Successor Basin Assemblages
Cenozoic molasse in the northern Alpine and Pyrenean Aquitaine and Ebro foredeeps consists of late orogenic clastic wedges initiated by the first major uplift of the mountain belt and associated thrusting. Deposits in these basins, tens of kilometers wide and hundreds long, grade upward from autochthonous flysch, through a lower thin-bedded mudstone without turbidites, into an upper transitional paralic sequence with minor proximal conglomerates. The succeeding nonmarine to paralic molasse dominated the foredeeps and graded laterally into persistent marine deposits. During accumulation of molasse, sedimentation kept pace with subsidence, producing a surface near sea level that was susceptible to exchange of marine and nonmarine conditions. At times of reduced sediment supply, shallow marine transgressions spread across the lowlands.
Molasse is heterogeneous and lenticular. Its distinctive proximal, nonmarine facies, several thousands of meters thick, consists mainly of fanglomerates (nagelfiuhen) and repetitious sequences of fining-upward fluvial cycles. Conglomeratic deposits, commonly exceeding 1000 m in thickness, are characterized by closely packed, rather well rounded clasts. Fluvial cycles comprise immature lithic and feldspathic sandstone and drab to variegated mudstone with thin lignite and coal, caliche, freshwater limestone, and evaporite. In the subordinate marine facies rather persistent, well-sorted, locally conglomeratic sandstone predominates. The composition of conglomerate clasts and sandstone grains has provided basic data about the location and progressive stripping of source areas, the emplacement of younger nappes, and the dispersal of sediments within the foredeep.
Although uplift of extensive source areas initiated and dominated the relatively undeformed molasse, older proximal molasse was overridden by nappes and folded and faulted in a zone a few tens of kilometers wide. Moreover, uplift of the mountains and emplacement of nappes were episodic and shifted along the range, as both nappes and foredeep encroached farther onto the craton. In the end the molasse was reduced to nearly half its original width.
Molasse accumulation persisted for a few tens of million years, with a maximum average preservation rate of about 400 m/my and as low as 150 to 200 m/my overall. There were two or three times of pronounced uplift and accentuated filling at intervals of about 10 to 15 my. Regional elevation and erosion of both mountain belts and foredeeps ended the molasse phase.