Revista de la Asociación Geológica 64 (1): 109 - 123 (2009) 109

FROM TO SANTA FE: DARWIN'S OBSERVATIONS AND MODERN KNOWLEDGE

Martin IRIONDO and Daniela KRÖHLING

CONICET - Universidad Nacional del Litoral, CC 217 (3000) Santa Fe. E-mails [email protected], [email protected]

ABSTRACT During his historical voyage around the world, Darwin raided deeply in the South American interior, travelling over 600 kilo- meters from Buenos Aires to the north along the Río Paraná. During that journey, he crossed a vast plain characterized by aeolian sediments, something unfamiliar to a European naturalist. However, Darwin's acute observation powers and precise descriptions are noteworthy. After more than 170 years since his visit, modern geological knowledge identifies several sectors in the Buenos Aires-Santa Fe region. One of them (the Tertiary at La Bajada) he described admirably and others such as the Paraná flood plain were brilliantly abstracted in only two sentences. In short, Darwin traversed a first sector (Buenos Aires- Rosario) characterized by aeolian and paludal Early Pleistocene sediments. From Rosario to Santa Fe the plain is formed by Late Pleistocene aeolian and fluvial units. At La Bajada (presently Paraná city) lies exposed the marine Miocene and in SW Entre Ríos is a reconstructed loess-paleosol sequence generated at the Early/Middle Pleistocene transition. The Paraná flood plain and the littoral complex at the mouth (practically not observed by Darwin) underwent rather complex Holocene episo- des. Keywords: Ch. Darwin, Pampa, South America, Quaternary, Tertiary.

RESUMEN: De Buenos Aires a Santa Fe: observaciones de Darwin y conocimiento actual. Durante su histórico viaje alrededor del mundo, Charles Darwin incursionó profundamente en el interior de Sudamérica recorriendo más de 600 kilómetros hacia el norte de Buenos Aires a lo largo del río Paraná. Durante ese viaje, él recorrió una gran planicie caracterizada por sedimentos principal- mente eólicos, un caso poco familiar para un naturalista europeo. Sin embargo, sobresale su gran capacidad de observación y precisión en sus descripciones. Después de más de 170 años de la visita de Darwin, el conocimiento geológico moderno ha identificado varios sectores en la región Buenos Aires-Santa Fe, uno de los cuales (el Terciario de La Bajada) él describió admi- rablemente y otros, como la llanura aluvial del Paraná, reseñó magistralmente en solo dos frases breves. En resumen, Darwin recorrió un primer tramo (Buenos Aires-Rosario) caracterizado por sedimentos eólicos y palustres del Pleistoceno Inferior. Desde Rosario hasta Santa Fe la llanura está formada por unidades eólicas y fluviales de edad pleistocena superior. En La Bajada (hoy ciudad de Paraná) aflora el Mioceno marino y en el sudoeste de Entre Ríos se reconstruyó un sistema loess-pale- osuelos generado a lo largo de la transición Pleistoceno Inferior-Pleistoceno Medio. La llanura aluvial del Paraná y el comple- jo litoral de su desembocadura (prácticamente no observados por Darwin) sufrieron cambios dinámicos bastante complejos durante el Holoceno.

Palabras clave: Ch. Darwin, Pampa, Sudamérica, Cuaternario, Terciario.

INTRODUCTION kilometers westwards. It is formed by a ments and Early Quaternary aeolian and slightly elevated block characterized by swampy formations. The last environ- Charles Darwin made the journey from Early Pleistocene formations and margi- ment Darwin recorded during his jour- Buenos Aires to Santa Fe by cart and nal Holocene littoral deposits. The ney is the Paraná river system, composed horse. After several days of work at La second sector covers the Rosario-Santa of a wide flood plain and a littoral com- Bajada (presently Paraná city) he retur- Fe road and reaches 300 km west to the plex at the mouth ("Paraná delta"), both ned to Buenos Airs by boat along the Río hills in Córdoba. This system is a relati- of them Holocene in age. Paraná. Such a trip allowed him (surely by vely sunken region dominated by Late In order to visualize Darwin's experien- chance) to cross four major geographic Quaternary loess bounded at the north ces, this article is organized in the same and Quaternary geological environments by the South American region of Chaco. sequence of geological terrains that he of southeastern South America (Fig.1). The third area he visited forms the found during his journey (Darwin 1846, The first one stretches from Buenos southwestern sector of Entre Ríos pro- 1945). Aires city (in fact, it begins further south) vince; it is represented by a hilly landsca- When reading his journal or his geologi- to Rosario and reaches several tens of pe carved into Miocene marine sedi- cal observations of the region, Darwin's 110 M. IRIONDO AND D. KRÖHLING

layers of bones¨ covering the bottom of some tributaries. Very probably such great drought was a short late occurrence of the Little Ice Age (LIA) climate, which was characteri- zed by a marked dryness in the (Parra 1939, Iriondo and Kröhling 1995) and ended around 1800-1810. The Spa- nish naturalist Azara also described simi- lar events in the late 18th Century. The Little Ice Age provoked advances of gla- ciers in the Cordillera and related proces- ses in southern South America, but did not produce visible geological effects in the Pampas, probably owing to its relati- vely short duration and not so severe dryness. On the other hand, a longer dry period occurred between 3.5 and 1.4 ka BP and formed a thin aeolian mantle which co- vers the Pampas and surrounding re- gions, i.e., the San Guillermo Formation (Iriondo 1990, Fig. 2). It is an aeolian gray silt that generally tops the sedimen- tary sequence in the large interfluves. Figure 1: Map of the region showing the localities visited Most of the sediment is originated lo- by Darwin. cally, by deflation of the A-horizon of soils and the subsequent deposition of great observation power and exceptional THE GREAT DROUGHT dust. In Santa Fe this mantle is ability for synthesis appear immediately formed by coarse silt with scarce propor- evident, notwithstanding the limitations An interesting environmental issue was tions of very fine sand and clay, brow- of geological theory at that time. For ins- described by Darwin in relation to the nish gray in color (10YR 5/1). It is fria- tance, the nature and origin of loess (the trip to Santa Fe. It was the "Gran Seca" ble, porous, permeable, and moderately dominant sediment in the Pampas) was (the great drought) that particularly af- structured. Typical thickness varies from discovered several decades later. It is evi- fected the north of Buenos Aires and 25 to 55 cm; ceramic shards and Indian dent that Darwin did not feel comforta- south of Santa Fe from 1827 to 1832. boleadora balls are included in the upper ble with such "muds", "clays" and similar. During those five years rain was excep- part of this deposit at some localities. In order to describe and interpret those tionally scarce and the plain underwent extensive deposits of the plain, he prefe- major changes: vegetation disappeared, BUENOS AIRES/ROSARIO rred to hold on to strata and concretions minor rivers dried up and the entire ELEVATED BLOCK of tosca (caliche), intercalated in the sec- country took the aspect of a dusty road, tions. His skill as a paleontologist in com- dust clouds continuously dominating the Darwin rode a 250 km long first journey parison with his abilities as a sedimento- air. The effect on animals was catastro- from Buenos Aires to Rosario in a logist is also clear. phic; cattle migrated massively to the straight northwestern direction. All that Besides, it is interesting to discover in south and assembled there in a giant time, the road borders the right banks of Darwin's writings the existence of quali- flock; droves with thousands of wild the Río de la Plata and Río Paraná and fied observers - i.e., people with some horses hurried furiously to the Paraná to runs atop a 10 to 20 m high cliff. This scientific knowledge - among the local drink and later were unable to climb back cliff is almost continuous, interrupted in population in those early years of Argen- up the slippery cliff and drowned. a few places by small Holocene estuaries tina. Some of them were British traders, Hundreds of thousands of animals floa- of minor tributaries (Luján, Areco, Arre- but others were local inhabitants that ted downstream to the Plata estuary. A cifes and others; Iriondo 2004). The cliff were great observers of Nature. few years later Darwin observed ¨true is tectonic in origin and exposes two From Buenos Aires to Santa Fe: Darwin's observations ... 111

Darwin this observation contradicts the insistent assertion of the French natura- list d'Orbigny on the general absence of stratification in the Pampean sediments. The academic point was then whether the Pampean System was diluvial or not: if stratified, not diluvial.

