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Abstract
AbstractThePoRiverBasin, where accumulation and preservation of thick sedimentary packages are enhanced by high rates of tectonic subsidence, represents an ideal site to assess the relations between vertical changes in stratigraphic architecture and sediment accumulation rates. Based on a large stratigraphic database, a markedly contrasting stratigraphy ofLatePleistocene andHolocene deposits is reconstructed from the subsurface of the modern alluvial and coastal plains. Laterally extensive fluvial channel bodies and related pedogenically modified muds of latest Pleistocene age are unconformably overlain byHolocene overbank fines, grading seaward into paralic and nearshore facies associations. In the interfluvial areas, a stiff paleosol, dating at about 12.5–10 cal kyBP, marks thePleistocene–Holocene boundary. Across this paleosol, aggradation rates (ARs) from 16 radiocarbon‐dated cores invariably show a sharp increase, from 0.1–0.9 mm year−1to 0.9–2.9 mm year−1. Comparatively lowerPleistocene values are inferred to reflect fluvial activity under a low‐accommodation (lowstand and early transgressive) regime, whereas higherARs during theHolocene are related to increasing accommodation under late transgressive and highstand conditions. Holocene sediment accumulation patterns vary significantly from site to site, and do not exhibit common trends. Very high accumulation rates (20–60 mm year−1) are indicated by fluvial channel or progradational delta facies, suggesting that extremely variable spatial distribution ofHoloceneARs was primarily controlled by autogenic processes, such as fluvial channel avulsion or delta lobe switching. ContrastingARbetween uppermostPleistocene andHolocene deposits also are reported from the interfluves of several coeval, alluvial‐coastal plain systems worldwide, suggesting a key control by allogenic processes. Sediment accumulation curves from adjacent incised valley fills show, instead, variable shapes as a function of the complex mechanisms of valley formation and filling.
Highlights
The contrast between Late Pleistocene and Holocene facies architecture is a basic motif of the late Quaternary stratigraphy of several coastal and alluvial systems world wide (Amos & Knoll, 1987; Blum & T€ornqvist, 2000; Blum & Aslan, 2006)
1994; Blum & Aslan, 2006) and from northern Europe (Busschers et al, 2005; Vis & Kasse, 2009) outline the stratigraphic architecture of Late Pleistocene paleovalleys, developed during Marine Isotope Stages (MISs) 4 to 2, as composed of: (i) basal, amalgamated channel fills, which result from degradation, lateral migration and subordinate aggradation during sea level fall and subsequent low stand; and (ii) upper, nonamalgamated valley fills, reflecting multiple episodes of aggradation during sea level rise and highstand
The general aim of this research is to verify if this abrupt change in stratigraphic architecture at the Pleistocene/Holocene transition was accompanied by a similar sharp change in aggradation rates (ARs)
Summary
The contrast between Late Pleistocene and Holocene facies architecture is a basic motif of the late Quaternary stratigraphy of several coastal and alluvial systems world wide (Amos & Knoll, 1987; Blum & T€ornqvist, 2000; Blum & Aslan, 2006). Late Pleistocene (glacial) deposits differ substantially from their Holocene (interglacial) counterparts in a batch of parameters including facies distribution and mode of sediment accumulation and preservation (Allen & Posamentier, 1993; Hori et al, 2002; Sarkar et al, 2009; Tanabe et al, 2013; Dung et al, 2014; Yoo et al, 2014) These parameters may vary in distinct ways, as a function of tectonic setting (Posamentier et al, 1988) and regional position within the basin (Zaitlin et al, 1994), the Late Pleistocene/ Holocene dichotomy remains a widely recognized trait. Sedimentation started again in these areas during the Holocene, after complete valley filling
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