One way or another there has to be a punctuation problem. My first instinct is to read it as :

molasse sous couverture quaternaire peu épaisse - molasse beneath thin Quaternary overburden.

To read it as "molasse sublayer" etc. would normally require it to read "sous-couverture molasse", non?

Though "molasse, sous-couverture quaternaire peu épaisse" might amount to the same thing.

I think - think - Quaternary is subsequent to the period of deposition of molasse, so barring massive geological upheaval, it would be logical for the Q. deposits to overlie the molasse.

The Penguin Dict of Geology states that molasse relates to a period of orogeny, and the table on orogeny goes from Precambrian to Tertiary ....