Publication Date:
2017-08-04
Description:
Continental shelves represent areas of highest economical and ecological importance. Nevertheless, these
sedimentary systems remainpoorly understood due to a complex interplay of various factors and processeswhich
results in highly individual construction schemes. Previous studies of sedimentary shelf systems have mainly
focused on a limited number of cores, retrieved from Holocene fine-grained depocentres. As such, the relation
between shelf architecture and sedimentary history remains largely obscure. Here,we present newdata fromthe
NW Iberian shelf comprising shallow-seismic profiles, a large number of sediment cores, and an extended set of
radiocarbon dates to reveal the Late Quaternary evolution of a low-accumulation shelf system in detail.
OntheNWIberian shelf, threemain seismic units are identified. These overly a prominent erosional unconformity
on top of the basement. The lowermost Unit 1 is composed of maximal 75-m thick, Late Tertiary to Pleistocene
deposits. The youngest sediments of this unit are related to the last glacial sea-level fall. Unit 2 was controlled by
the deglacial sea-level rise and shows a maximumthickness of 15 m. Finally, Unit 3 comprises deposits related to
the late stage of sea-level rise and the modern sea-level highstand with a thickness of 4 m in mid-shelf position.
Two pronounced seismic reflectors separate these main units from each other. Their origin is related to (1)
exposure and ravinement processes during lower sea level, and (2) to reworking and re-deposition of coarse
sediments during subsequent sea-level rise.
According to the sediment core ground-truthing, sediments of the Late Tertiary to Pleistocene unit predominantly
display homogenous fine sands with exceptional occurrences of palaeosols that indicate an ancient exposure
surface. Fine sands which were deposited in the run of the last sea-level rise show a time-transgressive
retrogradational development. The seismic reflectors, bounding the individual units, appear in the cores as 0.1 to
1-m thick deposits consisting either of shell gravels or siliceous coarse sands with gravels. The modern sea-level
highstand stage is characterised by zonal deposition of mud forming a mud belt in mid-shelf position, and
sediment starvation on outer shelf zones. Radiocarbon ages indicate that this mud beltwas the main depocentre
for river-supplied fine material on the NWIberian shelf at least over the past 5.32 ka BP. The initial onset of this
depocentre is proposed to be related to a shift in the balance between rate of sea-level rise and amount of
terrigenous sediment supply.
Various other stratigraphical shelf reconstructions reveal analogies in architecturewhich indicate that timing and
shaping of the individual units on low-accumulation shelves is fundamentally controlled by eustatic sea-level
changes. Other factors of local importance such as differential elevation of the basement and the presence of
morphological barriers formed by rocky outcrops on the seafloor have additionally modifying influence on the
sedimentary processes.
Type:
Article
,
PeerReviewed
Format:
text
DOI:
10.1016/j.margeo.2008.12.008
Permalink
