Deflection of the progradational axis and asymmetry in tidal seaway and strait deltas: insights from two outcrop case studies

Deltas represent the major sediment source for tectonically confined, tide-dominated seaways or straits. Modern examples show how along-shore tidal currents are able to modify the impinging delta shape, generating asymmetrical coastal plains, deflected delta fronts and elongate sandbanks. Seaway or strait deltas can become become tide-influenced or tide-dominated, assuming physical attributes that may depart from classical models. Ancient deltas in seaways and straits can also reveal unexpected facies stacking and stratigraphies, which can be misinterpreted or attributed to different depositional settings. Two ancient analogues of deltas that prograded into elongate basins dominated by amplified tidal currents are presented here. A common element in these deltas is the progressive-upwards change in the dominant process of sediment dispersion recorded in the delta facies. Early stages of progradation are dominated by river- and wave-influenced lithofacies, whereas late deltaic advancements occur with a dominance of tidal current circulation on the delta fronts and the consequent morphologies are deflected/elongated in the direction of tidal flow. This study provides the basis for a preliminary stratigraphic framework for the depositional style of these types of delta. The studied deposits also suggest analogies with the spatial distribution of many hydrocarbon reservoirs investigated along the margins of confined, narrow, linear basins, the interpretation of which is still debated.