# Rhythmites preserved in intertidal flat successions of the hypertidal Santa Cruz – Chico River estuary (southern Patagonia, Argentina)

Bernadette Tessier, Thibaud Lortie, José I Cuitiño, Jean-Yves Reynaud, Roberto A Scasso, Sebastián M Richiano, Léo Pancrazzi, Edward Salameh, Michel Condomines, Laurent Dezileau, et al.

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## To cite this version:

Bernadette Tessier, Thibaud Lortie, José I Cuitiño, Jean-Yves Reynaud, Roberto A Scasso, et al. Rhythmites preserved in intertidal flat successions of the hypertidal Santa Cruz – Chico River estuary (southern Patagonia, Argentina). Jean-Yves Reynaud; Eric Armynot du Châtelet. Short Papers from the 19th French Congress of Sedimentology, Lille, 84, Association des Sédimentologistes Français, pp.97-102, 2025, 2-907205-83-8. 10.70665/JFEB5133. hal-05005453

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Distributed under a Creative Commons Attribution 4.0 International License
RHYTHMITES PRESERVED IN INTERTIDAL FLAT SUCCESSIONS
OF THE HYPERTIDAL SANTA CRUZ – CHICO RIVER ESTUARY
(SOUTHERN PATAGONIA, ARGENTINA)

Bernadette TESSIER<fnref n="1" />, Thibaud LORTIE<fnref n="1" />, José I. CUITIÑO², Jean-Yves REYNAUD<fnref n="3" />, Roberto A. SCASSO⁴, Sebastián M. RICHIANO², Léo PANCRAZZI<fnref n="1" />, Edward SALAMEH<fnref n="1" />, Michel CONDOMINES<fnref n="5" />, Laurent DEZILEAU<fnref n="1" />, Malala LEKANDER⁶

Corresponding author: bernadette.tessier@unicaen.fr

**Abstract:** Rhythmites observed and cored in the hypertidal (12 m tidal range) Santa Cruz – Chico River estuary (Southern Patagonia, Argentina) are described. The estuary is made of two estuarine arms (the Chico River arm; the Santa Cruz River arm), both converging into an intermediate subtidal basin connected to the Atlantic Ocean. Tidal dynamics are dominant along the low fluvial discharge Chico River arm. In contrast, the water discharge of the Santa Cruz River is much higher so that the influence of tidal dynamics quickly drops down a few kilometers upstream. Extensive sandy to muddy tidal flats develop along the outer portion of the two arms. Upper intertidal sediment successions frequently reveal well-developed rhythmic layering, resembling tidal rhythmites. Cores (1m long) were retrieved in these successions, one from the tide-dominated Chico River arm, one from the tide-influenced Santa Cruz River arm. Core analyses show that the Chico River sediment succession is made of a fairly regular stacking of typical semi-diurnal sand-mud couplets and neap-spring-neap tidal rhythmites, and represents a 3-year continuous record. A seasonal modulation is probably superimposed to the tidal signal. Semi-diurnal tidal couplets are visible on the Santa Cruz arm succession, but the fortnightly cyclicity is altered, due to the combined influence of fluvial dynamics, westerlies-induced wave agitation and aeolian sand transport. 210Pbexc measurements confirm for both cores very high sedimentation rates. Although 137Cs data are difficult to exploit due to the very low fallout in the Southern Hemisphere, they could suggest that the successions are several decades old, meaning that the studied tidal flats are no longer aggrading.

**Keywords:** Tidal rhythmites, estuary, cores, hypertidal, fluvial influence, Patagonia.

# 1 GENERAL INTRODUCTION AND STUDY SITE PRESENTATION

Upper intertidal sediment successions within tide-dominated estuaries frequently contain well-preserved tidal rhythmites, recording basically semi-diurnal to neap-spring-neap cycles, eventually annual cycles in the uppermost part of the successions. Such rhythmic sediment records provide valuable information on hydrosedimentary processes and sedimentation rates (Tessier, 2023).

