A fan delta succession rich in water escape structures (Upper Turonian, Brandenberg, Austria): Possible record of paleoseismicity

The Upper Turonian of Brandenberg (Austria) is based by a transgressive fan delta succession rich in water escape structures that, at least in part, m ay have formed in association with earthquakes. The investigated fan delta is among the oldest deposits of the Lower Gosau Subgroup (Upper Turonian to Lower Campanian), a terrestrial to neritic succession that unconformably o verlies older carbonate rocks. In its subaerial part, the fan succession co nsists mainly of conglomerates deposited from mass flows, interlayered with red claystones to siltstones. Along the fringe of marine transgression, be achface/channel mouth conglomerates and bioturbated siltstones to fine sand stones accumulated. The marine part of the fan delta succession consists mainly of cross-lamina ted and hummocky cross-laminated arenites deposited in a wave/storm-dominat ed shoreface. Excellent preservation of sedimentary lamination throughout a nd near absence of bioturbation indicate (intermittently) rapid sediment ac cumulation. Intercalated shoreface conglomerates are present as compound ch annel-fills, and as thin sheets alongside and off channels. Offshore transp ort of gravels to cobbles into the shoreface may have been driven by river floods (in the most proximal positions) and by storm rip currents (farther seaward). Towards the top of the succession, conglomerate sheets disappear, and the arenites become bioturbated. In the succession of shore zone arenites, abundant water escape structures include distorted/convoluted lamination, short fluidization planes, tabular fissures (some associated with offset of beds), pods and lenses of interna l breccias, pillow beds up to more than 1 m thick, and hitherto undescribed , cyclindrical structures ("onion structures") built by concentrically arra nged planes interpreted as water escape routes. The tabular fissures, inter nal breccias and the pillow beds are closely similar to water escape struct ures documented from historical earthquakes and from inferred paleoearthqua kes. Stormwave loading or wave-induced microseisms are considered less prob able triggers of the larger dewatering structures. Water escape structures represent an hitherto unappreciated, although not strictly diagnostic, indi cator of syndepositional tectonism in the Upper Cretaceous of the Eastern A lps.