Baradero Section (NE )

The section on the Río Baradero north of the city of Baradero (100 km from Buenos Aires), is formed by the Punta Gorda Formation (5 m thick), confor- mably overlain by the Hernandarias For- mation (Iriondo 1980, 7 m thick). From Figure 2: Map of the San bottom to top: Guillermo Formation, a late Holocene environmen- 0 - 1 m Punta Gorda Formation. Bed of tal equivalent of Darwin´s clayish silt, dull brown in colour, modera- big drought (after Iriondo tely consolidated. It contains numerous 1990). Fe-oxide segregations, CaCO3 concre- Quaternary formations described by Both minerals were transported by tions of different sizes and large rhizo- Darwin as "pale and red Pampean mud". southwesterly winds from the Andes concretions. It is interpreted as a paludal Both units were identified and studied Cordillera. The Punta Gorda Formation deposit. later in the Twentieth Century. was deposited around 1 Ma BP during 1 - 5 m Punta Gorda Formation. Bed of González Bonorino (1965) refered the the most important Quaternary glacia- a silty grain-size composition, pale brown sections outcropping in the Buenos Aires tion occurring in South America (Mercer in colour, massive in general and in some area as the Pampeano Formation. Ac- 1976). places with a weak medium to coarse cording to its mineralogy, the author dif- The Red Pampean Mud/Hernandarias horizontal stratification. Medium to high ferentiated two zones separated by a clear Formation is a large playa deposit depo- consolidation (highly calcareous); carbo- boundary: the upper zone characterized sited by the Río Uruguay at the Early- nate (tosca) concretions. Fine root by plagioclase and illite and the lower one Middle Pleistocene transition. It forms moulds covered by black films. The de- dominated by quartz and montmorilloni- the surface of most of Entre Ríos pro- posit has a general aeolian origin, with te. Fidalgo et al. (1973) later described the vince and below the surface reaches into local rill reworking and krotovinas in Pampiano Formation and Riggi et al. neighboring regions in Santa Fé and some places. Remains of Glyptodon were (1986) discriminated the Ensenada For- Buenos Aires to the southwest (Iriondo found in the measured section. At the mation and the Buenos Aires Formation 1980). The typical thickness of this for- top, a marked horizontal paleosurface is in the same sections. mation varies from 20 to 40 m; it is com- recognized, locally joined to a residual Both units appear along all the distance posed of loam and silty loam with mont- material of 0.60 - 0.80 m thick, with few to Rosario, but have better outcrops on morillonite as main clay mineral, and local pedogenic features. the left bank of the Paraná, in Entre Ríos quartz and very fine sand containing gyp- 5 - 12 m Hernandarias Formation. It is province. Here they were defined as Pun- sum at the base. More than 90 % of the composed of clayish silt to a silty clay, ta Gorda Formation (Iriondo and Kröh- clay minerals, i.e., montmorillonite, bei- olive gray in general colour and with va- ling 2008) and Hernandarias Formation dellite and nontronite, show expan- riations to brown and olive; with abun- (Iriondo 1980). Pale Pampean Mud/Pun- sion/contraction properties. dant large and irregular carbonate con- ta Gorda Formation is an Early and Mid- The contact between both formations is cretions. Frequent Fe-Mn sesquioxide dle Pleistocene loess-paleosol sequence. concordant - a feature correctly observed segregations appear. It is less consolida- It is composed of the Andean plagiocla- by Darwin, who stated: "I could clearly dis- ted than the underlying unit and is orga- se-illite association and has an origin si- tinguish in this fine line of cliffs horizontal lines nized in poorly defined coarse strata, and milar to the classical Last Glacial Max- of variation both in tint and compactness" interpreted as a paludal deposit. Nume- imum Pampean loess (Kröhling 2001). (Darwin 1846, p. 87). As remarked by rous dessication cracks appear along all 112 M. IRIONDO AND D. KRÖHLING

the section. The deposit includes me- tans and ferriargillans. This paleosol co- concretions, 2.50 to 3 m thick. The unit dium to coarse lenticular to tabular strata rresponds to the unit G attributed by is covered by the paludal facies of the of pale brown silt, with internal lamina- Imbellone and Cumba (2003) -Geosol El Hernandarias Formation, with frequent

tion and segregated CaCO3. Such lenses Tala-Hisisa-. The Hernandarias Forma- CaCO3 concretions. are interpreted as aeolian, locally re- tion forms the rest of the exposed sec- transported material. In some sectors ap- tion, with general paludal characteristics Arroyo Pavón and Arroyo Seco Sec- pear lenses of retransported botrioidal and interbedded discontinuous soil hori- tions (Southern Santa Fe province)

CaCO3 concretions. zons in the middle sector of the unit. The forms of erosion of both forma- Numerous large krotovinas appear in this Entering the territory of Santa Fe pro- tions are very different and distinctive formation. vince, Darwin described a cascade about along the river cliffs. They were also twenty feet high in the Pavón creek, a affected by several gullies. Nabel et al. Vuelta de Obligado and Ramallo Sec- small tributary of the Paraná. He recor- (1993) studied the magnetostratigraphy tions (NE Buenos Aires province) ded there two varieties of tosca-rock (li- of two sections at Baradero, finding re- mestone). verse polarity (older than 0.78 Ma) in the The section of the cliff at Vuelta de Today the Pavón Cascade is located a few lower section of the cliff. Those authors Obligado is represented by the loessic hundred meters from Provincial High- identified two of the lenticular paleosols facies of the Punta Gorda Formation way 11. The cliff downstream the casca- and named them El Tala (Brunhes paleo- (6.50 - 8 m thick), overlain by a calcrete de begins at the base with a brown sandy magnetic chron) and Hisisa Geosols (2.50 m thick). The unit includes a 1.30 m aeolian deposit (1.5 m thick) covered by

(Matuyama chron > 780 ka). Nabel et al. thick paleosol at the base, composed a loessic unit with segregated CaCO3 (1993) discriminated eight litoestratigra- from top to bottom as follows: (1.5-2 m thick). A paleosol (1 m thick) is phic units at Baradero section: units I to 0 - 0.35 m B2t horizon. Brown clayish silt recognized on top of it (it very probably V interpreted as loess and paleosols de- (7.5 YR 5/4), moderately to well structu- correlates with the soil exposed at the posits and units VI to VIII associated to red in very firm, fine to medium prisms, bottom of the Vuelta de Obligado sec- limnic environments. The age of the de- bounded by fine fissures. Common clay tion). The section ends with the loessic posit in the region was attributed to the cutans and fine root moulds. Very fine to facies of the Punta Gorda Formation

