Abstract
Event beds recording Late Cretaceous synsedimentary seismicity occur in the Cametá Subbasin, northern Brazil. The deposits consist of nearly 40 m of kaolinized, fluvio-estuarine deposits. The lower fluvial (tidal-influenced) complex, which carries a high-quality commercial kaolin, includes genetically related channel fill, floodplain, crevasse splay, and mud plug deposits. The upper estuarine deposits contain trace fossils indicating marine-influenced settings ( Thalassinoides, Ophiomorpha, Teichichnus, Diplocraterion, Skolithos, Cylindrichnus, Rhizocorallium, and Planolites), and include heterolithic, sandy, and bioturbated units, interpreted as tidal-flat, storm-overwash, and low-energy estuarine bay deposits, respectively. In the upper portion of the estuarine lithosomes, four successive events (events 1–4) of synsedimentary brittle and ductile deformation and large-scale sediment failure, anomalous in estuarine settings, were recognized and attributed to earthquakes. Event 1 is recorded by heterolithic and bioturbated deposits with vertical to subvertical fractures that disappear into underlying undeformed deposits. Event 2 is recorded by thin, sharply bounded sections composed of heterolithic mudstone, and very fine-grained sandstone that shows convolute folds and pillow-like structures. Event 3 is documented by intraformational conglomerate and breccia, and minor mudstones and sandstones, with abundant trace fossils ( Skolithos and Thalassinoides). Event 4 is characterized by vertical to subvertical fractures and faults with homogenized sandstone intrusions along their planes, mostly developed in the upper parts of the deposits corresponding to Event 3. A detailed analysis of facies relationships, combined with consistent orientations of brittle deformation structures and the regional tectonics of the Cametá subbasin, suggests earthquakes as the triggering mechanism for sediment deformation and large-scale mass failure.
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