High-frequency sequential analysis and mixed facies in the Upper Cretaceous of the Iberian Ranges (Spain)

The Mesozoic Iberian Basin was a narrow and shallow intracratonic basin located within the Iberian microplate. During the Upper Cretaceous several carbonate platform with mixed siliciclastic-carbonate depositional systems, in the western coastal areas, were developed in this basin.

The recognition of high-frequency, 4th- and 5th-order parasequences in these coastal successions and the reconstruction of the stratigraphic architecture of the late Cenomanian (1), latest Cenomanian-early Turonian (2), early Turonian (3) and late Turonian-early Coniacian (4) 3rd-order sequences allow to explain the location and sedimentary significance of the mixed facies.

Siliciclastic and carbonate facies were deposited during the same 4th-order depositional event, but during different stages (lowstand, transgressive, highstand or regressive), representing distinct system tracts. Siliciclastic sediments were usually supplied into the Basin during lowstands; meanwhile carbonate facies were mainly deposited during platform progradation and extension during highstands. The mixed siliciclastic-carbonate facies were restricted to the transitional stages (transgressive, forced-regressive), between lowstand and highstands. In these stages, and as a consequence of facies belt retrogradation or progradation, a reactivation and remobilization of sediments in the platform occurred and then the sedimentary mixing took place.

Nevertheless, the precise distribution of these facies is different within the studied 3rd-order sequences. Thus, in Sequences 1 and 4, either the siliciclastic facies (representing lowstands) or carbonate facies (highstands) were broadly predominant within the entire sequence, being the mixed facies very scarce or even absent. On the contrary, in Sequences 2 and 3, mixed facies were well-developed and transgressive and forced-regressive system tracts can be easily recognized. These differences in the vertical facies distribution is related to the main transgressive (sequences 1 and 4) or regressive (sequences 2 and 3) depositional trend related to longer, low-frequency, 2nd-order megasequences in which those 3rd-order sequences were deposited.