Ensenadense Mammal Age (Tonni et al. fine Fe-mottles and Fe-Mn oxides segre- with abundant CaCO3 concretions (2 m 1999). Later, Kemp et al. (2006) recons- gations frequent. thick). Some kilometers to the north, the tructed the sequence of pedosedimen- 0.35 - 0.57 m B3 horizon. Brown silt Punta Gorda Formation forms the banks tary processes in the same locality based moderately organized in very firm, fine of the Arroyo Seco (9 m high at the tow- on soil science methods and identified to medium prisms. Scarce argillocutans n's camping grounds); locally differentia- paleosols embedded in swamp and aeo- and fine root moulds. ted is a 3 m thick brown silty strata, palu- lian sediments. Their ages as recorded by 0.57 - 0.87 m BC horizon. Brown silt, dal in origin. luminescence methods (OSL dates range weakly structured in firm and fine blocky from ca. 114 to 14 ka BP) differ conside- peds. Abundant very fine Fe-Mn mottles ROSARIO/SANTA FE BLOCK rably from previous paleomagnetic and fine root moulds. results. 0.87 - 1.29 m C horizon. Light brown cla- From Rosario to Santa Fe and beyond to yish silt with paludal characteristics (very the north and west, stretches another San Pedro Section (NE Buenos Aires fine resistant blocky peds, frequent Fe- sector of the Pampas, dominated by Late

province) mottles, common CaCO3 irregular and Pleistocene geological formations. The platy concretions). landscape is there shaped by two wides- In a quarry excavated into the Río Paraná At that locality, a well developed 1.5 m pread aeolian units: Tezanos Pinto For- cliff at San Pedro the following section high notch appears at the base of the mation and San Guillermo Formation. appears from bottom to top: a paleosol cliff; that feature indicates erosional ac- However other units originated in fluvial developed on top of the Punta Gorda tion by waves during the Middle Holo- environments have been described. The Formation and truncated by erosion cene marine ingression. Such a feature stratigraphic sequence is the following: (1.20 m thick) and constituted by brown also appears in the same position at Fray silt, weakly structured in peds. It contains Bentos, in the Banda Oriental (Uruguay). Section of the Paraná cliff between numerous large root moulds generated The Río Paraná cliff at the locality of Puerto Gaboto and Rosario (Santa Fe

by the lixiviation of CaCO3 rhizoconcre- Ramallo is formed by the Punta Gorda province) tions. Pedological features are: fine root Formation, represented by the loessic

moulds covered by black films, argillocu- strata with frequent CaCO3 vertical platy According to Kröhling (1999) the expo- From Buenos Aires to Santa Fe: Darwin's observations ... 113

sed stratigraphic column on the cliffs zaingó Formation. Darwin continues: cies is common too), outcrops at the base along the right bank of the Paraná and “The rest of the cliff at Gorondona, is formed of the cliff of the Río Paraná from its Coronda rivers, between the cities of of red Pampean mud, with, in the lower part, junction with the Río Coronda up to the Puerto Gaboto and Rosario, begins at the many concretions of tosca, some stalactiformed, city of Rosario, with variable thickness base with the Plio-Quaternary Ituzaingó and with only a few in the upper part… contai- between 0.5 m and 4.5 m. Locally the Formation, deposited by the Río Paraná. ning mammiferous remains close to its base” formation is covered by the Timbúes The sedimentary record of the unit is (Darwin 1846, p. 87). It corresponds to Formation separated by an erosional represented by mature and well selected the general features of the Puerto San unconformity, but in general the overl- sands, typical of channel facies of a flu- Martín Formation. ying unit is the Puerto San Martín vial system of high discharge. The condi- The main characteristics of the mentio- Formation. tions that favored the accumulation of ned formations exposed in the area visi- Main characteristics of the Ituzaingó the unit changed toward the top of the ted by Darwin were taken from Kröhling Formation in its type area (Corrientes formation to a fluvial regime of lower (1998) and are presented below: province) are presented in Georgieff et energy. Ituzaingó Formation (De Alba 1953, al. (2005). The Ituzaingó Formation is separated by Herbst 1971) Timbúes Formation (Kröhling 1998) an erosional unconformity from the It is composed of very fine quartz sand, It is composed of silty very fine to fine overlying Late Pleistocene Puerto San sandy silt to silty clay, grayish yellow in sand, opaque orange in colour (7.5 YR Martín Formation, formed by the dis- colour (2.5Y 6/2) with variations to olive 7/4), with visible micaceous minerals continuous accumulation of fine aeolian yellow (5Y 6/3). The sediment has abun- (frequency of 5-20%). It is organized in sediments. The accumulation environ- dant ferric segregations; also the postde- coarse to very coarse, internally lamina- ment was of semipermanent swamps, in- positional rubefication is in general dis- ted strata, lenticular to tabular in form, tercalated with periods of subaerial con- cordant to the stratification. The upper with irregular concordant to erosional ditions (loess). The existence of Ck-hori- part of the unit has abundant brown Fe- contacts among them. Deformational zons of truncated soils near the top of sesquioxides mottles (7.5 YR 5/8), loca- structures are common. There are Mn- the unit suggests more benign condi- lly concentrated in centimetric levels. The segregations, root moulds filled by sand tions. In some sites, between the two consistence of the sedimentary mass ran- and leaf moulds covered by Mn-films. mentioned formations there are interca- ges from friable to poorly consolidated Platty CaCO3 concretions appear at the lated several paleochannels infilled with non-calcareous. Segregations of CaCO3 contact between some strata, locally for- alluvial and paludal deposits. These were form irregular to botrioidal concretions. ming a calcareous net of phreatic origin. formed by successive avulsions of the Locally, in the lower part there are platty Large elliptic krotovinas are frequent. last segment of an important tributary in CaCO3 concretions forming an orthogo- At the Paraná cliff near the locality of the region - the Río Carcarañá (Timbúes nal net or ferruginous concretions of 30 Puerto San Martín, a B horizon of a pale- Formation). to 50 mm diameter. The unit is arranged osol appears at the erosional contact bet- The landscape developed over the Puer- into medium to coarse massive lenticular ween the Timbúes Formation and the to San Martín Formation was later cove- strata with 7-10 m of lateral extension, overlying Tezanos Pinto Formation. It is red and smoothed during the Last with variable concentrations of very fine 0.30 to 0.60 m thick, reddish brown in Glacial Maximum by the loess of the to medium Fe-mottles (up to 40%) and color and weakly to moderately structu- Tezanos Pinto Formation. The stratigra- Mn-segregations and mottles. Abundant red in medium angular prisms. phic sequence ends with the Late Ho- fine root moulds with black segregations. The Timbúes Formation (Late Pleisto- locene San Guillermo Formation, which In some sectors, the top of the unit is cene in age) outcrops at the cliff of the lies over the buried Holocene Optimum marked by a moderately developed B ho- Paraná and Coronda rivers, exhibiting a Climaticum soil developed on top of the rizon and by calcareous concretions thickness of 5 to 6.5 m and lateral exten- loess. bellow it. Locally, the truncated paleosol sions of tens of meters. The sedimentary Darwin described the Paraná cliff in the is replaced by medium cuneiform strata characteristics indicate a fluvial origin for area of Estancia Grondona, north of composed of fine sand, ochre-yellow in this unit. It is interpreted according to Rosario and near the last and more re- colour, that form a set with diagonal stra- mineralogical data as a deposit generated cent segment of the Río Carcarañá. The tification. Upwards they are replaced by by the Río Carcarañá and indicating the exposed section was characterized by medium horizontal strata composed of position of its previous mouths in the Darwin as follows, from the base to top: intraclasts in a sandy matrix or by inter- Paraná. “it consists of a pale yellowish clay, abounding nally laminated strata. Puerto San Martín Formation (Iriondo with concretionary cylinders of a ferruginous This unit, represented by the fluvial 1987) sandstone”. It is interpreted as the Itu- channel sedimentary facies (paludal fa- The unit is practically continuous along 114 M. IRIONDO AND D. KRÖHLING

