Sedimentology and Stratigraphy of Silicified Fossil Trunks of the Missão Velha Formation (Late Jurassic-Early Cretaceous), Araripe Basin, Northeastern Brazil: Paleo Climatic Implications

The present paper describes sedimentological, stratigraphic and climatic implications of silicified fossil trunks of the Missão Velha Formation (late Jurassic-early Cretaceous), Araripe Basin, Northeastern Brazil. This rich fossiliferous plant assemblages is dominated by conifers restricted to Agathoxylon and relates only to the Missão Velha Formation (MVF) coarse sandstone and conglomerate beds deposits. They are predominantly embedded in fluvial sediments deposited during the Early Rift phase of Araripe Basin. Silicified fossil woods and trunks of the MVF were recognized since 1863. Several works have discussed its morphology, internal structure and palaeogeographic implications. Regarding the taphonomy, these macrofossils have been interpreted as hypautochthonous in relation to their growth sites. Some authors, however, have interpreted these fossils as allochthonous in origin. Despite these important features, the sedimentological and stratigraphic characteristics of theses associations had received little attention on previous works. Recent investigations have demonstrated that fossil trunks belong solely and exclusively to the MVF, restricted mainly to the upper parts above an internal unconformity. Two main taphofacies were found: wood logs, and common wood fragment. Although, samples of wood fragment occur in most of nearby sandstone outcrops of the upper part of the unit, the larger wood logs occur particularly in a coarse sandstone to conglomeratic bed strata in the top of the MVF deposits. The main aspects of the trunk preservation include uncompressed fragments with poorly rounded edges, with the cell walls preserved by petrification and spaces permineralized by silica, suggesting short transport and fast burial. Silicified fossil woods are disposed in foresets of planar and though cross-stratifications, which bearing coarse-grained to conglomeratic sandstones, whose frequency is very abundant, sometimes constituting anomalous concentration as in Grota Funda outcrop. Trunk dimensions range from 5 cm up to 2 m long, including particularly a large existing (approximately 1.70 x 1.40 m). Some fossil trunks are concordant with the strata, sometimes other dissenting position, but most investigated mainly occurs without preferential orientation. The trend pattern of orientation analysis of 11 stems with geological observations suggest a predominant south paleocurrent direction trend, originated during monsoon storms, with a near source area on North, possibly in a shallow channel system context, generating the assemblages by putrefaction processes and late fragmentation and size-selection by fluvial transport. The transport of wood was relatively short, as proved by the edges, no preferential position and the size of the trunks. Thus, it is likely that the trunks came from own basin, to the detriment of the area of the basement, hence the hypautochthonous character. The host unit and the plant remains display strong resemblance with most of the same age deposits in Northeast Brazilian basins, as Recôncavo-Tucano-Jatobá, Sergipe-Alagoas and Camamu-Almada basins, and Gabon Basin in Western Africa. Evidence from the trunk taphofacies, stratigraphy and sedimentology of the Missão Velha Formation is consistnt with a warm, seaonal, hot and humid climate, suggestive for monsoon climate, as in other conifer forests in Gondwana during the Mesozoic.