Abstract

Basinwide hiatal discontinuity has been generally accepted as firm evidence of a distinctive allogenic event such as tectonic movement or a complete eustatic cycle. Here we show through theoretical and experimental modeling that a largescale, hiatal discontinuous surface can be produced autogenically in fluviodeltaic successions under no changes in external dynamic forcing, i.e., constant relative sea-level rise (rate r_slr) and constant sediment supply (rate q_S per unit width). This “autogenic hiatus” occurs where (1) the hinterland slope is steeper than the subaqueous slope of the delta, and (2) the initial downstream length of the feeding alluvial river exceeds a critical magnitude L_crt that is specified primarily by q_Slr_slr. In this topographic setting, the existing depositional system becomes transgressive and nondeltaic as soon as sea level starts to rise. The existing subaqueous surface is starved of sediment and progressively extends landward until alluvial length is reduced to L_crt. After the retreating river has attained this critical length, the shoreline still retreats but the depositional system is restored to deltaic sedimentation. Because of this, the subaqueous surface, which was previously starved of sediment, becomes overlain by delta foreset deposits after a significant time gap. Thus a steady sea-level rise can produce all of the following: a) the strata underlying the hiatus, b) the hiatus itself, and c) the strata overlying the hiatus. Changes in r_slr or q_S are not required to account for the presence of a hiatus of this particular type. Numerical simulations suggest that autogenic hiatuses are likely to be produced in most natural river deltas if a sea-level rise such as occurred during latest Pleistocene to Holocene (i.e., 0.01 km/kyr for 10 kyr) is available. This further implies that autogenic hiatuses may well exist in stratigraphic records of river deltas, especially of Quaternary age. An understanding of autogenic hiatuses, when combined with the theory of shoreline autoretreat, provides an alternative view of the origin of some stratigraphic breaks.