The rhythmic successions described here were deposited in the Santa Cruz - Chico River estuary (SCCR), located in South Patagonia, Argentina (Fig. 1A). The SCCR is a complex ria-type system combining hypertidal dynamics, high fluvial discharge, and wind-, wave-induced processes (Piccolo & Perillo, 1999; Pancrazzi et al., 2024; Tessier et al., 2024). In the details, the estuary comprises two arms, the Santa Cruz River arm and the Chico River arm, both converging into a 30 km – long subtidal basin connecting the system to the Atlantic Ocean through a 2 km wide narrow inlet (Fig. 1A). Tidal range reaches up to 12 m at Punta Quilla, located 4.5 km upstream of the inlet (Servicio de Hidrografía Naval: www.hidro.gov.ar). The tidal regime is semi-diurnal with very low diurnal inequality. Tidal currents are commonly faster than 1.5 m s<fnref n="1" />, reaching up to 4 m s⁻<fnref n="1" /> in the inlet. The flood is generally the dominant current, but the ebb is dominant in the downstream half of the intermediate basin. Fluvial discharge has little influence on the intermediate basin water levels and bottom velocities which are instead controlled by tidal dynamics. Along the low fluvial discharge (mean annual 30 m<fnref n="3" /> s⁻<fnref n="1" />) Chico River arm, tidal dynamics are dominant, with a tidal limit located at about 20-25 km from its mouth. In contrast, tidal dynamics quickly drops down a few kilometers upstream the Santa Cruz River arm due to a much higher water discharge (mean annual 750 m<fnref n="3" /> s⁻<fnref n="1" />). Moreover, water outbursts regularly occur (every 2-4 years) resulting in higher discharge peaks reaching up to 2500 m<fnref n="3" /> s⁻<fnref n="1" /> (Pasquini & Depetris, 2011). Sediment discharge is accordingly higher from the Santa Cruz River, especially for sand fraction but suspended sediment concentration is much higher in the Chico River tidal channels (Depetris et al., 2005). The Patagonian climate is characterized by intense and dry winds blowing almost permanently from the west. In the estuary, they provoke substantial sand transport on the tidal flats at low tide, and an important agitation on the water surface. Mean temperature in winter season (from May to September) is frequently below 0°C, with extreme less than -10 °C in July. It is around 20°C in summer (Nov.- Feb.) (Servicio Meteorológico Nacional Argentina: www.smn.gob.ar).

<footnote n="1">¹ UMR M2C CNRS, Univ. Caen-Rouen, France; ² CONICET-CENPAT, Puerto Madryn, Argentina;</footnote>

<footnote n="3">³ UMR LOG CNRS, Univ. Lille, France; ⁴ IGEBA CONICET, Universidad de Buenos Aires, Argentina ;</footnote>

<footnote n="5">⁵ UMR Geosciences Montpellier, France ; ⁶ Rio Grande, Tierra del Fuego, Argentina</footnote>
Along the edges of the outer portion of both the Chico and Santa Cruz River arms, extensive sand- to mud-flats develop.

<figure><img src="image_3.png" /><figcaption>Figure 1 – A: The Santa Cruz – Chico River estuary, in South Patagonia, Argentina (in Tessier et al., 2024). Red circle black dots indicate core location; B: zoom on the core C1 site (Google Earth image) sheltered behind a gravel barrier at the mouth of the Chico River arm; C: cross-section of the cored succession; D: zoom on the core C4 site (Google Earth image) 14 km upstream of the Santa Cruz River arm mouth; E: cross-section of the cored succession. White arrow for North on A, B and D.</figcaption></figure>

## 2 MATERIALS AND METHODS

In the uppermost intertidal part of the flats, below salt marshes, sediment sections made of heterolithic facies comprising sand-mud couplets are frequently exposed thanks to cutbanks along secondary tidal channels or creeks (Fig.1B to E). Some of these successions show clear cyclic patterns suggesting tidal rhythmites. Salt marsh successions may display as well very regular bedding corresponding likely to annual rhythmites (Tessier, 1998; Archer, 2013). In order to define the origin of the upper intertidal rhythmites, cores were retrieved manually, using PVC tube, at two sites on top of upper intertidal bankss: 1) at the mouth of the tide-dominated Chico River arm (Fig.1B, C). The site is sheltered behind a small gravel barrier (Core C1 – 114 cm); 2) at about 14 km upstream of the mouth of the tide-influenced Santa Cruz arm (Fig.1D, E). The site is fairly exposed to wind and wave agitation (Core C4 – 85 cm). On the basis of Sentinel-2 image examination, the elevation of the two coring sites can be set at about 10-10,5 m above lowest tide level, and the frequency of their inundation by tides is around 16% (cf. principle in Salameh et al., 2024).

