Genetically Related Incised Valleys and Deltas: Sequence and Modern Analogue Modeling From the Nonmarine Santa Barbara Coalfield, Late Pliocene, Italy

Abstract

Abstract The morphodynamics of coeval incised valleys and deltas are relatively well understood in marine-paralic environments, but not in the nonmarine, lacustrine counterparts. Here, we analyze the morphology and depositional and sequence architecture of a small-scale valley to deltaic complex preserved in a late Pliocene intermontane coalfield in the Northern Apennines, Italy. Outcrop exposures are integrated with 160+ well logs in an area of less than 1.5 km2. The main goal of this study is to demonstrate that spatially restricted yet fully preserved nonmarine systems are well suited for depositional, sequence, and modern analogue modelling, and deserve more attention in the literature. In this study, outcrop and borehole data are merged into isopach maps, highlighting the sedimentary response to a discrete cycle of lake-level retreat and re-expansion. Falling-stage and lowstand phases were characterized by incision of a meandering valley onto a previous highstand shelf, and by the avulsive progradation of an attached, shelf-edge delta. The supply-dominated character of the depositional system along with the limited wave fetch favored in this phase the preservation of the maximum regressive surface across the entire incised-valley to deltaic transect. The following lake-level re-expansion involved estuarine valley back-filling, and the transition from river-dominated to wave-dominated deltaic deposition. Enhanced wave ravinement and associated shoreline-independent prodelta forestepping promoted the development of a transgressive shelf-break bypass. These results are compared with reference sequence and accommodation models, as well as with two modern Ethiopian lakes undergoing acute forced regression. The similar scale and morphology of modern stranded highstand shelves and shelf-edge deltas provide a parallel for assessing the original topographic gradients and depositional dynamics of the late Pliocene case study. Modern and ancient analogues also shed light on the amplification of climate records in endorheic lakes, on the potentially strong avulsive character of falling-stage deltas, and on the sedimentary response to short-lived lacustrine fluctuations in rapidly subsiding basins.