Correlation and dating of Quaternary littoral zones along the Patagonian coast, Argentina

Quaternary Science Reviews, Vol. 8, pp. 213--234, 1989. Printed in Great Britain. All rights reserved.

0277-3791/89 $0.00 + .50 © 1989 Pergamon Press plc

CORRELATION AND DATING OF QUATERNARY LITTORAL ZONES ALONG THE PATAGONIAN COAST, ARGENTINA Nat Rutter,* Enrique J. Schnack,t¶ Julio del Rio,t Jorge L. Fasano,t§ Federico I. Islat§ and Ulrich Radtke:l: *Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada tCentro de Geologia de Costas, Universidad Nacional de Mar del Plata, 7600 Mar del Plata, Argentina :~Universitiit Diisseldorf, Geographisches Institut, 4000 Diisseldorf, F.R.G.

Six areas of the Patagonian coast, Argentina, were investigated in order to determine the n u m b e r , characteristics, geomorphology and development of Q u a t e r n a r y littoral zones. It became apparent that utilizing D/L ratios of aspartic acid and leucinc of various molluscan species would be the most useful in correlation, relative age dating and estimating the ages of these zones. T h e oldest littoral zone is found at elevations between 24 and 41 m above m e a n sea level and is judged to be older than Oxygen Isotope Substage 5e in age based on relatively high amino acid ratios and extrapolated from non-linear kinetic models. A n intermediate aged littoral zone may be present at some locations based upon beach ridges or platforms varying in elevation between 16 and 28 m above m e a n sea level. D/L ratios are generally lower than those for the oldest zone but show a greater variation. This zone m a y represent the Substage 5e sea level stand. Well defined young beach ridges 8-12 m above m e a n sea level are found in most locations and have been ~4C dated, and verified by amino acid ratios, as being Holocene. T h e presence of Q u a t e r n a r y aged e m e r g e d littoral zones at roughly the same elevation suggest that the glacio-eustatic contribution is the primary cause of the high sea level stands whereas secondary variations are attributed to other factors.

INTRODUCTION The coastal area of Patagonia displays a number of raised littoral zones, consisting largely of raised beach ridges and deposits, at varying elevations. There is very little accurate information on the number, age, characteristics, geomorphology and development of these features. Although the causes of elevation differences have been speculated upon there is little evidence. Most are considered to be Quaternary age. The authors investigated six areas from San Blas southward to Tierra del Fuego, a distance of over 1500 km, during 1986 and 1987 in order to elucidate the characteristics of these littoral zones with the primary objectives of correlating, relative age dating, and estimating the absolute ages of the beaches and an explanation of the causes of sea level change. The areas include San Bias, San Antonio Oeste, Caleta Valdes, Bahia Bustamante, Puerto Deseado and Tierra del Fuego (Bahia San Sebastian). Although other raised beaches and deposits are present, the areas investigated were judged to display some of the best sequences and were also easily accessible. In the course of field investigations, it became apparent that utilizing D/L ratios of amino acids of molluscs would be the principal tool in correlating, relative age dating and estimating absolute ages of littoral zones. The technique was particularly attractive because most deposits of varying age contain abundant fossil molluscs, are superbly preserved, many in living ¶Career Scientist, Commission for Scientific Research of the

Province of Buenos Aires. §Career Scientist, National Research Council of Argentina.

position, and were subjected to a similar diagenetic history. Although some radiocarbon dates are available, the D/L ratios of amino acids appear to be more reliable and useful. The results presented here are preliminary, and many problems remain. Research on the Quaternary evolution of the Patagonian coast is continuing. PREVIOUS WORK Previous work has centered on the identification, number and age of emerged shorelines (Witte, 1916; Feruglio, 1950; Auer, 1959; Codignotto, 1983; Fasano et al., 1983; Rabassa et al., 1984; Porter et al., 1984; Rabassa, 1987). The work by Feruglio (1950) provides a framework of the entire Patagonian coast. On the basis of the topographic position, geographic distribution and a thorough analysis of fossil invertebrates, mainly molluscs, he distinguished six marine terraces named as follows: Terrace I 170-186 m (above mean sea-level); Terrace II 105-140 m; Terrace III 70-80 m; Terrace IV 35-40 m; Terrace V 15-18 m; Terrace VI 8-10 m. Terraces IV, V and VI are composed of modern fauna. Terrace VI is the youngest, which he refers to as 'recent'. Later work by Codignotto (1983) using radioc'~rbon dating confirms a Holocene age for this terrace which extends along most of the Patagonian coastal plain and correlates with equivalent shorelines in the Pampas area to the north. Terraces IV and V were judged to be of Pleistocene age but do not extend

213

214

N. Rutter et a/.

i BOtlvl A

i

\...,-,,..~./.~,-_~/~ ,: i ..... ~ i "',. "q@O~, £"~ '

.t:

f

/ i

Santiago I

t~,

"~U

O

°

".'--'-

c.,

:]

~"

J l:

~. _aBos_

BRAZIL

~!

~] !

i

>

0

o<<,

SanBias "t ~E~ 3anAntonioOeste C, ~3DCaletaValdes /,.

~l~' o/"~

~ahiaBustamante

. :

PuertoDeseado

~"

+

_

~.

('~"~"~

N

~: ~o

0I

600 I km

1200 I

FIG. 1. Location map of areas investigated along the Patagonian coasl, Argentina.

beyond the 'last interglacial' (Feruglio, 1950). The former ('Littoral ridge with Mactra isabelleana at Bahia Bustamante') is deeply dissected (Cionchi, 1983) and represents the oldest high-Pleistocene stand, Previous work indicates that there is general agreemerit that sea levels were higher than present at least during the 'last interglacial' (Oxygen Isotope Substage 5e) and during the Holocene (early to mid). Sea levels lower than present are associated with the most recent glacial maximum during the late Wisconsinan (Fray and Ewing, 1963; Oxygen Isotope Stage 2). There is also agreement that elevations of the marine terraces of Patagonia are higher than the equivalent levels in the Pampas area to the north in Buenos Aires Province (Fasano et al., 1983). S E A LEVEL DEVELOPMENT

Several factors control the position of sea level in any particular regional setting and tectonic environment. Detailed investigations throughout the preceding twenty years have demonstrated that the concept of uniform, worldwide, eustatic sea level changes fails to explain the elevations of many late Quaternary strandlines (e.g. Clark et al., 1978). Along the coast of Argentina, the dominant factors controlling the position of sea level through time are glacioisostatic

adjustment of the area surrounding Antarctica, the complex interaction of the geoid surface, crust and uppermost mantle to the shifting of ice, marine water, and sediment loads induced by dissipation of the ice masses in the northern hemisphere, and vertical displacement induced by both local factors and lithospheric plate movements. The history of isostatic adjustment of the region surrounding Antarctica, and the extent of the areas affected by the glacially-generated marginal depressions and forebulge complexes are poorly known. Modelling of the Antarctic Ice Sheet and associated montane glaciers between 18 ka BP and the present (Hollin, 1968; Hughes et al., 1981) suggests that ablation of these ice masses contributed approximately 25% of the total increase in the global volume of marine water in the latest stages of the Quaternary (e.g. Clark and Lingle, 1979). The cumulative effects of glacioisostatic depression and sea level rise along the Australian/New Zealand sector of the Antarctic coast resulted in elevated shorelines 70-80 m above present sea level following deglaciation (Cameron and Goldthwait, 1961; Clark and Lingle, 1979). Deformation of the geoid surface, crust, and upper mantle followed deglaciation in the northern and southern hemisphere. Removal of the ice sheet loads, and application of the weight of newly produced water

Quaternary Littoral Zones to the nearshore marine areas, resulted in complex deformation, with some regions characterized by constant submergence, others by emergence, and still others by more complex patterns (Clark et al., 1978; Bloom, 1980; Clark, 1980; M6rner, 1984, 1986). Deglaciated areas are characterized by constant emergence, whereas in the regions surrounding the northern hemisphere ice sheets and Antarctica, collapse of the forebulges would result in steady submergence throughout the Holocene (Clark et al., 1978; Clark and Lingle, 1979). Between these two regions, a transition zone along the ice margin is characterized by initial emergence followed by submergence. Continental margins beyond the extent of the forebulge regions are characterized by constant emergence, as the continents rise and the oceans sink under the weight of the additional marine water produced by glacial ablation. In the presence of alteration by local or regional tectonically-induced movements, the coast of Tierra del Fuego would be expected to be characterized by constant submergence throughout the Holocene, as this area lies within the collapsing forebulge region of the Antarctic Ice Sheet delineated by Clark and Lingle (1979). Most of the Patagonian coastline lies to the north of the forebulge region, and thus would be predicted to undergo constant emergence throughout the Holocene, in the absence of neotectonic modification. The degree of emergence should be greater along the southern part of the coastline. Models developed by Clark and Bloom (1979) predict 2-5 m emergence for Holocene times along the east coast of South America, where tectonism and glacioisostatic effects are minimal, When considering the melting of the Patagonian Ice Sheet the model predicts minor emergencies. The margin of the forebulge region was probably located in the southern part of Patagonia. However, models developed considering the postglaciai redistribution of ice and water loads do not satisfactorily explain the altitudes of raised beaches of the southern hemisphere. Delaying the melting of the northern hemisphere ice sheets by 2 ka years (Clark et al., 1978), delaying the Antarctic melting 5-7 ka or thinning the lithosphere from 196 to 71 km (Peltier, 1988), were the methods proposed to explain mid-Holocene sea level data from the southern hemisphere, Neotectonic movements have been observed to affect shorelines developed on Tierra del Fuego (Porter et al., 1984; Rabassa, 1987). Tierra del Fuego is situated northwest of the triple junction between the Antarctic, South American and Scotia Plates (Dott, 1976). Right lateral movement along the northern side of the transform fault system bordering the South American Plate began during Palaeocene time, and is currently displacing Tierra del Fuego towards the east. The transform fault system is connected to the PeruChile Trench along the western coast of Tierra del Fuego and Isla Santa Ines. The Antarctic and Scotia Plates are separated by a northeast-southwest trending spreading ridge system. All of these plate boundaries are characterized by active movement, resulting in

215

regional uplift throughout the triple junction region on the order of 1-2 mm/year (e.g. Rabassa, 1987). Tierra del Fuego has, therefore, been undergoing tectonically-induced uplift since South America and Antarctica were split in the triple junction region during the early Miocene (Pitman, 1974; Dott, 1976). Immediately following deglaciation, the subsidence induced by collapse of the Antarctic forebulge would temporarily be in excess of the tectonically-generated uplift, resulting in subsidence of the area. Subsidence would continue until the constantly decreasing rate of forebulge collapse equalled the tectonic uplift rate. During the later Holocene, continued tectonic activity would result in progressively increasing emergence, at a gradually accelerating rate. This scenario is in accord with the dated sea levels reported by Porter et aL (1984) and Rabassa (1987). The coast of Argentina north of Tierra del Fuego lies on the tectonically stable trailing edge of the South American Plate. Global tectonic effects would therefore be expected to be minimal along this coastline. Local subsidence, induced by sediment loading in estuarine and deltaic regions or autocompaction of saturated or organic sediments, could act to produce submergence in estuaries and embayments.