the Río Paraná cliff from the mouth of bed formed by CaCO3 pebbles. A mode- fine silty sand, and silt with very fine the Río Coronda up to the city of San rately structured B horizon of a trunca- sand, dull orange in colour (7.5 YR 6/4). Lorenzo. Its thickness varies from 4 up ted soil tops this unit. It has very thick horizontal strata, not to 10 m. The unit is overlain by the Te- The sedimentary unit is interpreted as a well defined and generally with concor- zanos Pinto Formation, separated by an continuous sequence of strata represen- dant contacts. The sediment varies in unconformity that indicates an irregular ting a temporary swampy environment consistency between friable and consoli- paleosurface. alternated with typical aeolian facies that dated and it has a fine to medium blocky It is a yellowish brown silty deposit, orga- suggest an important accumulation of structure. In general, it is non-calcareous. nized in coarse to very coarse horizontal dust in the region. Taking into account It is affected locally by bioturbations, strata with poorly defined contacts. At the reference of Darwin about the pre- such as krotovinas (up to 0.75 m diame- Puerto San Martín - the type locality - the ponderant number of fresh-water spe- ter) and root casts (up to 0.60 m long and section is as follows from bottom to top: cies (Polygastrica and Phytolitharia) scraped 0.10 m diameter). In some places sedi- 0-0.60 m: bed composed of brownish from a tooth of one of the mastodons mentary structures are visible. These are olive silt, structured in peds, paludal in found in the red Pampean mud, we infer characterized by an irregular wavy pat- origin. 0.60-1.60 m: bed of yellowish that this corresponds to one of the cha- tern marked by differences in concentra- brown silt, massive, aeolian in origin. racteristics beds of paludal origin of the tions of colloidal materials, namely dissi- 1.60-2.10 m: bed formed by brownish Puerto San Martín Formation. pation structures. This unit forms the olive silt, organized in peds and accumu- Tezanos Pinto Formation (Iriondo 1987) lower sections of cliffs along the Río lated in a swampy environment. 2.10- This formation is the typical LGM loess Carcarañá, forming gentle to subvertical 3.30 m: yellowish brown loess. 3.30-4.80 of the northern Pampas that overlies the slopes, which reflect its resistance in m: bed composed by yellowish brown Puerto San Martín Formation, with comparison with the upper units. The silt, moderately structured, paludal in ori- thickness ranging between 1 and 4 m. maximum outcrop thickness is approxi- gin. 4.80-5.10 m: brownish olive silt, San Guillermo Formation (Iriondo 1987) mately 5.50 m. It is unconformably over- weakly structured, paludal in origin. 5.10- The brownish gray silty deposit (Late lain by the Tezanos Pinto Formation in 6.40 m: stratum of yellowish brown silt, Holocene in age) ends the sedimentary the interfluves and by the Lucio Lopez weakly structured in angular peds. sequence on a truncated soil developed Formation in the main fluvial valleys of

Locally a level with CaCO3 rhizoconcre- on top of the LGM loess. the area. tions appears immediately below the A representative section of the unit is ex- erosional unconformity at the top of the Sections of the lower Río Carcarañá posed along the cliffs of the Río Cañada unit indicating the existence of a Ck ho- (Santa Fe province) de Gómez, near the Río Carcarañá. From rizon of truncated soils. bottom to top: At the Río Paraná cliff in front of the Darwin was right in making his observa- 0.00-2.00 m: Bed formed by very fine, city of San Lorenzo, two sectors can be tions along the banks of the lower Río slightly silty sand, bright reddish brown differentiated: the lower one is 4 m thick Carcarañá because its basin constitutes a in colour when moist, with dissipation and formed by olive sandy silt with com- favorable area for research on geomor- structures. The sedimentary mass is non- mon ferruginous mottles. The upper part phology and Quaternary stratigraphy of calcareous and contains medium to coar- carries calcareous precipitates of phreatic the northern Pampas plain (Kröhling se and hard Fe-Mn sesquioxide nodules.

origin. The upper sector is 7 m thick and 1999).The exposed sedimentary column There are rhizoconcretions of CaCO3 composed of massive, yellowish brown of the lower Carcarañá basin reveals and abundant very fine macropores. silt, including a pedogenic horizon mar- fluctuating environmental conditions, Concordant upper contact.

ked by CaCO3 concretions. characterized by dry intervals associated 2.00-4.50 m: Bed sedimentologically The Puerto San Martín Formation out- with episodes of accumulation and re- similar to the underlying one, without crops at sectors of the Río Carcarañá mobilization of aeolian sand or dust ac- sedimentary structures, and with less

cliff near the town of Oliveros. On the cumulation during the stages of Glacial resistance to erosion. Numerous CaCO3 left bank the unit is also differentiated Maximum. These alternate with humid rhizoconcretions appear at the contact into two sectors separated by a concor- intervals characterized by pedogenesis between the units. It includes krotovinas dant horizontal contact. The lower silty and the development of fluvial belts. The up to 0.50 m in diameter. Near the top sector, olive in colour, comprises the Ck sedimentary record of the lower basin there is a level formed by a succession of horizon of a truncated paleosol. The up- was reported by Kröhling (1999) and is erosional geoforms, 0.50-0.80 m wide per sector is represented by a brownish explained below (Fig. 3). and 0.40-0.50 m high. to reddish brown silt with diffuse crossed Carcarañá Formation (Kröhling 1999) Relicts of a palaeosol appear disconti- stratification. It includes a lenticular fine This unit is composed of very fine to nuously at the bottoms of gullies within From Buenos Aires to Santa Fe: Darwin's observations ... 115

The Carcarañá Formation is Late Pleisto- cene in age (OIS 3; a TL dating in the middle section indicates an age of 52.31 ± 1.2 ka BP). The mainly aeolian forma- tion is the result of the reworking by ero- sion of a dune field generated during the OIS 4. Primary structures of the dunes appear in sections located immediately to the south of the area. Locally different paludal and alluvial facies of the Carca- rañá Formation are present. According to Darwin, “on the banks of the Carcarañá, a few miles distant (of the Paraná cliff at Grondona; next to the Villa La Ribera -Rosario-Santa Fe road-), the lowest bed visible was pale Pampean mud, with masses of tosca-rock, in one of which I found a much decayed tooth of the Mastodon: above this bed, there was a thin layer almost composed of small concretions of white tosca, out of which I extrac- ted a well preserved, but slightly broken tooth of Toxodon Platensis: above this there was an unu- sual bed of very soft impure sandstone” (Dar- win 1846, p. 88). The section of the Río Carcarañá described by Darwin is inter- preted as the Carcarañá Formation. Tezanos Pinto Formation (Iriondo 1980) It is the typical Late Quaternary unit of the Pampas plain, composed of aeolian silts (silt: 71-81%; clay: 13-29%; fine sand: 1-6%). At the base and the top, ero- sional unconformities connect these deposits with the Carcarañá Formation and with the San Guillermo Formation respectively (Kröhling 1999). A primary loess facies or aeolian facies of the Tezanos Pinto Formation has the greatest areal representation, with a typi- cal outcropping thickness of 2-4 m, and more developed on the interfluves (6-8 Figure 3: General stratigraphic section of the Carcarañá river basin, typical of the Rosario-Santa Fe area (after Kröhling 1999). m thick). It is a loose deposit, coarse silt with subordinate clay and fine sand, light the fluvial palaeovalleys of the region. concretions of phreatic origin occupy brown in colour (7.5 YR 6/4). It is a ho- Lateral variations are common in that coarse fissures between peds. Locally, mogeneous, porous and permeable de- soil: the structure changes from coarse to this level is represented by palaeogullies posit with a coarse granular to medium- very coarse blocky (strong) up to angular filled with retransported soil fragments. coarse blocky fabric. The sedimentary very coarse prismatic (strong, defined by At the top of the Carcarañá Formation mass is calcareous; it contains powdery coarse fissures, partly filled by CaCO3 another soil was developed. It is repre- concentrations and hard concretions of precipitates). The sediment is weakly cal- sented by a poorly developed B horizon, CaCO3 (in a variable frequency; varied careous to calcareous. There are nume- 0.40 m thick, dark brown in colour, forms and centimetric sizes). The loess rous epigenetic carbonate concretions weakly structured and containing CaCO3 body is crossed by fine rhizoid ramified produced by partial dissolution. CaCO3 rhizoconcretions. canalicula. It is stable in steep walls, in 116 M. IRIONDO AND D. KRÖHLING

