VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
Pleistocene and Holocene sedimentary facies in the Prestige
wreck area (SW Galicia Bank, NE Atlantic Spanish margin)
B. ALONSO1*, G. ERCILLA1*, D. CASAS1*, F. ESTRADA1*, M. FARRAN1*, M.GARCIA1*,
D. REY2* and B. RUBIO2*
1
CSIC. Instituto de Ciencias del Mar. Paseo Marítimo de la Barceloneta 37-49. 08003 Barcelona. Spain
e-mail: belen@icm.csic.es
2 Universidad de Vigo. Facultad de Ciencias del Mar. 36310 Vigo. Spain
* UA, Unidad Asociada de I+D, UV-CSIC
ABSTRACT
The detailed sedimentological study carried out on the Prestige wreck area (SW Galicia Bank)
allows us to define the main lithofacies and their vertical distribution on three sedimentary environments (fault scarp,
sedimentary lobes and inter-lobe channels) during the Holoceneand Late Pleistocene. The dominant lithofacies are the
turbidites and at least four turbidites events have been correlated between 8,9 and 19,7 ka. The highest turbidite event
frequency occurs on the inter-lobe channel environment (about 1 turbidite/2,000 yrs) and the lowest is on the crest of
sedimentary lobes. The prevailing sedimentary processes are gravity flows and less common are the vertical settling
and slow lateral advection.
1. INTRODUCTION
The Galicia Bank has been the subject of different geophysical and geological studies from
the 70’s to the 90’s (Montadert et al., 1974; Groupe Galice (1979). These works have mainly
focused in the sedimentary evolution during the Upper Cretaceous and Cenozoic, while the
recent most sedimentary history has not attracted the scientific interest until now. The present
paper intends to fill this gap, presenting a detailed study of the Late Pleistocene to Holocene
sedimentary history in different sedimentary environments on the Prestige wreck area (SW flank
of the Galicia Bank). This contribution provides a new understanding about the type of
processes and factors (local/global) governing the recent most sedimentation in a deepwater
structural bank, isolated from hinterland sediment supply sources. This study is included in the
framework of the researches associated to the Prestige wreck (Project VEM2003-20093-CO3).
2. RESULTS AND DISCUSSION
Nine gravity cores up to 3 m long were collected on different sedimentary environments
surrounding Prestige wreck (fault scarp, sedimentary lobes and inter-lobe channels) of the SW
flank of the Galicia Bank. Physical properties, sedimentological logs, textural analysis, and sand
fraction composition analysis were made of all cores. The cores stratigraphy was established
based on age control by 14C dating. Four main lithofacies are identified: Turbidites, which
include carbonate-rich biogenous muds (TB), biogenous muds (TB+Q), terrigenous muds (TT)
and biogenous sands (TAB); Hemipelagites, including normal hemipelagites (H) and
hemiturbidites (H/T); Pelagites (P), and Debrites (D). Figure 1 shows the predominance of the
turbidite lithofacies and the differences in the vertical successions of these four lithofacies in the
sedimentary environments. Thus the prevailing sedimentary processes are sediment gravity
flows that represent the dominance of downslope processes at the SW flank of the Galicia
Bank. Less common are the vertical settling and slow lateral advection.
As the turbidites lithofacies are dominant in almost all the cores, we focus the discussion on
two topics: (a) sediment source of turbidites, and (b) frequency and distribution of the turbidite
events. Nature of turbidites can be inferred from changes in components of their sand fraction.
In this sense, we interpret that the biogeneous turbidites (TB and TB+Q) located in the
sedimentary lobes and inter-lobe channels may be attributed to low terrigenous input and result
from re-deposited sediments coming from hemipelagite lithofacies of the near-surface fault
scarp sediments. Less abundant turbidites are terrigenous in nature and they may be deposited
when an important erosion of outcropping of the fault scarp occurs. In other hand, at least four
individual turbidite events have been correlated (4 to 1 from older to younger) between 8,9 ka
and 19,7 ka (Fig. 1). The turbidite event 4 (oldest) consists of TB lithofacies trough the deepest
sector of inter-lobe channel and adjacent sedimentary lobe. The turbidite event 3 is defined by
VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
TT lithofacies and affects the inter-lobe channels environment. The turbidite event 2 consists of
TB+Q lithofacies and affects to inter-lobe channel and sedimentary lobe environments. The
turbidite event 1 is characterised by TB lithofacies and occurs along the inter-lobe channel
environment.
In addition, we can attempt the frequency of emplacement of turbidites assuming that each
unit corresponds to a single event. In general, the frequency ranges from about 1 turbidite event
each 2,000 yrs to 4,000 yrs. The distribution of this frequency is different on each sedimentary
environment being the highest frequency on the inter-lobe channel environment and the lowest
frequency on the crest of sedimentary lobe environment. This variability in the frequency of
turbidite events could be the result of the interplay between the mesotopography (evacuation)
and lateral supply of sediment from erosion of the depositional lobe wall environment. In
addition, from the correlation of the four turbidite events, we have observed lateral/vertical
constant values of mean grain size. This aspect suggests a homogeneous hydrodynamic
regime of turbidity flow. It could be possibly explained in part because of the short distance
(study area of 5 km x 11 km) to the source area (fault scarp) and the proximity of between core
sites (60-100 metres). Finally, the turbidite deposition interrupted by hemipelagites lithofacies
could be explained by different pulses of sedimentary instability. Theses pulses should be
related to a combination of accumulation rate and oversteepening that induce instability on most
of the fault scarp and/or the occurrence of microearthquakes.
Figure 1: Lithofacies types, core correlation panel and 3-D bathymetry of the area where the stern (S)
and bow (B) are located. Letter/Numbers refer to the four correlable turbidite (T1 to T4) events. The age
(ka) of sediments are indicated. Legend: Turbidites: TB, carbonate-rich biogenous muds; TB+Q,
biogenous muds; TT, terrigenous muds; P, Pelagites; H, hemipelagites; H/T, Hemiturbidites; and D,
Debrites.
REFERENCES
Groupe Galice (1979). The continental margin off Galicia and Portugal: acoustical stratigraphy
and structural evolution. In: J.C. Sibuet, W.B.F. Ryan, et al., Init. Repts. D.S.D.P., 47 (2),
Washington, U.S. Govt, Printing Office, pp. 622-633.
Montadert, L., Winnock, E., Delteil, J.R., y Grau, G. (1974). Portugal and Bay of Biscay. In:
Burk, C.A. and Drake, C.L. (Eds.), The Geology of Continental Margins, Berlin, Heidelberg,
New York, pp. 323-342.