RECOGNITION OF PALAEO-SEA LEVELS Assessment of the positions and times of formation of former sea levels along the coast of Argentina is complicated by inaccuracies involved in 14C dating of marine mollusc shells, and in determining the relative position of sediments deposited below the palaeoshorelines, Many species of marine molluscs incorporate carbonate derived from the substrate or from the water mass in their shells. Estimates of the errors induced by these effects in living molluscs vary between 250 and 600 years (e.g. Mangerud, 1972). These errors are compounded by remobilization of carbonate conrained in the marine water mass during sediment and shell deposition. In the south Atlantic Ocean, the combined magnitude of the carbonate reservoir effects has been estimated at 1.5 ka (Angino, 1963). Postdepositional modification by circulating groundwater also can have a major effect on the accuracy of ~4C determinations from marine mollusc shells, especially in sediments containing detrital inorganic carbonate or those underlain by carbonate bedrock (e.g. Occhietti and Hillaire-Marcel, 1977). In nearshore marine, estuarine, or deltaic environments, the range of water depths tolerated by most benthic organisms (especially molluscs) generally exceeds the range of elevations above sea level noted for Quaternary shorelines. Determination of the depth of deposition of the dated sediments below the palaeo-sea level, therefore, depends upon detailed sedimentological analysis in the absence of fossils of depth-specific organisms. Submerged offshore banks and shallow tidal inlets

216

N. Runer el al.

are common along the coast of Patagonia, where mesoto macrotidal conditions prevail (Schnack, 1985). Mollusc shells obtained and dated from mesotidal coastal sediments could, therefore, have been deposited as much as 10 m below the level of the marine strandline. Shell bank deposits are subject to wave and current reworking during intense storms, occasionally resulting in the transport of disarticulated shells and shell fragments inland to elevations above sea level, The shell banks are composite structures of transported shells, some of which may be reworked from older submerged marine deposits. Marine shells found in sediments deposited in association with mesotidal conditions could, therefore, be deposited several meters above or below the palaeo-sea level, or could represent material reworked from older submerged units. In cases where the shell-bearing sediments are not clearly laterally gradational into a strandline, saltwater marsh or non-marine deposit, assessment of the precise position with respect to palaeo-sea level may not be possible, The problems discussed above are magnified when macrotidal environments, such as those present at Bahia Bustamante and San Antonio Oeste, are considered. Spring tidal and storm reworking of the extensive shell banks developed offshore of these coasts transported disarticulated shells and shell fragments into the lagoons, tidal saltwater marshes, and occasionally onto the shore above sea level (during extremely powerful storms). Deep tidal inlets with rapid currents also transport shells to the nearshore areas from the offshore. Reworking of older, submerged shell banks is also more prevalent along macrotidal coasts, Most of the problems associated with transportation and reworking of shells can be minimized if care is taken to sample and date only articulated shells, preferably those found in growth position within the nearshore sediments. The depth tolerance of many nearshore marine molluscs, however, can effectively preclude precise assessment of the water depth at the time of deposition. Generally, the depth can only be determined to a precision finer than 5 m if the shellbearing sediment grades laterally into strandline, saltwater marsh, or non-marine deposits. As most of the deposits studied in the area of Patagonia are related to beach environments under meso- and macrotidal conditions, caution has been taken in distinguishing foreshore and storm-beach deposits. Therefore, altitudinal positions are not referred to palaeo-sea levels as such. Instead, the topographic position of the surface of the marine terraces is used as relative reference level or the height of the fossil-bearing layer in relation to present meansea level, Estimation of palaeo-sea levels is not attempted, partly due to the fact that precise levellings have not been made yet. In addition, shell layers do not offer enough certainty for relative sea-level assessment, even if they are well-preserved and in growth position, as

tidal and wave conditions may have changed through time. All these factors require further analysis. CORRELATION ANDDATING As a first approximation of the number and the distribution of equivalent littoral zones along the Patagonian coast, D/L ratios of aspartic acid and leucine of molluscs were utilized, in conjunction with traditional geomorphic and stratigraphic methods. The D/t. ratios of a variety of species from the same sampling site were interpreted because the same variety of molluscs was not always found at all the sampling sites of varying age. In other words, interpretation of relative age was determined from a number of species, not from just one form. In order to avoid erroneous interpretation from reworked specimens, whole shells, and those believed to be in living position, were used where possible. An estimation of absolute ages of littoral zones was made by comparing the t)/L ratios from this study with D/L ratios of known ages from other investigations and by adopting non-linear kinetic models developed by others (Wehmiller and Belknap, 1982). D/L ratios were compared with radiocarbon dates on Holocene beaches where they were judged to be accurate, and then extrapolated to undated Holocene beaches. Radiocarbon dates available for older beaches are suspect. o/L ratios of other amino acids, such as valine, alanine, proline, glutamic acid and phenylalanine were also determined. In addition, kinetic studies are being carried out on several species in order to determine racemization rates. These results will be reported in a later report.

Analytical Procedure More than 200 samples were prepared as outlined in

Rutteretal. (1979), using acidified ethanol and pentafluoropropionic anhydride to derive the purified amino acids obtained from the shell samples. Each sample was prepared only once, unless it was found to be too weak to analyze and then it was prepared a second or third time with greater mass of shell. This was usually a necessary procedure with older samples. The amino acids were separated by gas chromatography using a Chirasil-Val capillary column, with a temperature program that ensured baseline resolution between the aspartic acid D and L peaks on the chromatogram. The standard mixture was analyzed daily, the O/L ratios calculated and, if necessary, the temperature program was adjusted to produce standard ratios between 0.9800 and 1.0500, Accuracy of instrument performance was tested by analyzing an interlaboratory shell standard. The ratios were found to be within the expected range. Each sample preparation was analyzed three times. The D/L ratios for up to seven amino acids were calculated by dividing the computer integrated peak areas and then dividing by the standard ratio to adjust for fluctuations in instrument performance. It was not

Quaternary Littoral Zones possible for the integrator to calculate accurate peak areas for all amino acids in each sample. Shells with greater amounts of threonine deterioration showed interference with alanine peaks. Shells with a greater concentration of serine, proline, glycine, isoleucine, threonine and possibly bacterial amino acids such as ~-alanine, produced chromatograms with peaks obscuring the o-leucine peak. WehmiUer (1984) reports similar experience with leucine DIE analysis. Valine converts from the L form to the D form very slowly, so the D peak is very small compared to the rest of the chromatogram and is susceptible to baseline disturbances, and crowding by larger peaks. The most suitable amino acid was aspartic acid which eluted in an isolated position on the chromatogram, and often occurred in higher concentration than the other amino acids. Leucine is reported also, although the ratios are not as consistent as aspartic acid because of poorer chromatographic separation. The main use of the leucine ratios is to compare them with the results of others, such as Wehmiller and Belknap (1982), in order to utilize their non-linear kinetic models based upon leucine racemization, For each amino acid D/L ratio determined, the mean and standard deviation were calculated for the three runs. If the standard deviation for aspartic acid was greater than 0.0900, additional runs were performed for that sample, and the mean ratios were recalculated, Standard deviations fell between 0.0400 and 0.0005 for most of the samples. The % error between runs can be calculated by dividing the standard deviation by the mean D/L ratio and multiplying by 100. Obviously a higher ratio (e.g. 0.8000) gives a smaller % error than a lower ratio (e.g. 0.1000) with the same standard deviation. Aspartic acid racemizes at a greater rate than the other six amino acids which are routinely analyzed, and also tends to demonstrate a lower standard deviation between runs.

Sampling of Specimens Listed below are the 15 species of molluscs analyzed in this study: Phylum Mollusca Class Bivalvia Class Gastropoda

Amiantis purpurata Aulacomya magallanica Brachidontes rodriguezi Chlamys p a t r i a e Glycymeris longior Mytilus edulis Pitar rostrata Protothaca antiqua Samarangia exaibida

Adelomedon ancilla Buccinanops sp. Crepidula dilatata Lucapinellahenseli Patinigeriamagallanica Zidona dufresnei

Variation in D/L ratios of amino acids of different genera of the same age have long been recognized (Wehmiller, 1980, 1982). The differences are largely inherent in the shell structure. Wehmiller (1980) hypothesized that the difference observed in the relative rate of racemization between genera is caused by

217

the greater stability of aspartic acid and peptide bonds in carbonate matrices. That is, genera with a greater proportion of aspartic acid, and containing a greater proportion of 'hydrolysis resistant' peptide bonds are expected to racemize more slowly than genera with less aspartic acid. In our study we compare and utilize a variety of species of several genera of bivalves and gastropods. We recognize and expect that there will be variations of D/L ratios of amino acids for different species of the same age. However, it was anticipated that the variation would not be enough to mask trends or groupings of D/L ratios between species with considerable age differences. It would be desirable to cornpare the different rates of racemization of each species when analyzing D/L ratio data, but this information is available for only three of the 15 species used. Longterm experiments are presently being conducted by this laboratory to determine the relative rate of racemization of many of the molluscs analyzed in this study. Less well understood is the variation of DIE ratios found within a single specimen of a certain species. Although subtle differences in temperature history and burial history may contribute to the variations observed, the principal cause is variations in the proportion of each amino acid. D/L ratios and the absolute concentrations of amino acids can vary from one anatomical region of a bivalve to another. Hare (1963) demonstrated this clearly with Mytilus californianus. Certain acids were absent in the calcified parts whereas organic matrices from the calcite layers have a consistently higher ratio of acidic to basic amino acids than the aragonite shell units. The uncalcified shell units, periostracum and outer ligament, have very few acid residues. However, certain species of mollusc show more variation than others, and in some the variations are so slight that any anatomical part can be used to get an accurate reflection of the amount of racemization that has taken place since death. Until there is a better understanding of the cause of DIE variation and the amount to be expected in a certain anatomical part, it is best to sample from the same region of the shell in order to produce consistent results. Among others, Brigham (1985) found that the hinge area gave the best results and considerable scatter of results is obtained if samples for analysis are taken from the anterior or posterior growth edge. Wehmiller et al. (1978) sampled from a single structural layer to achieve consistent results in layered molluscs such as Protothaca. As Mytilus was one of the most common molluscs sampled, it was used to test intershell and intrashell variations in aspartic acid D/L ratios of bivalves. A single valve of a modern specimen was sampled in five different anatomical parts and compared to the matching valve which was ground to powder and sampled as a whole. Also, several whole valves from the same sampling location were compared. Results showed very good reproducibility between specimens and insignificant intershell variation, except for samples taken from the posterior growth edge of the valve. Reproducibility was much poorer in older shells however, and even

218

N.

Rutter

though the growth edge was not included in the region routinely sampled for this study, Mytilus showed variation in apparent racemization rate within the sample site. To limit intrasheil variation within each sampling area in this investigation, we sampled from the same anatomical region, commonly the beak area or umbo in all bivalves, although in some of the smaller specimens it was necessary to use the entire valve (Fig. 2).

el al.

