Aggradational to progradational sequences of the slope and sand delivery to basin center in the Makó trough: Integration of 3D seismic volumes and well-logs

Orsolya Sztanó (TXM @ ELTE), Péter Szafián, Gábor Bada, Anita Horváth (TXM)

Investigating the manner and pace of shelf-margin progradation is a key to understanding the depositional history of basin-fill sequences. The sequences constructed as the slope deposits of the Algyő Formation were studied on an extensive, high resolution 3D volume of 45-50 km long seismic sections acquired in the Makó Trough, parallel to the paleo-transport direction. Corresponding lithology of the slope and basin center deposits were given by seven well-logs.

Sediments from the source area were partly accumulated on the wide, flat-lying shelf of Lake Pannon, while other portions were passing through the slope to the deep basin center. The height of the slope was 250-300 m based on correlated well and seismic data. In plan view, the slope was a 5-7 km wide belt, with a gradient of 1-2°. The transport processes, rate of deposition, and rate of sand delivery to the basin in front of the slope were determined by the slope gradient as well as by grain-size composition and the volume of sediments. The result was the sequential construction of the shelf margin: aggradation with subordinate progradation, followed by high-rate progradation.

In “traditional” models of sequence stratigraphy, the main stage of sand deposition is the shelf during periods of aggradation, and the basin during periods of progradation. In the Makó Trough, however, the opposite trend was observed. During aggradation – indicated by rising shelf-margin trajectory – the shelf builds up from superimposed deltaic lobes comprising coarsening-up units a few dozen meters thick. Meanwhile, the shale-prone slope is constructed mainly from prodelta and subordinately shelf-margin delta sediments, resulting in continuous high amplitude reflections. The majority of the sand, however, is transported further to the basin trough canyons and channels by clayey-sandy effective turbidity currents, and deposited either as extended or slope-detached thick turbidite lobes up to a distance of 20 km. Thus the vertical accretion of the basin floor equals that of the shelf region. During progradation, when the shelf margin trajectory is approximately horizontal, the slope is shown by weak-amplitude, reflection-poor, chaotic units made up of alternating shales and sandy units. Deposition occurs on the lower part of the slope and at the slope-toe region. Reflections downlap within a distance of 10 km. Thus the sediments of the shelf-margin deltas are sand-rich with low clay content, producing non-effective turbidity currents of short-distance transport. Consequently, the thickness of coeval basin-centered sediments remains negligible.

The sequence-stratigraphic subdivision of Lake Pannon sediments has been a matter of debate for more than 20 years. Based on our observations in the Makó Trough, it seems safe to say that neither third nor fourth-order sequences built up of LST, TST and HST as “traditional” models predict can be designated. Instead, the shelf-margin is constructed of alternating aggradational and progradational units. The sequence boundaries are represented by surfaces of maximum regression, these overlain by 1-2 reflections thick transgressive units, thick aggradational early highstand, and progradational late highstand reflections. The relative lake level rose continuously at varying rates. Relative level drops larger in amplitude than the seismic resolution did not occur, thus lowstand fans and wedges did not develop either. Sand delivery to the basin center is characteristic during the early highstand, while the slope-toe becomes the main locus of deposition during the late highstand and maximum regression.