The 'aggradational highstand systems tract' (AHST): A peculiar feature of highly-supplied growth fault-bounded basin fills

Normal fault-bounded basins commonly show a sequence-stratigraphic architecture that is deviated from classic models derived from passive margins. In particular, depositional sequences containing well developed shallow-marine, mostly aggradational intervals resting on back-barrier to shallow marine transgressive intervals are common in highly-supplied growth fault-bounded basins. These aggradational intervals have been called ‘aggradational highstand systems tract’ (AHST), and result from the balance between the rate of sediment supply and the rate of creation of accommodation following a transgression. A distinct maximum flooding surface is not recognizable at the base of the AHST, which can be considered as an extremely expanded maximum flooding zone. The development of this type of sequence is favored in contexts characterized by a relatively rapid and constant fault-controlled subsidence and a high rate of sediment supply that just compensates the rate of creation of accommodation. A rapid change in the tectonic regime may lead to the abrupt interruption of sedimentation and to the formation of main unconformities. For the observed linkage between the depositional sequence architecture and changes in the tectonic regime, AHST recognition may be useful to discriminate between a dominant tectonic or eustatic control in sequence development.