AREAS INVESTIGATED

San Blas Setting

The village of San Bias is located about 230 km east of San Antonio Oeste and about 200 km south of Bahia Blanca on Isla del Jabali. The area is characterized by a broad, relatively low relief area consisting of littoral sands and gravels and wind-blown dune sand, with elevations varying from about 4 to 12 m above mean sea level. Spring mean tide is 2 m and mean neap tide about 1.5 In (Fig. 3). [-" ..... "'-',~ e°~.r u ~ . On the basis of morphological expression and elev"k, ~ ~ ations, Witte (1916) defined five step-like coastlines: sp .... [ ~ Stages I, I1 and III of Pleistocene age and Stages IV and ~Iii ~ | V of Holocene age. The deposits consist of sand, gravel and abundant marine fossil remains. Stages I and 11 represent poorly defined coastlines with altitudes of c.~,~n~~.~,,h tdgJ >8 m above mean sea level whereas Stages III and IV Lip represent well defined marine terraces at about 2 and 8 m above mean sea level respectively. Stage V repreFIG. 2. Schematic representation of gastropod and bivalve shells sents modern beaches. showing regions sampled for analyses. Trebino (1987) analyzed the principal features of the San Blas area. He recognized three levels of marine terraces. The youngest terrace (Level III) is at about The gastropods used in this study show more 3 m above mean sealevel and yielded radiocarbon ages variations in D/L ratios of aspartic acid within a speci- between 217(/ _+ 110 and 3450 + 110 years BP. The men than the bivalves. Initially, Adelomedon ancilla remaining two, of Pleistocene age, are at about 10and Zidona dufresnei were tested. Samples were taken 11 m (Level II) and 12-14 m (Level I) above mean sea from the spire, body whorl and columella (Fig. 2). level. He also mentioned beach ridges at Isla del Jabali, These three regions showed significant intershell vari- 7 m high dated at 4100 and 5370 BP. Towards the west, ations in ratios by as much as 35%. However, of the undifferentiated mollusc remains yielded radiocarbon three values obtained for each shell, two of the ratios ages between 28,400 and 29,120 BP. They were were always similar ( + 8 % maximum difference). For sampled from fossil spits at about 9-10 m above mean example, Adelomedon ancilla produced the following sea level. results: Only two sites were accessible that contained suitable Apex of shell asp D/L = 0.15 molluscs for amino acid analyses. One is located at Body whorl asp D/L = 0.24 Faro Segunda Barranca (SB-I), 26 km south of the Columeila asp D/L = 0.26 village of San Bias on the coast, and another located The values for the body whorl and the columella are 23 km southwest of San Blas (SB-2, Fig. 3). At site within 6% of each other and are more likely to be SB-I, a wave cut cliff displays about 9 m of section reliable than the apex which differs by 35°/,,. above the modern beach (Fig. 3). The section consists This trend continued with other species of Adelo- of loess containing a palaeosol over 6 m of well-sorted medon analyzed, that is two similar values and one sig- and bedded beach gravels with some sand that overlies nificantly different. Most shells subsampled in this bedrock. The beach gravels are interpreted to repremanner showed less than 3% difference between the sent a foreshore facies of a transgressive sea. Molluscs two most similar ratios. The columella and the body are found along distinct beds at several stratigraphic whorl had the most consistent values. In all cases in this positions within the section. Samples for amino acid investigation we used the D/L ratios of aspartic acid in analyses were taken from the lower part of this section the body whorl. It should be mentioned that Trophon where whole shells in living position were embedded in geversiamus, although not reported here, showed ex- sand (Fig. 3). The other site, SB-2, exposes reworked cellent consistency in intershell ratios (+2%). silt and gravels over about 1.2 m of beach gravels All gastropods showed consistent amino acid content containing whole shells about 12-13 m above mean sea except for Crepidula dilatata. This species showed level, The beach gravels at SB-I are judged to be variable amino acid content, low concentrations and Pleistocene, part of a raised beach ridge that has been many shells were not large enough to provide extra truncated and eroded by a Holocene transgression mass for analysis (>500 mg). Although the results for (Stage I, Witte, 1916; Level I, Trebino, 1987). Later Crepidula are reported here, they should be interpreted loess was deposited, a soil developed, then more loess with_ caution, deposition and finally sand dune formation.

219

Quaternary Littoral Zones

GEOLOGIC MAP OF S A N BLAS AREA

..... 'Y" ,'/,¢/,.,(

....

"(/ /

F~Faro Segunda Bon'anco

•

LEGEND Sampled sites

PunlO Ruble

Dunes

:::': i;.....!....... • • o • . • ~ o~

Dunes Loess Soil Loess

,t

SB-I Sands 8 Gravels

Mode~'n beach ridge (Stage V )

Ho~,cel~ beach ridge (Stage IV ) L ~ ' ~

Pleistocene beaches ( Stocjes I~ll anti Ill ) Tertiary deposits

4' "'~*~ ~& -'.Segundo Barro~ca

~

~°o.

;B-I z

(~3 Samples ~"

L

Continenlal

Geologic mop after Witte (1916) COrt,M,Juli~noe~

0

~o4sI

FIG. 3. Geological and site location map, San Bias area. (Modified from Witte, 1916.

Results Mytilus edulis and Pitar rostrata were analyzed from the lower part of the section at SB-1 and yielded aspartic acid ratios of 0.80 and 0.65 and lower leucine ratios respectively (Figs 4 and 5). Buccinanops sp., Pitar rostrata and Zidona dufresnei sampled from what is believed to be at a stratigraphically higher position, gave aspartic acid ratios of 0.49, 0.59 and 0.54 and lower leucine ratios respectively. The M. edulis ratio indicates relative antiquity whereas the P. rostrata ratios, which are relatively high, indicate that they are about the same age but the stratigraphically higher sample slightly younger than the other. Z. dufresnei shows lower ratios than the others. Although it is

difficult to interpret the results because of the limited number of ratios available and the relatively wide range of racemization ratios, it is suggested that the ratios indicate two ages of Pleistocene deposits. San Antonio Oeste Setting San Antonio Bay lies at the head of San Maffas Gulf about 330 km southeast of Bahia Blanca (Fig. 6). It represents one of the common Patagonian embayments flooded repeatedly by marine transgressions. During these transgressions beach ridges were constructed, spits developed and wind action formed littoral bar-

Sen Bles :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: • I~o~oDl sp

[:

~ ~

X Zkkma dufresnet

~ i!i!iii!::iiiiiii•iiii!iiiiiiI::iiiiiiiiiiiiiiiiiiiiii!::iiiiii::iiiiiiiiiii!iiiiiiiiiiii.i ........................................iisi!!!!iiii!i

~ !~.::'E~.::"i i~~.~'~E.::'.::'ili~-.-'?.:?:'?~:" :'i!.::'~~:'?!~.~.::::~:i:i:i:i:!:!:~:~:~:~:i:i:-:i:i:i:i:!:~:~:~:i:i:i IIli:!.'i:i:i:.:?~i~i.:?.:?i?i?~?~?~i~il t;l'l~ ......................... i'lll'l J:II~ .............. i ........................... gJ...........................

0.3

0.4

0~,

0.6

i ........................... ",'......................... "~ 0.7

0.8

D/L. ratios of aspartto ~okl ~ 1 leuot~e

FIG. 4. Plot of D/L ratios of aspartic acid and leucine of each mollusc species according to site location, San Bias area.

220

N. Rutter et al. • 12

SB2

.................. o .................

"E

~

2

Aspavrtic acid

SB 1

1

ff

R~hge of data

l

i...................o ....................!

o'.~

o'.,

o'.~ I'~

FIG. 5. Plot

of

o'~

o'7

o'.B

D / L raHo$

mean ])/t. ratios of aspartic acid and leucine ot all molluscs according lo silc location, San Bias area.

riers. The Holocene drop in sea level left broad tidal flats, marshes and large sand banks flanking the main ebb channel. This particular geomorphologicai situation is responsible for its large tidal ranges: 8 m for spring mean tides and 5 m for neap tides, resulting in relatively warm water for the Argentine coast, Between 10 and 12 m above mean sea level, Feruglio (1950) found sands and gravels with molluscs similar to extant species in the peninsula where the city of San Antonio Oeste is situated, and on the peninsula east of the Bay. As the molluscs are similar to living ones, the deposits were thought to be postglacial, Angulo et al. (1979) recognized two different iithostratigraphic units: Baliza San Matias (Pleistocene) and San Antonio (Holocene) Formations, based on morphology, degree of lithification and stratigraphic position. The Pleistocene is typically a conglomerate with shells in a muddy matrix, whereas the Holocene forms distinct beach ridges composed of gravels in a sandy matrix. However, the San Antonio Formation yielded radiocarbon ages older than 27 ka BP (Fidalgo et al., 1980). During the 1984 IGCP Project 200 field meeting, San Antonio outcrops were visited. Discussion centered around the reliability of the radiocarbon dates and the degree of CaCO3 cement coating the pebbles in supposedly Holocene sediments. Many thought that too much cementation had taken place for them to be Holocene (Pirazzoli and Schnack, 1985). The area provided many sampling sites of a variety of forms. Two sites, assumed to be equivalent in age are located on abrasion platforms at below mean sea level, at Caleta Falsa (SAO-2) and Las Grutas (SAO-5) (Fig. 6). Whole shells were collected from highly indurated sand and beach pebbles, Most other sites are between 8-12 m above mean sea level. At SAO-1, Baliza San Matias, the section lies about 11 m above mean sea level and consists of sand dunes with anthropogenic middens that overlie up to 1.5 m of loess over a thick sequence of beach gravels (Fig. 6). The beach gravels dip to the northwest, are well to moderately sorted, and consist mostly of pebbles between 2-4 cm in diameter with a sand and silt matrix. Archaeological middens which have been 14C dated at about 2 ka BP form the upper part of the beach gravels. The section at SAO-3, Puerto de Vialidad,

about 6 km northwest of SAO-1, is at about the same elevation as SAO-1, and exposes about 8 m of beach gravels. The gravels are dipping northwestward, imbricated, well sorted and bedded, consisting mostly of pebbles between 2-4 cm in diameter. Whole, broken and highly abraded or worn mollusc shells were collected from the upper 3 m. Another location between 8-12 m above mean sea level is SAO-6, La Rinconada (on the west side of San Mat/as Gulf: Fig. 6). Here, a 50 cm section of beach gravels overlies a marine platform and underlies a soil developed in silt and fine sand (loess?) under sand dunes. Whole shells were collected from the beach gravels. Two sites are located away from the coast. SAO-4, Baliza Camino, is about 24 m above mean sea level (Fig. 6). Whole mollusc shells were collected from beach gravels that are overlain by reworked loess. The other site, SAO-7, is located on the highest beach ridges, presumably the oldest, at elevations between 23 and 25 m above mean sea level. Whole shells were collected at the surface as well as within sands and gravels on the ridges and in intervening depressions.

Results" Eleven species of molluscs were collected from the San Antonio Oeste area for amino acid analysis. These includeAdelomedon ancilla, Amiantispurpurata, A ula-

comya magallanica, Brachidontes rodriguezi, Buccinanops sp., Chlamys patriae, Crepidula dilatata, Glycymeris longior, Mytilus edulis, Pitar rostrata and Samarangia exalbida. Typically, not all of the species are represented at each of the six sites. In addition, complications in interpretation are exacerbated by resistant mollusc shells that can be commonly reworked. Although Figs 7 and 8 display a wide range of D/L ratios for the various forms analyzed at the various sites, there is a certain amount of order that aids in relatively dating the various beach deposits and ridges. Three species, C. patriae, S. exalbida and G. longior are particularly useful and represented at most sites. With the exception of site SAO-5, the D/L ratios of aspartic acid are fairly well grouped varying from 0.430.62 for C. patriae, 0.60-4).77 for S. exalbida and 0.740.88 for C. longior, whereas leucine ratios show more of a spread. This suggests that all sites except SAO-5

221

Quaternary Littoral Zones

GEOLOGICAL SKETCH OF SAN ANTONIO OESTE AREA

I~ 9

44041. 40041'

+

~lO-ll ,w.i.

~...,

/-~

m

,~'_~.

~'..-

U

--

40~41'

~

~

• SOmpldesitll SandytidalLEGENDflots

r",'l

( e a l i z a S a n MaliOS F o r m a t i o n ) Formotiafl - Miocsfle

Qs°9'

m

64041'

j

¢,wo,i ~,k~e~Ikl

0

FIG. 6. Geological and site location map, with selected sections, San Antonio Oeste area. (Modified from Angulo et al., 1979.)

San AntonJo 0este

I~.......... ..- ..................... ...,..=............... p e a ::::::::::::::::::::::::::::::::::::::::::::::

24/Is~041

.............................

"!::'~-" . . . . . . . . . . . .

+e

~

¢. +

¢

mLb...,,..:l :::::: • ~ ' ~ ' ~ ' ~ "

. . . . . . .

t

=m,

e A m f a m f l s IZ~.Lrom'ata

JsJsJ~ ~:~;~::;;?::~::~:~::~i~!~:~;=:~:~:~:~;~i~;~::i!:~i.~.;;~.~.~.~:~:~+.~.~.:.~.~.~.~:~:::~i ::~i *..~.,p~,p~'~" ~::~::i :~-~ ,o~,,o ~!~i::ii ~i:=::i~i:=ili~i i i !!ii!i~i!i i::~i;:i i ~i~:=l" • " !:=i~!!i~ili!iiiii~ ° ~ , . m , o a ~ . -

!::!::i::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::i::::iiiiiii::ili::E~3iii::i!iiiiii::::::i ::i::i::i::i::i::]"~" -~ ol~ ::...... ~i::ii!iiiill ~:=:::::::::::! .................... ! ........ ::::::::::::: ........ ::::::::::::::: .~,~,-~,,, L~

::!:!:!:!:i:~:i:~:~:~:~:~:~:~:~:~:!:i:~:!:!:!:!:!:!:!:i:i:~:~:i:~:~:~:~:~:i:!:!:!:!:!:!:!:i:i:i:~:~:~:~:~:~:~:~:!:!:~:!:!:!:!:!:!:!:!:!:!:i:i:i:~:~:~:~:~:~:i:i:L~m ~

oo.

