FOSSIL-DIAGENESIS AS AN INDICATOR OF PALEOENVIRONMENTAL CONDITIONS: INDICATIONS OF ARIDITY AT THE TRIASSIC-JURASSIC BOUNDARY

CARINA COLOMBI; ROGERS, RAYMOND

Mendoza

Congreso; IV International Paleontological Congress; 2014

Permineralization of vertebrate fossils is typically a story of calcite and Fe-rich mineral paragenesis. Detailed study with a focus on a broad range of minerals using scanning electron microscope (SEM) has revealed a more complex history of diagenesis related to paleoclimate and tectonics. Eight paleovertebrate assemblages from the Ischigualasto and Marayes Basins, ranging in age from Middle Triassic to Lower Jurassic, were analyzed. Five different diagenetic pathways were discerned, and compared with sedimentological, pedogenic and macro-scale taphonomic studies. The bones from Middle Triassic Chañares Formation (Ischigualasto Basin) are characterized by minor alteration of bone at the microscale, and precipitation of Fe-rich minerals, presumably during a short interval of anaerobic decay. Permineralization ceased when bone voids filled with secondary apatite. The paucity of authigenic minerals potentially reflects the impermeable nature of host smectite-rich volcaniclastic sediment. Bones from the Upper Triassic La Peña and Cancha de Bochas Members of the Ischigualasto Formation (Ischigualasto Basin) are characterized by calcite and Fe-rich minerals, and this is consistent with recovery from oxidized calcic paleosols. A second stage of calcite precipitation was also documented, and this presumably reflects interactions with phreatic solutions after burial. This diagenetic history is consistent with a seasonal semiarid climate, and also potentially reflects low rates of sedimentation during the early burial stage. In contrast, fossil bones from the Upper Triassic Valle de la Luna Member (Ischigualasto Formation) and those recovered from the base of Los Colorados Formation (Ischigualasto Basin) are characterized by advanced bone alteration and a thick coating of Fe-rich minerals, indicating potentially longer anaerobic decay under humid conditions. It is also important to note that the fossils from the Valle de la Luna Member are permineralized with silica and barite, and this possible reflects volcaniclastic input in the uppermost Ischigualasto Formation. Interestingly, fossil bones from the base of the Los Colorados Formation also show evidence of halite precipitation, which is consistent with a return to arid conditions. This trend is also evident in the top of the Los Colorados Formation (Norian), where halite is accompanied by dolomite and rare sulfates in bone voids. Remarkably, fossils from the contemporaneous Quebrada del Barro Formation (Marayes Basin) also preserve halite and dolomite, which suggests widespread aridity played a role in bone fossilization during this time. This study of bone diagenesis from eight vertebrate fossil assemblages is consistent with other lines of evidence that suggest an increase in aridity across the Triassic-Jurassic boundary.