Palynostratigraphy and environment of “Strunian” and “late” Visean diamictites from South America (Western Gondwana)
How to read the poster During the “Strunian”, the warmest sea-surface temperatures in the tropical belt (1) coincide with the development of polar cold and dry climate on Western Gondwana (devoid of vegetation and probably of ice), a climate which apparently started earlier nearer to the pole (Parnaiba Basin) than around 60°S (3,4). The development of ice cap reaching sea level (usually called “glacial” period) depends on maybe less cold but wetter periods allowing vegetation cover and diamictite deposits in several regions (4) during the LE?- LN Zones when the climate having become also more unstable, oscillated between glacial and interglacial phases (1,2). The existence of such “glacial” climate at the end of late Famennian (VCo-LL Zones) is poorly demonstrated so far (6,7) and its persistence into the earliest Tournaisian, still unproved (5). A confusion between “Strunian” and “late” Visean diamictites (6) or their close superposition (7) may occur. Diamictites often contain (much) more reworked than indigenous miospores (8,9) and, in the absence of reliable independent Gondwanan faunas, their dating depends on the correct identification of the youngest first occurring miospores in the calibrated miospore/conodont/forams scale of Western Europe (1, 10) . Detailed analysis of tillites and rhythmites demonstrates the almost contemporaneity of the indigenous vegetation and frontal stabilized moraines (8,12). A review of the “late” Visean miospore assemblages in South America suggests that several miospore zones must probably be recognized, ranging from early?/late Visean to Serpukhovian (11). Intercalations of the warm Paraca Megaflora (14) within this 4 Myr long period is well possible (13) but is in contradiction with the recent suggestion that ice masses persisted during the Visean (10).
1
High resolution record of oxygen isotope ratios of conodont apatite is given from postera to sandbergi conodont zones and compared with miospore zones across the Devonian/ Carboniferous boundary in southern Europe. Significant positive carbon isotope excursion (not shown here) from the Middle and Upper expansa zones coincides with a decrease in the oxygen isotope values (Kaiser et al. 2008). Interpretation data (on the left of the main chart) taken from the same authors are compared with the miospore zonation of western Europe and its equivalent in Western Gondwana. Western Gondwanan climates are based on our (Streel) assumption that the development of polar cold and dry periods (devoid of vegetation and probably of ice as suggested by Ormiston & Oglesby, 1995) coincide with the hottest sea-water in tropical belt, the development of ice cap reaching sea level (“glacial” period) depending on probably less cold but wetter climate when mid-latitude cyclonic activity allowed sufficient polar transportation of moisture to form large snow cover (see the proposal of Wright 1990 of a deflection of rain-bearing winds into Gondwana during the Tournaisian, explaining aridity in the Cornubian Basin).
2
The bathymetryand palynofacies of the Stockum section in Western Europe indicate that, during the miospore LE -LN Zones, the climatic system might have become more unstable oscillating back and forth between glacial and interglacial phases.
3
Comparison between Amazonas and Parnaiba latest Famennian diamictites.Several additional wells have been studied in Brazil since 1993, particularly in the AmazonasBasin (see Melo & Loboziak 2003). Note the absence of other Famennian miospores below the diamictites in the ParnaibaBasin.
4
South American localities (A to M) with latest Famennian diamictites shown on figs 3, 5, 6, 7. Note on figs. 3 and 8 the absence of other Famennian miospores below the diamictites or reworked in the diamictites, in the ParnaibaBasin, suggesting, as shown on fig. 1, that a cold climate without ice and vegetation started slightly earlier at 70° than at latitude 60° S. It should be also noted that there are another extensive record of diamictites in the eastern part of the Solimões Basin, in the subsurface of the Juruá Sub-basin (Jaraqui Member of the Jandiatuba Formation), yet of a slightly younger age than those discussed herein (late middle-to early late Tournaisian, BP-PC Zones, according to Loboziak et al., 1995).
5
The early Tournaisian age of sample 9b is here challenged and might as well belong to the uppermost Famennian (LN* Zone, see fig. 2).This may prove to be important because the Tournaisian part of Glacial Episode 1 of Lopez-Gamundi (1997) is based on this material (see also Iannuzzi & Pfefferkorn 2002, here below). Samples -9a and -4a in the Colpacucho Fm of the Villa Molino section belong to the “late” latest Famennian LN Zone and should indicate that the thick sandy part of that formation does correspond to a gap between the Cumana Fm and the Kasa Fm in the two sections ( ). However, correlations are shown differently by the authors. Should these author correlations be confirmed, then the succession LE-LN will prove to be unusable in this area. The scarcity of the miospore Verrucosisporites nitidus might render the distinction between these two zones unrealistic in the northern Brazil basins (Melo & Loboziak, 2003).It was the reason why they subsequently selected Vallatisporites vallatus as a marker for the LN Zone (see LVa in fig. 1) which has been proved ( Streel & Traverse, 1978; Higgs & Streel 1984) to first occur in the highest part of the Retispora lepidophyta range.