,

o~

,

o,

,

,

o'~

DIL ratios of aspart~

0'8

~=

rostrata

+ S ~ t n ~ r a n ( l t a exalbi(~q

,I~M

FIG. 7. Plot of D/L ratios of aspartic acid of each mollusc species according to site location, San Antonio Oeste area.

llS*O 7 ::::::::::::::::::::: _ .......v.....v...-.-...-.-.....v c

IIS~ [i

1 I: . . . . . . u i.':':':':':':'~.' . . . . . . . . . . . .

• X -..-. . . . . . .

sm +'Z:::::::::::::~:q . = .....=...........,_:.:.:.:.:..........,:.:.:.:.:..~ ÷

O .............

x AullmimUl

El~l~illink:a

. . . . . . . . :.:.:.:. -" =============================1' • Bu©c/lanODS $p

:::::::::::::::::::::::::::::::::::::::::::::::::::::::, ~:~:~::~::i:~i~i~i!i i!i!i!i!il !i!!!!!i ii!ili ilii !if!i!ii!iiiliiiiiiiiilili iiii ii]

0

~!i!:i:!:!:i:i:~!~!:!~!i!~!:!~!:~i~i~i~i~i~i~i~!~i:!~i~!~i~i~i.~i!i-~i!i!~!~!~!~!~!~!~:~i~i~i~!i~i~i~i~i~i~!~i~i:!~!~!~!!!~!!~:!:~!~!~!:!~!`

o.o

o.2

o.4 D/t. ratk/of

o.~

.°.o.,.,.,.,.,.

[] ~g H1us ,dulis W Pitar rostrata ÷ Samaran9 ~ exalbida

o.e

ku~fne

FIG. 8. Plot of D/L ratios of leucine of each mollusc species according to site location, San Antonio Oeste area.

222

N Rutter et al.

are approximately the same age. Interpreting the mean O/L ratios of aspartic acid and leucine for all forms as shown in Fig. 9, the case could be made that sites SAO-4, 7 and 2 are somewhat older than sites S A O - 1 , 3 and 6. SAO-5, Las Grutas, an indurated platform, appears to be the only site that is relatively young. It is concluded, therefore, that there must be three ages of beach deposits in the San Antonio Oeste area. The platform at SAO-5 is relatively young although it was previously interpreted by the authors as being probably relatively old based on topographic positions and its relationship to the indurated platform at SAO2. The beaches at SAO-1, 3 and 6 are judged to be the same age but older than SAO-5, whereas the platform at SAO-2, the beach deposits at SAO-4 and the beach ridges at SAO-7 represent the oldest littoral zones in the area. Shells that have been ~4C dated from SAO-4, Baliza Camino, yield dates of greater than 27 ka BP (Fidalgo

24-

SA0 4

i

SAo7 I

°-

....

........................

0'.0

0:2

~......... ...-"

i

Setting

Caleta Valdes is located on the east coast of Peninsula Valdes about 230 km southeast of San Antonio Oeste. The area consists of a series of northwestsoutheast trending beach ridges flanked on the west by a pediment below the Patagonian plateau (Fig. 10). The beaches are typically pebbly, with the younger beaches truncating the older and displaying a well developed set of ridges. Spring tide range is about 5 m

............

-- ........

i............. O -.~ ....... ~

I

~

Caleta Valdes

=,

I

-°~ 10-

et al., 1980). The D/L ratios suggest that the beach deposits at SAO-4 are very old, much older than 27 ka. Therefore, it is suggested that the highest level ridges and deposits at 24 m and the platform at SAO-2 is 'older' Pleistocene, the 10 m beach deposits are 'younger' Pleistocene and the platform at SAO-5 is Holocene in age.

o

o ,

a

Leucine

•

Aspart~c acid

*

I

"

01'4

01.6

01.8

Hean D/L rat~o!;

FIG. 9. Plot of mean D/1. ratios of all mollusc species according to site location, San Antonio Oeste area.

/

4/

•CV - 5 p1

LEGEND

o

Morshes and mudflots

/

N.oo r,O0.. Pleistocene beochu

P2 I CV-3

j,~ ~" ~.~/

CV-2

~CV-1

4

~

Elongoted ponds

i

Pediments ond llope depolits

~

Tehuelche grovels

Ilkm

Cort, Monlco Tomo$

FIG. 10. Geological and site location map with topographic profile location (Fig. ll),,Caleta Valdcs area. (Modified from Fasano et al., 1984.)

Quaternary Littoral Zones and neap tide about 3 m at Puerto Madryn, about 70 km west of Peninsula Valdes. The first reference on the littoral environments of Caleta Valdes was made by Rovereto (1920). He described two beach ridges with an altimetric difference of 5 m, one due to old tidal action and the other related to present tidal conditions. He believed the difference in altitude was due to eustatic change or to lesser extent wave energy caused by less frequent winds from the south and southeast during the upper Pleistocene. Feruglio (1950) published a list of bivalves and gastropods from Punta Norte, found in the lowest raised beach ridge. Five beach ridges have been identified, although identification is tenuous because the four highest are roughly at the same elevation varying between about 26 and 28 m above mean sea level (Fasano et al., 1983; Fig. ll). They are separated on freshness of morphology and minor elevation differences. The highest beach, locally called System I, reaches over 35 m dropping to about 26 m seaward, which is about the same elevation as Systems II, III and IV. The youngest raised beach, System V, is between 8 and 14 m above mean sea level, Samples for amino acid analysis were taken from all beach ridges except from System II where no good exposures were available. At CV-1, whole shells were collected from within 1 m of the surface from a 7 m exposure of well rounded, well sorted beach sands and gravels of System I (Fig. 10). The beds dip to the southeast and are capped in places by loess. Site CV-2 is located in System I sediments also where mollusc shells in living position were collected from 2 m below the surface. Further along the coast, at CV-3, shells in living position as well as bits and pieces of shells were collected from within 3 m of System III surface. Further to the north, a variety of molluscs, both broken and whole, were collected from well developed, and 'fresh' TOPOGRAPHIC sw

223

looking beach ridges of System IV. Although elevations of System IV ridges are similar to the older systems, the ridges are less subdued, and display higher amplitudes than the others. CV-5 is located on the lower System V complex, which displays a well developed, 'fresh' looking series of gravel beach ridges with amplitudes up to 5 m. The only molluscs seen and collected were close to the sea and may have been deposited as a result of present-day storms.

Results Nine species of molluscs were collected from the Caleta Valdes area. They include Mytilus edulis,

Brachidontes rodriguezi, Pitar rostrata, Protothaca antiqua, Crepidula dilatata, Aulacomya magallanica, Buccinanops sp., Zidona dufresnei and Adelomedon ancilla. Not all species were collected from t h e s a m e site or from all sites. D/L ratios of aspartic acid and leucine plotted against elevations, as shown in Fig. 12, suggest that beach ridges of Systems I and III are the oldest and about the same age, whereas System IV is younger, and System V younger yet. C. dilatata, P. antiqua and M. edulis analyzed from most ridges show consistent ratio increase with increasing age (Fig. 12). The mean value of all specimens show the same consistent relationships (Fig. 13). Fasano et al. (1984) reported radiocarbon dates from shells sampled from low-energy deposits of System I, ranging between 41 _+ 4 and 34 +_ 1.7 ka BP. These dates are suspect and not compatible with the relatively high D/L ratios of aspartic acid and leucine. Fasano et al. (1984) pointed out that although several radiocarbon dates determined along the Patagonian coast by other authors are systematically coherent with these ages and suggest a mid-Wisconsinan sea level stand, this is a matter of debate because of the suspect dates. In Caleta Valdes the same authors suggested that the PROFILES

P 1

NE

m M

IC | 6 4 2

,m.m i

t I00

m

• 200

i

I 300

I

I 400

w

o

I

I 500

I

I 6(30

i

I 700

I

I 800

I

P2

Lz

z.4

~s

4.e

s.o

I 900

I

I I000

I

I IlOOm

E

7.;~

e.4

9.6

v

,o.~'t=

FIG. 11. Topographic profile across beach ridges in the Caleta Valdes area. See Fig. 10 for locations. (After Fasano et al., 1983.)

224

N. R u t t e r et al. Caleta Valdes

;.:.;.:.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.:.:.:.; i

~

`ispt~?!:~:~!i:]!~i!!!i~i~i~i:i!i!iii!i!iiiii!i!ii~!iiiii!~i~i~i~i:i~i~i~m~:::::::::::::::::::::::::::::::::::::::::~ • Adet0medon an¢illa C V l - 2 k,~ : :.:.:. :. :. :. :. .: .: .: .:.:.:.:.:.:.:. :. :. :. :. .: .: .: .:.:.:.:.:.:.:. :. : : : : : : : : : : : : ................................................... ::::::::::::::: × Aulle0mU.t. magallanic~

$tJstem I

. . . .

2 6 - 2 8 Sy s~:em Ill "V. 3 "E o~

~ 4 System IV ~V

, , , ,

....

:'

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: • ~l~*~t~l~..~s sp

~ .........ii'"'..'~" ;~'"~" l:.:,*..:.:-:-:*:-:.:-:.:.:.:,:.:.:.:.:-:.:.:-:.:.:.:.:.: :::::::::::::::::::::::::::::::::::::::::::::::::::::• c ~ _ ~ .............................

dit,ta*a

l.gp [i.ii. ~.i._....i....,..../. ,. . .

l+u [ : : ~ ! i ! i i i ! i i i i i i i i i i i i i i i i i i i i i i i ! i ~ Htjtt_!l~e~lis :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: m P . . . . . . ,~.ta -+12 SgstemV

CV5

i:!.~.~ ======================================================================================================================X ======~,d0n~ ==============dufresnei ========== leu ~:~.~::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

asp

t-...-.....-.-.-,.....,..-....,...,-...,.-...-..,-.-,..-,..,.-,,.,/.,,.,-,,/.,.-,-..,,.....-.,.-...,,-.-,..,..,,.,,,.w ,,,-..,,r 0.0

0.2

0.4

0.6

08

D/L ratios of Aspartic acid and Leucine

FIG. 12. Plot of D/L ratios of aspartic acid and leucine ()lI each mollusc species according to site location, Caleta Valdes arem

sv.*--

I

2 6 - 2 I Ivs'~imlll

j .~.

~

cv,-2

:Vi; I

i

:

•

'

: ." . -.n.--~

i

S V s ' ~ IV ~ 4

:

,

A s p a , ' ~ add

o

te4iotne

I

o ...................,."

•

i'"'": 4"12 g v s t m ~ V

i

I.-a.'.l

:vs •

0.0

FIG.

,~

13. Plot of m e a n

off.

= -

R~ge of 41t6

| .

02

-

.

-

0.4 Mere D/IL r ~

.

,

0.6

OJ

-

ratios of all mollusc species according to site location, Caleta Valdes area.

beach ridges could correspond to the Sangamon Interglacial, Absolute ages from System V beaches were reported by Codignotto (1983) and ranged from 5720 + 105 to 1330 + 80 BP. In addition, he published a mollusc radiocarbon date of 38.7 + 2.7 ka BP sampled from the highest beach ridge• From the amino acid data, therefore, Systems I and III are judged to be 'older' Pleistocene, System IV 'younger' Pleistocene and System V Holocene in age.

