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

Abstract

The Upper Turonian of Brandenberg (Austria) is based by a transgressive fan delta succession rich in water escape structures that, at least in part, may 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 overlies older carbonate rocks. In its subaerial part, the fan succession consists mainly of conglomerates deposited from mass flows, interlayered with red claystones to siltstones. Along the fringe of marine transgression, beachface/channel mouth conglomerates and bioturbated siltstones to fine sandstones accumulated. The marine part of the fan delta succession consists mainly of cross-laminated and hummocky cross-laminated arenites deposited in a wave/storm-domainated shoreface. Excellent preservation of sedimentary lamination throughout and near absence of bioturbation indicate (intermittently) rapid sediment accumulation. Intercalated shoreface conglomerates are present as compound channel-fills, and as thin sheets alongside and off channels. Offshore transport 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 internal breccias, pillow beds up to more than 1 m thick, and hitherto undescribed, cyclindrical structures (“onion structures”) built by concentrically arranged planes interpreted as water escape routes. The tabular fissures, internal breccias and the pillow beds are closely similar to water escape structures documented from historical earthquakes and from inferred paleo-earthquakes. Storm wave loading or wave-induced microseisms are considered less probable triggers of the larger dewatering structures. Water escape structures represent an hitherto unappreciated, although not strictly diagnostic, indicator of syndepositional tectonism in the Upper Cretaceous of the Eastern Alps.