Facies analysis of a Pliocene river‐dominated deltaic succession (Siena Basin, Italy): Implications for the formation and infilling of terminal distributary channels

Abstract

The recent recognition and description of terminal distributary channels in ancient delta-front deposits have led to important progress when it comes to understanding: delta construction processes; the plan-view shapes of both modern and ancient river-dominated deltas; and sediment dispersal within deltas. Deltaic deposits fed by terminal distributary channels can be observed in the extensive outcrops (approximately up to 300 m wide and 50 to 70 m thick) in the central sector of the Pliocene Siena Basin in Tuscany, Italy. Stratigraphic constraints indicate that the deposition occurred in relatively shallow marine settings, was not affected by tidal effects and was influenced only locally by wave action. As a consequence, these rock exposures provide an opportunity to investigate the facies assemblage of an ancient delta complex, which was built up almost exclusively as the result of river-related processes. The studied deltaic succession is composed of vertically stacked parasequence-like units, which are invariably characterized by the lack of coarse-grained distributary channel deposits at the top and an extensive network of terminal distributary channels that controlled the deltaic deposition. These deposits were interpreted previously as deep-water turbidites. This study provides new elements for the correct distinction between hyperpycnal-dominated delta fronts and basinal turbidite deposits. The terminal distributary channel facies documented in this study are more complex than those investigated in previous research. The channels are often passively filled during abandonment phases, and not just due to the aggradation and lateral/upstream migration of mouth bars, as classically proposed. Moreover, the frequent occurrence of mudstone beds just above the basal channel scours may be indicative of an abandonment phase after the channel incision, and before its infilling, with subsequent implications for the formational and infilling mechanisms of terminal distributary channels. Furthermore, the channel fills are often the result of alternating depositional and erosional phases, leading to a complex stratigraphic arrangement

Deltaic coastline of the Siwalik (Neogene) foreland basin: evidences from the Gish River section, Darjeeling Himalaya

“…Decimetre‐thick marl beds are inferred to have been formed under the phases of low clastic sediment supply, and carbonate precipitation might have been aided by trapped vegetal matters (Berner, ). Sharp‐based, millimetre‐scale, rippled/wavy laminated siltstones are probably distal part of episodic hyperpycnal flows generated by major river floods or distal storm‐surge deposits (Pullham, ; Mulder et al ., ; Bhattacharya and MacEachern, ; Martini and Sandrelli, ). This facies association is inferred as prodelta sediments.…”

“…Interlayering of unidirectional (F3) and oscillatory flow generated structures (F4 and F5) demonstrate a delta mouth setting. Low‐relief subtle basal erosional surfaces, lack of prominent basal lag and dominant southward mode of palaeocurrent are inferred to indicate deposition from subaqueous, distalmost part of the distributary channel system termed as terminal distributary channels (TDC, Table ) (Olariu and Bhattacharya, ; Martini and Sandrelli, ). Lack of pronounced erosional relief and absence of prominent coarse basal lag indicate reduced turbulence associated with the subaqueous channel flow (McKee, ).…”

Driving processes and recognition criteria for complex subaerial unconformities in a non‐marine succession: Implications for basin margin shaping during forced regressions

“…Particularly, distal and proximal mouth‐bar deposits exhibit the vertical and lateral coexistence of river and wave influenced processes. Meanwhile, FA4 and FA5 are assigned to terminal distributary channels (Olariu & Bhattacharya, ; Li & Bhattacharya, ; Martini & Sandrelli, ) and subtidal to intertidal plain deposits (Fielding, ), and both FAs are interpreted as having accumulated in lower delta‐plain settings. In FA4, mudstone and siltstone laminae, and the heterolithic intervals, are rhythmically interbedded; this may suggest alternating high‐frequency tractive and suspensive processes in response to tidal current influence (Goodbred & Saito, ).…”

Syn‐rift sequence development in a fault‐controlled embayment (Early Permian Irwin River Coal Measures, Northern Perth Basin, Western Australia)

“…Facies St is interpreted as dunes filling shore normal rip channels (e.g. Yagishita, ; Martini & Sandrelli, ). Abundant soft sediment deformation structures and the absence of bioturbation support rapid sediment emplacement.…”

“…Hardened scours lined by pebble conglomerate point to higher energy storm surges where erosion and bypass prevailed and concentration of large clasts was possible (e.g. Martini & Sandrelli, ). Storm‐generated ripple and dune crests tend to be oriented parallel to the local shoreline and slope (Plint et al ., ); therefore a north‐west‐trending palaeoshoreline parallel to the basin margin is inferred from palaeoflow measurements (Figs and E).…”