Modeling Falling Stage to Early Transgressive Sedimentation in Linked Incised Valley and Deltaic Complexes: Outcrop and Borehole Data from the Nonmarine Santa Barbara Coalfield (Pliocene, Italy)

Alessandro Ielpi and Massimiliano Ghinassi

During a forced regression, landward exposure takes place, involving erosion and sediment bypass. The basinward areas below the falling base level are instead still capable to host thick sediment wedges. During the following base level rise, lowstand and transgressive deposits accumulate onto the older deposits, both as unconformable (landward areas), and as conformable (basinward areas).

Although this basic model has been widely used in marine realms, it was rarely applied to lacustrine deposits. This research aims at modelling the morphology and sequence architecture of a falling stage to early transgressive fluvial to deltaic complex. It is based on the integration of borehole and outcrop data, collected in a 8 km2 large Pliocene coalfield located in the Upper Valdarno Basin (Italy). Open pit exploitations of two coal seams, carried out during the 1950s-1990s, were assisted by the drilling of some 1600 boreholes. A selected dataset is here investigated, in order to assess the morphology of the incised valley and deltaic complex underlying the upper seam. The borehole mapping allows to trace the incised valley and deltaic boundaries, and to outlines the maximum lakeward shoreline shift. Integrative outcrop data from three key-outcrops represent upstream vallive, downstream vallive and deltaic domains, respectively.

During the falling stage, valley incision occurred, with the basinward accumulation of a deltaic wedge. The following lacustrine rise was initially outpaced by the sediment supply, leading to persisting lakeward shoreline shift, and parallel aggradation of vallive deposits. When the rise rate outpaced the sediment supply, the coastline started shifting landward leading to the upper coal seam deposition. The integration of outcrop and borehole data allowed to extrapolate the architecture of this spatially restricted system, providing significant insights on the spatial distribution and geometries of the Falling Stage and Lowstand Systems Tract deposits.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013