Tephras in lacustrine sediments of the Sarliève marsh (French Massif Central): age and preservation

Abstract

The Sarliève marsh sediments (Massif Central, France) contain two tephras. The first tephra [13.7 ± 0.4ka(2 δ), ca. 12 000 BP], regionally well known, enables to date the beginning of lacustrine infill to the Lateglacial. The second tephra, the 'tephra de Sarliève', the emitting volcano of which is unknown, would be dated to around the Early Subboreal from pollen data. This occurrence, after the discovery of the 'tephra de Beaunit', emphasizes that volcanic eruption(s) occurred in the 'Chaîne des Puys' or in the volcanic Cézallier more than 1000 years after the last known eruption (Pavin) in the 'Chaîne des Puys' at around 6.6/6.7 ka (5800/5900 BP). In the Sarliève piles, these tephras, well preserved in thick and more silicated deposits of deltas, were not observed in carbonated basin sediments where they were altered. The abundance of authigenic zeolites formed during the Lateglacial in restricted depocentre lacustrine waters allows us to detect initial CF1 tephra occurrence. . Le plus ancien [13,7 ± 0,4ka(2 δ), environ 12 000 BP], bien connu dans la région, permet de dater le début du remplissage sédimentaire de la cuvette lacustre du Tardiglaciaire. D'après les assemblages polliniques des sédiments qui le contiennent, le « tephra de Sarliève », émis par un appareil volcanique non identifié, pourrait dater du début du Subboréal. Cette découverte, faisant suite à celle du « tephra de Beaunit », suggère une (des) éruption(s) volcanique(s) dans la chaîne des Puys ou dans le Cézallier volcanique, plus récente(s) d'environ 1000 ans que la dernière éruption connue dans la chaîne des Puys (Pavin). À Sarliève, les tephras, bien conservés dans les dépôts plus épais et silicatés de deltas, s'estompent dans les dépôts carbonatés de bassins où ils ont été altérés. L'abondance de zéolites authigènes formées au Tardiglaciaire dans les eaux lacustres confinées des bassins est un bon traceur de la présence initiale du tephra CF1. Pour citer cet article : A. Fourmont et al., C. R. Geoscience 338 (2006).

Contrasted Late Glacial and Holocene hydrology of Sarliève paleolake (France) from sediment geometry and detrital versus biochemical composition

“…14 C ages obtained on seeds and charcoal are in stratigraphic order and are consistent with previous chronologies on pollen (de Beaulieu et al 1988;Guenet and Reille 1988;Argant and Cubizolle 2005;Argant and Prat in Fourmont et al 2006), whereas bulk sediment dates appear more than 1 ka too old (Fig. 6).…”

“…Zeolites are present in B1 (maximum 22%), and occur occasionally in B4 and B6 (up to 10%). As specified in Fourmont et al (2006), B1 unit contains abundant analcite in the middle part of the depocenter, whereas clinoptilolite occurs around this area. Main habits observed by SEM are the following: quartz: (Q1) silt-sized worn and broken fragments, and (Q2) often indistinguishable even if abundant, probably forming micro and/or nanocrystals mixed with clays; feldspars: (F1) worn and broken as well as pristine plagioclases, and (F2) potassic feldspars; zeolite (only analcite was observed): (Z1) worn and broken silt-sized clasts, (Z2) filling of lens-shaped cavities, and (Z3) pristine silt-sized cubic crystals.…”

“…Zeolites were observed in B1, B3 and B4-B6 units. According to Fourmont et al (2006), particularly abundant zeolites (clinoptilolite or analcite) encountered in the B1 unit have an authigenic origin: Z2 (filling of lens-shaped cavities) and Z3 (pristine cubic crystals) habits would have resulted from in situ crystallization due to reactions between CF1 tephra and brackish to saline and/or alkaline waters, with lowstand water levels, as shown in different endorheic basins (Teruggi 1964;Hay 1966;Sheppard and Gude 1968, 1969Surdam and Parker 1972;Surdam and Eugster 1976). This explains why CF1 tephra is generally absent in the B1 unit, while zeolites with Z1 habit (worn and broken clasts) observed in B4-B6 units probably result from detritism.…”

“…They are 4-5 m thick and located in the median part of the depression. From base to top, deposits are made up of several units: D1, beige to greenish carbonate-rich clayey-silts (CS) containing encrusted vegetal macrorests; T1, black pyroclastic sands corresponding to unreworked CF1 tephra (Vernet and Raynal 1995;Fourmont et al 2006) as shown by heavy minerals and geochemical composition specified in Vernet (2005); D2, alternation of dark tephritic sandy layers reworked from CF1 tephra, with beige carbonate-rich sandy to silty layers; D3, heterometric sandy colluvium topped by a paleosoil; D4, decimeter-thick ochre sandy layer; D5, homogenous greenish calcitic CS, including a 2-cm-thick pale pink tephritic ash layer, T2; T2, called "téphra de Sarliève" (Miallier et al 2004;Fourmont et al 2006); D6, charcoal-rich black calcitic CS; D7, greenishgray to brown calcitic CS. Moreover, S177 pit (Fig.…”

The Volcanic Chaîne des Puys: A Unique Collection of Simple and Compound Monogenetic Edifices

“…However, the most recent eruption in the central part took place on the cones of la Nugère and Lassolas, with trachybasaltic lava fl ows, while the last eruption formed the Lake Pavin maar to the south with trachyandesitic tephras. A few tephras without identifi ed sources suggest that more recent but small eruptions may have occurred between 6 and 4 ka (Juvigné et al 1986 ;Fourmont et al 2006 ), although this remains to be confi rmed.…”