parts altered by subcutaneous subfusion tely structured in very coarse firm prisms complex of the middle section was gene- and shaped by columnar disjunction. bounded by very fine fissures. This depo- rated during the Holocene Optimum Cli- TL datings gave ages of 31.69 ± 1.62 ka sit lacks nodules or mottles, but contains maticum. The formation was generated BP and 32.0 ka BP in samples of the aeo- numerous root moulds, abundant macro- during the Holocene and possibly even lian facies of the formation in the area, at pores, very fine and fine canalicula and the Late Pleistocene too. 4 m below the top of the unit (LGM, tubes generated by bioturbation. The se- This unit correlates with the Luján For- OIS 2). diment mass is non-calcareous. The unit mation defined by Fidalgo et al. (1973), Buried soil (Kröhling 1999) is partly the product of deflation of the representing the Late Quaternary - The top of Tezanos Pinto Formation is A horizon of the underlying soil and the nental infilling of the fluvial valleys of marked by a partially eroded soil, 0.30 - subsequent deposition of dust. The typi- NE Buenos Aires province. Toledo 1.10 m thick, typically represented by a cal thickness is 0.30 m, with a maximum (2005) presented a sequence stratigraphic Bt horizon. In a minor geomorphological of 0.55 m. model in the Lujan type section of this unit (fluvial valleys of the region) that The San Guillermo Formation generally unit, based on detailed stratigraphic ob- soil forms an accretionary pedocomplex forms the top of the sedimentary se- servations and C14 datings on mollusks. of argillic horizons separated by the ac- quence in the natural sections of the The author refered ages from > 40 ka BP cumulation of Andean volcanic ash, Pampas. It was deposited during a Late to >11 ka for the Guerrero Member (flu- mainly concentrated by alluvial proces- Holocene dry period that occurred bet- vial-paludal deposits or Lujanense) and ses. At the interfluves of the region, the ween 3.5 and 1.4 ka BP (Iriondo 1990). ages from 10.7 to 3.3 ka BP for the Río Bt-horizon is formed by a dark brown Lucio Lopez Formation (Kröhling 1996) Salado Member of the Luján Formation (7.5 YR 3/4) clayish silt, with intense illu- It constitutes a complex sequence diffe- (grayish to whitish silty facies or Platense). viation and formation of cutans. It has a rentiated into three sectors. It includes a Prieto et al. (2004) gave ages ranging strongly to moderately developed struc- clayish silt with scarce very fine sand (silt: from 11 to 3.5 ka BP for the Luján For- ture in medium angular prisms, strong 58-77%; clay: 19-37%; sand: 3-6%), orga- mation in the Río Luján. consistence, with very fine and fine nized in fine strata with variations in cracks between peds. It lacks nodules or colour from olive to grey; pedogenic LA BAJADA SECTION mottles, and the silt fraction is non-calca- horizons are intercalated in the middle reous. There are abundant root moulds. section (pedocomplex). Locally, it exhi- The town La Bajada is located across the The lower boundary is wavy in form and bits high proportions of biogenic mate- Río Paraná from Santa Fe. At present, well marked. The Bt horizon is compo- rial (mainly gastropods, ostracods, dia- that locality (nowadays Paraná) is the ca- sed of a dull brown (7.5 YR 5/4) slightly toms and abundant plant remains). The pital city of Entre Ríos province. Such clayish coarse silt. This horizon shows a Lucio Lopez Formation outcrops along area is crossed by an old transcurrent weakly to moderately well-developed the cliffs of the main rivers of the nor- fracture, the Tostado-Gualeguaychú fault, structure, formed by medium blocks, ve- thern Pampas region, with most typical which elevated its southern block some ry firm, limited by fine and very fine fis- characteristics and the greatest thickness 40 m in the Middle Pleistocene (Iriondo sures. Nodules or mottles are absent, but on the Río Carcarañá cliffs. The thick- 1989), thus exposing Tertiary marine fine macropores are present. The silt ness varies from 0.50 to 5.00 m. The for- strata. Darwin payed special attention to fraction is non-calcareous. The C hori- mation unconformably overlies the Car- those strata, collected an interesting fos- zon shows characteristics transitional to carañá Formation in sections located in- sil assemblage and made an excellent the underlying loess. side the fluvial valleys of the region. The stratigraphic description of the section. The buried soil was generated during the contact indicates a palaeotopography of Darwin quoted that "In Entre Ríos, the Holocene Optimum Climaticum and it is channels and gullies. In general the for- cliffs, estimated at between sixty and seventy feet a distinct pedostratigraphic marker in dif- mation is covered by contemporary or in height, expose an interesting section: the lower ferent areas of the Pampas plain. recent deposits of fluvial origin; locally half consists of Tertiary strata with marine San Guillermo Formation (Iriondo 1987) the unit forms the top of the section. shells, and the upper half of the Pampean for- A younger loessic formation overlies the The formation, with a clear paludal ori- mation. The lowest bed is obliquely laminated, buried soil on top of the Tezanos Pinto gin, constitutes the infilling of erosional blackish, indurated mud, with distinct traces of Formation, separated from it by an ero- landforms located in a recent belt of the vegetable remains. Above this there is a thick sional unconformity. It is composed of a main rivers and at the bottom of the bed of yellowish sandy clay, with much crystalli- brownish grey (10 YR 5/1) coarse silt major fluvial palaeovalleys of the region. zed gypsum and many shells of Ostreae, with scarce proportions of very fine sand A TL date in the upper section of the Pectens, and Arcae: above this generally comes and clay. The unit is massive, friable in formation (a cineritic stratum) indicates an arenaceous crystalline limestone, but there is general, porous, permeable, and modera- an age of 1.32 ± 0.12 ka BP. The soil sometimes interposed a bed, about twelve feet From Buenos Aires to Santa Fe: Darwin's observations ... 117