Work on cores comprise sediment facies description, grain-size analysis (1 cm interval), and bed thickness measurements. 210Pbexc and 137Cs measurements were also performed.

## 3 RESULTS

**Upper intertidal rhythmite description:** Grain-size along both cores has a very narrow range centered around very fine sand (Fig. 2). High frequency fluctuations are related to the heterolithic nature of the successions. No significant trends are observed from base to top. Core C1 is slightly finer-grained along the upper 25 cm, only the surface being coarser. Along Core C4, the bottom is slightly finer, and the top is very slightly coarser with a better sorting featured by a narrower grain-size range.

Cores C1 and C4 both show sand-dominated beds alternating with mud-dominated beds – mdb (red arrows on Figs. 2, 3). Thickness of «mdb-to-mdb» packages ranges from a few mm to a few cm. Some of the packages clearly contain sand-mud couplets, of varying thickness and shape (examples: zoomed areas outlined in light blue, Fig. 2A-1 and 2B-1). These packages are typical tidal rhythmites recording neap-spring-neap (semi-lunar – or fortnightly) cycles. Other packages do not clearly show sand-mud couplets, and mdbs look amalgamated (examples: zoomed areas outlined in pink, Fig. 2A-2 and 2B-2). Such packages are well expressed in C1. In C4, mdbs are locally poorly discernible.

**Rhythmite thickness measurements (Fig. 3A, 3B):** The only individual sediment units present along the whole successions are the mdb-to-mdb packages, sand-mud couplets being discernible discontinuously. Thus, thickness measurements were applied only on the mdb-to-mdb packages. Thickness evolution shows a cyclic pattern (smoothed red curves on Fig. 3A, B) with successive thickening and thinning. The cyclic
pattern is especially well expressed along core C1 (Fig. 3A), with an average number of 7 mdb-to-mdb packages per cycle. Along core C4 a cyclic evolution is barely detected (Fig. 3B). Cycles contain between 4 and 9 packages.

<figure><img src="image_4.png" /><figcaption>Figure 2 – Core C1 (A) and core C4 (B) photography and grain-size evolution (heatmap). Red arrows indicate mud-dominated beds (mdb). On A and B: Zoom 1 on mdb-to-mdb packages with distinct sand-mud couplets; Zoom 2 on mdb-to-mdb packages without distinct sand-mud couplets and with amalgamated mdbs. fslt, mcslt: fine, medium to coarse silt; vf, f, m, c: very fine, fine, medium, coarse (sand).</figcaption></figure>

210Pbexc and 137Cs measurements (Fig. 3C): In core C4, 210Pbexc values are very small and without consistent variation with depth. 137Cs is present with maximum values between 15 and 55 cm depth. For core C1 there is no significant 210Pbexc, and no sign of 137Cs.

4 DISCUSSION

According to their elevation of 10 to 10,5 m, only spring tides can reach the sites, and not all spring tides, only those reaching at least this elevation. Present-day high tide level variations (for the period 2021-2024) show typical semi-annual cycles (equinox-solstice-equinox), comprising ca. 12 semi-lunar cycles (Fig. 4). Above an elevation of 10 m (purple curve on Fig. 4), less than 10 spring tide periods on average, in some case only 7, over 12, are effective.

