| Abstract |
The evolution of the sedimentary regime that prevailed on the Armorican margin from Late Cretaceous to late Tertiary is herein discussed and interpreted. The sedimentation budget of four components of the sediment : calcium carbonate, quartz, "less than 10 m non carbonate material,"and biogenic silica are quantified as is the composition of the coarse fraction and the textural properties of the sediment. Variation of the coarse fraction components is directly related to change in the oceanic environment (terrigenous supplies, productivity, calcium carbonate dissolution). Sediment texture is a function of many factors, comprising lithology, dissolution action, and hydrodynamic control. The medians of the silt fractions from bulk and decalcified sediments are used as indicators of variations of the bottom current intensity averaged over long periods (n x 10² to n X 10 yr) of time. From the study the following aspects of the evolution of the sedimentary regime are characterized : 1) establishment of an active bottom current circulation during the early Eocene, concomitant with a clear thermal stratification ot the water masses ; 2) development of a highly fluctuating bottom current regime during the Oligocene, spicule-rich "contourites" being implaced during periods of active bottom currents ; 3) changes in the sedimentary regime at the early-middle Miocene boundary characterized by decreasing bottom current and incrcusing sedimentation rates ; 4) active bottom current, decreasing dissolution and marked increase of the sedimentation rates in the early Pliocene. that contributed to active building of a sedimentary ridge at the foot of the margin ; 5) it appears that the lowermost Upper Cretaceous hiatus and the upper Eocene hiatus are related to changes in the physiographic (and hydrologic ?) environment at an oceanic rather than regional scale. [NOT CONTROLLED OCR] |
