, 1995) and abundant Mediterranean and Normark, 1988; D!ıaz et al., 1990; Park et al., deltas, such as the Nile delta (Stanley and Warne, 1990; D!ıaz and Ercilla, 1993; Ercilla et al., 1995; 1993), the Rhone# delta (Gensous et al., 1993), the D!ıaz et al., 1996; Nittrouer et al., 1996; Hart et al., Tiber delta (Bellotti et al., 1994), the Ebro delta 1998). The pattern of erosion/accumulation and (Checa et al., 1988; D!ıaz et al., 1990; Somoza et al., the vertical build-up of the clinoform structure are 1998) and the Alboran Sea deltas (Hernandez-! also controlled by the intensity of oceanic pro- Molina et al., 1994; Fernandez-Salas,! 1996; cesses (Nittrouer et al., 1996). Fernandez-Salas! et al., 1996), among others. However, the detailed study of some Holocene In this paper, we examine the stratigraphic deltas has revealed a more complex vertical architecture and recent evolution of late Holocene structure. The cyclic character of deltaic sediment- highstand deposits associated with deltaic bodies ation was described by Scruton (1960), who stated from the southern Iberian margin, from a high- that the evolution of deltas can be subdivided in resolution seismic stratigraphy perspective which two phases: the first is constructive and prograda- has been supported by sedimentologic analysis. tional and the second is destructive and retro- According to this, the main goals of this work are: gradational. In these studies, channel avulsion and (1) to determine the governing forces and pro- delta lobe switching are amongst the most cesses which have controlled the resulting strati- important controlling factors in determining the graphic pattern and internal structure of middle to internal structure of Holocene deltas (Aksu et al., late Holocene deltaic sequences; (2) to test the ARTICLE IN PRESS L.M. Fernandez-Salas! et al. / Continental Shelf Research ] (]]]]) ]]]–]]] 3 relative significance of autocyclic (sediment sup- pathway of the AI describes two anticyclonic ply) and allocyclic (climatic, glacio-eustatic gyres, generating a mean east-northeastward flow changes) factors, and; (3) to explain the existence over the shelf (Cano and Garc!ıa, 1991). of regional variability of such deposits. According to Ercilla (1992), Ercilla et al. (1992), Hernandez-Molina! (1993) and Hernandez-Molina! et al. (1994), the late Quaternary depositional 2. Regional setting and oceanography sequences in the northern margin of the Alboran Sea is made up of the following deposits, from Our research has focussed on two sectors of the bottom to top: (1) A lowstand progradational continental shelf of the southern Iberian Peninsu- wedge in the shelfbreak, interpreted as the low- la. The Guadalhorce prodelta is located in the stand systems tract (LST); (2) A set of two northeastern Alboran Sea (Mediterranean Sea) backstepping units, one progradational and an- and the Guadalquivir prodelta is located in the other aggradational associated with the TST; (3) A Gulf of Cadiz (Atlantic Ocean). Both regions are progradational wedge attributed to the HST connected by the Straits of Gibraltar (Fig. 1a). composed of four units which represent prodeltaic bodies. 2.1. The Alboran Sea continental shelf The Alboran Basin is located in the westernmost 2.2. The Gulf of Cadiz continental shelf Mediterranean Sea. We have studied a sector of the Spanish continental shelf of the Alboran Basin, The studied sector of the Gulf of Cadiz is located between the city of Malaga and Calaburras located between the Guadalquivir river mouth to Cape (Fig. 1c). The coastal plain is very narrow, the north and the town of Zahara de los Atunes to because the Betic Mountains are very close to the the south (Fig. 1b). The most important river shoreline. As a consequence, several relatively feeding this shelf is the Guadalquivir river, whose steep and small rivers (Guadalhorce, Guadalme- mouth is located at the northern edge of the study dina and Fuengirola rivers), characterised by a area. It is 560 km long and its basin covers over pronounced seasonal variability in the sediment 57,121 km2, with a mean water discharge of supply regime, feed this sector of the Alboran 160 m3/s (Van Geen et al., 1997). The coastal plain shelf. The most important is the Guadalhorce of the Guadalquivir river is characterised by wide River, which is 160 km long and has a drainage salt marshes with very low gradients. basin that extends for about 3158 km2.