Microbial laminite versus rooted and burrowed caps on peritidal cycles: Salinity control on parasequence development, Early Cretaceous isolated carbonate platform, Croatia

Abstract

Some parasequences in the interior of the Early Cretaceous isolated carbonate platform, Croatia, which formed under a greenhouse climate, lack the typical regressive tidal flat laminites. Instead, these bioturbated emergent parasequences have burrowed and rooted upper parts, capped with thin greenish paleo­ sols, and are reminiscent of Triassic “Lofer cycles.” We propose that the bioturbated cycle tops reflect coastal salinities that are normal marine, brackish, or metahaline, whereas the transgressive (rare) and abundant regressive tidal flat laminites formed under hypersaline coastal waters. Thus, these features may be used to track gross salinity changes of coastal waters on the platform through time. In addi­ tion, we suggest ways to evaluate whether the salinity changes relate to hypersaline embay­ ments on the platform or changes in the cli­ mate at the parasequence scale. We propose that the bioturbated emer­ gent parasequences could have formed under semiarid conditions, seaward of subtle , shallow embayments on the platform. Con­ temporaneously, in the bayheads, micro­ bial laminites developed—these would require time­equivalent updip and downdip laminate­capped and bioturbated emer­ gence–capped parasequences. However, the para sequences could also have been con­ trolled by climate changes. The laminite­ capped parasequences could have resulted from salinity increase with shallowing of the platform interior under semiarid condi­ tions (stable climate). In contrast, the bio­ turbated emergence–capped parasequences could have formed during times of more humid climate punctuating the overall semi­ arid climate. The humid phase would have favored brackish to normal marine salinities in the coastal zone, along with macrophytes, and intense animal and plant bioturbation, which would suppress development of lami­ nite caps. Thus, these parasequences provide a means of tracking gross salinity of coastal waters, and if climate induced, then they have implications for fluctuating climates in greenhouse worlds.