Bahia Bustamante Setting Bahia Bustamante is located in east central Patagonia, about 100 km northeast of C o m o d o r o Rivadavia (Fig. 14). The area consists of an irregular shoreline with many peninsulas and entrances. Coastal features include gravel beach ridges, dunes, marshes and abrasion platforms. Spring tide range is about 5 m, whereas neap tide is about 3 m at Comodoro Rivadavia. Three systems of Quaternary beach ridges have been distinguished by morphological characteristics, degree of consolidation of sediments, development of drainage networks and elevations (Feruglio, 1950). Cionchi (1983) named them as Systems I, II and III, respectively, each related to a transgressive event. In the most landward system (I) between 50 to 1000 m from the

coast, there is a series of littoral ridges forming a band with a maximum width of 2500-3500 m and reaching a height of 34-41 m above mean sea level. They display a high degree of erosion and are masked by younger detrital material and consist of sediments with a fairly high degree of consolidation. The second system, below and seaward to System 1, consists of a series of ridges, 25-29 m above mean sea level, displaying more pronounced relief and less consolidated material than System I. These ridges are at variable distances from the coast 2000-3000 m in the north and next to the coast at Caleta Malaspina. The pattern of this system parallels the present coastline• The lowermost system (III) is attached to present gravel beaches. Their ridges usually reach heights of 8-10 m above present mean sea level and are composed of loose sediments and features typical of modern beaches. Although there are no radiocarbon dates reported from this immediate area, there are dates available for shells from beaches thought to be equivalent from nearby areas. Samples collected by Codignotto (1983, 1984) yielded dates of 36 + 2 ka BP and 37.3 ___ 2.4 ka BP for the oldest beaches; 30.9 + 1.1 and 31.8 + 1.4 ka BP for the intermediate beaches and 2030 + 85 and 2880 + 90 BP for the youngest. In the Bahia Bustamante area, nine sites were sampled. The sediments consist, in general, of poorly to well sorted beach gravels. Most pebbles are between

Quaternary Littoral Zones ® Co.Conclor t;;z

225

)o o

LEGEND

~'\.,,

~l~pta e~

IL Lobe4

. Ezquerm

'

Bahia

~

Volley floor

System

Dunes

System I I

Marshes

~

System Ill J

o

Bustamonte

Present boochel I

J ........ ~__I

: ;

I Slope deposits

•

Sampledsites c°vered)

-5Penirmulo ~

,-:: '. laalasplnai

./'/

Volcanic rocks(total or partially

~

"

~

~enineu~ Aristizobol

/

jJ

-x~\G

?

I

SCALE

£k.

/ POrt.t~iC(I Tomas

FIG. 14. Geological and site location map, Bahia Bustamante area. (Based on Cionchi, 1983.) 2 and 4 cm in length. Shells are abundant in all locations, both broken and whole. Some were collected from near the surface in beach ridges whereas others were collected from sections well below the surface. At localities BB-1 (System I) and BB-2 (Systems I or II) a quantity of molluscs were collected from weathered areas on the beach surfaces (Fig. 14). At BB-3 (System II) molluscs were collected from a 2 m outcrop composed of alternating coarse sand and gravel. The deposit unconformably overlies volcanic rocks and underlies a well-cemented gravel deposit. At BB-4, 2 m of beach gravels were exposed below about 2 m of silty and stony silt. Molluscs were collected from the beach gravels. At BB-5, BB-7, BB-8 and BB-9 (System III) beach gravels are well exposed in section. In all cases, shells were collected from 1-2 m below the surface. The last two sampling sites were 4--5 m above high tide water. BB-6 is located (System I?) beyond the immediate area about 4 km southwest of site BB-7. Samples of mostly broken molluscs were taken from stream bank exposures - - one about 3 m of indurated gravel with overlying colluviated silts and underlying rhyolitic bedrock and the other from younger, loose gravels about 5 m thick, about 600 m downstream from the first sampling site.

Results Ten species of molluscs were collected from the nine

sites in the Bahia Bustamante area. Once again not all species were collected from each site or from all sites. Species include Mytilus edulis, Brachidontes rodriguezi,

Protothaca antiqua, Crepidula dilatata, Aulacomya magaUanica, Patinigeria magallanica, Adelomedon ancilla, Lucapinella henseli, Samarangia exalbida and Pitar rostrata. D/L ratios of aspartic acid and leucine plotted in Figs 15 and 16 show essentially two groups of ages. This is also apparent on Fig. 17 which displays the mean of all ratios of all species. It appears that samples taken from deposits or surfaces that could be either part of System I or System II are the same age as System I. This is substantiated further if the same species such as S. exalbida, P. rostrata and B. rodriguezi are compared between the two systems. There is little doubt that sites BB-5, 7, 8 and 9 are essentially the same age and all belong to System III, and are much younger than the older sites. The relatively low D/L ratios of aspartic acid from P. magallanicataken from the surface at site BB-1 could be from a younger shell that has been transported to the older beach surface. It should be noted that sites BB-2 and BB-3 are located on the geological map (Fig. 14) in an area mapped as System II beach ridges. On the ground it is difficult to determine which System you are actually on. Site BB-4 is a section well below the levels of System I and System II beach ridges. The deposits are judged to be part of the System I complex.

226

N.

Rutter

el ul

Bahia Bustamante

System

-

I

or System II

010

06

0.4

0:2

0.8

D/L ratios of Asprrtic acid

FIG.

Ii;. Plot of D/I

ratios of aspartic

acid 01 each mollusc

species according

Bahia

to site location,

Bustamantc

area.

BB 1 System I

BB6

BE2 Systrm I 883

or System II

BB 4

I-BBS

BB 7

0

H

System Ill

BE 8

BE 9

0.0

0.6

0.4

0.2

0.8

D/L rattos of Lcuctnr

FIG.

16. Plot of

ratios

D/L

of leucine

of each mollusc

Bahia

v

j

0

BE6

I

Bustamante

b-l.

I

a

Bahia

Bustamante

Be 1 system

to site location.

species according

by

:

i

BB 2

. .._...__..,.:

0

Lwcinc

.

Aspartic acid

w

o...i

l-4

+!+

i_

i_ i_

1

0

0

H

Ranngcof data

j

0

,..,.

0

&-I

. . . . . . . .

0.2

..,.

i

0.4

0.6

0.8

Mean D/L ratios

FIG.

17. Plot of mean

D/L

ratios of all mollusc

species according

to site location.

Bahia

Bustamante

area

area

227

Quaternary Littoral Zones

The radiocarbon dates available from System I and II beach ridges, mentioned above, are considered to be incompatible with the amino acid dates. The dates are suspect and much too low for the amino acid ratios obtained. System I beach ridges are believed to be 'older' Pleistocene. System III beach ridges are Holocene in age, supported by radiocarbon dates and amino acid ratios.

Puerto Deseado Setting The city of Puerto Deseado is located at the mouth of Rio Deseado, about 250 km southeast of Comodoro Rivadavia (Fig. 18). The north-south coastline forms a boundary with the open ocean and then swings westward along Rio Deseado. The spring mean tide at Puerto Deseado is about 4 m and the neap tide range about 3 m. Raised beaches and platforms are present within the adjacent area of dissected bedrock resulting in local relief of over 100 m. Five systems of beaches and marine platforms have been recognized (Feruglio, 1950). The oldest three, locally called Systems I (Cerro Laciar Terrace), II (Cabo Tres Puntas Terrace) and III (Cerro Alonso Terrace) form platforms observed well beyond the coastline. System I is approximately 170186 m above mean sea level, System II is 50-70 m below System I but above System III which is approximately 75 m above mean sea level. Their ages are unknown but it has been suggested that the highest two are Pliocene or early Pleistocene. Neither one conforms to the present coastline configuration. System III may reflect an older interglacial. The three lower beaches, locally called Systems IV, V and VI are at approximate elevations of 38-40 m, 20-25 m and 8-10 m respectively. These three beaches have been informally correlated with the three levels at similar elevations at Bahia Bustamante. No radiocarbon dates have been reported from Puerto Deseado. Codignotto (1983, 1984) collected

samples from Puerto Mazarredo near the boundary between the provinces of Santa Cruz and Chubut, 100 km northward from Puerto Deseado. For the 38-45 m high beach (System IV) radiocarbon dates range from 25 to 35 ka BP. Gravel ridges equivalent to the 20-25 m beach ridge level (System V) yielded dates of 31 to 34 ka BP and for the lower ridges, 8-10 m above mean sea level (System VI) 1550 to 6630 BP. At Punta Cavendish, sampling site PD-1 (System IV), about 4 m of beach gravels and sand are exposed at about 38 m above mean sea level. The poorly to moderately well sorted sediments consist of medium to coarse gravels. Both broken and whole mollusc shells were scarce. PD-2 (System V) consists of over 4 m of well rounded, southwesterly dipping beach gravels with pebbles, mostly between 2 and 4 cm in diameter. Whole mollusc shells were collected from the top 3 m of the outcrop. The lower beach, site PD-3 (System VI), about 10 m above mean sea level consists of well rounded, well sorted gravels, with pebbles mostly between 4 and 8 cm in diameter. Pebbles from this site were more rounded than those from PD-1 and PD-2, suggesting recycling of older sediments. Mollusc shells, some in living position were collected from the upper 1 m of the 2 m exposure.

Results Six species of moiluscs were collected from three sites at Puerto Deseado. None of the sites had all six species, but some were present at more than one site. Species collected include Adelomedon ancilla, Aula-

comya magallanica, Brachidontes rodriguezi, Crepidula dilatata, Mytilus edulis and Patinigeria magallanica. O/L ratios of aspartic acid and leucine plotted against elevation and therefore, age, show fairly consistent results with higher O/L ratios increasing with increasing age (Figs 19 and 20). The exception is the O/L ratio of aspartic acid of C. dilatata which has a relatively low ratio for the oldest system represented when compared with the results of the other species. However, in

r~o~s'w

r~4",,' w

~ m o . - . . ccmtourline ira)

Rood

o

1

,2

~

•

?--

d"

/

SCALE

S

~Grt ~. F I r ~ l

FIG. 18. Topographic and site location map, Puerto Deseado area.

228

N. R u t t e r et al. Puerto Oeseado

38-45

~ysl:en', IV PD I

x ::::::::::: : : : : : : : : :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: :: : : .A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .J. q l :::::::::. • A ~ | o m e d o n ~ncJna ...... . . . . . . . . . . . . . . . . . . . ....... ============================================================================ x #t~ll~¢omq~ r n a g ~ l l a n i c a ~i!~!i!~iii~:~i~i~?~i~:~i~:~:~:~:~:~:~:~:~:i:i:i~:i:i:i:i:i:i:i:i:.:i:i:i:!:i~:i:i:i:i:!:!:i:i:!:i:!:!:!:!:i:i:i:!:~:!?i:i:i~

• " -

Sgstem

V PD 2

rodriguezi

. na [ ~ l l n u s e d u l i s

~sp . . . . ......:,:.:,:.:.:.:.:.:.:.:.:.:,:.:.:.:.:.:.:.:................ 20-25

<> B r e c h k l o n t e s

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::/,~ ::::P::l:t:t:n:|:q::e::r:~::a:m::~::g::a:1::1::.:n:i:c::a:

~0

..-,-.....-.-.-.-.-.....-, .-. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . s:.:.:.:.:~:.:~:~:.:~:~:~:~:~:~:.:~:~:.:.:.:.:~:.:~:.:~:.~:~:~:.:~:!~:::~:::~::~:::::::::~::::~::::~::::::::~::::::::::::~::::::::.1

•

ii i i i i iii ii iiNiii iiiiiiii!ii iiiii!iiii!iiii!!iii!ii!iii!ii!i!iii!iiiiiii ii!!i i!U!i i !i

ili , iiiiiiiiS !iiiiii'iii,ii! iii!iii iiiiiiii,iili,iii!iiiiiiiiiiiiiiiiiiii iiliiiii !iilii'iiit

,.°

0.0

0.2

0.4

0.6

0.8

D / L r'&tJo$ o r ~ s p ~ r t J ¢ acid ~ n d leucine

F I G . 19. Plot of D/t. ratios of a s p a r t i c acid and leucine of each mollusc species a c c o r d i n g to site location, P u e r t o D e s e a d o area.

