Stratigraphic architecture of a santonian mixed siliciclastic-carbonate succession (Catalonian Pyrenees, Spain)

At Collades de Bastus, Catalonian Pyrenees, a Santonian mixed siliciclastic -carbonate succession indicates two proximal-distal gradients, and records two styles of stratigraphical development upon relative sea-level change. T he succession consists of four small-scale sequences (5.1 to 5.4) within th e highstand systems tract of the "Vallcarca-5" depositional. sequence of Si mo (1993), and is topped by a drowning sequence (small-scale sequence 5.5). The investigated succession (Collades Member) accumulated near the margin o f the south-Pyrenean shelf, shortly before development of the south-vergent Boixols thrust system. Deposition of the Collades Member commenced with mo derate sea-level rise accompanied by increased siliciclastic input. In the larger, eastern outcrop sector the Collades Member consists of a succession of neritic marls with four intercalated intervals each deposited from a ca rbonate shelf. Each carbonate interval consists of stacked upward-shoaling cycles interpreted as parasequences. From bottom to top, most parasequences consist of a coral-sponge-rudist bioconstruction, a rudist biostrome, and bioclastic limestones. Depositional sequences 5.1 to 5.4 developed by overs tep of shelf carbonates with neritic marls, corresponding to the transgress ive systems tract (TST) and to part of the highstand systems tract (HST). T he carbonate facies tract of the HST consists of stacked parasequences that become thinner up-section and record a westward component of progradation. Each highstand carbonate interval is overlain by a stack of carbonate para sequences that become thicker up-section and, down depositional dip, by ner itic marls. Together, the upward-thickening parasequence stack and the late rally adjacent/overlying succession of neritic marls comprise the TST and p art of the HST of the successive sequence. The sequence boundary is the lev el of maximum shoaling within each carbonate shelf interval. The uppermost sequence 5.5 is a drowning sequence (cf. Simo 1993). In the western outcrop sector, the Collades Member consists of hummocky cro ss-laminated to bioturbated sandy calcarenites, of neritic marls and of rel atively thin intervals of coral-sponge-rudist limestones. Sequence developm ent may have started with deposition of sharp-based bedsets of sandy calcar enites that both eastward and up-section become thinner and grade into neri tic marls. Together, the succession of sandy calcarenites and neritic marls may comprise the TST and, possibly, part of the HST. In the HST neritic ma rls and, locally, coral-sponge-rudist bioconstructions accumulated. Deposit ion of some calcarenite bedsets seems to have started near or closely after maximum progradation of each carbonate shelf in the eastern part of outcro p. The stratigraphic architecture of the Collades Member indicates, for the eastern outcrop sector, an east-west proximal-distal gradient, whereas the western sector records a west-east gradient. The opposite gradients result from outcrop intersection subparallel to oblique to general northward depo sitional dip, across two distinct shelf depositional systems.