Journal of South American Earth Sciences最新文献

Trace fossils produced by fossorial mammals are relatively common in the North American fossil record. Yaviichnus iniyooensis consists of a system of chambers and tunnels marked by characteristic paired-groove marks. This text aims to analyze four burrows with well-preserved bioglyphs to determine whether one or more individuals produced them. The burrows were found in the Chilapa Formation (early Oligocene), which outcrops on the outskirts of Santiago Yolomécatl town, northwestern Oaxaca. Bioglyphs are visible on the external surfaces of the tunnel casts. These marks are short, straight, and paired, and are attributable to the incisor marks of Gregorymys spp. They may have been produced during the same ecological period. Analysis of the measurements of the width of these paired marks revealed two distinct groups: one ranging from 2.2 to 3.2 mm (mean 2.95 mm; n = 12) and another ranging from 3.4 to 5.3 mm (mean 3.99 mm; n = 31). A Mann-Whitney U test indicated statistically significant differences between the median widths of the groups. The presence of two different groups of incisor marks within a single burrow system suggests that multiple individuals were the producers. This supports the hypothesis that several individuals of Gregorymys veloxikua and G. mixtecorum were the attributable producers of Yaviichnus iniyooensis. The most plausible explanation is that burrows were reused by specimens of both species of Gregorymys, possibly linked to aridity conditions and non-solitary behaviour.

The integration of ICP-OES, XRD, chemical analysis, and scanning electron microscopy was used to investigate the source area, weathering, paleoclimate conditions, and provenance of the siltstones of the Santa Maria Formation (Middle to Upper Triassic, Southern Brazil). This unit is regionally subdivided, from base to top, into the Passo das Tropas Member (Late Anisian–Middle Ladinian) and the Alemoa Member (Middle Ladinian–Carnian). The Chemical Index of Alteration (CIA) was calculated for both members to highlight the weathering processes of primary minerals. The CIA results indicate moderate to extreme weathering of the source rocks for the siltstones. The extreme chemical alteration observed in the Passo das Tropas siltstones (CIA = 76.97–90.83) and in the mid-section of the Alemoa Member (CIA = 87.96–89.63) suggests weathering under more humid conditions, related to an increase in paleorainfall. Paleoclimate was interpreted as semi-arid to arid, with alternating wet and dry seasons. These results align with the geochemical behavior of uranium and thorium. Samples with higher CIA values show higher Th/U ratios, indicating a shift toward a more humid climate. X-ray diffraction analysis of the fine fraction (FF < 2 μm) of the samples identified mixed-layered illite-smectite (I/S) clay minerals with minor amounts of quartz. Regarding provenance, Th/U ratios coupled with Th vs. Sc and La-Th-Sc plots, suggest an upper crustal protolith for the southern Santa Maria siltstones, consisting of uplifted rocks from the crystalline basement of the Sul-Rio-Grandense shield and sedimentary rocks of the Camaquã Basin. The Th/Sc, Sc/Th, Co/Th, and La/Sc ratios, along with the contents of Al₂O₃, SiO₂, TiO₂, Co, V, Ni, and Sc, indicate a more felsic composition for the Passo das Tropas Member. Meanwhile, the source rocks of the Alemoa Member sediments demonstrate a more mafic composition in the mid-section, transitioning to a felsic composition in the uppermost succession.

A Cambrian sedimentary sequence with a predominant lithology of limestone, sandy limestone, and quartzite pertaining to the Bolsa and Abrigo formations is exposed in the Sierra Los Ajos to the east of Cananea city (northeastern Sonora, Mexico). Two fossil assemblages from Guzhangian layers of the Abrigo Formation are described herein. The trilobites identified comprise Tricrepicephalus texanus (Shumard), Crepicephalus sp., Coosia ariston (Walcott), Coosia sp., Cedaria cf. eurycheilos Palmer, Coosella sp., Llanoaspis modesta Lochman, and Llanoaspis? sp., representing the Cedaria and Crepicephalus zones (Miaolingian, Guzhangian). The Cambrian rocks of northeastern Sonora (Sierra Los Ajos, El Tule, and Mesteñas hills), southern Arizona, and Texas, were deposited as part of a thick sedimentary sequence on the southwestern margin of Laurentia, associated with an inner shelf marine environment. The distribution of the trilobites identified confirms the existence of a widespread Cambrian faunal province that includes the southeastern United States of America and northwestern Mexico.