The average number of 7 mdb-to-mdb packages per cycle found along core C1 approaches the number of spring tide periods during which the site can be inundated. Based on this comparison with present-day tidal data, we thus assumed that all mdb-to-mdb packages in core C1 represent the sedimentary record of semi-lunar cycles, and the cycles of increasing-decreasing thickness of mdb-to-mdb packages are semi-annual. Semi-lunar cycles are necessarily incomplete as the site is not inundated by neap tides. This interpretation leads us to conclude that C1 succession, characterized by a coherent cyclic signal of thickness evolution and the absence of obvious discontinuity along the core, represents a 3-year (Y1 to Y3 on Fig.3A) sedimentary record. Within this record, semi-lunar cycles with amalgamated mdbs occur during one half of years 2 and 3 (Fig.3A, pink rectangles). Therefore, we suggest that their occurrence could be of seasonal origin. However, the reason explaining amalgamation, i.e., the deposition of successive highly mud-dominated tidal “couplets” and that are not individually distinguishable, is matter of debate. Deposition of successive thick mud beds
during very high suspended fluvial load period, better preserved when spring tides are not the strongest, i.e., close to neap tides, could be an explanation. Homogenization of mud-dominated couplets due to freezing during winter can as well be evoked. We could indeed observe during a field survey in September ice-crystal cracks on the upper tidal flats along the Chico River estuary, as well as a very high density fracturing of tidal bedding due to these cracks in cross-section.

<figure><img src="image_5.png" /><figcaption>Figure 3 – Thickness evolution of mdb-to-mdb packages (red arrows) along core C1 (A) and core C4 (B). Red numbers: number of packages per cycle (smoothed red curve). Y on A for year (see text for explanation). Pink rectangles on A indicate mdb-to-mdb packages with amalgamated mdb; C: Results of 137Cs (blue) and 210Pbexc (red) measurements on C1 and C4.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 4 – Present-day high tide level variations for the period 2021-2024 at Punta Quilla (data from at the Hydrographic Navy Service of Argentina). Purple curve: high levels above 10 m (above lowest tide level), green rectangles: solsticial minimum.</figcaption></figure>

Compared with C1, the signal in core C4 is more difficult to decipher. The succession obviously contains individual tidal couplets (semi-diurnal couplets), as well as amalgamated mdbs, or at least thick mud dominated layers, that could sign very high suspended fluvial load episodes. However, mdb-to-mdb packages are not easily individualized along the whole succession, so that thickness measurements were difficult. The presence of discontinuities is probable but impossible to discern. The site is exposed to waves and eolian sand supply induced by permanent westerlies, and is influenced by the highly fluctuating fluvial
discharge and water levels of the Santa Cruz River. We thus believe that C4 succession records a very disturbed tidal signal.

According to the tidal signal, the sedimentation rate along C1 can be estimated around 30 cm/yr. This very high rate of deposition, typical of tidal rhythmite successions, is confirmed by the extremely low 210Pbexc values resulting from dilution by the high sediment flux. 210Pbexc values along C4 support similar very high rate. It is probably much lower than that estimated along C1, according to the combined wave and tide signal it contains, with discontinuities, but it certainly approaches a few cm per year. The apparent lack of 137Cs in core C1 could simply result, as the low 210Pb excesses, from an extreme dilution due to the very high sedimentation rate. Moreover, 137Cs fallout in the Southern Hemisphere was much less than in the Northern Hemisphere. For instance, very low values are detected in lagoonal cores in Australia, with undefined peaks (Pfitzner et al., 2004). Based on the values of 137Cs measured in surface sediment of a core from the Rio de la Plata River, Argentina (Di Gregorio et al., 2007), and assuming a similar 210Pbexc flux, values lower than 0.06 mBq/g of 137Cs can be anticipated for C1, which are close to the detection limit and hardly measurable. The higher 137Cs activities in C4 would then result from the lower sedimentation rate. Alternatively, the lack of 137Cs in core C1 could suggest that this succession was deposited before the 1960s, and the 137Cs "peak" in C4 could correspond to the 1963 atmospheric maximum. Although, on the field, C1 and C4 cross-sections were looking "fresh", satellite image comparison shows the very high stability of the channel and flat system position, especially on the core C1 site, during the last 40-50 years at least, signifying that this tidal flat is probably no longer active.

# 5 CONCLUSIONS

The two studied rhythmite successions well reflect the contrast in hydrodynamics and deposition processes that characterize the tidal flats along the edge of the Santa Cruz-Chico River estuary. The succession cored in the sheltered flat at the mouth of the tide-dominated Chico River estuary arm contains well expressed tidal rhythmites representing a 3-year record composed of neap-spring-neap and semi-annual cycles. A seasonal modulation is probably recorded through the deposition and preservation of thick mud beds during half of year, and related to very high suspended fluvial load. The succession cored along the tide-influenced Santa Cruz River arm obviously contains tidal beddings but fortnightly and semi-annual cycles are hardly detectable. The site, although subject to tidal dynamics, is exposed to the intense westerlies and induced agitation, giving rise to the record of an altered tidal signal. 137Cs measurements did not reach in concluding weather the studied successions were deposited very recently or date back a few decades or more. Satellite data comparison does not evidence sedimentological and morphological changes of the studied sites since at least 50 years, indicating that tidal flat aggradation is very poorly active nowadays.