thick, of dark green, soapy clay, weathering into small angular fragments. The limestone, where purest, is white, highly crystalline, and full of cavities: it includes small pebbles of quartz, bro- ken shells, teeth of sharks, and sometimes, as I was informed, large bones: it often contains so much sand as to pass into a calcareous sandsto- ne, and in such parts the great Ostrea patagoni- ca chiefly abounds. In the upper part, the limes- tone alternates with layers of fine white sand. The shells included in these beds have been named for me by M. d'Orbigny (a list of fossils follows) - M. d'Orbigny has given a detailed des- cription of this section, but as he does not men- tion this lowest bed, it may have been concealed when he was there by the river. There is a consi- derable discrepancy between his description and mine, which I can only account for by the beds themselves varying considerably in short distan- ces" (Darwin 1846, p. 88-89). In fact, the major discrepancies between both naturalists appeared because Dar- win worked in the elevated block of the fault and d'Orbigny researched in the downthrown block, where the marine Figure 4: The Paraná Formation outcropping in La beds appear only during low waters. Be- Bajada. A: Type profile sides, as Darwin correctly stated, the stra- (Iriondo 1973). B: Darwin´s ta vary in short distances. However, Dar- profile. win's description fits considerably well them are platy angular fragments without bodies are short lenses with internal dia- with the type section of the Paraná For- rounding. Major axes of plates are para- gonal lamination, up to 1.60 m thick. mation, formally defined there 150 years llel to stratification planes. Mean grain Loose, quartzose and yellow in colour. later by Iriondo (1973). size around 0.2 mm, finer towards the Sandy bodies are generally composed of The marine beds of Bajada were rena- bottom. Transitional upper contact. 5-15 cm thick internal units with diago- med as Paraná Formation in the 20th - Second bed - Darwin: Yellowish sandy nal stratification. The fine sediments are Century. This formation represents the clay, with much arenaceous crystalline gypsum plastic, gray clayey silts, forming conti- last widespread marine transgression and Ostreae. nuous strata 1 to 30 cm thick and at least occurring in the interior of South Ame- - Type section: 3 m. Massive green sand, 50-60 m long; internal laminae of sand rica (Yrigoyen 1969, Herbst 1971, Irion- without internal bedding, containing up are common. In parts with contorted in- do 1973, Marengo 2005) and the Bajada- to 10 cm long elliptic clay intraclasts. ternal lamination. Silt strata conformably Paraná area is considered the type loca- 20% of the sediment mass is composed cover the underlying sand lenses. Grain lity. Darwin's section, indeed, can clearly of chaotically distributed small platy size of the fines is 30 microns at the bot- be recognized in the type section of the intraclasts. Grain size around 0.25 mm tom, diminishing upwards to clay-sizes. formation (Iriondo 1973, Fig. 4): with variable dispersion, which grows The upper section of this bed changes to

- Both sections are composed of five upwards. Lightly cemented with CaCO3. white sandstone with dune structures beds and have similar thicknesses; a com- The detachment and fall of intraclasts containing well preserved bivalves. parison shows the following: produces numerous small hollows in out- Further on, the bed passes to a fine con- - The lowest bed - Darwin (profile B of crops. Neat upward contact. glomerate. Fig. 4): Obliquely laminated, blackish, indura- - Third bed - Darwin: 4 m. Dark green, - Fourth bed - Darwin: Sandy crystalline ted mud, with distinct traces of plant remains. soapy clay weathering into small angular frag- limestone. Where purest, it is white, highly Type section: 3 m. Green quartz sand in ments. crystalline, and full of cavities: it includes small 30-50 cm thick strata, including a large - Type section: 6 m. Sequence of interes- pebbles of quartz, broken shells, teeth of sharks proportion of clay intraclasts, most of tratified sand and clayish silt. The sand and sometimes large bones: it often contains so 118 M. IRIONDO AND D. KRÖHLING

much sand as to pass into a calcareous sandsto- characterizes the upper half of the sec- both cases, the main source of sediment ne, and in such parts the great Ostrea patagoni- tion in all the basin. The lower half appe- has been the Andean region, that produ- ca abounds (Darwin 1846, p. 89). ars as composed of green sand. ced fine materials originated by nival pro- - Type section: 1.5 m. White calcareous A research borehole drilled in the center cesses and volcanic eruptions. sandstone with dune structures and un- of the basin in the northern Pampas (San The older cycle, defined as the Punta dulose strata. Lateral and upward enrich- Guillermo locality; 30°15'S, 61°50'W) Gorda Group, is composed of three ment with shells of bivalves and scarce recorded 40.5 m of the Paraná Forma- units accumulated in paludal and sub- oysters. Bivalve shells underwent an ad- tion covered by 60.3 m of Quaternary aereal environments during the Early vanced degree of dissolution and re-pre- loessic formations (Kröhling and Iriondo Pleistocene. The second sedimentary cipitation, remaining often only as 2003). cycle occurred during the Late Pleisto- moulds. Oysters are well preserved. Following Darwin, "The upper half of the cene and Holocene (basically at the Last - Fifth (upper) bed - Darwin: The limesto- cliff in La Bajada, to a thickness of about Glacial Maximum) and is the Pampean ne alternates with layers of fine white sand. thirty feet, consists of Pampean mud, of which Aeolian System traversed by Darwin Type section: 5.5 m. Most of the outcrop the lower part is pale-coloured, and the upper from Buenos Aires to Santa Fe. It com- is covered by debris. The lower sector is part of a brighter red… Close above the mari- prises two formations accumulated in characterized by white sand, incipiently ne limestone, there is a thin stratum with a con- subaereal environments. A short period cemented by the carbonate provided by cretionary outline of white hard tosca-rock or of aeolian remobilization occurred the local dissolution of shells; the upper marl…" Three Quaternary formations during the Late Holocene. section is composed of fine quartz sand. can be recognized in that reference: The Several episodes of pedogenesis and lo- Green in colour. upper one is Tezanos Pinto Formation, cal erosional unconformities were recor- The general scenario suggested by the the main loessic deposit of the Last Gla- ded in both major sedimentation periods, section in both approaches (Darwin´s cial Maximum. The "lower mud" is the particularly in the older one. The long and modern) is one of a tidal environ- Hernandarias Formation, an Early Pleis- sedimentation hiatus, covering all the ment, with subtidal sands and gravels, tocene playa deposit of the Río Uruguay. Middle Pleistocene and a half of the Late intertidal muds, changing tidal channels The tosca-rock is a well developed level, Pleistocene constitutes a remarkable and coastal currents. Two different sedi- which thickens southwards up to 9 m and regional feature. mentary mechanisms can be deduced: a) is known as Puerto Alvear Formation. Important similarities were found betwe- Transport and accumulation of sand by en both sedimentary cycles: i) the same tractive currents, probably from nearby SOUTHWESTERN source of sediments; ii) the same long beaches if one considers the negative ENTRE RÍOS distance transport agent (wind); iii) stri- skewness of the sediment. b) Floccu- king similarity in grain-size distributions; lation of fines in calm environments. A key area of the region is located in the iv) scarce contribution of materials from The fossil assemblage also points to a lit- southwestern part of Entre Ríos provin- the Brazilian shield and associated areas toral/neritic position in a warm climate. ce and outcrops along the eastern cliff of in the north. A general setting of the Paraná Forma- the Paraná flood plain. Darwin did not The age of the older cycle can be located tion is as follows: The Paraná Formation study such outcrops, but navigated along between the Brunhes - Matuyama mag- covers an important sector of a major them in his journey back to Buenos Ai- netic polarity change (the Upper Matuya- geological event occurring in the Neo- res. However, he made a few comments ma Chron > 0.78 Ma.) and the Jaramillo gene of South America. Around the on "the Punta Gorda in Entre Ríos" (diffe- Subchron (1 Ma). This conclusion coin- Middle Miocene occurred a generalized rent from the other Punta Gorda in the cides with recent datings in the "Great marine transgression in large regions of Banda Oriental), which is an interesting Patagonic Glaciation" (Ton-That et al. the continent. The Paraná Formation sedimentary cycle that occurred around 1999), which rendered ages from 1 Ma to comprises the portion included in the one million years before present. That 1.17 Ma. The following sedimentary Chaco-Paranense Basin, which extends locality is placed 60 km south of Santa Fe units outcrop at Punta Gorda: from the central lowlands of Argentina and has the best stratigraphic section of Punta Gorda Group (Iriondo 1980): Sedi- at 36° lat. S northwards to the Bolivian the area (Fig. 5) in Entre Ríos. mentary cycle 1 and Paraguayan Chaco. The Miocene sea Puerto Alvear Formation (Iriondo 1980) was a shallow, platform branch of the Punta Gorda Section This is a sedimentary body accumulated Atlantic Ocean. Mean thickness of the as infilling of an abandoned belt of the formation is about 100 m; the sediment The Quaternary geologic column of the Río Paraná. The unit was a non-perma- is dominantly sandy in the type area (Pa- region (Kröhling 2001) records two well nent swamp at the beginning of the raná; La Bajada in Darwin times) and defined aeolian sedimentation cycles. In Pleistocene. The formation lies uncon- From Buenos Aires to Santa Fe: Darwin's observations ... 119