38--45 ~st~m~ IV PID 1

I

i

• o .................... ! I

,~, .i

•

Asl:a~ ~kl

a

L,win*

~......... . ,.-. I l l , lie *¢ d,:t, ~

20-25 $ust*m V PD 2

~. . . . .

i

,,

_

LM

~H 8-10

SvstemVI

PD3

i-.o.~ |

0.0

.

,

0.2

.

j

0.4

-

,

0.6

0.8

Ple~ aM r*~le of D/L ratbs of aslMrt~o ~ k l aM l e u ~

F I G . 20. Plot of m e a n D/[. ratios of all mollusc species a c c o r d i n g to site l o c a t i o n , P u e r t o D e s e a d o area.

general terms, D/L ratios of C. dilatata are commonly inconsistent and produce relatively low ratios for r e l a tively old forms. P. magallanica and B. rodriguezi, analyzed in more than one site, show a good relationship between increasing o/e ratios with increasing age. The relatively high ratio of A. magallanica, 0.76, from System V may be the result of analyzing a reworked sample. As mentioned earlier, radiocarbon dates have been obtained from molluscs of Systems IV, V and VI about 100 m northward of Puerto Deseado. The dates are finite and about the same for System IV (25-35 ka BP) and System V (31-34 ka BP). These are considered suspect and are believed to be too young when compared to the relatively high o/e ratios. The Holocene radiocarbon dates for System VI appear to be compatible to the D/L ratios, In conclusion, it appears that o/e ratios of aspartic acid can separate three ages of deposits, although Systems IV and V may be close to the same age. System IV is considered to be 'older' Pleistocene, System V 'younger' Pleistocene and System VI Holocene in age.

TierradelFuego Setting Tierra del Fuego is the most southerly area investigated. H e r e the Andes swing to the east forming a boundary with the Beagle Channel and South Atlantic

to the south and the southern part of the Patagonian Plateau to the northeast. In this region, Pleistocene glaciers have reached the Atlantic Ocean and, therefore, have had an influence on the evolution of littoral zone development although it is still largely unknown. In this study, only the area of Bahia San Sebastian on the east side of the Patagonian Plateau was investigated (Fig. 21). Tidal ranges are extreme - - over 10 m for Spring tides whereas neap tides vary between 5 and 9 m. Near the sea in the Bahia San Sebastian area there are a number of remnant beaches, lagoons and extensive tidal flats. Most of the beaches are between 1-5 m above mean sea level and commonly contain archaeological sites. Further inland there are beach ridges between 8 and 10 m above mean sea level and one identified at about 16 m above mean sea level. Nordenskjold (1899) first proposed a former sea level reaching heights of 20-30 m above mean sea level for Tierra del Fuego. Diatoms and Spongia needles were collected at 10 m above sea level at Bahia San Sebastian. He also recognized marine terraces at O'Brien Island (Western Beagle Channel), Porvenir (Chile), Ushuaia and other areas. O t h e r workers recognized evidence for sea level changes outside the immediate area mostly to the south. These include Andersson (1906) who recognized marine deposits at a height of 3.5 m and Halle (1910)who observed terraces at 6 m and 20 m height. Later Feruglio (1950) worked

229

Quaternary Littoral Zones

i ' i i Imo 68 00' 53°00' i •

'

Legend Sampled sites

m

Fluvial Depoelts(Holocenc) Supratldal Mudflats(Holocene) '~ Beachrldges(Holocene)

C.SonSeboltidn A A• a a • • a !

aaJ

:v

I.a Sara

•.

Sebastian

4

Formation

~-~

T~oeroSur Drift

~

Upper Tertiary

I~

Undifferentiated

(Pleistocene)

(Pleistocene)

AaAI se*oo' r3

GEOLOGIC

o

,o

I

MAP OF

THE BAHIA SAN SEBASTIAN

,,o

~,,.

8CAKE

Modified after Co(lignMtoand Ualumloalt981)

AREA

elmMvlemmlme

FIG. 21. Geological and site location map, Bahia San Sebastian area, Tierra del Fucgo.. (Modified from Codignotto and Malumian, 1981.)

in the area and made observations on past marine incisions, In the San Sebastian area, Codignotto and Malumian (1981) distinguished the Pleistocene La Sara Formation and the Holocene San Sebastian Formation. La Sara Formation consists of sandy gravels with shells radiometrically dated as older than 40 ka BP. The San Sebastian Formation consists of biogenic materials related to shorelines that have been dated at 1310 + 100 and 2990 + 100 BP. A study of the sedimentological evolution of San Sebastian Bay gives a maximum age of 5270 years for postglacial development (Isla et

sand deposits. The beach gravels are moderately to well sorted, consisting mostly of pebbles 2 to 4 cm in diameter. Whole and pieces of mollusc shells were collected from 2 to 5 m below the surface.

al., in press),

beaches near the various sites at 1 and 2 m above mean sea level yield D/L ratios of aspartic acid and leucine between 0.01 and 0.05 (Fig. 22). At SS-1 and SS-3, O/L ratios of aspartic acid varied between 0.06 and 0.12. These are all Holocene sites that have been ~4C dated between 2 ka BP and 5 ka BP and are very close to mean sea level. The relatively high D/L ratio of aspartic acid of 0.36 derived from P. rostrata from the 16 m beach ridge at SS-3 (La Sara) shows a reasonable relationship in ratios with the same species at 3 m from SS-2. Unfortunately there are very few results to compare. The 16 m beach ridge has been t4C dated at 30 to 40 ka BP. These are similar to dates on other high level beaches but are considered suspect. In conclusion, it appears that the D/L ratios of aspartic acid are reasonable for modern and Holocene beaches. The relatively high ratio obtained from the site at SS-3 suggests that the beach is Pleistocene in age.

Mollusc samples for amino acid analyses are relatively scarce, so that only samples from a few sites were collected. At SS-1, the YPF archaeological site, whole molluscs were collected from a 60 cm section consisting of coarse beach sands about 2 m above mean sea level, The site has been dated at 5 ka BP and is situated south of a lagoon and behind regressive beach ridges that decrease slightly (less than 1 m) in amplitude seaward over a distance of about 800 m. Whole mollusc shell samples were collected from the modern beach in order to compare D/L ratios with those of the SS-1 site (Fig. 21). At SS-2, La Angostura, 20 km northwest of San Sebastian, whole shell samples were collected from an eroded section near the high tide mark. Site SS-3, about 9 km southeast of Estancia La Sara, is located in a gravel pit about 16 m above mean sea level. The pit exposes a 5 m section of foreshore beach gravels and

Results Six species of molluscs were collected from the various sites in Tierra del Fuego. These are Adel-

omedon ancilla, Aulacomya magallanica, Mytilus edulis, Patinigeria magallanica, Pitar rostrata and Chlamys patriae. Samples analyzed from modern

230

N. R u t t e r

et

al.

Yierra del Fuego

ss~

~,~:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: =~xt~anm~

~'

i iiiii !iiiii',i i',i',i',iii',iiiiii;ili ',!ii i',i',i',i',i',i i!i i',i;'i,iiiiii!i;i' i i ',i ,iii',i',iii',i',iiil ;:~iii];]i]i[![i::i............:.:.:.:::::::::: [i[i~i]i~i]i]i]i]i::i][]~]~]i ] ~]]]]]`]::i ] i ] ]~i : :i i ] i [ i ] i ] i : :i ] i : :i ] i ] i ] i ] i : :~:::::::]]]i i ] ]]i : :~]i ]i::i[ii]i]i]i]i]i]]i]i::i::i~i]i][:::[i]]]]]]~]~::]i[i]i~i:~ ::::: :::::::: ;:.:.:¥:.:.:::::: :::::: :::::::::::::::::::::::::::::::::::::::::: :5::::;::::::::::::::::::::::::: •" " "

o- Hodern

" .' " "

• ~I .................................................................................................................... ~!il ============================================================================================================================================================================== O.0

0.05

O.t

0 15

0.2

0.25

0.3

0,35

D/L ratios of Aspertic acid ~nd Leucine

F I G . 22. Plot o f D/L r a t i o s o f a s p a r t i c acid a n d Icucine of e a c h m o l l u s c s p c c i c s a c c o r d i n g to site l o c a t i o n , B a h i a S a n S e b a s t i a n a r e a , T i e r r a del F u e g o .

CORRELATION A N D R E L A T I V E A G E S OF L I T T O R A L ZONES Correlation and relative ages of littoral zones along the Patagonian coast is based upon the relative position of beach ridges above mean sea level, and the similarity of mean values of aspartic acid ratios on all specimens at a particular site. As has been demonstrated, relatively high mean ratios do not necessarily have to have been obtained from the highest beaches. Beach deposits at a relatively low elevation may be part of the same complex as beach ridges at a higher elevation, One of the principal factors affecting the D/L ratios of a fossil of a particular age is the temperature history that the specimen has been subjected to since death, The sensitivity of the analytical methods employed in this investigation appear to have masked any noticeable difference in D/L ratios due to varying temperature histories. Today's mean annual ocean coastal temperature along the Patagonian coast varies from about 13°C at San Bias to about 7°C at Tierra del Fuego (Sverdrup et al., 1942). Mean annual air temperature is about the same or slightly higher. From San Bias to Puerto Deseado where most samples come from there is only about a 3°C mean annual air and ocean temperature difference. It is unknown how long the specimens have been subjected to coastal water and ground temperatures, and to temperature differences caused by clima-

tic and circulation changes. Therefore, translating present data to the past is extremely difficult. All that can be safely said is that temperature history variation appears not great enough to question the D/L ratio groupings assigned to littoral complexes suspected of being the same age. The exception to this could be the Tierra del Fuego area where temperature variations are greater than areas further north but unfortunately there are few D/[. ratios available from older littoral zones of Tierra del Fuego to make comparisons. Table 1 summarizes the correlation schemes of equivalent Quaternary littoral zones observed along the Patagonian coast. This represents a first approximation - - not all complexes contained fossils and new complexes may be identified in the future. Three different aged littoral zones are recognized as well as the modern beach complex. The oldest zone is identifled in five of the six areas. D/L ratios are similar on the highest Quaternary beaches recognized in each area as well as deposits close to sea level that are considered part of the same complex. At Caleta Valdes, System I1 and III ridges at about the same elevation as System I ridges were considered younger than System I based on 'fresher looking' geomorphology. We now judge these three systems as being about the same age. At Bahia Bustamante, System II beach ridges at a lower elevation than System I beaches were considered younger, but D/L ratios indicate that these ridges are most likely

• 16-$$

SS 2

~o >

3

o

L~*ein*

•

Aspa-tk~ ~id

; i

; ]

R ~ l e o f data

3-

G

ss

1

,

i

0, i ....... o ........

0.0

i

0.05

0.1

0,15

0.2

0.25

0.3

0.35

Plean D / L r a t ~

F I G . 23. P l o t o f m e a n D/L r a t i o s o f all m o l l u s c s s p e c i e s a c c o r d i n g to site l o c a t i o n , B a h i a S a n S e b a s t i a n a r e a , T i e r r a del F u e g o .

