Halite pseudomorphs in the Crato Formation (Early Cretaceous, Late Aptian–Early Albian), Araripe Basin, northeast Brazil: further evidence for hypersalinity

Abstract

Pseudomorphs after hopper-faced halite occur at several horizons and in five distinctive morphologies in carbonate dominated members of the Late Aptian–Early Albian Crato Formation of the Araripe Basin in northeast Brazil. Some forms exhibit typical hopper-faced cubes with an outer veneer of marcasite with calcite and quartz in the centre. A second morph comprises compacted marcasite veneers exhibiting a collapsed cube morphology showing that compaction predated calcite and quartz precipitation. A third morph comprises four pointed ‘stars’ in a series of concentric circles, representing halite crystals that grew within the sediment and possibly tore apart a thin bacterial mat that bound the surface of the sediment. Pseudomorph size varies from 5–10 mm diameter for those replaced by several mineral phases to aggregates of up to 20 mm diameter for those replaced by silica. The presence of abundant halite pseudomorphs suggests that hypersaline waters prevailed during deposition of the Nova Olinda Member fossil Konservat Lagerstätte and were a contributing factor to the excellent preservation of its fossil assemblage.