Figure 5: Punta Gorda section, representative of SW Entre Ríos province (after Iriondo and Kröhling 2008). formably on the Paraná Formation (ma- ment. Numerous vertical large and bo- olive patches. Abundant Mn- and Fe se- rine Miocene), with a maximum thick- trioidal concretions indicate a post-depo- gregations and nodules are conspicuous. ness of 9 m. The most visible field featu- sitional remobilization of the carbonate. This formation is divided into two mem- re is a closed net formed by CaCO3 pla- The clastic component of the unit is a bers; the lower one is characterized by tes of phreatic origin, 0.5 to 4 cm thick silty clay to sandy silt with diffuse lamina- thick continuous partition walls, with a with a dominant horizontal develop- tion, light reddish brown in colour with wavy trace. The upper member includes 120 M. IRIONDO AND D. KRÖHLING

a similar horizontal carbonate net, The central section is formed by 4 m of formably accumulated over the eroded although formed by thinner and more homogeneuos light brown loess, with a soil. It is a brownish gray loess, 20 to 35

irregular platy precipitates. An internal vertical slope. A lattice pattern of CaCO3 cm thick, loamy to silty-loamy. This unit unconformity separates both members; precipitates appears in the exposures. is the result of deflation of the A-hori- in some places the top of the lower Three weak to moderate paleosols (B- zon of the soil on top of the Tezanos member are the Bw-and C horizons of a and Ck horizons) were recorded at the Pinto Loess and the subsequent deposi- weakly developed paleosol. That pedoge- typical site. These soils are laterally dis- tion of dust. Such an event occurred nic level is non-calcareous. continuous as a result of uneven erosio- from 3.5 to 1.4 ka BP. La Juanita Formation (Iriondo 1998) nal contact with the overlying loess. They This unit was formed in a paludal envi- are re-calcified from this material too. THE PARANÁ FLOOD ronment during the Early Pleistocene. It The upper section begins with a Ckm PLAIN lays unconformably on the Puerto Alvear horizon of another eroded paleosol, Formation, with a typical thickness of 2 covered by a one meter thick paludal Darwin travelled from Santa Fe back to to 3 m. The La Juanita Formation is deposit. Buenos Aires by boat along the Paraná composed of light brown to olive silt Tezanos Pinto Formation (Iriondo 1980): and made some remarks about the river, with scarce fine sand with horizontal dif- Sedimentary cycle 2 its banks and fauna. Particularly, he noted fuse bedding. Strata are 15 to 30 cm The unit constitutes the Peripheral Loess the large number of islands that are per- thick, with internal lamination. Fe- and Belt of the Pampean Aeolian System, manently eroded and reconstructed by Mn segregations and root moulds are lo- which was deposited during the OIS 2 the currents. cally abundant. The sediment is slightly (36-8 ka BP, Iriondo and Kröhling 1995). In fact, according to modern geomor- structured in firm blocks. Concretions, The aeolian dust was transported from phology the river flows there within a which form more than 10% of the sedi- Andean sources by S and SW winds. In large flood plain, approximately 600 km mentary mass, are rough vertical infi- SW Entre Ríos province the unit is yello- long, stretching from the Paraguay- llings of roots, 2 to 5 cm thick and up to wish brown loess, 2 to 3 m thick, compo- Paraná junction (27°25'S) to south of 20 cm long. The general tendency of the sed of powderish sandy silt, massive and Rosario (33°S). Downstream it passes

CaCO3 precipitates indicates infiltration friable. OSL ages of 32 and 24 ka BP transitionally to a well-developed Holo- in a non-permanent swamp environment were obtained for the loess in this region. cene littoral complex. The general direc- with a non-saturated bottom. There are The loess forms typical vertical slopes, tion of the flood plain is north-south, also infillings of horizontal fissures. Fre- with columnar disjunction. It contains crossing several tectonic blocks in a per-

quent Mn films cover the surface of con- small CaCO3 concretions and frequent pendicular or diagonal manner This rela- cretions, indicating a younger age for the rhizoconcretions (15.48 ± 0.19 C14 ka tionship has generated local segments mobilization of such oxides. BP).The unit received contributions of within the plain, with slightly different Punta Gorda Formation (Iriondo and Kröhling fine sediments from the Paraná headwa- directions and slopes. The width of the 2008) ters, located to the northeast. floodplain varies from 13 to 45 km, with This is a brown loess characterized by The clay fraction of the loess is a com- typical values between 25 and 35 km. It is pedogenesis in several levels and fre- plex mixture of illite, interstratified illite- composed of several internal geomor- quent local internal unconformities. Ac- smectites and kaolinite. Illite is a pampe- phological units, the most modern of cording to palaeomagnetic analyses, the an mineral whereas smectites and kaoli- them is the “bar plain”, that is developing age of the unit is Early Pleistocene. Car- nite are derived from Brazil. A partially at present (Iriondo 2007). bonatic cement is widespread in the for- eroded soil on top of the loess is repre- The bar plain is a belt of large elliptic mation. Local second order variations are sented by well developed Bt-horizon and bars composed of very fine sand, that the rule in most outcrops. Three sections C horizons. The Bt horizon is 35 to 55 encloses the main channel along the were described in the unit at the type cm thick, clayey silt dark brown in colour, whole flood plain. This belt is perma- locality (Punta Gorda, Entre Ríos pro- structured in very firm prisms; argillocu- nently modified by erosion and sedimen- vince). The lower one was unconforma- tans are frequent. The buried soil was de- tation and has a width of 2-7 km; it is bly deposited over La Juanita Formation veloped under humid subtropical condi- formed by islands inside the main (navi- and begins with a paludal deposit com- tions during the Holocene Optimum gation) channel and sand bars juxtaposed posed of clayish silt, light brown in Climaticum. to the margins. The large bars are crossed colour, with Fe- and Mn segregations. San Guillermo Formation (Iriondo 1980): by minor active channels, most of them This is conformably overlain by a petro- Sedimentary cycle 2 with intense lateral migration, resulting in calcic horizon formed by vertical cylin- The sedimentary sequence of Punta minor meander belts. The powerful brai- drical welded concretions 50 cm high. Gorda ends with an aeolian unit uncon- ded mainstream - with a mean discharge From Buenos Aires to Santa Fe: Darwin's observations ... 121

