Chronological and depositional model of the Holocene Paraná Delta: analysis of the highstand delta enhancing the discussion on diachroneity in global highstands

JOHN HOLLBROOK; JIMENA ROLDÁN; STIEFFEL RODNEY; JUAN PABLO MILANA; DANIELA KRÖHLING

Paraná

Congreso; XVII Reunión Argentina de Sedimentología y VIII Congreso Latinoamericano de Sedimentología; 2021

Asociación Argentina de Sedimentología y Universidad Autónoma de Entre Ríos

The Paraná Delta is one of the most studied deltas in South America, holding one of the most complete records of the Holocene in southern South America. The delta encompasses an area of ~17,400 km2 enclosed in the Rio de la Plata estuary, growing steadily at a rate of approximately 2 km2 yr-1 for roughly the past 6,000 yrs. It recorded a complex history of delta progradation and has shifted from fluvial, to wave-dominated, and then back to its present day fluvial dominated system (Milana and Kröhling, 2015) creating striking geomorphologic features across the delta (Iriondo, 2004). Aerial and satellite imagery, shallow boreholes, radiometric dating of shells and sand, and Georadar are used to define the distinctive sedimentary features of the delta. Although these studies appear to encompass the entirety of the delta, the uppermost part of the delta remained poorly studied. The purpose of this research was to establish accurate depositional and chronologic models for the upper, middle, and lower parts of the delta through the collection of sedimentary cores for facies analysis and radiometric dating. Dates were determined from OSL and 14C results of samples taken from discrete mappable delta landforms. Also, the analyzed depositional units were interpreted in a sequence stratigraphic context. Sedimentary cores from the upper delta date the early stages of delta growth. The characterization of morphostratigraphic and subsurface units of the upper delta, defined by stratigraphic correlation of lithofacies from boreholes, conducted to better understand the early delta processes and test preliminary estimates of delta initiation. The chenier plain comprises much of the upper delta with the first cheniers (at +6.5 m a.s.l.) developing parallel to a major coastal barrier, subsequently, advancing to the SE during the early stages of the highstand. This chenier plain constitutes the upper part of a large and complex strandplain attached to the coastal barrier. It is characterized by a series of long and arcuate ridges consist of sandy loam and loamy sand with ripple and trough cross lamination, and separated from one another by narrow, low lying muddy swales. The common crest of the cheniers are ca. 6 m a.s.l. in the uppermost reaches of the plain. The upper delta chenier plain is a medium gray (GLEY2 5/5 B6) to dull yellowish brown (10YR 5/3) loamy sand to loam with dark greenish gray (GLEY1 5/10GY) to bright yellowish brown (10YR 6/6) mottles and fine CaCO3 concretions. It extends from the surface down to about 5.5 m depth and overlies conformably the transgressive estuarine deposits that filled the incised valley of the Rio de la Plata. They are represented by dark grey to black, organic-rich, ripple and planar laminated silty clays and clays. AMS 14C dates of ca. 8 ka. cal. BP from the upper member of the transgressive clays beneath the shoreface deposits in the upper delta helps constrain the timing and landward extent of the maximum transgression in the upper reaches of the estuary, after the LGM. Three OSL dates between 8 and 8.1 ka. BP from the earliest generated cheniers come from ca. 2.5 m above the contact with the transgressive clays. These dates evidence initiation and rapid accumulation of highstand deposition by ca. 8 ka. BP. Stability of relative sea level until ca. 6.5 ka. BP. is indicated by another OSL date collected from the chenier plain in the SE part of the upper delta. A downstep of 1.5 m along the chenier plain (NW-SE transect) corresponds to a change in shoreline trajectory of the chenier plain and evidences a drop in relative sea level and the onset of the falling stage. Lowstand delta deposits are characterized by a ca.3 m fall in relative sea level from +5 m to +2 m around 4.5 ka. BP. Recent climatic changes have resulted in a slight rise in relative sea level beginning around 1 ka. BP until present. If the delta proves to in fact be at least 8.1 ka. in age, this would make it one of the oldest if not the oldest modern delta in the world. Considering that highstand deltaic sequences during the Holocene initiated between ca. 6.5 to 5.5 ka. BP globally, the Paraná delta would have been initiated before the global sea level peak and potentially before any comparable highstand shoreline. This open a discussion about the assumptions of synchronicity of highstands with eustatic sea level.