Discrimination of multiple episodes of meteoric diagenesis in a Kimmeridgian reefal complex, North Iberian Range, Spain

Stratigraphic relations, detailed petrography, and fine-scale geochemical a nd isotopic analysis of diagenetic phases formed within Kimmeridgian-age re efal carbonates of the Torrecilla en Cameros Formation of Northern Iberian Ranges in Spain indicate a complex history of alteration in marine! meteori c, burial, and uplift-related settings. The reefal succession is separated from the overlying Tithonian-Berriasian fluvial and lacustrine continental deposits by a single unconformity, which is marked by brecciation and karst ification. Nevertheless, a record of three distinct stages of alteration as sociated with this unconformity is preserved in the succession of calcite c ements present within the reefal carbonates, Recognition of temporally dist inct episodes of meteoric diagenesis associated with a single unconformity is atypical in carbonate rocks, but this study illustrates how such complex systems can be deconvoluted, The first episode of subaerial exposure and meteoric alteration resulted in neomorphism of marine allochems and precipitation of a first generation of nonferroan clear calcite (NFC) cement. Following local faulting and brecci ation, a second NFC precipitated throughout the reefal unit. Although simil ar in petrographic character, this phase of cementation is distinct in its isotopic composition reflecting changes in the regional climate of the Iber ian Peninsula during the early Tithonian, Another phase of meteoric alterat ion of the reefal unit is recorded by renewed corrosion, including dissolut ion of preexisting calcite cements and precipitation of prismatic calcite, prior to the deposition of Tithonian-Berriasian lacustrine and continental sediments. These continental units, in turn, record yet another episode of alteration by meteoric waters as NFC cements formed within intragranular an d dissolution porosity in the lacustrine limestones. Notably. these cements are also distinct on the basis of their petrographic and geochemical chara cter. Following subsidence and burial during Aptian to late Cretaceous times, mig ration of regionally derived fluids (perhaps in response to the onset of te ctonic deformation to the north) led to cementation of saddle ankerite and ferroan calcite throughout the sedimentary sequence, Alteration of these bu rial-related diagenetic phases has subsequently taken place in response to the regional uplift during the Tertiary. This final Episode of meteoric alt eration is indicated by replacement of the ferroan calcite and ankerite by a cloudy nonferroan calcite.