Holocene highstand deposits in the Gulf of Cadiz, SW Iberian Peninsula: A high-resolution record of hierarchical environmental changes

Abstract

A comparison of the internal structure of Holocene highstand shallow-marine deposits with emerged spit-bar systems in the Gulf of Cadiz margin (SW Iberian Peninsula) is presented, in order to increase the knowledge about the patterns of environmental fluctuations of sub-Milankovitch periodicity during the recent highstand period. A high-resolution seismic database of boomer profiles was interpreted, focusing on shallow-marine deposits of the Gulf of Cadiz shelf, such as the Guadalquivir River prodelta, the Guadiana River depositional system and the Faro–Tavira infralittoral prograding wedge. The results indicate that the internal architecture of recent highstand shelf deposits is complex, as the recognition of three main types of seismic unit motifs suggests the existence of a hierarchical pattern of environmental fluctuations: (a) Major-scale motifs: the most significant feature observed in the seismic records is the pervasive occurrence of two major-scale motifs (lower and upper) within the Holocene highstand systems tract. (b) Intermediate-scale motifs: each major-scale motif is composed of two intermediate-scale motifs, defining four intermediate-scale motifs (H 1 to H 4). (c) Minor-scale motifs: the internal structure of major- and intermediate-scale motifs reveals a high degree of environmental variability during the recent highstand, probably led by climatic cyclicities of submillenial scale. The lower major-scale motif is characterised by alternating progradational–aggradational minor-scale motifs, whereas the upper major-scale motif presents a more uniform internal structure dominated by progradational units. The influence of two distinct trends or asymmetric cycles of sea-level change seems to have been significant in the generation of the major-scale architecture. The transition between both major phases was probably related with a major sea-level rise. Other factors, such as changes in the magnitude and/or direction of storm events and climatically induced changes of sediment supply, are proposed to have controlled the generation of intermediate- and minor-scale architectures.