6
At Pando well, a Mississippian miospore assemblage is present 30 m above the top of a late? to latest Famennian diamictite. At Manuripi well, two diamictites are present. Rugospora polyptycha and other species indicate a Mississippian age, 10 m above the top of the late? to latest Famennian lower diamictite. Taking into account these miospore data and the lithological correlation between Pando and Manuripi wells suggested by Isaacson et al. (1995), it appears that the upper diamictite (?) at Manuripi might well be late Visean i.e. about the same age as the shaly part of the Pando well above the late? to latest Famennian diamictite.
7
The Saipuru Fm (also known as Ituaca Fm) contains thick diamictites. Their age is late/latest Famennian in Perez-Leyton 1991 and Wicander et al. 2006, observed at Bermejo, near Santa Cruz de la Sierra, or “early” Visean age in di Pasquo 2007, studied at BermejoRiver, at the Argentine border. These ages are not necessarily in contradiction and might well depend on the importance of the hiatus separating this formation from the underlying Los Monos/Iquiri Fm.
A reexamination of the material studied by Perez-Leyton (1991) suggests that the Saipuru Fm might even contain diamictites of different ages, ranging from his sample M19 (late Famennian) to samples M12, 13 which contain indeed several species of Vallatisporites with aMississippian connotation, instead of the formerly published “latest Famennian” LE Zone. Such a rather close succession of Famennian and Mississippian diamictites can be compared with the observation made in the Manuripi well of the Madre de DiosBasin (see Fig. 6).
8
Tillites and associated silts and laminites from well 1-TA-1-MA (G on figs 3 and 4) allow identifying 41 taxa of miospores. Indigenous miospores are represented by 12 species with Vallatisporites vallatus having the youngest first occurrence in latest Famennian Zone LN. 29 taxa (70 % of the total)are reworked from Givetian and Frasnian sediments. The most recent reworked material is late Frasnian. No reworked miospores restricted to early to late Famennian can be demonstrated. The reworked miospores identified in the laminites might be of Frasnian origin only, which could indicate that a more distant Givetian source area has been added to the reworked material in the tillites and associated shales. The laminates show a distinct alternation of sandy and silty layers, They contain an almost similar proportion of palynomorphs but the ratio miospores/miospores+acritarchs increases, in sandy layers, from doublet B to doublet D contrasting with a number of miospores /g decreasing simultaneously in silty layers,. This probably resultsfrom changing intensities of the spring water discharges. The laminites are true varves, where doublets B to D might correspond to one single very short climatic cycle. Carboniferous glacimarine rhythmites were described in Argentina (Milana & Lopez 1998, del Papa & di Pasquo 2007) but analysing the palynomorphs of the different layers of the rhythmites was not attempted.
9
Cores from shallow wells for construction of a dam on the TocantinsRiver (Parnaiba Basin, Brazil) presented sandstones, siltstones and dark gray diamictites, very similar to late Devonian tillites. The age of the miospores contained in the diamictites as well as in theoverlying silts is however Visean and most probably late or even maybe “latest” Visean. The material is comparable with, but not identical to, the miospore assemblage (Mag Z.) found in the Poti Fm (Melo & Loboziak 2003)..
10
The Brazilian Jandiatuba, Faro and Poti formations contain miospores from the Mag Zone (Melo & Loboziak 2003) equivalent to the Western Europelate Visean (Asbian) TC and MN Zones (now ME + DP in Clayton 2006). Those formations are bordered by unconformities which suggests the absence of late Tournaisian to middle Visean sedimentation. The climatic interpretation (cold but dry, after Streel date?) is not matched by preliminary 18O data (Joachimski et al. 2007), who suggest that ice masses occurred in the Tournaisian, persisted into the Visean and intensified during the Serpukhovian.
11
Other possible” late” Visean miospore assemblages from the western side of South America. Columbia, Llanos Orientales Basin.1: Duenas 2001, 2: Duenas & Cesari 2005. Peru, Altiplano 3, 4: Azcuy & di Pasquo 2005, 2006. Bolivia, Madre de dios Basin 5: Fasolo et al. 2006. Argentina, Tarija Basin 6: di Pasquo 2007.
Argentina, Central-western Basins 7: Sessarego & Cesari 1989, 8, 9: Cesari & Limarino 1992, 1995, 10: Pazos et al. 2005, 11: Amenabar2006, 12: Amenabar et al. 2006-2007, 13: Cesari & Gutierrez 2000, 14-16: Perez-Loinaze 2005, 2007, 2008.
> Serpukhovian: Mag Z. ? + pollens > latest Visean: Mag Z.? late Visean: Mag Z. > early? late Visean No Mag Z.
diamictites
12
A: Extremely cold condition (less cold but wet condition, Streel interpretation). The glacier advanced and deposited moraines at its front. B: Frontal moraines stabilized during a climatic amelioration that led to glacial retreat and incoming of sub-arborescent or arborescent vegetation at the beginning of a postglacial transgression. C: The improvement in climate continued leading to sea-level rise, the production of icebergs, the accumulation of muddy sediments with dropstones and the beginning of flooding of land close to the coast , causing a gradual destruction of the vegetation. D: Further climatic amelioration led to widespread flooding of lowland soils and continued retreat of the glacier.
13,14
Alternation of climates during 4 Myr allows glacial and warm climates in succession in western Gondwana
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