ABSTRACT

Stratigraphic relations, detailed petrography, and fine-scale geochemical and isotopic analysis of diagenetic phases formed within Kimmeridgian-age reefal carbonates of the Torrecilla en Cameros Formation of Northern Iberian Ranges in Spain indicate a complex history of alteration in marine, meteoric, 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 karstification. Nevertheless, a record of three distinct stages of alteration associated with this unconformity is preserved in the succession of calcite cements present within the reefal carbonates. Recognition of temporally distinct 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 brecciation, a second NFC precipitated throughout the reefal unit. Although similar in petrographic character, this phase of cementation is distinct in its isotopic composition, reflecting changes in the regional climate of the Iberian Peninsula during the early Tithonian. Another phase of meteoric alteration of the reefal unit is recorded by renewed corrosion, including dissolution 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 and dissolution porosity in the lacustrine limestones. Notably, these cements are also distinct on the basis of their petrographic and geochemical character.

Following subsidence and burial during Aptian to late Cretaceous times, migration of regionally derived fluids (perhaps in response to the onset of tectonic deformation to the north) led to cementation of saddle ankerite and ferroan calcite throughout the sedimentary sequence. Alteration of these burial-related diagenetic phases has subsequently taken place in response to the regional uplift during the Tertiary. This final episode of meteoric alteration is indicated by replacement of the ferroan calcite and ankerite by a cloudy nonferroan calcite.