Gymnosperm woods from interglacial deposits of the Late Carboniferous Hoyada Verde Formation, Argentina

PUJANA, ROBERTO ROMÁN; CÉSARI, SILVIA N.

Bariloche

Congreso; VII International Organization of Paleobotany Conference; 2004

MEF

Carboniferous fossil woods have been rarely recognized in Western Gondwana although the Upper Paleozoic floras are well known on the basis of frond remains. Nevertheless, the study of woods has became a key piece in the evolution of the Upper Paleozoic of the region not only by their stratigraphic value but also by the paleoclimatic information they provide. In this way, the finding of woods in the Hoyada Verde Formation that results into the first record of anatomically preserved stems in interglacial Carboniferous deposits of Western Gondwana is here presented. Hoyada Verde Formation represents one of the best records of the glaciation which took place at the begining of the Late Carboniferous in Argentina. The glacial sequences is dominated by subglacial and proximal glacial-marine sediments including different kinds of diamictites, pebbly mudstones and shales with dropstones. All these rocks are covered by postglacial transgressive shales bearing marine invertebrate fossils belonging to the early Late Carboniferous Levipustula Biozone. On the basis of lithological composition, sedimentary structure and architecture of depositional units, the glacial sequence has been here divided in five stratigraphic intervals. The lowermost (section 1, up to 10 m thick) is almost entirely composed of massive coarse diamictites very probably representing tilites and gravity-flow resedimented diamictites. Section 2 (up to 5 m thick) comprises shales with dropstones (up to 10 cm in diameter) deposited during an interglacial transgression that covered the basal diamictitic interval. Section 3 is the most important in this study because it includes the well anatomically  preserved woods. Basically this section is formed by 12 m thick shales, mudstones and thin levels of resedimented matrix-supported diamictites interpreted as deposited in proximal marine environment. The lack of dropstones and coarse diamictites suggests that ice bodies were not in contact with the sea during the deposition of section 3. In this context the major part of the trunks is found in mudstone levels, frequently forming carbonatic pseudoconcretions yellow in color. Section 4 shows similar lithological features to section 1 and is composed of coarse-grained massive matrix-supported diamictites that point out a new glacial episode important enough to form thick sequences of tilites. Finally, sections 5 and 6 represent a new postglacial transgression that firstly produced the sedimentation of shales with dropstones and thin levels of resedimentated diamictites (section 5) and later dropstones-free shales (section 6) including marine invertebrates of the Levipustula Zone. The woods, up to 15 cm in diameter and at least 60 cm long, are pycnoxylic, gymnospermous with centrifugal secondary xylem showing marked growth rings. The primary xylem has a mesarch protoxylem and the secondary trahceids have alternate, bi-multiseriate bordered pits with elliptic apertures exclusively in radial walls. The cross-fields have 4 to 15 circular areolate pits. The rays are homogenous, uniseriate and 1 up to 11 cells of height. The rings range between 0.70 to 0.98 mm in width suggesting low rates of growth (1.40 to 1.96 mm anual increase in diameter) probably as a consequence of temperate to cool temperatures. The low values of mean sensivity are possibly related to the constant water availability during the year. The lack of stems into the coarse diamictites (section 1) which correspond to the maximum glacial is remarkable. In the same way trunks were not found in shales with dropstones (section 2) deposited during earlier ice retreat when the glaciers were still in contact with the sea. The woods only appear in dropstone-free shales (section 3) very probably representing the vegetation developed in coastal regions during an interglacial interval. This flora would have been progressively flooded during the high-sea rise originating the migration of vegetation to higher regions. Neverteless, part of the vegetation was likely flooded and cosenquently massive death of specimens took place. Under this conditions at least part of the woods was transportated by flotation to marine setting and then deposited in the sea bottom. Similar situations of interglacial flooding of forests followed by trunk transport to marina or lacustrine envirnments have been described in proglacial Pleistocene deposits.