Dynamic stratigraphy of composite peripheral unconformity in a foredeep basin

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This study employs facies analysis and basic principles of sequence stratigraphy to correlate isolated outcrop sections and reveal the depositional history of the Chmielnik Formation – a prominent mid-Serravalian clastic wedge formed on the basinward forebulge flank of the Polish Carpathian Foredeep. The coarse-grained clastic wedge, up to 30 m thick and spanning ca 1·1 Ma within biozone NN6, consists of fluvio-deltaic, foreshore and shoreface deposits with a range of large littoral sand bars, all enveloped in muddy offshore-transition deposits. Its dynamic stratigraphy indicates rapid shoreline shifts and environmental changes due to the interplay of forebulge tectonism, sediment supply and third-order eustatic cycles. A similar interplay of tectonism and eustasy is recognizable in the whole middle Miocene sedimentary succession deposited on the forebulge flank, demonstrating an extreme case of an accommodation-controlled shelf and indicating tectonic cycles of the forebulge uplift and subsidence spanning ca 800 to 900 ka. The episodes of forebulge uplift correlate with the main pulses of orogen thrusting. The resulting composite peripheral unconformity differs markedly from the idealized model of a ‘steady-state’ stepwise onlap driven by forebulge continuous retreat. It is concluded that the foredeep peripheral unconformities, instead of being simplified in accordance with this idealized model, should rather be studied in detail because they bear a valuable high-resolution record of regional events and give unique insights into the local role of tectonics, eustasy and sediment supply.

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