231

Quaternary Littoral Zones

TABLE 1. Correlations and relative ages of littoral zones at various locations along the Patagonian coast, Argentina. The mean ratios are for aspartic acid SAN BLAS ~eLttive Age Name

SAN ANTONIOOESTE

(m) Pioan Nwlle Ratio SAO 4

Oldest

~termedtate

SB 1

SB2

Young

2

12

.73

.54

CALETAVALDES

(m) Hem Name Ratio 24

.72

(SI) CV 1-2

BAHIA BUSTAHANTE

Eievati~ Hean Name (m) Ratio 26-28

.65 .(;9

SAO 7 SA02

24 0

.64 .69 (SIII)CV3 26-28

SAO I

I0

.62

SA03

10

.55 (SIV)CV4 26-28

SAO 6

I0

.65

SA05

0

.34 (SV)CV5

12

Pkdern

:SI) BB :Sl) BB6 ',SII)BB 2 ',SII)BB3

Elevatio Hear Name (m) Rattc 34-41 34-41 25-29 25-29

.73 .81 .74 .74

SII)BB4 25-29

.68

.43

.15

PUERTODESEADO

Elevatiol~Hem Nane (m) Ratio

SIV) PD' 38-45

CSV)PD; 20-25

ZSIII)BB5 8-10

.29

ZSIII)BB7 8-10 ',SIII)BB8 8-10

.28 .22

~SIII)BB9 8-10

.2!

TERRA DEL FUEO0

SVI)PD3 8-10

(In) Heart Ratio

.66

.57 I TDF3

I(;

35

TOF2

3

.10

TDF I

3

.10

Plodern

0

.03

.30

eThe meam ratios are not the onkj eriter~m used tn assigning the relative position of litto?81 zones. The reader sh~uM refer to the ~ s i o n

of results

far each ~rea.

similar in age and, therefore, we consider the age of System II beaches the same as System I. The intermediate aged littoral zones are not as well defined as the oldest. Elevations or ratios vary between locations but considering all criteria the evidence favours an intermediate aged littoral zone (see discussion of results for each location). At San Bias, even though P. rostrata is found at both sites and has similar aspartic acid ratios (although slightly lower for the higher site) the elevation of the ridge, 12 m, is considered too low for the oldest littoral zone. The mean aspartic D/L ratios at San Antonio Oeste from the intermediate aged zone are similar to the oldest; however, the means are slightly lower. They are from beach deposits that fall just below 10 m ridges, once again too low for the oldest deposits. Admittedly, the evidence for calling these deposits younger than the oldest is weak. It may be that the ridges are younger but the deposits are equivalent to the oldest beach ridges and deposits in the area. At Caleta Valdes there is little doubt of an intermediate aged zone. Although the mean aspartic acid ratios are relatively low, individual species are higher and/or close to what is recorded from San Bias and Puerto Deseado. No intermediate aged littoral zone can be argued for at Bahia Bustamante. At Puerto Deseado, slightly lower D/L ratios of aspartic acid were obtained from molluscs in each deposit occurring below a platform that lies between 20-25 m above mean sea level than those for the highest deposits between 38-40 m above mean sea level. Once again, even though the D/L ratios are similar, intermediate aged deposits are suggested based mainly on their stratigraphic and geomorphic position. The results from Tierra del Fuego are not easily correlatable with the locations further north. This stems from very limited data, the possible influence of lower tempera-

ture histories for fossils and, therefore, lower D/L ratios for similarly aged molluscs from further north, and the influence of glaciation on sea level elevations through such processes as isostatic depression and rebound. All that can be said at this stage of our investigation is that beach deposits and ridges assigned to intermediate aged deposits crop out at about 16 m and are the highest deposits observed in the area. More amino acid analyses are needed, as well as a better understanding of the evolution of the area, before a more conclusive interpretation can be made. The youngest deposits (not including modern beach ridges) form ridges at about the same elevation (810 m) in many localities along the Patagonian coast. At some locations no fossils were available for dating whereas in other areas samples were taken from near sea level in the lower parts of the beach ridges or from similarly aged sediments in mud flats. Ratios are much lower than those from older deposits and so leave little doubt of littoral zones of considerably younger age. The ratios from Tierra del Fuego are the lowest. This is not unexpected considering the lower t e m p e r a t u r e histories that the samples have probably experienced. Modern forms have been analyzed at Tierra del Fuego for comparison. The low ratios that were obtained are what would be expected for modern beach ridges. G E O C H R O N O L O G Y OF T H E L I T T O R A L ZONES As mentioned throughout this paper, radiocarbon dates of molluscs have been obtained on many deposits below various beach ridges along the Patagonian coast. Most of the oldest dates have been between 25 and 40 ka BP. A few have been infinite dates, the greatest being beyond 40 ka BP. In addition, many Holocene dates have been obtained from the younger deposits.

232

N,

Rultcr

el a/.

The authors believe that the older ~4C dates are suspect

!

~

~

and are most upon likelythe older than what obtained. This is based following: (l) has mostbeen samples were determined on a liquid scintillation counter at the University of B u e n o s Aires. A high pressure counter

46-0.83_~ \ \ ~ ' J ~ x - - ' " ~ ~ 0.0.63_ ~ 0.7,u 0.78-

would have probably extended the dates b e y o n d the

£

limit of radiocarbon dating; (2) the oldest dates do not consistently date what are believed by other criteria, such as position above sea level, to be the oldest deposits in the area: (3) the high amino acid ratios arc judged to indicate much older ages than those of the older finite ~4C dates; and (4) dates on what arc believed to be Quaternary deposits associated with the highest sea level rise st) far identified in the area. the

ilI -0.75-°s° -045 -0.55 -e. --0.65

(:3

~,

g <

0.86 -

-- 0 . 8 5

50

,oo

~5o Time

: i 200

-~

'70 250

soo

ss0

400

(10 s yr B.P.)

FI(J. 25. Non-linear kinetic illotlcl for I'rotothaca modilicd from Wehmillcr and Belknap (198;2) to accommodate the comparisons of aspartic acid i;h ratios to Icucine D/i ratios for temperatures applicableto the Bahia Bustamantc and ('aleta Valdes areas.

Sangamon Interglacial or older, arc m u c h too young, We are presently analyzing material that we have dated by ESR and U/Th methods and which will bc reported later. At the present time, the amino acid ratio results and Holocene ~aC dates give the best closer to 13°(L As thc E)/I ratios of aspartic acid of indication of absolute age. Ages can be estimated for Protothaca from the oldest deposits vary from about the ratios obtained on the intermediate and oldest (I.58 to (}.76, this would make the deposits most likely deposits by adapting and modifying the non-linear well over 100 ka BP if 13°C is chosen its the average kinetic model of Wehmiller and Belknap (1982) for I)/I temperature history (Fig. 25). Therefore, it is suggested ratios of leucine on Protothaca. These workers pre- that the oldest deposits are older than Sangamonian in ferred to use D/L ratios of leucine in their studies. We age. compared our D/L ratios of aspartic acid to leucine, in Although there is a paucity of amino acid data for order to verify consistency of results. 50 data points Protothaca, a lack of convincing evidence for the were plotted from 13 species of molluscs collected from presence of intermediate aged deposits, and the unthe Bahia Bustamante and Caleta Valdes areas (Fig. certainty ot: estimating ages for lower ratios from the 24). The results are relatively consistent, including the kinetic model, it is suggested that the ratios of aspartic identification of the ratio reversal predicted by Kimber acid, 0.44 and (1.52, lrom the intermediate aged et al. (1986). As our aspartic acid ratios are considered deposits from Caleta Valdes arc over 50 ka BP. This more reliable and consistent than our leucine ratios, would then date the deposits as perhaps Oxygen we plotted our range of aspartic acid ratios for Isotope Substage 5c. Protothaca to Wehmiller and Belknap's (1982) timeThe '4(7 dates obtained in the youngest deposits temperature areas (Fig. 25). As discussed under the associated with beach ridges. 8-12 m above mean sea correlation and relative age section, the average level, all fall into the tfoloccnc range. Although the lemperature histories of our deposits tire difficult to resolution of the non-linettr kinetic model is poor for estimate. However. it is reasonable to assume that the}.' Protothaca with low ratios, there is no reason to suspecl would fall somewhere between 8 ° and 17° and probabl~ that the low ratios obtained arc not Holocene (Fig. 25). These ratios therefore, arc believed to be in the right order of magnitude for ttolocene ages and are sup~.o~ ported by the laC ttolocenc dates available. ..= ° ° ' . ~ i" ~ ,~ ~ o.s.

~

CONCLUSIONS

•

The objective of lhis investigation wits to elucidate the evolution of the Quaternary littoral zones on the Patagonian coast. Six ;,ll-Cas were chosen based on the

0.4.

o.o.

0.0

.

0.2

.

' 04 0.6 t,u=~n,t)/t

' 0 e

"

~o

Nott= tht. au:p.irti¢ aoid r , v , r s a l (KimbE.r. Griffin and Niln,s 1986) which

requir.,t-,gedirftrentl9,lop,dlines.... for0.0,*0.2S~,~o~,,0a,,~ one

for 0.30 to 0.90 leucine D/L.

Leu¢ine D / L 0 . 0 to 0 . 2 5 :

9 = 0.039 + 3.462x - 5.696x'2

L,~o~,,0/L0.S0t.o.go: u=,.e,4x-0.,se.-2

R = 0.92

.=o.gs

FIG. 24. ('omparison ot aspartic acid and lcucinc I~;'l ratios tot 50 data points from 13 species of molluscs collected lrom the Bahia Bustamante and Caleta Valdes areas.

and dating events that affected the various areas. 'Fhis has been achieved in a preliminary' way .... with much more work to bc done. Results indicate that the highest level littoral zones identified are older than Sangamonian in age based on amino acid ratios. 'Fhis littoral zone coincides to Feruglio's (1950) marine Terrace IV, which he considered would not go back m age further than the 'last interglacial" (Oxygen Isotope Substage 5e). There is evidence, although not always convincing,

Quaternary Littoral Zones

for an 'intermediate' aged littoral zone. Beach ridges at lower elevations than the oldest ridges are present in m o s t a r e a s but the underlying deposits are commonly about the same age as the oldest beach deposits based upon amino acid ratios. There are exceptions and, therefore, the intermediate aged beach deposits and ridges are considered to be more than 50 ka old and perhaps equivalent t o t h e Sangamon and Feruglio's (1950) marine Terrace V. An argument could be made that the 'intermediate' aged littoral zone is midWisconsinan (Oxygen Isotope Stage 3) in age. This is unlikely because (1) The radiocarbon dates ranging between 25-40 ka BP should be considered minimum dates, and (2) the oceanic isotopic record clearly shows n o substantial warming in that age range (Shackleton and Opdyke, 1976). There is no doubt of a 'young' littoral zone with ridges usually between 8-10 m above m e a n s e a l e v e l found in most areas investigated. These are Holocene and a r e w e l l dated by radiocarbon and amino acid methods. Feruglio's (1950) Terrace VI would be equivalent to these. in a g e

The rather persistent presence of the Pleistocene emerged littoral zones at roughly the same elevations, as well as the Holocene raised beach ridges, suggest that glacio-eustatic contribution (i.e. changes in ocean w a t e r volume from deglaciation) is primarily responsible for the high relative sea level stands. Other factors, such as tidal configurations, tectonics (including e u stasy and modifications in tidal range), may explain some local variations in the elevations of the littoral zones.

Future work will center on more detailed investigations of the depositional facies of the littoral z o n e s , m o r e a c c u r a t e dating by amino acid, radiocarbon, electron spin resonance and U/Th methods, and elucidation of the cause and mechanics of sea level changes

in this region. ACKNOWLEDGEMENTS The authors acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. Field activities were partially financed by the Commission for Scientific Research of the Province of Buenos Aires (CIC) and the Argentine National Research Council (CONICET). We thank G. Lyons, University of Alberta, for determining the D/L ratios of amino acids, M. Aguirre, National University of La Plata, for identifying the molluscs, J. Kowal, University of Alberta, for computer entry, N. Catto, University of Alberta, for background material, and Vicky Bernasconi, Juliana Bo, and Monica Tom,is, the cartographers of the National University of Mar del Plata, for map preparation. Murray Larson of M. L. Larson Geological Consultants Ltd., Calgary, Canada and Marcelo Farengo, University of Mar del Plata, ably assisted the authors in the field. Dr. Jorge Rabassa, CONICET, kindly provided vehicle and accommodation support in Tierra del Fuego. Additionally, Ulrich Radtke thanks the Deutsche Forschungsgemeinschaft (D.F.G.) for financial support.

