Quantification des flux sédimentaires et de la subsidence du bassin ...

tel-00790852, version 1 - 21 Feb 2013
Earlier studies also demonstrated that the Last Glacial Maximum (LGM) paleo-shoreline,
20 ka ago, is recorded on the outer shelf, at the top of 30 m-thick sand bodies and at a
present day bathymetry of 115 m (Aloïsi, 1986, Berné et al. :1998, Rabineau et al. 1998,
2005, 2006, Jouet et al. 2006, Bassetti et al., 2006, 2008). During the LGM, sea-level is at
least 100 m below present day sea-level, so the shelf-offlap break was lying at around 50
m bathymetry.
We therefore think that paleo-shelf-offlap break during Pliocene-Quaternary do not
represent the shoreline but the shelf break and that the height of clinoforms represents the
slope. Paleobathymetry of this offlap-break can be estimated to be between 50 and 150
m. We assumed that this paleobathymetry is the same for all identified shelf-offlap-
breaks. Topsets of clinoforms therefore represent several environments from subaerial,
fluvial to shoreline and continental shelf all eroded during lowstand. We consider that all
Pliocene-Quaternary Topsets are related to the same process and therefore represent the
same paleoenvironments and the same paleobathymetry at time of deposition.
Therefore offlap-breaks in sedimentary record a paleobathymetry of either 0 (shoreline
offlap-break) or 150m (shelf-offlap break). We also identified a toeset-offlap break at the
base of foresets that we will use to calculate accommodation (see below).
• Calculation of Pliocene-Quaternary Subsidence rates
We used two distinct methods to calculate total subsidence.
a) Accommodation calculation at two different points (A and B) on profile LRM18
between Zanclean (around 5,3 Ma) and Present Day (Figure 7).
Zanclean accommodation can be estimated using the height of Pliocene prisms
between offlap break and toeset break. This height is around 650 m at point A The
bottomsets of clinoforms are therefore deposited under a waterdepth of at least 650 m
for the most landward clinoforms. To this value we must add the paleowater depth of
the offlap break, which can be estimated by analogy to present day situation at 150 m
at the most, so that Zanclean accommodation at point A is 800 m. At point B it is
more difficult to estimate accommodation with this method because we are lacking
appropriate marker to estimate the paleowater depth of initial deposits (just above the
messinian). However, we can postulate that it is at least of the same order of
magnitude (800m, probably more). Note that this estimate is a regional estimate as
locally the messinian erosional surface is very irregular and can induce locally large
thickness variations.