# 6 ACKNOWLEDGMENT

The present study is part of the Hypertide project, funded by the ECOS-SUD - MINCYT program, and of the IRP Hypertide funded by CNRS-INSU. The authors thank Robert B. Dalrymple and Sergio Longhitano for their careful and helpful review. They would like to express as well their gratitude to Alain Trentesaux, Nicolas Tribovillard and Jean-Yves Reynaud, the head-organizers of the successful French Congress of Sedimentology, ASF Lille 2024.

# 7 REFERENCES

Archer, A. W., 2013. World's highest tides: Hypertidal coastal systems in North America, South America and Europe. Sedimentary Geology, 284, 1-25.

Depetris, P. J., Gaiero, D. M., Probst, J. L., Hartmann, J., Kempe, S., 2005. Biogeochemical output and typology of rivers draining Patagonia's Atlantic seaboard. Journal of Coastal Research, 21, 835-844.

Di Gregorio, D. E., Niello, J. F., Huck, H., Somacal, H., Curutchet, G., 2007. ²¹⁰Pb dating of sediments in a heavily contaminated drainage channel to the La Plata estuary in Buenos Aires, Argentina. Applied Radiation and Isotopes, 65, 126-130.

Pancrazzi, L., Tessier, B., Weill, P., Mouazé, D., Cuitiño, J. I., Reynaud, J. Y., Scasso, R. A., Duperron, M., & Montes, A., 2024. Architectures and dynamics of a late Quaternary coarse-grained barrier complex at the inlet of a hypertidal estuary (South Patagonia, Argentina). Journal of South American Earth Sciences, 146, 105073.

Pasquini, A. I., Depetris, P. J., 2011. Southern patagonia's perito moreno glacier, lake argentino, and santa cruz river hydrological system: an overview. Journal of Hydrology, 405, 48-56.

Pfitzner, J., Brunskill, G., Zagorskis, I., 2004. ¹³⁷Cs and excess ²¹⁰Pb deposition patterns in estuarine and marine sediment in the central region of the Great Barrier Reef Lagoon, north-eastern Australia. Journal of Environmental Radioactivity, 76, 81-102.
Piccolo, M.C., Perillo, G.M.E., 1999. The Argentina estuaries: a review. In: Perillo, G.M.E., Piccolo, M.C., Pino-Quivira, M. (Eds.), Estuaries of South America; Their Geomorphology and Dynamics. Springer-Verlag, New York, pp. 101–132.

Salameh, E., Desroches, D., Deloffre, J., Fjørtoft, R., Mendoza, E. T., Turki, I., Froideval, L., Levaillant, R. Déchamps, S., Picot, N., Laignel, B., Frappart, F., 2024. Evaluating SWOT's interferometric capabilities for mapping intertidal topography. Remote Sensing of Environment, 314, 114401.

Tessier, B., 1998. Tidal cycles: annual versus semi-lunar records. In: Alexander, C. R., Davis, R. A., & Henry, V. J. (Eds) Tidalites: processes and products. SEPM Special Publication, 61, pp. 69–74.

Tessier, B., 2023. Tidal rhythmites: Their contribution to the characterization of tidal dynamics and environments. In: M. Green, *J.C. Duarte* (Eds) *A Journey Through Tides*, Elsevier, pp. 283-305.

Tessier, B., Reynaud, J. Y., Cuitiño, J. I., Scasso, R. A., Pancrazzi, L., Duperron, M., Weill, P., Bout-Roumazeilles V., Armynot du Châtelet, E., Kuinkel, A., Lortie, T. Dezileau, L., 2024. The hypertidal Santa Cruz-Chico River Estuary (South Patagonia, Argentina): A hybrid ria-type system under extreme tides, arid climate and active uplift. Sedimentary Geology, 471, 106728.