REFERENCES Andersson, J.G. (1906). Geological fragments from Tierra del Fuego. Bulletin of the Geological Institute of Upsala, VIII, 169-183. Angino, E.E. (1963). Discussion: Quaternary marine sediments and their geological dates with reference to the geomorphology of

Kronprins Olavkyst. Antarctic Geology, S C A R

233 Proceedings,

79-80. Anguio, R., Fidalgo, F., Gomez Peral, M.A. and Schnack, E.J. (1979). Las impresiones marinas cuaternaries en la Bahia de San Antonio Oeste y sus vecindades, provincia de Rio Negro. VII Congr. Geol. Arg. |, Buenos Aires, 271-283. Auer, V. (1959). The Pleistocene of Fuego-Patagonia. Part III. Shoreline Displacements. Annales Academiae Scientiarum Fennicae, Series A3, Geologica--Geographica, 60, 247 pp. Bloom, A.L. (1980). Late Quaternary Sea Level Change on South Pacific Coasts: A Study in Tectonic Diversity. In: Mrrner, N.-A. (ed.), Earth Rheology, Isostacy and Eustacy, pp. 505-516. Brigham, J.K. (1985). Marine stratigraphy and amino acid geochronology of the Gublik Formation, Western Arctic Coastal Plain, Alaska. Ph.D. Thesis, University of Colorado, 234 pp. Cameron, R.L. and Goldthwait, R.P. (1961). The US-IGY contribution to Antarctic glaciology. International Association of Scientific Hydrology, Publication 55, 7-13. Cionchi, J.L. (1983). Las ingresiones marinas del Cuaternario tardio en la Bahia Bustamante (Provincia del Chubut). Oscilaciones del Nivel del Mar Durante el Ultimo Hemiciolo Deglacial en la Argentina, IGCP Project 61, Mar del Plata, April 1983, pp. 1-26. Clark, J.A. (1980). A Numerical Model of Worldwide Sea Level Changes on a Viscoelastic Earth. In: Mrrner, N.-A. (ed.), Earth Rheology, lsostacy and Euatacy, pp. 525-534. Clark, J.A. and Lingle, C.S. (1979). Predicted relative sea-level changes (18000 years to Present) caused by Late Glacial of the Antarctic Ice Sheet. Quaternary Research, II, 279-298. Clark, J.A. and Bloom, A.L. (1979). Hydro-Isostasy and Holocene emergence of South America. Proceedings of the 1978 International Symposium on Coastal Evolution in the Quaternary, San Paulo, pp. 41-60. Clark, J.A., Farrell, W.E. and Peitier, W.R. (1978). Global changes in postglacial sea level: A numerical calculation. Quaternary Research,9, 265-287. Codignotto, J.O. (1983). Depositos elevados y/o acrecion Pieistocena-Holocena en la costa fueguino-patagonica. Simposio "Oscilaciones del nivel del mar durante el ultimo hemiciclo deglacial en la Argentina". CONICET, CAPICG, IGCP 61, Mar del Plata, pp. 12-26. Codignotto, J.O. (1984). Estratigrafia y Geomorfologia del Pieistoceno Holoceno costanero entre los paraleios 53 30' Sur y 42 00' Sur. IX Congreso Geologico Argentino, Ill, Buenos Aires, pp. 513-519. Codignotto, J.O. and Malumian, N. (1981). Geologia de ia region al norte del paralelo 54 S de la Isla Grande de la Tierra del Fuego. Revistade la Asociacion Geologica Argentina, 36, 44-81. Dott, R.H. (1976). Contrasts in tectonic history along the Eastern Pacific Rim. American Geophysical Union, Monograph 19, 299-308. Fasano, J.L., Isla, F.I. and Schnack, E.J. (1983). Un an~disis comparativo sobre la evolucion de los ambientes litorales durante el Pleistoceno tardio-Holoceno: laguna Mar Chiquita (Buenos A i r e s ) - - Caleta Valdes (Chubut). Simposio "Oscilaciones del nivel del mar durante el ultimo hemiciclo deglacial en la Argentina". CONICET, CAPICG, IGCP 61, Mar del Plata, pp. 27-47. Fasano, J.L., Isla, F.I. and Schnack, E.J. (1984). Movimientos relativos de la interfase continental/oceanica en la sector oriental de la Peninsula Valdes, Chubut, Argentina: Evidencias aportades por depositos litorales cuaternarios. International Symposium on Late Quaternary Sea-Level Changes and Coastal Evolution. IGCP 200, INQUA Abstracts, Mar del Plata, pp. 32-35. Feruglio, E. (1950). Descripcion geologica de la Patagonia Y. P. F. III. Buenos Aires, pp. 74-196. Fidalgo, F., Figini, A.J., Gomez, G.J., Carbonari, J.E. and Huarte, R.H. (1980). Algunas dataciones absolutas en sedimentos marionos de la Bahia de San Antonio. Simp. Probl. Geol. Litl Atlant. Bon., CIC, Mar del Plata, pp. 243-251. Fray, C. and Ewing, M. (1963). Pleistocene sedimentation and fauna of the Argentine Shelf, I: Wisconsin Sea Level as Indicated in Argentine Continental Shelf Sediments. Proceedings of the Academy of Natural Sciences of Philadelphia, 115, 113-126. Halle, Th. G. (1910). On Quaternary deposits and changes of level in Patagonia and Tierra del Fuego. Bulletin of the Geological Institute of the University of Upsala, IX(17/18), 63-117. Hare, P.E. (1963). Amino acids in the proteins from aragonite and calcite in the shells of Mytilus californianus. Science, 139, 216-217. Hollin, J.T. (1968). The Antarctic Ice Sheet and the Quaternary history of Antarctica. Palaeoecoiogy of Africa, 5, 109-138.

234

N. Rutter el al.

Hughes, T.J., Denton, G.H., Anderson, G.B., Schilling, D . H , Fastook, J.T. and Lingle, C.S. (1981). The last great ice sheets: a global view. In: G. H. Denton and T. J. Hughes (eds), The Last Great Ice Sheets, pp. 263-317. Wiley and Sons, New York. lsla, F.I., Vilas, F.E., Bujalesky, G., Ferrero, M., Gonzalez Bonorino. G. and Arche Miralles, A. (in press). Gravel drift, storm and wind effects over the microtidal San Sebastian Bay, Tierra del Fuego. Chapman Conf., Sediment Transport Processes in Estuaries, AGU. B. Blanca, 13-17 June, 1988. Kimber, R.W., Griffin, C.V. and Milnes, A.R. (1986). Amino acid racemization dating: Evidence of apparent reversal in aspartic acid racemization with time in shells of Ostrea. Geochimica et Cosmochimica Acta, 50, 1159-1161. Mangerud, J. (1972). Radiocarbon dating of marine shells, including a discussion of apparent age of recent shells from Norway. Boreas, 1, 143-172. M6rner, N.-A. (1984). Differential Holocene sea level changes over the globe: Evidence for Glacial Eustasy, geoidal eustasy and crustal movements. International Symposium on Late Quaternary Sea-level Changes and Coastal Evolution, IGCP. Mar del Plata, Sept 30-Oct 6, 1984, pp. 70-73. M6rner, N.-A. (1986). Global neotectonics, arcs and geoid configuration. In: Wezel, F.C. (ed.), The Origin of Arcs. Development in Geotectonics, 21, 79-9l. Nordenskjold (1899). Ober die postterti~iren Ablagerungen der Magellanl~inder. Wissenschaftliche Ergebnisse der Schwedischen Expedition nach den Magellanliindern 1895-1897, Stockholm. Occhietti, S. and Hillaire-Marcel, C. (1977). t4C Chronology of the palaeogeographic events in Quebec for the past 14,000 years. G~ographie Physique et Quaternaire, 31, 123-133. Peltier, W.R. (1988). Lithospheric thickness, Antarctic deglaciation history, and ocean basin discretization effects in a global model of postglacial sea level change: a summary of some source of nonuniqueness. Quaternary Research, 29, 93-112. Pirazzoli, P.A. and Schnack, E.J. (1985). Introduction: Late Quaternary sea-level changes and coastal evolution. Quaternary of South America and Antarctic Peninsula, 3, 161-173. Pitman, W.C. (1974). Sea Floor Age Chart. Geological Society of America. Porter, S., Stuiver, M. and Heusser, C.J. (1984). Holocene sea-level changes along the Strait of Magellan and Beagle Channel, Southernmost South America. Quaternary Research, 22, 59-67. Rabassa, J.O. (1987). The Holocene of Argentina: A Review. INQUA Programme with Abstracts, July 1987. Ottawa, Canada, p. 247. Rabassa, J.O., Heusser, C. and Stuckenrath, R. (1984). On-going research on deglaciation chronology and Holocene sea-level changes in the Beagle Channel, Tierra del Fuego, Argentina. In:

"'lnternational Symposium on Sea-level Changes". Mar del Plata, October 1984, Abstracts. Rovereto, G. (1920). Studi de geomorfologia argentina: V, La penisola Valdez. Bolletino della Societa Geologica ltaliana, 40(5), 147. Rutter, N.W., Crawford, R. and Hamilton, R. (1979). Dating methods of Pleistocene deposits and their problems IV: Amino acid dating techniques. Geoscience Canada, 7, 122-128. Schnack, E.J. (1985). Argentina. In: Bird, E. and Schwartz, M. (eds), The WorM's Coastlines, pp. 69-78. Van Nostrand-Reinhold Co., New York. Shackleton, N.J. and Opdyke, N.D. (1976). Oxygen-isotope and paleomagnetic stratigraphy of Pacific Core V28-239 Late Pliocenc to Latest Pleistocene. Geological Society of America, Memoir 145, 449-464. Sverdrup, H.U.. Johnson, M.W. and Fleming, R.1t. (1942L The Oceans. Prentice-Hall, 1087 pp. Trebino, L G . (1987). Geomorfologia y evolucion de la costa en los Alrededores del Pueblo de San Bias, Provincia de Buenos Aires. Asociacion geologica Argentina, XLII(1-2), 9-22. Wehmiller, J.F. (1980). Intergenetic differences in apparent racemization kinetics in mollusks and foraminifera: implications for models of diagenetic racemization. In: Hare, P.E., Hoering, T.C. and King, K. (eds), Biogeochemistrv of Amino Acids, pp. 341-355. Wehmiller, J.F. (1982). A review of amino acid racemization studies in Quaternary molluscs: stratigraphic and chronologic implications in coastal and interglacial sites, Pacific and Atlantic coasts, U.S., U.K., Baffin Island and tropical islands. Quaternary Science Reviews, I, 83-120. Wehmiller, J. F. (1984). Relative and absolute dating of Quaternary mollusks with amino acid racemization: evaluation, applications and questions. In: Mahaney, W.C. (ed.), Quaternary Dating Methods, pp. 171-193. Elsevier, Amsterdam. Wehmiller, J.F. and Belknap, D.F. (1982). Amino acid age estimates, Quaternary Atlantic Coastal Plain: Comparison with U-series dates, biostratigraphy and palaeomagnetic control. Quaternary Research, 18, 311-336. Wehmiller, J.F., Lajoie, K.R., Sarna-Wojcicki, A.M., Ycrkcs, R.F., Kennedy, G.L., Stephens, T.A. and Kohl, R.A. (1978). Amino acid racemization dating of Quaternary mollusks, Pacific coast United States. In: Zartman, R.E. (ed. ), Short Papers of the Fourth International Conference, Geochronology, Cosmochronology and Isotope Geology 1978, U . S Geological Survey Open File Report, 78-701, 445~48. Witte, L. (1916). Estudios geologicos de la region de San Bias (Partido de Patagones). Rev. M~eo de la Plata, XXIV, La Plata~ pp. 7-99.