Multiple-scale incision-infill cycles in deep-water channels from the lacustrine Transylvanian Basin, Romania: Auto- or allogenic controls?

Abstract

Many hierarchical schemes have been developed to describe multiple-scale cut-and-fill architecture of deep-water channel systems and analyse their evolution. A widely applicable genetical-based scheme is not on the horizon yet. Near seismic-scale exposures of two deep-water channel systems from the late Miocene of the Transylvanian Basin, Romania offer a fresh insight into the dynamics of erosional and depositional processes. Both channel systems are underlain by mudstone-prone successions; the channel-complex scale incisional surface can only be observed at Tău. Channel-form surfaces interpreted as the base of channel elements and storeys are present in both outcrops. At Tău, the shallow, base-of-slope channel complex consists of amalgamated sandstones separated by thin mudstones. The axis of the deep, slope channel at Daia is characterized by pebble lags, thick amalgamated sandstones that thin towards the margin, and thin mudstones. Backsets, possible cyclic steps, and erosional features, such as rip-up mud-boulders, detachment edges, and ghost bedding also occur. Cycles of incision, bypass with coarse lag accumulation, deposition from gravelly then sandy turbidity currents were repeated numerous times with differing amplitudes, which were followed by the abandonment of the channel complex. Novel incision-infill graphs were created to analyse and compare the cyclicity pattern of deep-water channels. Successive incision depth of channel-form surfaces, and infill height of channel-form bodies aids in distinguishing hierarchical levels, vertical and lateral offsets. Daia and Tău channel complexes show a remarkably similar cyclicity pattern, which might indicate an inherent character of this turbidite system. To investigate whether this pattern is universal, incision-infill graphs of two Laingsburg Karoo channel systems were constructed, based on published data. In all the four examples, a pattern of lateral-then-vertical offset of channel elements was present, and a similarity between channel element-complex and channel storey-element aggradation was found, suggesting a fractal behaviour. Two models are applicable to interpret the revealed pattern of incision-infill graphs, for the Tău and Daia channels: 1) three scales of waxing-waning flow magnitude cyclicity could create the architecture of channel complexes, or 2) the presence of autocyclic channel element incision-infill could eliminate one scale of flow magnitude changes, but two still need to be governed by allocyclic factors. The late Miocene turbidite system in the Transylvanian Basin developed in Lake Pannon, in close relationship with the uplifting Carpathians fringing the basin. As the duration of cycles is shorter than the time range of possible changes in uplift rate or fault activity, tectonically induced cyclicity is discarded. Climatically induced cycles are the most probable controls on varying the flow magnitude that created the complex architecture of channel systems in this lacustrine setting.