Sediment recycling and adjustment of deposition during deglacial drowning of a low-accumulation shelf (NW Iberia) Hendrik Lantzsch n , Till J.J. Hanebuth, R ¨ udiger Henrich MARUM—Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, P.O. Box 330 440, 28334 Bremen, Germany article info Article history: Received 2 February 2010 Received in revised form 17 June 2010 Accepted 23 June 2010 Available online 1 July 2010 Keywords: Late Quaternary Siliciclastic shelves NW Iberia Sedimentary facies Continental shelf processes Shelf sedimentation abstract Based on a well-established stratigraphic framework and 47 AMS- 14 C dated sediment cores, the distribution of facies types on the NW Iberian margin is analysed in response to the last deglacial sea- level rise, thus providing a case study on the sedimentary evolution of a high-energy, low-accumulation shelf system. Altogether, four main types of sedimentary facies are defined. (1) A gravel-dominated facies occurs mostly as time-transgressive ravinement beds, which initially developed as shoreface and storm deposits in shallow waters on the outer shelf during the last sea-level lowstand; (2) A widespread, time- transgressive mixed siliceous/biogenic-carbonaceous sand facies indicates areas of moderate hydro- dynamic regimes, high contribution of reworked shelf material, and fluvial supply to the shelf; (3) A glaucony-containing sand facies in a stationary position on the outer shelf formed mostly during the last-glacial sea-level rise by reworking of older deposits as well as authigenic mineral formation; and (4) A mud facies is mostly restricted to confined Holocene fine-grained depocentres, which are located in mid-shelf position. The observed spatial and temporal distribution of these facies types on the high-energy, low- accumulation NW Iberian shelf was essentially controlled by the local interplay of sediment supply, shelf morphology, and strength of the hydrodynamic system. These patterns are in contrast to high- accumulation systems where extensive sediment supply is the dominant factor on the facies distribution. This study emphasises the importance of large-scale erosion and material recycling on the sedimentary buildup during the deglacial drowning of the shelf. The presence of a homogenous and up to 15-m thick transgressive cover above a lag horizon contradicts the common assumption of sparse and laterally confined sediment accumulation on high-energy shelf systems during deglacial sea-level rise. In contrast to this extensive sand cover, laterally very confined and maximal 4-m thin mud depocentres developed during the Holocene sea-level highstand. This restricted formation of fine- grained depocentres was related to the combination of: (1) frequently occurring high-energy hydrodynamic conditions; (2) low overall terrigenous input by the adjacent rivers; and (3) the large distance of the Galicia Mud Belt to its main sediment supplier. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Continental shelf systems are of highest economical and ecological importance and represent the major sink for sediments on the sedimentary pathway from the coast to the deep sea. Nevertheless, the scientific knowledge on the sedimentary shelf evolution is still limited due to the complex buildup of these systems as a result of the interplay of various forces which control distribution and deposition of sediments. During the past decades, several studies aimed at explaining the sedimentary construction of shelf systems and their material distribution patterns. On the one hand, pure geophysical approaches were used to illustrate the general aspects of shelf- strata development. These studies were usually characterised by a lack of sedimentary ground-truthing and sufficient age control. Studies corroborating such seismic interpretations by a compre- hensive sediment coring program are still a rare case (e.g., Bahr et al., 2005; Grossman et al., 2006; Berne ´ et al., 2007). On the other hand, studies dealing with the sediment distribution patterns were often restricted to the modern surface situation (e.g., Ogston et al., 2000; Dias et al., 2002; Brooks et al., 2003) or few sediment cores (e.g., Maldonado et al., 1983; Dı ´az et al., 1990; Buck et al., 1999) which might not adequately represent the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2010.06.013 n Corresponding author. Tel.: + 49 421 218 65193; fax: + 49 421 218 65219. E-mail address: lantzsch@uni-bremen.de (H. Lantzsch). Continental Shelf Research 30 (2010) 1665–1679