Print this pagePorosity Associated with Sequences Boundaries in the Lower Cretaceous Carbonate Platforms of Subalpine Chains (Alps, France)
CARRIO-SCHAFFHAUSER, ELISABETH B. and ANNIE M. ARNAUD VANNEAU
Two carbonate platform episodes occurred during the lower Cretaceous in the subalpine chains: the first one during the Berriasian-lower Valanginian and the second during the Barremian-lower Aptian. The sequences boundaries identified in these carbonate platforms are associated with a long period of emersion, and below them, various primary or secondary porosity can be observed. The porosity is related to the duration of exposure, the paleotopography, the paleoclimatology, the vegetal cover and the diagenesis evolution. The following case histories can be recognized:
1. porosity occurring under the sequence boundary of the upper Berriasian (Sb B3) corresponds to the early void preservation of root molds. It is due to early cementation before disappearance of withered roots.
2. paleokarstification occurring under a tectonically enhanced upper Barremian sequence boundary (Sb B4) is developed only on the crest of blocks that tilted once more at that time.
3. moldic porosity of foraminifera tests occurring under an upper Barremian sequence boundary (Sb B5) was formed by selective dissolution by humic acid derived from upper Barremian vegetal cover.
4. lack of a final stage of cementation occurring under a lower Aptian sequence boundary (SB A1) can be assigned to specific aquifer system.
To summarize, the origin of this well preserved porosity developed beneath sequence boundaries depends on a combination of local factors.
It is interesting to note that the porosity is always preserved today on outcrops despite a long period of burial, then aerial erosion.