The role of sediment supply in large-scale stratigraphic architecture ofancient Gilbert-type deltas (Pliocene Siena-Radicofani Basin, Italy)
Ivan Martini ⁎, Elisa Ambrosetti, Fabio Sandrelli
Dipartimento di Scienze Fisiche, della Terra e dell'Ambiente, Università di Siena, Via Laterina 8, 53100 Siena, Italy
Aggradation, progradation and retrogradation are the main patterns that define the large-scale architecture ofGilbert-type deltas. These patterns are governed by the ratio between the variation in accommodation spaceand sediment supply experienced during delta growth. Sediment supply variations are difficult to estimate in ancientsettings; hence, it is rarely possible to assess its significance in the large-scale stratigraphic architecture ofGilbert-type deltas. This paper presents a stratigraphic analysis of a Pliocene deltaic complex composed of twocoeval and narrowly spaced deltaic branches. The two branches recorded the same tectonic- and climateinducedaccommodation space variations. As a result, this deltaic complex represents a natural laboratory fortesting the effects of sediment supply variations on the stratigraphic architecture of Gilbert-type deltas. Thefield data suggest that a sediment supply which is able to counteract the accommodation generated over timepromotes the aggradational/progradational attitude of Gilbert-type deltas, as well as the development of thickforeset deposits. By contrast, if the sediment supply is not sufficient for counterbalancing the generated accommodation,an aggradational/retrogradational stratigraphic architecture is promoted. In this case, the deltaic systemis forced to withdraw during the different phases of generation of accommodation, with the subsequentflooding of previously deposited sub-horizontal topset deposits (i.e., the delta plain). The subsequent deltaicprogradation occurs above these deposits and, consequently, the available space for foresets growth is limitedto the water depth between the base-level and the older delta plain. This leads to the vertical stacking of relativelythin deltaic deposits with an overall aggradatational/retrogradational attitude.
