* Corresponding author. Tel.: 003459 350599. fax: 0034 59530175, e-mail: borrego@uhu.es Quaternary Science Reviews 18 (1999) 769 — 788 The Holocene transgression into the estuarine central basin of the Odiel River mouth (Cadiz gulf, SW, Spain): lithology and faunal assemblages J. Borrego*, F. Ruiz, M.L. Gonzalez-Regalado, J.G. Pendo´n, J.A. Morales Departamento. de Geologı ´ a, Universidad de Huelva, Campus de la Ra & bida, 21819-Palos de la Frontera, Huelva, Spain Abstract Six estuarine facies were distinguished recording the Holocene history of sea level rise relating according to the start and development of the Holocene transgression which flooded the coast of Huelva: Facies 2 (gravels), Facies 3 (grey clayey silts), Facies 5 (well sorted sands), Facies 4 (silty sands), Facies 5 (black clayey silts), and Facies 6 (red muds). This group overlies Neogene sediments (Facies 1). Three faunal assemblages (Open bay (OB), Central estuary (CE), and Wave domination (WD)) including remains of macrofauna, foraminifers and ostracods plus depositional features, are identified in a sedimentological log constructed from a borehole with a continuous core, sunk in the central basin of the Odiel River estuary, Huela coast, SW. Spain. The OB assemblage requires shallow and protected zones controlled by low energy tidal currents; the CE assemblage is located within an intertidal zone, where reworked marine forms of foraminifers, ostracods and scattered macrofauna co-exist with small estuarine foraminifers and ostracods; the WD assemblage comprises tests of marine macrofauna with fractured shells plus large marine foraminifers and estuarine ostracods. During the first stage of continuous sea level rise (8720$260 BP to 5390$155 BP), estuarine accretion, high energy tidal currents and wave action took place successively, allowing the development of the OB, CE and WD assemblages. The second stage (5390$155 BP to Present), with a stabilized sea level, comprised a vertical decrease of energy, with tidal currents favouring deposition in shallow tidal channels and marsh zones, leading to less energetic CE assemblages within the estuarine central basin. High sediment supply resulted in deposition during this stage, which shows a regressive nature.  1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Over the past ten years, many studies have investi- gated sedimentation in Holocene estuaries. Various fa- cies models have been proposed (Frey and Howard, 1986; Dalrymple et al., 1990; Nichols et al., 1991; Dalrymple et al., 1992; Allen and Posamentier, 1993; Cooper, 1993; etc.) Most of these studies were based on lithofacies distributions and their relationship to the energy regime in different estuarine domains. However, there are very few studies of the biogenic component of the sediments, only those aspects related to bioturbation and directly related to sediment burroughing have been studied in depth. In these few exceptions, ecostratigraphy is usually based on only one faunal group, such as benthic foraminifers (Bloyd and Honig, 1992; Cearreta, 1992, 1994) or ostracods (Penney, 1985, 1987). Articles on the Huelva coast describe sedimentary en- vironments and their anthropogenic influence (Dabrio et al., 1980; Borrego and Pendo´ n, 1989), Holocene estuarine facies (Borrego, 1992; Borrego et al., 1993, 1995a, b; Morales, 1995; Pendo´n and Morales, 1997), coastal geomorphic evolution (Rodrı´guez Vidal, 1987a, b; Ojeda, 1988) and their relationship to sea level fluctuations during the last 10,000 yr BP (Mene´ ndez Amor and Flour- schu¨tz, 1964; Zazo et al., 1996a, b). Very little is known about the fauna in littoral sediments, only some microfaunal groups such as foraminifers and ostracods have been studied (Gonza´lez-Regalado et al., 1990; Ruiz Muno  z et al., 1994; Ruiz Mun  oz, 1995; Gonza´lez—Re- galado et al., 1996). The few macrofaunal studies avail- able are about Mollusca, Crustacea, Annelida, and Bryozoa (Pe´rez Quintero, 1989; Lo´pez de la Cuadra, 1990; Gonza´lez—Gordillo et al., 1990; Rordı´guez and Vie´itez, 1992). This paper compares the lithology and biological content (macrofauna and microfauna) in sediment 0277-3791/99/$ — see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 9 7 ) 0 0 0 8 5 - 1