between 16,000 and 20,000 m3/sec - con- ges were lower than today in all the quent. c) Lake-and-swamp facies, com- tinuously reshapes islands and banks. system and tributaries did not reach the posed of dark gray silt with abundant This is the environment described by collector, deposing the sediment loads in organic matter in different degrees of de- Darwin. lateral positions inside the flood plain; composition. It is compact, with low po- Other important geomorphological units the Paraná River itself was also smaller rosity; sand and clay are scarce. A really of the flood plain are the hindered drai- then than it is today. That resulted in rela- modest proportion of the sediment volu- nage plain and the deltas of tributaries. tively short and wide delta-like sedimen- me is represented by a fourth facies, i.e., The hindered drainage plain was descri- tary bodies. Most of such deltas (Los aeolian dunes composed of very fine bed by Darwin as a labyrinth of small Amores, del Rey, Corrientes) are located sand, which appear in isolated spots in branches separated by low islands cove- north of Santa Fe, a region not visited by the northern half of the flood plain. red by forest, he observed this landscape Darwin owing to health problems. Te- when crossing the flood plain from Santa rraces also appear in the north, in Chaco THE LITTORAL COMPLEX Fe to La Bajada. This unit is characteri- and Corrientes . AT THE RÍO PARANÁ zed by a morphology of flat areas with The sediments of the flood plain are MOUTH numerous shallow ponds, swamps and very homogeneous. Practically all the se- small adventitious channels. Such chan- dimentary mass is composed of only Darwin noted that "Some leagues downstre- nels are tortuous, active only at the begin- three sedimentary facies: a) Channel fa- am Rosario begins, at the western margin of the ning and end of floods, conveying water cies, which are composed of fine and Paraná, a line of perpendicular cliffs that into ponds during rising and back to the very fine quartz sand, generally in 10-40 extends beneath San Nicolás…The banks of main channel at low levels. The number cm thick horizontal strata with diagonal the river are formed by very soft earths; in conse- of ponds and shallow lakes is huge - internal structures. Such sand was origi- quence the waters are muddy…". The author about 5,000 according to Paira and nated in Mesozoic sandstones of the up- has entered the Paraná Delta, which in Drago (2007). This system was genera- per basin and comprises the major part fact is a large littoral complex of Holo- ted under a hydric regime different than of the buried sediments. b) Levee facies, cene age formed by several geomorpho- the present one. The deltas of tributaries which are characterized by silty-clayish logical and sedimentary units sequentially are areas of sedimentation formed inside very fine sand with intermediate plasti- generated. Four main phases have been the flood plain during a recent dry clima- city; ochre to greenish-gray in color, mot- identified there by Iriondo (2004, Fig. 6): te period (from 3.5 to 1.4 ka BP, accor- tled, with numerous pores, tubes and 1) A fluvial period represented by river ding to the latest datings). Water dischar- root moulds. Gley processes are fre- flood deposits; 2) A marine ingression

Figure 6: Map of the Holocene littoral complex at the Paraná mouth (after Iriondo 2004). 122 M. IRIONDO AND D. KRÖHLING

with development of a sand barrier, a the geological contributions of Darwin Asociación Geológica de Córdoba 2(1-2): 19- lagoon, minor tributary deltas and estua- and the present knowledge on the region, 23. ries and well-developed regression depo- the authors of this contribution necessa- Iriondo, M. 1980. El Cuaternario de Entre Ríos. sits; 3) An estuarine phase characterized rily follow the general structure of Revista de la Asociación de Ciencias Natu- by extensive tidal deposits in the central Darwin's books, a traveler's diary enri- rales del Litoral 11: 125-141, Santo Tomé. area; and 4) The present fluvial period, ched by digressions on Science, Politics Iriondo, M. l987. Geomorfología y Cuaternario with channel deposits and a deltaic unit and History. de la Provincia de Santa Fe. D'Orbignyana advancing into the Río de la Plata. The 4: 1-54, Corrientes. present dynamics of the deltaic complex WORKS CITED IN THE TEXT Iriondo, M. 1989. Major fractures of the Chaco- is dominated by the Paraná floods, the Pampa plain. Bulletin of the INQUA Neo- floods of the Uruguay and Gualeguay De Alba, E. 1953. Geología del Alto Paraná - tectonic Commission 12: 42-46, Stockholm. rivers, Atlantic tides, and floods produ- Relación con los trabajos de derrocamiento Iriondo, M. 1990. A Late Holocene dry period in ced by the southeasterly winds. entre Ituzaingó y Posadas. Revista de la Aso- the Argentine plains. Quaternary of South The southeasterly wind (Sudestada), inde- ciación Geológica Argentina 8:129-161. America and Antarctic Peninsula 7: 197-218. ed, is a major factor in the water dyna- Darwin, C. 1846. Geological observations on Iriondo, M. 1998. Excursion Guide Nº3: Pro- mics and navigation in that area and South America. Being the third part of the vince of Entre Rios. International Joint Field downstream in the Río de la Plata. The geology of the voyage of the Beagle, under Meeting: Loess in Argentina: Temperate and wind can be strong and blow sometimes the command of Capt. Fitzroy, R.N. during Tropical. INQUA - PASH - CLIP - UNER - during several days, provoking a rise of the years 1832 to 1836. Smith Elder and Co. CECOAL 3: 1-19, Paraná. the water level. Records of up to 2.5 m 280 p., London. Iriondo, M. 2004. The littoral complex at the Pa- above the terrain have been reported at Darwin, C. 1945. Viaje de un naturalista alrede- raná mouth. Quaternary International 114: several places. During such periods, navi- dor del Mundo. Librería El Ateneo, 118 p. 143-154. gation in boats and small vessels stops; (Spanish translation of the second English Iriondo, M. 2007. Geomorphology and Quater- surely, Darwin underwent such a meteo- edition), Buenos Aires. nary of the Flood-Plain. In Iriondo, M., rological phenomenon: "At night, the wind Fidalgo, F., De Francesco F. and Colado, U. Paggi, J. and Parma J. (eds.) Limnology of the is scantly favorable and we stop; the next day (1973). Geología superficial de las Hojas Middle Paraná River: A Subtropical Wetland, blows a strong wind…" Castelli, J.M. Cobo y Monasterio (provincia de Springer Verlag, 382 p., Berlin, Heidelberg, Buenos Aires). 5° Congreso Geológico Ar- London. COMMENTS AND gentino (Córdoba), Actas 4: 27-39. Iriondo, M. and Kröhling, D. 1995. El Sistema CONCLUSIONS Georgieff, S., Anis, K., Orfeo, O., and Rizo, G. Eólico Pampeano. Comunicaciones del Mu- 2005. Architecture of Ituzaingo Formation seo Provincial de Ciencias Naturales "Floren- Undoubtedly, the short incursion of the (Pliocene), a comparison with deposits of tino Ameghino" (NS) 5(1): 1-68, Santa Fe. great naturalist to the interior of the Paraná River, Argentina. University of Tech- Iriondo, M. and Kröhling, D. 2008. Cambios Argentine plains was a very modest por- nology, 8th. International Conference on ambientales en la cuenca del Uruguay (desde tion of his famous trip around the world, Fluvial Sedimentology, Abstracts: 107, Delft. el Presente hasta dos millones de años atrás). with plenty of discoveries, experiences González Bonorino, F. 1965. Mineralogía de las Colección Ciencia y Técnica, Ediciones UNL and amazing landscapes such as Tahiti or fracciones arcilla y limo del Pampeano en el 358 p., Santa Fe. Tierra del Fuego. The attention paid by área de la ciudad de Buenos Aires y su signifi- Kemp, R., Zárate, M, Toms, P. King, M., Sa- Darwin to these flat plains in his subse- cación estratigráfica y sedimentológica. Re- nabria, J. and Arguello, G. 2006. Late Quater- quent studies was necessarily far more vista de la Asociación Geológica Argentina nary paleosols, stratigraphy and landscape modest than those devoted to the Gala- 20: 67-148. evolution in the Northern Pampa, Argentina. pagos Islands or the Andes Cordillera. Herbst, R. 1971. 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