CHRONOLOGICAL AND DEPOSITIONAL MODEL OF THE HOLOCENE PARANA DELTA: ANALYSIS OF THE HIGHSTAND DELTA ENHANCING THE DISCUSSION ON DIACHRONEITY IN GLOBAL HIGHSTANDS

STIEFFEL RODNEY ; JOHN HOLBROOK; DANIELA KRÖHLING; JUAN PABLO MILANA

Parana

Congreso; XVII Reunion Argentina de Sedimentologia y VIII Congreso Latinoamericano de Sedimentologia - Parana; 2021

Asociacion Argentina de Sedimentología

The Paraná Delta is one of the most studied deltas in South America, holding one of the most complete records ofthe Holocene in southern South America. The delta encompasses an area of ~17,400 km2 enclosed in the Rio de laPlata estuary, growing steadily at a rate of approximately 2 km2yr-1 for roughly the past 6,000 yrs. It recorded acomplex history of delta progradation and has shifted from fluvial, to wave-dominated, and then back to itspresent day fluvial dominated system (Milana and Kröhling, 2015) creating striking geomorphologic featuresacross the delta (Iriondo, 2004). Aerial and satellite imagery, shallow boreholes, radiometric dating of shells andsand, and Georadar are used to define the distinctive sedimentary features of the delta. Although these studiesappear to encompass the entirety of the delta, the uppermost part of the delta remained poorly studied. Thepurpose of this research was to establish accurate depositional and chronologic models for the upper, middle, andlower 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 theupper delta date the early stages of delta growth. The characterization of morphostratigraphic and subsurface unitsof the upper delta, defined by stratigraphic correlation of lithofacies from boreholes, conducted to betterunderstand the early delta processes and test preliminary estimates of delta initiation. The chenier plain comprisesmuch 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 theupper part of a large and complex strandplain attached to the coastal barrier. It is characterized by a series of longand arcuate ridges consist of sandy loam and loamy sand with ripple and trough cross lamination, and separatedfrom one another by narrow, low lying muddy swales. The common crest of the cheniers are ca. 6 m a.s.l. in theuppermost reaches of the plain. The upper delta chenier plain is a medium gray (GLEY2 5/5 B6) to dull yellowishbrown (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 andoverlies conformably the transgressive estuarine deposits that filled the incised valley of the Rio de la Plata. Theyare represented by dark grey to black, organic-rich, ripple and planar laminated silty clays and clays. AMS 14Cdates of ca. 8 ka. cal. BP from the upper member of the transgressive clays beneath the shoreface deposits in theupper delta helps constrain the timing and landward extent of the maximum transgression in the upper reaches ofthe estuary, after the LGM. Three OSL dates between 8 and 8.1 ka. BP from the earliest generated cheniers comefrom ca. 2.5 m above the contact with the transgressive clays. These dates evidence initiation and rapidaccumulation of highstand deposition by ca. 8 ka. BP. Stability of relative sea level until ca. 6.5 ka. BP. isindicated by another OSL date collected from the chenier plain in the SE part of the upper delta. A downstep of1.5 m along the chenier plain (NW-SE transect) corresponds to a change in shoreline trajectory of the chenierplain and evidences a drop in relative sea level and the onset of the falling stage. Lowstand delta deposits arecharacterized by a ca.3 m fall in relative sea level from +5 m to +2 m around 4.5 ka. BP. Recent climatic changeshave resulted in a slight rise in relative sea level beginning around 1 ka. BP until present. If the delta proves to infact 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 comparablehighstand shoreline. This open a discussion about the assumptions of synchronicity of highstands with eustatic sealevel.