The upper Miocene diatomaceous sediments of the northernmost Mediterranean region: A lamina-scale investigation of an overlooked palaeoceanographic archive     

Luca Pellegrino  Francesco Dela Pierre  Richard W. Jordan  Kenta Abe  Yuta Mikami  Marcello Natalicchio  Rocco Gennari  Francesca Lozar  Giorgio Carnevale 《Sedimentology》2020,67(7):3389-3421

During the late Miocene the Mediterranean experienced a dramatic intensification of opaline accumulation, recorded by the deposition of diatomaceous sediments. The fine lamination of these deposits potentially records annual to sub-annual palaeoceanographic processes that occurred during a critical phase of the geodynamic evolution of the Mediterranean basin, which eventually led to the Messinian salinity crisis. The diatomaceous facies has been interpreted by previous researchers as the product of intensified upwelling currents and of bottom anoxia formation in the Mediterranean basin. However, until now, no efforts have been made to unravel the sedimentological and micropalaeontological content of these deposits at the lamina-scale. This paper presents the first case study of a systematic scanning electron microscope-based morphological investigation of the diatomaceous sediments deposited during the late Miocene at the northernmost offshoot of the Mediterranean basin (Piedmont Basin, north-west Italy). Using a non-invasive analytical approach, six faciological components (laminae, laminated packets, non-laminated intervals, burrows, opal-rich aggregates and mixed pelletal structures) and their relationships are described and interpreted herein. Following the lamina-scale study of these sediments, an annual sedimentary cycle could be identified and an accumulation rate (ca 50 cm kyr⁻¹) inferred that is atypical for a setting actively influenced by upwelling. The role played by the entanglement of diatom valves in creating a physical barrier to the bioturbation is here emphasized as the main process responsible for the preservation of the laminated fabric of diatomaceous sediments, challenging the supposed role of deep anoxia. These results suggest that the late Miocene diatomaceous deposition in the Piedmont Basin cannot be univocally considered as a by-product of upwelling intensification and seafloor oxygen depletion.  相似文献   

U–Pb ages of granitoids around the Kofu basin: Implications for the Neogene geotectonic evolution of the South Fossa Magna region,central Japan     

Yusuke Sawaki  Hisashi Asanuma  Mariko Abe  Takafumi Hirata 《Island Arc》2020,29(1)

The Japanese archipelago underwent two arc–arc collisions during the Neogene. Southwest Honshu arc collided with the Izu‐Bonin‐Mariana arc and the northeast Honshu arc collided with the Chishima arc. The complicated geological structure of the South Fossa Magna region has been attributed to the collision between the Izu‐Bonin‐Mariana arc and the southwest Honshu arc. Understanding the geotectonic evolution of this tectonically active region is crucial for delineating the Neogene tectonics of the Japanese archipelago. Many intrusive granitoids occur around the Kofu basin, in the South Fossa Magna region. Although the igneous ages of these granitoids have been mainly estimated through biotite and hornblende K–Ar dating, here, we perform U–Pb dating of zircon to determine the igneous ages more precisely. In most cases, the secondary post‐magmatic overprint on the zircon U–Pb system was minor. Based on our results, we identify four groups of U–Pb ages: ca 15.5 Ma, ca 13 Ma, ca 10.5 Ma, and ca 4 Ma. The Tsuburai pluton belongs to the first group, and its age suggests that the granite formation within the Izu‐Bonin‐Mariana arc dates back to at least 15.5 Ma. The granitoids of the second group intruded into the boundary between the Honshu arc and the ancient Izu‐Bonin‐Mariana arc, suggesting that the arc–arc collision started by ca 13 Ma. As in the case of the Kaikomagatake pluton, the Chino pluton likely corresponds to a granodiorite formed in a rear‐arc setting in parallel with the other granodiorites of the third group. The U–Pb age of the Kogarasu pluton, which belongs to the fourth group, is the same as those of the Tanzawa tonalitic plutons. This might support a syncollisional rapid granitic magma formation in the South Fossa Magna region.  相似文献