Sedimentology and stratigraphy of silicified fossil trunks bearing Missão Velha Formation (late jurassic-early cretaceous), Araripe Basin, Northeastern Brazil: Paleoclimatic implications

Abstract

The paper focuses on the sedimentological, stratigraphic, and climatic implications of silicified fossil trunks found in the Missão Velha Formation (MVF) of the Araripe Basin in Northeastern Brazil. The rich fossiliferous plant assemblages in the MVF are primarily dominated by conifers, specifically a group known as Agathoxylon, and are associated with coarse sandstone and conglomerate fluvial beds. The MVF represents a late Jurassic-early Cretaceous formation deposited during the Early Rift phase of the basin. Silicified fossil trunks and woods from the MVF have been known since 1863, and previous studies have discussed their morphology, internal structure, and paleogeographic implications. Taphonomically, these macrofossils have been interpreted as hypautochthonous, meaning they originated from their growth sites. However, some authors have suggested an allochthonous origin for these fossils. Despite their significance, the sedimentological and stratigraphic characteristics of these fossil associations have received limited attention in previous research. We have collected the fosil trunks exclusively in the MVF, and they are restricted to the upper parts above an internal unconformity. Two main taphofacies were identified: T1 comprises coarse-to medium-sandstones, locally conglomeratic, with planar and trough cross-stratifications, with abundant fossil wood and trunks, and T2 defined as well-stratified pebbly conglomerates, with horizontal to cross bedding, with larger common fossil wood and trunks fragments. T1 taphofacies occur in most of the cross-stratified sandstones of the upper part of the MVF, T2 taphofacies (larger wood logs) are primarily concentrated in a coarse sandstone to conglomeratic bed strata at the top of the MVF. The preservation of the trunks is characterized by uncompressed fragments with poorly rounded edges. The cell walls are preserved by permineralization, with the spaces filled by silica, indicating short transport and rapid burial. The silicified fossil woods are found in foresets of planar and trough cross-stratifications, associated with coarse-grained to conglomeratic sandstones. These sandstones are highly abundant, sometimes forming anomalous concentrations, such as in the Grota Funda outcrop. The dimensions of the fossil trunks range from 5 cm up to 2 m in length, with some exceptionally large trunks measuring approximately 1.70 × 1.40 m. The investigated trunks exhibit various orientations, with some concordant and others dissenting from the surrounding strata. However, the majority of the fossil trunks do not show a preferential orientation. Orientation analysis of more than twenty logs suggests a predominant south paleocurrent direction trend, likely resulting from monsoon storms originating from a source area to the north. The assemblages were likely formed through putrefaction processes and late fragmentation, followed by size-selection through fluvial transport. The relatively short transport distance is indicated by the preserved edges, lack of preferential position, and the size of the trunks. Consequently, it is inferred that the trunks originated within the basin itself rather than being transported from the basement area, supporting their hypautochthonous character. The MVF and the associated plant remains bear resemblance to deposits of similar age in other Northeast Brazilian basins, such as the Recôncavo-Tucano-Jatobá, Sergipe-Alagoas, and Camamu-Almada basins, as well as the Gabon Basin in Western Africa. Evidence from the trunk taphofacies, stratigraphy and sedimentology of the MVF is consistent with a warm, seasonal, hot, and humid climate, indicative of a monsoon climate similar to other conifer forests in Gondwana during the Mesozoic era.